The weak, cutoff low that has been meandering around the eastern Pacific at about 30 degrees north is going to come across the Southwest this afternoon through tomorrow. The water vapor image above (from 14 UTC this morning, 24 April 2013) indicates thickest, upper-level moisture is well ahead of the circulation, with a west-to-east band already present across southern Arizona. The blended PW product from CIMSS at Univ. of Wisconsin (below for 12 UTC this morning) indicates very little moisture with this system.
The early run of Atmo's WRF-GFS this morning indicates that most of the thicker, mid-high clouds will actually move across southern Arizona this afternoon and tonight. The above is forecast composite radar from the model valid at 6 pm MST this evening. Tomorrow the model keeps the radar echoes over the northern half of the state. The forecast sounding for TUS (also valid at 6 pm today - below) indicates VERY dry air below about 520 mb with strong southwesterly winds. What moisture there is is confined to the upper-half of the troposphere. So, perhaps some virga and mammatus late in the day. The model forecasts no precipitation at the surface with this system, except for light sprinkles tonight and tomorrow over the northwestern quarter of the state - so the long dry spell continues (only 0.25" of rain here at the house since February 20th).
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