** Winter storm watch Wednesday night and Thursday **

Overview: A winter storm watch is in effect for Wednesday night into Thursday as a significant storm is expected to impact the area and the broader East Coast. If you missed our first detailed look at the storm earlier today, a very brief summary is that we at CWG see a pretty high chance for the storm to reach or surpass 4 inches of snowfall across the area — 60% western spots, 50% in the I-95 corridor. If models, like tonight’s incoming runs, continue to show a significant snow threat, these numbers might be on the rise.

Note: None of the model discussion should be taken as a forecast. We’ll do a quick update on where we stand around 11 p.m. following model talk.

11:15 p.m. update: All systems still go for snow. The trend tonight is clearly one that solidifies our odds of seeing a sizable snowstorm. Details are of course to be worked out regarding any potential mix, and ultra-specific timing of storm progression as it passes by. In general, the story of a Wednesday evening start and a Thursday afternoon exit remains on target. Under the current expected scenarios, travel is likely to become quite difficult as we get through the night and then lasting through Thursday.

Our updated snowfall odds across the area are below. Just keep in mind, we’re still about 48 hours out from start, so there could be some large changes either way. If we’re looking like we do tonight tomorrow night, a big snow becomes nearly a lock:

Probability of 1″+: DC/Balt/Fredericksburg: 80%, Frederick/Loudoun/Warrenton 85%

Probability of 4″+ DC/Balt/Fredericksburg: 60%​, Frederick/Loudoun/Warrenton 70%

Probability of 8″+ DC/Balt/Fredericksburg: 45%​, Frederick/Loudoun/Warrenton 55%

I’ll leave you with this. Even though the GFS did not “go big” like some other models, the chart from it below is about as classic a 500mb (mid-level) chart as you’ll see as it passes the area. That “orange circle” just to our southeast is one thing to look for if you’re hoping for a snow day. (check out the full run)

We’re back with much more tomorrow, including an update from Wes Junker!

11:02 p.m. update: The newest UKMET, an “also quite good” model from over the pond has a big storm Thursday morning, in a solid spot if you like snow:

10:55 p.m. update: As you can see in the comparison below, the current (left) GFS run is wetter than the last run (right). CWG’s Jason Samenow just let me know it drops 0.67 inches of precipitation at D.C., all snow. In a 10:1 ratio that’s about 6 inches.

10:48 p.m. update: The GFS brings snow into the area late evening Wednesday (probably later the NAM, closer to 10-midnight). It’s looking like a slightly better version of the last run if you like snow as it’s more expansive in its precip shield and stronger with the low. Overall, it remains a bit weaker with the low than some other guidance and is thus a little drier. More as I look over the run…



GFS forecast for 7 a.m. Thursday. (Weatherbell.com)

10:25 p.m. update: The GFS is running. It’ll come out a lot quicker than the NAM did. Should have info soon!

10:10 p.m. update: We’ve got a bit of a lull before the 0z GFS runs around 10:30 p.m. You can load it up and get ready in the meantime.

The bottom line thus far is no major changes to our initial thoughts. While the models that came out so far are a bit east of “perfect” for a snow lover, they also minimize the risk of changeover. The global models upcoming may have a better handle on the track as well in the end. Also, a cold start.. so likely we won’t be waiting for hours for snow to accumulate like happened in some recent busts:



igh resolution NAM temperature forecast, with snow over the area late Wednesday evening. (Weatherbell.com)

10:00 p.m. update: Some folks were asking for SREF snow plumes at other locations. Here’s BWI, IAD, and FDK. If you zoom way in on the map on the page you can select any location that has a dot.

9:50 p.m. update: A question was asked on the radio show, “Is there a chance for thundersnow?” Wes Junker’s answer, “I think there is.” He’ll have more on that here in future updates on the storm.

9:40 p.m. update: On the NAM, the storm is in by late Wednesday evening and out around mid-afternoon on Thursday. This run cut back precipitation totals a bit in the area, but the last run was pretty insane and likely too wet to begin with. Here are the precip and snow maps from the run. It’s right around 1 inch liquid equivalent in D.C. (generally, snow is 10:1 ratio compared to liquid, but that’s dependent on a lot of factors), and likely cold enough for mostly snow given the track. (note: snow maps are more for fun than anything else in many cases!)

9:30 p.m. update: The NAM is out far enough to see the track of the storm is offshore in the region on this model. It moves from near the Outer Banks early on Thursday to off the Delmarva. More as details come out shortly, but it looks quite snowy on the whole.

9:20 p.m. update: On the radio show, CWG’s winter weather expert said, “I don’t think there’s a chance we get nothing,” while discussing this storm. Snow lover’s can worry a little less perhaps!

9:15 p.m. update: SREF has a neat “plume” diagram, which shows what each member of the ensemble system is showing to get the mean results shown in the 8:50 p.m. update below. Here’s the newest one for D.C., focusing on snow. There are some clear outliers that should probably be tossed, but the mean of the whole group is over 7 inches for the city. You can tinker with it here.



21z SREF plume diagram for snow at D.C. (SPC)

9:00 p.m update: The American Weather Forum is running a special model-watching radio show tonight as well, and participants include CWG’s winter weather expert Wes Junker as well as CWG occasional contributor/seasonal forecast lead Matt Ross. Listen live!

8:50 p.m. update: First up among the most skillful models in the evening suite is the North American Mesoscale Model (NAM). It has started running, and we should be in range of the storm in about 30-40 minutes. Before the NAM runs, its cousin the Short Range Ensemble Forecast (SREF) comes out. Sometimes SREF can be a first look at what to expect from the NAM.



Short Range Ensemble Forecast (SREF) comparison. Most recent (21z) run at left. (NOAA)

In short, the SREF shows the storm sitting just off the Outer Banks of NC at 7 a.m. Thursday, swinging up toward “the benchmark” (70W/40N) as the day progresses. A pretty classic I-95 region snowstorm track. The newest run also increased precipitation totals to around 1 inch into the immediate area, as well as east. It’s also plenty cold, and probably mostly snow in the area.