Thursday, July 31, 2008
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.
Monday, July 28, 2008
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:
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.
Thursday, July 24, 2008
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.
Tuesday, July 22, 2008
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.
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.
Monday, July 21, 2008
Madweather post: Weather Summary Last Three Days - July 18 through 20
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.
Figure 1 Figure 2 Figure 3
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.
Monday, July 07, 2008
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.
Saturday, July 05, 2008
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!)
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.
Friday, July 04, 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.
Thursday, July 03, 2008
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.