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* <a href="https://forecast.weather.gov/showsigwx.php?warnzone=MDZ503&warncounty=MDC031&firewxzone=MDZ503&local_place1=2%20Miles%20SSE%20Barnesville%20MD&product1=Winter+Storm+Warning&lat=39.1905&lon=-77.3695#.X9klBGhKg2w" target=_blank>Winter storm warning</a> for northern Fauquier, Loudoun, northwest Montgomery and western Howard counties and areas to the north and west Wednesday and Wednesday night *

* <a href="https://forecast.weather.gov/wwamap/wwatxtget.php?cwa=lwx&wwa=winter%20weather%20advisory" target=_blank>Winter weather advisory</a> for the District and Prince William, Fairfax, southern Montgomery and eastern Howard counties *

* <a href="https://forecast.weather.gov/showsigwx.php?warnzone=MDZ013&warncounty=MDC033&firewxzone=MDZ013&local_place1=Greater%20Upper%20Marlboro%20MD&product1=Flood+Watch&lat=38.8303&lon=-76.7527#.X9j9PWhKiUk" target=_blank>Flood watch</a> along and east of Interstate 95 *

* <a href="https://www.washingtonpost.com/local/school-closings-and-changes-in-the-dc-region-for-wednesday-dec-16/2020/12/15/0776882e-3f18-11eb-8bc0-ae155bee4aff_story.html" target=_blank>School closings and changes</a> on Wednesday *

After examining the latest forecast models, the forecast we present below seems about right. There are some models which suggest snowfall amounts may end up on the low side of the given ranges due to mixing with sleet, freezing rain and rain, but some other models suggest it would be unwise to lower our forecast any more than we have already.

We’ll be back with updates throughout Wednesday. This will be a very dynamic storm that will have us on our toes and we expect some surprises with it.

8:30 p.m. — Flood watch expanded into District, I-95 corridor

The <a href="https://forecast.weather.gov/showsigwx.php?warnzone=VAZ053&warncounty=VAC059&firewxzone=VAZ053&local_place1=Dunn%20Loring%20VA&product1=Flood+Watch&lat=38.8933&lon=-77.2303#.X9ljtWhKg2w" target=_blank>flood watch</a> has been expanded west to cover the District and its close-in suburbs near the I-95 corridor due to the likelihood of one to two inches of rain which could cause pockets of flooding. In a highly unusual situation this area is under both a winter weather advisory and flood watch at the same time. But both hazards could occur from this same storm.

A period of snow and sleet at the storm’s onset Wednesday late morning into the early to mid-afternoon could cause slick travel followed by heavy rain into the evening that could cause streams/creeks to overflow and high water in poor drainage areas. Then a second period of snow and sleet could come at the storm’s tail end overnight Wednesday.

A powerful, moisture-infused coastal storm is set to charge through the Mid-Atlantic on Wednesday, unloading a sloppy mix of precipitation in the Washington region. Heavy snow is expected toward the Interstate 81 corridor and the mountains, with heavy rain east of I-95 and a combination of snow, ice and rain in between.

Winter storm warnings are in effect for northern Fauquier, Loudoun, northwest Montgomery, Frederick and western Howard counties where a combination of snow, sleet and freezing rain will lead to very difficult travel Wednesday afternoon and night. Up to a foot of heavy snow is possible farther to the north and west, toward I-81, also under a winter storm warning.

In the District and its immediate western and northern suburbs, a winter weather advisory is in effect. Here precipitation may start and end as snow, but should change to a mix of sleet, freezing rain and rain in between. Roads could be slippery during any bursts of frozen precipitation between late Wednesday morning and midafternoon, and again overnight Wednesday as the storm pulls away and temperatures drop.

East of I-95, potentially heavy rainfall — between one and two inches — could cause flooding near creeks and streams and poor drainage areas, prompting a flood watch.

The storm will be relatively fast-moving, with precipitation lasting about 15 hours in most locations, starting around midday Wednesday and ending in the predawn hours Thursday or even a bit earlier. Here’s how we see the storm evolving:

Midmorning to early afternoon Wednesday (9 a.m. to 1 p.m.): Snow develops, possibly mixed with sleet and rain along and east of I-95. Rain develops in Southern Maryland. Light snow accumulation is possible, especially southwest of Washington. Temperatures 30 to 35 from northwest to southeast.

Wednesday afternoon (1 p.m. to 5 p.m.): Snow mixes with and changes to sleet and rain, heavy at times, along and east of I-95. Snow mixes with and changes to sleet and freezing rain west of I-95 (in area under winter weather advisory). Snow, heavy at times, in our far western and northwest areas (in area under winter storm warning), possibly mixed at times with sleet and freezing rain. Temperatures 30 to 37 northwest to southeast.

Wednesday evening (5 p.m. to 9 p.m.): Rain from I-95 and eastward, heavy at times. Rain and freezing rain west of I-95. Snow, freezing rain and sleet in our far western areas (under winter storm warning). Temperatures 30 to 40 northwest to southeast.

Wednesday overnight (9 p.m. to 1 a.m.): Freezing rain and rain gradually change back to sleet and/or snow from northwest to southeast, to around I-95, before ending. Precipitation ends in areas south and southwest of Washington. Temperatures 28 to 36 northwest to southeast.

Early Thursday morning (1 a.m. to 5 a.m.): Snow tapers off around Washington and to the west and north. Temperatures 25 to 32 northwest to southeast. Slick spots possible around region Thursday morning.

While most locations near and west of I-95 will start as snow on Wednesday morning into the early afternoon, a strengthening area of low pressure moving from the Carolinas toward a position west of Norfolk will draw in relatively mild air from the Atlantic Ocean in the lower levels of the atmosphere. This warm air will surge westward, on top of D.C. and maybe even all the way west into our normally snowier suburbs. As such, we have lowered snowfall amounts some since Monday.

A coating to a couple of inches seem most likely right around Washington and just to the south (including southern Fairfax, central and northern Prince George’s, and eastern Prince William counties). There are two possible periods of snow: when the storm begins and when it pulls away. Confidence in snow accumulation when the storm pulls away is low, and any initial snow may be washed away be rain.

In Washington’s immediate north and west suburbs (including western Prince William, northern Fairfax and central Montgomery counties), about one to three inches seem most likely, although some of this could be compacted or washed away by rain and freezing rain.

Farther to the northwest, three to six inches could fall in the normally colder areas of northern Montgomery, Loudoun and northern Fauquier counties. Totals would be higher, but we think precipitation could mix with and change to sleet and freezing rain at times.

For mostly snow, you’ll probably have to travel to the north and west of Frederick in Maryland and toward the I-81 corridor in Virginia. But even here, some sleet could mix with the snow. At least six inches are likely in this zone, with a foot or more not out of the question, especially into the mountains.

The National Weather Service, like the Capital Weather Gang, has lowered its snowfall amounts since Monday right around Washington. Its snowfall map is shown below. It still predicts somewhat heavier amounts than we do, in general, north and west of Washington.

In the zone between the heavy snow in our far western areas and heavy rain (east of I-95), we may see a corridor of freezing rain. Because temperatures are not expected to be far below freezing (around 30 to 32 degrees), the freezing rain may have some difficulty accumulating. However, a glaze could develop on elevated surfaces, tree limbs and some untreated paved surfaces — especially ramps, bridges and overpasses.

We’re most concerned about the freezing rain potential west and north of the Beltway, from around Manassas to Fairfax, Rockville, Columbia and just to the west into parts of Fauquier, Loudoun and perhaps eastern Frederick counties.

<a href="https://www.washingtonpost.com/news/capital-weather-gang/wp/2017/01/04/introducing-capital-weather-gangs-winter-storm-impact-scale/" target="_blank">Capital Weather Gang’s winter storm impact scale</a>, which categorizes the threat posed by events like this, shows the anticipated level of disruption varying considerably depending where you reside in the region. And conditions could change quickly over short distances.

Here’s the storm rating we’ve assigned to different areas:

The I-81 corridor and to the west: Category 4. Six to 12 inches of snow, possibly mixed with sleet, will cause snow-covered roads and make travel very difficult. Some areas could become impassable. Snowfall rates of at least one inch per hour at times could greatly restrict visibility. Schools that are open for in-person learning will likely close. This zone includes Winchester, Martinsburg and Hagerstown.

The zone from western Loudoun County to Frederick, extending west to near I-81: Category 3: Three to six inches of snow, along with sleet and freezing rain, will make for snow- and ice-covered roads, and difficult travel. Schools that are open for in-person learning will likely close. This zone includes Frederick, Harpers Ferry, Berryville, Purcellville and Middleburg.

The zone from central Loudoun County to eastern Frederick County, extending east into western Prince William, western Fairfax and northern Montgomery County: Category 2. Snow and/or freezing rain will cause slick travel, especially on untreated roads. Schools that are open for in-person learning may close. Airport delays are possible at Washington Dulles International Airport. This zone includes Warrenton, Gainesville, Ashburn, Leesburg, Sterling and Clarksburg.

Along and just west of I-95: Category 1. Patchy slick spots are possible on roads during frozen precipitation when the storm is beginning and ending. Schools that are open might dismiss early Wednesday afternoon or delay Thursday morning. This zone includes Fairfax, McLean, Northwest Washington, Bethesda and Silver Spring.

This is a very complicated forecast, and a lot could go wrong. Conditions may change dramatically over just a small distance.

Two competing influences on the air temperature will battle between I-95 and I-81. Cold-air high pressure in southeast Canada will funnel a wedge of cold air southward. At the same time, roaring winds off the Atlantic will send in a surge of mild air.

For areas just west of I-95, after snow changes over to freezing rain and rain, a degree or two will make the difference between just cold rain and significant iciness.

For areas farther west, toward Leesburg, a one-degree change in the temperature about 5,000 feet high will make the difference between more of a snowstorm and more of a wintry mix.

“The big forecast problems are all related to the really potent easterly flow that will be coming in off the warm Atlantic and how far west it penetrates,” said Wes Junker, Capital Weather Gang’s winter weather expert. “Unfortunately, computer models haven’t been able to solve them.”

National Weather Association, Digital Seal of Approval

University of Virginia, BA in environmental science, 1998

University of Wisconsin, MS in atmospheric science, 2000

Jason Samenow is The Washington Post’s weather editor and Capital Weather Gang's chief meteorologist. He earned a master's degree in atmospheric science and spent 10 years as a climate change science analyst for the U.S. government. He holds the Digital Seal of Approval from the National Weather Association.

Jason Samenow

Society of Environmental Journalists "Award for Outstanding Explanatory Reporting," 2018

Tufts University, BA in political science; Columbia University, MA in Climate and Society; Fletcher School of Law and Diplomacy, MA 

Andrew Freedman edits and reports on extreme weather and climate science for the Capital Weather Gang. He has covered science, with a specialization in climate research and policy, for Axios, Mashable, Climate Central, E&E Daily and other publications. 

Andrew Freedman

washpost

Mid-Atlantic faces heavy snow, ice and flooding Wednesday from nor’easter

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