Saturday, August 29, 2020

High PW Today And Flash Flood Watch

Pre-sunrise color on altocumulus drifting overhead this morning. Down at bottom is view of another colorful sunrise today.

The NWS has issued a Flash Flood Watch for this afternoon and tonight, as per above. Some models forecast the heaviest rains tomorrow, but logic for focus on today and tonight is well explained in the forecast discussion. 

Thunderstorm activity yesterday was very isolated in southern Arizona (CG flash plot above for 24-hours ending at 7:30 am this morning - from Atmo and Vaisala), although there was heavy activity in Sonora during the night. Very limited rainfall measured in the ALERT network for 24-hours ending at same time (below),

The TWC sounding this morning (above) is very moist, with over an inch and a half of PW and much increased CAPE wrt to last few days. However, there are several inversions above 500 mb that could come into play, depending on depth of mixed layer this afternoon. Steering flow is from the southwest, which is often not favorable for storms here at the house.

The forecast sounding below (from the 06 UTC WRF-GFS run at Atmo) is valid for TWC at 6:00 pm MST - sounding indicates convection has already occurred in the area by this time.

Forecast above from the 06 UTC WRF-GFS indicates heavy storms in the east part of the metro area at 8:00 pm today, while forecast below from the 06 UTC WRF-NAM indicates storms approaching our area at midnight tonight.

Second below is the WRF-GFS forecast from same run as above valid for total rainfall through Sunday night at midnight. 

Looks to be an interesting weekend, although I did note that the 00 UTC run of the WRF-GFS forecast a distinct donut hole over the Tucson metro area.

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