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** Winter storm warning through Thursday afternoon for immediate D.C. area, then winter weather advisory 5 p.m. tonight to 1 a.m. Friday morning | Winter storm warning through 5 a.m. Friday north and west of the immediate metro area | Winter weather advisory through noon southern Maryland, then again 5 p.m. tonight through 1 a.m. Friday morning **

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Key points:

Another area of snow, possibly beginning as rain or mixed precipitation in the city and points east, is likely to develop by late afternoon into the early evening. It could start as early as 2 or 3 p.m. and should move out between 8 and 10 p.m. from west to east.

The most likely time for steady snow in the region is between around 5 and 8 p.m.

Additional snow accumulations of 1-4 inches or so are possible, with 2-4″ most likely west of town, lesser amounts likely elsewhere

Yesterday I discussed how a band of precipitation will sometimes form to the north of a storm’s upper level disturbance (or low pressure center) and well west of its surface low pressure center. These bands sometimes produce a second shot of accumulating precipitation, which sometimes falls intensely.

Today’s big forecast problem is attempting to get a handle on whether, where and when such a band might impact various parts of the area. The models are in strong agreement that a band of precipitation will redevelop to the west of the city this afternoon and will then press eastward across the region.

The models continue to predict that the “wrap around” precipitation will swing across the region later this afternoon and evening. On the 11:45 a.m. radar image below you can see how the initial band of heavier precipitation has mostly lifted well to the north and west of Washington, leaving most of us dry or with a light drizzle or sleet. The models really did good job of picking up on the dry slot now across the area.



Radar view at 11:45 a.m. (National Weather Service)

The image also shows the seeds of our next bout of winter weather that is expected later this afternoon and evening. Note all the echoes across West Virginia and southwest Virginia. That precipitation will be pressing across the region this afternoon.

This morning’s NAM model simulated radar image still has the dry slot over us early this afternoon (below left) with only light mixed precipitation west of town and splotchy light rain east. By 7 p.m., a more solid band of precipitation has spread back into D.C. and, according to the model, a decent band of snow develops west of the city. The band should exit the western suburbs by 9 or 10 tonight but could linger near the Chesapeake Bay to around midnight.

Snowfall amounts with these bands are tricky. They’re especially tricky today because temperatures have risen to the mid-to-upper 30s in areas east of the city. Today’s GFS and NAM models are both forecasting another 0.20 to 0.30″ of liquid equivalent across the region from this area of precipitation – which would convert to 2 to 3 inches of snow (if temperatures are sufficiently cold). The RAP and last night’s European model were even a little heavier. All have temperature profiles which suggest the precipitation will change back to snow.

The big fly in the ointment is the surface temperatures which are likely to be above freezing in the city and points east – this could limit accumulations. Right now we’re calling for 2-4 inches over the colder locations west and north of the city and maybe a slushy inch or so elsewhere. This is a dynamic, energetic system moving through, so surprises are possible.