Monday, March 24, 2014

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Midweek Madness

One more day of spring before winter's icy grip returns to the Sierra Tuesday afternoon.  Confidence continues to increase in a moderate snow event for the Sierra focused on the Tahoe Basin, a rare and welcome occurrence this season. Unseasonably cold air associated with this system will provide more snow and less rain with snow levels crashing to Lake level before the bulk of the precip arrives.

Current thinking is still on track with 18-24" along the crest, 12-15" mid-mountain and 4-8" at Lake level.  
The Hydrometeorological Prediction Center (HPC) is giving it a 20% chance for 18 inches of snow along the crest through Wednesday at 1700 PDT. 
 An impressive cold front will move over the Sierra Tuesday afternoon/evening with some snow beginning for the highest elevations at that time.  Depending on the model, snow will either continue through the overnight or take some time to become widespread.  Either way, the best dynamics look to move into the area Wednesday morning as the left exit region of a 125+ kt cyclonically curved jet moves overhead.
The forecasted Jet Stream location and intensity Wednesday morning at 800 PDT
 The left exit region is notorious for producing copious amounts of precipitation due to enhanced lift through a deep layer of the atmosphere.  Simultaneously, the cyclonic, or counter clockwise, curvature of the jet provides favorable vorticity, or spin, further intensifying the lift.  For those of you physicist out there, just think about the "Right Hand Rule." For everyone else, more spin equals more lift and that means the snow piles up deeper and faster.
The forecasted mid-level vorticity Wednesday morning at 800 PDT

With the dynamics in place, the real question is: "What about the moisture?"

That is where the bulk of the uncertainty remains among the models.  Luckily for us, the models continue to trend towards the wetter GFS model. The folks at HPC are beginning to buy into that trend and have ramped up their precipitation forecast for the Sierra.
HPC precipitation forecast for through Friday at 1700 PDT with 2.8 to 1.5 inches along the Sierra Crest.
The bulk of the storm will come to an end late Wednesday night /early Thursday morning but not before it leaves a fresh blanket of surprisingly light Marchmellow fluff.

 Lingering moisture and instability will combine to produce scattered snow showers through the afternoon with maybe an inch or two of additional accumulation for a few lucky spots.

Another forecast will be issued on Tuesday before the system is slated to arrive.  Hopefully models continue to trend wetter with this system and we get in on a little March Madness of our own.

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