Southern US states began digging out on Thursday after severe storms killed at least 11 people, and Mississippi declared a state of emergency in areas pounded by tornadoes.

With about 100 million Americans expected to travel over the Christmas holiday, the National Weather Service forecast isolated severe thunderstorms from the mid-Atlantic region to the Gulf Coast and record warmth in New York.

The storm system packed high winds and triggered more than 20 tornadoes in Arkansas, Illinois, Indiana, Mississippi, Tennessee and Michigan on Wednesday, authorities said.

A large tornado tore a 100-mile (160-km) path through northern Mississippi, demolishing or heavily damaging more than 100 homes and other buildings before plowing into western Tennessee, authorities said.

Mississippi Governor Phil Bryant declared a state of emergency in areas affected by the storm, saying 14 tornadoes had touched down in the state. Bryant said seven people were killed and one person was missing.

"Everybody is pulling together here in Mississippi today to help respond to this disaster," Bryant said on CNN.

He said shelters had been set up and the full extent of the damage would not be known for several days. Mississippi authorities said some 40 people were injured and a 7-year-old boy was among those killed.

Three people died in Tennessee and an 18-year-old woman was killed in Arkansas when a tree crashed into her house, authorities said.

Thirteen counties in Tennessee suffered severe damage, with a post office destroyed and a state highway washed out.

Emergency crews in Mississippi, Alabama, and Tennessee were searching for several people reported missing. Scores were injured in the region.

A rare tornado touched down in Canton, Michigan, and about 15,000 homes in the state and neighboring Wisconsin were without electricity. The weather service issued a gale force wind warning for Lake Michigan, where waves could reach 15 feet (4.6 meters).

About 500 flights were delayed or canceled at Atlanta's Hartsfield-Jackson airport as the area was hit by a thunderstorm.

Meanwhile, much of the northeast enjoyed balmy weather on Thursday, including New York, which surpassed its record for the warmest Christmas Eve reaching 71 degrees Fahrenheit (22 C).

Cold and snow were forecast on Christmas Day for the US Northwest, including temperatures in the teens in Montana and snow likely in Washington, Oregon, northern California, and Nevada.

Of the 100 million Americans traveling over the holiday, 91 million will use cars, according to the American Automobile Association.

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