* Winter storm warning 2 a.m. to 8 p.m. Wednesday for entire region | PM forecast update *

11:40 p.m. update

The last model we’ll look at tonight is the American (GFS). The heaviest precipitation occurs east and north of the Beltway and it advertises about 3 to 6 inches of snow for the close-in D.C. area. However, some of the snow it portrays is falling during the afternoon when it may have a hard time accumulating – so realistically you might cut an inch or so off these totals.


GFS model snowfall forecast.

We’ve posted a lot of updates since our original post early this afternoon, but our general thinking about this event hasn’t changed when we consider all of the information and models that have since come in. Some summary points:

  • Snow and/or sleet start in the predawn hours. Temperatures should be low enough for accumulation to begin relatively quickly.
  • Snow is likely, possibly heavy at times, through the morning into the early afternoon. This is our best window to get accumulation and when travel may be hazardous, especially in the northern parts of our region.
  • By mid-to-late afternoon, the combination of the snow easing in intensity, temperatures warming a couple degrees, and the high March sun angle, will greatly lower accumulation prospects.
  • We’re sticking with our outlook for 2 to 5 inches of snow from downtown Washington and to the south, and 4 to 8 inches to the north.

We’ll be back with a new post at 5 a.m. Wednesday, and will post updates on the storm as long as it lasts. Have a good night!

11:00 p.m. update

This storm could start with a bang in a few spots predawn Wednesday. The latest radar image from southwest Virginia (see below) shows a heavy band of precipitation pointed at the D.C. region, probably arriving between 3 and 6 a.m. It could come in the form of sleet and/or snow.


Radar at 10:45 p.m. (National Weather Service)

Thundersnow was reported as this band came through Roanoke so don’t be shocked to hear a bit of thunder around here early in the morning.

10:30 p.m. update

Two more models are in this evening, the NAM and its companion, the high-resolution NAM. Neither suggest massive snow totals, but more moderate amounts and paint a picture pretty consistent with our forecast. The totals shown in the map below cover the entire event, ending in the evening (unlike the HRRR simulation shown in our 9:45 p.m. update, only through 2 p.m.). They may be biased a hair on the high side because some of the snow projected would likely compact and/or melt.

NAM


Snowfall forecast from NAM model.

High-resolution NAM


Snowfall forecast from high-resolution NAM model.

9:45 p.m. update

We’re now getting into the range in which short-term models can give us sneak preview as to what we might expect from the storm in its opening act. Below is a snowfall simulation from the HRRR model, which shows widespread 3 to 6 inch amounts through 2 p.m. Of course, snow may continue for a few hours after that, so it may be an underestimate of the total storm snow potential. Also, the model probably wouldn’t be able to pick up on where localized bands of particularly heavy snow might set up. On the other hand, some of the snow shown in this model would likely compact and/or melt falling in the middle of the day during March, so taking everything together, it may be a reasonable projection.


HRRR model snowfall simulation 2 a.m. to 2 p.m. Wednesday.

8:40 p.m. update

The Capital Weather Gang, in 2017, developed its winter storm impact scale to rate these events from 1 to 5 based on how severely they impact the region. It takes into account a lot of variables, including not only how much snow, but also the duration of the event, temperatures, wind, weight of the snow, whether the storm coincides with rush hour, and so forth.

Rating north of downtown Washington

In our northern areas, north of downtown Washington, where we expect 4 to 8 inches of snow, this storm has earned a rating of level 3, significant.


Significant events bring snow-covered roads, some of which could become impassable at times. They also result in widespread school closings and air travel disruptions. This event scored a level 3 due to the amount and intensity of snow expected, combined with its weight, and for coinciding with a heavy midweek commuting period.

Rating in downtown Washington and south

From downtown and to the south, where 2 to 5 inches of snow are expected, this event is rated level 2, disruptive. Such events typically produce slick roads and some school closings. This event scored a 2 in this region for moderate snow accumulation and intensity, and for coinciding with the Wednesday morning commute.


5:55 p.m. update

Models have presented a real dilemma this afternoon with respect to whether to increase the snow forecast for Wednesday. They are in two camps. The first camp is represented by the European model which showed a crippling snowstorm with double digit accumulations. But the American models, the NAM and GFS, showed a moderate event, more along the lines of our original map posted early this afternoon.

Less than 12 hours before the storm starts, model forecasts suggest as little as 2 inches and as much a foot could realistically fall in the region Wednesday.

We have decided not to increase our snow forecast, but to expand the chance and intensity of the boom scenarios, or the likelihood snowfall exceeds our forecast, to reflect the European model’s extreme solution.


3 p.m. update

The latest European model forecast that came in this afternoon predicts very heavy snowfall over the D.C. area Wednesday, substantially more than we have forecast. However, before we alter our forecast, we would like to review some more models coming in this afternoon. We plan to release one more snowfall projection for this storm between about 6 and 6:30 p.m. We’ll post it right above this update.

Original post from 1:00 p.m.

The Washington region had to wait until the second day of spring, but, finally, a significant snowstorm appears likely to materialize. A storm will rapidly develop off the Mid-Atlantic coast Tuesday night into Wednesday, drawing moisture from the ocean into cold air seeping in from the north.

Two to 5 inches of snow is most likely in downtown Washington and to the south, with 4 to 8 inches just to the north. Areas farther to the south are most likely to receive 1 to 3 inches. Especially from the Beltway and to the south and east, the snow may be mixed with sleet when the precipitation starts.


These amounts would mark the biggest snow event in the region since the January 2016 blizzard, and the most this late in the season since at least 1964.

While the most accumulation will be on grassy areas, this wet snow will stick to roads when it falls steadily Wednesday morning. Into the afternoon, accumulation on roads will become more difficult because of the high sun angle and slowly rising temperatures near and perhaps just above freezing. However, conditions could still be hazardous during heavier bursts, even into the afternoon.

It is not out of the question that a band of very heavy snow sets up over the region during Wednesday, most likely on the north and/or east side. Snowfall amounts could easily exceed forecasts where such a band develops.

The heavy wet snow may weigh down tree branches and take down power lines, causing outages, especially east and northeast of the city.

The entire region should expect disruptions Wednesday morning, including school closings, a difficult commute, and air travel delays and cancellations. Conditions should slowly improve Wednesday, but perhaps not until late in the day.

Storm timeline Wednesday

Midnight to 4 a.m.: Light mixed precipitation. Not much accumulation. Temperatures: 31 to 35 (northwest to southeast).

4 a.m. to 8 a.m.: Steady snow and/or sleet develops and accumulation begins. Temperatures: 30 to 34 (northwest to southeast).

8 a.m. to noon: Snow, possibly moderate to heavy at times, and accumulating. Temperatures: 30 to 33 (northwest to southeast).

Noon to 4 p.m.: Snow. Accumulation slows but still possible during heavier bursts. Temperatures: 31 to 35 (northwest to southeast).

4 p.m. to 8 p.m: Snow diminishes from southwest to northeast. Little additional accumulation. Temperatures: 31 to 34 (northwest to southeast).

8 p.m. to 11:59 p.m.: Lingering snow showers/flurries, mainly northeast. Temperatures: 30 to 33 (northwest to southeast).

Some answers to frequent questions

Will this storm affect my flight? Flights could well be affected if you’re departing/arriving between early morning Wednesday and late afternoon Wednesday — especially between midmorning and early afternoon — when snow may be heaviest. This probably won’t be a snow event that cripples airports and shuts down everything, so check your flight status.

If you’re flying very early in the day, you might escape before delays really start kicking in. By late afternoon or evening, the storm will be winding down, but there may be backlog delays. Also consider that hubs farther up Interstate 95 will be dealing with snows, which will affect the overall status.

Will this storm affect schools? Very likely. We will post a SchoolCast late Tuesday afternoon.

When will the heaviest snow fall? We think the heaviest, accumulating snow could occur anytime between early and midmorning Wednesday and early afternoon. This is when snow is likely to cause the most travel problems by reducing visibility and sticking to roads, especially during heavy bursts.

What about the high March sun angle? Will the snow stick? The sun angle combined with temperatures within a degree or two of freezing will make it harder for snow to accumulate between midmorning and midafternoon. However, snow falling heavily can overcome this. But when the snow lightens up in intensity, accumulation will be lost due to melting and it will be tough for the snow to stick to pavement. More snow will stick to grass than pavement.

What other factors could limit snow accumulation? It’s possible that there is a period of sleet before snow takes over, especially from the Beltway and to the south and east. This would reduce snowfall totals. Also, if the storm is slow to develop or tracks a little more to the north than forecast, the snow intensity would be lighter and temperatures would be warmer, both of which would reduce amounts. The bust scenario in this storm would involve a weaker storm, farther north, with less precipitation, and warmer temperatures.

What factors could increase snow accumulation? If the storm really cranks up and stays on the right track, it could really dump on the region even with the March temperature. Localized heavy bands of snow could result in snowfall rates in excess of 1 inch per hour.  Some double-digit totals would not be out of the question, especially north and northeast of the city. This would be our boom scenario.

Do we need to worry about power outages and tree damage due to the weight of the snow? This will tend to be a heavy, wet snow, and there could be isolated to scattered outages where some of the heavier totals occur due to its weight on limbs and wires. However, if we end up getting high-end amounts, it will probably mean temperatures cool enough to lighten the snow’s weight a bit. The biggest concern for outages would be east of the city, where temperatures could be 33 or 34 with heavy snow falling; this is the kind of snow that can really weigh down branches.


GFS model forecast of storm evolution.

Capital Weather Gang’s winter weather expert Wes Junker weighs in….

The models are really coming into agreement on Wednesday’s snow event. They now have a strong upper-level disturbance that takes an ideal track to place the D.C. area in a band of heavy snow Wednesday morning. This disturbance spurs the development of a coastal storm far enough south and close enough to the coast to keep feeding moisture over our area.

While significant accumulating snow looks likely across much of the area, there are still a few factors that make the forecast of accumulations difficult in and around the city.

1) Surface temperatures will be hovering right around freezing early in the day but, as the day wears on, are likely to edge above freezing especially across the city and points south and east. That makes forecasting additional accumulations during the afternoon a crapshoot.

2) If there are any pauses in the snowfall, the temperatures could edge up a bit reducing accumulation, only to drop again when the intensity picks up.

3) The late March sun angle also complicates the forecast. During any lulls in the precipitation, snow may start melting on hard surfaces and accumulation on roads during the afternoon hours may be unlikely except during bursts of intense snowfall.