A major winter storm pummeled large parts of the U.S. on Tuesday, killing two people as it dropped heavy snow on the Southern Plains and spinning off damaging winds and tornadoes in warmer areas southward. A state of emergency was declared in Mississippi.

Treacherous holiday travel was expected to become even more of an ordeal by the time the storm arrives in the Northeast later this week.

Read more from weather.com

A weather map of the U.S. looked like a child's coloring book, with a variety of advisories, watches and warnings spreading across the middle and the southern half of the country:

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• Blizzard warnings were in force for parts of Indiana, Texas and Oklahoma. Two people were killed Tuesday when high winds toppled trees in Tomball, Texas, near Houston, and in Richland Parish, La., while 21 cars and tractor-trailers crashed in a massive pileup on roads coated with freezing rain in Oklahoma City.

• Winter storm warnings stretched from those states north and east to Arkansas and Ohio. Almost 150,000 customers were without power late Tuesday across Arkansas, Entergy Arkansas said.

• At least 31 tornadoes were believed to have been spotted across the South from Texas to Alabama, the National Weather Service reported. Numerous tornado-related injuries were reported in Alabama, Mississippi, Louisiana and Texas, but, remarkably, none of them were believed to be serious, according to preliminary reports.

Hundreds more flights canceled as Christmas storm moves east

The worst of the tornadoes hit Tuesday afternoon in Mobile, Ala., where about 21,500 customers remained without power Tuesday night, Alabama Power said.

"The people of Alabama are strong," state governor Robert Bentley said in a statement released by his office. "We will recover together. First responders are doing a tremendous job helping people in areas impacted by the storms, and those efforts will continue."

The statement added: "I also want to offer my prayers for everyone impacted by these storms. We will work on the state level to do everything we can to help communities across the state."

Rick Cauley's family was hosting relatives for Christmas when the tornado sirens went off in Mobile. Not taking any chances, he and his wife, Ashley, hustled everyone down the block to take shelter at the athletic field house at Mobile's Murphy High School in Mobile.

It turns out, that wasn't the place to head.

"As luck would have it, that's where the tornado hit," Cauley told The Associated Press. "The pressure dropped and the ears started popping and it got crazy for a second." They were all fine, though the school was damaged, as were a church and several homes, but officials say no one was seriously injured. 

Suspected tornadoes also damaged homes and other structures Tuesday in or near Centreville, McNeill, Maxie and Janice in Mississippi; and in or near Luverne, Wilmer and Riderwood in Alabama, according to The Weather Channel.

A state of emergency was declared in Mississippi late Tuesday by governor Phil Bryant, the Mississippi Emergency Management Agency told NBC News in a statement. Homes, roads and businesses were damaged in at least nine counties.

Greg Forbes, a meteorologist with The Weather Channel, said thunderstorms with damaging winds and hail would make their way across the Deep South into Wednesday.

By the time it leaves the New England coast Friday, the storm will have left snow from coast to coast.

The National Weather Service said blizzard conditions were possible in parts of Illinois, Indiana and Kentucky, which could get 4 to 7 inches of snow. Whiteout conditions were forecast for stretches of Interstate 44 in Missouri, Oklahoma and Texas into the evening.

Ten to 15 inches of snow was expected in parts of Indiana. In Indianapolis, Mayor Greg Ballard ordered "non-essential" workers to stay home Wednesday.

Read more from weather.com

With more than 93.3 million people expected to take to the road during the holiday season, according to AAA, many travelers made last-minute changes of plans and decided to hit the road before the snow hit.

 

"You definitely have to worry about everyone while you're driving, especially out here," Dallas resident Jerdal Whitaker told NBC 5 of Dallas. "We're not used to the weather that comes, especially when it's ugly, so you definitely have to drive slow."

Travel delays could persist into Thursday morning along the East Coast because of "low clouds, wind, and potential changeover to light snow," The Weather Channel reported.

More than 500 U.S. flights had been canceled by 11 p.m. ET, the travel site flightaware.com reported. Many of them were into and out of Dallas, where as much as 3 inches of snow fell Tuesday, NBC 5 of Dallas reported.

NBCDFW.com: White Christmas brings delays, cancellations to DFW airport

A low-pressure system intensified as it moved across the Southern Plains toward the Lower Mississippi Valley on Tuesday. That sucked in arctic air from the north and the west to mix with warmer, wetter air in the southern half of the country, the National Weather Service said. 

The system is forecast to track east-northeast, getting stronger by the hour, as it moves into the Mid-Atlantic. From there, snow and freezing rain are expected to spread quickly northeast, reaching New England by Thursday morning. As much as a foot of snow was forecast later in the week across western and upstate New York.

Ioanna Dafermou and Meredith Placko of NBC News, Jim Cantore, Greg Forbes and Scott Kurtz of The Weather Channel and The Associated Press contributed to this report.

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Residents from New England to Washington, D.C., were cleaning up on a much-milder Sunday after storms spawned two New York City tornadoes and toppled thousands of trees as well as power lines, cutting electricity to tens of thousands.

Below's a look at the damage by area.

NEW YORK CITY
A twister with 70 mph winds formed over water near the Rockaway Peninsula of Queens Saturday morning, then hit the beachfront neighborhood of Breezy Point.

A second, 110-mph tornado followed seven minutes later about 10 miles away.

Residents got advance notice but still the storm took people by surprise, and 1,100 electricity customers lost power. No injuries were reported in either.

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At Breezy Point, roofs were ripped off cabanas at the Breezy Point Surf Club. The storms also scattered deck chairs and left a heavy metal barbecue and propane tank sitting in the middle of a softball field, at least 100 yards from any home, the Associated Press reported. 

"It picked up picnic benches. It picked up Dumpsters," said the club's general manager, Thomas Sullivan.

"We were all scared," added club worker Caitlin Walsh, 24. "The windows on the whole building broke, the lights went out, and the rain was pouring in. Everything was shaking. It was really crazy."

Related: NBCNewYork.com reports on the twisters

Broken flower pots, knocked-down fences and smashed windows littered the community of seaside bungalows.

Half an hour later, the weather was beautiful, but Sullivan had to close the club to clean up the damage.

The roof of Bob O'Hara's cabana was torn off, leaving tubes of sunscreen, broken beer bottles and an old TV set exposed to the elements, the Associated Press reported.

O'Hara, who has spent summer weekends at the Breezy Point club for his entire 52 years, tried to be upbeat. "We got a new sunroof," he said, and "the TV was getting thrown out anyway." 

The second twister hit to the northwest, in Brooklyn's Canarsie section and also near the water. Several homes and trees were damaged.

Tornadoes in the New York City area have occurred with more regularity in recent years. A small tornado uprooted trees on Long Island last month.

In September 2010, two tornadoes knocked down thousands of trees and blew off a few rooftops in Brooklyn and Queens. A weaker tornado struck the same year in the Bronx.

In 2007, a more powerful tornado damaged homes in Brooklyn and Staten Island.

WASHINGTON, D.C., AREA
Tornado-like funnel clouds were reported in Fairfax County, Va., and in Prince George's County, Md., but had not yet been confirmed.

A dozen people were injured while trying to evacuate the Prince George's County Fair, and six people were evacuated from a Washington apartment building when a tree fell on it.

Fairfax County officials reported three home cave-ins because of downed trees, a water rescue in the Potomac River and dozens of electrical wires down.

Related: NBCWashington.com reports on the storms 

Area storms left more than 108,000 households and businesses in Virginia without power, while over 60,000 electricity customers in the District of Columbia and the Maryland suburbs were hit by outages.  

By Sunday morning, more than 16,000 customers in the region were still without power.

NEW ENGLAND
The storms later moved into New England with wind gusts up to 60 mph, downing trees and flooding roads.

In Fall River, Mass., floodwaters reached up to car windshields and stalled out dozens of vehicles, the Associated Press reported. A day care center was evacuated and St. Anne's Hospital's emergency room flooded. 

In New Hampshire, WMUR-TV reported 4,000 power outages. The storm reached every county in Vermont, all within a two-hour window, but mercifully left the state without any extraordinary damage, according to early reports.

The Associated Press and Reuters contributed to this report.

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ATLANTA -- Most of the victims of last year's epic tornado outbreak in Alabama had at least one thing in common: They knew the storm was coming.

A year after the onslaught of dozens of twisters killed at least 250 people in Alabama and more elsewhere in the South, federal researchers are completing a study of who died and where they were when it happened. Among the conclusions so far: Nearly half of the people who died had been advised to take shelter. Indeed, most of them did.

But many of the tornadoes were so fierce that few structures were able to withstand them.

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These were catastrophic winds that could destroy pretty much anything in its path," Cindy Chiu, an epidemic intelligence service officer, said in reporting preliminary findings this month at a Centers for Disease Control and Prevention conference in Atlanta.

Unlike in other tornado outbreaks, the largest group of people who died were in single-family houses — not mobile homes — the CDC analysis found.

The April 27, 2011, outbreak involved 62 tornadoes that stretched along ground-hugging tracks that covered more than 1,000 miles. Fatalities were reported from central Alabama to far north Alabama.

While many who heard the warnings sought shelter, others took their chances and lost.

The American Red Cross shares disaster data with the CDC, including what was gathered in extensive interviews with families of the deceased.

Relatives of an 80-year-old woman from Lawrence County "notified her of impending storm — asked her to go to storm shelter next door. She refused, said if her time to go, she would."

The wife of a 35-year-old man from Franklin County heard the warning on TV, according to another vignette provided by Chiu. "She and sons went to basement of neighbors. He stayed in the home," the vignette states. "Tornado struck (at) 330pm and he was found 30 mins later near a tree. He was badly injured and died in the hospital."

The CDC has been examining reports of 255 deaths, including a few for which no Alabama death certificate has been found yet. It's possible a few people were injured in Alabama but died in hospitals in nearby states, Chiu said.

For 120 of those 255, the CDC determined whether the victims knew of the coming tornadoes ahead of time. And 105 were warned.

Watch the most-viewed videos on msnbc.com

Of those, 70 took some kind of protective action, like covering themselves or going to what they thought was a safer location or room — including 45 who sought proper shelter, like a basement or interior room on the lowest floor possible. Nineteen were in bathrooms, 10 in basements, 10 in bedrooms and 10 in hallways and smaller numbers in other rooms.

The average age of those who died was 50, and a third of the deaths were people 65 and older, the CDC found.

Being elderly is considered one of the greatest risk factors for death and injury in a tornado. Older people may be less mobile and have more difficulty getting to shelter. They may be frail, and more likely to die from an injury that might not kill a healthier and younger person.

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Updated at 6:38 p.m. ET: A long and wide swath of the central U.S. is in a severe storm danger zone on Sunday, following twisters that killed at least five people and caused damage across the Midwest on Saturday.

By early Sunday afternoon, at least two tornadoes reportedly briefly touched down in Nebraska. One caused damage to two buildings in Wheeler County. Oklahoma and Minnesota also each saw one suspected twister, though no damage was reported.

The areas "most likely" to see tornadoes on Sunday afternoon and evening, the National Weather Service said, are parts of Iowa, Minnesota and Wisconsin.

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"Severe storms are also possible in a band from Illinois and Missouri southward into Arkansas, northwest Louisiana and east Texas," the service added.

Weather.com severe weather expert Greg Forbes assigned a 70 percent chance for tornadoes in parts of Iowa, Minnesota and Wisconsin on Sunday.

Saturday saw 122 reports of tornadoes in the central plains, the Storm Prediction Center noted. Not all have been confirmed, and some might be the same twister reported in a nearby area, but the number of reports is unusually high.

Below's a look at the deaths, injuries and damage by state.

Oklahoma: In Woodward, a town of 12,000, five people were killed and 29 injured, five critically. Some 8,000 were without power, while 89 homes and 13 businesses were destroyed.

Lightning had apparently disabled the signal tower for the town's tornado sirens, Reuters cited Mayor Roscoe Hill as saying.

Two young sisters and an adult died at a mobile home park, while an adult and another young girl were killed just outside the city limits. [Earlier versions of this story reported that three young sisters were killed in the mobile home park].

"This thing took us by surprise," Hill said. "It's kind of overwhelming."

"On the west part of town it looks pretty bad," he added. "We still have search and rescue people out. We have people who are still missing."

One survivor emerged from a twisted SUV that had been tossed along the side of a road.

"The guy had blood coming down his face," the Associated Press quoted Marty Logan as saying.

Logan also saw people walking down the street covered in blood when he went to a hard-hit neighborhood. "It was scary, because I knew it was after midnight and a lot of people were in bed."

In April 1947, Woodward was hit by a tornado that still ranks as the deadliest in Oklahoma history, with 116 people killed, according to the National Weather Service.

Tornado hits Norman, Okla.

Kansas: A twister churned through parts of Wichita, where Brandon Redmond, a meteorologist with the Severe Weather Alert Team, said it passed over his vehicle and lifted it two feet off the ground.

"The tornado literally formed over our vehicle," he told Reuters. "I've never been that scared in my life. ... We had power flashes all around us and debris circulating all around the vehicle, sheet metal, parts of a roof, plywood."

No injuries were reported but damage includes the McConnell Air Force Base, a Boeing plant, other industrial properties and a mobile home park.

"I didn't think it was that bad until I walked down my street and everything is gone," the Associated Press quoted Yvonne Tucker as saying from her destroyed mobile home park. "I don't know what to do. I don't know where to go. I've seen it on TV, but when it happens to you it is unreal.

"I just feel lost."

Tornadoes also raced through north-central Kansas. Five homes in rural Saline County were damaged, but the tornado avoided towns and no one was hurt.

Gov. Sam Brownback on Sunday declared a state of disaster emergency. Some 11,000 people were without power, most in Wichita.

Kansas saw 97 tornado reports and about 40 percent of the state was under a tornado warning at one time or another on Saturday, Brownback noted.

Iowa:A hospital in Creston was damaged and two people were injured by a suspected twister. Patients were moved to nearby hospitals and most of the town of 7,500 people lost power.

In Thurman, a town of 250, homes were missing walls and roofs, while sidewalks and streets were covered with toppled trees.

An estimated 75 to 90 percent of the town's buildings and homes were damaged or destroyed, and yet only minor injuries were reported.

Nebraska: An apparent tornado near Oxford took a roof off a farm house and toppled a grain bin but no injuries or other serious damage in the area were reported.

Tornadoes briefly touched down earlier in Nebraska's Nuckolls County and Thayer County.

Areas in the state also saw baseball-sized hail.

The U.S. tornado season started early this year, with twisters already blamed for 62 deaths in 2012 in the Midwest and South, raising concerns that this year would be a repeat of 2011, the deadliest tornado year in nearly a century.

Some 550 people died in tornadoes last year, including 316 killed in an April outbreak in five Southern states, and 161 people in Joplin, Missouri, the following month.

The Storm Prediction Center, part of the National Weather Service, had warned of Saturday's developing system.

Director Russ Schneider said it was just the second time in U.S. history that the center issued a high-risk warning more than 24 hours in advance. The first was in April 2006, when nearly 100 tornadoes tore across the Southeast, killing a dozen people and damaging more than 1,000 homes in Tennessee.

Earlier warnings are now possible because storm modeling and technology have improved, said Chris Vaccaro, a spokesman for the National Weather Service. In the past, people often have had only minutes of warning when a siren went off.

The National Weather Service also announced last month that it would start using terms like "mass devastation," "unsurvivable" and "catastrophic" in tornado warnings in an effort to get more people to take heed.

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Updated at 11:55 p.m. ET: Forecasters are warning of a major tornado outbreak in Kansas and Oklahoma this weekend, with Oklahoma getting a first taste of it on Friday with a tornado touching down near the National Weather Service office in Norman.

A tornado touched down near the University of Oklahoma campus in Norman just after 4 p.m. local time - the same town that holds the National Storm Prediction Center, the National Weather Service confirmed.

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An operator at the University of Oklahoma said people had been warned to get to a basement or low floor.

"I was watching this tornado on TV, which was neat until I realized it was right here in Norman," said the operator, who did not give a name.

Video from television helicopters showed the tornado ripped roofs from buildings, downed power lines and uprooted trees in the city of about 100,000 about 20 miles south of Oklahoma City, but Oklahoma Department of Emergency Management spokeswoman Keli Cain said there were no reports of serious injuries.

"This is just a fraction of what's to come tomorrow," Chris Vaccaro, a spokesman for the National Weather Service, warned.

Norman Regional Hospital and an affiliate treated 19 people for mainly "bumps and bruises," and one patient remained hospitalized in fair condition late Friday, hospital spokeswoman Kelly Wells said.

Atmospheric conditions for the weekend will be similar to those that caused severe storms in parts of the Midwest and Southeast in early March that killed more than 50 people, said Steve Weiss, science support branch chief for the National Storm Prediction Center.

"We see potentially some ... very damaging tornadoes," Weiss said.

Oklahoma was already having severe weather Friday -- but the biggest storms are expected Saturday, said Weiss, who was watching heavy rain out of his office window in Norman on Friday afternoon. "It's not unusual to have successive days," he said.

Conditions favor strong thunderstorms in Kansas and Oklahoma on Saturday, with a few "supercell" storms with rotating updrafts, Weiss said.

"The potential is that some of the supercells could be long-lived, so if they produce tornadoes they could be on the ground for a while," he said.

Forecasters said the storms could start Saturday afternoon into the early evening and continue after dark.

"The really dangerous part is that it looks like it's going to be overnight," said Kurt Van Speybroeck, emergency response meteorologist for the National Weather Service. "It's a really bad combination to get tornadoes at night because they're harder to see. It could be a really bad evening."

Storms could strike heavily populated areas such as Oklahoma City, and Wichita and Topeka, Kansas, Weiss said.

The high-risk area is from about the I-40 highway in Oklahoma City going north along I-35 to I-70 in central Kansas, said Van Speybroeck.

Northwest Texas into Nebraska and parts of Iowa and Missouri are also at risk for thunderstorms and tornadoes this weekend.

Reuters and The Associated Press contributed to this story.

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Some church volunteers and construction workers complain that strict rules controlling access to the worst-damaged neighborhoods in Arlington prevent them from helping tornado victims.

City leaders say the process of requiring daily permits to get past police roadblocks is necessary to keep out looters and shady contractors.

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Contractors are required to go to a police station in South Arlington to apply for a permit to get into certain areas. A homeowner must verify the company has been hired, and the contractor must provide detailed identification information. Residents also must show proof of their address to get to their homes.

Don Tate, of Tate Roofing in Mansfield, said his company wanted to help put free tarps on damaged roofs.

"You can't help people if they won't let you in the neighborhoods," he said.

Another contractor, Gerardo Gonzales, of Royalty Roofing in Fort Worth, waited at a police station for city officials to verify with a resident that his company had been hired.

"The problem is, the homeowner is not answering to verify we actually have work to do with them, so it's holding us up," Gonzales said. "All we want to do is get in, get out, and help the homeowners out."

Ken Leonard, a volunteer for Churches of Christ, said he had a truck with water and other supplies to donate to victims, but he was unable to get a city permit.

"They're locked up pretty tight right now," he said. "I'm sure things will loosen up soon and we want to be there ready to go in when they let faith-based organizations like us in."

More tornado coverage by NBCDFW.com

City leaders defend the rules as necessary to keep out unscrupulous contractors and even looters.

"The process really is to make sure we have limited access to the secured areas," Arlington spokeswoman Rebecca Rodriguez said. "As you can see, there are so many houses that are vulnerable, and that means the homeowners are vulnerable too in so many ways."

She said some homeowners had complained about questionable characters going door to door to solicit business.

Nancy McNiel, whose home of 25 years was destroyed in seconds, said she had noticed several people hauling away metal debris which she believed they planned on selling to recyclers.

"How did they get in here?" she asked.

She praised the strict rules limiting access to her neighborhood.

"I think it's a good idea," she said. "We'd have a lot of fly-by-night people coming through."

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The storms that tore through North Texas produced not only destruction but some amazing stories of survival, including one babysitting grandmother who clutched her 18-month-old grandson as the wind tried to tear him away.

"It just felt like it was trying to just take him, you know just actually take him from me," Sherry Enochs of Forney told WFAA-TV as she held her grandson Lane. "I clutched him as hard as I could."

"She held onto his feet -- just by his feet -- and the wind kept taking him, but she hung onto him," her daughter Lindsey Enochs added in an interview with WFAA-TV.

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Sherry Enochs was babysitting her grandson and two other children when "I looked through the back glass of my bedroom window and saw the tornado coming."

She grabbed the children and huddled with them in a bathtub.

The tornado destroyed the home, and tore through dozens of other ones in Forney.

Sherry Enochs started screaming and a passerby helped her out of the debris.

"One of the little boys was walking around there in the backyard," she added, "and how he got there I don't know."

Lane was untouched and the worst injury was a scratch to one of the boys.

'Sounded like a bomb' at nursing home
In Arlington, a twister tore through part of a nursing home. Physical therapist Patti Gilroy saw it coming through the back door.

She had just finished herding dozens of patients into the hallway, The Associated Press reported, when "it sounded like a bomb hit. And we hit the floor, and everybody was praying. It was shocking."

The blast of wind lasted about 10 seconds, she said, adding that a co-worker was nearly "sucked out" while trying to get a patient out of the room.

Joy Johnston was visiting her 79-year-old sister at the time. "The windows were flying out, and my sister is paralyzed, so I had to get someone to help me get her in a wheelchair to get her out of the room," NBCDFW.com quoted her as saying. "It was terribly loud."

Children herded to safety at school
In Lancaster, the pastor of Cedar Valley Christian Academy herded more than 30 daycare children -- including some newborns -- into a windowless room when he saw debris swirling, NBCDFW.com said.

At the church's school, about 60 more children also took shelter in a windowless room.

An entire wall of the school was destroyed.

Pastor Glenn Young said he didn't know when the school might reopen. "I'm a little concerned," he added. "This is our livelihood."

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Tornado season is only just beginning, but already this year has seen dozens of destructive twisters from Illinois to Texas, where up to 18 might have touched town on Tuesday alone in the Dallas-Fort Worth area.

"We're at just the beginning of a very unusual" tornado season, NBC weather anchor Al Roker said on TODAY. 

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The numbers show just how unusual: March saw 223 twisters, up from an average of 80 from 1991-2010, according to the National Weather Service. February saw 63, compared to an average of 29; and January saw 97, compared to an average of 35.

So what's behind the outbreak?

"We've had record heat," weather.com meteorologist Greg Forbes told TODAY, and "that warmth is a big ingredient that provides the instability for the storms."

Last year started off slowly but then saw a record 758 tornadoes in April 2011, noted Roker. "Hopefully we're not on track for that this year."

U.S. forecasters have predicted a warmer than normal spring in the central part of the country, which could increase tornado threats. But countering that is the fact the cyclical La Nina weather pattern, which can help fuel twisters, is waning.

More than 800 homes hit in Dallas area
Grandma saves boy from twister pulling at him

Before Tuesday, the last big twister outbreak was on March 23, when tornadoes touched down in six states -- Alabama, Georgia, Illinois, Indiana, Kentucky and Missouri -- killing one person, injuring dozens and damaging hundreds of structures.

So far this year, tornadoes have caused 55 deaths, most on Feb. 29 and March 2 during outbreaks across the Midwest and the South. Through March of 2011, only 2 deaths were attributed to tornadoes.

The peak months for tornadoes are usually April, May and June, so this season is really just beginning.

Tuesday's outbreak suggests "we're on pace to be above normal," the Associated Press quoted National Weather Service meteorologist Matt Bishop as saying.

Some climate scientists expect more extreme weather if global temperatures continue to rise, while others say the science is not strong enough to make that conclusion about single events or even a single season.

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Updated at 6:08 p.m. ET: A woman was killed Friday when her mobile home rolled over during a severe storm in southern Illinois, authorities said.

Jefferson County, Ill., Coroner Eddie Joe Marks said Pat Brown, 61, was killed and a teenage boy suffered minor injuries when Brown's home toppled as heavy rain, hail, winds and possible tornadoes swept through the Opdyke area. It wasn't immediately clear whether a tornado hit the home.

The National Weather Service reported at least six confirmed tornadoes and relayed reports of numerous other unconfirmed twisters as storms moved across southeastern Missouri, southern Illinois and western Kentucky.

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Several homes were damaged in Fern Creek, Ky., near Louisville, including at least two that had their roofs torn off, NBC News reported.

 

Joe Sullivan, a meteorologist with the weather service, told the Weather Channel that the damage was consistent with a tornado with winds of 110 mph or even higher. 

No casualties were immediately reported in the Kentucky storms, Jody Johnson, a spokeswoman for Louisville MetroSafe, told NBC News.

Even before Friday's tornadoes, 2012 was off to an unusually active beginning of tornado season.

Greg Forbes, a severe weather specialist for the Weather Channel, said 117 tornadoes had been confirmed this month through Thursday, well above the 10-year March average of 87. January and February were also above their 10-year averages.

"Keep in mind we are only at the beginning of the tornado season," Forbes said.

The peak months for tornadoes are usually April, May and June, and "while we do not know exactly how active the rest of the tornado season will be, you should make sure you are prepared," he said.

The Weather Channel: Tornado risk by month and location

John Baiata, Ioanna Dafermou, Naima Lynch and Ziad Jaber of NBC News contributed to this report by M. Alex Johnson of msnbc.com.

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Severe storms overnight in Nebraska, Texas and Ohio damaged homes and tossed rail cars as well as at least one tractor-trailer, and the threat continued Monday with a large part of the central U.S. on alert.

The greatest damage overnight was just outside North Platte, Neb., where two confirmed tornadoes tore roofs off several homes, downed power lines and injured two people.

One twister crossed Interstate 80, flipping a tractor-trailer in its path. The truck's driver was hospitalized.

A rail yard also was hit, with 15 cars derailed or knocked over, the North Platte Telegraph reported. One worker there was hit by flying debris, treated at a hospital and then released.

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In central Ohio, tornado sirens went off as large hail and high winds swept through Sunday night. In Gardendale, Texas, two people were hurt when high winds flipped over their mobile home. No tornadoes were reported in either state.

The mix of warm weather in recent weeks with cold pockets across the Midwest and central U.S. has led to an early start to the tornado season.

"It has been an active season already for tornadoes, and that's part of the reason we've scooched up our siren testing starting in March," Paul Johnson, emergency manager for Douglas County in North Dakota, told KETV.

Tornado watches have been issued for parts of Texas and Oklahoma for Monday, while the rest of the central U.S. is under severe weather warnings that include the possibility of large hail and high winds.

 

The threat will shift slightly to the east on Tuesday, weather.com reported, with parts of Illinois, Arkansas, Louisiana and Texas seeing the biggest threat.

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