Strong 500 mb Shortwave To Move Across Southeast Arizona

First, another morning with freezing temperatures here in the north part of the City of Tucson. The low here at the house again dropped to 30F, making two days in a row with below below freezing temperatures. Other parts of southeast Arizona were not as cold this morning as yesterday. Temperatures tomorrow morning will be very dependent upon wind conditions during the night and early morning hours. The Univ. of Arizona, Atmo, WRF-GFS run from midnight last night indicates that winds will continue through much of the coming night across the metro Tucson, keeping low temperatures in the upper 30s and low 40s. So, it could be a morning with drastically varying low temperatures, depending on small-scale wind conditions.

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The top figure shows this morning's 500 mb analysis from NCAR RAP. Note the extremely strong baroclinic zone across the Southwest, associated with the shortwave trough. (Would certainly be nice to have a return of upper-air data in southern Nevada!) There is a strong vorticity maxima over southern California this morning and it is forecast to swing eastward across southern Arizona this afternoon (this morning's NAM 500 mb forecast for 5 pm this afternoon is shown in bottom graphic). Unfortunately, the strong forcing for vertical motion will be acting on an air mass that is extremely dry (this morning's GPS PW being only about 5 mm). So, we can expect some wind and perhaps some mountain clouds and sprinkles. The main impact of this system in the Southwest will be a period of strong, Santa Anna winds in California as the 500 mb ridge builds over the Great Basin during the next 48 hours.