Overturned cars left streets unnavigable and flattened trucks clogged highway shoulders. Preliminary estimates were that six to 12 twisters had touched down in North Texas, senior National Weather Service meteorologist Eric Martello said. But firm numbers would only come after survey teams checked damage Wednesday, he said.

In suburban Dallas, Lancaster police officer Paul Beck said 10 people were injured, two of them severely. Three people were injured in Arlington, including two residents of a nursing home who were taken to a hospital with minor injuries after swirling winds clipped the building, city assistant fire chief Jim Self said.

“Of course 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,” said Joy Johnston, who was visiting her 79-year-old sister at the Green Oaks Nursing and Rehabilitation Center. “It was terribly loud.”

Dallas-Fort Worth International Airport canceled hundreds of flights and diverted others heading its way. Among the most stunning video was an industrial section of Dallas, where rows of empty tractor-trailers crumpled like soda cans littered a parking lot.

“The officers were watching the tornadoes form and drop,” Kennedale police Chief Tommy Williams said. “It was pretty active for a while.”

The confirmed tornadoes touched down near Royce City and Silver Springs, said National Weather Service meteorologist Matt Bishop.

April is the peak of the tornado season that runs from March until June. Bishop said Tuesday’s storms suggest that “we’re on pace to be above normal.”

Johnston said her sister was taken to the hospital because of her delicate health. Another resident at the nursing home, Louella Curtis, 92, said workers roused her out of bed and put her in the hall.

“The hallways were all jammed,” Johnston said. “Everyone was trying to help each other to make a path for others. I’d say everybody was out of their rooms within 20 minutes.”

Most of Dallas was spared the full wrath of the storm. Yet in Lancaster, television helicopters panned over exposed homes without roofs and flattened buildings. Broken sheets of plywood blanketed lawns and covered rooftops.

A pastor at one Lancaster church saw debris swirling in the wind, then herded more than 30 children, some as young as newborns, into a windowless room to ride out the storm. Nearby at the church’s school, about 60 more children hid in another windowless room near the women’s bathroom.

An entire wall of Cedar Valley Christian Academy wound up being taken out in the storm. Pastor Glenn Young said he didn’t know when the school might re-open.

“I’m a little concerned,” Young said. “This is our livelihood.”

Residents could be seen walking down the street with firefighters and peering into homes, looking at the damage after the storm passed.

Devlin Norwood said he was at his Lancaster home when he heard the storm sirens. He said he made a quick trip to a nearby store when he saw the funnel-shaped tornado lower, kick up debris and head toward his neighborhood.

“I didn’t see any damage until I got back home. We had trees destroyed, fences down, boards down, boards penetrating the roof and the house, shingles damaged,” said Norwood, 50, an accountant and graduate student.

The storm pushed cars into fences and toppled trees. Branches and limbs scattered across lawns and residential streets, and in one driveway, a tow-behind RV was left torn apart and crumpled.

“Obviously we’re going to have a lot of assessments to make when this is done,” Dallas County spokeswoman Maria Arita said.

American Airlines canceled more than 450 arriving and departing flights at its hub airport by late Tuesday afternoon, and 37 other incoming flights had been diverted to different airports.

DFW Airport spokesman David Magana said more than 110 planes were damaged by hail. It wasn’t clear how many belonged to American Airlines, but American and American Eagle had pulled 101 planes out of service for hail-damage inspections.

Flights also were canceled at Dallas Love Field, which is a big base for Southwest Airlines. That airline canceled more than 45 flights in and out of the airport by Tuesday evening.

Meteorologists said the storms were the result of a slow-moving storm system centered over northern New Mexico.

Officer Paul Beck in the Dallas suburb of Lancaster says 10 people were injured when a reported tornado touched down there on Tuesday. He says two of those injuries are severe but did not have further details.

Assistant Arlington fire chief Jim Self says three people suffered minor injuries there. They include two residents of a nursing home who were taken to a hospital after swirling winds clipped the building.

The National Weather Service has confirmed at least two tornadoes tore through the area Tuesday, and several others have been reported.

Tornadoes and violent storms crumbled the wing of a nursing home and peeled roofs from dozens of homes in a destructive start to a Texas tornado season that forecasters say will be livelier than normal.

Big-rig trailers tossed into the air spiraled hundreds of yards like football, overturned cars left streets unnavigable and flattened trucks clogged highway shoulders. The National Weather Service confirmed two tornadoes and issued new warnings as the severe weather system crept east toward Louisiana.

In suburban Dallas, Lancaster Mayor Marcus Knight said multiple residents in his city were injured, including some critically. Two residents of a nursing home in Arlington were taken to a hospital with minor injuries after swirling winds clipped the building, city assistant fire chief Jim Self said.

“Of course 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,” said Joy Johnston, who was visiting her 79-year-old sister at the Green Oaks Nursing and Rehabilitation Center.

“It was terribly loud.”

Dallas-Fort Worth International Airport cancelled hundreds of flights and diverted others heading its way. Among the most stunning video was an industrial section of Dallas, where rows of empty tractor-trailers crumpled like soda cans littered a parking lot.

“The officers were watching the tornadoes form and drop,” Kennedale police Chief Tommy Williams said. “It was pretty active for a while.”

The confirmed tornadoes touched down near Royce City and Silver Springs, said National Weather Service meteorologist Matt Bishop. A tornado watch remained in effect until 8 p.m. CST.

April is the peak of the tornado season that runs from March until June. Bishop said Tuesday’s storms suggest that ‘we’re on pace to be above normal’.

Most of Dallas was spared the full wrath of the storm. Residents could be seen walking down the street with firefighters and peering into homes, looking at the damage after the storm passed.

The storm pushed cars into fences and toppled trees. Branches and limbs scattered across lawns and residential streets, and in one driveway, a tow-behind RV was left torn apart and crumpled.

“Obviously we’re going to have a lot of assessments to make when this is done,” Dallas County spokeswoman Maria Arita said.

American Airlines cancelled more than 450 arriving and departing flights at it hub airport by late Tuesday afternoon, and 37 other incoming flights had been diverted to different airports.

DFW Airport spokesman David Magana daid more than 110 planes have been damaged by hail. It wasn’t clear how many belong to American Airlines, but American and American Eagle had pulled 101 planes out of service for hail-damage inspections.

Flights also were cancelled at Dallas Love Field, which is a big base for Southwest Airlines. That airline canceled more than 45 flights in and out of the airport by Tuesday evening.

Meteoroligists said the storms were the result of slow-moving storm system centred over northern New Mexico.

The findings can be found in Geophysical Research Letters online and in print in the September 3, 2009 issue.

“As the size of landfalling hurricanes from the Gulf of Mexico increases, we’re seeing more tornadoes than we did in the past that can occur up to two days and several hundred miles inland from the landfall location,” said James Belanger, doctoral student in the School of Earth and Atmospheric Sciences at Georgia Tech and lead author of the paper.

Currently, it’s well known that when hurricanes hit land, there’s a risk that tornadoes may form in the area. Until now, no one has quantified that risk because observations of tornadoes were too sporadic prior to the installation of the NEXRAD Doppler Radar Network in 1995. Belanger along with co-authors Judith Curry, professor and chair of the School of Earth and Atmospheric Sciences at Tech and research scientist Carlos Hoyos, decided to see if they could create a model using the more reliable tornado record that’s existed since 1995.

The model that they developed for hurricane-induced tornadoes uses four factors that serve as good predictors of tornado activity: size, intensity, track direction and whether there’s a strong gradient of moisture at midlevels in the storm’s environment.

“The size of a tropical cyclone basically sets the domain over which tornadoes can form. So a larger storm that has more exposure over land has a higher propensity for producing tornadoes than a smaller one, on average,” said Belanger.

The team looked at 127 tropical cyclones from 1948 up to the 2008 hurricane season and went further back to 1920 modifying their model to account for the type of data collected at that time. They found that since 1995 there has been a 35 percent percent increase in the size of tropical cyclones from the Gulf compared to the previous active period of storms from 1948-1964, which has lead to a doubling in the number of tornadoes produced per storm. The number of hurricane-induced tornadoes during the 2004 and 2005 hurricane seasons is unprecedented in the historical record since 1920, according to the model.

“The beauty of the model is that not only can we use it to reconstruct the observational record, but we can also use it as a forecasting tool,” said Belanger.

To test how well it predicted the number of tornadoes associated with a given hurricane, they input the intensity of the storm at landfall, it’s size, track and moisture at mid-levels, and were able to generate a forecast of how many tornadoes formed from the hurricane. They found that for Hurricane Ike in 2008, their model predicted exactly the number of tornadoes that occurred, 33. For Hurricane Katrina in 2005, the model predicted 56 tornadoes, and 58 were observed.

The team’s next steps are to take a look to see how hurricane size, not just intensity (as indicated by the Safir-Simpson scale), affects the damage experienced by residents.

“Storm surge, rain and flooding are all connected to the size of the storm,” said Curry. “Yet, size is an underappreciated factor associated with damage from hurricanes. So its important to develop a better understanding of what controls hurricane size and how size influences hurricane damage. The great damage in Galveston from Hurricane Ike in 2008 was inconsistent with Category 2 wind speeds at landfall, but it was the large size that caused the big storm surge that did most of the damage.”

Twisters are born in thunderstorms and are often accompanied by hail. Giant, persistent thunderstorms called supercells spawn the most destructive tornadoes.

These violent storms occur around the world, but the United States is a major hotspot with about a thousand tornadoes every year. “Tornado Alley,” a region that includes eastern South Dakota, Nebraska, Kansas, Oklahoma, northern Texas, and eastern Colorado, is home to the most powerful and destructive of these storms. U.S. tornadoes cause 80 deaths and more than 1,500 injuries per year.

A tornado forms when changes in wind speed and direction create a horizontal spinning effect within a storm cell. This effect is then tipped vertical by rising air moving up through the thunderclouds.

The meteorological factors that drive tornadoes make them more likely at some times than at others. They occur more often in late afternoon, when thunderstorms are common, and are more prevalent in spring and summer. However, tornadoes can and do form at any time of the day and year.

Tornadoes’ distinctive funnel clouds are actually transparent. They become visible when water droplets pulled from a storm’s moist air condense or when dust and debris are taken up. Funnels typically grow about 660 feet (200 meters) wide.

Tornadoes move at speeds of about 10 to 20 miles (16 to 32 kilometers) per hour, although they’ve been clocked in bursts up to 70 miles (113 kilometers) per hour. Most don’t get very far though. They rarely travel more than about six miles (ten kilometers) in their short lifetimes.

Tornadoes are classified as weak, strong, or violent storms. Violent tornadoes comprise only about two percent of all tornadoes, but they cause 70 percent of all tornado deaths and may last an hour or more.

People, cars, and even buildings may be hurled aloft by tornado-force winds—or simply blown away. Most injuries and deaths are caused by flying debris.

Tornado forecasters can’t provide the same kind of warning that hurricane watchers can, but they can do enough to save lives. Today the average warning time for a tornado alert is 13 minutes. Tornadoes can also be identified by warning signs that include a dark, greenish sky, large hail, and a powerful train-like roar.