By Daniel Lovering and Aman Ali

CAMBRIDGE, Mass./NEW YORK (Reuters) - The U.S. Northeast was digging out on Sunday after a blizzard dumped up to 40 inches (one meter) of snow with hurricane force winds, killing at least nine people and leaving hundreds of thousands without power.

As the Northeast cleared roads and shoveled out, another storm bore down on the Northern Plains and tornadoes threatened the Southeast in a weekend of extreme weather across the United States.

A tornado which appeared to be a mile wide touched down in Hattiesburg, Mississippi, causing significant damage, said Anna Weber of the National Weather Service. The twister also hit the nearby town of Petal where it destroyed a brick building.

In New York City trucks plowed through residential streets, piling snow even higher at the edges and leaving thousands of motorists to dig out their buried vehicles.

Another round of severe weather on Monday could bring more misery, with freezing rain and more snow predicted that would make the evening commute even more difficult.

In Boston, Mayor Tom Menino canceled school on Monday after touring neighborhoods throughout the city, where two feet of snow fell.

Utility companies reported that some 350,000 customers were still without electricity across nine states after the wet, heavy snow brought down tree branches and power lines.

Air traffic began to return to normal after some 5,800 flights were canceled on Friday and Saturday, according to Flightaware, a flight-tracking service.

Bradley International Airport in Windsor Locks, Connecticut, and New York state's Long Island MacArthur Airport reopened on Sunday morning. Boston's Logan International Airport reopened late on Saturday, according to the Federal Aviation Administration.

Rare travel bans in Connecticut and Massachusetts were lifted but roads throughout the region remained treacherous, according to state transportation departments.

In Cambridge, Massachusetts, residents were digging out their cars and driveways under clear blue skies on Sunday afternoon.

As the region recovered, another large winter storm building across the Northern Plains was expected to leave a footof snow and bring high winds from Colorado to central Minnesota into Monday, the National Weather Service said.

South Dakota was expected to be hardest hit, with winds seen reaching 50 miles per hour (80 kmph), which would create white-out conditions. The storm was expected to reach parts of Nebraska, North Dakota, Wyoming and Wisconsin.

South Dakota officials closed a 150-mile (240-km) stretch of Interstate 90 in the center of the state. They also closed 75 miles of Interstate 29 in the state's northeastern corner near North Dakota.

Officials said motels and other facilities along Interstate 90 were filling up with travelers trying to avoid the heavy drifting and near-zero visibility.

"Travel will be difficult to impossible at times on other highways in many areas of South Dakota," state transportation officials said in a statement.

GREAT LAKES TO THE ATLANTIC

The mammoth storm over the weekend stretched from the Great Lakes to the Atlantic and covered several spots in the Northeast with more than 3 feet of snow. Connecticut, Rhode Island and Massachusetts took the brunt of the blizzard.

The Connecticut town of Hamden had 40 inches and nearby Milford 38 inches, the National Weather Service said.

New York Governor Andrew Cuomo said 675 pieces of equipment and 975 personnel had been dispatched to help Suffolk County, making up the eastern half of Long Island, dig out of 3 feet of snow. Continued...