Thursday, July 24, 2008

Bad RRS Sonde Data Complicate Today's Forecast

Difficult forecast for today.

Low-level moisture has been sloshing around in different ways last night and this early morning, with decreases in the southeast from the east; increases in south central Arizona from the southeast; and increases in lower Colorado Basin due to weak surge which should continue and increase in strength and depth. Diurnal wind regime may return today in the Tucson area, which would increase the low-level moisture.

Entire forecast problem is acerbated by the TWC sounding which is apparently about 7.5 mm too wet this morning. I am guessing the bad data are at and below 850 mb and modifying the sounding accordingly. My estimated sounding for this afternoon has boundary layer well-mixed moisture at 10 g/kg and cloud bases at 650 mb. This gives moderate CAPE with only slight CIN above, assuming no further drying from east.

At 500 mb a weak inverted trof extends from the north end of GoC to SE Colorado, with some cooler air on the east side of this feature. At 200 mb a strong inverted trof extends from west of Albuquerque, south between El Paso and Tucson and into the vast Mexican data void. This setup gives the Tucson area good steering level flow this late afternoon, but little vertical shear. If storms organize along outflows, they could move westward faster than their anvils.

NAM model indicates better wind profiles tomorrow. Thus, good potential for organized and some severe storms either this afternoon or tomorrow, with the answer hiding in the devilish, smaller-scale details.

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