Monday, September 03, 2012

Bit More PW On Labor Day

Yesterday there were more storms around eastern Pima County than on Saturday. The storm here at house brought 0.35". Winds were strong, but I only saw small branches down when I walked this morning. Across the ALERT network, 24 sites reported rainfall (about 25% areal coverage). There were 8 sites with more than a half inch and the station at Brawley Wash and Highway 286 reported 2.09". I saw that the RAWS site on Mt. Hopkins reported 24-hour rainfall at 2.34". So, there was definitely an increase in activity around the metro area yesterday.

The PW values have inched up a bit for today, but with highest values remaining in northwest Mexico and GoC. Tropical Storm John is helping keep PW high to the south, but it will probably take another Sonoran MCS this evening to push higher PW north of the border.

At 500 mb (12 UTC analysis from NCAR RAL above) the anticyclone is hanging in over the Southwest. It has at least two circulation centers, one over western Oklahoma and the other over western New Mexico. The western circulation has considerably cooler 500 mb temperatures than does the lobe over Oklahoma - this is positive for Arizona. A weak ridge-line is nearly overhead, and winds within most of the anticyclone are quite weak. The morning Tucson sounding (skewT plot below from SPC) also indicates weak winds  aloft, along with small CAPE at low elevations. CAPE over the mountains will be considerably larger and mountain storms will propagate toward wherever there is instability at lower elevations.

The Atmo, high-resolution WRF-GFS run from midnight is available this morning. It forecasts PW to increase some during the day. Forecast of composite radar echoes at 5 pm MST is shown above. Strongest storms are forecast for the Borderlands. Graphic below is forecast rainfall through midnight tonight, again with heaviest amounts along the Borderlands.

Have a nice finish to the Holiday weekend, if you're lucky enough to actually have the day off.

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