* Winter storm watch Tuesday night to Wednesday night for potential of at least 5 inches of snow for entire region*

1:55 p.m. update: The National Weather Service expanded the winter storm watch to cover southern Maryland (Charles, Calvert, and St. Mary’s counties). Model data (such as the European) coming in early this afternoon support our forecast below. We will live blog the evening model runs tonight between 9 and 11 p.m. so be sure to tune in.

From 11:47 a.m.: A wound-up, intense late winter storm marches into the Mid-Atlantic Tuesday night and Wednesday, promising widespread accumulating snow. Heavy snow is possible, especially in far western parts of the Washington, D.C. metro region and northwest Virginia, with some accumulations likely reaching double digits.

In the District and inside the beltway, a reasonable first snow accumulation estimate, subject to change and refinement, is 3-8 inches by Wednesday evening.

Higher amounts, exceeding 6 inches, are likely in D.C.’s western suburbs west of I-95.

Accumulation forecast map - first call, subject to revision

Key points:

* Around the city, due to temperatures near or slightly above freezing, snow may not stick and pile up as quickly, so amounts are less certain, especially as precipitation may begin as rain, or a mixture of rain, sleet, and snow late Tuesday night. Precipitation is likely to change to mostly snow by dawn Wednesday but some sleet/rain mixing is possible during the day.

* From western Montgomery county, western Fairfax county, northwest Prince William county and northern Fauquier county, west towards the Blue Ridge, most precipitation should fall as snow. 6-12 inches of snow are favored, with locally higher amounts above 1,000 feet.

* In west central and northwest Virginia, along I-81, and in the Blue Ridge, 8-15 inches of snow, or even more along mountain ridges, are favored.

* This is the kind of storm will snowfall may vary significantly within a very small area and where a few degrees will make a big difference. For example, it’s conceivable upper northwest Washington, D.C. will receive several inches more snow than downtown.

* The above snowfall amounts are *most likely* but weather forecasting is not a perfect science and a range of possibilities are possible. Snowfall could easily be above or below the ranges given. Below is a graphic which shows the range of possibilities in the immediate metro region, and far western suburbs.

* The least amount of snow is expected around the Chesapeake Bay and southern Maryland due to warmer temperatures. 1-4 inches is most likely in these areas, with a rain and/or mix gradually changing to snow.

* Because of the heavy, wet nature of the snow, it will accumulate on trees and power lines, potentially leading to power outages, particularly in areas which receive at least 6-10 inches of snow. Strong, gusty winds, particularly at higher elevations, will exacerbate this potential.

* Very heavy snowfall rates of at least 1-2 inches per hour are possible at the height of the storm, reducing visibilities below one half mile, and creating hazardous travel. Thundersnow is possible due to the dynamic nature of the storm.

* There is a strong chance of school closures Wednesday and possibly Thursday. We will issue our SchoolCast starting tomorrow.

* Flights are likely to be affected Wednesday, especially at Dulles Airport where heavy snow totals are more likely than National.

* This storm will have serious effects at the coast, with large waves, coastal flooding and beach erosion Wednesday into Thursday. Winds may gust to 50 mph at the beaches, with a storm surge of 2 to 4 feet.


4-10 p.m. Tuesday: A 50 to 30 percent chance of light mix of precipitation developing from southwest to northeast. Temperatures 35-40 (coldest west). No travel issues or accumulation expected.

10 p.m. Tuesday to 4 a.m. Wednesday: Widespread mix of precipitation developing; mostly snow in far western suburbs; rain/sleet changing to sleet/snow immediate D.C. area; a mix east of I-95. Tempertures 31-36 (coldest west).

4 a.m. to 10 a.m. Wednesday: Snow, heavy at times, in western suburbs. Mix to mostly snow immediate D.C. area; a mix east of I-95. Temperatures 30-35 (coldest west).

10 a.m. to 4 p.m. Wednesday: Snow, heavy at times. Snow may mix with sleet and rain east of I-95. Temperatures 31-35 (coldest west)

4 p.m. to 10 p.m. Wednesday: Snow tapers off west to east. Temperatures 29-34 (coldest west).

Frequent questions:

Who will see the heaviest snow? Northwest Virginia appears to be the most likely sweet spot, with totals over 12 inches a decent bet around Winchester.

When will the heaviest snow fall? Right now, it appears the window with heaviest snowfall would be between 5 a.m. and early-to-mid afternoon.

Could the storm bust and we get little or no snow? The bust potential is highest in the immediate D.C. area due to marginally cold enough temperatures for snow. There’s about a 20-30 percent change it becomes too warm and precipitation is not sufficiently heavy for accumulating snow. In the western suburbs, at least some accumulating snow is a pretty safe bet unless there are radical changes to this storm system.

Could the storm overperform and give the immediate Washington, D.C. area a foot of snow? Yes. In fact, some models suggest that will happen because this is a dynamic storm that will produce a lot of precipitation. If heavy snow bands develop and sit over the D.C. area, that will overcome the marginal temperatures and there is about a 20-25 percent chance this becomes a serious, immobilizing storm.

What’s Wes Junker’s, CWG’s winter weather expert, take on the situation? “The models have narrowed the possibilities to two different scenarios. Each presents a potent storm off the coast of North Carolina or southeast Virginia supporting very heavy precipitation somewhere between Richmond and Washington.”

“ The GFS and NAM models continue to suggest that this could be a significant snowstorm for much of the D.C. area except areas east of the city where the GFS wraps enough warm air into the system to prevent much accumulation around and especially east of the city at times during the day on Wednesday.”

“All models suggest a significant snowstorm for Blue Ridge and Shenandoah Valley.”

“If the worst case scenario pans out, intense snowfall rates could lead to heavy accumulations even in the city and would likely produce double digit accumulations within 60 miles of the city.”