A strong upper level low pressure trough over the western U.S. moved into the central states during the day on the 12th.  The situation was quite similar to the event 2 days prior, though the severe weather was farther north.  The 850 mb front was stretched across Iowa, with a strong surface front south of the state.  A significant cool layer was in place over Iowa with surface temperatures remaining in the 40s through the day.  Significant instability was in place south of Iowa with most unstable CAPE values of 1000 to 2000 J/kg in place.  A considerable amount of moisture was transported north into the central U.S. on a 60 kt low level jet.  Precipitable water values were between 1.5 and 1.75 inches.  The freezing level was in the 12,000 to 13,000 foot range.  CAPE in the -10 to -30 C layer of the atmosphere was between 100 and 400 J/kg.  There was significant shear present, with the effective shear around 60 kts.  Thunderstorms formed during the predawn hours of the 12th and moved across Iowa during the morning of the 12th.  Once these storms moved out, another area of thunderstorms formed along the 850 mb front.  Most of the storms produced hail up to dime size.  Some of the stronger thunderstorms produced quarter size hail with one producing 1 1/4 inch diameter hail in Marshall County.  Several of the storms also produced large amounts of hail, with reports of the ground being covered by hail.  Successive rounds of thunderstorms continued to push into central and northeast Iowa.  Most hail was below the size of quarters from the late afternoon rounds.  Heavy rain became a problem with spotty flash flooding reported.  Flooding took place in Butler County due to rainfall of nearly 3 inches that fell just a day after a general 1 to 2 inch rainfall occurred.  Flooding was largely limited to agricultural lands.  In Polk County, heavy rainfall of around 2 inches fell after the area had received about an inch in the 36 hours prior.  Flash flooding occurred on the east side of the Des Moines metropolitan area.  A ramp on Highway 65 was flooded by high water and an intersection southeast of Altoona was washed out.  As the storms moved through the Wavery area, lightning struck a house causing s small fire.  It was extinguished quickly, resulting in little damage.

Flash Flood in BUTLER, IOWA
From 12-MAY-10 22:52:00 to 13-MAY-10 03:00:00 (CST-6)
Location: CLARKSVILLE
Flood cause: Heavy Rain
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Water running through fields and ditches and also ponding of water near Clarksville.

Hail in FRANKLIN, IOWA
From 12-MAY-10 13:37:00 to 12-MAY-10 13:37:00 (CST-6)
Location: COULTER
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Newspaper

Lightning in BREMER, IOWA
From 12-MAY-10 20:05:00 to 12-MAY-10 20:05:00 (CST-6)
Location: WAVERLY
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Newspaper

Hail in MARSHALL, IOWA
From 12-MAY-10 13:15:00 to 12-MAY-10 13:15:00 (CST-6)
Location: STATE CENTER
Magnitude: 1.25 
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Marshall County EM relayed a report of quarter to walnut size hail covering the ground at State Center.

Hail in GUTHRIE, IOWA
From 12-MAY-10 13:31:00 to 12-MAY-10 13:31:00 (CST-6)
Location: BAGLEY
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Guthrie County EM reported nickel to quarter size hail on Highway 141 between Highway 24 and Bagley.

Hail in GREENE, IOWA
From 12-MAY-10 13:48:00 to 12-MAY-10 13:48:00 (CST-6)
Location: COOPER
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Pea to quarter size hail covering the ground.

Hail in WRIGHT, IOWA
From 12-MAY-10 13:35:00 to 12-MAY-10 13:35:00 (CST-6)
Location: DOWS
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in TAMA, IOWA
From 12-MAY-10 13:54:00 to 12-MAY-10 13:54:00 (CST-6)
Location: TRAER
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A trained spotter reported quarter size hail covering the road at the intersection of Highways 96 and 63 about 7 miles west of Traer.

Hail in BOONE, IOWA
From 12-MAY-10 14:09:00 to 12-MAY-10 14:09:00 (CST-6)
Location: BEAVER
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law enforcement reports hail up to quarter size at the intersection of Highway 30 and 169 north.

Hail in BOONE, IOWA
From 12-MAY-10 14:38:00 to 12-MAY-10 14:38:00 (CST-6)
Location: RIDGEPORT
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law Enforcement reports up to quarter size hail at the intersection of E18 and Nature Road.

Hail in ADAIR, IOWA
From 12-MAY-10 15:23:00 to 12-MAY-10 15:23:00 (CST-6)
Location: ARBOR HILL
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in POLK, IOWA
From 12-MAY-10 15:30:00 to 12-MAY-10 15:30:00 (CST-6)
Location: ALLEMAN
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Public

Quarter size hail reportedly covering the ground southeast of Alleman.

Hail in MARSHALL, IOWA
From 12-MAY-10 17:52:00 to 12-MAY-10 17:52:00 (CST-6)
Location: MARSHALLTOWN ARPT
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in MARSHALL, IOWA
From 12-MAY-10 17:55:00 to 12-MAY-10 17:55:00 (CST-6)
Location: MARSHALLTOWN ARPT
Magnitude: 0.88 
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hailstones just smaller than quarters.

Hail in GRUNDY, IOWA
From 12-MAY-10 18:18:00 to 12-MAY-10 18:18:00 (CST-6)
Location: ZANETA
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in MARION, IOWA
From 12-MAY-10 18:40:00 to 12-MAY-10 18:40:00 (CST-6)
Location: KNOXVILLE
Magnitude: 0.75 
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail was reported at the intersection of East Marion and South Attica Road.

Hail in BLACK HAWK, IOWA
From 12-MAY-10 19:22:00 to 12-MAY-10 19:22:00 (CST-6)
Location: WASHBURN
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Flash Flood in POLK, IOWA
From 12-MAY-10 19:00:00 to 12-MAY-10 22:00:00 (CST-6)
Location: ALTOONA
Flood cause: Heavy Rain
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Law Enforcement

NE 38th at 24th St. near 6th Ave SE intersection was washed out.

Flash Flood in POLK, IOWA
From 12-MAY-10 19:30:00 to 12-MAY-10 22:30:00 (CST-6)
Location: HASTIE
Flood cause: Heavy Rain
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Highway 65 ramp at Vandalia Road south of Pleasant Hill was impassable due to high water.

Hail in AUDUBON, IOWA
From 12-MAY-10 12:33:00 to 12-MAY-10 12:33:00 (CST-6)
Location: BRAYTON
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Hail in HAMILTON, IOWA
From 12-MAY-10 13:07:00 to 12-MAY-10 13:07:00 (CST-6)
Location: BLAIRSBURG
Magnitude: 0.88 
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Spotter reported dime to nickel size hail.

Hail in HAMILTON, IOWA
From 12-MAY-10 13:07:00 to 12-MAY-10 13:13:00 (CST-6)
Location: BLAIRSBURG
Magnitude: 0.88 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Public

The Blairsburg Post Office reported pea to nickel size hail lasting for 5 to 7 minutes, beginning at around 1307 CST.