Friday, July 29, 2016

Big Storm Day - Will It Just Miss Tucson?

Beautiful sunrise this morning behind Thimble Rock, after a very suppressed, as expected, day yesterday. The plot below shows detected CG flashes though 7:00 am MST this morning, with southern Arizona almost totally devoid of thunderstorm activity.

The morning soundings at TWC (above) and also PHX are quite unstable. Here at TWC there is a moist surface layer up to about 800 mb, but with an elevated mixed layer (EML - an old boundary layer - BL) from higher terrain above. The EML essentially caps the surface based BL for much of the afternoon, allowing it to really heat up in the deserts. But, when heating and convergence acts to merge the two layers into a single BL late in the day, serious storms are likely.

There is some drier air (light blues) to the east, northeast, and southeast - below is blended PW analysis for 12 UTC from CIRA at Colorado State. This drier air is the wild card for Tucson, since storms will likely develop in the convergence zone between the dry and moist air masses.

The 06 UTC WRF-GFS forecasts the dry air to be over the east metro by 5:00 pm (above). The NAM version forecast pre-sunrise storms over Tucson, and I didn't look at it in detail. The forecast surface plot for 7:00 pm below indicates dewpoints as low as 42F along I-10 east. There is some convergence over the City - but will it trigger storms here? The WRF-GFS forecast of composite radar echoes valid at the same time (bottom) develops the storms just west to northwest of the metro area - this is a scenario that we often experience here. So, a careful weather watch is the best approach for this afternoon.

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