Tuesday, March 5, 2013

March 5, 2013: Heavy Snow Accumulation In Metro Region Still Uncertain


My forecast for 4"-6" is still on track!   In fact, the NWS has lowered their accumulation totals across the Maryland Piedmont (compare this new afternoon forecast with the one issued this morning, below):

The Sweet Spot is still 100-150 miles to our southwest, over the high terrain.  There, the threat of power outages from heavy snow and high winds is very real.

Once again, arguments can be made that enough mild air will flow into the system, on E-NE windst after midnight, to keep the precip all rain over the metro area for several hours.   Once the cold air does arrive from the north, there will be sufficient moisture to generate several inches of snow, which will start off wet and slushy.   Locally higher amounts may fall if and where a snowband sets up.   The 4"-6" range is borderline Winter Storm Warning criteria.   Winds will gust up to 30 mph from the north tomorrow night, and then from the NW on Thursday, as the low pressure system exits (the storm is intensifying as it exits).

Timing on the snow is shifted a bit:  Rain should transition to all snow later tomorrow morning - by around 9 AM.  Moderate snow will fall in the afternoon.  Snow will start to taper around 6-7 pm, but may linger a few hours into the evening, adding perhaps another inch - and it may not be as wet as the earlier period of snow.


My forecast for 4"-6" heavy wet snow is still on track across the Metro region, for all the reasons I gave yesterday.  NWS is struggling to sort through competing processes, some of which would cool the atmosphere and favor more snow, some of which would warm things up and favor rain mixing in.

My own assessment:  There is no cold air mass in our region that can sustain heavy snow.  To get heavy snow, you either have to go up in altitude - where it's cold enough - or the storm has to manufacture its own cold air supply.  It can do this, under the right circumstances, through vigorous uplift of air - rising air cools.   Dry air beneath the cloud layer promotes evaporation, and this also chills the air.  But at the same time, the storm will pull in milder air off the Atlantic, and the condensation and freezing of oceanic moisture releases heat inside the clouds, warming them.
It boils down to a delicate balance of warming and cooling processes, which some of the forecast models do not have the best handle on.  We may not know until the event is underway, which process will win out.

There is a range of professional opinion on how much snow our region will pick up. Other weathercasters and meteorologists this morning are calling for 3"-7" across the Metro region.  Some are going for higher amounts.  The divergence of opinion reflects our lack of understanding of the temperature balancing act that will play out tomorrow.

Here is the latest snow accumulation map put out by NWS Sterling.  They have upped the totals from yesterday...mainly because there is one prediction model, the NAM, which is hanging on to a cold and snowy solution.   One of the other main models, the GFS, has warmed up, decreasing snow totals across the metro area.  The NWS may have raised their totals, to accommodate the cold bias in the NAM, as a worse-case scenario:

As new model runs come in, totals will get re-adjusted.  This process will continue throughout today and tonight and even tomorrow as the event gets under way.   This is not a high confidence forecast for our region, so stay tuned.  Next update today around 4 PM.

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