A broad cyclonic circulation was in place over Iowa with the surface low over northeast South Dakota.  A trough of low pressure extended south into eastern Nebraska.  A cyclonic jet in the mid latitudes was nosing into Iowa.  The atmosphere became unstable with most unstable CAPE between 1000 and 2000 J/kg.  Downdraft CAPE was 800 to 1000 J/kg with CAPE available in the -10 to -30 C layer of the atmosphere between 400 and 500 J/kg.  The freezing level was quite low, between 9,500 and 11,000 feet.  There was a decent amount of shear available with 40 to 50 kts at the beginning of storm development and the LCL was around 1250 meters.  Precipitable water values were around 1.1 inches.  Thunderstorms fired rapidly in central Iowa along an old boundary.  The storms fired rapidly all the way from southwest into central and parts of northeast Iowa.  With the low freezing level, the primary severe threat was hail.  Many of the thunderstorms produced penny to quarter size hail.  Golf ball hail occurred in Union, Polk, and Story County.  Elsewhere the hail was an inch or less.  There were numerous reports of funnel clouds, but with the LCL above 1000 meters, none of them touched down.  During the later part of the event, thunderstorms began to produce high winds with winds of 60 to 70 MPH reported in Hardin, Marshall, and Grundy Counties.

Funnel Cloud in POLK, IOWA
From 20-MAY-13 17:50:00 to 20-MAY-13 17:50:00 (CST-6)
Location: DODGE FLD ARPT
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Employee

Two funnel clouds were observed.

Hail in POLK, IOWA
From 20-MAY-13 18:00:00 to 20-MAY-13 18:00:00 (CST-6)
Location: HERROLD
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

At the NWS office in Johnston.

Hail in POLK, IOWA
From 20-MAY-13 18:33:00 to 20-MAY-13 18:33:00 (CST-6)
Location: FARRAR
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Nickel to quarter size hail at the intersection of 620th Ave and 340th St.

Hail in STORY, IOWA
From 20-MAY-13 18:25:00 to 20-MAY-13 18:25:00 (CST-6)
Location: NEVADA
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: County Official

Hail in STORY, IOWA
From 20-MAY-13 18:38:00 to 20-MAY-13 18:40:00 (CST-6)
Location: MAXWELL
Magnitude: 1.75 
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Hail reached quarter size by 1838 CST.

Hail in POLK, IOWA
From 20-MAY-13 18:17:00 to 20-MAY-13 18:17:00 (CST-6)
Location: ORALABOR
Magnitude: 1.75 
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Amateur Radio

Hail in WARREN, IOWA
From 20-MAY-13 19:33:00 to 20-MAY-13 19:33:00 (CST-6)
Location: MARTENSDALE
Magnitude: 1.00 
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Winds of 50 to 60 MPH also occurred.

Hail in DALLAS, IOWA
From 20-MAY-13 17:22:00 to 20-MAY-13 17:22:00 (CST-6)
Location: VAN METER
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in FRANKLIN, IOWA
From 20-MAY-13 19:45:00 to 20-MAY-13 19:45:00 (CST-6)
Location: POPEJOY
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in UNION, IOWA
From 20-MAY-13 17:45:00 to 20-MAY-13 17:45:00 (CST-6)
Location: CRESTON
Magnitude: 1.75 
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Thunderstorm Wind in POLK, IOWA
From 20-MAY-13 19:50:00 to 20-MAY-13 19:50:00 (CST-6)
Location: SANTIAGO
Magnitude: 55.00 EG
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Mitchellville Fire Department reeported several trees down in town.  Also 8 to 10 inch diameter branches snapped.

Hail in UNION, IOWA
From 20-MAY-13 17:40:00 to 20-MAY-13 17:40:00 (CST-6)
Location: CRESTON
Magnitude: 1.75 
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Public

Golf ball size hail was reported on the northeast side of Creston.

Funnel Cloud in POLK, IOWA
From 20-MAY-13 18:01:00 to 20-MAY-13 18:11:00 (CST-6)
Location: ANKENY
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Three HAM operators reported a rotating funnel cloud which lasted 10 minutes.

Funnel Cloud in STORY, IOWA
From 20-MAY-13 18:48:00 to 20-MAY-13 18:48:00 (CST-6)
Location: COLLINS
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Rotating funnel cloud observed to the north near Collins.  Dust stirred up at the ground below the funnel, but rotation was not observed.  Lasted about 20 seconds.

Thunderstorm Wind in GRUNDY, IOWA
From 20-MAY-13 19:22:00 to 20-MAY-13 19:22:00 (CST-6)
Location: IVESTER
Magnitude: 61.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Utility lines bent to the east.

Thunderstorm Wind in MARSHALL, IOWA
From 20-MAY-13 19:11:00 to 20-MAY-13 19:11:00 (CST-6)
Location: BANGOR
Magnitude: 52.00 EG
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Tree fell and took out a power line near Bangor.

Thunderstorm Wind in HARDIN, IOWA
From 20-MAY-13 19:10:00 to 20-MAY-13 19:10:00 (CST-6)
Location: UNION
Magnitude: 52.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Pea to dime size hail also.

Thunderstorm Wind in MARSHALL, IOWA
From 20-MAY-13 19:15:00 to 20-MAY-13 19:15:00 (CST-6)
Location: STATE CENTER
Magnitude: 52.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Pea to dime size hail with the wind.

Thunderstorm Wind in MARSHALL, IOWA
From 20-MAY-13 20:23:00 to 20-MAY-13 20:23:00 (CST-6)
Location: LURAY
Magnitude: 61.00 MG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Mesonet

Hail in POLK, IOWA
From 20-MAY-13 18:14:00 to 20-MAY-13 18:14:00 (CST-6)
Location: ANKENY
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in STORY, IOWA
From 20-MAY-13 18:24:00 to 20-MAY-13 18:24:00 (CST-6)
Location: NEVADA
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Mainly pea size hail with some stones up to quarter size.

Hail in UNION, IOWA
From 20-MAY-13 17:35:00 to 20-MAY-13 17:47:00 (CST-6)
Location: CRESTON
Magnitude: 1.75 
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Public

Golf ball size hail covering the ground.  Lasted 12 minutes.

Hail in POLK, IOWA
From 20-MAY-13 18:16:00 to 20-MAY-13 18:16:00 (CST-6)
Location: ANKENY
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter