Threat of wintry mess Sunday grows; ice storm potential western areas

Since Monday, we’ve been reasonably confident the region would experience a wintry mix of precipitation.  As of Tuesday, we said the event in the immediate metro area may just be a conversation starter or nuisance.  But today, the data are hinting at a more significant winter storm, especially for areas west and northwest of the District.

crystal_ballThis does not look to be a major snowstorm, but – due to a healthy supply of cold air as the storm moves in – some accumulating snow does seem possible before changing over to mixed precipitation.

Next accumulating snow chance: Sunday morning
Probability of more than 1″ of snow: 30% (40% western suburbs)
Probability of more than 4″: 5% (15% western suburbs)

Ice may be the bigger story, from along and west of a line from Manassas to Fairfax to Rockville to Columbia (Md.), and particularly along the I-81 corridor from Harrisonburg to Hagerstown.

Storm setup

Arctic air is building in the western and central U.S. On Friday morning, lows in parts of Montana and the Dakota may dip below -20 F. We won’t see anything like that, but – starting Saturday – cold air will ooze into the Mid-Atlantic as an area of arctic high pressure is anchored to our north. Along a front draped over the region, low pressure will develop bringing precipitation to the region starting Sunday morning.

Graphical summary of storm set up (National Weather Service)

Graphical summary of storm set up (National Weather Service)

Overview of how the storm may play out

Precipitation may begin as snow and/or sleet between around 4 and 8 a.m. Sunday morning, with some accumulation possible before a changeover to sleet and freezing rain late morning into the early afternoon.

GFS model shows strong high pressure - feeding cold air into the area - anchored over southern Canada Sunday morning while snow and sleet is moving into the northern Virginia. (WeatherBell.com)

GFS model shows strong high pressure – feeding cold air into the area – anchored over southern Canada Sunday morning while snow and sleet is moving into the northern Virginia. (WeatherBell.com)

Then, as warmer air moves in high altitudes late Sunday morning into early Sunday afternoon, a period of sleet and freezing rain is likely with some ice accumulation possible.

By late afternoon and early evening, precipitation may change to rain close to and east of I-95. But in our far western suburbs, icing may continue for several more hours, with a chance of more significant ice accumulation.

By late at night, even the coldest areas (towards the Shenandoah Valley) should rise above freezing and precipitation will taper off.

“The cold air looks like it will hold on a longer than I was thinking yesterday as the models are holding the [arctic] surface high to our north in longer putting it in a very favorable position for cold air damming [draining south over area] during the day on Sunday,” says Wes Junker, Capital Weather Gang winter weather expert. “This is now definitely a system to monitor as there is potential for a period of snow and for icing problems lingering into Sunday night especially west of the city.”

Regional impacts

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

1) This event will largely focus on a Sunday when there is no rush hour, reducing the impacts on commuting and schools.
2) Sunday morning religious schools/services may be impacted as well as to travel to Sunday NFL homes games for the Redskins and Ravens.
3) Temperatures will be close enough to freezing that treated roads should be ok to travel on. The problem may be untreated roads, bridges, ramps and overpasses.
4) Most precipitation tapers off by Monday morning and temperatures should rise above freezing most places by then.

zones-dec8Zone 1 impacts: Potential for light to moderate accumulations of snow and/or sleet, followed by significant icing. Precipitation may change to rain late Sunday night. Significant ice build-up could pose a power outage risk.

Zone 2 impacts: Potential for brief period of snow/sleet (light accumulations), and some icing, before likely changeover to rain in the late afternoon or evening Sunday.

Zone 3 impacts: Potential for brief wintry mix before likely changeover to rain late Sunday morning or in the afternoon.

Uncertainties

We won’t be able to nail down specifics on exactly how much snow and ice falls in each zone and how disruptive the storm will be until Friday into Saturday. Nor will be able to narrow down the timing of the precipitation and precipitation-type changes.

Those specifics depend on temperature details which are hard to pin down right now. If the storm comes in later in the morning and temperatures are not quite as cold as models are currently simulating and/or precipitation isn’t as heavy, we would see less snow and ice.

On the other hand, models often underestimate the amount of cold air at the surface in these kinds of cold air damming events, which may allow icing to last longer than forecast.

We will have another detailed update tomorrow.

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