Friday, October 07, 2011

Much Colder But Little Rainfall Yesterday

Yesterday's cold front and bands of light showers produced dramatic temperature changes, but only small amounts of rainfall. Above is view toward Catalinas at 1 pm MST yesterday afternoon. Below is time series of T and Td at Atmo, showing the big temperature drop around 11 am. Here at the house the temperature was hanging around 52 to 55F from noon to early afternoon. This morning's low was 44F (lowest Fall temperature before this morning was 56F on several days). Rainfall here was only a trace. Across the ALERT network 24 of 93 sites had rainfall (~25% areal coverage), but amounts were generally light, except in the Catalinas where 1 site had more than half an inch. Rainfall was generally restricted to the northwest part of the network and the Catalinas.

This morning significant cloudiness and moist air below 700 mb remains present over eastern Arizona - as can be seen in 1415 UTC visible satellite image above. The 500 mb trough axis remains west of Tucson this morning, with a significant vorticity maximum located over the Yuma area. The maximum will move across southern Arizona today, and also bring cooling in the middle levels. The 500 mb NAM forecast from this morning (below) indicates that the vorticity maximum will be centered north of Tucson at 5 pm MST this evening. So, the question for today is whether the strong forcing for vertical motion will squeeze out some showers before the moisture shifts away from Pima County. Both the NAM and early Atmo WRF-GFS indicate isolated showers today, but mostly east and north of Tucson.

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