Response to AJ-NWS

Response to AJ of NWS - see AJ's comment on the previous post.

AJ makes a very good point here and one that serves to emphasize how difficult the forecast challenge is for the coming 84 hours. The three principal models each have different solutions, and we have little in the way of observational data to help evaluate the model initializations, since the key features analyzed by the models are within a great data void.

First, if one looks at the NAM, the GFS, and the ECMWF model runs from this morning – which I hadn’t when I wrote the original blurb – it is clear that each of the three models has somewhat different solutions for the initial conditions.

As per at 500mb:

The GFS analyzes an IT from south of GoC into north central MX where it connects with a trailing trough from the S/W over the central Missouri River Basin.

The ECMWF has this feature analyzed further north, extending from the south central GoC to SE Kansas.

The NAM is similar to the ECMWF except that it has several additional vorticity maxima further south and southeast which neither of the global models have.

We do know that such a feature is present and that at 12Z this morning it extended from Nebraska south-southwestward entering Mexico in the western Big Bend country.

Each model goes it own way in the forecast fields. The most important issue relates to whether or not there is a feature in the actual atmosphere that is going to spinup into a tropical system off Baja. Only the NAM indicates this evolution. The NAM solution would result in a very deep intrusion of subtropical moisture into the Southwest. At this time we mostly know that there has been ongoing convection in the regions where the NAM forecasts tropical development.

Both the GFS and ECMWF strengthen the inverted trough over north central Mexico and forecast this as the main feature that affects southern Arizona on Saturday and Sunday. The GFS stalls this feature over north central Mexico, while the ECMWF is more aggressive and keeps it further north and moving westward. The wind shear profile forecast on Sunday by the GFS is not nearly as favorable for organized storms (assuming that the key ingredients of moisture and CAPE have moved/developed north of the border). The ECMWF is probably in between the two other models wrt wind shear.

Current IR and water vapor satellite imagery indicate that the feature over north central Mexico has a clear circulation now and that it is moving westward. Significant convection is developing ahead of this feature over the Sierra Madre Occidental.

So where does all this lead? The forecast situation appears even more complicated than what I originally wrote. The most likely scenario for the next 84 hours is not at all clear at this time, and the evolving situation merits careful monitoring.

Challenging Forecast for the Weekend - Aug 23 and 24 2008

It appears that the large-scale setting here in the Southwest will be changing rapidly during the coming three days as very warm air in middle levels pushes into the Great Basin. Note the very warm 500 mb temperatures of only -4C at Medford OR and -3C at Desert Rock NV this morning. This warm intrusion builds the anticyclone back to our north and helps setup a more typical late summer flow regime.

Pressures have fallen and are quite low over the interior Southwest and may drop another couple of mb over the next 36 hours. There is also a weak inverted trough over the lower end of the GoC that has widespread convection associated with it. As this first disturbance shifts westward, there should be a low-level surge of moisture back into the lower Colorado River Basin. This moisture surge could begin as early as morning tomorrow but should be quite pronounced by mid-day Saturday.

The NAM forecasts that a tropical storm will develop just southwest of the tip of Baja and this feature, if it materializes, would deepen the moisture inflow to Arizona on Sunday and Monday. The NAM also forecasts very favorable wind shear profiles for organized convective storms in southeastern AZ both on Saturday and Sunday afternoons. Thus, both days could be very active if there is CAPE available.

It appears that the crucial aspect of the weekend synoptic setting will the temperatures in the middle levels. If the very warm air to the north and northwest spreads over southern Arizona and northern Mexico there would be little CAPE to realize, and it would likely be fighting an inversion aloft. However, if the easterly flow that develops at 500 mb has a fetch from the central and southern Plains by Saturday afternoon, the 500 mb temperatures would be much cooler and supportive of storms.

I think that the more favorable situation will evolve and, depending on the details, either Saturday or Sunday afternoon and evening should be quite active with strong and organized storms.

Slide shows re Severe Storm of 13 August 2008

Still bringing the new PC online, so none of the photos I took of damage produced by the early nigttime storm of Wednesday August 13th have been posted in the gallery. However, each of the Tucson newspapers have slide shows attached to their online articles about the storm.

http://www.azstarnet.com/sn/hourlyupdate/252838.php

http://www.tucsoncitizen.com/ss/local/93782.php

Computer Problems

I have been struggling with a failing PC here at the house and have been just able to get online every once in awhile. Thus, everything I usually do is to some extent on hold. We are going to replace this PC, but I may be out of touch for much of the next week or two, until a new computer is up and functional here. Bob

"When it rains it usually pours, but sometimes it just spits."

Rex Block
Private Investigator
Forensic Meteorologist

Sunday Morning Postmortem

Well my guess for yesterday afternoon was about as bad as I can do. Obviously the negatives dominated the local area around Tucson - only 7 of the 91 Alert gauges had rain yesterday, and the highest amount was 0.59 way down south at Tubac.

So what went wrong - I think that it was mainly related to the dry air in the upper-troposphere curling across southeast Arizona and keeping there from being much activity to the east - as per the WRF forecast. Although there were more storms to the east than the WRF indicated, they were not widespread and didn't produce any significant, organized outflows. With the GoC surge and increasing low-level moisture, the Tucson sounding at 00Z yesterday afternoon had substantial CAPE, but needed mesoscale convergence to kick it loose. That didn't happen.

As for today - moisture is way up as expected and there is substantial CAPE at both TUS and PHX this morning. El Paso is fairly stable so we have to assume that the air over the southeastern mountains is somewhat like the Tucson sounding. Morning wind shear from middle to upper levels is unidirectional but the NAM indicates that the wind profiles may improve by evening. Mainly a shear zone affecting southeast Arizona today and with no data from the south, it is hard to have much confidence in the model predicted winds aloft. So, there will be a lot of small and mesoscale interactions driving the evolution of storms today - good potential for more severe across all of southern Arizona and Phoenix could get hit by a strong, severe storm event. The chances are probably better for here at house today, but, even though it's not a binary call like yesterday, the shear profile could throw anvils over this part of town. So it's wait and see, as per many to most storm days down here.

The other wild card at play here is that there is a lot of strong morning storm activity out to the west of Tucson. If these storms continue to build produce early, cool outflows, that would complicate things greatly. I got a couple photos as sunrise showers moved by, producing an early morning rainbow. I'll post these to the blog later today.

Storm Outlook for Today

Visually spectacular storms yesterday evening - if anyone has photos for pasting on the blog, let me know.

Hot boundary layer temps and pocket of cooler 500 mb air, plus slight increase in moisture allowed storms, particularly at higher elevations, to overcome the strong inversion near 400 mb. Rain reported at 23 of the 91 Alert gauges in eastern Pima County - again mostly in mountains and foothills - the 1.50" at Agua Calienta Park seems an outlier, but there was spectacular lightning off that way. One wind damage report out at Pinal Airpark. Afternoon cells did not seem to have much in the way of precip shafts but then things obviously changed before dark.

Here at house - lightning, thunder and brief spit of rain shortly after 9 pm.

Situation today is very difficult with a number of competing plus and minuses.

Negatives first:

Very dry air to east in NM

Morning soundings continue with marginal IPW and boundary layer moisture.

Best difluence ahead of approaching upper-level, inverted trough appears to remain south of border.

Upper-level winds weak as we remain near the center of upper anticyclone.

Cool pocket at 500 mb has moved on to our west and northwest.

Pluses:

Nice push of subtropical moisture occurring into lower Colorado Basin, with some signature in PHX am sounding and Yuma VAD showing the influx to be about 4,000 ft deep. GPS IPW shows nice increases at the stations around the northern GoC.

Outflows from MCS over Mexico last night have pushed Td up across all of southern AZ and helped the GoC surge along.

Tropical low spinning off Baja in good location to keep low-level moisture flow going for a couple of days.

Very nice steering winds for storms that will tend to leave anvils to rear of strong cells.

Approaching inverted trough that extends down to 700 mb.

Summary - Looks good for strong storms with likely tropical squall line organization and somesevere storms and heavier rains, but today or tomorrow? I think things will come together today better than one might expect and that afternoon and evening should have strong storms - tomorrow will of course be highly affected by what does or does not happen during the next 12 hours. Chance of rain here at house probably at least 50% this evening.

Art Douglas Photos

Art has sent some digital photos from southeast Arizona - four of these have been posted on the Blog Gallery of weather images at www.madweather.com

Art's photos can be viewed directly at

http://www.squidinkbooks.com/madweather/photos-art-douglas.htm

Today's Weather - Southeast Arizona

Have been off-line for last two days due to dsl problems. So, it was the nice cool mornings that told me I shouldn't have doubted the NWS models and forecasts in keeping a dryout over Arizona for most of this week.

This morning there are, however, some indications of changes underway that will bring storms back into this part of the state. This morning there were quite a few ACC drifting around beneath the cirrus at sunrise. Since there has been no NWS sounding taken at TWC the last two sounding times, I have to guess about what a sounding here would look like this morning. There does appear to be some increase in east-northeasterly winds around 500 mb, along with some cooler temps advecting over southeastern Arizona today. Deep, subtropical moisture remains south of Hermosilla, but hot surface temps, along with the cooler middle level temps indicate that high-based storms should return to some of the sky island mountains of southeast Arizona late today, producing some lightning and gusty outflows.

The widespread convective activity over the southern half of GoC, coupled with falling pressures in the lower Colorado Basin, as well as the 500 mb wave moving slowly westward across the lower GoC all are good indicators that a northward push of the subtropical air is imminent. My guess is that we should see a push of low-level moisture into southwest and south central Arizona within 24 to 48 hours, followed by increasing storm activity for the next few days.

MORE BAD SOUNDING DATA AT TUCSON

I have just been perusing the morning sounding data and find that the TWC 1200Z sounding for today - Monday 28 July 2008 - poses a very substantial forecasting dilemma.

The sounding indicates IPW of 1.65" and the SPC analysis of this sounding indicates a forecast CAPE this afternoon of 1180 J/Kg. However, the GPS IPW from FSL indicates that precipitable water is probably between 1.3 to 1.4" - the difference between the sounding value and the GPS value being more than 0.25" - a huge difference and one that could seriously affect the convective forecast. See:

http://www.rap.ucar.edu/weather/upper/twc.gif

Note that the Td increases substantially between the surface value and the first level of sonde data - this aspect of the TWC soundings has been noted off and on - particularly on calm mornings - for the past several weeks. The Td values from the sonde then decrease nearly linearly to 715 mb, even though the sonde was passing through two elevated, residual boundary layers. The Td trace does not appear to be physically realistic and is probably too wet in each of the residual layers. Thus, I conclude that the Td data are bad and that the atmospere's dewpoint structure is quite different than that measured by the RRS sonde.

The reasons for the questionable data are not clear - but two possibilities come to mind - 1) local conditions on the roof of the NWS building may be affecting the first couple of data points and 2) the RRS sonde humidity sensor for this flight had a slow response time for Td and did not properly respond to Td changes below about 715 mb.

So, one is left wondering exactly what the local conditions might actually have been at the time of the sounding. So it goes.

Bad RRS Sonde Data Complicate Today's Forecast

Difficult forecast for today.

Low-level moisture has been sloshing around in different ways last night and this early morning, with decreases in the southeast from the east; increases in south central Arizona from the southeast; and increases in lower Colorado Basin due to weak surge which should continue and increase in strength and depth. Diurnal wind regime may return today in the Tucson area, which would increase the low-level moisture.

Entire forecast problem is acerbated by the TWC sounding which is apparently about 7.5 mm too wet this morning. I am guessing the bad data are at and below 850 mb and modifying the sounding accordingly. My estimated sounding for this afternoon has boundary layer well-mixed moisture at 10 g/kg and cloud bases at 650 mb. This gives moderate CAPE with only slight CIN above, assuming no further drying from east.

At 500 mb a weak inverted trof extends from the north end of GoC to SE Colorado, with some cooler air on the east side of this feature. At 200 mb a strong inverted trof extends from west of Albuquerque, south between El Paso and Tucson and into the vast Mexican data void. This setup gives the Tucson area good steering level flow this late afternoon, but little vertical shear. If storms organize along outflows, they could move westward faster than their anvils.

NAM model indicates better wind profiles tomorrow. Thus, good potential for organized and some severe storms either this afternoon or tomorrow, with the answer hiding in the devilish, smaller-scale details.

Severe weather pattern setting up last three days of week

Note that we've posted a weekend summary and some recent photos on the madweather web page, with links posted on the blog this morning.

Conditions remain very moist this morning; however, the TWC sounding is about 5mm too high in IPW according to the FSL comparison site. The 12Z sounding is quite strange, exhibiting the off-the-surface moist spike that Mike Leuthold pointed out about a week ago. Many of the 12Z soundings for last several weeks show this same artifact - particularly on calm mornings. This morning's sounding exhibits an adiabatic layer off the surface and then a sharp inversion to about 890 mb. This same layer has the strange Td structures. These data in lowest levels probably reflect the local conditions on the roof of the NWS building, and the moisture spike may result from similar contamination.

Regardless, moist and unstable and a calm troposphere again this morning. Only 11 of the Pima County Alert gauges had precip yesterday - making it the driest day since last Friday - with the significant amounts being on the mountains. The amount of anvil that the early mountain storms produced was truly amazing, but of course left all the lower elevations under thick anvil for much of the afternoon. Looks like anvils will tend to spread toward south and southwest today also.
Upper-level difluence increases this afternoon and evening as the inverted upper trof approaches. This feature directly affects southeastern Arizona tomorrow afternoon and steering level winds finally pick up. The 500 mb temperatures with this feature are -8 to -9C, i.e., very much cooler than have been observed at TWC for many days!

The best vertical shear for organized, propagating MCSs is forecast by the NAM on Thursday afternoon/evening as the winds at high levels - 200 mb - turn to south-southwest as remnants of Dolly approach the Big Bend. Wind shear predicted by the NAM remains very favorable on Friday afternoon. Very difficult to call which day will bring the most intense storms to southeast Arizona for rest of week, since local and small scale features and the previous day's events will be playing a big role each day. But it will be very interesting to observe what actually evolves.

Weather Summary for Past Weekend

We've posted a summary of the weather observed over this past weekend directly on the web page.

http://www.madweather.com/

It can be found under BLOG IMAGE GALLERY Weather Summary for July 18-20

We've also added some other July storm photos here, including lightning photos from near El Paso. these were provided by Mike Hardiman.

Weather Summary Last Three Days - July 18 through 20

Or - this is a direct link to the summary and photos.

Madweather post: Weather Summary Last Three Days - July 18 through 20

CURRENT WEATHER, MONDAY JULY 21, 2008

This morning conditions over most of Arizona remain very moist and the TWC morning sounding (Fig. 1 - link below) indicates a subtropical air mass with just over 50 mm of IPW. Although there is abundant CAPE, the winds aloft are light and variable through most of the troposphere. Thus, the situation is much like yesterday when there were many storms which remained mostly over higher elevations. The large subtropical anticyclone at 500 mb has three distinct sub-centers. The main one is over Arkansas with weaker centers in the west over southeastern Arizona and southwestern Wyoming (see Fig. 2).

The most interesting aspects of this morning's charts and the model forecasts relate to what will occur on Wednesday through Friday. There is a strong upper-tropospheric shear zone at 200 mb from somewhere near the southern end of the Gulf of California that extends east-northeast through the Mexican data void, and then across southeast Texas and southern Arkansas. The NAM initializes this feature a bit to the north and weaker than it appears to be. The NAM forecasts the west end of this feature to become a distinct upper-cyclone that moves westward across northern Mexico on Wednesday and Thursday. As this occurs, the 500 mb steering flow becomes east-northeast. The flow at 200 mb becomes more diffluent, initially from the east-northeast on Wednesday and then from the southeast on Thursday. Thus, a more favorable shear profile returns for propagating and organized storms.

It then appears that the remnants of TS/Hurricane Dolly (see Fig. 3) will affect southern Arizona on Friday and perhaps Saturday. Currently, the models and NHC forecast Dolly to follow a track similar to that of Claudette during the period July 8-17th in 2003. All-in-all it should be an interesting weather week.

Figure 1 Figure 2 Figure 3

Weather Summary - 30 June Through this Morning

The last seven days have been quite amazing here in southeastern Arizona. There have been thunderstorms somewhere in the Tucson metro area every day except for the 30th, which was suppressed due to strong, downslope east winds.

Yesterday was the day with the most widespread rains. A large are of heavy cloud at sunrise acted to suppress local storm development. But shortly after dark outflow winds converged over Tucson, helped by two weak short waves - one from the southeast and one from the northwest - triggering a large MCS. Every one of the Flood Districts 91 gauges reported measurable rainfall except for three - which I suspect may be out of order. Largest accumulation was 2.60 at White Tail on the Catalinas. Here at the house we had 0.60".

Today the two middle-level waves continue to interact and the atmosphere is very wet with high IPW amounts. The widespread rains will mean a shallow boundary layer later today that will need strong outflows into lower elevations if storms are to develop away from the mountains. The longer range forecasts continue with very wet forecasts out past 7 days as both depression Five-E and an upper cyclone over eastern Mexico move slowly this way and continue the flow of subtropical moisture into the Southwest US.

Some summaries for the past seven days:

Douglas Tstms on the 1st, 3rd, 5th, and 6th total rainfall 0.17"

Art Douglas reports storms and rain at Ash Canyon (just south of Sierra Vista)
on the 2nd through the 5th (and probably yesterday also) with rainfall of 1.00"

Nogales: Tstms every day with total rainfall 1.31"

Tucson (TUS) Tstms on 1st, 3rd, 4th, 5th, and 6th total rainfall 0.45"

Here at house Tstms on 1st, 3rd, 4th, 5th, and 6th total rainfall 1.39"

Phoenix (PHX) Tstms on the 3rd and 4th total rainfall only a Trace

Note that Phoenix has had high temps of over 105F every day of this period and that 3 days were over 110F. While temps were over 100F in the afternoons the dewpoints were often in the middle 50s to low 60s. Thus, the heat stress and misery gradient between Tucson and Phoenix this last week has been very strong, probably maximizing between Picacho Peak and Casa Grande.

Spitting Rain

Someone asked me if my use of "spitting rain" was a technical term and, if so, what does it mean?

Obviously, it's not very technical, but here is my attempt at a definition.

Subjective - if it is spitting rain, you can almost run between the drops.

More quantitative - if you observe spitting rain falling onto a porous surface, such as concrete or flagstone, you'll note the following. The drop splats and wet spots remain separate from one another while it is spitting rain. If the surface becomes completely wetted, then light rain, or heavier, has begun.

So that's it - not copyrighted; feel free to use.

Watch a video (Madweather's first attempt at doing this!)

Storms in Tucson on the Fourth of July

While the central and south parts of the state were very active yesterday with several severe storm reports, the nearby mountains here were suppressed relative to the day before. The boundary layerby late afternoon was only well-mixed in moisture up to 600 mb. This layer required substantial lift for its CAPE to be released.

A strong cluster of storms near Green Valley produced a deep outflow that was visible on the KEMX base scans. It passed TUS at 6:46 pm when winds shifted to the south with gusts to 38 mph. Thunderstorms were developing along this outflow as it moved north across the city. We had thunder and light to moderate rain here at house from about 7:00 to 7:40 pm with total rainfall of 0.35". The lightning was quite spectacular with CG bolts jumping out to the north of the cells through clear sky. Quite a dangerous situation.

Significant moisture remains in place but winds aloft over southeastern Arizona are becoming nearly light and variable in the NAM forecasts through midnight. My guess is that this afternoon's well-mixed, in moisture, boundary layer will have theta w of about 24C - a very favorable value for moderate CAPE and heavy storms. But the situation will probably be like yesterday and the air mass over the deserts will require a good kick from outflows from storms at higher elevations. Timing will depend on heating and how quickly storms develop over higher elevations. Also appears as if the northwestern half of state will have an active day.

Interesting Weather Continues in the Southwest

Weather Update - 8:00 am Friday July 4, 2008

Yesterday was a very active day with most reporting stations in southeast and south central reporting thunderstorms and wind gusts of 30 mph or greater. Douglas reported a gust to 75 mph, and others of note were: 53 at Mt. Hopkins RAWS, 45 at Scottsdale airport, and 44 at TUS. The Pima Flood Control District monitors rainfall at 91 sites in eastern Pima County and 56 of these sites reported measurable rain for the 24 hours ending at 6 am this morning. Notable amounts: 4.61" at Manning Camp in the Rincons (this is from a RAWS site and may be suspect) and 1.77" at Oracle Ranger Station. The most rain reported in the main part of the City was 0.47" at Pantano and Houghton. Of the 18 City gauges only 6 reported measurable rain, showing that the higher elevations did much better. Here at the house we had lots of lightning and thunder, several outflows and a couple of brief spits of rain, amounting to a trace. But the cool outflows and strong smell of rain made for a great evening.

This morning the moisture contents over southern Arizona have continued to increase. The TWC sounding is very moist and indicates the most CAPE present for the season so far. The IPW products and surface observations show that the surge of GoC moisture which began midday on Wednesday has pushed north almost to Las Vegas. IPW at Tucson is 38 mm this morning and Phoenix has 41 mm.

Winds in the upper and lower troposphere are light and variable, but steering level winds from 400 to 600 mb are from the east-northeast at 15 to 30 kts. There are no synoptic features of note today, except for the moderate wind flow around the midlevel anticyclone and perhaps a bit of midlevel cool advection. Expect that it will be another active day across much of southern Arizona. With higher moisture and lower cloud bases, the threat of heavy rain is greater today and the possibility of severe downbursts remains significant. The increased CAPE will allow better propagation of mountain storms into the deserts during the afternoon and evening.

So, another active day likely with lightning fireworks for the Fourth.

Weather Discussion for Thursday July 3rd 2008

Prepared Thursday July 3rd 8:30 am

Low-level moisture has continued to push into the lower elevations of all of southern Arizona during the night. Moisture increases and southeasterly winds are most spectacular over the lower Colorado River basin as per Yuma surface observations, the KYUX Doppler velocities (See Figure 1 and Figure 2) and the GOES IPW (Figure 3). Winds at 1500 UTC this morning are gusting 20 to 30 kts at Yuma and the dewpoint has risen into the lower 70s. The VAD shown in Figure 2 indicates that the surge of GoC air may bextend up to 4,000 ft AGL.

The Tucson 1200 UTC sounding this morning shows a slight increase of low-level moisture and is a bit dry relative to GPS IPW. There is a very deep residual boundary layer from yesterday and some CAPE indicated over low elevations, with CAPE over the higher mountains likely to be quite substantial. This afternoon's CAPE at lower elevations will be determined by how the low level moisture is advected by low-level winds, any further increase in moisture, and also by any warming that might occur between 500 and 400 mb - so estimating desert CAPE is toughest aspect of this afternoon's forecast.

Steering winds are likely to be northeast to east at 20 to 25 kts around 0000 UTC, while upper-level winds will blow the anvils off to the southeast - an excellent shear configuration for severe thunderstorms in southeastern Arizona. With diurnal afternoon low-level winds from the west-northwest, the shear profile below 500 mb will be supportive of strong, mesoscale outflows.

The threat for severe macroburst winds today is high, and depending on how the CAPE evolves there may also be some heavy rains in lower elevations and possibly hail. Hail is quite likely in higher elevations.

All-in-all looks like a good setup for a significant thunderstorm day here in southeast Arizona and also in much of the Southwest. As is often the case here in southeast Arizona, the outlook for tomorrow will be strongly impacted by how convection evolves late today and tonight - the NAM indicates even more activity tomorrow but this could be impacted by strong outflows later today and tonight. An interesting 4th regardless.

Weather Discussion - 9:00 am Sunday June 29, 2008

There are many interesting things happening over the Southwest and so this discussion.

For the past week or so there have been daily thunderstorms over southeast, and sometimes central, Arizona. Moisture values and CAPE have generally remained marginal with IPW values of around 25 mm. Thursday afternoon (the 26th) IPW increased briefly above 30 mm and there was ample CAPE over lower elevations of southeast Arizona for severe storms and heavy rains to occur in the Tucson metro area. The street flooding here on the north side of town was quite impressive, even though at the house there was only 0.33" of rainfall. Temperatures in middle levels have remained quite cool through this period, with temps at 500 mb of -8 to -10 C, and thus there has been CAPE around even though moisture values were low.

On Thursday the high, subtropical values of IPW (>37.5 mm) had inched northward toward the end of the GoC and the borderlands. This morning the subtropical moisture has retreated southward and covers only about the southern third of the GoC.

This morning the Tucson sounding indicates that there will probably be CAPE present this afternoon at lower elevations in southeastern AZ. If we end up with 4 g/kg in the well-mixed boundary layer (BL) there will be slight CAPE, but it looks like the value could be 5 g/kg or even greater. Steering-level winds are easterly at about 20kt. There is a weak upper-level SW over New Mexico and the NAM forecasts part of this feature to move southwestward and interact with the northern end of a quite pronounced tropical wave. So the potential for storms appears enhanced today, especially given that 500 mb temperatures remain in the -8 to -10 C range across all of the southwest. There is also pronounced westward advection of higher moisture values across southern New Mexico. By afternoon there should be substantial moisture convergence over southeastern Arizona as the afternoon diurnal northwesterly winds butt into the easterly flow from New Mexico. Thus, there appears to be high potential for a significant storm day over southeastern AZ today, with severe macrobursts likely - given the high cloud bases and very hot subcloud BL.

In the longer term it finally appears that a wave of significance will move across the lower GoC. The models indicate substantial cool advection at 700 mb with the tropical wave that is now over Mexico and indicate significant storm activity likely in Sinaloa and southern Sonora - perhaps even moving out over the GoC. It would be nice to have some sounding observations over Mexico to help one evaluate the model forecasts, but all of northern Mexico has been without upper-air data for almost a month now. The evolution and movement of the wave, as predicted by the models, as well as the cool advection and rainfall would trigger a low-level surge of high moisture values into southern Arizona sometime between early morning on Tuesday and mid-day on Wednesday.

All in all, it looks like a number of very interesting days are on tap for us here in southeastern Arizona.

II. Problems With the TWC Soundings on 23 June 2008

Problems with the data from the RRS sondes are again starting to impact the analysis of the soundings taken at TWC and the forecasting of convection here in southeastern Arizona.

The FSL time series comparison of GPS IPW versus TWC IPW since the 19th of June indicates that 5 soundings were apparently too dry, while one sounding (1200 UTC 19 June) was very wet relative to GPS IPW. See Figure 1. Careful monitoring of the remotely sensed IPW continues to be the best way to make an initial assessment of how accurate a given TWC sounding might be.

The 00Z sounding last evening (see Figure 2) indicated a deep, well-mixed boundary layer extending up to 500 mb; however, the moisture structure was quite questionable, with very dry conditions off the surface and no recovery to a well-mixed value. This led to the "dry" sounding wrt IPW.

The 12Z sounding this morning (see Figure 3) indicates a very deep residual boundary layer, but the moisture structure is extremely erratic. The IPW values correspond well. Given the unrealistic physical nature of the Td data, I suspect that the agreement of the two IPW values is essentially a coincidence this morning. If the afternoon BL mixes toward 5 g/kg, then there is a small amount of CAPE present; however, if it mixes toward 4 g/kg then there is no CAPE present. Given the erratic Td data trace it is very hard to make a subjective forecast of the late afternoon sounding.

The Haboob of Saturday Evening June 21st 2008

This was probably the first significant event, for southeast Arizona, of the 2008 summer thunderstorm season. Widespread convection occurred over parts of southeast Arizona and particularly over southwest New Mexico and northern Mexico. See Figure 1.

High cloud bases and a very hot boundary layer led to an organized, mesoscale outflow that affected much of southeastern Arizona. Passage here in Tucson occurred about 4:30 pm local time. The outflow was well-depicted as a fine line on the KEMX reflectivity display see Figure 2.

Note that Douglas, Arizona, reported a severe convective gust of 51 kt at 3:32 pm local time after the haboob had moved passed. This severe storm event did not make it into SPC's list of wind events for the 21st.

We were returning to Tucson from Chicago and landed about 10:00 pm local time. The approach was very bumpy and winds were still gusty from the east to southeast. The air was heavily laden with dust, giving the 3/4 moon a very strange and dirty yellow-brown appearance.

REST OF MONTH LOOKS INTERESTING FOR SOUTHWEST

Both GFS and NAM model forecasts continue to be quite interesting for the Southwest.

Appears that there may be high-based, dry lightning storms over the mountains of far southeast AZ this weekend. Definitely not good, given the extreme dry conditions of last two and a half months. Only 0.09" of rain here at house since March 17th!

In the longer term, the GFS ensemble members keep a strong, middle-level anticyclone north Mexico/southern Plains, and then tend to shift it northward during last 10 days of month. Throw a tropical disturbance into this pattern and who knows what we might see down here.

I'm off to Wisconsin where it will be green and wet and probably muddy.

OVERVIEW of RRS SOUNDINGS 6 JUNE 2008

I have been involved in other things last few days. Have just taken a look at today's soundings and report the following:

0000 UTC soundings with superadiabatic layers aloft apparently due to wetbulbing:

UIL SLE BOI OTX FWD OAX SHV MSP TLH BMX APX MFL

ABR has an extreme super layer aloft but it is not due to wetbulbing. LBF flight missing.

12000 UTC soundings with superadiabatic layers aloft apparently due to wetbulbing:

UIL SLE ILX OTX DVN SGF

OUN sounding appears very strange for unknown reasons.

SUSPECT RRS DATA FOR 23 MAY 2008

Numerous data problems continue in the RRS soundings.

Overview of suspect data for 0000 UTC 23 May 2008:

Superadiabatic layer(s) aloft:

LBF UNR SLE TWC FWD LIX TLH LCH MFL BUF

Additionally - the LBF flight was incomplete; the MHX flight was missing all winds except that at the surface; the LIX flight also exhibited a warm spike and extremely noisey data; the JAN flight was terminated.

Overview of suspect data for 1200 UTC 23 May 2008:

Superadiabatic layer(s) aloft:

LBF UNR TWC FWD OUN TLH MHX DVN BMX BNA APX RNK

Additionally - the UNR and MFL flights exhibited extreme noise in the RH data and the bizarre TLH flight was terminated.

MANY SUSPECT SOUNDINGS THIS MORNING

While there were lots of bad or suspect soundings last evening, this morning's data appear quite bad. At least for the number of flights with obviously suspect layers. I guess this reflects the important weather systems affecting the continental US today - Thursday 22 May 2008.

Visually obvious problems appear in the sounding data as per:

Warm spikes at GGW RIW

Superadiabatic layers aloft at GJT GGW(3) SHV TOP OMA AMA JAN TLH MFL(2)

Suspect layer (Opposite of wet bulb superadiabatic) at UIL SLE MFR APX

Some of these are quite amazing. The sounding data can be easily viewed at Univ. of WY's upper air page:

http://weather.uwyo.edu/upperair/sounding.html

Note that two of the suspect soundings are from non RRS stations - TOP which flew into an MCS and MFR which showed a weak Op-Super layer from its VIZ instrument.

I call the strange layers "Op-Super" that occur when sonde exits cloud into drier and warmer air and T increases rapidly but RH stays at 100% for a while. This likely indicates a hygrister problem vs wet bulbing under same conditions which indicates a wetted thermister.

A Quick Look at This Morning's 1200 UTC Soundngs

Friday Morning 16 May 2008

I have had time to take a quick pass through the morning soundings.

Soundings having a super-adiabatic layer aloft include:

MAF BRO LIX TLH MHX GYX

Soundings having a warm spike aloft between 0 and -10C include:

GJT LIX IAD

Too busy today to take a careful look at any of these.

Super-adiabatic Layers This Morning

The following soundings exhibit elevated super-adiabatic layers this morning for the 1200 UTC flights:

AMA FWD BNA LBF LZK JAN LCH

The problem at LBF was due to an intense warm spike.

The too cool layers in several of these soundings cause computed CAPE to be inaccurate.

I cross-checked the NCAR RAP plots with those at Wyoming this morning, and all the above
problem soundings were similar at each web page.

LCH is the most bizarre sounding of the bunch. The instrument apparently was launched into
a thunderstorm or deep convection.

RE Flagstaff Sounding Previous Post

The bad sounding shown for Flagstaff was apparently due to the following:

Data were missing for for a short period.

Operator interpolated data for the missing period.

A saturated point was interpolated with T/Td spread shown as //

The NCAR Rap sounding decoder did not interpret the // correctly,
producing the very strange sounding on their web page.

The decoders at Wyoming and SPC plotted the data correctly.

Suspect Soundings - Two Recent Examples

The figures below show soundings from Flagstaff AZ yesterday morning at 1200 UTC and from Ft. Worth TX this morning at 1200 UTC. Both have suspect layers, but the cause of the problems are not obvious.

Figure 1 - KFGZ

This sounding indicates a very dry layer of almost 100 mb vertical extent just above 700 mb. It had been raining for several hours and at the time of release Flagstaff was reporting moderate to heavy snowfall. The snow continued for several hours after sounding time. The dry layer certainly does not seem physically realistic, given the steady precipitation event that was underway.


Figure 2 - KFWD

This morning's sounding at FWD indicates a strongly super-adiabatic lapse at about 710 mb. This excursion causes a layer that appears to be several degrees too cool from about 710 to 550 mb. The net result is that sounding analysis software routines would produce CAPE values that were likely too great. Since the sonde did not, apparently, rise into a very dry layer, the reason for the super-adiabatic layer aloft is not clear.

Both of these soundings may illustrate random instabilities within these two RSS sonde's internal electronics, but this is just a guess on my part.

Problem Soundings 0000 UTC May 9th

My quick look at the soundings last evening identified two RSS flights that exhibited strongly super-adiabatic lapse rates in shallow layers aloft. These soundings were taken at Birmingham, AL, and Sterling, VA.

Note that the RRS operational procedures indicate that layers such as these should be removed via manual editing by the operator, and provide instructions for how this should be done.

Figure 1 (00Z BMX)

The layer of interest here extends from about 630 mb to 610 mb. The sonde apparently had moisture on the thermistor, and perhaps on the hygristor. When the instrument exited a deep, cloudy layer it encountered very dry air aloft and evaporative cooling of the sensors probably produced the super-adiabatic layer.

Figure 2 (00Z IAD)

The layer of interest here is only about 10 mb deep and is located near 740 mb. The reason for this super-adiabatic layer is not obvious, since the entir atmosphere appears to be quite moist. The sounding apparently did not exit cloud into a very dry layer above. It is possible that the instrument was in a saturated updraft and exited the updraft suddenly. It is also possible that the hygristor did not respond adequately to a layer that was considerably drier than the data indicate.

During the next few months I'll be paying particular interest in identifying soundings in which bad data have complicated the diagnosis of CAPE or potentially caused forecasting difficulties.

If readers note soundings that seem to be candidates, as per above, for closer examination, please feel free to alert me to them. Thanks, Bob bob@squidinkbooks.com

BLOG will be active now for the spring/summer

I am beginning again to monitor the RRS Sippican sounding data, as I start preparing a formal paper. This paper will document the often poor, and physically bizarre, soundings produced by the Sippican radiosondes the NWS has switched to at most sites in the US.

Today's winning example of a strange sounding is that taken at Ft. Worth TX at 1200 UTC.

http://www.rap.ucar.edu/weather/upper/fwd.gif

Cold Cutoff in the Southwest

Yet another success for the long-range models, especially theECMWF, which correctly forecast the development of the cold, midlevel low currently over the Southwest - beginning from about 5 to 6 days out.

The system produced more precipitation than I was expecting, given the very low precipitable water values. Here at the house, I measured 0.29" through 7 am this morning and light showers remain in the area. Low this morning was 33F.

Precipitation yesterday was from convective showers in the cold, unstable air. I did not hear any thunder here at the house. During midafternoon northern and western parts of the Tucson metro area had heavy showers of graupel (AKA snow pellets) and the ground was covered in some spots.

Perhaps some more of this today, before the system moves on east. It's hard to know how unstable it is this morning, since the somewhat critical Tucson upper-air sounding was missing both last evening and again this morning.

The NWS forecast this morning (see below) mentions the possibility of showers, thunder, and sleet. While showers accompanied by graupel may occur, the liklihood of sleet is extremely small, today or with any winter weather event in the Tucson metro area, since sleet results when raindrops fall through a layer of subfreezing air at or very near the surface. Definitions of graupel and sleet are also below.
-------------------------------------------------------------------
Today: A chance of rain showers possibly mixed with sleet, mainly after 11am. Some thunder is also possible. Partly sunny, with a high near 53. South southeast wind 9 to 14 mph becoming west northwest. Chance of precipitation is 30%. Little or no snow accumulation expected.

From the Glossary of Meteorology:

snow pellets (Also called soft hail, graupel, tapioca snow.) Precipitation consisting of white, opaque, approximately round (sometimes conical) ice particles having a snowlike structure, and about 2-5 mm in diameter. Snow pellets are crisp and easily crushed, differing in this respect from snow grains. They rebound when they fall on a hard surface and often break up. In most cases, snow pellets fall in shower form...

sleet or grains of ice, generally transparent, globular, solid grains of ice that have formed from the freezing of raindrops or the refreezing of largely melted snowflakes when falling through a below-freezing layer of air near the earth's surface....

DEVELOPING WEATHER EVENT

Finally looks like we may have some clouds and weather here in SE AZ during the weekend.

The ECMWF and the GFS ensemble members are all pretty much in agreement now, indicating a strong wave digging down the west coast, with the eventual cutoff affecting us from about 96 to 144 hours - from 00 UTC model runs last evening.

There are about 5 waves strung out across the northern Pacific and it's not clear which one kicks up the ridge into AK, forcing the wave down the coast. Obviously all the models have keyed on one of these to be the agent of change.

The models appear to predict about a quarter to half an inch of precip at lower elevations by late Monday afternoon. Uncertainties this far out - how long will the wave affect SE AZ - some model runs keep it more isolated from the main flow to the north; how much moisture will the system pickup (the ECMWF hints at better moisture flowing northward toward the end of the event, while - surprise - the GFS members stay drier). Only 10 of 12 GFS ensemble members predict measurable rainfall at lower elevations in SE AZ during the event.

Current fcst POPs for both TUS metro and also top of Catalinas max out at 30% for Sunday night. PHX has issued a hazardous weather outlook for this event already (see the nice early am FD from PHX). Will be interesting to watch how this event evolves.

Different topic - My total precip for Dec/Jan/Feb here at house was 5.84 inches -not bad for a la nina winter! Actually not bad for any of past ten winters - beaten only (I think) by the 97-98 el nino winter.

BRIEF SUMMARY OF JANUARY 2008

The weather for the winter continued fairly wet and cold here at the house during January 2008.

There was measurable rainfall on only 4 days but the total, mostly from the storms of the 7th and of the 27th/28th, was 1.00 inch. The storm late in the month produced much flooding and runoff after very heavy rains at higher elevations. The Rillito wash is still running today.

After a very cold December (our gas bill for heating in December was the highest ever during our ten years in the house), January was also fairly cold. There were 18 mornings with low temperatures of 32F or colder. Half of these days had lows of 27F or colder.

The coldest mornings occurred on the 18th - 20th with readings of 20F, 20F, and 21F. This morning, the 31st, was also cold with a low of 22F. There was a period of 14 consecutive days from the 10th through the 23rd with lows of 32F or colder.

So, probably another big gas bill.

Appears that February will also start out unsettled with perhaps more rain during this strange la nina winter.

INTERESTING BUT TOUGH FORECAST FOR LATER IN WEEK

Very interesting situation for the next six days as the complex middle level low off of California coast this morning swings around to south and then minors out across the Great Basin, with stronger wave replacing it. There is also a persistent very tight cyclone far off to the WSW that models may be trying to swing around the south end of the larger-scale trough.

This morning's NAM indicates a precip period Wednesday night through Friday morning. Interestingly, the operational GFS (from 00Z last evening) shows little at Tucson during this period. The GFS tends to keep the subtropical moisture to the south as the first middle level low swings around and minors out, while the second system digs in. The role, if any, of the system out around 140W and 30N is not clear in the model forecasts.

The GFS and the ECMWF (both from 00Z last evening) indicate that the second wave will pull the subtropical moisture much further north and provide the most significant event out toward the weekend. The precip forecast of these long range models is fairly different. The GFS indicates the main precip near Tucson to occur Saturday night and to amount to around a quarter to half an inch. Whereas, the ECMWF indicates that the main precip event would also be Saturday night, but rainfall amounts near Tucson of a half to a bit more than an inch are predicted by this model.

So, the summary is that there may be two distinct precip events over southeastern Arizona with the weekend event likely being the more significant one. Will be interesting to watch how this very complex situation actually evolves. Regardless we're definitely having a more interesting la nina winter than one might have expected!

A WET AND COLD DECEMBER 2007 HERE IN TUCSON

The following observations taken here at the house - north end of Country Club about a half mile south of the Rillito Wash - document what was an unusual early December rainy period that was followed by a very cold last half of December. Note that the part of Tucson we live in is considerably lower in elevation than the official NWS observation site at the airport on the far south side of the city. On clear and calm nights low temperatures here at the house are routinely at least 8 to 10 degrees F colder than the "official" readings at the airport. I do not record maximum temperatures here at the house because I have never been able to screen my max/min thermometer adequately to avoid contamination by daytime solar heating.

December 2007 Precipitation (I read my gauge at about 7 am each morning so that some rainfall may have actually occurred after midnight and technically be for the next day):

Dec. 1 - 1.84" No thunderstorms observed at house
Dec. 2 - 0.05"
Dec 7 & 8 - 0.62" We were out of town from Friday afternoon until Sat. evening.
Dec. 9 - 0.04"
Dec. 10 - 0.23" Strong thunderstorms occurred late afternoon on Monday and pea size hail fell here at the house at dusk.
Dec. 11 - 0.40"
Dec. 12 - very dense fog until about 11 am in the morning.
Dec. 21 - 0.05" In addition to the light rain there was a brief burst of snow pellets here at the house about 9:30 am. Weather broadcasts erroneously referred to the occurrence of sleet around town.

Total December precipitation: 3.23 inches

Minimum temperatures:

24 days had lows of 39F or colder
16 days had lows of 29F or colder
3 days had lows of 19F or colder

An extended period of very cold morning lows (at least for here in Tucson) began the morning of the 15th - as per:

15th - 24F The first hard freeze with very heavy frost here at house.
16th - 24F
17th - 29F
18th - 29F
19th - 29F
20th - 27F
21st - 36F
22nd - 21F
23rd - 19F
24th - 24F
25th - 26F
26th - 18F
27th - 23F
28th - 19F
29th - 22F
30th - 24F
31st - 24F

The extended period of cold temperatures caused fairly heavy plant damage here at house, and I hauled out about 100 lbs of broken and fallen cacti arms this morning.

My summary of the entire year, highlighting significant weather events, will follow by the end of the week.

Happy New Years to all!

Henriette and Today's Weather

Rain from Henriette

Here at the house we received only 0.03" of rain on the afternoon of the 5th (Wednesday) but very heavy storms occurred in the vicinity (see Photo). We experienced considerable lightning and thunder and gusty outflow winds but little rain. Then, during the early morning hours of Thursday from about 5 am to 9 am there were several moderate showers with no lightning or thunder here. These produced an additional 0.41" of rainfall. Henriette has left behind a very wet low-level atmosphere and the forecast for today is of interest.

Today's weather

There is a large pool of very moist air lingering over much of Arizona this morning, along with considerable low and middle level cloudiness.

Winds in the lower half of the troposphere remain L/V while upper-level winds and shear are quite strong (this due to strong height gradients between the anticyclone to the southeast and a trough off the Pacific coast). The upper-levels appear to be divergent over southeastern Arizona.

The morning TWC sounding indicates substantial CAPE is present below 350 mb. The questions for the day are: how believable is the TWC sounding, and how will drying during the day affect the CAPE? In the morning soundings in the layer between 400 and 300 mb there is a pronounced inversion between the moist lower atmosphere and the dry upper-levels. At TWC the inversion is about 7C warmer than at any other upper-air site in the Southwest. To me, the TWC sounding appears suspect between 520 to 280 mb. My guess is that the elevated inversion is not as strong as indicated here, and that it will not strengthen due to the dry flow aloft, so that there is at least moderate CAPE present.

I expect that strong storms are likely this afternoon over the southeastern third of Arizona, especially where the clouds break and there is decent solar heating. The storms should have the potential to produce strong, moist downbursts and some hail, especially at higher elevations.

All this assumes that I have been able to properly unravel the uncertainties in the upper-air data.

Forecast Remains Difficult

See yesterday's message about tough forecasts!

Today we have moisture and hopefully CAPE will remain through the day -we are getting some mid-level warming now. Upper-level winds are difluent between the hurricane and the system to NW. Winds are very light in lowest half of troposphere - again. Low-level winds become southwesterly during the day and there is more IPW to the west. So, there should be storms around this afternoon with the potential for heavy rains.

Tomorrow is much tougher and it's possible that it will be an all or nothing situation here in the Tucson area. Who remembers Nora?

NAM forecasts a decent rainfall event, but keeps the really heavy rain with the core of what was Henriette. It is of note that there were soundings this morning at La Paz and Guaymas for the first time in ages, so the model actually had some observations in its initialization.

Negatives tomorrow are that NAM forecasts winds to become strong downslope below 500 mb into the local area. This is coupled with what appears to be fairly strong confluence aloft. The GFS members forecastsignificant rain event also but, of course, local terrain really isn't in that model.

If the remains of Henriette move northward to the west of the NHC and NAM tracks, get out the sandbags!

So, I conclude that, with so many uncertainties in the next 24 hours, the only thing to do is to watch the observations closely and see what actually transpires during the afternoon and night.

Henriette Leads to Rapidly Changing Conditions over Southern Arizona

A Very Difficult Forecast Situation

The last two days have been extremely suppressed and very dry in southern Arizona, with dewpoints yesterday falling into the upper 30s. This was the third dry-down of the summer that came in on easterly to southeasterly winds.

This morning a number of things have changed. There has been a northward surge of low-level moisture into southwest and south-central Arizona during the late night and early morning hours. This was likely triggered by strong winds to the northeast of Hurricane Henriette. At 7 am the moisture had increased dramatically at Yuma, Sells, and Sasabe. The VAD Doppler radar data at Yuma indicate that this surge is about 3,000 ft deep. It may provide enough destabilization for at least mountain storms over the southern third of the state this afternoon.

The track of Henriette will then be the key determining factor for the weather over southeastern Arizona. The models now are in fair agreement that Henriette should move into the southern GoC. Tomorrow there will be a strong isallobaric wind blowing down the GoC and that may slosh today's low-level moisture back toward the south. Then, the main issue becomes how far north the deep moisture and precipitation fields associated with the weakening storm will come.

It appears that the current trough in the northwest will move rapidly over the top of the middle-level anticyclone leaving some remnants of Henriette blocked and decaying over northern Sonora and the GoC. This would allow the moisture associated with the storm to move directly into at least southeastern Arizona and give the potential for a significant rain event.

Yuma Storm on Saturday

I have checked the Yuma newspaper this morning and there are two stories online that describe the intensity of the severe thunderstorm that struck Yuma Saturday evening. There are numerous damage photos available - and perhaps even a video (I couldn't get it to play though).

http://www.yumasun.com/

A Big Storm Day in Southern Arizona and California

Yesterday afternoon , Saturday September 1st, turned out to be a big storm and severe storm day across southern Arizona and into southern California. There were several small tornadoes in California, and events in Arizona were mostly from hybrid downbursts, although there was one hail report. Most notable was a measured gust to 84 mph out at Yuma, Arizona. I have not yet chased down many photos, but will try to post some damage shots tomorrow.

Strong, drying easterly winds today have led to mostly suppressed conditions across southern Arizona, with significant storm activity focused in southern California.

Data from TWC yesterday provide a good illustration of how difficult it is to use RRS soundings with suspect data in making a forecast.

In my previous post I noted that the 12 UTC (1 Sept. 2007) sounding at TWC was suspect and that it was difficult to estimate what the CAPE might actually be in the afternoon. I guessed that a well-mixed afternoon BL might have a mixing ratio of ~9 g/kg. But I also mentioned that 1 g/kg either way would have substantial implications for the evolution of afternoon storms. The TWC sounding at 00 UTC (2 Sept. 2007) apparently entered a storm updraft - see Figure 1. The data are mostly bad or suspect, except for the temperature trace from the surface to about 650 mb. However, the updraft data from 500 to a bit above 400 mb have theta w values of 24 to 25C. This implies that the BL air at cloud base had a mixing ratio of 10.5 to 11.5 g/kg.

My morning estimate, using the sounding with the suspect Td trace, was clearly way too low. It is fairly obvious that moisture advection, given the winds below 650 mb for the morning and evening soundings, was not a major player during the day. Figure 2 shows the morning sounding with lines added showing q = 10 g/kg and theta w = 23.5C. The Sippican hygrister appears not to have responded well at all in the residual layer and indicated conditions that were too dry.

It is likely that the sounding problems yesterday contributed to forecasts that significantly under-estimated the extent of the severe storm outbreak. And so it goes in the new era of NWS RRS soundings - a serious crap shoot for forecasters that sometimes has public safety implications.

Weather Discussion 1 Sept. 2007

There have been many interesting changes from yesterday morning to this morning.

Atmosphere has moistened up some wrt IPW, but the soundings are very hard to interpret.

Last evening's TWC sounding (see Fig. 1) had a dry spike off the surface and then seemed to indicate a BL with a bit less than 8 g/kg and IPW of 25.8 mm (this was extremely dry wrt the GPS value). The evening sounding appeared to have no CAPE - but storms developed to the south of the airport around 0200Z and TUS carried a brief tstm (nice lightning visible from here with this cell).

This morning's sounding (see Fig. 2) has IPW of 37.5 mm, quite a large increase of almost 12 mm in 12 hours according to the soundings, but GPS data (see Fig. 3 from FSL) show a 12 hour increase of only about 2.5 mm. So, once again, the first forecasting dilemma for today is that of trying to figure out what the local thermodynamic conditions actually are.

The morning sounding has NO residual BL in moisture but a very deep residual BL in temperature. I'm going to guess that it's there (a residual BL in moisture) and that the mixing ratio we're heading toward is about 9 g/km. This would give a decent amount of CAPE this afternoon. But if I'm off by either +/- ~1 g/kg, the difference goes from a marginal day with mostly mountain storms to a very good day at low elevations with moderate CAPE available for storms. The deep BL and high cloud bases leads to either the potential for dry microbursts, or to severe hybrid downbursts at lower elevations - all depending upon what the afternoon BL actually looks like.

Am I frustrated trying to use bad sounding data to make a forecast? VERY!

Other interesting aspects of today's forecast:

Surface pressures are down 2 to 3 mb in south-central AZ compared to 24 hours ago.

An upper-level S/W trough is swinging southward across New Mexico, giving much of AZ flow aloft that is quite difluent.

Easterly low-level winds are still present but have diminished quite a bit. It will be important for the evolution of storms this afternoon if WNW diurnal winds can develop - the lower deserts to west and northwest are more moist than the local area.

Steering level winds are around 10 to 15 kt from the north, but anvil level winds are also northerly at about 20 kts. Thus, unless storms can propagate rapidly southward, anvil cloud will spread out ahead of the cells - not favorable for storms at lower elevations.

Thus, it's a very difficult call for lower elevations. Mountain and higher elevation areas should be very active. Will significant storms occur at lower elevations of southeast AZ? It's probably a coin toss, and we'll just have to see what kind of event unfolds this afternoon and evening.

As for TS/possible Hurricane Henriette, the NAM and the GFS have much different scenarios predicted for the storm. The NAM takes the storm fairly quickly into the area around the tip of Baja. This would not trigger a surge of low-level moisture into Arizona until the storm remnants actually moved to the north end of GoC or northern Sonora. The GFS takes the storm northward considerably further to the west, along a track that would trigger a significant surge. The GFS then brings the storm remnants right into southern AZ for a big rain event. However, it appears that only the operational GFS has this solution, and all the other GFS ensemble member forecasts from 0600 UTC hardly indicated that Henrietta exists. So, there are more coin tosses in the long-term outlook also.

August Weather Summary

A quick summary of what August brought here at the house.

Rainfall - 2.21" (less than half of July's 4.94")

Measurable precip - 8 of 31 days

Trace or more - 13 of 31 days

Thunderstorms in general area - 19 of 31 days

Severe thunderstorms in Tucson area occurred on: 10th, 14th, and 24th

Heavy tstm rains on the 4th produced severe flash flooding and several fatalities in Sabino Canyon.

All in all an active month, but much less so than was July.

The WRF run yesterday indicated that we might see a considerable increase in storm activity today. So, I'm about to see what the morning observations and products are saying. I will note that at sunrise today there were patches ofmiddle cloud floating around in all quadrants with some ACC present toward the south. This is much different than yesterday's sunrise, when there were only some wispy cirrus and lenticular clouds.

Last Weekend's Storms Near San Diego

I have been away from the weather for most of the past week, as I had to prepare a book catalogue for mailing before the Labor Day weekend. I now have a bit of time to catch up on weather events and the like.

Jim Means, who is an avid storm watcher and monsoon follower, sent information at the first of the week, as per:

"Finally a big monsoon today over here in Southern California. The combination of an upper level disturbance offshore from San Diego and a deep moisture field (PW ~ 2") from the remnants of Hurricane Dean gave San Diego County its best monsoon day in quite a while. Lifting caused by the upper level disturbance initiated storms before sunrise, and the NWS in San Diego had to issue an unusual Flash Flood Warning for the area around Escondido, about 25 miles north of San Diego. A strong (but not severe) storm produced flash flooding and some areas got more than 2 inches of rain in about an hour and a half. Another storm occurring at the same time produced a waterspout offshore from San Clemente. Daytime heating later generated another flash flood producing storm in the area of Anza Borrego Desert State Park, where people had to be rescued from their cars in what are normally dry washes. This storm generated 3 inches of rain in some places and the NWS reported the top at 60,000 feet."

Jim, who lives in Alpine CA - which is east of San Diego along I-10, has sent a couple of photos of interest:

Photo 1, taken by Jim at sunrise on Sunday 26 August, shows the early morning storm near Escondido, north of Alpine.

Photo 2 shows Escondido Creek still running in flood on Monday morning.

More Information on Missing Winds in the NWS Upper-Air Soundings

I took a quick look at the soundings from yesterday evening and from this morning. At least the following soundings have missing wind data:

0000 UTC 24 August 2007

RAP sfc - 620 mb
LCH sfc - 400 mb
SHV sfc - 600 mb
TLH sfc - 720 mb
MHX sfc - 680 mb

1200 UTC 24 August 2007

BOI sfc - 600 mb
TFX sfc - 700 mb
MAF sfc wind is only wind for entire sounding
TLH sfc - 680 mb
MHX sfc - 780 mb
IAD sfc - 750 mb
BUF sfc - 780 mb

I browsed though the soundings at the Storm Prediction Center (SPC) site:

http://www.spc.noaa.gov/exper/soundings/

These soundings now display only the actually observed winds aloft.

I would imagine that such serious losses of wind data would be having negative impacts on the models and also upon forecast operations at places such as the SPC.

Discussion of the TWC Sounding for 0000 UTC 24 August 2007

The TWC sounding taken yesterday afternoon Figure 1 illustrates several of the serious problems with data from the NWS RRS Sippican sondes. I discuss the sounding layer by layer beginning at the surface:

* Abrupt jump in Td at release - reason for this is unknown. See my earlier post on the Madweather webpage regarding the roof-top launch conditions at TWC.

* The afternoon boundary layer (BL) in T extends upward to 700 mb; however, the Td trace from 900 to 780 mb does not depict a well-mixed BL in moisture. My guess is that this reflects a slow response of the hygrister back toward actual values, after the abrupt Td excursion at release.

* A realistic portion of the BL is is sampled from 780 to 700 mb.

* The sonde apparently entered a towering cumulus at about 660 mb and experienced severe wetting of the thermistor and perhaps of the hygrister. The data are extremely bad from 600 to 400 mb affected by evaporation, and perhaps freezing, and sublimation. Note - I was inthe immediate vicinity of the launch site and there were no thunderstorms present; however, a deep and thick mesoscale anvil covered the sky from the WSW through the ENE. The equally absurd ASOS surface observation from TUS at this time was "CLR."

* From 400 to 300 mb the sensors are apparently slowly recovering toward values present in the actual atmosphere.

* Above 300 mb the data remain suspect; are extremely smooth; and do not reflect the presence of the ice saturated anvil cloud.

* The temperature data from at least 600 to 300 mb appear too cool. Since the thermodynamic data are used to compute the heights of the pressure surfaces, it is not surprising that heights at and above 300 mb appear 20 to 40 m too low.

Thus, it is likely that this sounding had physically realistic data for a single layer that was only about 80 mb deep!

I certainly hope that these data do not end up in the NCDC sounding data archive.

Winds Aloft Problem with RRS sondes - Today's Worst Example

While I didn't have time to plow through the details of all the NWS RRS Sippican soundings this morning, the worst example was not hard to find. The upper-air sounding at Boise, ID (WMO 72681), 1200 UTC, 23 August 2007, observed no winds above the surface.

I keep hoping that the NWS will formally alert external users of upper-air data, whether the users be proviate forecasters or the university and research community, of the problems that are plaguing their new RRS soundings.

As far as I know, such information has not been shared with the external user community - if my assessment is wrong, please let me know. Thanks. Bob

Overview of Problems with Data from the RRS Sippican Sondes

I have seen a copy of an internal NWS issue paper on the RRS sonde, including "talking points" for the media. After reading this document, I have a number of concerns:

The sonde problem is apparently being approached by higher levels of the NWS as if it is essentially a problem related to the performance of the Sippican hygrister in dry, desert environments.

In my opinion, nothing could be further from the truth.

After looking at many RRS soundings during the past month and a half, I conclude that there are serious problems with both the hygrister and the thermistor and, probably, with the sonde's internal electronic components. This last statement is just a hypothesis, since I don't know much about the technical details.

I am collecting bad RRS soundings from all over the country so that I can provide an extensive documentation of the problems. I provide here a quick overview of theproblems I have seen to date:

1 - Extreme dry layers above the surface with a slow recovery, leading to soundings that are too dry. (Note that this "too dry in low levels" problem was first detected by NWS personnel in the Southwest. However, the NWS instruction manual for operators of the RRS system indicates that other of the sonde's problems were detected during evaluation; however, implementation proceeded regardless.) See Fig. 1.

2 - Slow response to dry layers aloft when sonde is launched in a relatively humid environment, leading to occasional soundings that are too wet. See Fig. 2 and post of August 8th.

3 - Both the thermistor and hygrister are apparently poorly vented and one or both are easily wetted, leading to a plethora of problems caused by the latent heat effects of condensation, evaporation, freezing, and sublimation. (The wetting problems apparently relate to the compact size of the new overall instrument package that was evidently developed initially for military field use.) See Fig. 3 and Fig. 4.

4 - The RRS soundings are sometimes missing winds through significant layers (the ultimate example of which I distributed the other day - for a MN sounding, hardly a desert environment!) - See a previous post made today. The problems the missing winds can cause are many, some of which have serious implications for forecasting. This problem may be related to the sonde's internal electronic components. See both Fig. 1 and Fig. 5.

5 - Other problems, again probably related to internal electronics, cause high to veryhigh frequency noise leading sometimes to many many "special" points - in bothT and Td. See Fig. 6.

Avoiding GEMPAK Interpolation when Processing Upper-Air Soundings

The following information on how to avoid interpolation within the GEMPAK code has been provided by David O. Blanchard - many thanks!

The fix for the interpolation of winds and temperatures in GEMPAK is not difficult.

GEMPAK can store upper-air data in two formats: merged and unmerged.

Unmerged data sets will store the TTAA, TTBB, and PPBB data as separate parts. There is no interpolation of the data.

Merged data sets combine the TTAA, TTBB, and PPBB parts and contain values for each parameter (P, T, Td, Dir, Spd, and Z) at each level and interpolates the data.

To avoid interpolation, the data must be saved unmerged. This is specified with the "MRGDAT" variable in GEMPAK when originally creating the upper air file.

Most sounding programs that use GEMPAK upper-air data will call SN_RDAT to read the data; SN_RDAT calls SN_MERG to merge the TTAA/TTBB/PPBB data if they are not already merged; SN_MERG calls MR_UADT which calls MR_MISS to interpolate the data.

Only MR_UADT needs to be modified. Copy the MR_UADT source code and the required INCLUDE files to your sounding software directory. Edit MR_UADT and comment out the line that calls MR_MISS. Compile your sounding source code and the modified MR_UADT source code.

The necessary code is found in the following locations:

./gempak/source/gemlib/mr/mruadt.f
./gempak/include/GEMPRM.PRM
./gempak/include/MCHPRM.PRM (rename MCHPRM.machineType to MCHPRM.PRM)

This patch prevents interpolation of the upper-air data in your sounding program but makes no changes to GEMPAK and its programs and libraries. Your sounding program must handle the uninterpolated data arrays
(i.e., it must be able to handle missing data flags of -9999.)

Example of merged TTAA/TTBB/PPBB data with interpolation:
603.0 3.8 -45.2 298.7 4.4 4375.2
589.0 2.8 -46.2 295.5 4.7 4565.5
558.0 -1.1 -36.1 288.1 5.4 4999.7
546.0 -0.3 -49.3 285.2 5.6 5173.3
545.4 -0.4 -49.4 285.0 5.7 5182.0
505.0 -5.1 -54.1 280.6 5.2 5791.9
500.0 -5.5 -53.5 280.0 5.2 5870.0
485.7 -6.2 -54.2 260.0 4.1 6096.0
477.0 -6.7 -54.7 261.5 4.6 6237.0
431.2 -12.6 -58.4 270.0 7.2 7010.0
416.0 -14.7 -59.7 252.0 6.3 7284.9
414.3 -14.8 -59.4 250.0 6.2 7315.0
400.0 -15.9 -56.9 255.0 7.7 7580.0

Example of merged data TTAA/TTBB/PPBB without interpolation:
603.0 3.8 -45.2 -9999.0 -9999.0 4375.2
589.0 2.8 -46.2 -9999.0 -9999.0 4565.5
558.0 -1.1 -36.1 -9999.0 -9999.0 4999.7
546.0 -0.3 -49.3 -9999.0 -9999.0 5173.3
545.4 -9999.0 -9999.0 285.0 5.7 5182.0
505.0 -5.1 -54.1 -9999.0 -9999.0 5791.9
500.0 -5.5 -53.5 280.0 5.2 5870.0
485.7 -9999.0 -9999 .0 260.0 4.1 6096.0
477.0 -6.7 -54.7 -9999.0 -9999.0 6237.0
431.2 -9999.0 -9999.0 270.0 7.2 7010.0
416.0 -14.7 -59.7 -9999.0 -9999.0 7284.9
414.3 -9999.0 -9999.0 250.0 6.2 7315.0
400.0 -15.9 -56.9 255.0 7.7 7580.0

Missing Winds from RRS Soundings - An Extreme Example

On Monday afternoon, August 13, 2007, Rob Dale and Eric Hanson alerted me to the strange characteristics of the special 1800 UTC sounding from MPX (Chanhassen, MN, an RRS/Sippican sonde site).

The unbelievably smooth hodograph is what had caught their attention. The sounding, as analyzed by software analysis programs at the Storm Prediction Center (SPC) is shown in Fig. 1. Note the perfectly smooth hodograph and the winds that back continuously from the surface to 150 mb. This seems a very strange sounding indeed!

However, examination of the sounding at the FSL RAOB site indicated what the problem was. The MPX sounding at FSL (Fig. 2) indicates that the Sippican sonde apparently lost GPS tracking immediately off the surface and did not recover the signals until 150 mb - i.e., the winds were actually missing from the surface to 150 mb.

So where did the "data" plotted at SPC come from? It turns out that many sounding analysis programs use, as part of their code, GEMPAK. The GEMPAK procedure interpolates data into layers where observations are missing - assuming probably that such layers are never very deep.

The sounding plot from the Univ. of Wyoming site (Fig. 3) shows interpolated winds that are similar to those on the SPC plot. However, Lance Bosart alerted me to look at the same sounding at the UCAR/RAP upper-air site (Fig. 4). Their plot shows a completely different wind profile through the troposphere! Apparently, RAP software uses a different interpolation procedure. The winds in their "answer" for the deep layer of missing observations essentially keep the direction at 150 mb constant, while decreasing the speed smoothly until near the surface, where a shallow layer of backing winds is shown.

While I'm not sure what impact this comedy of errors might have had on forecasts during the afternoon of August 13th, it does vividly illustrate how serious the Sippican missing winds problem can be. I have been alerted by a number of people around the country about the missing winds problem, and I will document it further at a later time. However, this is a particularly insidious problem for non-NWS users of the data. If one suspects that a sounding may have missing winds, then a time-consuming examination of the sounding at different sites has to be made for accurate evaluation.

Severe Storm Complexes Two Nights in a Row

I have chased down some photos of damage that occurred in Casa Grande on Monday night. See Photos 1-3.

As for yesterday afternoon (Tuesday Aug. 14) - I was out on east side when storms rolled through around 3:30 to 4 pm. Thought I saw several wall clouds, and there was a spectacular hail shaft out over Oro Valley around 4:30 pm. Of course my camera was back here at house! I see that there was wind damage reported in Oro Valley with the afternoon storm.

I note on the SPC page that there was severe hail NW of Flagstaff yesterdayafternoon and also a damaging microburst in Riverside CA!

I was very surprised by the early storm developments yesterday afternoon. Obviously, I did not properly adjust and then forecast the morning sounding - at least there's now another "out" available, given the bad Sippican sounding data!

MCSs developed again during the night (Tuesday night) and the activity continues this morning in south-central AZ. Tucson, Casa Grande, and Phoenix Skyharbor all reported gusts over 50 mph near or after midnight. Phoenix reported a severe gust to 59 mph,and there was wind damage near the Tucson airport.

The question for today is whether we'll experience similar intense and severe storms for the third afternoon/night in a row? The soundings this morning appear similar to yesterday (TWC again too wet wrt GPS IPW) with at least moderate CAPE available given decent heating, and the low-levels remain cooled by the recent outflows. The steering winds remain relatively strong and there is better directional shear aloft with another inverted trough approaching. I am again thinking that activity should be late and after dark, but the NAM seems to convect early and to move another organized line across southern AZ during the mid-afternoon. We will see, and I'll keep my camera at hand today.

The weekend could be interesting if part of the tropical system over Gulf of Mexico breaks off and comes across Sonora and southern AZ.

Additional Commentaries on the Damage from Friday's Severe Thunderstorm

From Jim Toth:

Driving west on Orange Grove, I did not see anything significant until approaching the intersection at Oracle. The Park Place apartments are on the north side of OG, and that's where you start to see large trees down.

North on Oracle, the damage sticks out on both sides of the road all the way north to Ina. Aside from trees down and stripped green leaves obscuring parking lots, the most impressive thing I saw was the Long Realty building on the west side of Oracle. They had a large number of clay roof tiles peeled back and settled at odd angles.

At the Safeway parking lot at Oracle and Ina, there were a few large mesquite trunks snapped. These were out near Oracle. West on Ina, at the UMC medical offices, just east of Safeway, there were two large trees down near the entrance. Farther east on Ina, and then south on First, I did not see anything.

Steve Mullen reporting from Boulder:

My oldest son Ryan got caught in the teeth of the macroburst that nailed the NW side yesterday. He was driving along Ina between La Cholla and Oracle before heading southbound Oracle to Orange Grove around 6:10 to 6:15. Ryan reported two snapped stop signs, a leveled ~100-year saguaro, too many downed big trees to keep track. The sign in front of HiFalutin Restraurant was completely snapped. It was raining and blowing so hard near Ina and La Canada that he had to pull off the road.

Ryan shortly thereafter experienced some small hail. The Tucson Citizen had a blurb stating that winds were 60 mph over the NW side. I agree that they were 60 mph, for a moment, before gusting faster. It would take far more than 60 mph to down all that got leveled last evening.

Forecasting SE AZ Convection using Unreliable Upper-Air Sounding Data from NWS/TWC

The last two soundings taken at TWC illustrate how frustrating it can be trying to use unreliable data from the new NWS Sippican sondes. The sounding from 00 UTC on 11 August is shown in Fig. 1, and the 12 UTC sounding on 11 August is shown in Fig.2.

The FSL time series comparing GPS IPW with TWC sounding IPW is shown in Fig. 3. The sounding last evening was too dry by 5 mm wrt GPS IPW and this morning's sounding is too wet by 5 mm wrt GPS IPW. The forecaster apparently didn't notice this problem this morning:

AREA FORECAST DISCUSSIONNATIONAL WEATHER SERVICE TUCSON AZ945 AM MST SAT AUG 11 2007 DISCUSSION...THE TUCSON 12Z UPPER AIR SOUNDING INDICATED A MORE MOIST BOUNDARY LAYER COMPARED TO YESTERDAY. THIS ADDED LOW LEVEL MOISTURE COMBINED WITH GOOD SURFACE HEATING SHOULD YIELD MODERATE TO STRONG INSTABILITY FOR THIS AFTERNOON...ESPECIALLY NEAR TUCSON AND WESTWARD.

RE: Fig. 1

The afternoon sounding yesterday is particularly difficult to resolve meteorologically.
First, the surface super-adiabatic contact layer seems to extend upward to 900 mb. Possible I guess, but it also may be because a dry-spike data point was eliminated by the person who processed the sounding. I have no way to determine whether this was the case.

Second, where is the missing moisture located in the vertical? Since the well-mixed BL in theta extends up to 700 mb and the well-mixed q BL extends only to about 780 mb, I suspect that the "missing" moisture was likely in the upper-part of the BL. This sounding was taken in proximity, in both time and space, to a severe thunderstorm, I think that the CAPE was probably greater than the 314 indicated by the sounding analysis software. These sounding problems may have affected the timeliness of the severe thunderstorm warning that was issued at 6:22 pm.

Obviously, unless this sounding is somehow flagged as suspect in the data archives, a future researcher could consider it a valid severe thunderstorm proximity sounding, leading to………..

RE: Fig 2

This morning's sounding is also very difficult to analyze wrt to the likelihood of afternoon storms. Question is where is the "extra" moisture hiding in this sounding?
It is probably not in the spikey (noisy) data above 620 mb (some Sippican soundings are characterized by extremely noisy T and q data points for unknown reasons). The strange layer from 900 to about 840 mb is my guess for the layer that is too moist. I estimate that a BL well-mixed in q this afternoon would be one with q of ~ 11 to 12 g/kg.

While the mountains will have storms today, it is not clear how the lower elevation BL may evolve. It will likely have CIN above it's top, and if warming occurs in mid-levels, may end up not supportive for storms trying to move to lower elevations. But this is all guess work - since I don't know what the thermodynamic structure is below 700 mb to begin with, it is very difficult to try to predict what the afternoon structures are likely to be.

As for my forecast for our house - that's much more simple. Regardless of what happens to the south, unidirectional southerly flow is very unfavorable for storms and measurable rain at this particular spot. So, I would say that POPs here at our house are near zero today.

Storms in Tucson Metro Area on Friday 10 August 2007

Storms did develop over much of southeastern and south central Arizona yesterday. However, Cochise County in the far southeast was relatively suppressed, perhaps because dry air from New Mexico and northern Mexico invaded that part of the state.

Here at our house, we began hearing thunder from storms on the Catalina Mountains by 4:30 pm. I took Photo 1 of the initial developments over the Catalinas at 3:45 pm, looking NNE. My estimate is that cloud bases were at 700 mb or a bit lower, which fits with what one would expect from yesterday morning's TWC sounding.

The evening sounding was quite dry and this morning's (Saturday 11 August) is too moist. More on the problems of trying to forecast storms using the flawed data from the NWS/Sippican sonde in the next post.

I took Photo 2 at about 5:15 pm, again looking NNE. At this time frequent lightning was visible, along with lots of rumbling thunder. Updrafts had built southward from the mountains. There was another cell off to the ESE that had come off the Rincon Mountains, and new updrafts were forming between them. Photo 3 shows the Catalina storms in a view to the N, i.e., a bit to west of my Photo 2. The storms from the Rincons and the Catalinas apparently merged over the north side of town and then moved westward as a severe thunderstorm, producing damaging downburst winds.

Here at house we experienced strong wind gusts of 40 to 55 mph (estimated) from about 5:50 pm until about 6:20 pm. Winds were initially from the east with lots of dust, then from the northeast, and finally from the north (I think, since visibility was near zero in heavy rain at this time). Rain accumulation here was only 0.29". Jim Toth reported 0.97" a couple of miles to our NW.

Photo 4 shows the view of the storm from campus at 5:54 pm looking N. The ragged appendage, ahead of the outflow, showed some rotation as I observed it from the house. Note that the updraft and leading edge of rain shaft show green coloring, and small hail was indeed reported with the storm. Photo 5 shows some of the wind damage produced by the storms outflow, several miles to our NW.

All in all, quite an interesting afternoon here!

Interesting Storm Day Developing over Southern Arizona

Looks like a very interesting day setting up locally.

Very nice push of low-level moisture into most of southern Arizona after midnight, makes today quite different than yesterday thermodynamically.

The TWC morning sounding exhibits a low-level structure that's often associated with such nighttime moisture pushes, and I think that it is a reasonably good representation of morning conditions - one of the few times this week. The sounding, with the middle-level cooling and added moisture, indicates moderate CAPE lower elevations and even more unstable conditions higher elevations. The strong storms to our south and southeast last night crashed as they moved toward lower elevations. Today should be better, with storms coming into the lower elevations.

The morning NAM indicates a good setting by evening with upper-level difluence, and 15 to 20 kt steering winds at 500 mb. All of this due to an upper-level inverted trof brushing over the top of the malingering weak wave at lower levels. The dilemma of course is that the NAM has been forecasting stronger middle level winds than have been observed the past few days - so we have to wonder whether conditions will be as good this evening as NAM suggests.

The VAD winds this morning show L/V winds aloft - except for KYUX which shows that the surge extends up to at least 6000 ft agl. So, the evolution of the wind field today will likely determine how aggressively storms move into lower elevations. A good day to watch the Doppler winds and the Ft. Huachuca profiler winds closely.

My guess is that today will be quite active SE third of state with possibility of strong to severe hybrid downbursts and locally heavy rains. Storms here at house? Probably at least a 50% chance or better.

An Examination of Suspect Thermodynamic Structures in the Tucson Upper-Air Sounding AT 12 UTC 8 August 2007

Those who have read my previous posts are aware that I have been documenting data problems in upper-air soundings taken by the NWS using the new Sippican sonde. This morning's TWC sounding illustrates a suspect thermodynamic structure that I have observed several times here during the last few weeks.

First, a brief discourse on boundary layer (BL) structures. The afternoon BL is usually, but not always, well-mixed in both potential temperature (theta) and in moisture mixing ratio (q). Such BL layer structure is illustrated in the Phoenix 00 UTC sounding from last evening (Fig. 1) and in the TWC 00 UTC sounding (Fig. 2), ignoring the dry spike in contact layer. TWC uses the new Sippican sonde, whereas, Phoenix uses a Vaisala sonde with a different type hygristor. Both afternoon soundings depict a well-mixed BL extending from just above the surface to about 750 mb.

When winds are light and there is little advection or turbulent mixing during the night (the situation present over southeastern AZ during the past night), a cool and stable nocturnal BL develops near the ground, with the residual BL from the previous afternoon remaining in place above the nocturnal BL. The Phoenix 12 UTC sounding this morning (Fig. 3) indicates a residual BL present from about 850 to 700 mb.

However, the TWC 12 UTC sounding this morning (Fig. 4) indicates a much different structure aloft over Tucson this morning. Note that the theta profile indicates a residual BL aloft in about the same layer as found in the Phoenix sounding. However, the structure of q does not indicate a residual BL in the moisture field. Above the nocturnal BL q decreases steadily from about 12 g/kg to about 8.5 g/kg at 750 mb and then jumps up to 10 g/kg at 700 mb. This structure is physically very suspect, although I can not prove that the q values are bad, I feel strongly that they are.

Based upon the TWC afternoon sounding at 00 UTC, one would expect to find a residual BL well-mixed in moisture, as in the Phoenix sounding, with nearly constant q values of about 10 g/kg. The Sippican sonde seems to have responded slowly to the abrupt change in q above the nocturnal BL decreasing steadily from 900 to 750 mb and then jumping back up to the expected value of ~ 10 g/kg at 700 mb.

Interestingly, although I am personally convinced that the TWC morning moisture profile data points are erroneous from 900 to 700 mb, the sounding's total IPW agrees very well with the GPS IPW. Thus, at times the GPS-derived data may indicate that a Sippican sounding is accurate wrt IPW, even though there may be serious problems within the details of the Sippican data.

Errors such as the ones likely present in this morning's TWC sounding can have serious implications if one is trying to forecast the likelihood of strong convective storms.

Weather Update for Next Couple of Days

The last week has been very interesting with very high IPW combined with very warm middle level temperatures. It has seemed like Central America. Several very heavy rain events occurred with little or no cloud to ground lightning. There were three fatalities here in the Tucson area due to flash flooding.

Yesterday was quite interesting as skies cleared and IPW decreased steadily. Winds aloft here in southeast AZ were very light. With the abundant sunshine, a well-mixed afternoon BL built up to about 750 mb. So, the drying was apparently a result of weak advection plus subsidence - particularly from 700 to 400 mb. The BL has retained fairly impressive moisture, and dewpoints remain high in the SE part of state.

The TWC sounding has suspect structures this morning - more on this in a separate blog post later this morning - but it appears to me that with heating and slow deepening of BL, coupled with at least some degree of cool advection in middle levels, deep convection will return very quickly. This afternoon we may see Cbs over the nearby mountains. Then things get more complicated.

There is a substantial tropical wave moving westward into the eastern Pacific. Very impressive convective complexes occurred over the lower GoC and further south last night. Thus, with falling pressures in lower Colorado River Basin and southern AZ (about 2 mb during last 24 hours), southerly winds, and moisture advection, should increase up the GoC.

The NAM has had great difficulty last two weeks over Mexico. Disturbances persist in analyses and forecasts with apparently a lot of latent-heating induced spinup leading to stronger middle level winds in the model than have likely been present in the real atmosphere. The lack of observations over most of Mexico lets the model feedback on itself and develop structures that may or not be real. For example, this morning's run takes the main tropical wave to the west-northwest, but it also breaks off a piece to the north up the GoC - winds around this feature may again be too strong, but it is certainly an interesting situation worthy of careful monitoring.

Meanwhile, I'm really enjoying the return of some sunshine to the desert!