Tuesday, August 30, 2016

Higher PW From East May Bring Isolated Storms

Suppressed conditions continue over the West on our side of the Continental Divide - above (from weather.graphics and Vaisala) is 24-hour CG flash density ending at 7:00 am this morning. The lack of thunderstorm activity is somewhat amazing.

This morning more moist air has pushed in with higher pressures and easterly winds from New Mexico. It was a very warm morning with easterly, downslope winds along the Rillito. The morning sounding's skewT plot (above from SPC) shows PW now above an inch, with a moist, well-mixed layer below 850 mb. The troposphere continues to be split between between a deep surface layer that extends almost to 500 mb and an upper half with west to northwest wind flow and a warm layer centered just above 500 mb. At 500 mb there are two distinct anticyclone centers - one over the Great Basin and another far to the east over Arkansas. The trough/shear line separating these centers is over the Arizona - New Mexico border this morning, which leaves the middle-level flow light and variable. All-in-all a fairly mundane sounding, with slight CAPE at lower elevations.

The WRF forecast model runs from Atmo at 06 UTC mirror the mundane morning conditions with a forecast of at best isolated thunderstorms over Pima County this afternoon. The skewT above is from the WRF-NAM and is valid at 5:00 pm MST. Mixed layer CAPE for this sounding is only 87 J/Kg and PW is 26 mm. The warm air at mid-levels limits CAPE and there is little steering flow. The model's forecast of composite radar echoes valid at 4:00 pm below reflects the lackadaisical synoptic setting with its forecast of very isolated storms across southeast Arizona. 

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