American model simulation of snow event in the weekend. (PivotalWeather.com)

Snow has become increasingly possible Saturday afternoon through Sunday afternoon but exactly how much is still a big question mark.

Based on the latest information, a light-to-moderate snow event seems most probable — about two-to-four inches, but less or more could easily fall. Because the storm is still three to four days away, predicted amounts could easily be adjusted — upward or downward.


(Washington Post)

The latest model simulation suggests the worst conditions may focus more on Sunday, whereas earlier forecasts had suggested Saturday night.

At the moment, we believe there is about a 60 percent chance of at least an inch, a 40 percent chance of at least three inches and a 10 percent chance of at least six inches within a county radius of the District.

Like the snow event on Dec. 9, the heaviest amounts are possible south and southwest of Washington. Places such as Charlottesville, Richmond and Roanoke could see four-to-eight inches.

There may be a cutoff in accumulating snow toward the far northern part of our region, toward the Mason-Dixon Line. However, predicting exactly where accumulating snow starts and stops is challenging this many days ahead of the storm.

The timing of the snow is another tricky aspect of this storm prediction. The snow may be strung out over a rather long duration starting Saturday afternoon and continuing through Sunday afternoon. But there may be breaks or long periods during which the snow is falling very lightly and is not all that disruptive.

Models generally agree that Sunday morning into the afternoon is the period during which the steadiest accumulating snow may fall — which could make roads and sidewalks slick. Below-freezing temperatures at this time should allow the snow to stick. There may also be a short period of steadier snow Saturday afternoon — close to the onset of the storm — but we have less confidence in that, and the snow that falls initially will not accumulate as fast (with temperatures closer to freezing).

Storm scenarios

Based on model projections and our own forecasting experience, we’ve developed three scenarios of what we think is most likely to happen:

  1. Light to moderate snowstorm (two-to-five inches) — 45 percent chance: Light snow develops during Saturday afternoon with temperatures of 30-to-35 degrees. Even if temperatures rise a few degrees above freezing Saturday, they would probably fall quickly when snow begins, allowing it to stick. Light snow would probably continue through the night into Sunday morning, although it may pause at times. The heaviest snow would probably fall Sunday morning into the afternoon, and roads would become snow-covered and slick. Accumulations would probably be in the two-to-five-inch range, with the heaviest accumulations south of the city.   
  2. Some light snow (a dusting to two inches) — 45 percent chance: The storm would be suppressed far enough south to only offer light snow accumulation. The snow would probably pause or be very light at times due to dry air associated with the strong high pressure system to the north. Even so, icy patches could develop on roads and sidewalks — especially Sunday morning, when the steadiest snow would probably occur.
  3. Moderate-to-heavy snowstorm (five-to-10 inches) — 10 percent chance: In this scenario, the storm system would track close enough to us to support a period of moderate-to-heavy snow Sunday morning perhaps into the afternoon. Lighter snow would be possible before and after that (starting Saturday afternoon and ending late Sunday). Even in this scenario, snowfall amounts would probably remain below blockbuster levels because the storm will be moving along, and there is no feature to the east or northeast to block its forward motion. 

Storm discussion

If you’re itching for snow, the weekend’s storm system features several favorable ingredients:

  1. A strong area of high pressure to the north to keep cold air feeding into the region.
  2. A low-pressure system that will track from the Gulf Coast states northeast to off the coast of the Carolinas, drawing moisture across the Mid-Atlantic region.
  3. A favorable wind pattern for snow at high altitudes.

But the snow forecast also has complications. Much like the storm that passed just south of the District on Dec. 9, bringing heavy snow to Charlottesville and Richmond but missing the city, strong flow from the northwest will stream across the Northeast United States, pushing dry air south. How much that flow relaxes will determine how far north significant snow will spread.

The model simulations that maintain that northwesterly flow over the Northeast keep the storm weak enough and far enough south to only offer light snow. But the simulations that weaken that flow and allow the storm to lift toward us offer us a more substantial snowstorm. A few even suggest heavy amounts in the five-to-10-inch range.


Snowfall simulations from American modeling system for the weekend. (WeatherBell.com)

Most model simulations predict light to moderate accumulating snow on the order of two-to-five inches, but have the snow falling lightly during an extended period. That always complicates forecasting snow amounts because flake size can be small during periods of light snow, which sometimes produces less accumulation than one would normally expect.

But, if we get a period of moderate snow, flake size would probably be larger allowing for snow to pile up more quickly — especially given temperatures which should fall into the 20s Saturday night into Sunday.

Model summary

American model: It takes the storm on a favorable track for around four inches of snow. It forecasts mostly light snow beginning Saturday afternoon and continuing well into Sunday. It does predict a period of moderate snow Sunday morning through the afternoon when most of the accumulation would occur.


American model snowfall forecast for this weekend. (PivotalWeather.com)

Canadian model: It forecasts mostly light snow between Saturday afternoon and Sunday afternoon, equating to about two-to-three inches from the Beltway north. Amounts increase some to the south into Southern Maryland, but milder temperatures there might eat into the accumulation.


Canadian model snowfall forecast for the weekend. (PivotalWeather.com)

European model: It forecasts patchy light snow from Saturday afternoon to early Sunday morning. The steadiest, accumulating snow would occur between early Sunday morning and midday, before tapering off in the afternoon. It predicts roughly three-to-four inches within a county radius of the District, but four-to-six inches just to the southwest.


European model snowfall forecast for the weekend. (WeatherBell.com)

We’ll be continuing to monitor this storm system and will provide an update on it again Thursday.