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Odds of D.C.-area snow accumulation by Friday morning are increasing

* Winter weather advisory in effect from 4 a.m. to 7 p.m. Friday *

Our confidence in accumulating snow and snow-covered roads across most of the region by early Friday morning has grown since we issued our first-call forecast Wednesday. While not set in stone just yet, disruptions to the Friday morning commute, as well as delays and cancellations, are becoming more likely.

How much snow is a trickier question, and depends on where you are in the region. We think 1 to 2 inches is the most likely accumulation, but a little more is possible the farther north you are, and a little less the farther south you are. It’s unlikely to be more than 4 inches in the region.

Because the Washington region is toward the southern edge of the accumulating snow, small changes in model predictions could require us to tweak our snowfall forecast upward or downward.

On Capital Weather Gang’s winter storm impact scale, this rates at a Category 2 out 5 for the Beltway area and to the north — meaning it’s a disruptive but not major storm that will cause slick roads and affect schools, businesses and flights. Although the amount of snow predicted is not particularly high, its impact will be heightened because of its timing during the Friday morning commute and the cold air in place before and after the storm.

Whatever falls will stick around this weekend, with Saturday highs only in the 20s and Sunday highs struggling to reach freezing. But the snow will steadily melt away by the middle and latter part of next week when a major warm-up is expected to send highs into the 40s, 50s and possibly higher.

10 p.m. Thursday to 2 a.m. Friday: Possible flurries or a snow shower. Little or no accumulation. Temperatures: 28 to 35 degrees (35 to 40 in Southern Maryland).

2 to 5 a.m. Friday: Areas of light snow develop. Dusting possible. Rain could mix with snow in Southern Maryland. Temperatures: Upper 20s to low 30s (mid- to upper 30s in Southern Maryland).

5 to 8 a.m. Friday: Light to moderate snow, accumulating around 1 inch. Snow-covered roads likely. Temperatures: Upper 20s and low 30s (low to mid-30s in Southern Maryland).

8 to 11 a.m. Friday: Snow could become lighter or intermittent, especially south of the Beltway. Accumulations of around a half-inch possible. Snowy or slushy roads likely. Temperatures: Upper 20s and low 30s (low to mid-30s in Southern Maryland).

11 a.m. to 2 p.m. Friday: Periods of light to moderate snow with the heaviest north and east of the Beltway, accumulation around 0.5 to 1 inch. Snow-covered side roads likely; some treated roads may be just wet or slushy. Temperatures: Low 30s (low to mid-30s in Southern Maryland).

2 to 5 p.m. Friday: Light snow gradually tapers off from southwest to northeast. A dusting to a half-inch possible. Temperatures: Low 30s (mid-30s in Southern Maryland).

Most models now show enough snow, and cold enough temperatures, to whiten the ground by around 5 to 7 a.m. Friday across most of the region. That makes us more confident that we’ll see snow-covered roads during the morning commute — except toward Southern Maryland, where lighter snow amounts and higher temperatures may lessen the impact.

The amount of total snow accumulation expected is less certain. Models increased their snow amounts Thursday after having decreased them Wednesday. Such fluctuations aren’t unusual, and the models may wobble back and forth again. We’ll tweak our forecast if necessary.

The biggest wild card is how much drier air pushing in from the southwest might lighten the snow, or even cut it off for a time late Friday morning. In general, the farther north you are the better chance for steadier, heavier snow and higher amounts, and thus higher forecast confidence. The farther south you are, the better chance the snow could be lighter or intermittent at times, and thus lower forecast confidence.

County-by-county forecasts

The District, Alexandria and Arlington, Fairfax, Prince George’s, Prince William, Fauquier and Anne Arundel counties: Snow should develop around 2 to 4 a.m., and untreated roads are likely to be snow-covered by 6 to 8 a.m. Snow could become lighter or intermittent between 8 and 11 a.m. The snow may pick up again around 11 a.m. to 2 p.m., but that’s less certain than it is for areas farther north. Temperatures in the upper 20s to near 30 throughout the event.

Montgomery, Loudoun, Howard, Frederick and Carroll counties: Snow should develop around 2 to 3 a.m., and roads are likely to be snow-covered by 5 or 6 a.m. The snow could become lighter between 8 and 11 a.m., especially in southern portions of the counties, but will probably pick up again around 11 a.m. to 2 p.m. before gradually tapering, thus the higher snow totals and higher forecast confidence. Temperatures in the upper 20s to near 30 throughout.

Charles, Stafford, Calvert, St. Mary’s and King George counties: Snow should develop around 2 to 4 a.m., perhaps mixing with rain at the onset. Side roads and sidewalks could be snow-covered by 6 to 8 a.m. Treated main roads may only be slushy since temperatures will be a little warmer, in the low to mid-30s, compared with areas farther north. Snow will probably become lighter or intermittent between 8 and 11 a.m. It may pick up again around 11 a.m. to 2 p.m., but that’s less certain than it is for areas farther north.

Capital Weather Gang’s winter weather expert Wes Junker describes the storm as “a moisture-starved, clipper-type system,” which limits how much snow it can produce.

“Clippers generally produce a stripe of no more than around 1 to 3 inches of snow to the north of the system’s low-pressure center, unless a second area of low pressure develops near enough to the coast to tap Atlantic moisture, which seems like a long shot,” Junker said. “Small differences in the track can determine whether you end up with 2 or 3 inches or only a dusting to an inch. We’re likely to be near the southern edge of the more solid snow shield, which is always a dicey place to be near.”

Jason Samenow contributed to this report.

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