Winter storm Sunday: briefly snow, then ice; major ice storm possible in western areas

December 6, 2013
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A wintry mess is on tap for Sunday and Sunday night with a mix of snow and ice, and then rain. The storm has the potential to produce a significant ice storm for areas west of I-95.

Questions remain concerning how quickly any snow might change to ice and on the timing of the eventual transition from ice to rain.  While there is the potential for a light accumulation of snow Sunday, our primary concern is ice and how long it might last into Sunday night especially north and west of the city.

Snow could be falling on the drive to Fed Ex field with the snow changing to sleet and freezing rain during the game.  Same goes for the Ravens game in Baltimore.

Along the I-81 corridor in western Virginia and western Maryland -from Harrisonburg to Winchester to Hagerstown – a winter storm watch has already been issued for 2-4 inches of snow and over 0.25″ of ice accumulation.  Expect watches to be expanded closer to the metro area by Saturday morning.

UPDATE, 3 p.m.: A winter storm watch has been issued for much of the D.C. and Baltimore metro region Sunday morning to Monday morning for snow and ice. More information here: Winter storm watch issued for much of D.C. area Sunday into Monday

How the storm may play out

The onset of precipitation across the area is most likely between mid-morning and noon with the precipitation probably starting as snow but changing to sleet and or freezing rain by late afternoon (in most spots). 

The snow accumulation forecast is tricky. In this kind of event, accumulations often end up lower than suggested by the models.  They should mostly be in the 1 to 2 inch range with amounts probably being closer to 1 inch around the city and up to 2 inches west and northwest of the beltway. (Caveat: accumulations will heavily depend on the setup of an initial band Sunday afternoon, the location of which is uncertain – it might even set up south of D.C.)


Sometime during the early evening hours any sleet should transition to freezing rain setting the stage for a possible ice storm for areas especially along and west of a line from Manassas to  Fairfax to Rockville to Columbia.  To the east, temperatures are expected to slowly rise to above freezing sometime between around midnight and the pre-dawn hours, though the transition may occur a bit earlier near the Chesapeake Bay.

The figure below provides a rough approximation of the timing of how the precipitation might transition from snow to rain across the area.


Considerable uncertainty continues about how long the freezing rain and icing will linger west of I-95.  The duration temperatures stay below freezing is always tricky in classic cold air damming cases like this one, where cold air funnels southward from high pressure to the north.  Out towards Shenandoah Valley, along I-81 and locations like Martinsburg and Hagerstown, freezing rain and icing will likely continue for much of the night, possibly changing to plain rain by dawn.

Possible storm impacts

 1). Since the precipitation is not expected to start until mid morning, impacts to religious schools/services should be minimized.

2). Snow is likely for the commute to FedEx field (and for the Ravens game in Baltimore) with the snow changing to sleet and freezing rain during the game.  Sleet or freezing rain is likely for the drive home.

3). Because temperatures around the city will be only a degree or two below freezing most treated roads will probably be OK.  However, 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 especially in the colder areas west and north of the city.

4). Ice accumulating on tree limbs and power lines could lead to power disruptions especially in areas west and north of the city but there is even a threat of such disruptions happening inside the Beltway.

5). Icing issues could continue into the early morning rush in the far western and northern suburbs if the cold air holds on a little longer than forecast.

6). During Sunday night into Monday morning fog may develop.

7). Iciness could linger into Monday morning for areas in Zone 1 (below), affecting schools (we’ll debut SchoolCast for this storm tomorrow).

Here’s what we expect to happen, by zone:

 


Zone 1: Light to moderate snow accumulation Sunday afternoon – probably and inch or two – before transitioning to sleet and freezing rain towards evening.  Freezing rain might change to rain before dawn.  Significant icing is a good bet.

Zone 2: Light snow accumulation Sunday afternoon (an inch or so), mixing with and changing to sleet and freezing rain by evening.  The sleet and freezing rain may change to plain rain after midnight and is likely transition to plain rain by dawn. Light to moderate icing possible.

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

(In Zone A: Significant icing is very likely, with high potential for downed tree limbs and power outages.)

Why are we expecting so many precipitation types?

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The high pressure system to our north at 1 p.m Sunday is in a really nice place to facilitate  cold air damming (see 4-panel figure above).  The location of the surface high just to our north keeps low level air funneling south on the east side of the mountains.  Our precipitation on Sunday will come from an arctic front sagged south of our area in low levels, but – at the same time – our flow above the surface will be pumping warm air that will be lifted across that boundary.

Southerly and southwesterly winds will warm the atmosphere to above freezing between 5,000 and 10,000 feet even as the high pressure to the north is supplying cold air Sunday afternoon (see figure to the right). The resulting elevated warm layer helps switch our snow first to sleet and then to eventually to freezing rain.

What makes timing the switch to freezing rain so tough? 

If you look again at the four-panel precipitation type figure (above), you’ll notice that it also depicts the surface pressure pattern.  Note that the surface high that was located in a near perfect position for damming early Sunday afternoon but then slides eastward. By 7 p.m. Sunday it is located in Maine, while a low pressure system has moved to Lower Michigan.

Such a pressure configuration guarantees that there will be an easterly component to the surface winds bringing warmer air off the ocean while also acting to shut off the supply of cold air funneling down from the north.  The closer you are to the mountains the slower the temperatures are to moderate.  Complicating the forecast is the fact that in the past models have sometimes eroded the shallow below freezing layer out towards the mountains too quickly.

The bottom line

A winter storm is likely to impact the area Sunday. The biggest threat from the storm is the possibility of significant icing.  We hope to have a better handle on the timing of the various changeovers as we get closer to the storm.

(Jason Samenow contributed to this post)

RELATED:  Extreme weather of 2013

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Jason Samenow · December 6, 2013