Weekend tornadoes killed at least five people, including three children, in the US South, authorities said on Sunday, as a massive storm system with damaging wind and hail moved across the country and began drenching East Coast states.
A tornado touched down and spun toward Enigma, Georgia, on Sunday, after 17 twisters were reported on Saturday and earlier on Sunday across the South from Texas to Alabama.
"We'll be seeing severe weather from Florida to New York, with the most unstable parts so far in Georgia," said meteorologist David Roth of the National Weather Service (NWS) Weather Prediction Center.
Tornado watches, some lasting through early Monday, were issued for most of West Virginia, most of North Carolina, Virginia, Pennsylvania and Maryland and parts of Ohio and New York. The watches mean conditions are favourable for tornadoes to form.
The affected areas will get heavy rains, winds with gusts of up to 70 mph (110 kph) and the possibility of hail, Roth said.
"Whenever you have a tornado watch, you also have threats for severe thunderstorms," he added. More than 100 million people from the middle of the United States to the East Coast were at risk of extreme weather, facing warnings of heavy thunderstorms and another round of tornadoes, said NWS meteorologist Bob Oravec.
Nearly 2,300 US flights were cancelled by Sunday evening, more then 90 per cent of them at airports in Chicago; Houston, Texas; Charlotte, North Carolina; Pittsburgh; Columbus, Ohio and a dozen major airports on the Eastern Seaboard, according to FlightAware.com.
The storm's cold front also brought snow to Chicago on Sunday, with 1 to 3 inches (2.5-7.6 cm) reported in central Illinois.
Two children, siblings aged 3 and 8, were killed on Saturday when a tree fell on the car in which they were sitting in Pollok, Texas, said a spokeswoman for the Angelina County Sheriff's Department.
A third child, Sebastian Omar Martinez, 13, drowned late on Saturday when he fell into a drainage ditch filled with flash floodwaters near Monroe, Louisiana, said Deputy Glenn Springfield of the Ouachita Parish Sheriff's Office.
In another storm death nearby, an unidentified victim's body was trapped in a vehicle submerged in floodwaters in Calhoun, Louisiana, Springfield said.
In Mississippi, Governor Phil Bryant said one person was killed and 11 people injured over the weekend as tornadoes ripped through 17 counties and left 26,000 homes and businesses without electricity.
"It could have been much worse," Bryant told reporters after issuing an emergency declaration for the affected areas, including Monroe County where a curfew was ordered from 8 pm Sunday through 6 am Monday.
In addition, three people were killed when a private jet crashed in Mississippi on Saturday, but Bryant said it was uncertain whether it was caused by the weather.
Soaking rains could snarl the Monday morning commute on the East Coast before the storm moves off to sea.
"The biggest impact rush hour-wise probably will be Boston, around 7 to 8 o'clock in the morning, and around New York City around 5 or 6 o'clock, before sunrise," Oravec said.