Hot, Dry Air To The Northwest

There remain at best only slight changes to the large-scale pattern this morning. The 500 mb analysis (from NCAR for 1200 UTC 23 August) below shows the huge anticyclone that covers much of the U.S. and northern half of Mexico. Note that the height gradients are very weak, so that light winds continue over the Southwest and northern Mexico.

The morning Tucson sounding (above from Univ. of Wyoming) shows the weak winds below 450 mb. The BL is fairly dry (afternoon BL mixing ratio will probably be around 8g/kg), but the high temperatures actually lead to an increase in CAPE this afternoon. There was a slight uptick in storm activity at higher elevations yesterday - as evidenced by 7 ALERT sites reporting light rain. Skies here at house were covered with orphan anvils by evening. The Atmo early run of WRF-GFS forecast indicates increased activity this afternoon along the Borderlands, and particularly to the south and southwest of Tucson. Interestingly, the WRF forecast also brings a second round of storms into southeastern Arizona well after midnight tonight. There is much drier low-level air off to the northwest toward Phoenix, and the WRF forecast indicates that Pima County is a battle zone, with the dry air trying to wedge in across the Tucson metro area by evening (850 mb Td forecast below is valid at 7 pm this evening).

This morning's NAM forecast holds some hopes for improved winds in the middle-levels. The 500 mb forecast below is valid at 5 pm MST tomorrow afternoon. The inverted trough moving across the middle GoC, and a strengthening of the pressure gradient to north of it, leads to increased winds along the border. Will see what tomorrow brings. Off to the east, Hurricane Irene churns along on a course that will threaten much of the East Coast.