Computer models have come into reasonably good agreement that the big winter storm that will sweep through the Midwest and South and up the East Coast will produce a sloppy mix of heavy precipitation in the Washington region.

It will start as snow for a time Sunday afternoon and evening, before changing to sleet, freezing rain and plain rain. This will create very hazardous driving conditions for a time Sunday as roads will be frozen from cold air ahead of the storm. Highs on Saturday will be only in the 20s.

If you’re traveling in the region Sunday, be aware that the snow is most likely to begin between about 1 and 4 p.m. and may be heavy at times, especially from about 4 to 7 p.m. Between 7 and 10 p.m., the transition from snow to sleet to rain is anticipated from southeast to northwest in the immediate area. This timeline could shift some as the storm draws nearer and we’re able to refine the forecast.

Conditions may gradually improve overnight Sunday into Monday as temperatures moderate and the conglomeration of snow and ice turns more to slush; in some areas, especially east and southeast of the District, the snow could be completely washed away by heavy rain. However, there’s some chance that rain and mixed precipitation will change back to snow briefly before ending, so we can’t rule out some slick spots Monday morning.

Our best bet is that one to four inches of snow and sleet will fall in the immediate D.C. area before precipitation flips to the more fluid form. Areas north and west of the Capital Beltway may be on the higher end of that range, or even exceed it in places like northwest Montgomery County, Leesburg and Frederick. These areas may also see the snow change to sleet and freezing rain rather than plain rain, meaning a longer duration of hazardous conditions.

In the Blue Ridge, near Interstate 81 and into the Appalachians, where precipitation may remain mostly snow, at least 6 to 12 inches are possible.

There are still significant questions as to the exact track of the storm, which will have a huge bearing on how long precipitation remains in the frozen form before flipping to rain. But, at the moment, it seems very likely that areas from Interstate 95 and eastward will see a changeover from snow to rain.

Here’s our initial accumulation outlook for the District (on Friday, we will provide a snowfall forecast map for the region):

  • Chance of at least one inch: 70 percent
  • Chance of at least three inches: 40 percent
  • Chance of at least six inches: 15 percent

Storm scenario breakdown and model discussion

The big forecast question pertains to the track of the storm as it comes up the East Coast. Will it track right over the city, the Chesapeake Bay, or even farther east or west? The farther west the track, the quicker the freezing line rockets north and west and snow changes to sleet and rain. The farther east the track, the longer the cold air holds and the more snow that falls.

A track over the bay and especially farther east would allow for colder air to hang on longer — this would mean more snow before the switch to sleet. A period of freezing rain would also possible.

A track near or west of the District would push the rain-snow line west really quickly, minimizing how much snow might accumulate along and east of the track. Some snow would still fall, followed by sleet, but temperatures would probably warm up enough for plain rain to take over.

The possibility of a track east of the bay along the Atlantic Coast, which would produce more snow and less rain, is very low based on current model simulations

We’ve boiled down the likely storm outcome to two main scenarios, but the first is far more probable than the second.

Scenario 1: Slop — snow changes to sleet and rain. 1 to 4 inches. (85 percent chance)

In this scenario, the storm tracks inland from the Carolinas northward and passes somewhere between the bay and the District. Snow would begin Sunday afternoon, with the snow mixing with and changing to sleet and rain along and east of Interstate 95. The farther north and west of the city you are, the longer snow would hold on and the higher the possibility of its being replaced by sleet and freezing rain rather than plain rain. The precipitation could be heavy at times.

Historically, such a track keeps snowfall amounts along and east of I-95 around 1 to 3 inches with 2 to 4 inches just to the west and at least 4 to 6 inches toward I-81; locations in the mountains would probably see double-digit totals.

There is a slight chance that the rain will change back to snow briefly as the storm pulls away early Monday, but most models track the storm far enough west that a dry slot passes through the region with no changeover.

The question of whether locations west of the storm track hold on to temperatures below freezing after the snow is over is always a tricky one. Models are often are too aggressive in eroding the cold air, and rather than plain rain, freezing rain occurs after the snow ends. We’ll try to refine this aspect of the forecast in next day or two.

Scenario 2: Precipitation remains mostly snow with at least 4 to 8 inches. (15 percent)

None of the primary runs of the major computer models present this scenario, but a few of their alternative, lower-resolution simulations do. The fact that the European, American, UKMET and Canadian models all have an inland track suggests that our snowiest scenario has very long odds against it.

But, if such a track were to occur, the immediate D.C. area would likely end up with at least 4 to 8 inches of snow, though some mixing with sleet could still occur.

How much snow do the models project?

Here are the latest snowfall totals projected by the main computer models for the District:

  • American (GFS): 5 to 6 inches. Probabilities from its group of 30 simulations: chance of at least 1 inch: 80 percent; chance of at least 3 inches: 50 percent; chance of at least 6 inches: 30 percent.
  • European: 4 to 6 inches. Probabilities from its group of 50 simulations: chance of at least 1 inch: 90 percent; chance of at least 3 inches: 60 percent; chance of at least 6 inches: 10 percent.
  • Canadian: 1 inch. Probabilities from its group of 20 simulations: chance of at least 1 inch: 80 percent; chance of at least 3 inches: 30 percent; chance of at least 6 inches: 5 to 10 percent.
  • UKMET: 2 to 3 inches.

On Friday, we’ll produce our own snowfall forecast map and detailed timeline for this event.