Sunday, March 3, 2013

March 3, 2013: Significant Snowstorm Possible This Wednesday

It may be the start of Meteorological Spring, but it appears that there is still a lot of winter to get through, as the next 7-10 days offer up the possibility of significant, late-season snow.

The next threat is a strong coastal storm - which has not yet formed - but all computer models take dead aim at the Mid Atlantic, this Wednesday. 

Let me start by saying that the next few days are going to be very frustrating, because of the great uncertainty in storm track, intensity, speed, moisture content and availability of air cold enough for snow.

Statistically, major March snowstorms are not as likely as in February and January.  However, there have been several powerful, even crippling March Nor'easters and heavy snowfalls, including the 1993 Storm of the Century and the 1962 Ash Wednesday Storm.

But there is consensus among the forecast models slowly emerging, one that tracks a developing coastal low to our south, emerging off the coast of North Carolina, rapidly intensifying along the way.
The two forecast charts below illustrate the evolution of this system:

The track and intensification put us in a space-time "sweet spot" for significant snow accumulation - namely, on the northwest side of the storm, during the timeframe of its rapid deepening.

Here are factors in favor of heavy snow in the metro region:

1)  Storm track and intensity - not just at the surface, but at the jet stream level, which involves the optimal phasing of a northern and southern stream piece of energy;
2)  Ample moisture, pulled in off the Atlantic;  some models are generating upwards of 2" of liquid precipitation equivalent - making this potentially a very wet storm;
3)  Strong dynamics in the middle and upper atmosphere, which favor a process called "dynamic cooling" - that is, air forced to ascend vigorously cools strongly, enough to couteract surface temperatures that area a few degrees above freezing.

Now, there are plenty of uncertainties involved in this forecast;  here are some of the factors that would argue against a heavy snow in the metro region:

1)  Time of day and time of year - with some of the heaviest precipitation falling during the day on Wednesday, and the ever-increasing solar angle, it may be tough to get surface temps below freezing;
2)  Ground temperatures that remain too warm - especially at the onset - ameliorating heavy snow accumulation on roadways;
3)  No major source of sub-freezing air at the surface:   Lacking a strong, cold anticyclone over New England and Eastern Canada, surface temps in the metro region will be marginally cold enough for snow (this is why the heavy snow bulls eye may set up in the higher, colder elevations to our west and north);
4)  As the storm intensifies, enough warm air may get pulled in at low levels to change snow to sleet, cutting down on accumulations.  The strong, warm air inflow argues for mainly a rain storm along the Eastern Shore.

Here is the 2 PM Sunday guidance on heavy snow likelihood, from the NWS Hydrometeorological Prediction Center - note the bulls eye in the Central Appalachians:

So at this juncture, in the metro region, there are many issues revolving around (1) timing involving type of precipitation;  and (2) gradients in precipitation type i.e. the exact setup of a rain-snow line.   But there are enough "positives" for a significant snow event, here in the metro region, such that this storm should be taken seriously, and preparations started for possible big impacts.   The likelihood of seeing some accumulating snow is increasing.  The likelihood of a moderate- impact event i.e. 4"-6" is significant.  There is even a possibility that this storm will deliver a crippling blow to our region - a heavy, wet snow lasting for hours, combined with wind gusts of up to 35-40 mph, which could create widespread, multi-day power outages.

Here's a scary forecast scenario, consistent with a crippling snow - which is based on the GFS forecast model from 2 pm this afternoon.  Yes...Washington and Baltimore get the bulls eye of 12"-18" of snow!!!!

This is only one of several models, and one of many more forecast cycles to come, as the predictions become better refined.   This clearly is the most extreme of the spectrum of forecasts being generated.

In terms of impacts, this one is going to have us all on the edge, right up to 6-12 hours before the event.

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