Oklahoma City - US weather forecasters issued an unusually strong tornado warning for Saturday, saying conditions are ripe for an outbreak in the heart of the country that could be a "high-end, life threatening event."

It was only the second time in U.S. history that the Storm Prediction Center issued a high-risk warning more than 24 hours in advance, said Russ Schneider, director of the center, which is part of the National Weather Service. The first time 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.

Storms were already kicking off in Oklahoma, where a twister whizzed by the nation's tornado forecasting headquarters but caused little damage.

The strongly worded message came after the National Weather Service announced last month that it would start using terms like "mass devastation," ''unsurvivable" and "catastrophic" in warnings in an effort to get more people to pay attention. It said it would test the new warnings in Kansas and Missouri before deciding whether to expand them to other parts of the country.

Friday's warning, despite the strong language, was not part of that effort but just the most accurate way to describe what was expected, a weather service spokeswoman said.

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 tomorrow will be a very busy and dangerous day in terms of large tornadoes in parts of the central and southern plains," Vaccaro said. "The ingredients are coming together."

The worst weather is expected to develop late Saturday afternoon in Oklahoma and Kansas, but other areas also could see severe storms, forecasters said. The warning issued Friday covers parts of Illinois, Iowa, Kansas, Missouri, Nebraska, Oklahoma and Texas.

The weather service confirmed a tornado touched down about 4 p.m. Friday near the University of Oklahoma campus in Norman, where it is based. Non-essential personnel at the storm center and students were ordered to take shelter, officials said.

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," Vaccaro 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.