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Major snow storm Tuesday: Accumulations of 5-10 inches

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12:10 a.m. update: Temps are running a bit warm around the District at this hour (still 41 at midnight).  While we fully expect temperatures to rapidly drop off once snow moves in, a little accumulation could be lost early in the storm if temps aren’t solidly below freezing especially given relatively warm ground. This is most likely to be an issue near downtown and the Potomac.

This is our last update for the night.  Our next post will go up at 5 a.m. Tuesday, with regular updates starting as soon as snow moves into the region.

 11:00 p.m. update: After taking in all of tonight’s data, we have bumped up snow totals.  Our revised map is below.

In summary, we’re calling for a general 5-8 inches of snow in the D.C. area, with 6-10″ just north and northeast of town.

Important note: Isolating where heavy snow bands might set up is difficult and double digit totals, even up to a foot or so, cannot be ruled out in some locations in the D.C. area.

Our overview points at the bottom of this post, in terms of the start time and impacts of this storm, all remain valid.

One other point of emphasis is the extreme cold combined with wind that will follow this storm.  Temperatures will fall into single digits to near zero late Tuesday night, with wind chills below zero.  A wind chill advisory may well be posted.  Blowing snow on the ground will be a possibility even after the snow ends, especially in open areas.

10:40 p.m. update: Snow lovers will surely like the GFS model run.  It puts out about 0.58″ of liquid equivalent precipitation for D.C., which would convert to just shy of 9 inches of snow.

10:35 p.m. update: The regional version of the Canadian model paints a picture similar to the NAM: significant snow for D.C., but heaviest north/northeast of town, and lower amounts into central Virginia.

10:25 p.m. update: Just want to drill into people’s heads that around 4 p.m. tomorrow – the beginning of the afternoon commute – may be a really bad time to be on the road.  That’s when the SREF model indicates the region has the highest chance (above 50 percent) of 1 inch per hour snowfall rates and visibilities below half a mile.  See the graphic below.

See this neat/useful Web site to see more information about snow impacts, using the SREF model: SREF Derived Winter Weather Impact Graphics

10:09 p.m. update: Multiple school systems have already announced they will close tomorrow due to the predicted snow, including Fairfax Co., Prince William Co., Manassas City, and Frederick Co. (Md).  List of closings.

10:00 p.m. update: (Note: The next model to come in will be the GFS, around 10:30 p.m.) One of the reasons our forecast confidence for this storm is higher than usual is the amount of cold air to our north feeding it.  Already, temperatures have dropped from 58 at 3 p.m. to 40 at 10 p.m. here in D.C. By dawn we should be at or below freezing.  Winds are coming in from the north and the incoming cold air mass means business.Temperatures are below zero from upstate New York through the Upper Peninsula of Michigan and into northern Minnesota, where some areas could drop to 40 below Tuesday morning.

9:41 p.m. update: Where will the heaviest snow fall? Examining the high resolution NAM model, it concentrates the heaviest snow in two bands: 1) northwest of the District, 2) east and northeast of the District. See the map below.

I wouldn’t take this map verbatim, as it’s difficult for models to pinpoint the  heaviest bands ahead of time and they’re often wrong.  But a heavy band northwest of the city (due to it being colder and at a higher elevation) is a common feature in these kinds of storms and the idea of a heavier area of snow to the east also makes sense since the storm will be rapidly developing over the Atlantic.  Snow lovers in central Va. may be in for some snow disappointment if this representation has legitimacy as snow decreases rapidly south of northern Va.

I wouldn’t take the numeric totals above verbatim  as they may actually be underdone since the snow is likely to be more powdery and stack up more than this model assumes.

9:25 p.m. update: When does it start? This – of course – has implications for the morning commute, schools, and so forth. Both the NAM and SREF models suggest snow begins in the 7 to 11 a.m. time frame, beginning first in Loudoun and Frederick counties and last near the Chesapeake Bay.  Inside the Beltway, light snow could begin between 8 and 10 a.m., but the steady stuff that causes roads to deteriorate may hold off until after the morning rush – probably between 10 a.m and noon.

9:13 p.m. update: The NAM model is in and predicts 0.49″ of liquid precipitation for D.C.  This converts to about 7 inches of snow (assuming a 15 to 1 snow to liquid ratio) – which is at the high end of our earlier forecast.  The heaviest snow falls between 1 and 7 p.m. and then rapidly tapers off.

9:05 p.m. update: The SREF model simulations (a set of 22) predicts an average of about 0.7″ of liquid precipitation for D.C..Converting that to snow and assuming 15 inches of snow for every inch of liquid (which is a reasonable assumption since temperatures will cold and the snow will be fluffy) would suggest about 10.5″ of snow for D.C.  Some of its simulations are unrealistically wet/snowy, so I think that may be on the high side. But even the least snowy simulation out of SREF’s 22 gives us 4 inches. Most of its simulations are in the double digits.  This is a good start for snow lovers.

Overview, 8:55 p.m.: We have fairly high confidence that a major winter storm will affect the entire Washington and Baltimore metro regions Tuesday, beginning between 7 and 11 a.m. and ending between 7 and 10 p.m. (west to east).  Our initial accumulation forecast, issued early this afternoon, called for 4-7 inches of snow.  Late afternoon data suggest that this forecast (below) could be conservative and that some areas could even flirt with double digit snowfall totals.

We are considering bumping up our accumulation forecast, and this evening’s computer model simulations will help us settle this matter. High drama.  Stay with us as we live blog these models this evening as they stream in.  By around 11 p.m., we hope to have reached a conclusion about how much snow to expect Tuesday.

A few important things to note about this storm:

* Temperatures will fall through the 20s during the day, and this will be a fluffy, powdery snow.  There is no chance of rain. The snow should stick – except perhaps for a brief period initially in milder areas near the city.

*  The morning rush may be ok, but it’s a close call as snow may start to get going in western areas at that time and could even start flying in the metro area towards the tail end.

* We expect the heaviest snow and worst travel generally to start between 10 a.m. and noon, and end by around 7 p.m.  Roads will become snow-covered and slick in this time window, and flight delays and cancellations are quite possible.

* During the late afternoon and evening, winds will start to crank, reducing visibility and possibly leading to blowing and drifting snow.  The afternoon and evening commute could well be hazardous.

* Extremely cold air follows the storm Tuesday night, with temperatures dropping into the single digits and below zero wind chills.  Highs Wednesday will stay in the teens.