Thursday, August 21, 2014

Thick Clouds From TS Lowell This Morning

There were isolated thunderstorms yesterday in eastern Pima County, mainly over the high elevations. The CG flash plot above is for 24-hour CGs up to midnight last night.  The most storm activity occurred out to the west. Several storms near the Colorado River and also near Quartzsite had quite strong radar echoes.

Across the ALERT network in eastern Pima County 10 high-elevation sites had rainfall and three of these exceeded half an inch. Keystone Peak, west of Green Valley, had the most rain at 0.75".

Sunrise this morning had a bit of blue to the east but with heavy cloud cover moving overhead. Photo above was from along the Rillito looking toward Redington Pass at about 5:40 am MST. The visible satellite image below (for 7:30 am) shows that these clouds extend southwestward to TS Lowell. The big forecast issue for the day will be whether or not the clouds will break enough to allow significant BL heating.

The Tucson morning sounding (skewT below from SPC) is fairly moist through entire troposphere and cooling of a couple of degrees has occurred in layer above 500 mb. This cooling, with the increased moisture,  results in a modest amount of CAPE today, even at low elevations. Winds are strong in upper-troposphere ahead of the cyclone spinning near San Diego this morning - this feature will only move slowly eastward today. Winds in the lower troposphere are southerly but quite light - an increase in the low-level winds would improve the shear profile to one that might support organized thunderstorms.

The early WRF forecasts from Atmo keep strongest storms well to the west this afternoon. Both versions forecast isolated to scattered storms in eastern Pima County, with the GFS version having more activity in its forecast. So it is a day characterized by a lot of "ifs" at this time.

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