Season’s first winter storm Thursday will bring slop of sleet, snow and rain. Some slick travel possible.

Season’s first winter storm Thursday will bring slop of sleet, snow and rain. Some slick travel possible.

NAM model forecast radar at 10 a.m. Thursday. (

4:05 p.m. – Winter weather advisory issued for D.C. area; winter storm warning toward Interstate 81

The National Weather Service has issued a winter weather advisory for the District and surrounding area from 4 a.m. to 1 p.m. Thursday. It is forecasting up to an inch of sleet and snow and the possibility of a light (0.1 inches) glaze of ice.

This advisory extends until 4 p.m. in our far north and west suburbs, including northwest Montgomery, northern Fauquier, and Loudoun counties, where frozen precipitation is likely to last longer due to colder temperatures. In these areas, there is higher potential for an icy glaze (up to 0.2 inches) on top of the snow and sleet. In Frederick County, Md. and to its east across northern Maryland, the Weather Service forecasts one to two inches of snow and sleet accumulation.

Advisories and warnings in effect on Thursday. (National Weather Service)

Even farther west, along the Interstate 81 corridor from Harrisonburg, Va. to Hagerstown, Md. and to the west, a winter storm warning is in effect from 4 a.m. Thursday to 4 a.m. Friday for up to 2 to 5 inches of snow and sleet and an icy glaze up to 0.25 inches thick.

Advisories are not in effect in Southern Maryland, where mostly rain is predicted.

From earlier…

An unusually cold November storm brought snow and sleet as far south as Houston on Tuesday and West Monroe, La., on Wednesday. On Thursday, the storm will arrive in the Washington region, probably offering a smorgasbord of precipitation types: snow, sleet, rain and freezing rain. Some of this precipitation will fall heavily.

The period of greatest concern is between about 7 a.m. and 11 a.m., coinciding with the morning commute, when temperatures will be lowest and the chance of frozen precipitation highest. This is your typical Washington winter weather event in which conditions will tend to deteriorate as you head into the colder areas north and west of the Beltway, where the chance of snow and sleet accumulation and slick spots is greatest.

Precipitation may begin briefly as light rain in the predawn hours before changing to snow and/or sleet by dawn. Any snow is likely to be short-lived, as temperatures at high altitudes will be increasing, but sleet and pockets of freezing rain may continue through the morning. However, in locations south and east of the Beltway, any frozen precipitation should change to plain rain as the morning wears on.

During the late morning and afternoon, precipitation from the Beltway and to the northwest should gradually transition to plain rain. Frozen precipitation will last longest in our far northern and western areas, perhaps well into the afternoon west and north of Warrenton, Leesburg and Frederick. Some of these areas are under a winter storm watch because of the potential for accumulating snow and sleet, as well as a glaze of freezing rain on top.

Forecast radar from NAM model early Thursday through Friday morning.

Inside the Beltway and to the south and east, air and road temperatures should be high enough so that frozen precipitation that falls, mostly in the morning, melts. However, a few slick spots cannot be ruled out during any heavier bursts of frozen precipitation — which are possible during the morning commute Thursday.

Farther to the north and west, the possibility that sleet and snow coats roads increases, especially those that are untreated, as well as bridges, ramps and overpasses.

Precipitation will be heaviest across the region between about 9 a.m. and 3 p.m., but areas of light rain and drizzle (possibly freezing rain and drizzle in our coldest locations) will linger through Thursday night (rain may mix with or change to wet snow briefly before ending in our northern areas). The equivalent of at least one inch of precipitation (from melted snow and ice, and rain) should fall in most areas. If Washington receives this much precipitation, it will move into the fourth spot among the wettest calendar years on record.

This is an event where just a few degrees will make a big difference in how much snow and wintry mix occurs and where, so surprises are possible if not likely. The setup makes it “tricky to tell how long the below freezing surface temperatures will hold across the area north and west of the Beltway,” said Wes Junker, the Capital Weather Gang’s winter weather expert.

This type of winter weather in Washington so early in the season is uncommon. If accumulating snow or sleet manages to materialize in Washington, it will mark the first time in November since 1996.

Forecasts by zone

As conditions will vary widely over the region, from mostly rain in Southern Maryland to substantial amounts of frozen precipitation toward the Interstate 81 corridor, we’ve divided the region into four zones, shown on the map below.

(The Washington Post Staff/Washington, D.C.)

Here is how we see the event evolving, zone by zone, and the expected impact.

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Zone 1: Northern Maryland, eastern panhandle of West Virginia and northwest Virginia

(Includes Winchester, Hagerstown, Frederick and Westminster)

Moderate accumulations of snow and sleet (one to three inches) and a glaze of freezing rain are possible in this region, as temperatures will be lowest here, and precipitation is likely to remain in a frozen form for a good portion of the event.

Precipitation should begin as snow and sleet early Thursday morning, and then transition to more sleet and freezing rain as the morning wears on. Some areas could change to plain rain during the afternoon, but pockets of freezing rain may linger even into the evening in some of the colder valleys in the western parts of this zone.

Forecast temperatures between Thursday morning and Thursday night from high-resolution NAM model.

“The high-resolution models suggest that the temperatures at the surface could stay below freezing for an extended period of time in our typical colder locations north and west of the city,” Junker said.

A winter storm watch is in effect in the western part of this zone, along and west of the Interstate 81 corridor, where temperatures will be lowest. Because of freezing rain, a glaze of up to 0.25 inches could build up on trees and power lines, leading to outages. Untreated roads, driveways, parking lots and sidewalks could turn hazardous.

Ice accumulation forecast on Thursday. (National Weather Service)

The expected conditions rate as a Level 2 out of 5, or “disruptive” winter weather event on our winter storm impact scale. The effects will be magnified by the fact that the wintry mix will coincide with the morning commute and that this is the first winter storm of the season — meaning people have less recent experience dealing with such conditions.

(Washington Post)

Zone 2: Washington’s and Baltimore’s north and west suburbs

(Includes Warrenton, Leesburg, Rockville, Columbia and Towson)

Mixed precipitation should begin just before dawn and snow and/or sleet is likely to put down a coating to an inch through midmorning or so. More sleet than snow is likely to fall. Road surfaces, especially those that are untreated, could turn slick.

Some areas may change to plain rain by late morning, and most areas should do so during the afternoon, but there may be a few colder spots (sheltered valleys) where temperatures hardly budge — which would allow for a slick glaze of freezing rain to develop.

The freezing rain risk may be mitigated by the fact that precipitation will fall heavily at times and make it hard for ice to adhere to surfaces such as trees and power lines. Also, temperatures in this zone may hover right around freezing (between 30 and 34 degrees) — and would need to be lower for a more serious ice buildup.

On our winter storm impact scale, this event is Level 1 — a nuisance winter storm (note that in our assessment this was close to Level 2, so it is a high-end nuisance event). Although widespread hazardous conditions are unlikely, pockets of slick travel are a good bet, and some school delays and cancellations cannot be ruled out.

(Washington Post)

Zone 3: The immediate Washington area and city of Baltimore

(Includes Gainesville, Manassas, Fairfax, Bethesda, Laurel and Annapolis)

Precipitation may begin as light rain predawn but transition to a wintry mix of sleet and snow around sunrise through the midmorning hours. Then the wintry mix may transition to rain. However, it’s possible that pockets of sleet and freezing rain linger into the early afternoon in some of the colder areas.

Temperatures in this zone will be near if not above freezing, generally limiting accumulation. However, slick spots could form during any heavy bursts of frozen precipitation.

Even if icy travel does not materialize, roads will be wet, and visibility will be reduced because of the messy mix of falling precipitation — so allow extra time to reach your destination.

Storm conditions do not merit a rating on our impact scale in this zone. We generally do not expect delays and cancellations in this zone, unless conditions turn out to be worse than expected.

“For snow lovers, the combination of being November and the upper-level storm track being a tad north of where you want it pretty much killed our chances for any meaningful snow around the city,” Junker said.

Zone 4: Far southern suburbs and Southern Maryland

(Includes Fredericksburg and La Plata)

Expect mostly a cold rain in this zone. A little sleet or wet snowflakes could mix with the rain during the early morning, but roads should mostly remain just wet.