Accumulating snow becoming more likely in D.C. area Friday night

Our forecast confidence in exact amounts and where remains only at a medium level because the overall storm track could shift a bit, with the potential for a localized zone of heavier snow of 4 inches or more. But it’s too soon to say whether and where such a zone might be.

Overall, we’d place the odds of at least an inch falling in the area at 70 percent, a 20 percent increase from 24 hours ago.

The storm is a “clipper” system that will come through from west to east. The anticipated track, with its low-pressure center passing to our south, should produce mostly or all snow for most of the region, although it may briefly start as rain or a rain-snow mix around downtown Washington and to the south and east.

The overnight timing means that snow is likely to accumulate on untreated roads, even though temperatures will start out above freezing. However, any travel disruptions should wane by mid- to late Saturday morning.

8 p.m. Friday to midnight Saturday: Snow arrives from west to east, possibly starting as rain or a rain-snow mix around downtown Washington and to the south and east. Temperatures: Mid-30s to near 40.

midnight to 4 a.m. Saturday: Snow could be moderate to heavy at times, accumulating on roads. Temperatures: Upper to 20s to mid-30s.

4 to 7 a.m. Saturday: Snow tapers from west to east. Temperatures: Upper 20s to low 30s.

7 to 10 a.m. Saturday: Improving road conditions. Temperatures: Rising through low and mid-30s.

Note that the higher end of the above temperature ranges will set up between downtown Washington and Southern Maryland. The lower end of the ranges will be north and west of the Beltway, where snow should generally accumulate with greater ease.

• A several-hour stretch of snow seems probable across the region Friday night. Accumulations look to be in the range of a coating to 3 inches. There’s some chance that a localized band of heavier snow produces as much as 3 to 6 inches, but we can’t yet say whether it will materialize or where that might be.
• Temperatures across the northern half of the region, roughly north of Interstate 66, D.C. and Route 50, should be cold enough for most or all of the snow to accumulate. But there’s a chance the snow cuts off earlier in the night in the region if the precipitation veers to the south, as some models suggest.
• Slightly warmer temperatures across the southern half of the region, including the District, may delay how long it takes to drop to near freezing and reduce how much snow accumulates, making the forecast more uncertain there. On the other hand, that region could see the snow last longer. A boom scenario of 2 to 4 inches or so is possible in this zone if a localized band of heavier snow sets up and lowers temperatures.

• Models have been fairly consistent in their predicted storm track, and this type of storm tends to be more predictable than the nor’easter that drove meteorologists crazy earlier this week. But there’s always a chance the track could shift, which would shift the position of the accumulation bands in our forecast map.

“Two big question marks remain: How quickly will temperatures fall to near freezing and, in areas where the temperature only falls to 33 or 34, will the snow fall intensely enough to allow for efficient accumulation,” said Wes Junker, Capital Weather Gang’s winter weather expert. “The other big question mark is whether and where a band of heavier snow might develop. If the development of such a band becomes more evident, then we might need to bump up our forecasts.”

Jason Samenow contributed to this report.