American GFS model projects light rain changing to light snow next Tuesday as Arctic air spills into the region.

An unusually strong blast of Arctic air for November will sweep over the D.C. region by next Tuesday, perhaps setting the stage for the season’s first snowflakes.

Right now, this doesn’t look like a big snow event but, perhaps, a situation where we have light rain on Tuesday that changes to light snow before ending. Little or no accumulation is the most likely scenario.

The chance for a light accumulation will increase in colder areas north and west of the Beltway and especially into the mountains, where accumulation is likely between Monday night and Wednesday.

As we’re still six days away from this event, we could see forecasts trend toward a more significant snow event or a nonevent with no snow at all.

Whether it snows, temperatures Tuesday and Wednesday are likely to be much below normal with highs in the 30s to low 40s, and lows in the 20s.

Discussion

The probability of snow for next week hasn’t changed in the past day. Yes, there is still a chance of snow next week, but the details are iffy and any snow is more likely to be conversational (falling from the sky and melting) than high-impact (sticking to the roads).

The air spilling into the region behind the cold front will have Arctic origins, so it will be significantly colder than normal for this time of year. The outstanding question is whether the precipitation associated with the cold front shuts off before the cold air is entrenched enough to support snow.

The potential for snowfall is complicated by the fact that there is no high-pressure system to our north, so the cold air has to come over the mountains. In these situations, it often arrives later than forecast by computer models.

The past two American (GFS) model forecasts predict that the precipitation will start as rain before it transitions to a period of snow as temperatures plummet to below freezing. End the precipitation more quickly and delay the arrival of the cold air, and we might not see any snow. But if the precipitation hangs around longer once the cold air moves in (and if the cold air moves in quickly), there could be enough snow to accumulate, especially in colder areas.

The European modeling system indicates we have about a 20 to 30 percent chance of getting an inch of snow, but we would put it closer to 10 to 20 percent, considering the event is still six days away. (Those probabilities increase as you head west and northwest.)


Likelihood of an inch of snow between 1 a.m. Tuesday and 1 a.m. Wednesday from the European modeling system. (WeatherBell.com)

The GFS model would argue for snow hanging on long enough for accumulation, but the strung-out, rather weak look to the zone of low pressure passing to our south would argue against anything substantial.

The European model forecast is similar to the GFS, but it has slightly higher temperatures and doesn’t hold onto the precipitation as long, arguing that there might be a few snowflakes before the precipitation ends but little or no accumulation.


European model simulation of rain briefly changing to snow before ending in Washington on Tuesday. (PivotalWeather.com)

November snow history in Washington

Washington’s average snow in November is 0.5 inches, but measurable snow occurs only about once every two to three years, on average.

It snowed in Washington in November last year, when 1.4 inches fell on Nov. 15. It was the biggest snow during the month since 1989, when 3.5 inches fell just before Thanksgiving.

Washington’s biggest November snowfall on record occurred on Nov. 11, 1987, when 11.5 inches fell from a surprise storm.