Friday, June 22, 2012

Complicated Situation This Morning

Yesterday afternoon thunderstorms stayed mostly in New Mexico (visible images above and below are from 01Z 22 June 2012) and in Mexico south of about 28 degrees N. The moist, mT low-level air along the west coast of Mexico had pushed north about half way up the Gulf of California. A complicating factor was that surface air with higher dewpoints was also pushing westward across New Mexico, behind a back-door front. There was a moderate-sized MCS over southern Sonora at 04Z - see color IR image 2nd below. The weather story yesterday for southeastern Arizona was blistering heat and slowly increasing surface dewpoints.

The full product suite from the Atmo early run of its high-resolution WRF-GFS model is available this morning, and I will mostly discuss the model forecasts. However, we do start the day off with quite warm temperatures in the middle levels of troposphere, although the forecasts indicate a couple of degrees of cooling during the next 48-hours. The WRF forecasts for this afternoon and evening indicate moderate moisture convergence east of Tucson where upsloe flow from the low desert converges with weakening easterly flow behind the decaying, back-door front. The model forecasts high-based storms mostly over higher elevations to the east and the south, with evening storms in Santa Cruz County. Precipitation amounts are very light. Outflows are strong but not long-lived, nor do they seem to grow upscale into organized mesoscale boundaries.

Tomorrow (Saturday June 23rd) becomes more interesting as significant amounts of PW are forecast into southern Arizona from the GoC. The graphic below shows PW at 7 pm MST tomorrow evening - note that values climb above 40 mm in central Pima County. The forecast composite radar image at 7 pm (above) indicates storms in Santa Cruz and eastern Pima County. The model forecast CAPE remains in the 500 to 1000 J/kg range (with the model values typically a bit too optimistic), thus the reason for the spotty character of the storms. The storms tomorrow also tend to keep to the higher elevations. Rain amounts are quite light. However, the model does forecast a strong, mesoscale outflow to move westward across Pima County. The system in northern Sonora moves toward the border and eventually sends an outflow northward across the Tucson metro during the late night and early morning hours of Sunday. Appears to be an interesting start to the summer thunderstorm season here in southeast Arizona. The weather on Sunday will of course depend strongly on what actually transpires during the next 48-hours.

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