UPDATE: Tornadoes, storms kill 11 in Texas; six die in floods in Missouri

(CNN) — Crews were busy Sunday scouring debris for possible storm victims and assessing the damage in North Texas, following the fatal storms and tornadoes that ripped through Dallas suburbs Saturday evening.

The National Weather Service in Dallas-Fort Worth determined Sunday that the destruction left behind in hard-hit Garland was the work of an EF-4 tornado, which typically has wind gusts of between 166 and 200 mph.

The agency also said the damage left behind in nearby Rowlett was from a tornado that was at least EF-3, which have gusts of between 136 and 165 mph.

An EF-2 tornado struck Copeville, northeast of Dallas, the National Weather Service said.

At least 11 people died in the severe weather that barreled through the region, with Garland suffering the most casualties, authorities said.

Eight of the deaths were in Garland, Garland Police Lt. Pedro Barineau said Sunday morning. Fifteen people were hurt and 600 structures were damaged.

Another three deaths were reported in Collin County, said Lt. Chris Havey, a spokesman for the sheriff’s office. Two of the deaths were in the city of Copeville, CNN affilate KRLD reported, and an infant died in Blue Ridge.

Texas Gov. Greg Abbott in Austin declared a disaster in four counties, including Dallas County, which covers Garland, and Rockwall County, which covers Rowlett.

Collin and Ellis counties were also declared a disaster. The Fort Worth office of the National Weather Service said there was an unconfirmed report of a tornado in eastern Ellis.

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 in two counties in southwestern Arkansas. It said a tornado was observed on the ground in Hope, Arkansas.

Six people died in flash floods on rural Missouri roads overnight, Pulaski County Sheriff Ron Long said Sunday, as a result of the same storm system that hit Texas.

Rising flood waters carried away two vehicles in separate incidents. Two adults were in one vehicle and at least four were in the other, 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.

Three adults and two children died after their car was swept into a creek swollen by heavy rains in Marion County in southern Illinois, 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.

Marion County was under a flash flood watch Sunday.

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.”

Airlines are 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.

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.

The National Weather Service issued blizzard warnings 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.

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.

CNN’s Chuck Johnston, Faith Karimi, Shanna Pavlak, Rachel Aissen, Steve Almasy and Chandler Friedman contributed to this report.