Last Updated 5:42 p.m. ET
(CBS/AP) OKLAHOMA CITY - Baseball-sized hail was breaking windows and tearing siding off homes in northeast Nebraska, while tornadoes were spotted in Kansas and Oklahoma on Saturday as forecasters warned residents across the nation's midsection to brace for "life threatening" weather.
Tornado sirens sounded across Oklahoma City before dawn, and at least three possible tornadoes were reported west and north of the city, said Oklahoma Department of Emergency Management official Michelann Ooten. Some homes were damaged, though no injuries were immediately reported in any of the states.
But the most dangerous weather was expected later in the day, and National Weather Service officials issued a stern warning for residents to prepare for overnight storms that could spawn fast-moving tornadoes. Officials said a large area could be at risk for dangerous storms.
A tornado watch is in effect for parts of Iowa until 9:00 p.m. CT; for parts of Nebraska until 8:00 p.m. CT; and in parts of Kansas and Oklahoma until 6:00 pm CT. The weather service called today's weather system "a particularly dangerous situation.
"The threat isn't over with tonight, unfortunately. Severe weather is possible again tomorrow from east Texas and Arkansas and up to into the Great Lakes," said Bill Bunting, chief of operations at the Storm Prediction Center, which is part of the National Weather Service.
"This could go into certainly to overnight situations, which is always of immense concerns to us," Ooten said.
In Nebraska, Boone County Sheriff David Spiegel said the large hail also damaged vehicles and shattered windows in and around Petersburg, about 140 miles northwest of Omaha. Two possible tornadoes were reported father south in Nebraska near the Kansas border, according to the National Weather Service, which confirmed that at least one rain-wrapped tornado touched down in southwest Kansas and another in Oklahoma.
One of the suspected tornadoes in Oklahoma hit near the small town of Piedmont, taking a similar path as a tornado last May that killed several people, Mayor Valerie Thomerson said.
"Because of last year, we've had a lot of new people put storm centers into place," the mayor said, adding that no major damage had been reported. "We're all very anxious about this afternoon."
The Storm Prediction Center, which is part of the National Weather Service, gave the sobering warning that the outbreak could be a "high-end, life-threatening event."
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.
It's possible to issue earlier warnings because improvements in storm modeling and technology are letting forecasters predict storms earlier and with greater confidence, 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.
"We're quite sure (Saturday) will be a very busy and dangerous day in terms of large tornadoes in parts of the central and southern plains," Vaccaro said Friday. "The ingredients are coming together."
The city of Norman, Okla., home to the University of Oklahoma campus, got a preview of the potential destruction on Friday when a twister whizzed by the nation's tornado forecasting headquarters but caused little damage. Norman Regional Hospital and an affiliate treated 19 people for mainly "bumps and bruises," hospital spokeswoman Kelly Wells said.