Major Band of Activity Headed Into DC Area

A line of strong radar returns is moving through the Fredericksburg area and heading north towards us.  Reports with this line have included heavy rain changing to sleet and then to snow fairly rapidly.

This is an unusual situation where you'll probably be able to watch as the thermal profiles in the atmosphere change and support frozen precipitation.

To add to the fun, there are numerous lightning strikes associated with the line.

Shouldn't be long now before the activity starts around here.

Current Radar

You can see the precipitation in western Virginia and developing now to our south.  The low pressure center is clearly defined in the swirl that, at this moment, is located just along the border of Virginia and North Carolina.  I'm expecting snow to start falling - perhaps initially as a bit of rain which should rapidly change to snow - within the hour.

Models Continue to Show More Snow

The short-range models and radar trends continue to advocate more snow for our area and an unusually high probability of convection - thundersnow - this afternoon.

5 inches or more seems likely at this point with higher amounts out to the west.  Portions of Fairfax and Loudoun counties could see almost 10 inches.

The Winter Storm Warning has now been extended for all but the southern portion of Maryland.  The second round of precipitation is moving into the Charlottesville, VA area now.  The center of the upper-level energy seems to be in western North Carolina.

I worry that the worst of this will be right around rush hour and if you're reading this and you're in the DC/Baltimore Metro area (especially if you're to the north and west of the cities), you might consider leaving early.  The Federal Government just announced that they will close 2 hours early.

From the National Weather Service:

...RAPIDLY DETERIORATING CONDITIONS FOR THE MID TO LATE AFTERNOON
AND EVENING RUSH HOUR COMMUTE...

AFTER A BREAK IN PRECIPITATION THIS MORNING...ACCUMULATING SNOW
WILL OVERSPREAD THE AREA THIS AFTERNOON AND EVENING. A MIX OF RAIN
AND SNOW WILL QUICKLY CHANGE TO ALL SNOW DURING THE AFTERNOON AS
IT BECOMES HEAVY. THE PERIOD OF HEAVY SNOW WILL COINCIDE WITH THE
LATE AFTERNOON AND EVENING RUSH HOUR COMMUTE ACROSS MOST OF THE
REGION...INCLUDING THE WASHINGTON DC AND BALTIMORE METROPOLITAN
AREAS. CONDITIONS IN SOUTHERN MARYLAND WILL DETERIORATE AROUND
SUNSET AS PRECIPITATION CHANGES TO SNOW.

THE HEAVY SNOW WILL REDUCE VISIBILITIES BELOW A QUARTER MILE WITH
SNOWFALL ACCUMULATION RATES AT TIMES OF 2 INCHES PER HOUR. THIS
WILL PRODUCE DANGEROUS TRAVEL CONDITIONS DURING RUSH HOUR. SNOW
WILL MOVE OUT OF THE AREA BY MIDNIGHT.

12Z Models Coming In

If anything, the NAM is snowier for us.  Need to see the rest of them but the NAM really lays down some moisture over our region tonight.  This is down to what we call "nowcasting" which is observing the trends that are happening now as a basis for prediction rather than putting so much stock in the models.

Still - look at the NAM total precipitation for the storm.  It's got us in a mini-bullseye - almost 1.25 inches liquid equivalent.  All snow and a 10:1 ratio would give us over 12 inches but with the ratios and losing some already this morning to rain and slush, it's less than that.

But how much less?  Let's see the trends once the snow starts falling.

Forecast Pretty Much on Track - Rain Coming, Then Snow

So after a night when the data showed some uncertainty to final snow totals for today, it seems that the general idea of the forecast is holding pretty well.

This morning, many of us have a coating of slushy snow on the ground thanks to an early-morning burst that has changed in some spots to sleet and in most spots to rain or drizzle.  Expect this to continue for a while - probably with a total changeover to rain before the atmosphere begins to cool.

By the afternoon the reverse of the morning change begins.  Rain and sleet will change to snow from west to east and should start falling with intensity - especially as the afternoon goes on.  By rush hour, travel conditions could be pretty bad both with heavy snow falling and accumulations.

With the thermal profiles in the atmosphere and a changeover-based forecast, location is going to be everything in this storm.  As an example: the National Weather Service has Washington, DC under a Winter Storm Warning for 3-6 inches of snow but has Arlington County in a Winter Weather Advisory for 2-4 inches of snow.

But I expect some of these totals could change as the day goes on - especially once the snow starts seriously falling later this afternoon.

As far as timing:  it looks like the changeover will start during the mid-afternoon hours - I'd guess around 2 PM.  The main thrust of the storm should hit our area from around that hour until about 8 or 9 tonight.

2 PM Satellite Shot (GOES-13)

Shows our storm gathering strength along the Gulf coast and strong to severe thunderstorms moving through Florida:

WSW Extended to all Metro, Atmospheric Profiles Suggest Thundersnow Possible

Reviewing some of the data from this morning/early afternoon.  Because of the thermal profiles of the atmosphere and the amount of energy available with the storm and especially because the main impetus for our snow will be the upper-level energy which is what we call "cold-core" - there's a chance that we could see some convection tomorrow night and the exciting phenomenon of thundersnow.

The guidance (especially the NAM) is also trending earlier with the rain to snow changeover: more like mid-afternoon than "after sunset".

And the National Weather Service has extended the Winter Storm Warning to cover the bulk of the DC Metro area.