Updates: Severe thunderstorm warning looming over Johnson County as storms arrive

Several rounds of storms are pushing through eastern Iowa Tuesday, bringing with the potential for intense winds, flooding, hail, and even tornadoes.

Early morning thunderstorms delivered a brief initial blow before a second line of storms ripped through west and central Iowa, causing extensive damage in communities like Greenfield and Nevada .

The most severe weather is expected to impact eastern Iowa this evening.

Here's the latest:

What to know about Tuesday's severe weather threat

A weather system that cut through central Iowa and moved east was "expected to be very dangerous," the National Weather Service in Des Moines said in a Facebook post.

Around 7:15 p.m., the National Weather Service issued a severe thunderstorm warning for the area.

The NWS has designated the severe weather threat to the area as a "moderate risk," the second-highest rating on its five-point scale.

The National Weather Service has issued a tornado watch through 9 p.m. this evening as the threat of severe weather this afternoon and evening sweeps across the state. That alert includes Polk, Story, Linn and Johnson counties.

NWS is warning that "several strong tornadoes," "softball sized-hail," and "widespread" wind gusts up to 90 miles per hour are all likely.

On Tuesday morning, NWS also issued a flood watch through 9 p.m. The flood watch extends east to Johnson and Linn counties.

Here are the latest updates

7:15 p.m. : Severe thunderstorm warning looms over Coralville, Iowa City

A severe thunderstorm warning has been issued for areas at the core of Johnson County, including Iowa City, Coralville and North Liberty.

The warning warns of quarter-sized hail and 60 mph winds and stretches north and east through Cedar and Jones counties.

6 p.m.: After a quiet afternoon, warnings issued as storms head toward eastern Iowa

A flurry of warnings and watches is approaching eastern Iowa, part of a line of strong storms crossing the state Tuesday.

The National Weather Service in the Quad Cities projects that the storms will develop by approximately 7 p.m. this evening.

Thunderstorm warnings have been issued for communities in Black Hawk, Henry and Tama counties to the west of Johnson County.

The storm brings with it the potential for tornadoes, two inches of hail and wind gusts up to 75 miles per hour.

2 p.m.: When does a storm become a derecho?

The National Weather Service's definition of a derecho stipulates that “if the wind damage swath extends more than 240 miles and includes wind gusts of at least 58 mph or greater along most of its length."

Tuesday’s storm, with a tornado watch now stretching nearly the entire 310-mile length of the state, is going to be “a significant event no matter what you call it," Johnson County Emergency Management Director Dave Wilson told the Press-Citizen.

From March: How University of Iowa researchers adapted after a 2023 tornado destroyed their lab

A storm must fit "a narrow set of circumstances" to be classified as a derecho, Wilson said.

"I think people get hung up on the definitions like 'derecho' or a tornado,” he said. “Anytime you get winds over 70 miles an hour that are spanning the length of the state, that's going to be a recipe for problems."

The NWS has issued a severe weather threat level of four for a large swath of eastern Iowa. The last time Johnson County was under that high of a threat level was March 2023, when a pair of EF-2 tornadoes swept through Coralville and Hills. The devastating storm system peeled roofing off homes and ripped through a University of Iowa research lab .

Wilson said Tuesday's forecast, which could produce large hail, damaging winds, driving rain, and strong tornadoes, is even more reason for residents to stay "weather-aware."

“People need to heed the warnings,” Wilson said. “Take time to just chill at home and keep your eye on that TV, the radio, and your weather outlets, and have a plan for where you're going to shelter.”

From 2023: A week after tornadoes, displaced Johnson County victims anxious to return home

1:45 p.m.: Preparedness ‘key,' sirens seem ‘inevitable,’ emergency management says

As a strong storm system rolls in from the west, the possibility that Johnson County residents will hear sirens this evening seems more like an inevitability, Johnson County Emergency Management Director Dave Wilson said.

The sirens are sounded for 70-mile-per-hour winds or golf-ball-sized hail as well as for the threat of a tornado, he said.

“(People) cannot be oblivious to your surroundings on a day like today, because it's a highly volatile situation,” Wilson told the Press-Citizen.

If the storm continues as forecasted, residents should expect power outages, downed trees and ringing fire alarms, he said.

“People need to take this seriously,” Wilson said. “We're encouraging people to get home, stay home and have a plan.”

1:10 p.m.: Tornado watch stretching across 80 counties issued until 9 p.m.

Parts of four states and the vast majority of counties in Iowa are under a tornado watch until 9 p.m. on Tuesday.

The National Weather Service said the storm system "is a particularly dangerous situation."

Eighty of Iowa's 99 counties are under the alert.

The NWS is warning of strong tornadic activity, "softball sized-hail" and wind gusts reaching 90 miles per hour.

12:45 p.m.: Severe thunderstorm warning for Tiffin, North Liberty, areas north

The NWS issued a severe thunderstorm warning that includes parts of Coralville and all of North Liberty, Tiffin, Oxford and Swisher in Johnson County that will last until 1:30 p.m. The warning also includes Cedar Rapids. The weather system is capable of producing quarter-sized hail and winds up to 60 miles per hour, according to the NWS.

12:20 p.m.: Small storm capable of hail, strong winds headed for Iowa City

The National Weather Service shared radar images of a small storm that is expected to pass through Iowa City between 12:30 p.m. and 1:30 p.m. The weather service said the system is capable of producing quarter-sized hail and strong winds. It will be the second true round of weather to sweep through the area after an early morning system passed near the Iowa City metro.

11:55 p.m.: Severe thunderstorm warning issued in Iowa County

The National Weather Service has issued a severe thunderstorm warning across Iowa County and areas east, including Marengo and Amana, until 12:45 p.m.

11:35 a.m. : NWS says severe weather threat 'expected to be very dangerous'

In a comment on a Facebook post, the NWS said this afternoon and evening's storms are "expected to be very dangerous" as they sweep across the state.

In Des Moines, school districts have announced that they will release students early, ahead of the arrival of more severe weather that has already dumped an inch of rain on the capital city. More than 3,600 in Des Moines were without power after this morning's thunderstorm.

10:30 a.m.: Second line of storms moving north and east

After thunderstorms shook up the morning commute just after sunrise, a second line of storms is headed toward Johnson County.

From Des Moines: Multiple rounds of severe storms to hit Iowa today. Flash flooding, tornadoes possible

The National Weather Service office in the Quad Cities is monitoring a line of storms that downed power lines, damaged trees and set off sirens in Des Moines this morning before dumping hail in Waukee.

Like the early morning system, the brunt of the storm is expected to largely sweep north and east, passing through Cedar Rapids and communities further north.

What's the difference between a severe weather watch and a severe weather warning?

A watch is issued when conditions are favorable for severe weather to develop. They often cover large areas.

Warnings are issued when severe weather is actually happening and Iowans need to take action.

What should you do during a severe thunderstorm warning or tornado warning?

During a severe weather warning, it's time to take action and follow your plan.

NWS has tips for what to do at different locations:

• Stay Weather Ready: Continue to follow local news or listen to a NOAA Weather Radio to stay updated about watches and warnings.
• At Your House: Go to your secure location, like the basement or lowest floor level, if you hear a warning. Take your pets with you if time allows.
• At Your Workplace or School: Stay away from windows. Do not go to large open rooms such as cafeterias, gymnasiums or auditoriums.
• Outside: Go inside a sturdy building immediately. Sheds and storage facilities are not safe. Taking shelter under a tree can be deadly.
• In a Vehicle: Being in a vehicle during severe weather is safer than being outside, but, drive to the closest secure shelter if there is sufficient time. Do not stop and park under a bridge or underpass.

The Register's Victoria Reyna-Rodriguez and Kate Kealy contributed reporting from Des Moines.

Ryan Hansen covers local government and crime for the Press-Citizen. He can be reached at rhansen@press-citizen.com or on X, formerly known as Twitter, @ryanhansen01.

This article originally appeared on Iowa City Press-Citizen: Updates: Severe thunderstorm warning looming over Johnson County as storms arrive