The tornadoes followed a weekend blizzard that shut down much of the Northeast, which was coping Monday with freezing rain in many areas.
The Hattiesburg tornado, damaging an area near the University of Southern Mississippi, was among twisters that touched down Sunday from southern Mississippi to southwestern Alabama, AccuWeather.com reported.
The university campus and a nearby high school were damaged along with areas throughout Hattiesburg, officials said.
Seven counties in Mississippi reported tornado damage, CNN reported.
Mississippi Gov. Phil Bryant declared a state of emergency for counties affected, allowing them to access state resources and assets to support local response effort.
"If there is a good thing about this, it happened on a Sunday when most of these structures were vacant," Hattiesburg Mayor Johnny DuPree said.
The threat of tornadoes lessened by Sunday night, but the possibility of flash flooding caused by thunderstorms remained into early Monday, AccuWeather.com forecasters said
As of Monday morning, about 4,000 power customers were without electricity, Mississippi Power said, down from the nearly 14,000 customers who were without power at one point Sunday.
In New York and Connecticut, rain spurred fears of flooding and roof collapses from melting snow as the Northeast struggled to dig out from a weekend blizzard.
Rain froze into glaze on contact with the cold ground and other surfaces before dawn Monday, but temperatures were forecast to rise into the mid- to upper 40s from New York City through Boston, with precipitation falling as snow and sleet farther north.
The rain, on the heels of a storm that dumped up to 40 inches of snow across New England and parts of New York, prompted Connecticut Gov. Dannel Malloy to warn residents Sunday to clean snow off their roofs amid dangers of roof collapses from the added weight of rain.
President Obama declared a state of emergency in Connecticut Sunday, ordering federal aid to supplement local emergency response efforts.
New York Gov. Andrew Cuomo ordered 675 pieces of equipment and 975 personnel dispatched to help Suffolk County, making up the eastern half of Long Island, dig out of 3 feet of snow.
Most public transit throughout the Northeast was expected to resume at least limited service Monday. Some New England transit agencies warned that heavily accumulated snow in some areas and unplowed snow on secondary roads would make it difficult for buses to travel safely.
The Massachusetts Bay Transportation Authority warned customers to "expect significant delays and plan extra time for their Monday morning commute."
The New York state Metropolitan Transportation Authority said most service on the Metro-North Railroad, the nation's busiest commuter railroad, and Long Island Rail Road, the second-busiest, would operate with limited service.
Amtrak said it restored limited service between New York and Boston and near-normal service between New York and the state capital in Albany.
Logan International Airport in Boston and the three major airports in the New York City area resumed flights.
Mail service suspended in all six New England states Saturday was to resume Monday in areas where it was safe to deliver, the U.S. Postal Service said.
Across eight states, fewer than 200,000 customers remained without power, a United Press International count of outages from utility reports indicated. This figure was down from a peak of nearly 670,000 reported by the U.S. Energy Department.
Most remaining customers should have power again by late Monday, utilities said.
As the Northeast sought to recover, a large winter storm across the Northern Plains dumped at least a foot of snow and brought high winds and whiteout conditions from Colorado to central Minnesota.
Officials closed Interstate 90 between Sioux Falls and Chamberlain, S.D., 140 miles west, and Interstate 29 between Sioux Falls and Watertown, 100 miles north. Interstate-29 was also closed in North Dakota from the state line to Fargo, near western Minnesota.