Monday, July 18, 2011

Monsoon Surge

The IR, precipitable water (contours) and wind vector loop below shows quite nicely the "surge" of moisture from the south and east into Utah over the last couple of days.

As seen in the 0100 UTC (0700 PM MST) regional radar composite below, precipitating convection was confined primarily to eastern Utah yesterday evening, but should be statewide (but scattered) today.

Source: NCAR/RAL
The GFS dynamic tropopause loop (top image below) shows nicely the large-scale evolution responsible for the surge.  We have the persistent long-wave troughing along the west coast and monster ridge over the midwest, but note also the cyclonic potential vorticity (PV) anomaly digging down the back side of the upper-level trough and another cyclonic PV anomaly (a.k.a., easterly wave) moving across northern Mexico in the easterly flow on the subtropical side of the ridge.

The net impact of this large-scale pattern is strong confluence between relatively dry air from over the eastern Pacific Ocean and relatively moist air originating over the Gulf of Mexico.  This produce a strong gradient in precipitable water over our region, which shifted northward and westward across Utah late yesterday and overnight.  This shift partly reflects the amplifying trough on the west coast and apparent retrogression of the large-scale ridge, which could be related to diabatic heating associated with convection over eastern Utah and Colorado yesterday.  The easterly wave helps to enhance moisture transport into the four corners area.   

Thus, after a nice weekend, we should see scattered showers and thunderstorms this afternoon in northern Utah.  One can never complain, however, when it is showery in Salt Lake in July.  

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