Friday, September 07, 2007

Henriette and Today's Weather

Rain from Henriette

Here at the house we received only 0.03" of rain on the afternoon of the 5th (Wednesday) but very heavy storms occurred in the vicinity (see Photo). We experienced considerable lightning and thunder and gusty outflow winds but little rain. Then, during the early morning hours of Thursday from about 5 am to 9 am there were several moderate showers with no lightning or thunder here. These produced an additional 0.41" of rainfall. Henriette has left behind a very wet low-level atmosphere and the forecast for today is of interest.

Today's weather

There is a large pool of very moist air lingering over much of Arizona this morning, along with considerable low and middle level cloudiness.

Winds in the lower half of the troposphere remain L/V while upper-level winds and shear are quite strong (this due to strong height gradients between the anticyclone to the southeast and a trough off the Pacific coast). The upper-levels appear to be divergent over southeastern Arizona.

The morning TWC sounding indicates substantial CAPE is present below 350 mb. The questions for the day are: how believable is the TWC sounding, and how will drying during the day affect the CAPE? In the morning soundings in the layer between 400 and 300 mb there is a pronounced inversion between the moist lower atmosphere and the dry upper-levels. At TWC the inversion is about 7C warmer than at any other upper-air site in the Southwest. To me, the TWC sounding appears suspect between 520 to 280 mb. My guess is that the elevated inversion is not as strong as indicated here, and that it will not strengthen due to the dry flow aloft, so that there is at least moderate CAPE present.

I expect that strong storms are likely this afternoon over the southeastern third of Arizona, especially where the clouds break and there is decent solar heating. The storms should have the potential to produce strong, moist downbursts and some hail, especially at higher elevations.

All this assumes that I have been able to properly unravel the uncertainties in the upper-air data.

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