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Powerful nor’easter to bring drenching rain, inland snow to Northeast, Mid-Atlantic

Over an inch of rain is likely from Washington to Boston, with the potential for heavy snow in interior New England

The American GFS model simulates total moisture associated with the Saturday storm. (Pivotal Weather)

The calendar flipped to meteorological winter Tuesday, and the atmosphere is going all in. A strong storm system could drench the coastal Mid-Atlantic and Northeast with a soaking shot of rainfall late Friday night into Saturday, while inland areas risk being blanketed by the first big snow of the season.

The impending storm’s exact track and impacts are far from worked out, despite our being less than two days from its arrival. One thing is clear, though: The system will contain a prodigious amount of moisture.

Washington winters are rapidly warming up and feeling more Southern

Places such as Washington, D.C., Baltimore, New York and Boston all could see excessive rainfall from the system, with two inches or more of water likely for some. Daily rainfall records could be challenged or broken, and there’s a chance Saturday could become a top-five wettest December day in a few cities.

A storm taking shape

The ingredients for Saturday’s storm system were already beginning to converge Thursday, their delicate overlap key to tapping into the atmosphere’s power. An area of low pressure was rolling along the Oklahoma-Kansas border toward the Ozarks after bringing as much as 10 inches of snow in northwest Oklahoma, while another disturbance was sagging south out of Ontario. The two will probably “phase,” or overlap, early Saturday, their forces combining into a more significant storm system.

If and where exactly they do so will determine how strong the storm becomes and exactly where it tracks, which will have important implications for how much rain and snow falls in different locations.

It’s a “rather complex upper air pattern which models are still struggling to resolve,” the National Weather Service in Boston wrote. “It all hinges on the complex interaction of northern and southern stream energy and potential phasing … [we] have to keep all options on the table.”

Storm begins taking shape late Friday, affects Mid-Atlantic into Saturday

Rain and a few thunderstorms will translate east with the storm over Georgia, the Carolinas and the Southeast on Friday and Friday night, with raindrops as far north as the nation’s capital by the afternoon. The storm will unleash heavy rainfall over Washington, Baltimore and potentially Philadelphia overnight Friday into Saturday as it “wraps up” into a classic comma-shaped system and pivots northeast. Rain should end in the Mid-Atlantic either Saturday morning or afternoon, depending on how fast the storm moves through.

New York City may be on the western fringe of the heavy rainfall late Friday night and Saturday. Until the eventual track of the storm is ironed out with greater confidence, it will be tricky to gauge just how wet the Big Apple will get.

A challenging forecast in New England

The storm will shift into southern New England on Saturday after dawn, with cold air entrenched in the mountains inland and borderline temperatures for snow near the Interstate 95 corridor. That could favor a wallop of snow for some, with a half-foot or more possible where the cold and the moisture sufficiently overlap.

The track of the storm will prove crucial in determining where the rain-snow line sets up. A classic New England snowstorm would require a slightly offshore track, with cold air spiraling into the system on its backside flipping rain to flakes. This system is more likely to be an “inside runner,” tracking near the Cape Cod Canal and potentially near the Maine coastline.

Such a track would favor 1.5 to 2.5 inches of rain for Boston and Providence, R.I., but could make things more interesting for Hartford, Conn., Worcester and Concord in Massachusetts, and points north and west, where enough cold air could be drawn in for wintry precipitation.

Elevation will favor rain during the day on Saturday in northwest Connecticut, including Hartford, but snow will be possible as the day wears on if temperatures drop and colder air works in behind the system’s center. Only minor accumulations are likely in Hartford proper, though the higher terrain northwest of town could pick up more.

The Worcester Hills, and in particular regions between the Connecticut River and the Blackstone Valley, including central Massachusetts, are the biggest wild card right now. There exists the potential for a widespread four inches of snow or greater Saturday and Saturday night, but further details are impossible until the exact track is ironed out.

Some places in western Maine and northern New Hampshire could see more than a foot if the storm reaches its potential between Saturday and early Sunday.

Along the coast of New Hampshire and into coastal Maine, rain should predominantly fall. But just inland, temperatures at the onset of the precipitation may favor rain mixed with a few snowflakes. If precipitation becomes heavy enough, it could drag down cooler air, with “evaporative cooling” also chilling the atmosphere. That may be enough to help the rain/snow line collapse toward the coast and bring about snow west of Interstate 95.

The storm will eventually become a big wind generator in eastern New England. Gusts could top 40 mph near the shoreline, with 50 mph gusts on Cape Cod and the Islands.

A potent early-week storm brought strong winds to Northeast, snow to South and tornadoes to Mid-Atlantic

Precipitation will come to an end across southern New England on Sunday morning as colder air filters into the region on northwesterly winds.

Snow will continue most of the day in interior northern Maine on Sunday as the system withdraws into the Canadian Maritimes.