Five of the deaths were in Kentucky, CNN reported. Three people drowned when the vehicle they were in drove into Rolling Fork River near New Hope. Two people escaped but were hospitalized for hypothermia.
In another crash, a rider on an ATV overturned into a creek near Carrollton and was trapped underneath, police said.
The Kentucky Emergency Management said a fifth person died because of flood waters in Ballard County, CNN reported.
Two deaths occurred in Mississippi, where severe weather damaged homes and knocked down trees, the Mississippi Emergency Management Agency said.
A traffic accident near Kansas City killed one person, the Missouri Department of Transportation said.
A woman was killed by a tornado in Arkansas, KARK-TV, Little Rock, reported.
The weather played havoc with air travel across the United States. Flightaware.com reported at least 6,848 delays at U.S. airports, with at least 616 cancellations at mid-afternoon Monday.
While thunderstorms and tornadoes plagued the Southeast, the Midwest and Central Plains were hit with snow, ice and plunging temperatures. Above-average temperatures were recorded in the Mid-Atlantic.
New York topped a record high for Sunday's date, National Weather Service meteorologist Ashley Sears said.
"[The record] was previously 63 degrees in 1998," Sears said of the Big Apple's warmth. "And we are currently sitting at 70 degrees, which we've been at for the past couple of hours."
Temperatures in New York are expected to fall to the freezing mark by Christmas, though, CNN said.
In New England, stretching from Maine to northern New York state, however, winter warnings -- including alerts for ice storms and freezing rain -- were in effect.
New York Gov. Andrew Cuomo declared a winter ice emergency in northern counties near the Canadian border.
Weather forecasters said Buffalo could expect up to an inch of ice, CNN said.
In western Michigan, extra utility workers were dispatched to help restore power following the weekend's ice storm, the Grand Rapids Press reported. As of Monday morning, at least 49,000 Consumers Energy customers were without power, part of more than 201,000 customers without electricity statewide because of the storm, the utility said in a release.
Officials estimate power won't be restored until Tuesday for some customers while other customers may have to wait until Christmas or the weekend.
The ice storm also affected Canada as more than 350,000 people in Ontario experienced power outages, 250,000 in the Toronto area. By Monday morning more than 115,000 customers were still without power in Canada's largest city, CNN said.
In the Pacific Northwest, the National Weather Service predicted coastal and valley rain, and 6- to 12 inches of snow in higher elevations.