Tuesday, August 27, 2013

Anticyclone at 500 mb Covers Most of U.S. and Mexico

Yesterday was much more active in eastern Pima County than any of the model forecasts I looked at indicated. This was very similar to the 22nd of August when models forecast CAPE holding over eastern Pima County but did not forecast much in the way of storms. Here at house there was a storm around 4:30 pm MST that produced strong outflows but only a trace of rain. Then, just at dark a cluster of storms moved from the Rincons across parts of town. Those storms produced a lot thunder and a brief moderate shower that left behind 0.03" of rain here. Lightning plot above shows CGs for 24-hours ending at 6 am this morning. Across the ALERT network during past 24-hours 47 sites had rainfall (a bit over 50% coverage). Amounts were again generally light with 12 sites having 1/4" or more and only 3 sites with 1/2" or more.

Models continue with dry forecasts today and precipitable water is down to around an inch, give or take a bit, across southern Arizona, except for the southwest corner where values were holding around 2 inches this morning. However, as per last week, both versions of the early WRF run at Atmo forecast reduce, but continued, CAPE late in the afternoon and evening but only indicate a couple of isolated, evening showers over the mountains.

At 500 mb a huge anticyclone covers the U.S. and Mexico - above 12 UTC analysis from NCAR RAL. There is an inverted trough across central Mexico that extends into north-central Texas. This feature will move into the Southwest during the next two days and bring increased storm activity tomorrow and Thursday. The early WRF-GFS forecasts PW to increase dramatically over southern Arizona during Wednesday night. The forecast of PW valid at 6 am MST Thursday morning (the 29th) below shows vales back up in the 50-60 mm range from Tucson westward.

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