(CNN) -- Baseball-size hail pelted parts of Nebraska Tuesday as severe thunderstorms swept through the region.
In Omaha, cars jammed streets with residents rushing home as a storm moved in, darkening afternoon skies over the city with ominous clouds, CNN affiliate KETV reported.
Forecasters warned that winds of more than 80 mph could tear through parts of the Midwest and heavy rains could cause flash floods.
In Nebraska, residents shared photos on social media showing large hail -- and damage the hail left behind.
An Omaha police sergeant posted a picture of what he said was an officer's dented car, its windshield shattered.
A wind gust of 100 mph was reported in Ringgold County, Iowa. By 10 p.m. ET, there had been 11 reports of possible tornadoes in Nebraska, Iowa, Wyoming and Kansas, the Storm Prediction Center said.
At one point, a storm churning through Nebraska was more than 10 miles tall, CNN meteorologist Chad Myers said.
And it's not over yet.
Storms were just gearing up Tuesday evening, Myers said, advising residents to stay indoors. Even if tornadoes don't develop, he said, damage from a derecho can be widespread and severe.
"If you're in Nebraska, Iowa, or Missouri, this is going to be a very long night," Myers said.
Sustained heavy rainfall could bring widespread flooding, Myers said.
KETV showed rescuers working to reach residents stranded by floodwaters in an Omaha neighborhood.
Police shared photos on social media of flooded and damaged roads.
In nearby Missouri Valley, Iowa, the storm ripped the roof off a hotel, the CNN affiliate reported.
Iowa Gov. Terry Branstad issued a storm disaster proclamation Tuesday night for Pottawattamie County, where the storms hit while voters were heading to the polls for primary elections.
CNN's Dave Hennen, Dave Alsup and Todd Borek contributed to this report.