Most Read: Local

Posted at 03:23 PM ET, 11/06/2012

First snowflakes of the season possible in Washington, D.C. Wednesday


A simulated radar image shows precipitation grazing the region Wednesday at 4 p.m. (NWS)
The Nor’easter poised to batter coastal areas from the Delmarva to New England may just graze the Washington, D.C. metro region. But with unseasonably cold air in place, any precipitation the storm throws back at us may contain snowflakes.

Most computer models simulate the edge of the storm’s precipitation field brushing the D.C. area Wednesday afternoon and night with heavier precipitation to the east and northeast. Baltimore and, especially, Philadelphia are likely to receive more precipitation and have a chance for some light snow accumulation.

For the D.C. area, we’d give the following odds for snow:

* 40 percent chance of no snow; just light rain showers, if anything
* 20 percent chance of snowflakes mixing in with rain, but no accumulation
* 20 percent chance of a slushy coating of snow, mainly on grassy areas
* 15 percent chance of a coating to 1” of wet snow, mainly on grassy areas
* 5 percent chance of 1-4” of wet snow

In short, we think there’s a 60 percent chance we’ll see snowflakes, but just a remote chance that they amount to anything. If you live south and/or west of the District, these odds go down, and if you live east and/or northeast of the District, these odds go up.

The most likely scenario is for some chilly light rain that mixes with or briefly changes to wet snow at times. But CWG’s winter weather expert Wes Junker stresses this is a delicate forecast situation.

“How much precipitation will fall across the D.C. area still varies considerably among models ranging from a decent hit to having the storm develop far enough east to have the precipitation completely miss us,” Junker said.

Limiting factors for snow are two-fold:

1) Moisture will be scarce since our area will be positioned so far west of the storm, centered offshore in the Atlantic ocean. The National Weather Service likened this storm to the “No-mageddon (snow hole)” storm of December 26, 2010 when more than 4 inches of snow was forecast, and less than a dusting occurred (but much more to the east and northeast) as moisture cut off over our area:

THOUGH NOT A ONE-TO-ONE COMPARISON...THIS EVENT DOES SHOW SOME SIMILARITIES TO A COASTAL STORM THAT DEVELOPED OFF THE MID ATLANTIC COAST LATE DEC 2010. THE PRECIP GRADIENT WAS TIGHT BUT MAINLY STOPPED AT THE BAY. OVER OUR [AREA THERE] WAS AN SIGNIFICANT AREA OF DOWNWARD MOTION AND LEFT VERY LITTLE IN THE WAY OF PRECIP

2) The amount of cold air needed for snow is marginal. Remember, it’s early November in Washington, D.C. when snow is rare. Furthermore, there’s no Arctic high pressure system to the north feeding cold air into the storm. For it to snow, we’re relying on a dynamic cooling process to pull down colder air from high in the sky down to the ground. This requires steady precipitation which is not guaranteed given 1) above.

“The one thing the models do agree on is that the low level temperatures will be an issue that could hinder snowfall from developing as model forecast surface temperatures generally range from the mid-30s to close to 40 degrees,” said Junker.


SREF model simulations (each line is a different simulation) favors light rain (green lines) but some simulations have snow mixing in (blue segments of green lines) throughout the storm, with the bulk of the precipitation falling late afternoon into early evening. X-axis is timeline, y-axis is accumulated precipitation amount. Subtract 5 hours from “Z time” to get EST.

The Short Range Ensemble Forecast System (SREF) - a package of numerous model simulations (shown above) - favors light rain for the most part but shows the potential for some periods of wet snow in some of its runs.


Odds of 1” or more of snow as indicated by color shades. The National Weather Service gives a 20-40 percent chance of 1” or more of snow in our region. Note: we think this is overdone and too far west in our area. (NWS)
The only scenario for accumulating snow to fall in the D.C. would be for the storm to develop explosively and closer to the coast than forecast by current models, throwing back heavy precipitation into the area that would tap cold air aloft.

We don’t think this is particularly likely, but Junker cautions uncertainty is still high.

“This remains a very tricky forecast as the models are struggling with how two strong waves in two separate streams of flow will interact,” Junker said.

By  |  03:23 PM ET, 11/06/2012

Categories:  Latest, Winter Storms

 
Read what others are saying
     

    © 2011 The Washington Post Company