A weak frontal boundary was draped across northwest Iowa.  This boundary was nearly stationary and had moved little over the past few days.  The airmass had become quite unstable with surface dew points in the mid to upper 60s over the region.  CAPE rose to 3000 to 5000 J/kg across the state.  Downdraft CAPE was quite high, in the 900 to 1300 J/kg range, while CAPE in the -10 to -30 C layer of the atmosphere was between 500 and 1000 J/kg.  Deep moisture had moved into the central U.S. with precipitable water values above the 99th percentile ranging from 1.2 to 1.7 inches across the state from north to south.  The LCL level was quite low over southern Iowa, around 750 meters, and sloped to 1500 meters over the north.  The shear was quite weak with only 20 to 30 kts available by later afternoon.  The freezing level was high for early May and was between 13,000 and 14,000 feet.  Thunderstorms developed over northern Iowa with several severe cells developing in the northern two tiers of Iowa counties.  The storms produced numerous reports of hail to around an inch in diameter  Golf ball size hail fell in Kossuth County from one of the stronger storms.  The storm in Kossuth County also produced 3.2 inches of rain in about 2 hours.  The thunderstorms weakened across northern Iowa late in the day as new convection fired up over southern Iowa and northern Missouri.  Most of the severe action remained south of Iowa, though nickel to quarter size hail fell in Adams County, and hail up to one and one quarter inch fell later into the night in Taylor County.

Hail in CERRO GORDO, IOWA
From 03-MAY-12 16:12:00 to 03-MAY-12 16:12:00 (CST-6)
Location: FREEMAN
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Quarter size hail reported near the intersection of 330th and Olive St.

Hail in WINNEBAGO, IOWA
From 03-MAY-12 15:24:00 to 03-MAY-12 15:24:00 (CST-6)
Location: FOREST CITY
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Storm Chaser

Hail in HUMBOLDT, IOWA
From 03-MAY-12 18:57:00 to 03-MAY-12 18:57:00 (CST-6)
Location: THOR
Magnitude: 1.25 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Storm Chaser

Hail in ADAMS, IOWA
From 04-MAY-12 02:00:00 to 04-MAY-12 02:00:00 (CST-6)
Location: PRESCOTT
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Hail in HUMBOLDT, IOWA
From 03-MAY-12 17:23:00 to 03-MAY-12 17:35:00 (CST-6)
Location: ARNOLD
Magnitude: 1.25 
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail fell for 12 minutes and covered the ground.

Hail in CERRO GORDO, IOWA
From 03-MAY-12 16:15:00 to 03-MAY-12 16:15:00 (CST-6)
Location: FREEMAN
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Storm Chaser

Hail in HUMBOLDT, IOWA
From 03-MAY-12 18:25:00 to 03-MAY-12 18:30:00 (CST-6)
Location: ARNOLD
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Quarter size hail lasted about 5 minutes.

Heavy Rain in KOSSUTH, IOWA
From 03-MAY-12 13:00:00 to 03-MAY-12 16:05:00 (CST-6)
Location: BANCROFT
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Storm total of 3.20 inches since rain began around 1300 CST.

Hail in HUMBOLDT, IOWA
From 03-MAY-12 17:23:00 to 03-MAY-12 17:23:00 (CST-6)
Location: RUTLAND
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in HUMBOLDT, IOWA
From 03-MAY-12 17:32:00 to 03-MAY-12 17:32:00 (CST-6)
Location: ARNOLD
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in ADAMS, IOWA
From 04-MAY-12 02:30:00 to 04-MAY-12 02:30:00 (CST-6)
Location: STRINGTOWN
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in KOSSUTH, IOWA
From 03-MAY-12 13:30:00 to 03-MAY-12 13:30:00 (CST-6)
Location: BANCROFT
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in KOSSUTH, IOWA
From 03-MAY-12 14:02:00 to 03-MAY-12 14:02:00 (CST-6)
Location: BANCROFT
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in KOSSUTH, IOWA
From 03-MAY-12 13:50:00 to 03-MAY-12 13:50:00 (CST-6)
Location: BANCROFT
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in KOSSUTH, IOWA
From 03-MAY-12 15:14:00 to 03-MAY-12 15:14:00 (CST-6)
Location: SWEA CITY
Magnitude: 1.75 
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in KOSSUTH, IOWA
From 03-MAY-12 15:13:00 to 03-MAY-12 15:13:00 (CST-6)
Location: SWEA CITY
Magnitude: 1.75 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Public

Torrential rainfall also reported.

Hail in CERRO GORDO, IOWA
From 03-MAY-12 15:56:00 to 03-MAY-12 15:56:00 (CST-6)
Location: (MCW)MASON CITY MUNI
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in CERRO GORDO, IOWA
From 03-MAY-12 16:01:00 to 03-MAY-12 16:01:00 (CST-6)
Location: (MCW)MASON CITY MUNI
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in WINNEBAGO, IOWA
From 03-MAY-12 15:23:00 to 03-MAY-12 15:23:00 (CST-6)
Location: FOREST CITY
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Hail in ADAMS, IOWA
From 04-MAY-12 01:34:00 to 04-MAY-12 01:34:00 (CST-6)
Location: PRESCOTT
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in TAYLOR, IOWA
From 04-MAY-12 02:38:00 to 04-MAY-12 02:38:00 (CST-6)
Location: LENOX
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in WINNEBAGO, IOWA
From 03-MAY-12 15:13:00 to 03-MAY-12 15:20:00 (CST-6)
Location: FOREST CITY
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Mainly pea size hail with bigger hail up to nickel size mixed in.  Lasted about 7 minutes and started to cover the ground.

Hail in ADAMS, IOWA
From 04-MAY-12 02:03:00 to 04-MAY-12 02:03:00 (CST-6)
Location: MERCER
Magnitude: 1.25 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Public