Friday, July 31, 2009

Nice Morning in Tucson

After two very humid and hazy mornings in Tucson, the sky was cleaner and relative humidity noticeably lower this morning. A quick look at 24-hour changes this morning indicates that, for the southern Arizona stations, dewpoints have decreased 5 to 12F and pressures have risen ½ to almost 3 mb – neither of these trends is very exciting. However, conditions are more moist at El Paso where the dewpoint change was +9F and the pressure was up 5 mb this morning. The El Paso sounding is considerably more moist than is the very dry Tucson sounding. To our south, the Guaymas sounding (see above), assuming it can be believed, is remarkably moist with almost 3 inches of precipitable water. So, the question remains: how long will the dry out persist before the subtropical air over Mexico is pulled north across the border?

The 500 mb anticyclone again has three circulation centers – one over central California, one over northwestern Mexico southwest of Tucson, and one southeast of Brownsville, Texas. There are several weak troughs separating the centers and the most pronounced, according to the NAM, lies over northern Mexico to the south-central Gulf of California. This morning’s NAM forecast consolidates the S/W over Mexico and moves it slowly westward across the central Gulf of California. So, the convection associated with this feature may push the subtropical moisture back northward. The NAM also gradually consolidates the 500 mb anticyclone over the New Mexico/Arizona border. The NAM keeps the dry out going through this forecast cycle, but moisture will undoubtedly return sooner that the model forecasts. So, all eyes to the south for the next day or two.

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