Sunday, August 23, 2009

Sunday Dawns Perfectly Clear

Activity was considerably down yesterday, compared to Friday, with most storms occurring at higher elevations and on the flanks of the mountains. The sunshine we got wasn’t enough to heat up a well mixed boundary layer and it was quite a cool day. Additionally, the S/W that had passed by on Friday night was much stronger than the model runs suggested. Measurable rain fell at 44 of the 93 Pima County Alert Gauges, with quite a bit of that occurring before noon. Only 3 sites had half an inch or more and Corona measured 1.46” in a heavy storm between 11 am and noon.

This morning there is dry air aloft above 500 mb; the lower troposphere continues moist with moderate amounts of CAPE; the deep moist plume is completely to the east of Arizona with clear skies prevailing; winds this morning are south-southwesterly to southwesterly not a very good shear profile for storms moving into the low deserts. Precipitable water has trended slowly downward with values over souther Arizona of 1.25 to 1.50" this morning. The 500 mb anticyclone is centered over the Oklahoma panhandle and is very large with a north to south orientation. A complicated situation once again.

The NAM forecasts the deep moisture plume to shift back westward, with deeper moisture moving up from the south. The 500 mb trough off southern California does not come ashore until tomorrow. The SPC has outlooked a slight chance for severe thunderstorms today over the lower Colorado River Basin – see above – where temperatures are cooler at middle-levels and winds are stronger. The NAM continues to forecast significant precipitation and storms across eastern Arizona both today and tomorrow as strong southerly winds, with embedded ripples, prevail. The model also forecasts a strong vorticity maximum to move northward up the Gulf of California (there is a very large MCS just south of the mouth of the Gulf this morning). There should be an upswing in activity today, but the details remain fuzzy given the wind aloft profiles forecast; heavily raining storms likely perhaps a downburst or two. Tomorrow depends very much on what today brings.

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