3:30 p.m. update: The National Weather Service has issued a winter storm watch for the entire region for the potential for at least 5 inches of snow Monday evening through Tuesday morning. It writes:

BEGINNING MONDAY EVENING…ROADS WILL BECOME SNOW COVERED AND [S]LIPPERY WITH VISIBILITIES BEING REDUCED TO ONE HALF MILE OR LESS AT TIMES. THE COMBINATION OF SNOW COVERED ROADS AND LOW VISIBILITY WILL MAKE TRAVELLING DANGEROUS.


Original post from 1:45 p.m.


GFS model shows snow ending over the D.C. area around 7 a.m. Tuesday morning. (WeatherBell.com)

Over the last day, computer models have come into strong agreement in predicting a moderate snow event for the D.C. area Monday night. Assuming no significant changes in the forecast, this should be the heaviest snowfall of the season so far.

About 4 to 8 inches of snow seems like a reasonable first estimate for the immediate D.C. metro area. However, our far northern suburbs – from Frederick to Baltimore and north – may receive a bit less, probably on the order of 3-6 inches. Some of our southeast suburbs – towards Southern Maryland and Virginia’s northern neck – could receive more, up to 5 to 10 inches of snow.

The snow is likely to begin between 4 and 8 p.m. Monday (southwest to northeast) and end between 4 and 8 a.m. Tuesday morning (southwest to northeast). The heaviest snow is likely to occur between about 8 p.m. Monday and 4 a.m. Tuesday throughout the area.

Snow amounts will be very sensitive to the exact track of the storm, which could change. If the storm shifts south (bust scenario), the immediate D.C. area may get just a couple inches of powdery snow. But if it shifts a little north (boom scenario), totals near 10 inches or so become a possibility.

“The storm – as currently forecast- is taking a very favorable track for accumulating snow in Washington,” says Wes Junker, Capital Weather Gang’s winter weather expert. “But a shift south would disappoint snow lovers, while a shift north would make this a pretty significant snowfall for the region.”

Models forecast a weak area of low pressure to track from northern Louisiana through South Carolina and eventually off the coast of Cape Hatteras. Very cold air is in place, so all precipitation should fall as snow and every flake will stick given the frozen ground.

Parts of the Northeast are deep into another major winter storm this weekend, with blizzard conditions in six states along the coastal region. (Reuters)

The snow is likely to be a powdery one. Models forecast roughly half an inch of melted liquid, which would normally would convert to about 5 inches of snow. However, as temperatures will fall into the upper teens and low 20s when the snow falls Monday night, the snow to liquid ratios will be closer to 12 to 1 or even 15 to 1 (and possibly higher in our northern areas). This is why we think up to about 8 inches of snow, or even a bit more in a boom scenario, is possible.

Given the amount of snow forecast and cold temperatures, school closings and changes in Federal government operating procedures are a decent bet Tuesday, unless there is a big change in the storm track to the south.

This looks to be a fairly polite snow – the bulk of it falling overnight Monday night – ending by the start of Tuesday morning’s commute.

Here’s a look at the various model snow forecasts – the level of agreement is pretty remarkable (note the models assume a 10 to 1 snow to liquid ratio, which is probably conservative in this case):

European model (7 inches for D.C)


(WeatherBell.com)

NAM model (5 inches for D.C.)


(WeatherBell.com)

GFS model (5 inches for D.C.)


(WeatherBell.com)

Canadian model (5 inches for D.C.)


(WeatherBell.com)

SREF model simulations, not shown, forecast an average of 7.5 inches

Finally, this is not the only snow in the forecast this week. A clipper may bring a bit more snow Wednesday and some snow or at least mixed precipitation is possible next weekend.

Programming note: We will live blog the models tonight and post a snowfall map.