Tuesday, August 25, 2009

Brief Postmortem On Yesterday (24 August 09)


There was more convection over Arizona than I had expected yesterday, particularly over mountains to the west of Tucson. Storms occurred all the way out to the Colorado River. Radar loops indicated that the storms were strongly locked to mountain peaks and did not generally move far off the mountains.
Here at the house we could see storms in all directions and several cells tried to develop nearby but didn't make it, except up over the Catalina Mountains. We did have a nice cool outflow again in the late afternoon, this time from the south. During the past 24-hours 25 stations in the ALERT network had rainfall, down only slightly from the 39 stations with rain on Sunday. Heaviest amounts were were in the Catalinas with 0.87" at Samiego Peak.
The biggest problem with my outlook yesterday was that I expected dry and warm, middle-level advection to reduce CAPE during the day, with the gradual downturn in precipitable water continuing. However, there was actually cool advection across southern Arizona during the day in middle-levels and PW recovered or held steady at around 1.25 inches. So, there was nice CAPE, especially over the mountains, and lots of mountain storms. Note that the Tucson RRS sounding was very dry last evening wrt GPS PW, it's not possible to know exactly how much CAPE was present, except by the storm activity.
The NAM initialization at 500 mb last evening (00Z on the 25th) shows a distinctly unfavorable flow pattern over the state with westerly winds and the strongest vorticity lobe well off to the east. But flow below 500 mb was light, allowing the mountains to percolate all afternoon. Yesterday illustrates well a point that I often try to make - given low-level moisture and CAPE, it doesn't really matter if the middle-level winds are not from a "classic monsoonal direction." We see this all the time, and it's why drying, middle-level winds from the northeast are sometimes present on big storm days.

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