A cold front moved slowly south into the state during the day on the 20th.  The flow was quite light with overall weak dynamics associated with the system.  The atmosphere became unstable by afternoon with around 2000 J/kg of CAPE available.  The downdraft CAPE was quite high in the 900 to 1200 J/kg range.  CAPE in the -10 to -30 C layer of the atmosphere was between 300 and 600 J/kg.  Precipitable water was around 1.3 inches, with a freezing level in the 13,000 to 13,500 foot level.  Thunderstorms developed rapidly along and near the 850 mb front extending from north of Omaha to near Dubuque.  They initially produced hail with penny size hail reported in Audubon County.  The storms formed into a scattered line of severe thunderstorms and produced hail up to golf ball size.  Ames received golf ball size hail across much of the city.  As the storms continued one of the cells began producing high winds.  Winds of up to 65 MPH occurred in Story County around Huxley and Maxwell.  The high winds damaged barns and outbuildings at several locations.  Heavy rainfall of 2 to 3 inches fell over Story County, resulting in flash flooding, especially in the Maxwell area.  Most of the rain fell in a one to two hour time period.  Many ditches and farm fields were flooded.  Highway 210 was under water near Maxwell for a period of time.

Hail in DALLAS, IOWA
From 20-MAY-14 17:43:00 to 20-MAY-14 17:43:00 (CST-6)
Location: WOODWARD
Magnitude: 1.75 
Property damage: 50.00K
Crop damage: 5.00K
Reported by: Trained Spotter

Large amounts of hail reported.

Hail in BOONE, IOWA
From 20-MAY-14 18:15:00 to 20-MAY-14 18:15:00 (CST-6)
Location: MADRID
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in TAMA, IOWA
From 20-MAY-14 17:15:00 to 20-MAY-14 17:15:00 (CST-6)
Location: TRAER MUNI ARPT
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in DALLAS, IOWA
From 20-MAY-14 17:45:00 to 20-MAY-14 17:45:00 (CST-6)
Location: WOODWARD
Magnitude: 1.00 
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in AUDUBON, IOWA
From 20-MAY-14 15:53:00 to 20-MAY-14 15:53:00 (CST-6)
Location: VIOLA CENTER
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: SHAVE Project

Hail in AUDUBON, IOWA
From 20-MAY-14 15:54:00 to 20-MAY-14 15:54:00 (CST-6)
Location: LARLAND
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: SHAVE Project

Hail in AUDUBON, IOWA
From 20-MAY-14 15:25:00 to 20-MAY-14 15:25:00 (CST-6)
Location: FISCUS
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in STORY, IOWA
From 20-MAY-14 17:02:00 to 20-MAY-14 17:02:00 (CST-6)
Location: STORY CITY
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in STORY, IOWA
From 20-MAY-14 17:03:00 to 20-MAY-14 17:03:00 (CST-6)
Location: AMES
Magnitude: 1.25 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in BOONE, IOWA
From 20-MAY-14 16:35:00 to 20-MAY-14 16:35:00 (CST-6)
Location: BERKLEY
Magnitude: 1.00 
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in STORY, IOWA
From 20-MAY-14 17:05:00 to 20-MAY-14 17:05:00 (CST-6)
Location: AMES
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in STORY, IOWA
From 20-MAY-14 17:08:00 to 20-MAY-14 17:08:00 (CST-6)
Location: AMES
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in STORY, IOWA
From 20-MAY-14 17:05:00 to 20-MAY-14 17:08:00 (CST-6)
Location: AMES
Magnitude: 1.75 
Property damage: 250.00K
Crop damage: 0.00K
Reported by: Social Media

Several reports via TV and social media of golf ball hail in Ames.

Hail in STORY, IOWA
From 20-MAY-14 17:15:00 to 20-MAY-14 17:15:00 (CST-6)
Location: GILBERT
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Social Media

Hail in STORY, IOWA
From 20-MAY-14 18:30:00 to 20-MAY-14 18:30:00 (CST-6)
Location: SLATER
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Hail in AUDUBON, IOWA
From 20-MAY-14 15:49:00 to 20-MAY-14 15:49:00 (CST-6)
Location: ROSS
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: SHAVE Project

Hail in AUDUBON, IOWA
From 20-MAY-14 15:24:00 to 20-MAY-14 15:24:00 (CST-6)
Location: FISCUS
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: SHAVE Project

Hail in AUDUBON, IOWA
From 20-MAY-14 15:50:00 to 20-MAY-14 15:50:00 (CST-6)
Location: ROSS
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: SHAVE Project

Hail in JASPER, IOWA
From 20-MAY-14 19:24:00 to 20-MAY-14 19:24:00 (CST-6)
Location: CLYDE
Magnitude: 1.00 
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in MARSHALL, IOWA
From 20-MAY-14 19:42:00 to 20-MAY-14 19:42:00 (CST-6)
Location: VAN CLEVE
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in AUDUBON, IOWA
From 20-MAY-14 15:51:00 to 20-MAY-14 15:51:00 (CST-6)
Location: VIOLA CENTER
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: SHAVE Project

Hail in DALLAS, IOWA
From 20-MAY-14 21:27:00 to 20-MAY-14 21:27:00 (CST-6)
Location: KENNEDY
Magnitude: 1.00 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Social Media

Hail in RINGGOLD, IOWA
From 21-MAY-14 00:00:00 to 21-MAY-14 00:00:00 (CST-6)
Location: KELLERTON
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Heavy Rain in STORY, IOWA
From 20-MAY-14 06:00:00 to 21-MAY-14 06:00:00 (CST-6)
Location: AMES
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

Total rainfall for the 24 hour period was 3.01 inches.

Flash Flood in STORY, IOWA
From 20-MAY-14 19:38:00 to 20-MAY-14 22:30:00 (CST-6)
Location: MAXWELL
Flood cause: Heavy Rain
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Highway 210 had water across the road between 670th and 680th ave, about 2 miles east of Maxwell.

Flash Flood in STORY, IOWA
From 20-MAY-14 20:18:00 to 20-MAY-14 22:10:00 (CST-6)
Location: IOWA CENTER
Flood cause: Heavy Rain
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Numerous country gravel roads had water covering them from earlier rainfall of 2.5 to 3 inches.

Thunderstorm Wind in STORY, IOWA
From 20-MAY-14 18:42:00 to 20-MAY-14 18:42:00 (CST-6)
Location: HUXLEY
Magnitude: 56.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Gust was estimated.

Thunderstorm Wind in STORY, IOWA
From 20-MAY-14 18:59:00 to 20-MAY-14 18:59:00 (CST-6)
Location: IOWA CENTER
Magnitude: 52.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Wind gust was estimated.  Some hail also fell at the time.

Thunderstorm Wind in STORY, IOWA
From 20-MAY-14 19:02:00 to 20-MAY-14 19:02:00 (CST-6)
Location: MAXWELL
Magnitude: 50.00 EG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Numerous 2 to 4 inch limbs were down in Maxwell.

Thunderstorm Wind in TAMA, IOWA
From 20-MAY-14 20:41:00 to 20-MAY-14 20:41:00 (CST-6)
Location: TAMA
Magnitude: 53.00 MG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Mesonet

Wind speed was measured by a home weather station.

Hail in AUDUBON, IOWA
From 20-MAY-14 15:47:00 to 20-MAY-14 15:47:00 (CST-6)
Location: ROSS
Magnitude: 0.88 
Property damage: 1.00K
Crop damage: 0.00K
Reported by: SHAVE Project