Radar & lightning:Latest D.C. area radar shows movement of precipitation and lightning strikes over past two hours. Refresh page to update. Click here or on image to enlarge. Or see radar bigger on our Weather Wall.

* Winter Weather Advisory for Montgomery, Howard, Frederick, Loudoun and Fauquier counties late tonight into Monday *

9:40 p.m. Update: The National Weather Service has expanded the Winter Weather Advisory to cover Montgomery and Howard counties from 4 a.m. to noon on Monday. NWS says “A PERIOD OF MODERATE TO LOCALLY HEAVY SNOWFALL IS EXPECTED TO FALL LATE TONIGHT AND MONDAY MORNING … ROADS WILL BECOME SNOW COVERED AND SLIPPERY… ESPECIALLY ACROSS ELEVATED SURFACES SUCH AS BRIDGES AND OVERPASSES.”

As mentioned below, we agree roads *could* become briefly snow covered should snow fall heavy enough – about a 50/50 chance in the advisory area.  Outside of the advisory area, while we can’t rule out accumulating snow, odds are less.

From 8:33 p.m.: Areas of light snow and flurries have overspread many parts of the Washington, D.C. metro region, but the time period when the steadiest snow may fall is between around 3 and 9 a.m. 4-10 a.m. (time frame: updated at 10 p.m.).  It’s *possible* snow could fall at a moderate to heavy clip in this window – especially north and west of the District, impacting travel and the Monday morning commute.  Be prepared for *possible* delays in the morning and definitely check conditions before heading out.

Link: Wintry slop moves in this evening and overnight; light accumulations possible

The storm system moving through the region is playing out in two rounds.  The first round is producing what we’re seeing out there now…which is mainly just light snow, mixed with rain in a few spots.  This activity may whiten the ground and produce some slushy accumulations, especially south of the District where it is (and has been) steadier/heavier.  In fact, well to the south, very heavy snow moved through Richmond and Williamsburg late this afternoon and early this evening, producing a quick 3 to 5 inches.  We will not see heavy snow like this during the first round as it has moved off to the east.  In fact, the light snow in the region right now should only continue for a few more hours before a let up in the action.

We are a bit concerned about the potential for accumulating snow in the pre-dawn hours through the morning commute – a so-called round 2.  The experience in Richmond from earlier this evening demonstrates if snow falls heavily enough, it can and will accumulate – temperatures don’t have to be below freezing (in Richmond, accumulating snow fell between 33 and 35 degrees).  While we may not see snow as heavy as Richmond, the snow will be falling at a time when temperatures will be coldest and the March sun angle won’t be a factor.  In other words, if it snows at a moderate to heavy clip, the snow will stick.

Based on model guidance, radar and experience, we favor the steadiest, heaviest snow generally north of the District during round 2 but northern Virginia could get in on some of the action.  We don’t know exactly where heavier and steadier snow bands will set up, but if/where they do, amounts could exceed our forecasts (hence the BOOM potential in the accumulation map).  Locations that miss out on steady snow bands will see little or no accumulation (because temperatures are not cold enough for snow to stick unless it snows somewhat heavily), hence the bust potential.

CWG's snow accumulation forecast for late Sunday through Monday afternoon. "Boom" indicates probability of more snow than predicted. "Bust" indicates probability of less snow than predicted.
CWG’s snow accumulation forecast for late Sunday through Monday afternoon. “Boom” indicates probability of more snow than predicted. “Bust” indicates probability of less snow than predicted.

It’s not even out of the question someone in the D.C. area could see  a quick 4 inches or more (10 percent likelihood). The National Weather Service’s Weather Prediction Center even says there’s a 40 percent chance of at least 4 inches in the area; we think this is overdone, but understand the rationale.

National Weather Service
National Weather Service

FedCast: (we have upgraded this from 1 dome in earlier update)

Stay tuned for additional updates as needed this evening, and we’ll have detailed forecast information tomorrow starting around 5 a.m.