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Heavy snow tonight, then mixed precipitation Thursday (LIVE FORECAST UPDATES)

Capital Weather Gang snow accumulation forecast, version 2, issued 2 p.m. Thursday

Capital Weather Gang snow accumulation forecast, version 2, issued 2 p.m. Wednesday

** Winter storm warning tonight through Thursday afternoon | Winter weather advisory for Calvert and St. Mary’s counties **

Cold air is in place, and the stage is set for a moderate to major snowstorm for the D.C. metro region.

Some key points about the forecast:

  • Snow falls heavily tonight, with 4-8 inches of accumulation by 7 a.m. Locally heavier amounts are possible.
  • Near I-95 (within 15 miles) and points east, snow likely mixes with and changes to sleet and freezing rain Thursday morning, between 5 a.m. and 9 a.m. from east to west.  Locations near the Chesapeake Bay and southern Maryland may turn to plain rain.
  • From western Fairfax County and western Montgomery county (from roughly Gaithersburg to Oakton to Manassas) and points west, precipitation remains mostly snow  during the day, although some sleet could mix in at times, especially during the morning. Another 4-6 inches of accumulation are possible.
  • In the afternoon, areas getting mixed precipitation may turn back to snow, with light additional accumulations of 1-2″ possible.  However, how heavy the snow is in this second round and whether temperatures are cold enough for more meaningful accumulation is uncertain.

We’re bringing you the live updates here. To discuss the forecast and more, sound off in the comments on this post.

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Temperatures are cold

When the snow starts falling tonight, most likely between 7 and 10 p.m., it will stick quickly, if not immediately.  10 a.m. temperatures are only in the low 20s across the metro region and will struggle to reach freezing today.  Arctic air is firmly in place ahead of this storm: look at those single digits to our north.

10 a.m. temperatures (National Weather Service)
10 a.m. temperatures (National Weather Service)

The question is: how long until it erodes Thursday?

Jason Samenow
February 12, 10:29 am
  • Jason Samenow February 12, 10:29 am

NAM model: Major snow thump, then mixing.

The NAM model continues to show heavy snow falling tonight, on the order of 10 inches or so by 7 a.m.  But after 7 a.m., from the District and points east, it simulates just a little additional snow during the day Thursday as the precipitation switches to sleet and freezing rain before possibly changing back to snow at the storm’s conclusion.  West of the District, additional snow accumulation is possible.  Here’s an animation showing the snowfall through 7 a.m. Thursday and again as of 1 a.m. Friday morning.

Total snowfall simulated by NAM model at 7 a.m. Thursday and 1 a.m. Friday (WeatherBell.com)

Total snowfall simulated by NAM model at 7 a.m. Thursday and 1 a.m. Friday (WeatherBell.com)

 

We continue to think these amounts may be somewhat overdone (as the model has a tendency to simulate too much precipitation), especially along and east of I-95 – but not certain.

Jason Samenow
February 12, 10:30 am
  • Jason Samenow February 12, 10:30 am

GFS model: Suggests at least 5 inches in D.C. before changeover

The latest GFS model, just in, simulates about 0.51″ of liquid equivalent which converts to about 5 inches of snow in D.C. between 7 p.m. tonight and 7 a.m. Thursday morning.

GFS model precipitation simulation between 1 a.m. and 7 a.m. (StormVistaWxModels.com)

GFS model precipitation simulation between 1 a.m. and 7 a.m. (StormVistaWxModels.com)

So – essentially – taking the NAM and GFS together – they forecast a range of 5-10 inches of snow in D.C. by  7 a.m. Thursday when a changeover to mixed precipitation may be starting.

Jason Samenow
February 12, 10:42 am
  • Jason Samenow February 12, 10:42 am

Mixed precipitation likely Thursday for I-95 corridor and east

By Wes Junker

The models seem to be reaching a consensus about a storm track as the NAM model has shifted west more in line with the last night’s Euro.  Both the GFS and NAM models have come west from yesterday.  The inland track raises the specter of a changeover to sleet or freezing rain for the city.

This morning’s NAM model forecast soundings illustrate just how close we are to the line where we might see almost all snow or have the precipitation mix with sleet or freezing rain.  The red line below shows the temperature of the atmosphere as you ascend upward.

profile

The figure gives the temperature and dew point from the ground level to a little above 9,000 meter. When the red line crosses over to the right side of the blue line, the temperature at that level is above freezing.   By 7 a.m., the NAM shows  a warm layer with the temperatures right at freezing, and by 1 p.m. the temperature in the warm layer rises to around 1.4 C.  Neither sounding supports rain except possibly near the airport if the surface temperature nudges above freezing. Instead they argue for snow to sleet to possibly freezing drizzle by 1 p.m. if the model forecast is perfect.   Slight changes in the storm track to the west could lead to a slightly quicker changeover to sleet.   If the models shift back east, then any changeover might be delayed.  Right now we favor a changeover to sleet in the city sometime toward rush hour.   The precipitation may never change to sleet westward towards Dulles.

Jason Samenow
February 12, 10:49 am
  • Jason Samenow February 12, 10:49 am

Dry slot Thursday midday along I-95 corridor and east?

By Wes Junker

After the thump of snow overnight, today’s NAM model brings a dry slot northward across our area late morning into early afternoon Thursday.   When it comes through, mixed precipitation may taper to freezing drizzle, or plain drizzle.

NAM model simulation shows dry slot develop along I-95 corridor and points east during the day Thursday (WeatherBell.com)

NAM model simulation shows dry slot develop along I-95 corridor and points east during the day Thursday (WeatherBell.com)

Above, note the lack of radar echoes from DC eastward to the Bay at 1 p.m. Thursday.  Also, note that a band of snow continues west of the city with the developing comma head.  That model swings that comma head feature across the area during the evening in the form of snow but with surface temperatures a little above freezing.  I’m not sold on the temperatures ever nudging above freezing at the surface west and north of the city. But east of the city, I think they might.

Jason Samenow
February 12, 11:03 am
  • Jason Samenow February 12, 11:03 am

When does it start?

Models have been pretty consistent in having the snow start between 7 and 10 p.m. tonight.  Here’s a graphic from our local National Weather Service office (in Sterling, Va.) which illustrates the expected start time from south to north:

(National Weather Service)

(National Weather Service)

Note: 18:00 is 6 p.m., 19:00 is 7 p.m., 20:00 is 8 p.m., etc.

Jason Samenow
February 12, 11:17 am
  • Jason Samenow February 12, 11:17 am

National Weather Service: Storm will start fast

From the latest National Weather Service forecast discussion:

IT IS IMPORTANT TO NOTE THAT ONCE THE SNOW DOES START…SNOW WILL QUICKLY BECOME MODERATE TO HEAVY WITH ACCUMULATION RATES OF 1-2 INCHES PER HR POSSIBLE TNGT. TRAVEL CONDITIONS WILL RAPIDLY DETERIORATE AS A RESULT. BY DAYBREAK…SNOWFALL AMOUNTS BETWEEN 3-6 INCHES ARE FORECAST AREA WIDE WITH HIGHER AMOUNTS APPROACHING 10INCHES POSSIBLE ACROSS THE CENTRAL FOOTHILLS.

Jason Samenow
February 12, 11:21 am
  • Jason Samenow February 12, 11:21 am

Devastating ice storm expected in Atlanta, parts of South

A paralyzing coating of ice, perhaps up to one inch thick, is forecast for parts of northern and central Georgia and into the Carolinas.

Here’s how National Weather Service in Atlanta is characterizing the threat:

This is a storm of historical proportions with potentially catastrophic…crippling impacts.

In case it hasn’t been made clear already…these totals could result in widespread power outages that may last for days. If residents have not completed their preparations it may be too late.

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Eric Holthaus, at Slate, has more: Today’s the Day Atlanta Could Lose a Quarter of Its Trees

Jason Samenow
February 12, 11:42 am
  • Jason Samenow February 12, 11:42 am

Strong agreement about heavy snow tonight

Our confidence is becoming high that at least 4 inches of snow will fall overnight alone, and perhaps double that in areas.  The RGEM model (regional version of the Canadian model) – like the NAM and to a lesser degree the GFS – simulates very heavy snow overnight – with 0.85″ liquid equivalent precipitation by 7 a.m. – which would equate to over 8″ of snow.

Here’s a screenshot of the model at 5 a.m.:

RGEM model shows very heavy snow over D.C. around 5 a.m. (Environment Canada)

RGEM model shows very heavy snow over D.C. around 5 a.m. (Environment Canada)

Jason Samenow
February 12, 12:01 pm
  • Jason Samenow February 12, 12:01 pm

Snow emergency declared in D.C.

From the Post’s Mike DeBonis:

The District plans to declare a snow emergency at 6:30 p.m. Wednesday, the city’s public works director said.

The declaration, which restricts street parking on major city thoroughfares, will allow crews to more thoroughly plow streets, William O. Howland said.

A snow emergency violation involves a $250 ticket plus towing and storage fees. This will be the first snow emergency declared in the city since 2010, Howland said.

“We’re going to impound every snow emergency vehicle,” he said.

A map of snow emergency routes is available here: http://dcps.dc.gov/DC/DDOT/On+Your+Street/Traffic+Management/View+All/Snow+Emergency+Route+Map

Jason Samenow
February 12, 12:10 pm
  • Jason Samenow February 12, 12:10 pm
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