A wintry mess is  likely in the Washington, D.C. metro area Sunday and Sunday night, with the risk of a significant ice storm in western areas.

Since yesterday, the models have trended somewhat colder with the storm, which increases the chances the region sees a little accumulating snow and also the amount of time we contend with icing.  We remain concerned about significant ice accumulation Sunday evening in our western areas, along a line from Manassas to Fairfax to Rockville to Columbia and points west.

This does not look like a big snowstorm, but odds of enough snow to sweep or shovel have increased slightly since yesterday.

“The cold air damming [draining of arctic air from the north into the region] continues to look impressive and now it looks like the D.C. area has a chance of seeing its first inch of snow sometime on Sunday,” says Wes Junker, Capital Weather Gang’s winter weather expert.

Next accumulating snow chance: Sunday late morning into afternoon
Probability of more than 1″ of snow: 45% (higher in western suburbs)
Probability of more than 4″: 10% (higher in western suburbs)

(Note: these probabilities could increase or decrease. We are erring on the side of being conservative with them since the storm is still about 3 days away)

The other big change in the forecast since yesterday is a delay in the onset of the precipitation.  Models are now suggesting precipitation begins around late morning Sunday rather than early Sunday morning.  I should stress the nailing down the exact timing remains tricky and could still change (earlier or later for precipitation onset).

“There continues to be considerable uncertainty about how quickly the heavier precipitation might arrive and uncertainty in how long surface temperatures might stay below freezing,” Junker adds.

How the storm may play out

Our best bet now is that precipitation begins between mid-morning and noon Sunday, probably as snow and/or sleet.  Snow and/or sleet is likely in the afternoon, mixing with and changing to mostly sleet, from southeast to northwest.  Snow accumulations should be light, but 1″ or so is certainly possible in the region, with somewhat higher amounts possible in the west and northwest suburbs.

By Sunday evening, snow will have changed to freezing rain and sleet in most areas.  A nasty wintry of mix of precipitation is likely through at least 8 or 9 p.m., but may gradually change to plain rain in eastern areas between 8 p.m. and midnight. Near and west of I-95, frozen precipitation may continue through midnight or even later.

After midnight, there is considerable uncertainty about precipitation type near and west of I-95.  It may remain icy or change to plain rain.  We will try to get a better handle on this in future forecasts.  For areas along a line from Manassas to Fairfax to Rockville to Columbia and to the west, icing stands a better chance of continuing through a good portion of the night before a possible changeover to rain.  Out towards I-81, from Harrisonburg to Hagerstown, freezing rain and icing likely continue for much of the night, possibly changing to plain rain by dawn.

GFS model shows moderate precipitation in the region through 1 a.m. Monday morning, with the freezing line (blue line labeled 32) in DC’s western suburbs. (StormVistaWxModels.com)

During the day Monday, everyone rises above freezing, with lots of slush and melting, and some additional light rain.

Important note/qualifier: Cold air often hangs in longer than simulated in models in this kind of storm set up.  We cannot rule out the possibility that the changeover from ice to rain takes until during the day Monday in a worst case scenario, even fairly close to the District.

 Storm impacts by zone

Here’s a general idea of how we see the storm playing out by zone. But, first, several things to keep in mind:

1) The delayed onset of precipitation may result in fewer impacts for morning religious services/schools.

2) Snow and/or sleet now looks like a decent bet for Redskins and Ravens home games (unless the onset time is delayed further).

3) Temperatures will be close enough to freezing that treated roads should be ok to travel on, but slick spots and reduced visibility are possible during periods of heavier snow/sleet Sunday afternoon and evening. The most hazardous areas will be untreated roads, bridges, ramps and overpasses.

4) As the colder air is now expected to last longer, inclement conditions could linger into Monday morning’s rush hour, especially in western areas.

5) Ice build-up on trees and powerlines Sunday night could light to power outages, especially in western areas (Zone 1, below).

Zone 1: Light to moderate snow accumulation Sunday afternoon into the evening, with sleet and freezing rain overnight Sunday, possibly changing to rain before dawn.  Significant icing possible.

Zone 2: Light snow accumulation Sunday afternoon, mixing with and changing to slight and freezing rain by evening.  Sleet and freezing rain possibly changes to plain rain after midnight and likely to plain rain by dawn. Light to moderate icing possible.

Zone 3: Brief period of snow Sunday late Sunday morning and early afternoon, mixing with and changing to sleet and freezing rain.  Sleet and freezing rain likely changes to plain rain mid-evening Sunday. Light icing possible.

(In region (A), west of zone 1, moderate snow and significant icing is likely; in  region (B), east of zone 3,  a brief wintry mix is possible, but mostly rain is expected)

As we are still three days away from the storm, and there is considerable uncertainty in the details as to how this plays out, consider this information our best bet overview which will be refined in the next two days leading up to the storm.