The forecast for today is quite tough, and has a very bimodal, yes or no aspect to it (I'm writing this a bit before 8 am). The upper-tropospheric air is very dry overhead, with a very sharp boundary between the Pacifc dry and the subtropical wet air masses. The top image shows the water vapor satellite data with the dry boundary (upper level dryline if you will) just off to the east of Tucson. The middle image shows the morning TWC sounding - if things were static, then this sounding should easily supprt deep convection fairly early as heating commences (it's quite clear here and to the west). However, the GPS PW data indicate a very strong gradient in PW from Tucson westward. Current PW values are: Safford 42 mm; TUS 33 mm; PHX 33 mm; Tohono O'odham CC 26 mm; Organ Pipe NM 24 mm; and Yuma 19 mm. So, if the winds pick up a westerly component below 500 mb there will substantial drying from west to east - somewhat like a Great Plains dryline. Where will the demarcation be between storms and no storms? At this time I'd have to guess, based on the NAM forecast winds and the strong drying trends in the GPS PW here and to west, that storms later today will occur primarily east of Tucson.
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Note that in the longer term the ECMWF has been advertising a significant dryout for the first part of the coming week. However, the NWS models seem to be forecasting that the 500 mb anticyclone will prevail and push the west coast trough back out over the ocean - so something else to watch. Finally, note that Tropical Storm Estelle is southwest of Baja moving west and will not be a player this far north.
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