Thursday night snow likely, then punishing wind and cold Friday

crystal_ballA complex weather system marching into the region Thursday and Thursday night and the cold in its wake will bring the D.C. metro region its most extreme combination of winter weather of the season, thus far.

Precipitation begins as rain or a mix of precipitation Thursday afternoon but changes to snow Thursday night as temperatures plummet.  The exact amount of snow  Thursday night is a tough call, but anything that falls from 9-11 p.m. and later will freeze on all surfaces with temperatures rapidly falling below freezing and eventually towards the teens.

Generally, we expect about a dusting to 2 inches in the region, with the highest amounts north and northeast of the city. But adjustments to this forecast may be needed.

Capital Weather Gang snowfall forecast for January 2-3. First issued January 1 at 1:30 p.m.

Capital Weather Gang snowfall forecast for January 2-3. First issued January 1 at 1:30 p.m.

Snow ends early Friday morning, but is followed by one of the coldest days in the last few years, with highs only 20-25, and winds gusting over 30 mph.  That means single digit wind chills.

The high res NAM model simulates temperatures in the mid-teens Friday morning.  This may be a few degrees too cold close to the city, but gives a sense of the frigid air mass moving in.   Wind chills are likely to be in the single digits, and perhaps below zero in the colder suburbs. (WeatherBell.com)

The high res NAM model simulates temperatures in the mid-teens Friday morning. This may be a few degrees too cold close to the city, but gives a sense of the frigid air mass moving in. Wind chills are likely to be in the single digits, and perhaps below zero in the colder suburbs. (WeatherBell.com)

Storm timeline/evolution

Thursday afternoon and early evening (1-6 p.m.): Patchy areas of light precipitation develop, likely as rain showers, but perhaps a mix or snow showers in colder spots to the north and west. Chance of precipitation 50 percent. No travel problems. Temperatures 37-43.

Thursday evening (6-11 p.m.): Rain showers gradually change to snow showers. Chance of precipitation 60 percent. A few slick spots could develop, especially in colder areas north and west. Temperatures falling through 30s.

Graphic showing storm evolution (National Weather Service)

Graphic showing storm evolution (National Weather Service)

Thursday night into pre-dawn Friday (11 p.m. Thurs, to 4 a.m. Fri.): Snow showers, with possible steadier snow north and northeast of the District. Chance of snow 50-70 percent, highest chance northeast of town, lowest southwest. Slick spots likely where snow falls and possible hazardous travel. Temperatures falling through the 20s.

Early to mid- Friday morning (4 a.m. to 9 a.m.): Snow ends southwest to northeast, becoming windy, very cold. Possible blowing snow. Temperatures 13-19.

Snow amounts

Suffice to say, this is an extremely challenging forecast.  As one area of low pressure heads into West Virginia Thursday, a new one will form off the coast of the Carolinas.  In these setups, the D.C. area can get caught in a dry slot or “snow hole”.  On the other hand, if the new coastal storm develops quickly enough, we can get dumped on.

“The storm system that will be approaching us is on a very favorable track to give us snow. The models are still showing an initial dry slot but then have precipitation developing across the region as a new low develops offshore,” notes Wes Junker, Capital Weather Gang’s winter weather expert. “How quickly it develops and whether we get into the comma head [area with heavy snow] associated with the low will largely determine how much precipitation/snow we get.”

Junker adds: “Some of my biggest busts have been trying to forecast these types of storms. We’re still leaning towards a dusting to 2 inches across the DC area with heavier amounts possible towards Baltimore. There is a slight chance that the system will develop quicker than forecast which could lead to a band of heavier snow developing.”

Computer models are all over the map with the forecasts, simulating as little as dusting of snow to 8 inches or more.

SREF model simulations show snow amounts for DC varying from a dusting or less to over 8 inches.  Its average is between 3 and 4 inches, but we think that is probably overdone. (NOAA adapted by CWG)

SREF model simulations show snow amounts for DC varying from a dusting or less to over 8 inches. Its average is between 3 and 4 inches, but we think that is probably overdone. (NOAA adapted by CWG)

The NAM, GFS and European model are in decent agreement… simulating around 1-4 inches of snow.

The high resolution NAM model forecasts 1-4" of snow in the region (WeatherBell.com)

The high resolution NAM model forecasts 1-4″ of snow in the region (WeatherBell.com)

The GFS model simulates a dusting to 2 inches in the region (WeatherBell.com)

The GFS model simulates a dusting to 2 inches in the region (WeatherBell.com)

The European model simulates 3-4 inches of snow in the D.C. area (WeatherBell.com)

The European model simulates 2-4 inches of snow in the D.C. area (WeatherBell.com)

Our forecast – with the most likely snow amounts between a dusting and 2 inches – is a bit more conservative. But there is considerable bust or boom potential here and we may increase (or decrease) forecast amounts later today.

In cases like this when there is large uncertainty, thinking about snow potential in terms of probabilities may be most useful.

For the D.C. area, here’s our thinking:

Chance of at least 1″ of snow: 50%
Chance of at least 2″ of snow: 25%
Chance of at least 4″ of snow: 10%

For the Baltimore area:

Chance of at least 1″ of snow: 75%
Chance of at least 2″ of snow: 50%
Chance of at least 4″ of snow: 25%

Stay tuned for additional updates today and tomorrow. The SchoolCast and FedCast for Friday will be posted tomorrow. (There are no concerns about Thursday’s day time weather and schools/governments will open on time.)

Also on Capital Weather Gang

D.C. area forecast: Calm start to 2014, but snow showers and bitter cold await