Tornadoes were spotted across the Midwest and Plains on Saturday as an outbreak of unusually strong weather seized the region, and forecasters sternly warned that "life-threatening" weather could intensify overnight.

Storms were reported in Kansas, Iowa, Nebraska and Oklahoma. Emergency officials in Iowa said that high winds or a tornado damaged a hospital in Creston, but no injuries were reported. Authorities also said about 75 percent of the small western Iowa community of Thurman was destroyed, with no injuries reported there either.

National Weather Service forecasters issued sobering outlooks that the worst of the weather would hit around nightfall, predicting that conditions were right for exceptionally strong tornadoes. Weather officials and emergency management officials worried most about what would happen if strong storms hit when people were sleeping, not paying attention to weather reports and unlikely to hear warning sirens.

When it's dark, it's also more difficult for weather spotters to clearly see funnel clouds or tornadoes.

"This could go into, certainly, to overnight situations, which is always of immense concern to us," said Michelann Ooten, an official with the Oklahoma Department of Emergency Management.

The Storm Prediction Center in Norman, Okla., which specializes in tornado forecasting, said that the outbreak could be a "high-end, life-threatening event" nearly two days before the weather hit.

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 southeastern U.S., killing a dozen people and damaging more than 1,000 homes in Tennessee.

While there were no fatalities as of Saturday evening, storms were erupting faster than spotters could tally them all. The danger began Saturday morning when tornado sirens sounded in Oklahoma City around dawn.

One of the suspected tornadoes in central Oklahoma touched down near the small town of Piedmont and followed a similar path the one last May that killed several people, Mayor Valerie Thomerson said. Later in the day, several tornadoes were reported to have touched down in the northeast part of the state. Aside from damage to a camper, the chaos was minor.

More than 5,000 people who had gathered in Woods County, Okla., for a rattlesnake hunt scattered when a tornado touched down, said county emergency management director, Steve Foster.

In Iowa, Thurman _ a town of about 250 people _ was severely damaged by a possible tornado. Fremont County Emergency Management Director Mike Crecelius said that about 75 percent of the town was destroyed, but there were no injuries or deaths. Crecelius said the town was on lockdown Saturday night, and that town officials and residents expect to start cleaning up on Sunday.

In Creston, about 75 miles from Des Moines, the Greater Regional Medical Center suffered roof damage and had some of its windows blown out by a storm, said John Benson, a spokesman for Iowa Homeland Security and Emergency Management. No injuries were reported. Medical center officials were calling other area hospitals to determine how many beds they had available in case they needed to move patients.

The National Weather Service in Des Moines also received reports of high winds that toppled at least five semis on Interstate 29.

In northeast Nebraska, Boone County Sheriff David Spiegel said baseball-sized hail had damaged vehicles, shattered windows and tore siding from houses in and around Petersburg, about 140 miles northwest of Omaha. In southeast Nebraska, an apparent tornado took down barns, large trees, and some small rural structures. Johnson County emergency director Clint Strayhorn said he was trying to determine the twister's duration and the damage it caused.

"I'm on a 2-mile stretch that this thing is on the ground and I haven't even gotten to the end of it yet," he said, walking the path of destruction near the Johnson-Nemaha county line. He didn't immediately know of any injuries.

Two possible tornadoes were reported father south in Nebraska near the Kansas border, and as many as 10 others were reported in largely rural parts of western and central Kansas, including one north of Dodge City that was said to be on the ground for a half-hour, weather officials said.

In Salina, Kan., tornado sirens sounded after a possible tornado was spotted nearby. National Weather Service meteorologist Mike Scott also said tornadoes were reported in the central and western Kansas counties of Pratt, Stafford, Rush and Hodgeman. There were reports of a home damaged in Rush County and an old schoolhouse damaged in Hodgeman County.

Tornado threats caused some weekend festivities to be called off. The threat prompted University of Nebraska-Lincoln athletic officials to cancel the annual spring football game minutes before Saturday's kick-off.

Forecasters warned once Saturday night's danger had passed, the threat from the storm system wasn't over. Severe weather was also possible for a significant band of the center of the country on Sunday.

"The threat isn't over with tonight, unfortunately. Severe weather is possible again tomorrow from east Texas and Arkansas and up into the Great Lakes," said Bill Bunting, chief of operations at the Storm Prediction Center, which is part of the National Weather Service.

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Associated Press reporters Grant Schulte in Omaha, Neb.; David Pitt in Osceola, Iowa; Sean Murphy and Rochelle Hines in Oklahoma City; Heather Hollingsworth in Kansas City, Mo.; Erin Gartner in Chicago; and Ed Donahue in Washington contributed to this report.