** Winter storm warning from 8 p.m. Wednesday night to 3 p.m. Thursday (southern Maryland excluded) **

In about 24 hours the snow should be moving into the region. How much will fall? When and where will it change to sleet and rain?

11:25 p.m. update: After reviewing this evening’s model runs, we’re not altering our forecast at all. We think the GFS model is probably a little underdone, and the NAM is probably a little overdone. The UKMet and Canadian models (shown below) probably have reasonable compromise solutions.

Here are some take home points from tonight’s data:

A winter storm warning is now in effect for the entire region Wednesday night into Thursday afternoon, except for southern Maryland

Snow, moderate to heavy at times, falls overnight Wednesday night (starting between 7 and 10 p.m.) into early Thursday morning, with at least 3 or 4 inches of accumulation by dawn, and possibly 6″ or so. Temperatures and the ground will be cold, and snow should accumulate quickly.

Precipitation continues during the day Thursday, but varies in intensity (not as steady as the previous night) and probably mixes with and changes to sleet/freezing rain/rain near the I-95 corridor and points east by mid-morning (if not a little earlier). Temperatures hover near freezing. In this area, we don’t expect a lot of additional snow accumulation during the day (perhaps 1-2″) – but this is the toughest part of the forecast with large uncertainty. We’ll try to nail it down better tomorrow.

A little mixed precipitation could reach locations towards the Loudoun and Frederick (Md.) county lines, but far western areas probably remain mostly snow. That’s where totals over 8 inches and into the double digits are most likely.

Areas getting mixed precipitation may end as a period of snow between late afternoon and early evening Thursday.

We’re standing by our accumulation map issued at noon today, per below.



Capital Weather Gang snowfall forecast, issued noon, Tuesday, 2/11.

The European model comes out around 1 a.m. We will not be live blogging that, but I imagine some of our readers may stay up for it…so feel free to discuss it in the comment area below. We’ll have our next forecast update at 5 a.m., and plan to provide frequent updates all day tomorrow. Thanks for joining us this evening.

11:10 p.m. update: Here’s one more model to share: the UKMet. It has some similarities to the NAM, forecasting heavy snow between 1 a.m. and 7 a.m. Thursday morning (over 0.5″ liquid equivalent, or 5″ of snow in those 6 hours). See below.



UK Met model simulates over 0.5″ liquid equivalent precipitation or roughly 5″ of snow, between 1 a.m. and 7 a.m. Thursday.

10:55 p.m. update: Like the NAM, the GFS model simulates its heaviest precipitation Wednesday night into early Thursday morning, when it’s cold enough for just snow. But it then suggests mixed precipitation near the I-95 corridor and points east beginning in the 7-10 a.m. time frame Thursday. Out towards Dulles airport, it suggests mostly snow during the day Thursday with perhaps some sleet mixing in. Overall, it forecasts significantly less precipitation (around 0.7″ liquid equivalent) than the NAM (1.26″ liquid equivalent) but keeps it going later in the day and perhaps into the evening.

10:40 p.m. update: The GFS model is mostly in (for the time periods we care about) and is not nearly as snow-friendly as the NAM. By 7 a.m. Thursday, it simulates about 0.41″ liquid equivalent precipitation which would convert to about 4″ of snow in D.C.. That’s less than half the amount simulated by the NAM (1.11″ liquid equivalent, or roughly 11″ of snow) at the same time. So some huge differences in the models.



GFS model precipitation simulation between 1 a.m. and 7 a.m. is around 0.27″ liquid-equivalent in D.C., which is about 3″ of snow. (StormVistaWxModels.com)

10:30 p.m. update: The GFS is just starting to run. The Regional version of the Canadian model, in the mean time, has come in and has a track a little east of its earlier run and is slightly snowier. It simulates in the neighborhood of 7-10″ of snow, but also introduces mixed precipitation around 9 or 10 a.m. Thursday along the I-95 corridor.



Canadian (RGEM) simulation of precipitation at 9 a.m. Thursday (Environment Canada)

10:20 p.m. update: Even though some of the models, show a changeover to some type of precipitation besides snow during the day Thursday, CWG winter weather expert says temperatures may not rise above freezing given the pattern and cold air in place ahead of the storm (as long as precipitation is falling). That could mean light accumulations of sleet or freezing rain.

“I wonder whether we ever will see temps above freezing at the surface until the precip lets up,” Junker says. “It might warm at 5,000 feet (850 mb) but having a low developing off the coast should keep the surface winds northerly. The low off the coast should act like the one did earlier during our freezing rain event. That airmass at the surface stayed colder than forecast and we’re starting with a colder air mass than that one.”

10:10 p.m. update: While we await the GFS model’s arrival (starting around 10:30 p.m.), it’s interesting (and somewhat reassuring) to see the National Weather Service’s forecast snow total map, issued at 7 p.m., is very similar to ours (issued at noon).



National Weather Service snow total forecast map issued at 7 p.m.

Compare what’s above with our map at the bottom of this post.

10:00 p.m. update: Reasons for snow lovers to be optimistic: The NAM model just simulated over 1 foot of snow in D.C., most of it falling overnight when temperatures are plenty cold enough for it to accumulate. By the time the atmosphere warms to the point snow could mix with sleet and/or rain, most of the snow has already fallen if it’s right. Reasons for snow lovers to keep their expectations in check: The NAM has a tendency to overdo precipitation amounts and other models don’t show as much. It’s also colder than the other models which suggest a greater risk of a changeover. The GFS comes next…

9:47 p.m. update: Here’s the total snowfall simulated by the NAM model.



Total snowfall simulated by NAM model (WeatherBell.com)

I’m going to go ahead and say I think this is probably overdone. Lest snow lovers get too excited, the model has a known bias of simulating too much precipitation. But, still, even if you cut its snowfall output in half, it’s a significant amount.

9:42 p.m. update: Here’s a simulated radar from the NAM model at 3 a.m. Thursday morning – dark blue is heavy snow.



High resolution NAM model simulated radar at 3 a.m. Thursday morning (WeatherBell.com)

That’s some heavy snow.

But look at the radar by 10 a.m. below, not nearly as impressive and even hinting some light rain could start mixing in:



High resolution NAM radar simulation at 10 a.m. Thursday (WeatherBell.com)

9:31 p.m. update: The National Weather Service has hoisted a Winter Storm Warning for the entire metro region Wednesday night (8 p.m.) through Thursday afternoon (3 p.m.). For the immediate metro area, it predicts 4-8″ of snow and sleet, and for western areas 6-10″ (except up to 12″ at high elevations over 800 feet). Nicely timed issuance given the NAM model’s aggressive forecast.

9:25 p.m. update: After 7 a.m. Thursday morning, snow starts to gradually decrease on the NAM model. In only spits out about 0.15″ liquid equivalent or 1-2 inches of snow at Reagan National between 7 a.m. and 1 p.m. For snow fans, it continues to stay cold enough for precipitation to remain mostly snow except near the Chesapeake Bay and into southern Maryland where sleet and/or rain could mix in. Overall, I’d describe the NAM as very “front-loaded” – with a really impressive burst of snow Wednesday night, followed by fairly low impact lighter precipitation during the day Thursday.

9:15 p.m. update: The NAM is rockin’ it – definitely the model of choice for snow lovers. By 7 a.m. Thursday, it simulates 1.11″ of liquid equivalent at Reagan National, which converts to over 10″ of snow. Wow.



NAM model simulated precipitation between 1 a.m. and 7 a.m. (StormVistaWxModels.com)

9:10 p.m. update: Thump! If the NAM model is correct, this storm starts strong. It projects about 3″ of snow by 1 a.m. Thursday morning. (0.32 inches liquid equivalent)



NAM model total liquid equivalent precipitation through 1 a.m. Thursday morning. (StormVistaWxModels.com)

9:06 p.m update: The NAM model is just starting. It implies snow will begin between 7 and 10 p.m. Wednesday night from south to north across the region. Models have been pretty consistent in this start time.

9:04 p.m. update: Here are our basic overview points as models start streaming in this evening. We’ll see if any of this information changes meaningfully as the new models stream in:



Capital Weather Gang snowfall forecast, issued noon, Tuesday, 2/11. Click to enlarge.

Basic information: