Friday, August 26, 2016

Surprise Early AM Shower Here

Light showers across parts of area occurred between about 3:00 and 4:00 am MST. There was 0.08" here at house at sunrise, and visible satellite image (above from 7:00 am) shows considerable cloudiness across much of central and southern Arizona. Rainfall data from the ALERT network (below for 6-hours ending at 7:00 am) indicates light rainfall measured at 5 sites. Storms and rainfall yesterday afternoon generally hung over the mountains.

The morning sounding (above from SPC) skewT plot has PW up to 1.41 inches and CAPE values around 750 J/Kg. The PW across southern third of Arizona is generally running about 1.00 to 1.50 inches. The winds are westerly above 500 mb and steering flow is indistinct - although, WRF forecasts trend toward westerlies through the CAPE layer this afternoon, which is not a very good steering flow for storms in the metro area. West winds at upper-levels will produce some anvil shading ahead of storm cells, which could lead to erratic propagation toward pockets of more unstable air. Generally a so so transition-type situation - current morning POPs from NWS for airport grid are 30% today and 40% tonight.

The WRF forecasts from 00 and 06 UTC runs were not very consistent; however, the 00 UTC WRF-NAM appeared best for the early morning showers. This morning's 12 UTC forecast from NWS NAM model (forecast below for 500 mb valid at 5:00 pm MST tomorrow afternoon) indicates a  short-wave breaking off of the Great Basin trough and moving across Arizona Saturday night and Sunday - continuing the Fall-like pattern.

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