Saturday, June 25, 2016

Difficult Forecast Today

Yesterday there was considerable thunderstorm activity primarily to our east and south as per the 24-h CG flash density for the period ending at 1330 UTC this morning. Most activity occurred over and near the Santa Rita Mountains and south to the Border. Over the ALERT network 15 sites had rainfall, with the most being 0.43" at Agua Caliente Park over toward Redington Pass. Some outflow winds occurred across parts of the metro area.

This morning there is a distinct moisture gradient present over southern Arizona, easily seen in the upper-levels in the water vapor image above from 7:00 am MST and below in the CIRA blended PW analysis (greens are higher than an inch). The basic situation has become complicated by an inverted trough at 500 mb that is located along the Arizona/New Mexico border this morning. This feature is forecast to move westward today across southeastern Arizona.

The morning TWC sounding plot (from SPC above) is very similar to yesterday - there may actually be a bit more CAPE today - and winds at steering levels remain a bit chopped up. However, anvils would move off toward the east, which is a positive change in the wind profile.

The inverted trough is the wild card for today's forecast. The 06 UTC WRF forecasts move it across the Tucson metro area by early afternoon, which shifts the focus of the thunderstorm activity out to our west and southwest - considerably different than yesterday's scenario. Both versions do this and below is WRF-NAM forecast of composite radar echoes at 3:00 pm MST this afternoon.

Key to today's forecast will be the role and movement of the inverted trough - the new NAM forecast this morning is slower, and that movement would probably produce more activity at lower elevations of eastern Pima County. Important to check the new WRF forecasts from 12 UTC, since situation appears quite different today than it was yesterday. Storms have potential for wind and dust and perhaps some local heavy rains.

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