Friday, December 10, 2010

The upcoming snow density yo-yo

It's going to be a very interesting weekend with regards to changes in the density (a.k.a. water content) of falling snow in the Wasatch mountains.

To the first order, the water content of freshly fallen snow in the Wasatch Mountains is determined by the near-crest-level (700 mb) temperature.  The water content of storms at Alta, for example, tends to be higher in warmer storms and lower in colder storms.

Alta snow-water content on deep powder days.  Median is
red line, box denotes range between 25th and 75th
percentile, and whiskers the range to the last outlier
within 1.5 times the interquartile range
(Steenburgh and Alcott 2008)
There is obviously a lot of scatter, so there's more going on than just temperature.  Alcott and Steenburgh (2010) describe the gory details and develop an algorithm for predicting snow density at Alta (they predict snow-to-liquid ratio, which is the inverse, but does the job equally well).  Their techniques can be used to generate a snow-density forecast for Alta based on data from the NAM.

Courtesy Trevor Alcott
Today's (i.e., Friday's) storm is associated with the passage of an upper-level trough that will drop the 700 mb temperatures from about -3C this morning to -10C tomorrow morning.  Correspondingly, the predicted density of freshly fallen snow drops from about 13% this morning to about 7.5% by tomorrow morning.  In other words, this storm should be "right side up" with lower density snow falling on top of higher density snow.  That's what you want for hero snow.   If we get enough snow, the riding conditions should be quite nice on Saturday in areas where the wind hasn't hammered it (a concern for this storm).  

On the other hand, a warm front moves through the region on Saturday night and the 700-mb temperature rises to -1C by Sunday morning.  Correspondingly, the estimated snow density of freshly fallen snow increases to 16.5%.  This is a recipe for an upside down snowfall that is punchy and more challenging to ski.

The saving grace might be that the models are currently keeping us right on the edge of the precipitation on Saturday night.  Thus, the density of freshly fallen snow might be increasing, but accumulations may be limited.  On the other hand, if it snows alot, I expect turning conditions on Sunday to be interesting.  

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