* Winter storm warning Wednesday from 1 a.m. to 7 p.m. (except until 1 a.m. Thursday far west and northwest) | Winter weather advisory Wednesday for Stafford County and Southern Maryland from 1 a.m. to 4 p.m. | SchoolCast and FedCast for Wednesday | Announced school closings and delays *


NAM model forecast radar from 4 a.m. Wednesday to 4 a.m. Thursday.

9:30 p.m. - Snow on track to arrive before dawn; accumulation outlook lowered slightly

We’ve reviewed the majority of the late afternoon and evening models and the forecast generally remains on track. However, due to dry air over the region, the snow may take an hour or two later to develop than indicated earlier - starting around 6 a.m. perhaps, instead of 4 or 5 a.m. This shrinks the period of snowfall some since the transition to a wintry mix is still expected between mid-morning and midday.

In response to the shorter duration of snow, models have reduced their snowfall forecasts for the immediate area to around 2 to 3 inches. Here are the latest numbers from the afternoon and evening models for Washington:

  • NAM: 2 inches
  • SREF (average): 2.1 inches
  • HRRR: 2.3 inches
  • High-resolution NAM: 3 inches
  • European: 3 inches

Given the downtrend in model forecasts, we are also reducing our snowfall accumulation forecast from 2 to 5 inches around Washington to 2 to 4 inches, and from 3 to 7 inches in our northern and western areas to 3 to 6 inches.


Predicted snow amounts on Wednesday. (Capital Weather Gang)

Despite the lower high end for predicted amounts, we still expect accumulating snow, possibly heavy times, during the morning commuting period followed by a messy wintry mix. This will lead to difficult travel conditions. Our ideas and timeline described in detail below have not materially changed.

This is our last update of the evening. We’ll resume coverage around 4 a.m. on Wednesday.

6 p.m. - Forecast remains on track, models hinting at lower likelihood of snow ‘boom’ scenario

Since this article was originally published, a handful of additional model simulations have come in and they support the ideas posted below. They all predict about 2 to 4 inches of snow in the immediate region in the morning followed by a wintry mix. However, these numbers are down a bit compared to some forecasts made Monday or even earlier today. For example, here are the latest model snowfall forecasts compared to the same forecast made 24 hours earlier, in parenthesis:

  • NAM: 2.3 inches (3.4 inches)
  • High resolution NAM: 3.8 inches (3.5 inches)
  • GFS: 3.0 inches (6.5 inches)
  • SREF (average): 1.8 inches (3.6 inches)

We think the likelihood of “boom scenario” of more than 5 inches has dropped in the immediate area. That said, we still expect accumulating snowfall during Wednesday’s commute that will be create challenging travel conditions, followed by a mess of mixed precipitation.

Original post from the afternoon

For only the second time this year, a winter storm warning covers much of the D.C. region on Wednesday for the likelihood of a hazardous combination of snow and ice.

We are most concerned about Wednesday morning’s commute. Snow is expected to move in just before dawn and may rapidly become rather heavy, limiting visibility and turning roads treacherous during the heart of the typical commuting period.

Through midmorning Wednesday, the snow may quickly pile up (at a rate of around one inch per hour) and we anticipate widespread school closings. Airports are also likely to deal with delays and some canceled flights.

We encourage teleworking instead of commuting Wednesday morning to avoid large backups and to give crews room to operate. If you decide to drive in expect “very difficult” conditions, according to the National Weather Service.

The period of snow (without any mixed precipitation) may not last long as models suggest precipitation will transition to sleet and then freezing rain between the midmorning and midafternoon hours from south to north. The relatively fast changeover should limit snow accumulations to around 2 to 5 inches in the immediate area, with a bit more north and west, and a bit less south and east.


Forecast snow accumulation for Wednesday, Feb. 20. (CWG)

If the changeover to icy precipitation is delayed, snowfall could exceed our forecast. But if the transition happens on the fast side, we could see amounts more in the 1 to 3 inch-range.

Something else to watch for in this storm is a possible localized zone of heavier snowfall. Models vary as to where exactly this will be, but we cannot rule out some totals exceeding six inches. The overall pattern would support jackpot amounts in our colder areas west and northwest of the city but they cannot be ruled out in the immediate metro area.

Even after the precipitation changes from snow to ice, the threat of hazardous conditions is far from over, especially in our colder locations. Visibility does tend to improve when snow changes to sleet and sleet does not accumulate as fast as snow, but untreated roads and sidewalks will remain slick into the afternoon. The exception may be in our milder areas south and east of Washington where temperatures rise above freezing and precipitation turns to plain rain.


Forecast ice accumulation for Wednesday, Feb. 20. (CWG)

Later in the afternoon and evening hours on Wednesday, along and west of Interstate 95 (Zones 1 and 2 the above map), sleet will transition to more freezing rain. Freezing rain forms an icy glaze on contact with cold surfaces. In addition to causing slick untreated surfaces, it can buildup on utility lines and break tree limbs, leading to power outages.

Our far north and west zones (Zone 1) have the highest risk of significant ice buildup of at least a light glaze (or 0.1 inches), enough to cause tree damage and power outages. In these areas temperatures will remain near or slightly below freezing well into Wednesday night or even into Thursday morning in the coldest spots (toward northwest Virginia and northern Maryland). Near Interstate 95, we expect temperatures to edge slightly above freezing by Wednesday evening, mean freezing rain would change to plain rain before a more substantial ice accumulation.

We do expect rain and freezing rain to end everywhere by the predawn hours Thursday, but some school delays and closings are likely Thursday morning due to the earlier precipitation. Some untreated road, parking lots, and sidewalks are likely to remain slippery into Thursday.

Detailed storm timeline

3 to 6 a.m.: Snow developing southwest to northeast. It could start as sleet or a snow-sleet mix, but it should change over to snow. Temperatures 28 to 34, northwest to southeast. Accumulation of a coating to an inch or so.


NAM model radar simulation at 7 a.m. Wednesday. Blue is snow, purple is sleet, pink is freezing rain, and green is plain rain.

6 to 10 a.m.: Snow, possibly heavy at times. Could begin to mix with or change to sleet south of Washington. Temperatures 27 to 32, northwest to southeast. Accumulation of 2 to 4 inches, less south.

10 a.m. to 2 p.m..: Snow changes to sleet everywhere, except to rain in far south and southeast areas. Precipitation decreases some in intensity. Temperatures 28 to 33 northwest to southeast. Up to an inch or two of additional snow and sleet accumulation, highest amounts north of Washington.


NAM model forecast radar at 1 p.m. on Wednesday. Blue is snow, purple is sleet, pink is freezing rain, and green is plain rain.

2 to 6 p.m.: Sleet changes to freezing rain along and west of Interstate 95 (Zones 1 and 2) with cold rain elsewhere. Temperatures 30 to 35 northwest to southeast. Light glaze of ice possible in colder areas.

6 p.m. to midnight Thursday: Freezing rain in colder areas well north and west of Washington (Zone 1), with freezing rain changing to rain near Interstate 95 (Zone 2). Rain elsewhere. Temperatures 31 to 36 northwest to southeast. Coating of ice accumulating in colder areas (Zone 1).


NAM model forecast radar at 7 p.m. Wednesday. Blue is snow, purple is sleet, pink is freezing rain, and green is plain rain.

Midnight to 4 a.m. Thursday: Precipitation tapers off. Temperatures 32 to 37 northwest to southeast.

Models

Here’s how much snow the various models predict around Washington:

  • SREF (average of 24 simulations): 1.8 inches (range: trace to 4.5 inches)
  • NAM: 2.3 inches
  • GFS: 3 inches
  • European: 3 inches
  • High-resolution NAM model: 3.8 inches
  • HRRR: 3.8 inches
  • Canadian: 4.8 inches
  • Canadian high-resolution: 7.2 inches

Note that some of these model forecasts count sleet as snow, whereas sleet actually tends to limit snow accumulation.

“The big question mark is whether the heaviest band of snow fringes us and ends up to our north across Pennsylvania like the high resolution NAM model and to a lesser extent European model predict, or ends up right over us like the Canadian models predict,” says Wes Junker, Capital Weather Gang’s winter weather expert.

“In the former scenario with less snow, the precipitation focuses along a frontal boundary that forms to our north between the warm air that surges northward into Kentucky and western West Virginia,” Junker explained. “That often happens with these types of systems. We get a couple of hours with a thump of snow that changes to sleet and then lighter freezing rain and eventually rain.”

“The latter evolution, which would result in more snow, is rarer but can’t be completely discounted and seems to me to be the more unlikely scenario of the two. It would constitute a boom scenario which still is possible and helps explain why there still is so much disparity between the snow forecasts from different outlets.”

Forecasts from other outlets

National Weather Service


National Weather Service snowfall forecast for Wednesday. (NWS)

NBC4

FOX5

ABC7

WUSA9

Storm impact

(This section is unchanged from Monday’s assessment)

On Capital Weather Gang’s winter storm impact scale, this rates as a high-end Category 2 “disruptive” storm in the immediate metro area (Zone 2) but a solid Category 3 “significant” storm in our colder areas to the north and west

From a practical standpoint, we don’t expect a big difference in the effects in these two zones during the day Wednesday. Both will deal with widespread accumulating snow and some ice, and slick roads. But in Zone 1, the amounts of frozen precipitation will be somewhat higher, and the slick conditions will last longer into Wednesday evening and night.


(CWG)

In our milder areas east and southeast of town (Zone 3), expect one to three inches of snow or less, this is more of a Category 1 “nuisance” storm, where the effect of snow and ice will be more patchy and short-lived — with probably just a brief period of challenging conditions Wednesday morning.