Wednesday, July 22, 2015

Mixed-Bag Situation Continues

Photo from Mike Leuthold yesterday over in Paradise (which is just a bit northwest of Portal in the Chirichuas) showing a nice summer green-up underway there.

Plot of detected CGs for the 24-hours ending at midnight last night (below from Vaisala/Atmo) shows that thunderstorm activity again stayed to the east yesterday afternoon. There were some buildups and showers late afternoon southwest of Tucson (about where and when forecast by the WRF yesterday, but much weaker than indicated by the forecast). Only two ALERT sites reported rainfall yesterday and amounts were very light.

The battle between wet and dry continues over southeast Arizona. The 14 UTC water vapor image (above) shows upper-tropospheric moisture returning over the corner of the state. However, the time series of GPS-estimated PW on campus (below) shows that PW bottomed out yesterday afternoon at below one inch (drier than the 06Z WRF model forecasts indicated). Trend is now upward, with morning PW a bit over one inch. There is a substantial east-to-west PW gradient from El Paso (~1.50" to Tucson ~1.20" to Yuma < 1.00"). Models continue to forecast a westward shift of the higher values.

The sounding continues to exhibit a mixed-bag of mostly minuses this morning (12 UTC skewT above from SPC). There is only a sliver of CAPE, however, this would increase if PW continues to rise during the day. Winds below 300 mb are light and variable with no notable advective flow present - the low-level winds have just been pathetic for about past five days. The Pacific trough dominates in upper-troposphere with moderate southwesterly winds. Very dry air dominates from 600 to 300 mb, and the increased moisture depicted in the WV image above is occurring above 300 mb, and is not a significant factor in today's convective forecast. Primary issues are whether or not low-level moisture will increase and how much of a damper the current inversions between 700 to 400 mb will impose on developing convective towers.

Both versions of the WRF model run at 06 UTC at Atmo forecast the development of isolated thunderstorms over eastern pima County during the evening today, however rainfall mostly avoids the metro area in the forecasts - below shows WRF-GFS accumulated rainfall forecast valid through midnight. Both versions of the model forecast improved low-level moisture and a considerable increase in thunderstorm coverage tomorrow afternoon. Of course the devil is in the details and we'll have to wait and watch.

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