Sunday, July 16, 2006

Another postmortem needed!

The weather events of yesterday really need a fairly detailed postmortem analysis. The e-mail buzz was about how big a 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). What happened? There were intense storms over much of south central and 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 and the eastern third of watch was devoid of thunderstorms even.

What happened with such an outwardly "good" situation? The primary bugaboo seems to have been that the approaching sysnoptic scale system was strong enough to "override" the diurnal boundary layer flow regime, bringing pronounced downslope, drying, boundary layer winds to southeastern Arizona. Note that terrain elevations decrease from over 7,000 ft msl at the Continental Divide in southwestern New Mexico to about 2,500 ft msl in Tucson area.

In some ways yesterday was somewhat similar to events aassociated with TS Nora several years ago - much precipitation expected but none realized due to the strong east winds in low-levels. Interestingly, Nora is often referred to locally as "No rain at all."

I hope to post a numbe r of graphics and more discussion about July 15th later today.

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