A frontal boundary stretched across the northwest part of the state during the day on the 12th.  At the same time, a strong upper level low pressure was approaching from the southwest.  Afternoon temperatures warmed into the upper 70s to low 80s with dewpoint readings in the 60s.  The atmosphere became fairly unstable with around 2000 J/kg of CAPE available by mid afternoon.  Lifted indices fell to the -6 to -12 C range.  The shear was stronger than the previous day, but not all that impressive with 25 to 35 kts of effective shear.  Precipitable water values were also significantly lower than the previous day, around 0.9 inches.  With the approach of the upper low, freezing levels fell and were around 10,500 feet.  A significant amount of CAPE was available in the -10 to -30 C layer of the atmosphere.  Thunderstorms fired up along the cold frontal boundary and quickly produced hail.  The convection wasn't all that strong with most of the hail in the sub-severe category from west central into north central Iowa.  The largest hail was around an inch in diameter, mostly in west central and north central Iowa.

Hail in CRAWFORD, IOWA
From 12-MAY-11 19:49:00 to 12-MAY-11 19:49:00 (CST-6)
Location: DENISON MUNI ARPT
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Dime to penny size hail.

Hail in CALHOUN, IOWA
From 12-MAY-11 19:49:00 to 12-MAY-11 19:49:00 (CST-6)
Location: WIGHTMAN
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trained spotter reported quarter size hail west of Lake City on Highway 175.

Hail in GREENE, IOWA
From 12-MAY-11 20:18:00 to 12-MAY-11 20:18:00 (CST-6)
Location: ADAZA
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in HUMBOLDT, IOWA
From 12-MAY-11 20:31:00 to 12-MAY-11 20:31:00 (CST-6)
Location: DAKOTA CITY
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Pea to nickel size hail.

Hail in CARROLL, IOWA
From 12-MAY-11 19:26:00 to 12-MAY-11 19:26:00 (CST-6)
Location: RALSTON
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trained spotter reported prolonged pea size hail with a brief period of nickel size.

Hail in CALHOUN, IOWA
From 12-MAY-11 20:06:00 to 12-MAY-11 20:06:00 (CST-6)
Location: WIGHTMAN
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in GREENE, IOWA
From 12-MAY-11 20:33:00 to 12-MAY-11 20:33:00 (CST-6)
Location: CHURDAN
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Public reported brief period of nickel size hail with a prolonged period of smaller hail.

Hail in GREENE, IOWA
From 12-MAY-11 20:35:00 to 12-MAY-11 20:40:00 (CST-6)
Location: CHURDAN
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Public reported pea to nickel size hail fell for 5 minutes.

Hail in SAC, IOWA
From 12-MAY-11 18:55:00 to 12-MAY-11 18:55:00 (CST-6)
Location: EARLY
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Mostly dime and nickel sized hail with a few quarter sized stones.

Hail in CRAWFORD, IOWA
From 12-MAY-11 19:26:00 to 12-MAY-11 19:26:00 (CST-6)
Location: DOW CITY
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Mostly nickel size hail with a few quarter sized stones.

Hail in GREENE, IOWA
From 12-MAY-11 19:35:00 to 12-MAY-11 19:35:00 (CST-6)
Location: SCRANTON
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Public

Casey's reported quarter sized hail.

Hail in CARROLL, IOWA
From 12-MAY-11 19:25:00 to 12-MAY-11 19:25:00 (CST-6)
Location: RALSTON
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in FRANKLIN, IOWA
From 12-MAY-11 22:30:00 to 12-MAY-11 22:30:00 (CST-6)
Location: HAMPTON
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Quarter size hail was reported southeast of Hampton.

Hail in CERRO GORDO, IOWA
From 12-MAY-11 22:35:00 to 12-MAY-11 22:35:00 (CST-6)
Location: MASON CITY
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Pea to quarter size hail was reported in the town of Mason City.

Hail in WRIGHT, IOWA
From 12-MAY-11 21:09:00 to 12-MAY-11 21:09:00 (CST-6)
Location: EAGLE GROVE
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in WRIGHT, IOWA
From 12-MAY-11 21:28:00 to 12-MAY-11 21:28:00 (CST-6)
Location: CLARION
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: State Official

Quarter size hail reported in the town of Clarion.

Hail in SAC, IOWA
From 12-MAY-11 18:15:00 to 12-MAY-11 18:15:00 (CST-6)
Location: SCHALLER
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Public

Pea to quarter size hail covered the ground.

Hail in CRAWFORD, IOWA
From 12-MAY-11 19:27:00 to 12-MAY-11 19:27:00 (CST-6)
Location: DOW CITY
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Nickel to quarter size hail.