A day after fatal tornadoes and summer-like temperatures in the Dallas area, the immediate forecast called for freezing weather and snow, and the National Weather Service reported quarter-size hail south of the city.
The Texas panhandle was under a blizzard warning along with eastern New Mexico and parts of Oklahoma.
Winter storm warnings were in effect from Texas north to Michigan, and at least nine states farther east were on alert for flash floods, which already killed at least 13 in Missouri and Illinois.
The National Weather Service on Sunday confirmed three tornadoes were part of the storms that ripped through the Dallas area a day earlier, killing 11 people and damaging hundreds of buildings.
The governor declared a disaster in four Dallas-area counties Sunday, urging caution as a new wave of severe weather began to hit Texas in the afternoon.
An EF-4 tornado, which typically has wind gusts of between 166 and 200 mph, was responsible for the destruction in hard-hit Garland, the National Weather Service in Dallas-Fort Worth said. Eight of the 11 reported deaths happened there, Garland Police Lt. Pedro Barineu said, and 600 structures were damaged.
The damage in nearby Rowlett was from an EF-3 tornado, which has typical gusts of between 136 and 165 mph.
Three deaths were reported in Collin County, said Lt. Chris Harvey, a spokesman for the sheriff's office. CNN affilate KTVT said they included two deaths in Copeville
, where the National Weather Service confirmed an EF-2 tornado, and the death of an infant in Blue Ridge.
Texas Gov. Greg Abbott in Austin declared a disaster in Dallas County, which covers Garland; Rockwall County, which covers Rowlett; and Collin and Ellis counties.
The Fort Worth office of the National Weather Service said there was an unconfirmed report of a tornado in eastern Ellis.
More extreme weather to come
Parts of Texas were experiencing extreme swings in the weather. Dallas had a high of 82 degrees Saturday and was down to 41 on Sunday, with freezing temperatures forecast for Monday, according to CNN meteorologist Rachel Aissen.
Quarter-size hail was reported in Mansfield, south of Dallas, Sunday afternoon, the National Weather Service said.
Blizzard warnings were in effect through Monday evening in eastern New Mexico and the Texas and Oklahoma panhandles. The affected areas could receive more than eight inches of snow, the agency said, with possible snowdrifts higher than six feet.
That snow will eventually work its way down to Dallas, blanketing and freezing over communities in the middle of tornado clean-up. The Texas governor said almost the entire portion of the state north of Interstate 10, which runs through Houston and San Antonio, was already dealing with challenging weather conditions.
Abbott urged Texans to remain vigilant to perilous conditions north of I-10, including the threat of rising waters.
As of late Sunday morning, people in some parts of New Mexico had already seen more than 16 inches of snow fall with drifts as high as 8 feet, making roads impassable in several counties, according to the governor's office.
Gov. Susana Martinez declared a state of emergency Sunday in response to the snowstorm.
In addition to snow, icy conditions and strong winds are expected from central Oklahoma up into Kansas, making the roads dangerous for driving.
In Oklahoma, crews were busy treating highways and bridges with salt and sand, according to Oklahoma's Department of Energy Management. As of late Sunday morning, there were at least 8,000 power outages around the state, with the most being in Lawton, about 90 miles southwest of Oklahoma City.
Severe weather moves east
As a result of the same storm system, 20 states from New Mexico to Michigan and Georgia had some type of weather watch or warning Sunday, according to the National Weather Service
Roughly half of them involved wintry conditions like blizzards, ice and heavy snow, while the others involved other severe weather like tornadoes and flash floods.
The National Weather Service issued tornado warnings Sunday afternoon for a handful of counties in northeastern Texas, southern Arkansas and northern Louisiana. Tornado watches were in effect for much of Louisiana and Arkansas.
Three adults and two children died after their car was swept into a creek swollen by heavy rains in Marion County in southern Illinois, which was under a flash flood watch Sunday, County Coroner Troy Cannon said.
The car was traveling over a low-water crossing that normally has very little water trickling across the road, if any at all, Cannon said. With the recent heavy rains, that trickle grew into several feet of swiftly moving water.
Flooding and freezing forecast in Missouri
Missouri Gov. Jay Nixon declared a state of emergency
Sunday as heavy rain led to widespread flooding across much of the state, with more heavy rain and flooding expected through Monday.
At least eight people have died, Nixon's office said. They include six people whose cars were swept away by rising waters on rural roads Saturday night, Pulaski County Sheriff Ron Long said.
"Streams turn into rivers, and people sometimes don't see the road has flooded over when they are driving at night," Long said.
The Missouri State Highway Patrol has made dozens of water rescues and helped evacuate residents from flooded areas, Nixon's office said. Many roads were closed because of flooding and Nixon said people in flooded areas should avoid travel if possible.
Nixon urged caution in northern Missouri, where snow, freezing rain and increasing wind gusts were expected to accompany a drop in temperatures.
Widespread flash flooding also caused problems in Arkansas. In Benton County, which borders Missouri, 50 roads were closed and the county judge issued an emergency disaster declaration.
Air travel affected
The Federal Aviation Administration issued a ground stop for Dallas/Fort Worth International Airport because of the weather.
Southwest Airlines canceled 70 flights across its system Sunday, spokesman Dan Landson said. American Airlines announced the cancellation of 170 flights out of the airport Sunday. Both airlines are based in Dallas.
Praying under a mattress
In some neighborhoods in Garland, the storms ripped facades off houses, leaving gaping holes. Cars that had been in driveways ended up inside homes after the tornado barreled through, witnesses said.
Officials said earlier that five of the deaths were related to vehicles hit by a tornado in southeast Garland.
Garland resident Pat McMillian said the tornado left neighborhoods in darkness.
"All I heard was the roaring of the tornado, and my mom told us to get in the bathroom," McMillian said. "Then we went across the hall to make sure everyone was OK. The church across the street was destroyed."
Afterward, they left their house and sought shelter elsewhere.
"We are in our car now, and I'm not sure where we are going to go," McMillian added. "It's extremely hot, and there is no power."
Lafayette Griffin and his family hid under a mattress and prayed as the tornado hit.
"It was terrifying. It was terrifying," he said. "They didn't know if they were going to make it."