Tuesday, September 13, 2016

Storms Yesterday - Difficult Forecast Today

Very active thunderstorm day yesterday for eastern Pima County and much of the metro area. Mike Leuthold sent photo above of severe thunderstorm over the Catalinas late in the day. There were several reports of severe hail (see photo at bottom) and wind damage in the Oracle area on north side of the mountain. The plot of detected CG flashes below (from Atmo and Vaisala for 24-hours ending at midnight last night) shows the band of activity from Cochise across parts of Pima and into Pinal Counties. Here at the house we had thunderstorms twice during afternoon, with some gusty north winds to about 30 mph; little rain however, with total for both only coming to 0.20". Across the ALERT network more than 80% of sites had rain, but only nine of them reported more than half an inch, with none reaching an inch.

Very difficult forecast for today. The above forecast of composite radar echoes, valid at 4:00 pm MST this afternoon, is from the 06 UTC WRF-GFS model run at Atmo last night. It indicates a chance for storms around the edges of eastern Pima County, mostly over and north of the Catalinas.

The observed 12 UTC TWC soudning plot is a mixed bag. The SPC sounding analysis indicates considerable CAPE again today, but it is based on lifting the lower, moist layer that reaches only to about 800 mb. However, there is an elevated mixed layer above to about 550 mb. This amounts to a fairly formidable cap, not to mention the second potential cap produced by warm air above 500 mb. On the other hand, the veering, strong wind profile is typical of Plains severe thunderstorm environments.

Thus, if deep convection can overcome the capping layers, there is an excellent chance for severe thunderstorms. Looking at the forecast soundings from the WRF, I feel that they are not as favorable for storms as model seems to indicate. Based on my early look, I think storms are most likely from eastern Pinal and Maricopa Counties on up into the Rim Country. Will definitely want to take a look at the 12 UTC WRF forecasts.

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