Tuesday, August 18, 2009

Some Storms in the Far Southeast


There were a few thunderstorms along the far southeast borderlands yesterday afternoon. This is where there is a weak trough at 500 mb separating the anticyclone off Baja from a weakening center over west Texas. Again, the morning NAM does not have these two centers initialized well.
Deep westerlies prevail over most of Arizona and a weakening polar jet extends from northern Arizona eastward across Omaha into the Great Lakes Region. Upper-air soundings this morning are very dry and stable wrt moist processes. Surface pressures are falling (down about 1 to 1.5 mb in past 24 hours) across southern Arizona and dewpoints are also down some, except at Douglas where their morning dewpoint is up 6F relative to yesterday.
The NAM indicates that the western S/W trough (currently from central California notheastward toward Hudson's Bay) digs markedly next 60 hours (keeping Bill out in the Atlantic) and extends the westerlies across New Mexico. The models don't do much on returning subtropical moisture (see GOES 12Z precipitable water above) into the West until after 84 to 96 hours. The moist subtropical air mass extends southward from Guaymas across western Mexico. However, with weak cutoff at 500 mb approaching southern California coast and falling pressures in lower Colorado Basin, shallow surges of low-level moisture are likely next couple of days.
After 96 hours, the ECMWF is much more aggressive in bringing deep moisture northward around the west side of the subtriopical high than is the GFS. So, it looks mostly hot and dry until the weekend, with storm activity most likely along the eastern Arizona/Mexico borderlands.

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