Hundreds of flights across the area have been cancelled, and with portions of major highways flooded or snowed under, the storm system is wreaking holiday travel havoc for millions of Americans.
The National Weather Service is warning of blizzards, freezing rain and flash floods in the next days, all part of a powerful storm system fuelled by unseasonably warm air that began in the deep south last Wednesday.
The NWS said Monday that 21 states -- from New Mexico to as far north as Michigan -- are under a weather watch or warning.
The governors of New Mexico, Texas and Missouri declared states of emergency for all or parts of their states to handle storm damage on Sunday. Alabama, Mississippi and Georgia earlier took similar measures.
- Texas tornados kill 11-
At least 11 people were killed as tornadoes struck the heavily populated Dallas-Fort Worth metroplex in Texas, according to authorities who searched house-to-house Sunday for additional victims.
The rare December twisters flattened homes, knocked cars off of highways and flipped big-rig trucks like toys. The NWS said that at least nine twisters touched down in the region late Saturday.
Hardest-hit was the Dallas suburb of Garland, where authorities confirmed eight fatalities after a tornado packing winds of up to 200 miles (320 kilometres) per hour bore down on the city.
City officials said this is only the second time since 1950 that such a powerful tornado struck the area.
Aerial footage showed rows of flattened homes, while others had roofs blown off and windows shattered. Some 600 buildings were damaged or destroyed, officials said.
"We're going to look at every house and every car to try to make sure we find everybody," Garland Mayor Douglas Athas told CNN.
Tornados are common in the US midwest in the spring, but rare during winter.
Three other storm-related fatalities occurred northeast of Dallas, the Collin County sheriff's department told AFP.
State governor Greg Abbott told reporters that snow and ice was causing power outages in northwest Texas, while central Texas was facing flood risks and the east was bracing for the possibility of more tornados.
- Snow, ice, flooding -
The storm dumped rare heavy snow in eastern New Mexico, a situation Governor Susana Martinez described as "dire."
Martinez activated the National Guard for disaster assistance, and urged residents to stay off the roads.
The governor reported more than 16 inches (40 centimeters) of snow has fallen in parts of the state, "with drifts as high as eight feet, making roads impassable in several counties."
Authorities even closed down I-40, a major highway linking the city of Albuquerque with neighbouring Oklahoma.
State police said that parts of the highway were obstructed by stranded big-rig trucks. Police estimated that up to 600 trucks were trapped on the highway, some parked on the side, the Albuquerque Journal reported.
In Missouri, Governor Jay Nixon said that at least eight fatalities due to flash flooding were reported in his state.
"I urge Missourians in flood-affected areas to stay alert, avoid travel if possible and never drive into a flooded roadway," he said.
In nearby Mississippi, where Governor Phil Bryant declared a state of emergency to deal with flooding, "severe storms" are forecast through Monday, the state Emergency Management Agency said.
The agency also reported 10 storm-related deaths.
Illinois reported five deaths, while eight others were killed in southern states on and before Christmas Day.
And one person drowned Sunday in Alabama floodwaters, local officials said.
- More weather misery -
The NWS forecast blizzards and heavy snow for eastern New Mexico, Oklahoma and northern Texas, with snow expected to eventually reach the tornado-damaged Dallas area.
The temperature in Dallas on Saturday, when the twisters struck, was an unseasonable 82 Farenheit (28 Celsius) -- but weather forecasters said it will plunge to 32 F (00 C) by late Monday.
"A major winter storm system will continue to bring a plethora of weather impacts ranging from heavy snow and blizzard conditions to severe weather and flooding," the NWS said early Monday.
"The storm system will gradually pull away by Tuesday, but flooding and treacherous travel could linger into midweek."
Conditions are especially perilous in places unused to heavy snowfall such as New Mexico, where urban areas lack snow removal equipment, and few drivers are experienced at driving safely in snow and ice.
To deal with icy conditions pounding their state, Oklahoma closed some roads and sent out trucks to salt major arteries, local media reported Monday.
More than 200 flights were cancelled when the Federal Aviation Administration issued a ground stop for the Dallas International Airport on Sunday.
Flights were delayed and cancelled at the Will Rogers World Airport in Oklahoma, as in other airports across the affected region.