Friday, January 25, 2013

January 25, 2013: Uncertainty In Today's Snow Totals

In a word, snow is coming.  It will be dry and powdery.  It will rapidly stick.  But just how much, that is the question.

NWS has painted a broad-stroke prediction of 1"-2" across the metro region.  

I took a look at two of the experimental, high-resolution forecast models for this storm.  These are called mesoscale models and they capture the small-scale processes that the more established, regional forecast models often miss. 

Here is a snapshot from one of them, showing a moderate snowband setting up across the DC-Baltimore metro region late this afternoon:

High resolution forecast of snow accumulation rate, Friday afternoon.  NWS

Blue is light snow, green is moderate snow.  This could easily spell more than 1"-2".

However, here is the output from another mesoscale model, for the same timeframe.  It basically puts the metro region in a "snow hole" with nada:

Another high resolution forecast of snow accumulation, Friday afternoon.  NWS

In this scenario, there is a heavy snow band to the south, and also along the windward side of the Appalachians (where snowfall is favored, along the "upslope" ridges).

One thing that struck me is the incredibly dry air layer in place in the lowest 10,000 feet (even though it is overcast above this).  This means that a significant portion of the snow falling overhead will sublimate (turn to vapor) as it passes through this dry layer.   It will take a few hours to moisten this layer, such that snowflakes can survive all the way to the ground.   Snow will show up on radar early this afternoon, overhead, yet what it detects is virga - snow disappearing before it reaches the surface. Depending on how long it takes to saturate the dry air layer, this may cut significantly cut down on our accumulation.

Stay closely tuned, because until this storm actually sets up across the region, the forecast of any heavier snow essentially becomes a "nowcast".

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