Sunday, September 09, 2012

Downslope East Winds Upset Forecast Thunderstorms

The big news in Tucson this Sunday morning is last evening's football game - the Univ. of Arizona team upset Oklahoma State by 20 points! Who would have forecast that? I have to do a quick look post again this morning, but it looks like the weather upset the forecasts for a second day here in southeast Arizona. Forecasts were far too optimistic, and the NAM from 12 UTC had a very serious bust that really needs some closer examination back at NCEP.

I went back and checked the 12 UTC run of Atmo's WRF-GFS yesterday (below is 12 UTC forecast for total rain through midnight last night) - this forecast run mostly eliminated any rainfall across southeast Arizona (compare with the same forecast from the NAM in previous post!), and seriously beat the NWS NAM, but the odds favored the WRF-GFS. The strong easterly, downslope winds (east gusts of 20 to 30 mph from 9 am yesterday through the night at TUS) behind the significant, backdoor cold front shut things down for the second straight afternoon. There was a nice complex of storms over northwest Sonora and parts of central and west Pima County (see 5 pm MST radar coverage in 2nd graphic below). Over southeastern Arizona, areal coverage was essentially 0%, although I did find 0.07" at Safford and 0.12" at Nogales. But basically another suppressed afternoon and evening.

This morning's early WRF-GFS forecasts some afternoon storms in the west part of the ALERT network, but with the stronger activity again along the Borderlands from Nogales westward. The model forecast early morning storms over parts of the metro area and Catalinas, but there was no such activity - so again the model was not good in its first six hours, and the morning forecast may be the one to bet on, when it's available. Hopefully the strong east winds will relax by afternoon.

The morning sounding from TUS (below, with SPC analyses) shows the cool, low-level air that came in with the front yesterday. The sounding also indicates little afternoon CAPE, and continuing weak wind fields. The PW has gone down also, with Tucson down almost half an inch since Thursday morning - dewpoints are in the 50s. So, it would take some serious heating this afternoon to get things going. Significant, closed low at 500 mb now over northern Baja, but forecast to move little during the day. The new NAM run this morning forecasts considerably more storm activity again over southeast Arizona than does the early WRF-GFS.

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