Tuesday, November 25, 2014

Final Call for Pre-Thanksgiving Day Storm

11/27 Update:
Final Snow Accumulation Maps:

End Update

11/25 8pm
Here's my final call for tomorrow.  Didn't feel the need to make big changes...but adjusted a few smaller details.  

It’s amazing the difference 48 hours can make in weather.  After plentiful sunshine and southerly breezes helped climb temperatures up near 70 degrees on Monday, we'll be stuck in the 30s Wednesday under overcast skies and snow.  It’s the combination of a few well-timed features that will make this rare November snow event possible across western Virginia.  A piece of upper level energy diving deep into the southeast out of Canada will aid in the development of a coastal storm right off the mid-Atlantic coast.  At the same time, cold air continues to work in from the northwest following the passage of the recent cold front.  Where moisture from the coastal storm gets thrown west into this cold, drier air-mass working in is where potential exists for a white Thanksgiving.

Shenandoah Valley Breakdown:
  • precipitation breaks out 2-5am
  • we start as rain...but change over to snow towards dawn 5-7am
  • heaviest snows occur between 5am-1pm
  • precipitation tapers off during the afternoon
  • 3-5" likely across the entire Valley...with higher amounts across the higher elevations. My map reflects the elevation dependence of this storm.  Here's an excellent map showing topography across the area.  This should always be taken into account, but especially for a storm like this where low level temperatures need to cool to support snow.  The higher up you are, the quicker this occurs.
elevation map (via TerpWeather...I highly recommend this page if you live in Maryland by the way)
  • temperatures start off in the upper 30s, gradually dropping once precipitation starts.  A few important things to remember: it can snow with temperatures above freezing, snow can stick with temperatures above freezing, and even two days after temperatures near 70---snow can in fact accumulate---we've seen this plenty of times.  Of course it takes heavy snow rates for this to occur.  
  • Speaking of heavier rates...short range models (even the operational ones for that matter) have been hinting at a band of heavier precipitation forming well west of the main batch.  Below are examples using NMM and ARW models:

If this heavier band ends up over us...I don't see us having any problems changing over to snow and ending up on the upper end of my totals.  If it happens to form west of us, it will be a different story and we'll struggle to reach the lower end.  It is very hard to pinpoint where these bands will occur until they do in fact form.  
  • Just for fun, I'll go ahead and say the highest snow total in the state is reported somewhere up in the Blue Ridge Mountains...the Shenandoah National Park up on skyline drive seems like a good candidate.
  • After this storm we'll have a quick turnaround with a clipper diving south into the area early on Thursday.  This won't be a big deal but will bring additional light rain/snow showers across portions of the state.
Stay Safe.  Happy Thanksgiving.  Enjoy the Snow.

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