Thursday, July 08, 2010

Many Similarities To Last Week Continue

First a quick summary of yesterday - weak storms on mountains with limited outflows and not much rain. Convection was not very deep yesterday - just deep enough to generate some CGs at higher elevations. Very deep convection was limited to areas well east of Continental Divide and in Mexico south of about 27 degrees north (i.e., not very close to our area). I examined all the reporting stations, RAWS stations and ALERT gauges across southeast Arizona and could only find: a trace at Douglas, 0.01" at Trail Cabin, and an unknown but measurable amount at Safford. So, not much activity for a day in July.
Lot of things coming into play today make for a complicated forecast. Low-level moisture has increased from the east (as per a week ago there is a strong gradient from Tucson to El Paso with much cooler and more moist air on the east side of Continental Divide - see satellite image above). TD-02 is near Brownsville, Texas, a bit further north than was Alex. There is a pronounced upper-level inverted trough ahead of TD-02 but morning NAM indicates this feature will be squashed to the southwest, leaving the best upper-level difluence back in west Texas and southeast New Mexico. The current NAM forecast indicates that TD-02 will persist and make it into southeast Arizona by Sunday as a much weakened feature within the building ridge. Thus, the subtropical moisture with TD-02 is forecast to eventually come across the divide. Low-level flow is again this week fairly strong and downslope which is a negative.
The morning TWC sounding (lower image) has moistened considerably during past 24-hours, from the east, and is once again approaching PW of 30 mm. The TWC data and the GPS data are within a mm or so on PW this morning, with TWC just a bit more moist. Offsetting the low-level moistening is a deep layer of very strong warming from 500 mb to 300 mb, with the temperatures around 400 mb having increased more than 5C. Thus, even with increasing moisture during the day, it appears that low-elevation CAPE will be quite limited - a forecast cloud base of 600 mb with theta-w of 22C has only a small area of CAPE from 600 to 450 mb. So, it is likely that only higher elevation convection will have much hope of beating the warm air aloft, since I don't see any indications of cool advection in the upper-half of troposphere.
So a considerably mixed bag of pros and cons for today. However, we can hope that TD-02 will come closer to us than did Alex and bring improved chances for rain over the weekend. The upper-level inverted trough may also invigorate more intense convection further north in Mexico, which could help push low-level moisture up the GoC.

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