Monday, December 3, 2012

Strong Front, Big Water, Dense Snow

It was a wild and crazy evening last night as the cold front became convective over northern Utah and gave us a flurry of thunderstorm activity with very high snow-water equivalent rates in the mountains.  MesoWest and radar data show the front pushing into the Salt Lake Valley and northern Wasatch front at 0300 UTC (0800 PM MST) last night.

Source: MesoWest 
Source: NCAR/RAL
In the valley, the storm produced a heavy ice pellet shower at my house in the Avenues.  They were still clinging for life on my car this morning.

It was a crazy storm for December in upper Little Cottonwood Canyon.  Temperatures prior to frontal passage were obscenely high, 33ºF at Alta-Collins.  The heaviest snow-water equivalent (SWE) rates were just after frontal passage (0.35 inches in 1 hr) and about 2/3 of the 0.99" (SWE) total fell at temperatures at or above 30ºF.  

These warm temperatures, combined with the convective nature of the storm, led to very high snow densities, with Alta-Collins recording only 5" of snow.  That's a mean water content of close to 20%, although the last inch or two should be lower density.  Total snowfall likely decreases rapidly with decreasing elevation below about 8500 ft given the high snow levels during the storm.  

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