A strong surge of moisture and warm air took place during the overnight hours of the 28th into the morning of the 29th.  The low level jet was around 50 kts during the predawn hours.  Precipitable water values increased to 1.5 to 1.7 inches during the early morning hours.  The airmass became very unstable with CAPE values increasing to near 4000 J/kg just south of Iowa, with lifted indices around -8 C.  The atmosphere had a fair amount of shear with 50 to 55 kts available.  Downdraft CAPE was in the 500 to 800 J/kg range, with 300 to 600 J/kg of CAPE in the -10 to -30 C layer of the atmosphere.  During the night, the freezing level increased from under 13,000 feet at sunset, to between 13,500 and 14,000 feet by sunrise.  Thunderstorms initially formed in two areas.  One was in a west to east band, ahead of the warm front, south of Iowa.  The second was a small MCS that tracked along a mid level boundary in Nebraska.  As the night progressed, the storms in Missouri weakened as a new area of strong storms formed as the MCS began to interact with the low level jet.  Thunderstorms fired rapidly over southeast Nebraska and southwest Iowa just before sunrise.  The storms became severe as they moved across the southern third of Iowa. Initially, the storms produced hail with nickel to ping pong ball size hail reported at several places.  As the storms progressed, they formed into a more linear structure and the line began to accelerate.  As a result, the storms turned into more of a wind hazard.  The high winds blew a semi-tractor trailer off the highway near Knoxville.  A 69 MPH wind gust was recorded at the Oskaloosa Airport.  In addition, there were numerous reports of trees and power lines being blown down by the strong winds.

Hail in CLARKE, IOWA
From 29-MAY-11 08:04:00 to 29-MAY-11 08:04:00 (CST-6)
Location: MURRAY
Magnitude: 1.50 
Property damage: 10.00K
Crop damage: 10.00K
Reported by: Trained Spotter

Thunderstorm Wind in MARION, IOWA
From 29-MAY-11 06:55:00 to 29-MAY-11 06:55:00 (CST-6)
Location: KNOXVILLE
Magnitude: 57.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Semi blown off of Highway 14 about 1 mile north of Knoxville.

Thunderstorm Wind in WAPELLO, IOWA
From 29-MAY-11 07:50:00 to 29-MAY-11 07:50:00 (CST-6)
Location: OTTUMWA
Magnitude: 50.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Large tree over the road near the park on the east side of the city.

Thunderstorm Wind in MARION, IOWA
From 29-MAY-11 07:03:00 to 29-MAY-11 07:03:00 (CST-6)
Location: PELLA
Magnitude: 57.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Numerous trees and branches down in town.  Construction signs blown down.

Hail in POLK, IOWA
From 29-MAY-11 05:36:00 to 29-MAY-11 05:36:00 (CST-6)
Location: DES MOINES EKMAN ARP
Magnitude: 1.00 
Property damage: 1.00K
Crop damage: 5.00K
Reported by: Public

Thunderstorm Wind in MAHASKA, IOWA
From 29-MAY-11 07:55:00 to 29-MAY-11 07:55:00 (CST-6)
Location: OSKALOOSA MUNI ARPT
Magnitude: 52.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Spotter noted tree branches down on the way to Oskaloosa.

Hail in WAPELLO, IOWA
From 29-MAY-11 07:58:00 to 29-MAY-11 07:58:00 (CST-6)
Location: FARSON
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 10.00K
Reported by: Trained Spotter

Hail was accompanied by 50 MPH winds.

Thunderstorm Wind in WAPELLO, IOWA
From 29-MAY-11 07:54:00 to 29-MAY-11 07:54:00 (CST-6)
Location: OTTUMWA
Magnitude: 52.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A trained spotter reported a 60 MPH wind gust.

Hail in MADISON, IOWA
From 29-MAY-11 05:21:00 to 29-MAY-11 05:21:00 (CST-6)
Location: MACKSBURG
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 10.00K
Reported by: Public

Lots of penny to nickel size hail.

Thunderstorm Wind in MAHASKA, IOWA
From 29-MAY-11 07:55:00 to 29-MAY-11 07:55:00 (CST-6)
Location: OSKALOOSA MUNI ARPT
Magnitude: 60.00 MG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: AWOS

Hail in CLARKE, IOWA
From 29-MAY-11 06:16:00 to 29-MAY-11 06:16:00 (CST-6)
Location: MURRAY
Magnitude: 1.00 
Property damage: 1.00K
Crop damage: 5.00K
Reported by: Fire Department/Rescue

Mainly nickel size hail with a few quarters mixed in.

Thunderstorm Wind in MARION, IOWA
From 29-MAY-11 08:11:00 to 29-MAY-11 08:11:00 (CST-6)
Location: PELLA
Magnitude: 57.00 MG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Mesonet

Thunderstorm Wind in TAMA, IOWA
From 29-MAY-11 06:48:00 to 29-MAY-11 06:48:00 (CST-6)
Location: TAMA
Magnitude: 52.00 MG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Mesonet

Thunderstorm Wind in MARION, IOWA
From 29-MAY-11 06:55:00 to 29-MAY-11 06:55:00 (CST-6)
Location: KNOXVILLE
Magnitude: 57.00 EG
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Numerous trees reported down in Knoxville.

Hail in WAPELLO, IOWA
From 29-MAY-11 07:35:00 to 29-MAY-11 07:35:00 (CST-6)
Location: BLAKESBURG
Magnitude: 1.00 
Property damage: 1.00K
Crop damage: 10.00K
Reported by: Trained Spotter

Hail in MADISON, IOWA
From 29-MAY-11 05:22:00 to 29-MAY-11 05:22:00 (CST-6)
Location: MACKSBURG
Magnitude: 1.00 
Property damage: 1.00K
Crop damage: 5.00K
Reported by: Public

Quarter size hail reported.

Hail in CLARKE, IOWA
From 29-MAY-11 06:29:00 to 29-MAY-11 06:29:00 (CST-6)
Location: MURRAY
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 5.00K
Reported by: Fire Department/Rescue

Second round of hail with nickel size reported.

Hail in POWESHIEK, IOWA
From 29-MAY-11 06:50:00 to 29-MAY-11 06:50:00 (CST-6)
Location: MONTEZUMA
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 10.00K
Reported by: Trained Spotter

Quarter size hail, hail was in a disk-like shape.

Hail in UNION, IOWA
From 29-MAY-11 07:45:00 to 29-MAY-11 07:45:00 (CST-6)
Location: ARISPE
Magnitude: 1.00 
Property damage: 1.00K
Crop damage: 5.00K
Reported by: Trained Spotter