Tuesday, September 03, 2013

Big Bend Inverted Trough Will Brush Southern Arizona Today

Yesterday was very suppressed after the early morning storms and MCS moved off to the northwest. There were 8 stations in the ALERT network that had rainfall from the morning storms and then coverage was zero though the afternoon and last night. There was one thunderstorm over in Cochise County that moved from near Benson toward the south end of the Rincons and then dissipated. Here at the house there was a morning sprinkle or two as the MCS passed by - so another Trace day.

                                              

This morning at 500 mb (above is 12 UTC NAM 500 mb analysis) there is an inverted trough over central Mexico that extends northward into the Texas Big Bend. This feature brushes southern Arizona late this afternoon and tonight, primarily in the upper-troposphere. Two main impacts, from the 12 UTC NAM forecasts this morning, are that winds at 500 mb increase some in speed and become more easterly, and that significant difluence in the upper-troposphere impacts northern Sonora and the Borderlands.


This morning's skew-T plot for Tucson (above) indicates mostly southeasterly flow with strongest speeds above 500 mb. When I modify this sounding for late afternoon, I estimate only a sliver of CAPE available at low elevations (similar to yesterday's situation). However, the early WRF-NAM sounding forecast for Tucson at 6 pm MST (below) indicates substantial CAPE (routine within the the model often over-forecasts available CAPE).



The early WRF-NAM (note that early WRF-GFS went seriously awry early in its forecast with a large, sunrise MCS forecast over southwestern New Mexico) keeps storm activity mostly west of Tucson area and along the Borderlands. However, the model forecasts a cluster of storms across the metro area just at dark (above forecast of composite radar echoes valid at 8 pm). Total rainfall forecast through midnight is shown below, indicating a nice swath of rain across southern metro and also a separate storm off the west of Catalinas into Oro Valley area. Will watch to see how things actually evolve.


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