Wednesday, July 26, 2006

Brief comments re yesterday and last night's storms

The influx of low-level moisture, triggered by Tropical Storm Emilia, produced very unstable thermodynamics over much of Arizona yesterday. CAPE values were about as high as we see out here, but there was also considerable CIN to overcome at low-elevations. Indeed, the Phoenix region became the eventual focus of storm action after dark last night.

While the atmosphere was very unstable for convection, the winds were quite light below 500 mb and there were no distinct steering flows, nor much in the way of wind shear within the lower half of the cloud-bearing layer.

Storms developed over high terrain in three regions: 1) east to south of Tucson and 2) north to northwest of Phoenix during mid to late afternoon, and then 3) northeast of Tucson/east of Phoenix during the evening. The three resultant MCSs produced outflows that converged into the Phoenix area shortly after dark.

The MCS south of Tucson was characterized by storm cells that seemed to move randomly in a variety of directions, before eventually drifting to the west. Some of these produced hail - see

The late MCS (number 3) produced a great lightning show between 9 amd 11 pm here in Tucson, but, alas, not much rainfall.

A final, congolmerate MCS formed directly over Phoenix as the gust fronts converged there, produced torrential rains, and drifted slowly westward during the night.

Outflows from the massive convective episode did not reach to Yuma. Because of the slow movements and lack of a long-lived, dominant outflow driving westward propagation, I would not call this event a tropical-like squall or desert derecho.

I took four photos - CLICK HERE - from the house. Two of these show my rain gauge at noon yesterday and then at sunrise this morning. The third photo shows a dusty outflow moving north across the Catlina Mountains toward Phoenix late afternoon yesterday, and the last shows stratus cloud along the flanks of the mountains this morning.

The PHX and TUS soundings are quite stabilized this morning from all the activity yesterday. The Tucson area can usually recover more quickly than can Phoenix - this should be especially true for this event, since rain amounts around Tucson were not great (0.14" here at house, Trace at both TUS and DMA).

My initial impression is that neither the NAM nor WRF forecasts forecast the complex evolutions of storm complexes that actually occurred.

More later on this event!

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