Winter storm watch: Significant snow possible late Monday into Tuesday, but amounts uncertain – Washington Post

(This post, originally published at 11:30 a.m. Saturday, was updated mid-afternoon and in the evening.)

* Winter storm watch 7 p.m. Monday to 2 p.m. Tuesday *

It took almost the entire season, but the D.C. area finally has a legitimate shot at a significant snowstorm late Monday into Tuesday.

If everything comes together just right, we could see a major storm from the District to New England, with heavy wet snow and strong winds that might cause power outages. That said, forecast confidence remains low at this time, and potential accumulations in the D.C. metro area range from just a little to a lot (sorry, folks, that’s the nature of snow forecasting around here, especially with more than 48 hours to go until onset).

The low forecast confidence is mainly because of uncertainties in the track and strength of the storm’s low-pressure center, which will affect how much snow we see and whether at least parts of the area see the snow change to rain. As is usually the case, slight changes in the storm’s track could be the difference between a bust and a blockbuster.

Because of the heavy snow potential, the National Weather Service has issued a winter storm watch for much the region Monday night through early afternoon Tuesday. A winter storm watch means there’s a 50-50 chance or better of at least five inches of snow. The watch is in effect for much of the D.C. and Baltimore region with the exception of southern Maryland (Calvert, Charles, and St. Mary’s counties).  The watch statement notes “some locations may see significantly higher accumulation” than five inches.

The region is part of a watch that extends all the way north into Maine.

Based on our analysis of the latest model information, here is the probability of different snow amounts, which could increase dramatically from southeast to northwest across the metro area:

Immediate metro area, inside the Beltway

  • One inch or more: 70 percent chance
  • Four inches or more: 50 percent chance
  • Eight inches or more: 40 percent chance

North and west of the Beltway

  • One inch or more: 85 percent chance
  • Four inches or more: 70 percent chance
  • Eight inches or more: 55 percent chance

South and east of the Beltway

  • One inch or more: 60 percent chance
  • Four inches or more: 35 percent chance
  • Eight inches or more: 20 percent chance

The most likely timing for snow is from Monday evening, probably starting after the commute home, through midday Tuesday. If the heavier amounts of snow materialize, then travel Monday night and Tuesday could become difficult, with many delays and cancellations. But that’s still a big “if” as of now.

While the models have reached a consensus that there will be a big storm, they still differ on the exact track, which has huge implications as to how much snow vs. rain and where. Generally speaking, areas north and west of the District have the best chance of seeing the most snow. Areas from D.C. to the south and east could also see significant snow, but they also have the highest possibility of warmer air reducing accumulation or changing some of the snow to rain.

The National Weather Service forecast office serving the D.C. region has made a first attempt at predicting snowfall totals for the region, and its shows a big differences in amounts depending on where you live, with “lesser east, greater west.” It’s calling for 10 inches in Leesburg, but just two inches in Annapolis.

In its latest forecast discussion, the Weather Service said it expects it will post a winter storm watch for the region overnight. It issues a winter storm watch for situations when there is the potential for at least five inches of snow.

What’s working  for snow?

  • Cold air in place ahead of the storm, with strong high pressure entrenched over southeastern Canada to help hold the cold air in place
  • Potential for an almost ideal storm track that’s close enough to our east to produce heavy precipitation, but far enough east to keep warmer air away
  • What looks to be the overnight timing for much of the snow (it’s harder for snow to accumulate during the day with the strong March sun)

What’s working against snow?

  • Possibility of a storm track too close to our east, which would push warmer air into at least southern and eastern parts of the area
  • Potential for the storm to be very intense with strong winds from the east blowing warmer ocean air into at least southern and eastern parts the area
  • Potential for a weaker storm that produces lighter snow that accumulates less because of warmer temperatures

What is the highest-impact scenario?

If all the ingredients come together just right, portions of the area could see 8 to 12 inches (or even a bit more) of wet, wind-driven snow. With temperatures forecast to be right around freezing, the snow could be quite wet, raising the possibility of tree limbs coming down and hitting power lines. The northern and western suburbs have the best chance of seeing this scenario happen, but we can’t rule out the same for the rest of the area with the right storm track.

What is the lowest-impact scenario?

The storm tracks too far west, bringing in warmer air which would limit snow accumulations or primarily produce rain. Or it tracks too far east to give us much precipitation at all (this seems to be the least likely scenario, but it’s not impossible). In this scenario, snow accumulations would be minimal east of Interstate 95, but a few inches would still be possible our colder areas to the west.

Can it really snow in March?

Absolutely. As Ian Livingston detailed yesterday, we’ve had plenty of big March snows. Most recently, March 2014 featured three storms that together produced just over a foot of snow in the District.

Why the uncertainty?

The image below from the European model shows why at this point there is still a lot of uncertainty in the forecast. The model is run 50 times, each with slightly different initial conditions, to account for our inability to measure the initial state of the atmosphere with 100 percent accuracy at all points in space and time. Each L on the map shows where each model run is predicting the storm’s low-pressure center to be by Tuesday morning.

Note how much spread there is between the different low positions. Any low over the Eastern Shore or southeastern Virginia would probably draw in enough warm air to change the rain to snow as far west as Dulles Airport. Alternatively, the lows located toward the eastern edge of the green shading would either keep the precipitation to our east or would cause the heaviest snow band to be east of the city. The lows positioned toward the center of the green would probably result in a big snowstorm for the entire metro area.

The bottom line is that, as of this morning, the models still have not reached a consensus concerning who in our area, if anyone, cashes in with a major snowstorm.


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