An unusually strong December coastal storm is set to batter the Eastern Seaboard, producing a huge swath of heavy, wind-driven rain. Strong winds, topping 60 mph in some instances, will batter coastal areas that also should expect shoreline flooding and beach erosion. The rain will be heavy enough inland to cause areas of flooding.

The moisture-loaded storm is organizing over the Gulf of Mexico on Saturday and will charge northward, progressing from the Georgia coast Sunday morning to the Mid-Atlantic coast Monday morning and to New England on Monday afternoon.

The National Weather Service is describing the storm as "intense" and "powerful."

The system begins with the risk of tornadoes Saturday night into Sunday across the Florida Peninsula. At least some tornado risk will spread north into coastal Georgia and/or the Carolinas on Sunday.

Ordinarily, a sprawling December storm system carrying this much moisture would feature a rainy side and a snowy side. That said, there's hardly any cold air to come by in the eastern United States, so mostly rain will fall until some snow showers wrap around on the storm's backside Monday and Monday night in the interior Northeast and the Appalachians.

There will be a lot of rain, too ― with widespread totals of 1 to 3 inches from Florida to Maine, with locally higher amounts as the storm draws moisture from the Atlantic, the Caribbean and even the Pacific. Some areas will be bombarded by a narrow jet of particularly heavy rain, not unlike the atmospheric rivers that affect the West Coast.

Flood watches have been issued for portions of Florida, New Jersey, eastern Pennsylvania and the Delmarva Peninsula (which consists of portions of Delaware, Maryland and Virginia); those watches affect more than 20 million people. The watches probably will be expanded to many other areas along the East Coast.

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Saturday-night tornado risk in Florida

Saturday night into Sunday will yield the risk of severe thunderstorms and a few tornadoes in Florida, particularly from Gainesville and Daytona Beach southward. The center of developing low pressure will move ashore in the Big Bend or Panhandle. Winds from the east and southeast will spread over most of the peninsula, drawing inland a filament of warm, humid and unstable air - storm fuel.

That will allow thunderstorms to sprout, and some may rotate. While winds will blow from the east and southeast near the ground, they'll be more westerly at higher altitudes.

That change of wind speed and/or direction with height, meanwhile, will cause storm clouds that span multiple layers of atmosphere to spin. That's why concern for tornadoes exists, particularly overnight in Florida.

On Sunday, the risk of severe storms - and a somewhat lower tornado risk - will shift toward the eastern Carolinas.

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Heavy rain for the East Coast

The moisture-infused storm will unload at least 2 to 3 inches of rain over a wide swath near where the storm center tracks, bringing the potential for pockets of flooding near creeks and streams and areas of poor drainage.

After dousing Florida Saturday night, the rain will spread over the Carolinas Sunday morning, reach D.C. and Baltimore by midday, Philadelphia by evening and work toward New York City Sunday night. In most areas rain will last for about 12 to 18 hours before ending.

Portions of eastern South Carolina and perhaps southern North Carolina could see some of the heaviest rain, potentially approaching 4 inches. That's where weather models are suggesting an influx of subtropical moisture that could contribute to "potentially heavy to excessive rainfall" according to the Weather Service. The agency notes that some computer models show the potential for rainfall rates up to 2 inches per hour Sunday afternoon in this zone.

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Strong winds

As the storm system intensifies while riding up the coast, it will inhale air from all sides. That vacuum effect will result in strong winds.

Gusts could hit 60 mph at the coast from the South Carolina/Georgia border north to the Gulf of Maine. A few weather models hint at gusts to 70 mph for the Outer Banks, Cape Cod and Downeast Maine.

Along the Interstate 95 corridor, gusts could reach 40 to 50 mph or even a bit higher in Southern New England. Some power outages could result and treefalls are possible as heavy rain loosens soils and makes trees more susceptible to strong gusts.

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Coastal flooding

A persistent onshore flow ahead of the storm center's arrival will pile water against the coastline. Minor to moderate coastal flooding is likely at minimum, and more significant coastal flooding may occur if the storm tracks nearer the coast and the easterly winds are stronger.

Moderate coastal flooding with water levels up to 2 to 3.5 feet above normal is expected in Charleston Sunday morning, the North Carolina Outer Banks Sunday afternoon and night, Virginia Beach Sunday evening and parts of the Delmarva Sunday night. This threat will spread over the Jersey Shore, Long Island and the south coast of New England on Monday.

There will also be 60 foot waves within 25 miles of the coastline for both the Outer Banks and Cape Cod. While waves that large will stay offshore, the seas will undoubtedly be churned up closer to the coast, resulting in beach erosion.

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Jason Samenow contributed to this report.