Short wave west of Baja (see MIMIC PW image for 12 UTC this morning - above) has a weak leading wave which will cross Arizona today. This feature has turned the Pacific moisture plume eastward across the Baja spur. So, this will be another moisture starved system, just as the models had predicted a couple of days ago. This morning's early WRF runs at Atmo are similar, with weak radar echoes forecast across eastern Pima County during middle of night - below is WRF-NAM valid at 11 pm MST tonight. Echoes remain aloft in the model forecasts. Gusty winds forecast for tomorrow afternoon.
Roger Edwards sent the above surface plot for around 3 pm yesterday afternoon, highlighting the extremely low dewpoint temperatures at Tucson and Douglas. A check of the surface observations shows that Tucson dropped as low as -39 F and Douglas -45 F. But, DM (only a couple of km from TUS) had a low dewpoint of only -9 F, while Mt. Hopkins hit -17 F and Rincon -14 F. So strange things going on at small scales. The TUS sounding below (released around 4 pm) does not show any air nearly so dry - except the layer right below 500 mb. So, if the dewpoint sensors at TUS and Douglas were accurate (? - I don't know how these sensors perform at ultra-low RH), then several pockets of strongly subsided air crossed portions of southeastern Arizona. Note that the dry adiabatic BL does extend upward to the dry layer aloft, so this scenario was possible.
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