A slow, south moving cold front and an outflow boundary from morning thunderstorms over northern Illinois, combined with a hot and humid environment to produce widespread showers and thunderstorms across eastern Iowa and northwest Illinois overnight. Several thunderstorms became severe, and produced large hail and damaging winds. Quarter sized hail was reported across Clinton and Iowa counties, while destructive winds of 70-90 MPH were reported Iowa and Washington Counties.
In addition, torrential rainfall amounts of 1 to 2 inches were reported along the Interstate 80 corridor as storms slowly moved across the area. This lead to flash flooding across many major roadways, including Interstate 80 near Coralville and Highway 965 in Oakdale.

Thunderstorm Wind in SCOTT, IOWA
From 16-MAY-19 21:15:00 to 16-MAY-19 21:15:00 (CST-6)
Location: BETTENDORF
Magnitude: 56.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Employee

A large 6 inch diameter tree branch was reported down near the intersection of Kimberly Road and Jersey Ridge Road.

Thunderstorm Wind in CLINTON, IOWA
From 16-MAY-19 18:29:00 to 16-MAY-19 18:29:00 (CST-6)
Location: CLINTON MUNI ARPT
Magnitude: 50.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: AWOS

Thunderstorm Wind in BENTON, IOWA
From 16-MAY-19 19:33:00 to 16-MAY-19 19:33:00 (CST-6)
Location: KEYSTONE
Magnitude: 58.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Thunderstorm Wind in IOWA, IOWA
From 16-MAY-19 19:57:00 to 16-MAY-19 19:57:00 (CST-6)
Location: WILLIAMSBURG
Magnitude: 68.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Measured by a home weather station.

Thunderstorm Wind in JOHNSON, IOWA
From 16-MAY-19 20:23:00 to 16-MAY-19 20:23:00 (CST-6)
Location: NORTH LIBERTY
Magnitude: 56.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Estimated winds of 60 to 70 mph were reported.

Thunderstorm Wind in WASHINGTON, IOWA
From 16-MAY-19 20:29:00 to 16-MAY-19 20:29:00 (CST-6)
Location: RIVERSIDE
Magnitude: 67.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Thunderstorm Wind in JOHNSON, IOWA
From 16-MAY-19 20:32:00 to 16-MAY-19 20:32:00 (CST-6)
Location: IOWA CITY MUNI ARPT
Magnitude: 54.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: AWOS

Thunderstorm Wind in JOHNSON, IOWA
From 16-MAY-19 20:34:00 to 16-MAY-19 20:34:00 (CST-6)
Location: TIFFIN
Magnitude: 61.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Torrential rain also reported.

Thunderstorm Wind in JOHNSON, IOWA
From 16-MAY-19 20:40:00 to 16-MAY-19 20:40:00 (CST-6)
Location: IOWA CITY MUNI ARPT
Magnitude: 56.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Thunderstorm Wind in WASHINGTON, IOWA
From 16-MAY-19 20:40:00 to 16-MAY-19 20:40:00 (CST-6)
Location: RIVERSIDE
Magnitude: 75.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Thunderstorm Wind in IOWA, IOWA
From 16-MAY-19 20:01:00 to 16-MAY-19 20:01:00 (CST-6)
Location: WILLIAMSBURG
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Power lines were blown down.

Thunderstorm Wind in JOHNSON, IOWA
From 16-MAY-19 20:27:00 to 16-MAY-19 20:27:00 (CST-6)
Location: WILLIAMSTOWN
Magnitude: 60.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

A small shed was blown away.

Flash Flood in JOHNSON, IOWA
From 16-MAY-19 20:43:00 to 16-MAY-19 21:43:00 (CST-6)
Location: OAKDALE
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Four to six inches of water briefly covered the eastbound lanes of Interstate 80 due to tremendous rainfall rates. Other major roads in the area were also impacted by water due to heavy rainfall rates.

Hail in BENTON, IOWA
From 16-MAY-19 19:25:00 to 16-MAY-19 19:25:00 (CST-6)
Location: KEYSTONE
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail in IOWA, IOWA
From 16-MAY-19 19:41:00 to 16-MAY-19 19:41:00 (CST-6)
Location: VICTOR
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in IOWA, IOWA
From 16-MAY-19 19:51:00 to 16-MAY-19 19:51:00 (CST-6)
Location: GENOA BLUFF
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Hail in CLINTON, IOWA
From 16-MAY-19 20:15:00 to 16-MAY-19 20:15:00 (CST-6)
Location: DEWITT
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Hail in CLINTON, IOWA
From 16-MAY-19 20:22:00 to 16-MAY-19 20:22:00 (CST-6)
Location: DEWITT
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in SCOTT, IOWA
From 16-MAY-19 20:22:00 to 16-MAY-19 20:22:00 (CST-6)
Location: GAMBRILL
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Employee

Winds gusted to 40 mph during the hail.

Hail in CLINTON, IOWA
From 16-MAY-19 20:26:00 to 16-MAY-19 20:26:00 (CST-6)
Location: LOW MOOR
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Social Media

Thunderstorm Wind in IOWA, IOWA
From 16-MAY-19 19:58:00 to 16-MAY-19 19:58:00 (CST-6)
Location: PARNELL
Magnitude: 70.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trees were blown down in the town of Parnell. The largest were around 2 feet in diameter and some were snapped off 3 feet above the ground.

Thunderstorm Wind in JOHNSON, IOWA
From 16-MAY-19 20:15:00 to 16-MAY-19 20:15:00 (CST-6)
Location: OXFORD
Magnitude: 56.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

Several highway signs were blown down near mile marker 230 on Interstate 80.