Saturday’s storm: more rainy than snowy in D.C.; some ice and snow west and north

* Winter Weather Advisory Saturday for Montgomery, Loudoun, Howard, and northern Fauquier counties, mainly elevated areas *

* Winter Storm Warning Saturday for Frederick and Carroll counties *

advisorieseUpdate: 2:45 p.m.: The National Weather Service has issued winter weather advisories for Montgomery and Howard counties Saturday, for 1-3 inches of snow and a possible glaze of ice at elevations above 500 feet, and less than one inch of snow elsewhere.  A similar winter weather advisory is in effect for Loudoun and northern Fauquier counties, but for most impacts above 1,500 feet in elevation. A winter storm warning is out for Frederick and Carroll counties for 4 to 8 inches of snow and sleet (we think that may be overdone except at high elevations, see our map below).

From 2:20 p.m.: Saturday looks more wet than snowy for most of the D.C. metro area, with little or no snow accumulation. There are two exceptions:

(1) from western Loudoun County into northern Fauquier county, a glaze of ice is likely due to subfreezing temperatures (following a possible brief period of snow, generally 1″ or less).
(2) along a line from near Leesburg, Virginia to Clarksburg, Maryland in northern Montgomery county to Eldersburg, Md in Carroll County, around 1 inch of snow is possible, possibly up to 2 inches or at higher elevations in this zone; from Frederick to Manchester in Maryland to the north and west towards the Mason Dixon line, 2-4 inches is possible (with a bit more possible at high elevations). Winter storm warnings are out for these locations.

snow_chances_12-14-13-2

Disruptive winter weather is not expected in the immediate D.C. area. Temperatures tonight are expected to dip near freezing but rise above freezing during the day Saturday. When precipitation arrives around late morning – perhaps as light snow – we expect above freezing temperatures.  By early to mid afternoon, any snow should gradually turn over to rain, with temperatures reaching the mid-to upper 30s. Snow should not accumulate on roads.

In the far northern suburbs, there could be some light snow accumulation and/or icing. Areas north and west of a line from Leesburg to Eldersburg (Md.) – including northern Montgomery County, need to watch their temperatures closely tonight and tomorrow as they have more winter weather potential than those of us living farther to the east. These areas could see an inch or two of snow, especially at higher elevations, and then frozen mixed precipitation.

In the far western and southwest suburbs, southwest of Leesburg, including the Shenandoah Valley, a brief period of snow is possible before a light mix of frozen precipitation which could leave a glaze.

In all areas, precipitation winds down from west to east between 10 p.m. and 2 a.m.

Storm evolution and model forecasts

A low pressure system is expected to track to Tennessee and Kentucky before a new low starts developing over North Carolina. There is a surface high in pretty good spot to supply cold air into the region but the air mass associated with it just isn’t that cold.

The system has a Miller B type evolution with the primary low tracking into Ohio and then filling (or dying out) as the new low takes over along the Atlantic coast. Often with such storms, the initial push of steady precipitation is to our north and west. The primary low tracking to Ohio also allows just enough easterly component to the low level winds to allow warmth off the Atlantic to keep our temperatures above freezing during the bulk of the precipitation. We typically get a quick shot of moderate precipitation before a dry slot develops as our winds turn westerly while the low really starts to deepen off the coast just to our north.

Today’s GFS and NAM models predict such an evolution of the storm. At 1 p.m. Saturday an area of steady snow is located to our north near the Mason Dixon line (and north) while only light precipitation spreads into the area. By evening, when the steadier precipitation moves in, warm surface temperatures lead to rain. By 1 a.m., the storm deepens to our north and the dry slot starts showing up in western Virginia. The low develops just far enough to the north to keep any wrap around precipitation from hitting us as the cold air starts feeding back towards the area. For us to get a decent snow from a Miller B event, we would need the upper level center to pass to our south and that is not in the cards.

(WeatherBell.com)

(WeatherBell.com)

The models are pretty unanimous about that general solution. Because of the Miller B evolution, all the operational models and the Short Range Ensemble members (see below) predict that surface temperatures will be well above freezing during the bulk of the precipitation. The NAM model is on the lower end of the range of the various ensemble members but it forecasts temperature rises to 37 by 1 p.m. Saturday afternoon, ditto for this morning’s GFS.

SREF ensemble members (simulations of the model) show temperatures above freezing for the duration of the storm Saturday in Washington, D.C.

SREF ensemble members (simulations of the model) show temperatures above freezing for the duration of the storm Saturday in Washington, D.C.

The bottom line

The event looks more like a rain storm than anything else for all but our far northern and western suburbs. The precipitation could start as snow but the snow doesn’t stick and should quickly yield to rain. However, the far western areas towards Purcellville and out to Winchester could have the snow change to freezing rain with the freezing rain holding on through much of the day, possibly even into the night over colder valley locations. Far northern suburbs north of Frederick and up towards the Pennyslvania state line could see a few inches of snow. Those are the areas of concern for winter weather, but good news for ski areas (Liberty, Whitetail, Roundtop!).

(Jason Samenow contributed to this post)

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