* Winter storm warning late Saturday night through Sunday night *

10:25 p.m. update — Forecast on track, snow set to begin before dawn Sunday

Having reviewed the latest models, observations and trends, the forecast presented below appears to be on track. Expect light snow to break out over the area between about 3 and 6 a.m. Its intensity should pick up by late morning into the afternoon. Temperatures throughout the region are current below freezing, meaning the ground is cold and the snow will stick.

The forecast radar shown below, from the HRRR, indicates the snow breaks out predawn and continues into the evening. However, the snow may mix with and change to sleet and freezing rain (and rain southeast of the city) when it begins to ease in intensity toward the evening. By 10 p.m., this model shows the precipitation cutting off, ending the first phase of the storm before the next phase potentially sets in Monday morning (read more about that below).

We’ll be back early Sunday morning with updates throughout the day.

Original article

Several inches of snow are set to paste the Washington region on Sunday, the most the area has seen in two years. Additional snow or mixed precipitation is then likely to fall Monday into the night, but amounts are uncertain and the best chance for significant additional accumulation will focus north and northeast of the District.

Due to freezing temperatures in advance of and during the storm, Sunday’s snow will quickly stick to roads and walkways once it begins falling in the early morning, making for hazardous travel throughout the region.

The Capital Weather Gang projects four to eight inches of snow in the immediate area by Tuesday, although amounts at the high end of that range are dependent on additional accumulations Monday, which may or may not materialize. The National Weather Service, which has issued a winter storm warning for much of the region, is also calling for four to eight inches of snow close to Washington but six to 12 inches to the north and northwest.

The D.C. area is expecting its largest snowfall in two years with 4 to 8 inches of snow beginning on Sunday, Jan. 31. More is possible early next week. (The Washington Post)

The storm will come in three phases:

  • Steady snow on Sunday, beginning between predawn and sunrise, that should produce about three to five inches of accumulation (two inches at the low end, around six inches at the most).
  • A lull Sunday evening into early Monday, when precipitation eases but patchy, light mixed precipitation remains likely. This mix of light sleet and freezing rain could add a glaze of ice on top of the snow, mainly along and west of Interstate 95. To the east, patchy light rain and drizzle may fall.
  • A round of mixed precipitation that may change to snow Monday into Monday night. It’s unclear how widespread and heavy the precipitation will become. Areas north and northeast of Washington have the best chance of seeing additional snow accumulation during this phase of the storm.

Snowfall on Monday is contingent on where and how quickly a low-pressure area develops and intensifies along the coast. Some computer model projections suggest it will form too slowly and too far north to generate much snow in Washington, but it may sock locations north and northeast of Baltimore. That said, a few reliable computer models do suggest a heavy snow band could wallop the immediate D.C. area during the second half of Monday.

The storm responsible for the predicted snow is the same one that unloaded up to nine feet of snow in the mountains of California and is poised to sweep through the Plains and Midwest through Sunday.

Forecast totals

Based on all of the model information and our forecast experience, we believe a moderate four- to eight-inch snowstorm is the most likely scenario in the D.C. area for this event, although we would lean toward the lower end of this range since the likelihood of additional accumulation on Monday has decreased some.

Amounts on Monday and Monday night are highly uncertain and could reasonably range from nothing to several inches depending on how the coastal storm evolves. Areas south of Fairfax County, in particular, have seen their chances of additional accumulating snow on Monday diminish.

North of Interstate 70, from Frederick to Baltimore, chances of additional accumulation on Monday and Monday night increase sharply. Some areas in this zone could see storm totals creep into the double digits.

For the District:

Chance of at least one inch: 95 percent

Chance of at least three inches: 65 percent

Chance of at least six inches: 40 percent

Chance of at least 12 inches: 10 percent

The National Weather Service’s snow map shows four to eight inches inside the Beltway and six to 12 inches to the north and west of Montgomery County.

Storm impacts

Considering the combination of snow and freezing temperatures on Sunday, we expect untreated roads to quickly become snow-covered and slick. Occasional bursts of moderate to heavy snow are possible between midmorning and midafternoon Sunday, which will reduce visibility. Major, well-traveled roads, especially those that are treated, should be passable but could still become slick when snow falls steadily. Expect airport delays and possibly some flight cancellations.

On Monday, everything will depend on how quickly the coastal storm develops. But the snow from Sunday along with possible light icing Sunday night will probably be enough for schools that would otherwise be open for in-person learning to close or switch to distance learning for the day.

Commuting conditions on Monday morning may be fine on treated, well-traveled roads, but untreated neighborhood roads are likely to still be slick.

Road conditions could deteriorate Monday afternoon and evening if snow redevelops over the region, with locations north and northeast of the District most likely to encounter problems.

On Capital Weather Gang’s winter storm impact scale, this storm rates at a Category 3 “significant” winter storm for immediate metro area, primarily due to the amount of snow projected, cold temperatures prior to the storm, and its duration. Our storm rating is consistent with the National Weather Service’s declaration of “moderate” storm impacts in our region.

Storm timeline

Temperatures described in this timeline will be coldest west and northwest of Washington and mildest to the southeast.

1 a.m. to 7 a.m. Sunday: Snow develops from southwest to northeast. Temperatures: 25 to 30 degrees.

7 a.m. to 1 p.m.: Snow, possibly moderate to heavy at times. Temperatures: 27 to 32.

1 p.m. to 7 p.m.: Snow, tapering off by late afternoon or evening and mixing with and changing to sleet, except rain southeast of the Beltway. Temperatures: 29 to 34.

7 p.m. Sunday to 7 a.m. Monday: Intermittent and patchy freezing drizzle and sleet along and west of Interstate 95. Patchy light rain to the east. Temperatures: 29 to 35.

7 a.m. to 1 p.m. Monday: Patchy mixed precipitation west of Interstate 95, and patchy light rain to the east, possibly becoming steadier midday and changing to snow. Temperatures: 30 to 35.

1 p.m. to 7 p.m. Monday: Chance of mixed precipitation changing to snow inside the Beltway and to the north and west, with chance of mixed precipitation to the east and southeast. Temperatures: 29 to 33.

7 p.m. Monday to 1 a.m. Tuesday: Chance of snow, especially from the Beltway north. Temperatures: 29 to 32.

1 a.m. to 10 a.m. Tuesday: Chance of snow showers, tapering off in the morning, mainly from the Beltway north. Temperatures: 28 to 31.

What we know and what we’re less confident about

What we know

  1. A storm will track from the Central Plains to the Ohio Valley region into Sunday.
  2. The circulation associated with the Ohio Valley storm will spread mild air eastward that will be lifted over the cold dome across the area due to the high-pressure system to the north, leading to overrunning snow during the day Sunday.
  3. Temperatures across the area Sunday morning at the onset of the storm will be in the 20s, which is plenty cold enough to quickly produce accumulating snow resulting in slick roads, walkways, etc.
  4. The snow Sunday from the first phase of the storm should produce several inches.
  5. A lull in the storm will occur during the night on Sunday into early Monday. Light sleet and snow or freezing rain may continue across portions of area west of the city, while areas to the south and east may see the snow or mix change to light rain as temperatures nudge above freezing.
  6. A new coastal storm will develop somewhere to our south, which will help hold the low-level temperatures near or a little above freezing through the storm.

What we are less confident about:

  1. How quickly and exactly where the new coastal storm will form. That will help determine how much temperatures rise or fall and whether a new band of snow develops Monday. If the storm forms too far offshore or north of our latitude, it won’t result in a solid band of snow forming across our area Monday.
  2. How far west and north snow may change to mixed precipitation late Sunday into Monday. Sleet, freezing rain and rain all tend to hold down accumulations.
  3. Whether a narrow band of heavy snow develops somewhere across the area Monday. That is the big wild card in the forecast. Some models are forecasting such a band that might produce an additional four to six inches or maybe even more during Monday afternoon and night. Without that band, an inch or two more, at most, might fall. Areas a little north of Baltimore look to have the best chances for cashing in on the wild card.
  4. Note: We probably won’t know with confidence if and where a heavy snow band might develop until sometime Sunday.

Snow predictions that rely on a new low rapidly developing near our latitude are tricky, and often don’t pan out quite right for D.C. But when they evolve in exactly the right way, major snow accumulations can result.