Saturday, September 12, 2015

Thunderstorms Return To East And South Of Tucson

Thunderstorms returned yesterday afternoon and evening to Cochise and Graham Counties, and also to the Borderlands of northern Sonora. Pima County remained almost totally devoid of storm activity, as forecast by the WRF runs yesterday. There was one particularly active storm out toward Wilcox between 7:00 and 8:00 pm MST, with CG flashes quite visible from the east part of Tucson.

Today brings an interesting situation, worthy of a closer look. The morning skewT plot from the 12 UTC sounding at TWC is shown above (from SPC). The sounding has substantial CAPE and a very favorable wind profile for organized storms, with strong easterly steering flow becoming strong west-northwesterly winds above 300 mb. The 06 UTC WRF-NAM forecast of the TWC skewT (below, valid at 5:00 am this morning) was fairly accurate wrt PW, CAPE, and the wind profile. Actual sounding is more moist below 500 mb and a bit warmer between 500 and 400 mb. But certainly an interesting situation at sunrise.

The fly in the ointment is that there is considerable dry air advection sneaking in behind a weakening back-door front from New Mexico - El Paso and Bisbee have PW values less than inch this morning, so that there is a strong moisture gradient across Pima County, with considerably higher amounts out to our west. This is indicated above by the 06 UTC WRF-NAM forecast of PW valid at 6:00 pm MST this evening. The forecast below is of composite radar echoes, also valid at 6:00 pm, which indicates storms to our west, and intense storms from Pinal County west-northwestward out to the Colorado River.

The WRF has a good track record in situations like the current one, where initial storms develop near or west of Tucson along the moisture gradient. So, question for later today will be that of exactly where do thunderstorms develop - nearly overhead or out to the west and northwest?

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