Wednesday, July 29, 2009

Humid Hazy Morniing in Tucson

A push of high dewpoint surface air (see 1340Z surface chart above) has occurred over southern Arizona during the late night and early morning hours. The push has brought very high dewpoints to the lower Colorado River Basin with Yuma reaching Td of 75F. The push has gradually worked its way north during the morning. Doppler radar VADs indicate that the surge at Yuma reaches to 3,000 ft AGL with south-southeast winds of about 20 kts in this layer. At Tucson and Phoenix the layer of southerly flow extends only to about 2,000 ft AGL. With strong westerly winds above, the moisture should mix out fairly quickly this late morning.

The upper-air charts are reminiscent of late May or early June this morning and the two features of most interest are a small vorticity maximum hanging over the central GoC in the NAM progs, and a weak S/W in the westerlies approaching northern Arizona from central California. There is cooler middle-level air with this S/W, but it will tend to avoid southern Arizona where the fetch is still around the huge anticyclone over Mexico.

However, the combination of very hot temperatures, some increase in moisture, and the approaching S/W will probably lead to a bit more storm activity over mostly higher elevations across the north and east portions of Arizona today.

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