Forecasters say to expect "life-threatening" storms throughout the Midwest starting Saturday afternoon, in a path ranging from Minnesota to Texas. Kansas, Nebraska and Oklahoma are projected to receive the brunt of the damage, which could include thunderstorms, tornadoes and hail.
Here are some details on why these early warnings - which came on Friday - are so unusual.
JUST HOW RARE? This marks the second time in U.S. history that the Storm Prediction Center has issued a high-risk warning more than 24 hours in advance.
WHAT HAPPENED LAST TIME: The first high-risk warning more than a day early came in April 2006, when nearly 100 tornadoes tore across the southeastern U.S. In all, a dozen people died and more than 1,000 homes were damaged in Tennessee.
WHY EARLIER WARNINGS: In the past, people often have had only minutes of warning when a siren went off. But improvements in storm modeling and technology let forecasters predict storms earlier and with greater confidence, the National Weather Service says. The Storm Prediction Center is part of the service.
NEW WARNING LANGUAGE: The weather service is now testing words such as "mass devastation," "unsurvivable" and "catastrophic" aimed at getting more people to take heed. The warnings are being experimented with in Kansas and Missouri. The "life-threatening" warning for this round of storms, despite the dire language, was not part of that effort but just the most accurate way to describe what was expected, a weather service spokeswoman said.
WHAT TO EXPECT: The worst conditions are projected to hit late Saturday afternoon between Oklahoma City and Salina, Kan. Other areas too could see severe storms with baseball-sized hail and winds of up to 70 mph. The warning includes parts of Illinois, Iowa, Kansas, Missouri, Nebraska, Oklahoma and Texas.
