Saturday 15 JULY 2006


The weather events of this past Saturday really beg for a fairly detailed postmortem analysis. The e-mail buzz on Saturday morning was about how big a storm day it would be in southeast Arizona; the local forecast had 40% POPs (as high as they've gone this summer I think); there was a severe thunderstorm watch issued for much of the southern third of the state (the 4th watch this year I think).

Storms developed over the higher Rim country north of Phoenix during mid-afternoon and also to the east of Phoenix over higher terrain. These storms moved rapidly to the west-southwest, eventually forming a fairly large MCS (for Arizona). The principal weather produced was lightning, wind, and dust storms. The MCS eventually moved across extreme southern California and northern Baja and on westward over the Pacific (as noted by Lance Bosart's mapper post). The MCS left behind a mid-level MCV that looked like a mini-tropical storm at midday on the 16th.

NOTE: click on each figure for a larger view

Figure 1                                   Figure 2    

Figure 1 presents radar-estimated rainfall for Arizona ending at 1200 UTC on the 16th, while Fig. 2 shows the cloud-to-ground lightning strike locations for the same period. The radar-rainfall shows that there weren't widespread areas of significant rainfall with the storms. Regions that experienced the heaviest storms were very remote. The figure also seems to indicate that the storm clusters from the mountains weakened, before re-developing and re-grouping into the dominant MCS over southwestern Arizona. The CG plot indicates this also, with a dearth of CGs over the Phoenix region (dare I call it the infamous circle of tstm death?) of suppression This region is somewhat circular and is of low elevation. The 0423 UTC IR satellite image (Fig. 3) shows that the Arizona MCS had raced to the southwest corner of the state by that time. The MCS in Sonora is much larger and more interesting, as is usually the case.

Figure 3

Rainfall reports from the Arizona daily station list show also that there was not much rain measured with the storms. Of 57 stations reporting 24-h precipitation amounts at 5 am on the 16th of July, only 14 had 0.01" or more, of these 7 stations had 0.10" or more, 3 of these had 0.25" or more, and 1 had 0.50" or more (Payson reported 0.62").

The intense storms occurred mostly over southwest Arizona, while the southeast portion of state was very suppressed. There were 4 severe storm reports that were in or near the western half of watch area but the the eastern third of the watch was devoid of thunderstorms even. Even though significant thunderstorm activity was expected over the southeastern portion of Arizona, this part of the state was very suppressed.


So how did an apparently "good" situation go so badly awry?

Figure 4

Let's focus on the upper-air soundings in this postmortem. Figure 4 is the skew-T plot of the Tucson morning sounding. The sounding shows a moist layer from about 600 mb to 300 mb (positive); wind directions at anvil level are backed 40 degrees or so relative to the "steering" winds in the 600 to 400 mb layer (good); well mixed boundary layer CAPE is indicated, although the amount of CAPE is not great (especially wrt severe storm soundings from the Plains), it is typical of amounts that often produce nice storms and downbursts out here; the standard-level charts (not shown) depicted an unusually strong upper-level cyclone centered in northern Mexico, south of the Texas Big Bend. This synoptic-scale system was moving west-northwestward toward southeast Arizona and this type scenario is often favorable for increased thunderstorm activity.

The primary bugaboo on Saturday seems to have been that the approaching synoptic-scale system was strong enough to "override" the usual diurnal boundary layer flow regime, bringing pronounced downslope, drying, boundary layer winds to southeastern Arizona. Note that terrain elevations decrease from 5-6,000 ft msl along the Continental Divide in southwestern New Mexico to about 2,500 ft msl in the Tucson area. If one examines Fig. 4 carefully, it shows that strong east winds were already present in the boundary layer at Tucson. Data from El Paso (not shown) also detected this situation. By afternoon on the 15th in Tucson east to southeast winds at the surface (the typical afternoon surface wind is from the west to northwest) were blowing at 10 to 15 kts, and the surface dewpoint had plummeted from 64F at 9am to only 42F at 5 pm. Interestingly, the morning sounding at Phoenix (Fig. 5) showed quite different low-level flow over central Arizona, where there would be strong storm activity later in the day. The 00 UTC sounding at Tucson (Fig. 6) indicates that the easterly, downslope flow had dried out the lower levels so that the evening sounding had virtually no CAPE, again explaining the dearth of thunderstorm development over all of southeaster Arizona.

Figure 5                         Figure 6

The break-through of easterly winds to the surface produces a curious, large mesoscale feature. The leading edge of the easterlies becomes somewhat analogous to a "mirror-image" of the dry line of the southern Plains. The most intense storms of both Saturday and Sunday developed on the moist (west) side of this dryline - shown most clearly in detailed, high-resolution WRF model forecasts (unfortunately I was too slow on the draw to capture examples).

The main conclusion that I make from this brief postmortem is that this non-event illustrates very nicely how terribly difficult summer forecasting in the Southwest can be. It is often extremely difficult for a forecaster to be able to anticipate accurately the thunderstorm threat for "this afternoon and evening!"

I have forecast for many locations over the past 40 years, and I can honestly say that I have not ever dealt with a more difficult region than the Southwest in summer!

The presence of a "synoptically evident" feature often muddies the evolution of a given day's weather - as per this event, when meteorological features were presumed to be providing a setting favorable for large scale upward motion, but in effect produced low-level flow that destroyed what CAPE that had been present before the onset of the downslope winds.

All of this leads me to ask why the NWS spends so much effort producing forecasts and forecast grids out through seven days, when it remains extremely difficult, at least in this part of the country, to forecast this evening's weather accurately? The challenges of the summer forecast in this part of the country also explain why the forecasts rarely deviate far from seasonal climatology - except when there is an obvious dry period.


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