This morning the anticyclones at 500 mb (12 UTC analysis above) and 250 mb (12z UTC analysis below) are spatially aligned in a way that provides an excellent shear profile for organized thunderstorms across southern Arizona. The 500 ridge axis stretches from Boise, Idaho, southeastward to Del Rio, Texas. The steering winds at 500 mb over southern Arizona are from the east-southeast, while the winds at 250 mb are from the southwest. The 250 mb anticyclone is centered somewhere over northern Sonora.
The 12 UTC skewT for the Tucson sounding (above) indicates small CAPE from the SPC analysis, however, it appears that a new boundary layer (BL) will try to develop today (assuming we get afternoon sunshine - see previous post). The new BL will reach only to around 800 mb (see early WRF-GFS forecast sounding for TUS at 3 pm MST below), but will have considerable CAPE. It will also require considerable lifting to reach its LFC (probably somewhat similar to yesterday). The soundings at TUS, PHX, and YUM this morning, and the overall larger-scale pattern, are very similar to the situations described by Smith and Gall (1989, Tropical Squall Lines of the Arizona Monsoon. Mon. Wea. Rev., 117, 1553-1569). The early WRF-GFS forecasts a very active day across almost all of southern Arizona, with several lines of thunderstorm activity continuing through the night. Much of the forecast activity depends upon both considerable sunshine and strong, mesoscale outflows - a situation that may or may not materialize given the current cloudiness. I noted that the early WRF appeared to do a good job forecasting the MCS that was south of Douglas at sunrise. Obviously we need to check back on the 12 UTC WRF-GFS forecasts and Mike Leuthold's discussion!
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