An upper level low was lifting northeast out of the southwest U.S. during the day on the 21st and was over the central High Plains by the evening hours.  The dry slot had punched northeast across Iowa during the day on the 21st, pushing much of the deeper moisture northeast of the area by afternoon.  Skies cleared, allowing the atmosphere to destabilize by evening.  CAPE values rose to around 2000 J/kg by late evening.  CAPE in the -10 to -30 C layer of the atmosphere was around 900 J/kg, with downdraft CAPE around 1100 J/kg.  The lifted index fell to around -9 C shortly before inception.  The freezing level was relatively low, around 10,500 feet.  The only missing ingredient preventing the system from becoming a widespread event was the shear was limited, generally under 30 kts.  Thunderstorms formed to the west and southwest of Iowa during the late afternoon hours.  The storms moved northeast and broke into two areas.  One hit the northwest part of the CWA by late afternoon.  The mode of severe weather was in the form of hail with numerous reports of one to two inch diameter hail.  Baseball size hail fell in Emmet County, east of Wallingford.  The second piece of the broken line moved into southwest Iowa during the evening, producing large hail and strong winds.  Numerous wind gusts of 60 to 70 MPH were reported.  An outbuilding was damaged and a window blown out of a house east of Bondurant in Polk County.  The hail was not as large with the southern area, generally under an inch in diameter.

Hail in GUTHRIE, IOWA
From 21-MAY-11 19:36:00 to 21-MAY-11 19:36:00 (CST-6)
Location: GUTHRIE CENTER
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Thunderstorm Wind in CRAWFORD, IOWA
From 21-MAY-11 20:05:00 to 21-MAY-11 20:05:00 (CST-6)
Location: SCHLESWIG
Magnitude: 61.00 EG
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Power poles blown down.

Thunderstorm Wind in WAYNE, IOWA
From 21-MAY-11 20:48:00 to 21-MAY-11 20:48:00 (CST-6)
Location: BETHLEHEM
Magnitude: 52.00 EG
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Pea size hail fell as well.

Hail in HUMBOLDT, IOWA
From 21-MAY-11 21:31:00 to 21-MAY-11 21:31:00 (CST-6)
Location: RUTLAND
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Thunderstorm Wind in BOONE, IOWA
From 21-MAY-11 20:41:00 to 21-MAY-11 20:41:00 (CST-6)
Location: ERICSON
Magnitude: 50.00 MG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Mesonet

United Community School report received from a KCCI Schoolnet site.

Hail in EMMET, IOWA
From 21-MAY-11 19:05:00 to 21-MAY-11 19:05:00 (CST-6)
Location: HUNTINGTON
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A non-rotating wall cloud moved overhead at 1910 CST.

Thunderstorm Wind in POLK, IOWA
From 21-MAY-11 20:41:00 to 21-MAY-11 20:41:00 (CST-6)
Location: SANTIAGO
Magnitude: 57.00 EG
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Public reported damage to an outbuilding and windows blown out.

Thunderstorm Wind in DECATUR, IOWA
From 21-MAY-11 19:53:00 to 21-MAY-11 19:53:00 (CST-6)
Location: (30I)LAMONI MUNI ARP
Magnitude: 52.00 MG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: ASOS

Hail in EMMET, IOWA
From 21-MAY-11 17:03:00 to 21-MAY-11 17:03:00 (CST-6)
Location: WELLINGFORD
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 5.00K
Reported by: CoCoRaHS

Quarter size hail covering the ground.

Thunderstorm Wind in MAHASKA, IOWA
From 21-MAY-11 21:15:00 to 21-MAY-11 21:15:00 (CST-6)
Location: OSKALOOSA
Magnitude: 52.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Estimated 60 MPH winds with minor tree damage and the skirting blown off the base of a mobile home.

Thunderstorm Wind in MAHASKA, IOWA
From 21-MAY-11 21:19:00 to 21-MAY-11 21:19:00 (CST-6)
Location: OSKALOOSA
Magnitude: 52.00 EG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Estimated wind gust.

Hail in PALO ALTO, IOWA
From 21-MAY-11 16:43:00 to 21-MAY-11 16:43:00 (CST-6)
Location: OSGOOD
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Pea to nickel size hail.

Hail in EMMET, IOWA
From 21-MAY-11 18:38:00 to 21-MAY-11 18:38:00 (CST-6)
Location: WELLINGFORD
Magnitude: 1.75 
Property damage: 10.00K
Crop damage: 3.00K
Reported by: Law Enforcement

Hail in EMMET, IOWA
From 21-MAY-11 18:42:00 to 21-MAY-11 18:42:00 (CST-6)
Location: WELLINGFORD
Magnitude: 2.75 
Property damage: 5.00K
Crop damage: 3.00K
Reported by: Law Enforcement

Hail in EMMET, IOWA
From 21-MAY-11 18:52:00 to 21-MAY-11 18:52:00 (CST-6)
Location: ESTHERVILLE ARPT
Magnitude: 1.50 
Property damage: 3.00K
Crop damage: 3.00K
Reported by: COOP Observer