A significant storm system is sweeping over the East Coast, delivering high-impact hazardous weather and setting the stage for cascading travel delays into the start of the workweek. A dramatic clash of air masses is waging war over the eastern U.S., with concerns for flooding rain, snow and potential severe thunderstorms.

Flood watches stretch from Virginia to Maine, with winter storm warnings for the interior Northeast, where a plowable amount of snow is likely. Coastal New England will deal with damaging winds, while a tornado risk could creep into the Carolinas and Virginia Tidewater.

It comes barely a day after the same system caused a deadly tornado outbreak in the Tennessee and lower Mississippi Valleys that left at least six people dead, and prompted the National Weather Service in Nashville to issue a "tornado emergency" for the first time in more than a decade.

The worst damage occurred when a tornado hit Hendersonville, a city of 61,000 in Sumner County just north of Nashville. The tornado was captured on traffic webcams as it carved across Interstate 65 and into the city.

The Hendersonville storm was preceded by a long-track tornado, which may have been on the ground for more than 40 miles, that tracked from north-central Tennessee to near Bowling Green, Ky. It's unclear if the path was continuous.

A child and two adults were killed in Clarksville, Tenn., near the border with Kentucky, the city's mayor said.

Other tornadoes hit Mississippi, Alabama and Florida. It's two years to the day (Dec. 10, 2021) since a high-end EF4 tornado on the 0-to-5 scale for intensity killed 57 people, mostly around Mayfield, Ky., during a nearly three-hour rampage that spanned over 150 miles.

While tornadoes aren't unheard of in December, there is some research to suggest that rising temperatures linked to human-caused climate change may be making it easier for them to form in the "offseason." Temperatures ahead of Saturday's tornadoes were 10 to 20 degrees above normal.

The basic setup

A broad, diffuse strip of low pressure was anchored along a cold front near the Carolina Piedmont on Sunday morning. To the east, southerly winds were allowing a warm, humid air mass to waft northward. Temperatures could approach 70 degrees even in Washington, D.C.

Behind the front and low pressure, temperatures were crashing. That's why snow is expected on the cold side, or back side, of the system.

A high-altitude disturbance, meanwhile, will intensify that new low-pressure system as it swings into the Northeast. That's why winds will ramp up dramatically Sunday night, while also tugging more cold air toward the coast and flipping rain to snow briefly in Washington, Baltimore and Philadelphia.

Heavy rain/flooding

A general 1 to 2 inches of rain is expected east of the Appalachians, with 2 to 3 inches near and east of Interstate 95 - including Washington, Philadelphia and New York.

Two zones could pick up more, with amounts locally approaching 4 inches:

- The Virginia Tidewater. That's where training thunderstorms, or storm cell that move repeatedly over the same areas, could locally boost rainfall.

- The Connecticut River Valley in central Connecticut and west-central Massachusetts, perhaps east to the Worcester Hills or even Blackstone Valley. Persistent downpours may also lead to greater accumulations.

While flood watches remain in effect, flood impacts aren't expected to be widespread in the Mid-Atlantic. Most places there are running 6 inches or more behind average for the year to date, which means the rain will be largely beneficial.

Tornado risk

The National Weather Service Storm Prediction Center has outlined a level 2 out of 5 risk of severe weather for a portion of northeastern South Carolina, eastern North Carolina and southeastern Virginia. Areas from Virginia Beach to Wilmington are included, as are places east of Raleigh and the Research Triangle.

It appears that a filament of warm, humid air will waft north into those zones, providing instability, or storm fuel. Changing winds with height will sculpt any individual storms into rotating supercells, assuming they don't merge into messy lines or clusters. An isolated tornado is possible, especially near the North Carolina-Virginia border northeast of Raleigh.

Heavy snows

- Accumulations inland

A significant snowstorm is possible in parts of the Blue Ridge and the Appalachians of Pennsylvania, New York State and northern/central Vermont.

Precipitation will begin as rain, but will switch to snow Sunday evening or overnight as colder air works in from the northwest. An inch or two is expected in parts of the Alleghenies, with minimal accumulations in Pennsylvania due to warm ground temperatures and earlier rainfall. Amounts stack up from interior New York into Vermont, as well as in northern New Hampshire and western Maine.

That's where 4 to 8 inches is expected, with a few of the higher elevations seeing 6 to 12 inches.

- I-95 changeover

There are also growing signs that a brief flip to snow may occur late Sunday night, probably after midnight, in parts of the Mid-Atlantic. That includes areas near and northwest of the Washington, D.C., metro, as well as Baltimore and Philadelphia.

No accumulation is expected on paved surfaces, though a coating can't be ruled out in grassy areas.

It's unlikely that the snow reaches New York City.

Windstorm in coastal New England

A zone of strong winds above the ground, known as a low-level jet stream, will be intensifying as it swings overhead Sunday night. That means winds will be screaming northward at 80 to 100 mph just half a mile above the ground, and some of that momentum will probably mix to the surface in the form of strong to locally damaging wind gusts.

Gusts around 50 mph are likely east of New York City northeast to a Hartford-to-Boston line. South and east of Providence, a few gusts to 60 mph are possible, with gusts of 70 mph not out of the question for outer Cape Cod and the Islands.

Downeast Maine could also see winds approaching hurricane force at the immediate coastline, with gusts up to 85 mph for coastal regions in New Brunswick around the Bay of Fundy. Gusts over 80 mph are also probable in southern Nova Scotia.

The winds will be worst Sunday night through midmorning Monday, probably leading to flight cancellations at Boston's Logan International Airport.

Scattered power outages are also possible, especially in eastern Massachusetts and Downeast Maine.