The cloud cover certainly won out yesterday. Slightly cooler temps, higher pressures and a retreat of the moisture back into Mexico dominated our weather. Radar precipitation estimates show that all of southeast Arizona was quite suppressed yesterday. There was no rainfall recorded in the ALERT network. The only uptick was a brief increase in moisture and the stubborn cloudiness. I’ve posted several data plots on the blog that illustrate the dismal precipitation that’s occurred during the summer so far across the state.
As for today – MCSs have formed and dissipated through several cycles in Mexico during the night, while propagating northward (see 1330Z IR image). The satellite imagery this morning shows a large cloud mass covering most of western Mexico that is moving northward.
The soundings this morning at Phoenix and Tucson have slight to no CAPE and thus the issue of whether cloud cover hangs around or not today is probably not of great importance. The nocturnal activity in Mexico appears associated with a distinct inverted trough at 500 and 700 mb. The NAM model continues to forecast an important precipitation event across much of Arizona as it forecasts this feature to move northward, split the anticyclone apart, merge with a weak upper S/W in the westerlies and then move eastward. Thus, the model forecasts an interesting interaction of features from the subtropics and westerlies, all accompanied by a large push of moisture and precipitation northward into the U.S.
It appears that this transition has begun and that it will affect southern Arizona beginning late afternoon or tonight and then continue into Friday, bringing the subtropical air mass finally back into Arizona.
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