A significant winter storm is set to unleash an array of snow, sleet and ice across much of the central and eastern Lower 48 this weekend. Major travel disruptions and hefty accumulations of wintry precipitation are likely, as the storm is expected to slip through the Upper Midwest, Mid-South and Eastern Seaboard to punctuate Martin Luther King Jr. Day weekend.

The Washington region is under a winter storm watch ahead of a Jan. 16 storm expected to bring snow, sleet and rain. Here’s what you need to know. (Amber Ferguson/The Washington Post)

The National Weather Service is already noting that the winter storm could have an “extreme” impact for some. The storm will deliver its wintry wallop in a three-act performance that could impact as many as 75 million Americans.

While the storm has yet to develop, it has been the subject of considerable forecast uncertainty for days. Weather models have been waffling in their simulations, resulting in doubts about precipitation type and totals along the Interstate 95 corridor.

Fortunately for forecasters, the instigating ingredients that will spur the system’s growth were in the process of moving ashore in the Pacific Northwest on Thursday morning, allowing them to be “sampled” by weather balloons. That will yield a better availability of data that can be used to help fine-tune computer model output.

The storm now

While the storm had not yet materialized as of Thursday morning, its precursors were already bringing active weather to parts of Washington state and Oregon. A shortwave trough, or lobe of cold air, low pressure and spin nestled within a dip in the jet stream, was cruising ashore, with dry air in its wake spelling an end to days of rainfall from a stubborn “atmospheric river.”

The mid-level low-pressure system will begin to acquire a negative tilt — or a slant resembling a backslash — that will give it more of a kick. That will help it energize a surface disturbance early Friday, ejecting east of the Rockies out of Montana onto the High Plains. High pressure over Manitoba and Ontario will reinforce an uninterrupted supply of cold air over the north-central United States, setting the stage for snow.

Plains and Upper Midwest

The low will tug north a ribbon of moisture that will “overrun” up and over a lip of cold, dense air near the surface. That will deposit a strip of snow from central North Dakota southeast over the Lake Traverse Reservation of northeastern South Dakota and the James River Valley. Southwestern Minnesota and most of Iowa will see moderate to major accumulations, too, as the snow continues over the Corn Belt. Sioux Falls, S.D., and Des Moines could be in the jackpot zone.

The key time frame to watch will be from sunrise Friday near the Canadian border to early Saturday along the Interstate 80 corridor of southern Iowa.

Winter storms have brought heavy snow and ice to Canada in the first weeks of January. (The Washington Post)

Winter storm watches and warnings have been hoisted in the narrow zone of real estate likely to pick up a good thump of snow, with a widespread 6 to 10 inches and localized one-foot totals slated to fall. Only watches were in effect over most of Iowa on Thursday morning, since warnings usually aren’t issued more than about 36 hours before an event.

Unlike in a number of recent events, strong winds aren’t expected to coincide with the falling snow, leaving blizzard conditions out of the question. High-wind warnings are in effect, however, across western South Dakota from roughly the Missouri River to the Black Hills, where gusts topping 55 mph are possible as a frigid air mass builds in behind the system on Friday night.

The snow will continue Saturday into northeastern and north-central Missouri, primarily well north of Interstate 70 and east of Interstate 35, although Kansas City could pick up several inches of snow Friday night into Saturday morning.

Ozarks, Tennessee Valley and Mid-South

From there, uncertainty increases. Forecasters are confident the storm will take a left, or eastward, turn as it approaches Arkansas and shifts toward the Tennessee Valley, but we still have yet to iron out exactly where that inflection in path takes place. That will have a bearing on who ends up on the warm side of the nascent storm system and who is thrust into the chilly air north of the low-pressure center. That, in turn, will determine precipitation type and amounts.

In cities such as Memphis and Nashville, the potential exists for a couple of inches of snow, with a low-end risk of a plowable snow. Northern parts of Middle Tennessee and the Cumberland Plateau will probably see more-appreciable totals, as will most of Kentucky.

There will be a sharp southern cutoff to the snow, so while places like Tupelo, Miss., Huntsville, Ala., and Atlanta could see a few flakes on Saturday night, the propensity for anything more appears rather limited.

Appalachians and East Coast

This is where the forecast continues to remain hazy. What transpires across the East Coast is predicated on the exact track of the system. Right now there are a couple of possibilities:

  • Least likely: an offshore track that would swirl cold air down across the Eastern Seaboard. That would reduce the amount of moisture present for snow but maintain cold-enough temperatures to keep it a predominantly snow event. This appears improbable right now.
  • Most likely: an “inside runner” track that would bring the low-pressure system up the East Coast. That would tug ashore a filament of warm air at the mid levels. While high pressure over Ontario is currently banking cold air in place across the East Coast, the incoming low will scour away that cold gradually. Thus, the Interstate 95 stretch from Washington, D.C., to Boston — including Baltimore, Philadelphia, New York, New Haven, Conn., and Providence, R.I. — may begin with a thump of snow Sunday afternoon or evening before flipping to sleet and eventually all rain. Accumulations would probably be limited to a couple of slushy inches that would quickly fade away.
  • Also possible: a second area of low pressure over the Hudson Bay could yank the storm farther north-northwestward, placing more of the East Coast on the warm side of the counterclockwise-spinning atmospheric eddy and maintaining a mainly rain event.

Areas west of Interstate 81 in the Mid-Atlantic should stay mostly snow, as will the Appalachians, leading to potential double-digit snowfall in places like the mountains of western North Carolina and eastern Tennessee, western Virginia, West Virginia, central Pennsylvania and parts of New York state. Eastern Ohio may see some snowfall with the “wraparound” on the back side of the departing low into Monday, which could receive some lake-effect enhancement. That’s something to watch for, Cleveland.

Meteorologists are also carefully tracking where a “dry slot” could end precipitation early and put a dent in realized totals.

Ice storm potential in Carolinas

Cold air draining down the Appalachians on Sunday may remain quite stubborn across central parts of extreme northern South Carolina and especially central North Carolina. With moisture and mildness riding up and over that shallow wedge of cold air hugging the ground, precipitation may fall as liquid rain before freezing on contact with the surface.

This is a particularly low-confidence element of the forecast, but ice storm conditions may be possible for some. We’ll fine-tune those predictions in the days ahead.