Saturday, December 07, 2013

Quick Look At The NAM

First - another frosty morning here along the Rillito in north Tucson - the low temperature has again dropped to 23 F this morning, with moderate frost.

Shown above is the NAM analysis of 500 mb heights, winds, and vorticity for 12 UTC this morning. There are several vorticity maxima associated with the short wave over the Northwest, with the strongest being just north of San Francisco. (Note that when NWS forecasters refer to "energy" in Forecast Discussions they are probably talking about the vorticity structures of features aloft.) The NAM forecast valid at 24-hours tomorrow morning (Sunday, December 8th) is below. Two vorticity maxima are forecast to move quickly by just to the north of the Arizona/Utah border, while a long vorticity tail develops over the Pacific from northern California westward. The longer range forecasts for this system, particularly those of some runs of the ECMWF, took this short wave too far south, with substantial height errors over southern Arizona.

The NAM forecast of total precipitation through 12 UTC tomorrow morning is shown below. The event is forecast to produce snow over the Rim country and along the northern border lands of Arizona, with amounts maximized over Utah and Colorado. So it goes.

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