The blockbuster snowstorm burying the mountains of California is beginning a cross-country journey that will end along the Mid-Atlantic and Northeast coastline early next week. The storm has the potential to produce a significant amount of snow in the Washington region, but perhaps only modest amounts will result if ingredients do not come together in exactly the right way.

Here’s the bottom line:

  • Computer model projections are in agreement that at least some accumulating snow will fall but differ as to how much.
  • At the low end, about 1 to 3 inches or so would fall on Sunday before precipitation changes to a light mix or even plain rain late Sunday into Monday. At the high end, at least 6 to 12 inches would accumulate, beginning Sunday and lasting into Monday night or Tuesday, perhaps mixed with sleet at times.

The key to the forecast: The amount of snow the D.C. area sees depends on the details of the development of a potential coastal storm to the south, which may crawl up the Mid-Atlantic coast. If this happens, the region will easily see its biggest snowstorm in two years and possibly since the blizzard of January 2016. But, if the coastal storm forms too far to the north and/or east, a more run-of-the-mill minor to moderate snow event would occur, with the heavier amounts falling north of the D.C. area.

The National Weather Service has placed the Washington region in an “enhanced” threat zone for a winter storm Sunday into Monday. As the storm is still three to five days in the future, substantial changes in the predicted timing and amounts of snow and mixed precipitation are possible.

Chance of at least one inch: 65 percent

Chance of at least three inches: 50 percent

Chance of at least six inches: 35 percent

Note these percentages are intentionally conservative because of the number of days until precipitation begins, taking into account the possibility the forecast could change and that models sometimes overdo snow totals at long lead times.

Possible storm scenarios

All available computer models track the initial storm from California toward the Ohio Valley, which would produce a few inches of snow across the region Sunday. Then the models show a new storm forming along the coast, southeast of Washington.

Where that second storm develops and tracks, how quickly it intensifies, and its forward speed will determine how much snow the area receives and whether the snow mixes with or changes to sleet or rain.

The European and UKMet models project that the new coastal storm will develop far enough south to hold in cold air and generate a second round of snow, some of it potentially heavy at times, from the coastal storm. But the American modeling system suggests only a front-end thump of several inches of snow before light rain and/or mixed precipitation takes over.

We broke down the various model projections, and added our forecast insights, into three scenarios. Each of these are about equally likely at this point:

Scenario 1: Significant snowstorm of 4 to 8 inches

In this scenario, the coastal storm would develop to our south faster than forecast by the American model but slower than the UkMet and European models project. This slightly slower coastal storm development would allow enough warm air to come into the region to change precipitation in areas along and east of Interstate 95 to sleet and/or rain before colder air arrives, flipping a wintry mix or rain back to snow.

Some snow could redevelop as the coastal storm cranked up, and amounts could pile up, especially in our colder areas. In this scenario, snow could last well into Monday or even Tuesday.

This scenario would tend to favor large amounts of snow, probably into the double digits, in northwest Virginia, northern Maryland and southern Pennsylvania.

Scenario 2: Major snowstorm of 6 to 12 inches or more

In this scenario, the coastal storm would develop to our south over the eastern portions of the Carolinas and slowly track northward just off the coast, possibly stalling as it is captured by an upper-level disturbance. If this were to happen, the region could see a prolonged period of snow, heavy at times, that could linger into Tuesday.

The latest UKMet model presented such a scenario and the European model has shown this scenario or something close to it since Tuesday. Snow would begin early Sunday, increasing in intensity through the morning. In this scenario, the snow might ease or even stop after an initial thump late Sunday, but then pick back up as the coastal storm strengthens. The snow would continue through Monday and possibly into Tuesday. A narrow band of heavy snow could develop, resulting in localized totals of a foot or more.

Depending on the exact track of the coastal storm, snow could mix with or even change to sleet at times, especially along and east of Interstate 95, which would lower totals some. But a strong coastal low would probably keep temperatures in the upper 20s to low 30s except into southern Maryland.

Scenario 3: Modest to moderate snowstorm of 2 to 4 inches or so

In this scenario, most of the snow would fall on Sunday, starting early in the day, from an initial slug of precipitation as the storm approaches from the west. About 2 to 4 inches of snow would be most likely, though up to 3 to 6 inches might accumulate in a few spots, most likely in our colder locations north and west of the Beltway.

This scenario is supported by the American (GFS) model, which tracks the storm into Ohio and suggests the new coastal storm will form either too far north or too far east to bring us much in the way of snow after the initial thump Sunday morning into the afternoon. Instead, a dry slot would move in and allow temperatures to warm above freezing, well into the 30s, resulting in just some remnant drizzle by later on Sunday into Sunday night.

The American model does eventually intensify the coastal storm enough to begin generating a new area of snow Sunday night into Monday, but it would be confined along and north of the Mason-Dixon Line, into southeast Pennsylvania. In this scenario, the Washington region would just be left with some spotty light rain, which might mix with or change to wet snow at times late Monday into Tuesday, without accumulating much.

Model differences

The amount of snow we will see is heavily dependent on what happens with the coastal storm. If it can develop near the coast of the Carolinas and quickly strengthen as it crawls up the Mid-Atlantic coast, the D.C. region will see a significant snowstorm. But if it develops near our latitude, too much warm air will flow into the region for significant snow, and the heaviest precipitation will end up falling to our north.

Models still present a wide range of possible coastal storm locations early Monday morning, and that’s shown in the image below, comparing simulations of the storm’s position at the same time period in the European and American modeling systems:

The storm locations, indicated by the red Ls (which stand for “low pressure”), are more focused near the coast of the Carolinas in the European modeling system but just offshore Virginia Beach in the American modeling system.

The two modeling systems have begun to converge on similar solutions, though they’re not all the way to a consensus yet, with relatively minor differences having large results on the forecast for whether we get a minor snow event or a more significant event.