(Washington Post/Washington Post)

On Tuesday, computer models suggested that the snow potential for late Thursday night and early Friday was pretty minimal, while Sunday night might offer better prospects for accumulation.

Today, they’ve traded places. Their forecasts indicate that the chance of accumulating snow very early Friday has increased a little while Sunday night’s snow outlook has dimmed.

Thursday night and Friday morning snow potential (↑)

European model shows weak low pressure crossing the Carolinas on Friday morning while cold high pressure is perched over New England. This combination could produce some light snow over the Mid-Atlantic region.

A rather weak area of low pressure will pass to our south while modestly cold air seeps in from the north. This combination may squeeze out light snowfall across the region.

Models have come into reasonably good agreement that light rain, snow or a mix of precipitation could begin after 10 p.m. Thursday. Temperatures above freezing mean precipitation should initially melt.

But into the predawn hours Friday, any rain or mixed precipitation should change to snow from the Beltway to the north and west as the air cools to near freezing. Some accumulation is possible here, especially in our colder north and west.

South and east of the Beltway, precipitation may end up as more of a wintry mix with little or no accumulation likely.

High-resolution Canadian model simulation of winter weather event late Thursday night into Friday morning.

This is expected to be a short-lived event, with the bulk of the precipitation over by Friday morning’s commute. That said, some delays and cancellations cannot be ruled out Friday.

Here’s how much snowfall the models project for the District:

  • NAM: No accumulation (but 1 to 3 inches north and west)
  • High resolution NAM: 0.4 inches (1 to 3 inches north and west; none south and east)
  • Canadian: 1.0 inches (2 to 6 inches north and west; less south and east)
  • GFS model: 1.8 inches (less than an inch north; 1 to 3 inches south)
  • European: 2 inches (1 to 3 inches area-wide)
  • High-resolution Canadian: 2.3 inches (2 to 6 inches north and west; less south and east)

The National Weather Service has issued an initial accumulation outlook (shown below), which we find reasonable. Its amounts are on the low end of models, but some of the snow predicted by models will melt at first with temperatures above freezing.

Snowfall forecast from National Weather Service for late Thursday night into early Friday. (National Weather Service)

The key to snowfall materializing will be precipitation falling fast enough to cool the air and ground. If the precipitation ends up being light and patchy, little or no accumulation is probable. But there is a chance of a moderate burst of snow, which would cool the environment enough for accumulation.

It’s premature to say where the heaviest snow will occur. Some models, like the NAM and Canadian, suggest that a heavier band of snow may form north of the District, while the GFS predicts it to set up south of town.

We would lean toward the most snow happening north and northwest of the District, simply because it is normally colder there. Considering temperatures may rise to near 50 on Thursday ahead of the event, dropping temperatures back down enough for accumulating snow to occur may prove a challenge around the city and south and east, unless the precipitation is heavy.

Overall, we think the chance of at least an inch inside the Beltway is about 40 percent but increases to 60 percent to the north and west.

We’ll attempt to draw a detailed snowfall forecast map in our update Thursday.

Sunday and Sunday night snow potential (↓)

As an Arctic front sinks south late Sunday while a storm develops along its southwest flank, the question for our region becomes: Which arrives first, the cold air or the precipitation? If the cold air seeps south quickly, before the bulk of the precipitation moves in, we could see a moderate snowfall. But if the precipitation begins before the arrival of the cold air, this will mostly be a rain event.

Models have gently shifted toward the latter and more of a rain event since yesterday.

The European model, which we put the most stock in, hangs the front back to our west as low pressure forms over the Tennessee Valley and scoots northeast over West Virginia. This draws enough mild air northward for rain beginning Sunday afternoon, continuing into the night.

The GFS model brings the cold front through fast enough so that rain, which begins Sunday morning, might mix with and change to sleet and then snow late Sunday afternoon into Sunday evening from northwest to southeast. A little accumulation would be possible in this scenario, especially in our colder areas to the northwest, where the changeover to snow happens quickest.

The Canadian model is in the European model camp, forecasting more of a rain event.

Canadian model simulation of the storm Sunday into Sunday night. The European model is similar. (PivotalWeather.com/PivotalWeather.com)

As we’re still more than four days away from this event, which is highly sensitive to small changes in the front’s timing and the track of the storm, we wouldn’t yet shut the door for snow.

“The models have been jumping from one solution to another over the past few runs,” said Wes Junker, Capital Weather Gang’s winter weather expert. “This jumpiness is always a caution for trying to be too specific about a forecast.”

He added: “Until the storm track and intensity of the precipitation is resolved, there is no way to say anything more about the forecast for the storm than there is a chance of rain or snow. "