Wednesday, July 22, 2009

Miscellaneous Ramblings - 22 July 2009

The atmosphere in the Southwest is currently disorganized and chopped up and so will be this message.

A very nice MCS developed southwest of Phoenix last evening (as per 05Z radar image posted on blog) – I thought that this system had some potential for continuing on toward the southeast, since it obviously had a developed a nice gust front. Alas, the active convection fell apart by 07Z and all it brought our way was a dusty outflow that lowered visibilities and produced a very dirty sky at sunrise.

The really big storm activity yesterday and last night occurred over many parts of New Mexico. However, there was a severe thunderstorm out at Needles, California, and the storms southwest of Phoenix were impressive on radar. After morning showers in the Phoenix area and sprinkles here in Tucson area, the afternoon yesterday was mostly suppressed, although the morning activity damped the hot temperatures down a bit. Only 6 of the 93 ALERT gauges in Tucson area had light rain for past 24 hours – whether these amounts were from the morning activity is not clear. Other things I note this morning:

The 500 mb anticyclone is shifting inland as per models and is centered over the California/Nevada Sierras this morning, on it’s migration toward Utah. A weak secondary circulation center remains somewhere in northern Mexico. This leaves a long, curving, weak trough inside the anticyclone that stretches from eastern New Mexico westward between Phoenix and Tucson and then northwestward to west of Las Vegas. This feature is somewhat stagnant across Arizona. The two ends of it are forecast to move away from each other, leaving the feature even more washed out.

Precipitable water has continued to increase slowly over most of Arizona due to weak advection and lots of recycling by higher elevation convection into lower elevations. Phoenix appears to be in the middle of the pooled buildup with about 1.8 inches of PW – I think the highest value I’ve seen since around the end of June. The dewpoints at 850 mb at Phoenix and Tucson are at 12C this morning and there is increased CAPE available at low elevations, and it’s just waiting for a good kick.

Winds aloft over southern Arizona are essentially light and variable. The winds will be shifting to easterly directions as the high migrates toward our north, but will probably remain quite light. There is some hope that a better-defined flow field will evolve tomorrow.

Finally, surface pressures are falling fairly markedly – most stations in Arizona are down 2 to almost 4 mb during the past 24 hours and lower pressures seem to be a positive for strong storms. As for the details of what the day will bring, time will tell as the various old outflows and cloud patches interact with themselves and the terrain.

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