Monday, December 20, 2010

Alta-Collins dumpage mystery

Precipitation rates at the Alta-Collins observing site (CLN) increased dramatically last night, exceeding 0.3 inches per hour of snow-water equivalent (SWE) between 0200 and 0600 UTC (7-11 PM MST).  The peak hourly SWE rate was 0.5 inches from 0400-0500 UTC (9-10 PM MST) This brings their storm-total-SWE to over 5 inches.

Given the frontal passage last night, one might assume that this increase in precipitation rate accompanied the frontal passage, but this is not the case.  In fact, the winds on Mount Baldy were southwesterly during the heavy precipitation period and frontal passage at the Salt Lake International Airport occurred just after 0700 UTC (Midnight MST).  

Instead, during the period of heaviest precipitation, the radar shows a persistent orographic convection band initiating in the southwesterly flow near Lone Peak and then training off downstream to Alta.  It is this band that produces the exceptionally high precipitation rates at Alta, and it occurs in the prefrontal environment.   

At issue are the processes responsible for this band and if it is an example of the orographic convection bands that have been documented over the Coastal Range of the Pacific Northwest (Kirshbaum and Durran 2005a, Kirshbaum and Durran 2005b). 

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