Thursday, August 31, 2006

Hurricane John and Arizona weather

Today in southern Arizona is much like yesterday wrt CAPE. However, the steering winds today are light westerly and there is little directional shear above 500 mb. Thus, another day favoring heavy storms over the mountains and lots of anvil cloud over many low elevation areas.

Hurricane John is moving toward southern tip of Baja as a strong Cat 3, or even a Cat 4, hurricane. Yesterday's discussion remains essentially valid. Initially, the pressure gradient at low-levels becomes cross or down the Gulf of California. The NAM actually predicts light northerly winds at 850 mb over southern Arizona tomorrow afternoon, but as John moves westward away from the tip of Baja, a rapid push of moisture up the GoC should occur, probably coming into the U.S. late Friday night. A turn toward the west sooner than NHC predicts would bring moisture northward more quickly.

On Saturday the moisture influx from John should interact with a strong back-door cold front coming westward across the Continental Divide, producing perhaps the most intense storms of the weekend.

There are too many complexities in the situation to have much forecast confidence beyond Saturday and even the details of Saturday are difficult to anticipate, since the flow may become strongly downslope in the local Tucson area.

A final possible complication to at least consider is that the GFDL model continues to predict that Hurricane John will move directly into the GoC and make landfall on the western coast of Mexico.

Wednesday, August 30, 2006

Tropical cyclones galore

First - yesterday had some thunderstorm activity in southeastern Arizona, especially along the Borderlands south and southwest of Tucson. Observed lots of thunder and lightning here at house but rain stayed on the north side of Rillito wash - as per usual.

Today's morning sounding indicates about 33 mm of precipitable water and some decent CAPE, especially over mountains. There are light and variable winds to above 400 mb, so this will likely be a day when storms form on mountains and then wander around wherever their outflows take them.

On the large-scale, the pattern has been changing significantly. At 500 mb the subtropical anticyclone has shifted far southeastward over northeastern Mexico. While from 300 to 200 mb it is not clear that there even is an over-land anticyclone, except over far south Mexico. Down at 700 mb there is an anticyclone over northeastern Mexico and a weak trough extends from southwest Texas westward to northen Baja. Very moist air lies in and south of this trough, with precipitable water values greater than 50 mm.

The big weather uncertainties are what will happen from now through Labor Day. Hurricane John (click for IR image) was found to be a Cat 3 storm by NOAA aircraft early this morning. Complicating the picture is the fact that nearby TS Kristy appears to be approaching hurricane intensity also. The two storms are quite close and it is not obvious to me how they will interact. There was a large ensemble spread yesterday, with the GFDL model actually predicting John would move up the Gulf of California.

The moisture surge situation becomes complicated if a strong hurricane moves into the mouth of Gulf or near the tip of Baja. Initially, the pressure gradient becomes north-to-south with low-level winds blowing across or down the Gulf. If the storm then moves westward, its winds and associated pressure rises can trigger the northward surge of moisture, or the storm can continue northward bringing its moisture field with it into the Southwest.

The current NHC forecasts indicate John to be moving from south of the mouth of the Gulf to near the tip of Baja as a strong Cat 3 hurricane on Friday. Thus, the situation is extremely complex and the timing of a strong push of tropical air into the Southwest is very uncertain. All of this complicated by the fact a strong cold front moves rapidly down the High Plains on Saturday and Sunday, trying to back-door into southern Arizona.

All that one can do with such a difficult mix of mid-latitude and tropical features is to watch the observations very very closely. Of course, most weather attention the next few days will be on the east coast events associated with Ernesto.

Tuesday, August 29, 2006

Significant moisture surge into southern Arizona

The moisture surge discussed yesterday has occurred and been quite spectacular across southern Arizona.

First, there has been a classic and very pronounced surge of low-level moisture northward up the Gulf of California. Pressures at Yuma dropped quite low yesterday afternoon and the moisture surge was observed near dark between 8 and 9 pm there. The passage of the surge was quite spectacular as indicated by the changes in surface conditions from 8 to 9 pm. It has continued through the night. The NWS Doppler radar data show that the surge extends upward to just above 3000 ft MSL at Yuma this morning.

However, across southeast and south central Arizona moisture has also increased dramatically over the night due to both outflows from northern Mexico and the moist flow behind the back-door front, also mentioned yesterday. The change in the soundings at TUS from yesterday to this morning is also quite spectacular. Strong moisture increases are apparent up to 600 mb. This influx of moisture occurred during the late night and early morning hours and was more gradual than the surge observed at Yuma. Surface observations from Douglas to Tucson to Sasabe to Sells all showed significant dewpoint increases during hte night.

The morning sounding at Tucson indicates that signifcant CAPE may be present by afternoon. If the low-level moisture continues to increase, potential CAPE by afternoon could become quite large for out here. Temperatures at 500 mb have fallen and there is some steering flow from the north-northeast. Thus, it appears that an interesting and stormy afternoon is possible over southeastern Arizona.

In the longer term, Tropical Storm John developed yesterday afternoon. The NHC expects it to reach hurricane strength this morning. The forecast by NHC anticipates that John will rapidly intensify into a major hurricane and track to the northwest along a track that would, if it verifies, bring another major push of moisture into the Southwest by Friday and Saturday. All told a very interesting week and Labor Day weekend shaping up!

Monday, August 28, 2006

Another difficult forecast day

A look at the observations this morning, as well as the NAM and GFS, indicates another difficult forecast situation for the Borderlands. The following is my brief discussion of the situation.

First, the middle and upper-level wave spinning over the northern Plains has a weak trough hanging back, east-to-west, over the western U.S. The models appear to move this feature southward across at least eastern Arizona tomorrow. There is currently cold advection with the S/W, and this would be quite favorable, especially given the very nice vertical wind shear profiles that develop during next 24-hours, for strong storms, if there low-level moisture and CAPE in place. There also appears to be an upper-level moisture plume from what was Ileana approaching the Southwest.

A gradual advection of low-level moisture is occurring across southern New Mexico and into southeastern Arizona - this is due to the back-door cold front that has coming down the Plains as it tries to push westward across the Continental Divide.

Now to the south. No observational data wrt to upper-air soundings, as per the last 7 weeks. The NAM initializes a strong inverted trough over the southern Gulf of California at 500 and 700 mb - the 700 mb trough appears to brush across southern Arizona tomorrow afternoon. This occurs about the same time the 500-300 mb weak S/W is positioned over eastern Arizona. Thus, the question is: how much of this is real and how much is happening only in the NAM world?

Satellite loops indicate that a pronounced wave is moving westward across the lower Gulf of California. So, the moisture surge predicted by the NAM may indeed be getting underway. Surface obs indicate very high dewpoints along the GoC and south winds at LaPaz and Loretto. The U 0f A GPS precipitable water time series page shows that a huge increase in precipitable water has occurred at Hermosilla during past nine hours, i.e., since midnight. Values there are almost 50 mm this morning.

So it appears that there is probably at least a 50/50 chance of everything coming together over next 24 to 36 hours to support a new round of summer thunderstorms over parts of southern Arizona. The dilemma here is that very dry air will be lurking very near. The key is likely to be tied to whether or not a GoC surge actually occurs by tomorrow morning.

Who said that summer forecasting is easy?

Sunday, August 27, 2006

Forecasting for the tropics today

Just a general remark this morning.

I'm certainly glad that I wasn't working the midshift at NHC this morning!

The models (at least the NAM and GFS) are at best ambigous in their solutions regarding what might evolve at low latitudes the next few days. Ernesto seems to go a variety of different directions and the NAM tries to bring another tropical system up toward Baja at end of period - last time it did this it was an artifact and nothing materialized.

The NHC does anticipate that the tropical wave coming off Central America will develop into a tropical system. Meanwhile, there is something sitting over the southern Gulf of California with considerable associated convection. It seems to be a weak, nearly stationary low, BUT with the absence of observational data over much of Mexico continuing, the models seem constrained mostly by their last forecast.

What a mess! Does anyone care that the missing sounding data have caused extremely unreliable model forecasts for much of southwestern N.A. this summer?

Saturday, August 26, 2006

Drying out slowly

The TUS sounding this morning shows west-northwest winds in the 600 to 400 mb layer and precipitable water values down to 33.5 mm. The PHX sounding is drier with stronger winds.
Interestingly, the TUS sounding still has a fair amount of potential CAPE, so storms continue likely over mountains to the east.

I have just checked the 12Z soundings for TUS for the last four mornings. The winds in the 600 to 400 mb layer have been west-southwest to west-northwest each of these days and precipitable water has ranged from 42.9 to this morning's 33.5 mm. One could ask:

Did the monsoon end four days ago?

What kind of storms have been occurring with baroclinic westerly winds and lingering high moisture and CAPE?

I would call these "hybrid" or "mixed flow regime" storms since they are occurring in a meteorologically mixed environment. There is a preprint article about hybrid storms at:

Drying out slowly

The TUS sounding this morning shows west-northwest winds in the 600 to 400 mb layer and precipitable water values down to 33.5 mm. The PHX sounding is drier with stronger winds.
Interestingly, the TUS sounding still has a fair amount of potential CAPE, so storms continue likely over mountains to the east.

I have just checked the 12Z soundings for TUS for the last four mornings. The winds in the 600 to 400 mb layer have been west-southwest to west-northwest each of these days and precipitable water has ranged from 42.9 to this morning's 33.5 mm. One could ask:

Did the monsoon end four days ago?

What kind of storms have been occurring with baroclinic westerly winds and lingering high moisture and CAPE?

I would call these "hybrid" or "mixed flow regime" storms since they are occurring in a meteorologically mixed environment. There is a preprint article about hybrid storms at:

Friday, August 25, 2006

Difficult forecasts down in the Borderlands

Copy of e-mail sent earlier today to local weather watchers.
Mike and all,

Well I sure goofed yesterday by not paying attention to the outflow/mesohigh spreading southeast over us. That's of course a real kiss of death. What happens when you're trying to do too many things at once.

Today seems really a mess - westerlies with drying in top half of troposphere - how far will it mix down? No matter how I fiddle with TUS sounding in low levels, I get afternoon max theta-w of 22C and thus only a tiny sliver of CAPE.

PHX sounding looks like it should be thundering there right now - maybe a bad sounding point? Definitely some CAPE up there but stronger west winds lower down bode ill. Guess that we can say mountains should be active yet again - so what's new?

The upper-air charts show cold advection west of trof in NW U.S. at 500 and 300 mb indicating southward movement as per models' forecasts. I guess that I am not sure what a fairly strong wave like this coming south over continent in summer might actually evolve to. The first waves that have come by here have come off Pacific. A piece of the northern S/W may end up abandoned by its mother wave and wander around in the subtropical ridge after tomorrow.

NAM shows two tropical and one baroclinic 500 mb vort centers at play through the weekend - we know two are real but third?????? The GFS hints at the end of the monsoon in about 7 days - although it may be happening next couple of days. Then the GFS let's it come back in at longer periods. But the GFS doesn't seem to know about TD 5, potential hurricane Ernesto. Of course, neither of these models seems to have had much reliability for weeks.

I'm plenty confused and so, I'll just sit and watch what happens!


Thursday, August 24, 2006

Satellite Precipitable Water Vapor

The determination of the impacts of hurricane Ileana have proved difficult to quantify, given the moist precursor conditions, as discussed yesterday. The Creighton GSI has been indicating weak surge conditions at Yuma for more than 24 hours - the values did go above 25 beginning last evening. Yuma is currently surging quite strongly with dewpoints into the 70s and SSE winds over 20 kts.

The satellite precipitable water this morning seems to indicate a distinct plume of high precipitable water extending from Ileana and lower Gulf of California all the way north into the lower Colorado River Basin - see graphic by clicking below.


Note that the storms that developed in south central Arizona a bit before sunrise appeared to develop at the nose of the higher precipitable water plume.

The high dewpoint air is now just south of Las Vegas and it is not clear whether or not the plume will impact storm probabilities in southern Nevada - the midshift forecasters there expressed concerns about the possible surge in their early am forecast discussion.

Finally, the next couple of days appear fairly complex as the S/W in the northwest does battle with a strong subtropical circulation and several tropical systems.

Wednesday, August 23, 2006

What will be the impacts of Hurricane Ileana?

Hurricane Ileana has strengthened considerably since yesterday morning and has moved northwestward faster than forecast yesterday. Early this morning Ileana was a strong Cat 2 storm with estimated winds of 100 kt with gusts to 120 kt. The official NHC forecast calls for her to be located near 19.9 N and 114.4 W at 24 August 0600 UTC as a Cat 3 storm. The track forecast is shown in Fig. 1 IR satellite imagery from 1030 UTC (Fig. 2) this morning indicates that Ileana may be moving northward slightly to the east of the forecast track. Also of interest is the associated large mass of clouds and thunderstorms to the northeast of Ileana over west-central Mexico and also south of the Gulf of California. All of the above indicates that Ileana should trigger a significant northward surge of moisture that would reach into the lower Colorado River region by tomorrow morning.

However, the dilemma here is that high amounts of moisture and precipitable water are already present over much of the southwest U.S. So what impacts might be expected? I am going to guess that Ileana will result in increased moisture advecting into the lower Colorado River Basin, southern Nevada, and the southern California deserts. A distinct surface surge may be more difficult to detect, but one is certainly possible. Watch Art Douglas' surge index on the Creighton Univ. web page at:

Tuesday, August 22, 2006

Brief review of yesterday and last night's weather

There was much more storm activity yesterday over south central and southeast Arizona than there was on Sunday. The "favorables" dominated yesterday apparently and intense storms had developed by noontime. For Arizona there were quite a few severe reports during the afternoon. See the reports at:

There were also a couple of measured severe gusts within the PRISMS mesonet in the Phoenix metro area that are not on the report map. A brief story and photo of damage in the Tucson area yesterday is at:

Here at the house there was much thunder from nearby cells early in the afternoon, but once again only wind and sprinkles and a trace of rain. However, the evening sounding was much moistened with considerable instability in all layers below 650 mb - nighttime/early morning storms developed and it was raining here at sunrise with 0.18" in the gauge.

The morning sounding at Tucson exhibits a very nice "onion" sounding with a deep, ice saturated layer producing anvil rainfall above the onion bulb. Sounding is at:

Today appears to favor a heavy rain threat in southeast Arizona, as the inverted trough at 700 mb that brought a favorable wind profile for severe storms yesterday has moved on to the west. The western lobe of the 500 mb subtropical high is over southeastern Utah this morning, and the weak trough separating this anticyclone from the larger anticyclone over the Southern Plains appears to be between here and Phoenix. Thus, winds aloft are very light and variable up to about 300 mb and storms should be very slow moving in this part of Arizona today.

Tropical Storm Eleana is predicted to become a hurricane shortly and the NAM and NHC forecast the storm to move northwestward and to come north of 20N. Thus, this storm may come close enough to the south end of Gulf oc California to trigger yet another push of deep tropical moisture into Arizona Friday or over the weekend.

Monday, August 21, 2006

Postmortem of yesterday and overview of morning observations today

Part 1 - Postmortem of yesterday: Storms were not widespread yesterday and a tropical-like squall did not materialize. So, yet another overly optimistic outlook - it's been a very tough summer for forecasting down here. Thunderstorms did move from the Catalinas into town last evening, and the airport got a direct hit by a storm with a moderate downburst and 0.33" of rain. I shot a photo of this storm at about 7pm when it was just west of the airport (i.e., looking south from our house) - CLICK HERE The simultaneous radar depiction of the storm CLICK HERE shows both the storm in the photo, and also the limited extent of local storm activity. As usual, we had about two hours of thunder here at the house, outflow, a sprinkle (trace of rain) and a great rainbow.

The most negative aspect of the day was the evolution of the thermodynamics during the afternoon (we knew we were dealing with a marginal wind profile). By evening, the Tucson sounding CLICK HERE showed drying had occurred during the day below 600 mb and that the afternoon BL was characterized by a moisture profile that was not well-mixed. This is a difficult negative to overcome, and the storm that made it to airport must have been propagating with a fairly strong downdraft (as the gust to 33 kt at TUS indicates).

Part 2 - Overview of conditions this morning: This morning's Tucson sounding CLICK HERE indicates moistening up to 500 mb, no residual BL is present (meaning substantial heating will be needed to build a deep BL over lower elevations), and there are fairly strong southeasterly winds present below about 650 mb (i.e., a downslope direction). Winds at upper-levels have veered again and are southwesterly around a weak trough over the West (so there is a good shear profile, except for the weak winds present from 500 to 350 mb). The key forecast issue is whether a well-mixed BL will be able to develop in the face of the downslope winds. (Mike Leuthold - let us know what the WRF model does if you can find time.)

On the larger-scale, standard level charts we find that:

At 700 mb: An anticyclone centered near Las Vegas is affecting most of Arizona, except the far southeast where the inverted trough over western Mexico appears to have resulted in southeast winds. The main anticyclone over southeast Oklahoma is separated from the western lobe (which actually has three weak centers) by a trough line extending from near Phoenix northeastward to east of Omaha. The 700 mb level is very moist at all sites upstream from Tucson (i.e., Albuquerque, El Paso, and Chihuahua MX).

At 500 mb: A western lobe of the anticyclone is centered in southern Utah, with a trough line separating it from the strong eastern anticyclone center over Oklahoma. This trough extends from near Tucson northeastward to central Nebraska. The southern part of this trough is an extension of the inverted trough over western Mexico - this trough appears to have moved westward away from the upper-level feature that has been nearly stationary for last couple of days over west Texas. Thus, the NAM may have it sort of right today, with the lower-level trough actually moving westward.

At 250 mb: The upper trough over west Texas/northeastern MX is very diffuse and hard to define - thus, this feature appears to be confined to the very upper troposphere, making it a likely non-player in the evolution of weather west of the Continental Divide. The TUTT seems very pronounced and shifted to a northerly location from central Mexico eastward across south Florida. Upper flow today is not nearly as diffluent as it was yesterday morning (a negative).

Why all this description? I think that today's 12 UTC observational data illustrate well how terribly difficult it can be to sort out all the potential interactions and evolutions of weak features which will come together to produce afternoon and evening thunderstorms. My gut says that today will be a much more active day than yesterday ended up being, but only time will tell which features dominate.

Sunday, August 20, 2006

Severe storms in southeast Arizona this afternoon?

Has been a busy week and I have not had a chance to do much here in blogland.

The morning observations today - Sunday August 20 - are very interesting and indicate some potential for a tropical-like squall and significant storms this afternoon.

The morning Tucson sounding, view at:

went bonkers above 350 mb, so we have to decide whether to accept the data below 350 mb as being good. If I accept the observations below 400 mb, it appears that CAPE is substantial for out here, that there are some easterly steering winds above well-mixed BL cloud base (~700 mb), and that anvil level flow is from the southwest (favorable).

The 500 mb anticyclone has a split off a lobe right over Phoenix this morning - not a favorable position. But, there is not a deep, barotropic anticyclone aloft over Arizona.There is a deep trof/shear zone from 700 to 200 mb over New Mexico between the large anticyclone to the east and features to the west. The flow above 300 mb appears highly difluent this morning (favorable). The weak flow in middle levels continues - the storms of August 8th got organized with similar weak flow, so we'll have to see what happens today.

The models at 0600 UTC provide little help - NAM continues with the near TS it has somehow genertaed off to the SSW. Thus, one has little feel for how much credance to give the predicted wind profiles. In general, the wind patterns predicted by GFS for this evening are less favorable than what's observed this morning.

Certainly should be a much more interesting day than have been the last couple of days, since the early morning deluge of Wednesday the 16th.

Sunday, August 13, 2006

Mike Leuthold said:

This is a re-post of Mike's comment with the urls activated.

The NAM has had terrible initializations for most of the last week with a semi permanent strong inverted trough in various random parts of northern Mexico. The result of this is that mid level forcast winds have been much stronger (up to 30kts) that what has actually happened, and these winds then caused storms in the model to organize and propagate into the lower elevations, which hasn't happened. The NWS had bit at these poor forecasts more than once with high pops for low elevations and even a mention by SPC sometime last week that the forecast shear was high.This is a link to a model sounding over Mt Lemmon made from the 12z NAM from the previous day (6 Aug).

This is the actual sounding.

Perhaps the NWS should put some effort and money into supporting northern Mexico soundings during the summer. The above cases show how poor the models perform when there are no soundings from there. It would be money well spent.
11:59 AM

Does Tucson really experience a summer monsoon?

The link at the bottom connects to three figures. Each figure shows (for 2004, 2005, and 2006 to August 10th) daily average GPS precipitable water at Tucson, daily average wind direction for the layer 600 to 400 mb from the TUS soundings, and number of cloud-to-ground lightning strikes detected each day over Arizona, including about 100 km outside the state border. Figures provided by Carlos Minjarez-Sosa.

The frequency of various wind directions during the "monsoon period" (i.e., the period with PW values consistently 30mm or greater) are shown in the table below:

* Note - 2006 data are through August 10th

It is clear that the observed wind directions, within the 600 to 400 mb layer, during the last three summer "monsoon periods" have been highly variable. If wind directions from the SSW to ESE are considered to be typical or favorable "monsoonal flow," then only 36% of the days at Tucson have exhibited "monsoonal" wind directions.

Interestingly, if the wind directions for all days shown in the table are averaged, the resultant mean wind direction during the monsoon periods was from the southeast. This is because the SW and NE winds essentially average to SE, and the result is not an accurate picture of what wind directions actually were during the three summers.

Indeed, these summer wind directions are not at all persistent. Although Tucson experiences a summer period characterized by high PW and significant thunderstorms and rains, the variable wind directions lead one to ask whether Tucson actually experiences a summer monsoon.

CLICK HERE -- for Monsoon Variable Comparison figures discussed above

The severe storms of Tuesday August 8th

I have been very busy the last week or so and have not gotten much posted.

Some of the most severe storms to strike the Tucson metro area occurred during the late afternoon of Tuesday August 8th. The synoptic setting was not one that seemed supportive of such intense storms. These storms displayed a distinct linear organization for about two hours as they moved across the southern half of the city. Some details and a photo are at this newspaper url:

Later this summer or Fall I will analyze the most severe events of the summer and contrast them with a newspaper description of what meteorological conditions bring Tucson the most severe storms during the summer.

Finally, the almost total lack of sounding data from much of Mexico since early July continues to frustrate attempts to analyze the larger-scale conditions over southwestern North America - the negative impacts on the models have been especially significant.

Monday, August 07, 2006

Three very different morning soundings

There has been a substantial increase in low-level moisture during the night over southeastern and south-central Arizona. This moistening was related to outflows from both local convection and a large MCS in northern Sonora, in combination with some larger-scale advection as a strong 500 mb cyclone pulls north into the western US.

The overnight increase of precipitable water at Hermosilla, MX, was quite spectacular - as per Pat Holbrook's comments last evening. The following is a link to the GPS precipitable water time series at U of A atmospheric science.

Thunderstorms moved into central and south Tucson last evening with thunder and light rain observed at Davis Monthan AFB and light rain (but no thunder or lightning) observed at NWS TUS. Lightning was clearly visible from our house but apparently no showers up here in the north metro.

The morning soundings at Tucson, Phoenix, and Yuma are quite different from each other.

CLICK HERE to see the 12z soundings for these 3 locations.

Both Phoenix and Tucson, with heating, have the most potential CAPE observed since the 25th of July. The winds are very light at Tucson above 800 mb, indicating a good potential for heavy rainstorms and possible hail over higher elevations. In contrast, Phoenix has a strongly veering wind profile that would favor strong storms and possible severe wind gusts - however, the Phoenix wind profile should trend toward that of Tucson. Phoenix may also be vulnerable to low-level drying, since drier air is very close to the west. Definitely a difficult forecast situation at Phoenix!

In contrast, Yuma has strong veering winds but remains very dry and stable. I still expect a dramatic low-level moisture increase at Yuma, as the air sample by GPS at Hermosilla moves northward into Arizona.

It will be interesting to see how things evolve today.

Sunday, August 06, 2006

Moisture surge knocking on the door

I have been e-mailing with Mike Leuthold this afternoon about the liklihood of an overnight surge of low-level moisture up the Gulf of California (GoC). It appears likely that the large areas of active convection over the southern GoC, and also south of the mouth of GoC have triggered at least a moderate surge. Expect the influx to show up at Yuma by early morning Monday. Moisture is also increasing from the east, so storm activity should increase next couple of days, even though mid-level temperatures are relatively warm.

The warm, dry air aloft that came in from the south on Friday really shut things down, since it eliminated what CAPE there had been. It was a nice example of how things here in southern Arizona can go totally suppressed, even though low-levels remained fairly moist. The situation was also interesting since the NAM runs on Friday kept showing precip here every 6 hour period through Saturday night. The NAM has been forecasting cyclones and waves south of Tucson for a number of days, but today is first day that observations show much. It's not clear whether there is any cooler mid-level air lurking over Mexico - there have been almost no soundings over much of Mexico since the third week of July. This may be the reason that the NAM initializations and forecasts have been so unreliable of late.

So, we'll see what tonight and tomorrow bring.

Thursday, August 03, 2006

The Week of Great Storms

The past several days have been so unusual, with such heavy precipitation and flooding, that I have been unable to do much except try to save data files and images so that, eventually, I can do careful analyses of the events.

There were significant MCSs over Arizona from the 25/26 through the 30/31. On some days there were MCSs early over the mountains and then later over lower elevations.

The episode began with a great northward push of low-level moisture into the state that was triggered by TS Emilia moving through the favored area southwest of Baja (see earlier post). I had anticipated this surge would reach Arizona 12 to 18 hours earlier than it did. I put too much confidence in the early track forecasts of NHC, which were too fast. Art Douglas warned me that he didn't think the TS would move northward as fast as the early NHC forecasts indicated and he was correct.

The first MCS event occurred during the night of the 25th (26th UTC time - note that Arizona is on MST and that midnight is 0700 UTC). An earlier post covers this first event in some detail.

Satellite IR images are shown for UTC times on the 26th through the 1st of August.

CLICK HERE for Figures 1 -8

As the week progressed, the large-scale pattern evolved from one dominated by a large, middle-level anticyclone over the west, i.e., a fairly typical July setting into one with a cyclone and several associated short-waves spinning over northeast Arizona and western New Mexico, remaining nearly stationary for a couple of days.

MCSs during the period first moved toward the west-southwest, and then toward the south, and eventually toward the southeast.

The rain events measured at our house are shown below - and, for a larger perspective total CG counts over the Arizona domain (domain has been shown in earlier posts). My 24-h rain measurements were taken around 1300 UTC each day and the 24-h CG counts ended at 1200 UTC each day.

Date / Rain Total / AZ CGs

7-26 / 0.14" / 36,256
7-27 / 0.17" / 26,696
7-28 / 0.39" / 14,614
7-29 / 1.64" / 25,047
7-30 / 0.40" / 10,013
7-31 / 1.39" / 14,907
8-01 / 0.15" / 5,652

Total rain measured here at the house was 4.19"! The most unusual aspect of these heavy rains, at least here locally, was that approximately 80% of the rain occurred between 2 am and 8 am local time. Thus, events in southeast Arizona for the week considered were mostly nocturnal and more like the summer rainfall climatology of Omaha than of Tucson.

More will follow.