11:15 p.m. update: So, after digesting the latest model info, I don’t see any reason to change our snowfall forecast for the D.C. area. Here’s a slightly modified snowfall accumulation map – which lowers amounts well to our southwest and increases amounts in parts of the Delmarva – but keeps amounts the same around D.C.

Capital Weather Gang snow map issued 11:15 p.m. Wednesday, January 1.

Given the likelihood for a sharp cutoff in the snow and, conversely, the chance that a heavy snow band forms if the storm develops rapidly, the bust and boom potential remain high. Also, I’d be remiss not to mention that temperatures above freezing in the early phase of the storm and a brief period of rain will cut down on snow totals. If temperatures are slower to cool than forecast, our accumulation prospects would take a big hit.

For D.C. (Reagan National Airport), I think it’s unlikely this storm ends the record long streak without a 2″ snowstorm. But stay tuned, this forecast could still change down to the wire tomorrow.

One thing is for sure, it’s going to be cold, really cold. To close, here’s the GFS model simulation of temperatures at 7 a.m. Friday, even colder than the NAM.

GFS simulation of 7 a.m. temperatures (WeatherBell.com)

10:50 p.m. update: Here’s the graphical representation of the GFS snow output.

GFS snowfall simulation (WeatherBell.com)

Compared to the NAM (scroll down), it shows slightly more in D.C. and for areas south of the District (it’s cutoff isn’t quite as sharp – but that may somewhat reflect the model’s lower resolution), and more or less the same in our northern suburbs to around Baltimore.

10:45 p.m. update: The total precipitation simulated by the GFS is very slightly more than the NAM. It shows 0.28″ liquid equivalent compared to the NAM’s 0.22″. If you convert this GFS amount into snow (taking into account rain at the onset and melting of snow given temperatures above freezing at first), it’s probably around 1.5″, maybe 2″ if I’m being generous.

Gridded output from GFS model at Reagan National Airport
Gridded output from GFS model at Reagan National Airport

Overall, the NAM and GFS models are in excellent agreement on the timing and amount of snow.

10:39 p.m. update: The GFS looks a bit snowier than the NAM – with less of a dry slot.

GFS model simulation showing surface weather map and 6-hour precipitation through 1 a.m. Friday (StormVistaWxModels.com)

10:25 p.m. update: The latest GFS model is starting to run. New analysis soon…

10:15 p.m. update: Earlier updates have talked a lot about the snow, but what about the cold simulated by the NAM model? It’s impressive – in a punishing kind of way late Thursday night into Friday.

At 7 a.m. Friday, it shows temperatures in the low-to-mid teens.

Temperature simulated by NAM model at 7 a.m. Friday (WeatherBell.com)

At the same time, it shows sustained winds are 18 to 26 knots or 20-30 mph, with gusts probably to near 40 mph.

Sustained winds simulated at 7 a.m. Friday morning by the NAM model, in knots (WeatherBell.com)

Assuming a temperature of 15, and wind of 25 mph, this would produce wind chills of around -4 F at 7 a.m. Friday.

Winds diminish some by Friday afternoon, but the NAM simulates highs only in the low 20s.

10:00 p.m. update: While we await the GFS model (coming in around 10:30 p.m.), I thought I’d post the National Weather Service’s snowfall forecast – which is not dissimilar to our first call (scroll to bottom of this post):

Snow forecast for Thursday night (National Weather Service)

The NWS also provides maps showing minimum (bust) and maximum (boom) snowfall scenarios on their excellent winter storm Web site.

9:50 p.m. update: For our friends up in Baltimore, you certainly have a better chance of shovelable snow than D.C. The NAM spits out 0.31″ of liquid equivalent precipitation, and about two-thirds of that probably falls as accumulating snow. So the NAM gives Charm City a solid 2 inches.

9:40 p.m. update: Here’s what the high resolution NAM snowfall output looks like in graphical form. Looks pretty reasonable to me…

Total snowfall simulated by the NAM model (WeatherBell.com)

What stands out is the sharp cutoff in the accumulating snow in southern Fairfax County. Whenever the cutoff is in our region, that makes me nervous because a slight shift (north or south) in the cutoff can result in a big forecast bust (less or more snow).

9:30 p.m update: The NAM model data for Reagan National Airport indicates 0.22″ of liquid equivalent precipitation (see below). Some of that – probably around half – will be probably fall as rain and/or melted snow (i.e. with temperatures above freezing). Temperatures fall below freezing around 10 p.m. if the model is right, then around 0.1″ liquid equivalent falls. Assuming a ratio of 10″ of snow per 1″ of liquid equivalent precipitation, the NAM model would give Reagan National about 1″ of snow.

Gridded output from NAM model
Gridded output from NAM model

Notice only 0.02″ liquid equivalent falls after 06z (1 a.m. Friday morning) when it starts to turn really cold. So the NAM definitely makes this a quick hitter – with the snow over by 2 or 3 a.m. But look at the 12z (7 a.m.) Friday temperature: 12F! Wow…

9:15 p.m. update: The NAM model is now in, and – if it’s right – it’s hard to see the immediate D.C. area managing much more than an inch of snow. The simulated precipitation shows an unmistakable dry slot over our region. We do get a few hours of snow Thursday night, but some of it falls before temperatures are below freezing, eating into the limited accumulation prospects. Areas to the east and northeast see more precipitation.

Simulated 36 hour precipitation (liquid equivalent) from the NAM model (StormVistaWxModels.com)

9:05 p.m. update: As we await the arrival of the NAM model, the SREF model – which has simulated the most snow of any – continues to put out some impressive totals. Of its 22 simulations, the average snowfall predicted is close to 4 inches. Most of it falls between 9 p.m. and 4 a.m. Thursday night (Friday morning).

SREF model simulations show snow amounts for DC varying from a dusting or less to over 8 inches. The simulations cluster in the dusting to 3 inch range. Its average is about 4 inches, but I think that is probably overdone. (NOAA)

I am skeptical of this model as it tends to overdo snow amounts especially in situations like this where there is a sharp gradient between areas that get a little snow and more significant amounts.

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

Overview: We’re expecting a quickly exiting coastal storm to throw back a dusting to a couple inches of snow Thursday night followed by some of the coldest air in years.  New model data comes in between 9 and 11 p.m. tonight. I’ll report what it’s telling us and whether we need to adjust any parts of our forecast.  By 11 p.m., I’ll post a revised snow accumulation map.