An unusual 500 mb pattern covers the U.S. - see above 12 UTC analysis this morning from NCAR RAL. A cyclone from the westerlies has morphed into a cyclone/inverted trough in the easterlies. The cyclone - over Missouri this morning - is moving southwestward, with abundant moisture immediately ahead of it, and it will be the main weather event this coming week. A very similar evolution of the large-scale pattern occurred back in the second week of August 2003 - that ended with a widespread outbreak of strong storms across most of Arizona. Of note this morning is that very warm and dry air at middle and upper levels is out ahead of this system - note the - 2 to -5 C temperatures over west Texas, southern New Mexico, and northeastern Mexico. This dry air will likely suppress storm activity even further over southeastern Arizona; however, it is a tough call, since there is significant PW gradient from El Paso to Tucson.
The TUS skew-T plot this morning - 12 UTC below - again indicates slight to moderate CAPE but requires substantial lift above the boundary layer if it is to be realized. At low elevations this requires a significant outflow. But again the winds aloft are very weak below 400 mb - note that the well-mixed boundary layer only reached to 750 mb yesterday. With some drying and heating today the BL will be deeper but with CAPE decreasing. So, a continuing story of a balancing act between weather pros and cons.
Yesterday there was rainfall at only 16 ALERT sites - mostly in the Catalina and Rincons - with only Mt. Lemmon catching more than a 1/4 inch (0.39"). So, summer rainfall is off to a very slow start over portions of southeastern Arizona - i.e., from the north part of metro Tucson northwestward and westward. Meanwhile Cochise County is wet to very wet. The above NWS graphic shows current rainfall versus average for the summer through 12 July. The Phoenix metro area (Phoenix Rainfall Index shown below) is still waiting for its first meaningful rain of the summer - so high humidity and heat up there but no rain.
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