A state emergency official says a sixth person has died after a tornado ripped through the Oklahoma town of Woodward.

The Oklahoma Department of Emergency Management spokeswoman Keli Cain could not provide details about the victim's gender or age Monday.

Two men and three children were also killed in the tornado that hit the town in the early hours of Sunday, part of a storm system that stretched from Texas to Minnesota and spawned more than 120 twisters.

Tim Crom picks up debris from a damaged home in Thurman, Iowa, on Sunday. Tornadoes caused major damage throughout the U.S. and killed at least six people. (Lane Hickenbottom/Reuters)

Authorities said warning sirens that did not sound in Woodward may have been disabled when lightning hit the control tower.

CBC meterologist Johanna Wagstaffe said the weather was remarkable for a number of reasons.

The National Weather Service's Storm Prediction Center in Oklahoma took the unusual step of warning the region more than 24 hours in advance of a possible "high-end, life-threatening event."

Wagstaffe said all of the ingredients were there for tornadic supercells: an incredibly sharp contrast between hot, humid air and dry, cool air, twisting in the atmosphere, and a cold front to act as a trigger.

It was only the second time in its history that the National Oceanic and Atmospheric Administration issued a "high risk" warning more than 24 hours in advance. The last time was April 7, 2006.

The number of people hurt or killed with the storms would have likely been much higher without these advanced warnings, Wagstaffe said.

The warnings were unusually stern: The weather service in Wichita, Kan., warned residents "could be killed if not underground or in a tornado shelter.

'People become used to those warnings. That is a dangerous complacency.'—Randy Duncan, emergency management director

"Complete destruction of entire neighbourhoods is likely," the impact-based warning said. "Mass devastation is highly likely, making the area unrecognizable to survivors."

In south central Kansas, Sedgwick County Emergency Management director Randy Duncan credited the dire language of the warnings for saving lives.

"People become used to those warnings. That is a dangerous complacency," Duncan said. "We need to break through the clutter of everyday noise to get people's attention."

A National Weather Service official said a "month's worth" of tornadoes were spotted in Kansas over the weekend. About 100 homes were damaged in a Wichita mobile home park, but no serious injuries or fatalities were reported.

"We knew well ahead of time that this was going to be ugly. People listened," Sedgwick County commissioner Tim Norton said.

Sirens an unreliable warning system

Many residents in Tornado Alley have grown up counting on sirens to warn them when a twister has been spotted on the ground, but emergency officials say that can be one of the least reliable methods, especially when a tornado hits at night.

"An outdoor warning system should never be the only way or even the primary way to receive a warning," said Rick Smith, a warning co-ordination meteorologist with the National Weather Service. "Our message that we preach is you have to have several ways to receive a warning."

Curt and Andra Raymer had taken steps to prepare, but thought they were in the clear until a television meteorologist warned Woodward residents to take cover just minutes before the storm hit.

A house sits ripped open from a tornado on Sunday in Thurman, Iowa. (Patrick Breen/The Journal-Star/AP)

"We heard the sirens yesterday afternoon, and they blew for 40 minutes," said Andra Raymer, 44, as she picked through the rubble of her home that was covered with insulation, broken glass and splintered wood. "Last night when this one came through, we didn't hear anything."

The couple and their dogs took shelter in an interior bathroom as the roof was lifted from their home and smashed in their backyard.

"We're just lucky to be alive," Curt Raymer said. "We walked out into the street and just couldn't believe it."

Emergency management officials urged residents to take advantage of weather radios, smartphones and television warnings to keep them up to speed when weather turns dangerous.

Sirens are not designed to wake residents who are sleeping or to penetrate the thick insulation in today's homes, said Albert Ashwood, the director of Oklahoma's Office of Emergency Management.

"Sirens are referred to as outdoor warning systems, and that's what they're there for — to tell people who are outdoors to come inside and find out what's going on," Ashwood said.

Wagstaffe said there is a slight risk for severe storms on Monday from the upper Mississippi Valley to the Texas Gulf Coast thanks to hot and humid air still in place.

With files from CBC News