Monday, December 22, 2014

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Cold and white Christmas?

Ski conditions this past Saturday were...interesting.  Things started out pretty good on Saturday morning at Alpine Meadows, with a steady light snow falling from top to bottom.  Visibility was poor at the top, but snow conditions were decent.  By about mid-day, a funky rain/snow/mist was falling on the lower half of the mountain, and the skiing deteriorated rapidly.   With temperatures warming throughout the day, the glorious wintry conditions the Sierra had experienced much of last week turned into a wet, sloppy mess.

Last weeks storms brought some nice refreshment to the ski resorts, but for the water year we are still in a bit of a hole in the Tahoe basin and down to the eastern Sierra.  The splitting nature of several of the December storms (caused largely by Rossby wave breaking off of the CA coast) brought much needed rain to a good chunk of CA, but the bulk of the energy associated with theses storms was deflected around the Sierra (at least the southern 2/3).

(Source: High Plains Regional Climate Center)

Things are starting to shape up for a small (by Sierra standards), but cold storm to impact the region on Christmas Eve into Christmas day.  Rain and snow could begin in the Sierra by late Wednesday afternoon, with possibly a few hours of moderate to heavy snowfall overnight.  Here is the initial forecast:

  • snow levels begin around 7,000-8,000'
  • snow levels fall rapidly down to lake level, and then down to valley floors (Reno) by the end of the event
  • 4-8" above 7,000' with locally higher amounts along the crest
  • 2-4" at lake level
  • a dusting to possibly an inch or two on the valley floors
It has been interesting to watch this storm evolve in the model runs, with the GFS indicating almost no snow for the Sierra a little over 24 hours ago, and now showing a nice set up for a fast moving mini-dump.  This leads to fairly low confidence in the storm, even only three days away.

A strong polar jet is currently barreling across the pacific, and the large scale Rossby wave is forecast to start breaking near Hawii over the next 24 hours.  The break in the wave is caused by the strong upper level ridge that is currently pumping up off of the coast of CA.

(Source: University of Hawaii)

The polar jet redevelops quite nicely, and the energy that was forced south during the wave break event is forecast to merge with the main branch of the jet.  This should benefit the Sierra by drawing in additional moisture from the south-southwest.  This is depicted very nicely by the 300 mb winds and column integrated water vapor on 18Z Wednesday morning (10:00 AM).

(Source: University of Washington)

There is also strong forcing and favorable dynamics associated with the frontal passage that should act to enhance Sierra snowfall totals.  A large vorticity core (local atmospheric spin) can be seen directly upstream of the Sierra, which will increase the upward vertical motions of the air parcels, and hopefully generate good snowfall.
(Source: RAP/UCAR)

If this storm was moving slower, it could potentially yield significant snowfall for the Sierra, but at this point it appears the wave will move through pretty quickly.  We will keep an eye on this storm and have another update soon.  For now, it looks promising for a white Christmas in the Sierra, and good chance of a least a dusting in the valleys.


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