Even as recent warmth may have residents of the Northeast looking ahead toward summer, a late-season winter storm is threatening to bring snow to portions of the region on Monday and Tuesday.

In an acute case of weather whiplash, at least 4 to 8 inches of snow could fall in hilly and mountainous areas of the interior Northeast amid temperatures up to 25 degrees below normal. Even the high elevations of the Mid-Atlantic (above 2,000-3,000 feet) could see snow accumulation.

“This late season system will have high water content (moisture) and will be quite heavy,” the National Weather Service wrote Sunday. “This could result in downed trees, power outages, and difficult shoveling. Hazardous travel conditions may develop due to slushy, snow-covered roads.”

Although computer models show a range of possible snow outcomes, they agree that a strong zone of low pressure will track up the Eastern Seaboard on Monday and Tuesday. The major forecast question is what areas see snow versus a cold rain. The storm will draw down cold air from Canada but is occurring at a time of year generally hostile toward subfreezing temperatures.

The lead-up

The past week has been quite warm across the eastern half of the United States, a result of the fast-flowing belt of air known as the jet stream being swept north into Canada.

But much cooler air has been lurking on the western side of this atmospheric boundary. Portland, Oregon, recorded an inch of snow on April 11, its latest measurable snowfall on record by a wide margin.

In the Northern Plains, winter’s siege continued on the 12th and 13th with a blockbuster North Dakota blizzard. More than 2 feet fell across a fairly wide swath of the state in a storm followed by record-breaking cold and harsh winds.

A jet stream realignment to start the weekend has sent this unusually cold air surging eastward. On Sunday morning, International Falls, Minnesota, dipped to 11 degrees, tying a record low for April 17.

By Sunday night, below-freezing temperatures are possible as far south as the central Appalachians. Frost advisories and freeze warnings are in effect west and northwest of Washington. This cold air mass will set the stage for the early-week winter storm.

Monday

The beginning of the workweek will see two zones of low pressure stretched out ahead of a dip in the restructured jet stream, one over the western Great Lakes and the other over the Southeast. An expansive zone of precipitation accompanying the two storms will push into the cool eastern U.S. air mass Monday morning, resulting in snow that will fall as far south as the highlands of western Virginia, western Maryland and West Virginia.

Snow will spread through the higher terrain of Pennsylvania on Monday afternoon and night before tapering to off to snow showers and flurries later Monday night into Tuesday.

In the higher elevations (above 2,500-3,000 feet) of northeast West Virginia, western Maryland and western Pennsylvania, 2 to 6 inches or so could fall, with locally higher amounts not out of the question. Little or no snow accumulation is probable below 1,000 feet.

From Richmond to Philadelphia, just a cold rain is expected Monday, but with temperatures only in the 40s to near 50, about 20 degrees below average.

Tuesday

As the two zones of low pressure merge near the coast of the northern Mid-Atlantic on Monday night, they will form an intensifying coastal storm or Nor’easter. Bands of precipitation will wrap inland, with moderate snow across higher elevations of interior New York and New England and steady, cold rain closer to the coast and at lower altitudes.

Where moderate snow is falling, it will reduce visibility and turn roads slippery for the morning commute. The dense, wet snow will also quickly begin to plaster onto trees and power lines, which could lead to scattered power outages.

Even outside of the interior and high-elevation portions of the Northeast, it seems possible that the intensity of precipitation could cause rain to change into snow. This could lead to difficult-to-predict bursts of heavy, wet snow closer to the coast.

Snow will begin to wrap north into interior New England by Tuesday afternoon. Where heavy snowfall is persistent, and where snow can fall for the longest without rain mixing in, more than 6 inches could well accumulate by Tuesday night. This will be most likely across the high terrain of eastern New York, Vermont, New Hampshire and perhaps interior Maine.

How unusual is this?

Interior Northeast snow in mid-April is not particularly uncommon, though this storm will be unusual in a few regards. Daily minimum temperature records are likely to be broken. On Monday, a number of locations in the Mid-Atlantic may see their coldest high temperatures on record for April 18. By Wednesday morning, in the storm’s wake, a few record-low temperatures for April 20 could materialize in parts of the interior Northeast.

The storm will also be fairly unusual for its overall output of precipitation. The equivalent of 2 inches of rain (or melted snow) could occur across the interior Northeast. This means where precipitation falls as snow, it will have an uncommon ability to produce damage to trees and power lines because of its heft.