Tornadoes, wind gusts of up to 110km/h and pounding hail remain threats from eastern New York and into New England, as the remnants of a deadly weekend storm push out to sea, the National Weather Service says.
"This is an ongoing threat," said meteorologist Brian Hurley at the NWS Weather Prediction Centre in Maryland on Monday.
"It's halfway through the eastern states now. By daybreak it'll be cutting through New York City, eastern Virginia and into Boston by 7 or 8am," he said.
The weekend storm brought tornadoes that killed at least five people, including three children, in the US South, officials said.
The massive storm system sped from Texas eastward with dozens of twisters reported as touching down across the South from Texas through Georgia into Pennsylvania.
More than 134,000 homes and businesses were without power early on Monday in Virginia and Pennsylvania, according to the tracking site PowerOutage.US, with thousands more outages reported in North Carolina, Michigan, Maryland and New York.
The affected areas will get heavy rains, winds with gusts of up to 110km/h and the possibility of hail, the NWS reported.
"There are short spin-ups, pockets of heavy rain and damaging winds that can still hit before this pushes offshore," Hurley said.
Nearly 2300 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.
But no major flight delays were reported on the east coast.
The storm's cold front brought snow to Chicago on Sunday, with 2.5-7.6cm 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 injured over the weekend as tornadoes ripped through 17 counties and left 26,000 homes and businesses without electricity.
In addition, three people were killed when a private jet crashed in Mississippi on Saturday, although Bryant said it was unclear whether it was caused by the weather.