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Snow likely Saturday with some accumulation, winter weather advisory in effect

Winter weather advisory for much of area, from midnight tonight until 4 p.m. Saturday | Winter storm warning for Southern Maryland *

(This post was updated at 4 p.m. to refresh forecast based on updated information and add winter storm warning for Southern Maryland)

A developing storm system across the South today will scoot northeast to a position off the Mid-Atlantic coast Saturday, throwing back snow over the D.C. region, for the first time this season.

See how this early winter storm coated parts of Texas, Deep South in a rare snow

This will be a wet, slushy snow as temperature hover near and slightly above freezing. We expect the bulk of the snow to occur between early Saturday morning and Saturday afternoon, when untreated roads could become slushy and slick. But some snow could occur somewhat before and after that (see more on timing below).

For snow amounts, 1 to 3 inches seems most likely in the immediate D.C. area, with totals up to 3 to 6 inches possible in Southern Maryland and near the Chesapeake Bay, where a winter storm warning was issued Friday afternoon.

Small changes in the position of the storm off the coast Saturday could result in significant changes to amounts. If the storm is closer to the coast, the immediate D.C. area could see totals closer to four inches, but if it drifts more out to sea, less accumulation would occur.

The latest modeling has tended to signal higher end snow forecasts are more likely than lower end and, in particular, has upped amounts in our western areas – which previously looked like they may not get much.

Capital Weather Gang's Jason Samenow tells us when the bulk of the snow is expected to fall in the D.C. area and how much to expect. (Video: Claritza Jimenez, Jason Samenow/The Washington Post)

Exact temperatures will also affect how much snow falls. While it has turned colder in recent days, the ground is not particularly cold, so any initial snow will melt. Temperatures will also be above freezing at times, especially when the snow first begins, and at times during the day on Saturday; this will cut back on how much snow sticks. However, where and when the snow falls steadily, we do expect temperatures to fall to 30-32 degrees, so there should be some accumulation.

Furthermore, the sun angle is near its lowest point in the calendar year, aiding accumulation prospects during the day Saturday.

Storm timing

For most of the region, the period we’re most concerned about is between about 5 a.m. and noon Saturday, when the steadiest, accumulating snow is most likely. Locally heavy snow could even briefly occur. Here’s how we see the event evolving:

  • Friday afternoon (noon to 6 p.m.): Light snow or sprinkles possible, mainly in our southern areas. No accumulation. Temperatures near 40.
  • Friday evening (6 p.m. to midnight): Steady light snow develops in Southern Maryland to around Fredericksburg — coating possible, mainly on grassy areas. Flurries possible elsewhere, especially southern areas. Temperatures falling into 30s.
  • Saturday predawn (midnight to 6 a.m.): Snow increases in Southern Maryland with a couple inches possible. Snow develops from southeast to northwest elsewhere — maybe a coating or so by dawn. Temperatures fall to near freezing.
  • Saturday morning (6 a.m. to noon): Snow, possibly moderate to heavy at times along and east of Interstate 95 with accumulation. Light snow and flurries west and north of Leesburg to Frederick. Temperatures 30 to 34.
  • Saturday afternoon (noon to 6 p.m.): Snow gradually tapers off from west to east. Some light additional accumulation possible. Temperatures 31 to 35.
  • Saturday evening: A few lingering flurries possible. Temperatures 30 to 34.

Impact analysis

Immediate metro area

Using Capital Weather Gang’s winter storm impact scale, which ranges from 1 to 5, this is a very typical Level 1 “nuisance” event for much of the metro region. It has the potential to create some slick travel and minor disruptions, but the impact is lessened by the fact that it’s occurring on a Saturday, when most schools and offices are closed. It may, however, have a small negative effect on commerce — occurring on a prime holiday shopping day, but major roads should be treated and in shape for careful travel.

Fredericksburg to Southern Maryland

In Southern Maryland and toward the northern neck of Virginia, including Fredericksburg, where up to six inches or so are possible, this event rates at the low end of a Level 2 disruptive event, mainly due to the amount of snow possible and the possibility of a period of heavier snow which could make travel difficult for some time Saturday morning. But the impact will be limited some by the storm timing.

Model discussion by Wes Junker

Over the past 24 hours the global models (the American GFS and European) have trended west with the back edge of their precipitation, increasing snowfall over the D.C. region, while the NAM model has shown more run to run variation and is not quite as heavy.

The models have also trended colder, probably due to the precipitation arriving late Friday night/early Saturday morning, the optimal time to allow the atmosphere to cool but also because models are now predicting heavier precipitation. The latter works through evaporation and the process of melting of the flakes to cool the atmosphere.

One of the big forecast problems today is that wherever a heavier band of snow sets up, which is impossible to predict until it’s almost happening, the temperatures are likely to drop a degree or two, dipping the temperature below 32 degrees. Wherever this happens, the snow could pile up at a good clip. This morning’s NAM model gives a sense how the intensity of the precipitation might help govern the temperature and how much and where snow might stick.

Note on the NAM forecast valid at 7 a.m. Saturday that where the darkest blue shades and heaviest precipitation rates are located on the east side of the Bay, the red line depicting where temperatures are forecast to be below freezing dips southward (see to the right).

Unfortunately, there are sometimes smaller scale bands of heavier snow that set up farther west of the heaviest precipitation shield. The high resolution NAM hints at such a structure (see below).

Note that while Washington is forecast to get the equivalent of 0.3 inches of liquid (equating to 2 to 3 inches of snow), that a band of 0.4 inches (3 to 4 inches of snow) extends from just west of Washington north-northeastward into Pennsylvania. Another aspect of the precipitation field to note on the NAM is the sharp cutoff in the heavier precipitation west of that implied heavy band, suggesting that dry air might try to keep eroding the western edge of the precipitation shield.

As noted before, the GFS has edged its precipitation shield west since Thursday (see below).

The most troubling aspect of its forecast is the implied band of heavier precipitation extending toward well to the southwest and west of Washington. Like the high resolution NAM, the GFS is suggesting there will be a separate smaller scale band of heavier precipitation. Unfortunately, the models differ on where it might set up.

Note the huge differences between high resolution NAM and the GFS implied band. The latter forecasts the equivalent of 0.50 inches liquid (4 or 5 inches of snow) while the former only predicts 0.10 or 0.20 inches of liquid (1 to 2 inches of snow) across the same area. Light snow at marginal temperatures might only garner a sloppy dusting to an inch.

We’re playing the snow forecast conservatively across the region because the models differ so much and there is the possibility of a sharp cutoff along the back edge of the precipitation as drier air tries to filter in.

Thursday night’s European model run (see below) also shifted the back edge of its heavier precipitation westward. It’s a viable compromise between the NAM and GFS.

Note how the dark green area (the 0.50 inches or more liquid equivalent) shifted from barely touching St. Mary’s County to almost extending northward to the District. That jump put the model’s forecast more in line with last night’s UKMET and today’s GFS than the previous run.

Friday morning’s European model run, just out, is very similar to last night’s — even bringing the heavier precipitation a little farther west.

Taking all of the information together, we lean toward the European model forecast as a compromise but with limited confidence.

A look at how much snow the D.C. area could get this winter. (Video: Joyce Koh/The Washington Post)
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