Thursday, September 15, 2011

Difficult Forecast For Tucson Metro Today

First, a bit of a precipitation update - the ALERT network came back online at midmorning yesterday. For the previous 24-hours 98% of the sites had rainfall and 15 sites had more than half an inch. So, a widespread event on Tuesday. This morning only 53% of the ALERT stations had rainfall in past 24-hours, so coverage was down substantially yesterday. Fives sites had more than half an inch and one site had more than 2 inches - these were mostly along the east edge of the network. There were a number of quite strong storms and several reports of severe scattered around. Here at house we had anvils and could see lightning at dark, but no thunder and no rain.

This morning's Tucson sounding (above) is quite unstable, even though there is very dry air above 600 mb. Middle level temperatures remain cold, and the 500 mb trough axis remains to the west. The sounding is a bit too moist wrt GPS PW - so not quite as unstable as it appears. Winds and vertical wind shear remain very impressive for here. The morning hodograph is shown below and it even indicates some potential for supercells - if storms develop.

The Atmo early run of the WRF-GFS forecasts storms this afternoon from central Pima County eastward. The composite radar echo forecast for 2 pm MST is shown below.

The above graphic shows total rainfall forecast by WRF-GFS through midnight tonight. The model forecasts a good rain event for the Tucson metro area. The new NAM forecasts from 1200 UTC this morning are similar to the WRF-GFS.
There is an uncertainty though that makes the forecast difficult. The WRF-GFS continues to show dry, low-level air invading southern parts of the state tomorrow. This dry air comes in from the south and southwest and without a Guaymas sounding the actual situation to the south of the border is somewhat uncertain. The satellite PW products seem to indicate that PW will stay at around an inch or a bit more today. Additionally, the strong shear in the dry air aloft will be hostile to developing towers. So, a mixed bag of pros and cons this morning.

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