How did tornadoes form across Iowa on Tuesday? What we know about the weather conditions

A map of tornado reports from the outbreak in Iowa on Tuesday, May 21. A total of 21 triangles indicate possible tornadoes from Red Oak to Nevada.

Multiple tornadoes touched down in Iowa on Tuesday night and devastated communities across the state. Wednesday was spent assessing the damage and starting a cleanup process that could stretch well into the summer.

Here's what we know about the storm system that swept across the Midwest.

Iowa tornado map: Where did tornadoes reported in Iowa on Tuesday?

Only a tornado in Greenfield had officially been confirmed, by 3 p.m. Wednesday. National Weather Service crews doing an initial survey reported at least EF3 level damage from the tornado in southwest Iowa.

The weather service's Des Moines office said three survey teams were in the field on Wednesday. The National Weather Service in Omaha also sent a team into southwest Iowa to complete surveys.

Unofficial reports from trained weather spotters throughout Tuesday included: A cluster of sightings in southwest Iowa in Cromwell, Prescott, Corning, Carbon, Red Oak and Villisca. Greenfield in southwest Iowa appeared to be a particularly hard hit. In central Iowa there were unconfirmed reports in Nevada, rural Story County and southeast of Polk City.

"The focus right now is just trying to get that assessment done but also be out of the way for [the community] to do their recovery," said Allan Curtis, a meteorologist with the National Weather Service in Des Moines.

Updates: Tornado, storm damage still being felt in Iowa as clean up begins

What caused the storm system that produced deadly tornadoes in Iowa?

A surface low-pressure system swept across the Midwest on Tuesday, triggering multiple storm formations, Curtis said.

That's not what made the system, unusual, though. The dangerous storms were driven by the way all of the "ingredients" lined up on Tuesday, he said.

More details: How do tornadoes form? Explaining the severe weather after dozens of recent Iowa tornadoes

"[We need] cold fronts, moisture and humidity. We had some sunshine that kind of helped give things more energy," Curtis said. "A big part and this is a little less intuitive for the public, we also look up into the atmosphere. We need some strong winds up there to kind of help keep those kind of storms going."

These types of storm systems are easy to spot in advance, Curtis said. By Monday, NWS meteorologists were forecasting an enhanced (Level 3) risk for severe weather, and began alerting the public via radio, online updates and social media. That increased to a moderate risk (Level 4) by Tuesday morning, hours before deadly storms hit the state.

The weather service's Storm Prediction Center issued a tornado watch for far western Iowa counties around 11:15 a.m. Tuesday. A second watch, issued around 1:15 p.m. covered the rest of the state. The watch put Iowans on notice hours ahead of the storms that dangerous weather was possible. The National Weather Service used a rare "particularly dangerous situation" label on the watch to signify the significant risk of violent, long-lived storms.

More: What is a PDS tornado watch? Understanding the label that describes 'rare situations' for tornadoes

When did severe weather enter Iowa on Tuesday?

An early wave of severe thunderstorms entered the state late Monday night and early Tuesday morning. A few severe thunderstorm and tornado warnings were issued in southwest Iowa shortly after midnight.

As forecast, another round of severe storms developed in Nebraska in the afternoon, bringing the most dangerous storms to Iowa, Curtis said.

By 1:30 p.m. storms had crossed the Missouri River into southwest Iowa, he said. Tornadoes were seen on the ground soon after in places like Prescott and Corning.

Greenfield, where storms were particularly severe, had tornadoes on the ground by 3:30 p.m. Around 4:50 p.m., severe weather had crossed the Des Moines metro and a damaging tornado touched down near Nevada in Story County. The storm was fast-moving. The last severe thunderstorm warning in eastern Iowa expired at 9:15 p.m. as storms crossed into Illinois and Wisconsin.

NWS says Iowa tornadoes highlight the importance of being weather aware

This spring has been active for storm systems, but Curtis said that isn't stopping anytime soon. Iowa is not out of severe weather season, "by any stretch."

  • Stay aware: If you have outdoor plans, stay aware of the weather. You can check forecasts at

  • Receive warnings and alerts: There are plenty of ways to stay up-to-date on weather warnings throughout the day. Here are some from the NWS:

    • Local television and radio stations routinely broadcast official "watches" and "warnings" issued by the National Weather Service as well as forecasts and storm tracks developed by on-air meteorologists.

    • Emergency notifications on your phone: Wireless Emergency Alerts, which will warn you when weather threatens. To learn more about alerts visit

    • NOAA Weather Radio is a nationwide network of radio stations broadcasting continuous weather information directly from the nearest National Weather Service office.

Editor’s note: An earlier version of this article incorrectly stated which river storms crossed to enter Iowa. It was the Missouri River.

Victoria Reyna-Rodriguez is a general assignment reporter for the Register. Reach her at or follow her on Twitter @VictoriaReynaR.

This article originally appeared on Des Moines Register: How did tornadoes form across Iowa? What weather conditions tell us.