DES MOINES, Iowa (AP) — Severe weather packing large hail and heavy rain rolled into Nebraska and Iowa on Tuesday as potentially dangerous storms targeted a swath of the Midwest, including states where voters were casting ballots in primary elections.

The National Weather Service said the highest risk for severe weather was centered in parts of northern and eastern Nebraska, western and southern Iowa, and northeast Missouri. Officials said there was the potential for a weather event called a derecho, which is a storm of strong straight-line winds spanning at least 240 miles.

“This is one of these days we can’t let our guard down,” said Bill Bunting, forecast operations chief at the Storm Prediction Center in Norman, Oklahoma. “It’s unfortunately panning out as we thought it would. This is shaping up as a very dangerous evening.”

An outbreak of severe thunderstorms is forecast for the afternoon into the early hours of Wednesday. It could pack winds of up to 80 mph, large hail and up to 4 inches of rain.

In Nebraska, weather officials said at least one severe thunderstorm capable of producing a tornado was reported near the city of Fullerton. A similar thunderstorm was reported in the western Iowa community of Elliot. Officials say they won’t be able to confirm if there were any tornadoes for several hours.

The storms also are expected to affect parts of Illinois, Kansas and South Dakota.

Severe weather in Omaha caused officials to close Eppley Airfield airport. Baseball-size hail also was reported in the northeast community of Norfolk, Nebraska.

The bad weather has already had an impact on the primary elections in Iowa, where some officials and voters were forced to seek shelter in Pottawattamie County around 5 p.m. County Auditor Mary Jo Drake said operations have been suspended in about 10 precincts out of 40. There are no reports of injuries.

“It’s nasty here, you can’t imagine,” Drake said. “It’s as black as the ace of spades.”

Iowa Secretary of State Matt Schultz also encouraged residents to vote early due to the expected severe weather.

In South Dakota, a thunderstorm forced Senate candidate Mike Rounds, who was flying to Rapid City, to land in Pierre. Secretary of State Jason Gant said he hopes the weather doesn’t hurt voter turnout, but he wants people to be safe. Bunting said Iowa residents planning to vote Tuesday should be cautious about when they head out to the polls.

“There may be in many areas time to go out and do things before the storms hit. I think the main thing is to listen to the forecast, get some sense of when storms are likely in your area and just make sure you’re in a safe place before the storms hit,” he said.

The severe weather threat arrives amid an unusually quiet late spring, with far fewer documented tornados in May than in many recent years.