A weak cold front passed through Iowa on the 28th and became stationary just south of the state.  Warm advection over the frontal boundary produced scattered thunderstorms through the night of the 28th into the day on the 29th.  Nickel size hail fell from one of the storms in Marshall County during the morning of the the 29th.  During the day the atmosphere became quite unstable with CAPE increasing to 4000-5000 J/kg and the lifted index falling to -11 C.  High temperatures reached into the 90s with dewpoint readings in the low 70s across southern Iowa.  Considerable downdraft CAPE was available, in the 1000 to 1500 J/kg range.  The soundings were relatively dry in the low levels with a LCL around 1500 meters.  Precipitable water was around 1.6 inches.  There was 700 to 1000 J/kg of CAPE available in the -10 to 30 C layer of the atmosphere, though with the freezing level just over 15,000 feet.  That limited the size of the hail.  The atmosphere became moderately sheared as a weak shortwave moved east along the front during the afternoon.  There was 45 to 55 kts of shear available.  A cluster of storms formed in western Iowa and advanced east.  They produced gusty winds and some hail.  One inch diameter hail fell in Dallas County at De Soto.  As the thunderstorms moved east, they intensified.  With the deeper storm depth, the strongest cluster of storms was able to realize the high CAPE values for both hail and downdraft CAPE.  Wind gusts around 60 MPH and hail of 1 to 2.5 inches in diameter occurred in Marion, Jasper, and Poweshiek Counties.  The winds blew down large branches in the area.  A 72 MPH wind gust in Poweshiek County south of Grinnell caused a semi-tractortrailer truck to jackknife.

Thunderstorm Wind in POLK, IOWA
From 29-JUN-12 14:34:00 to 29-JUN-12 14:34:00 (CST-6)
Location: DES MOINES
Magnitude: 50.00 EG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Five inch diameter tree limb down at the intersection of 5th Ave and Watson Powell.

Thunderstorm Wind in POWESHIEK, IOWA
From 29-JUN-12 15:55:00 to 29-JUN-12 15:55:00 (CST-6)
Location: BROOKLYN
Magnitude: 61.00 MG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Mesonet

Measured at the Brooklyn Schoolnet site.

Thunderstorm Wind in POWESHIEK, IOWA
From 29-JUN-12 15:47:00 to 29-JUN-12 15:47:00 (CST-6)
Location: GRINNELL
Magnitude: 52.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Three to four tree limbs three to five inches in diameter down.  One branch was 20 feet long.

Thunderstorm Wind in TAMA, IOWA
From 29-JUN-12 16:00:00 to 29-JUN-12 16:05:00 (CST-6)
Location: CHELSEA
Magnitude: 52.00 MG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Wind continued to gust between 50 and 60 MPH at 1605 CST.

Thunderstorm Wind in MARION, IOWA
From 29-JUN-12 15:09:00 to 29-JUN-12 15:09:00 (CST-6)
Location: RED ROCK
Magnitude: 50.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Measured on the Lake Red Rock Bridge.

Thunderstorm Wind in POWESHIEK, IOWA
From 29-JUN-12 15:45:00 to 29-JUN-12 15:45:00 (CST-6)
Location: BROOKLYN
Magnitude: 61.00 EG
Property damage: 75.00K
Crop damage: 0.00K
Reported by: COOP Observer

Many trees and tree limbs down across the city.  Some trees on roofs.

Thunderstorm Wind in MARION, IOWA
From 29-JUN-12 15:19:00 to 29-JUN-12 15:19:00 (CST-6)
Location: PELLA
Magnitude: 51.00 MG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Mesonet

Measured gust at the Pella Schoolnet site.

Thunderstorm Wind in MARION, IOWA
From 29-JUN-12 15:13:00 to 29-JUN-12 15:13:00 (CST-6)
Location: RED ROCK
Magnitude: 53.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Very strong winds knocked down several large tree limbs just north of Lake Red Rock.  Spotter estimated winds in excess of 60 MPH.

Hail in MARSHALL, IOWA
From 29-JUN-12 07:09:00 to 29-JUN-12 07:09:00 (CST-6)
Location: MARSHALLTOWN
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 5.00K
Reported by: Trained Spotter

Dime to nickel size hail was reported.

Hail in DALLAS, IOWA
From 29-JUN-12 14:06:00 to 29-JUN-12 14:06:00 (CST-6)
Location: DE SOTO
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 5.00K
Reported by: Trained Spotter

Estimated quarter size hail along with winds estimated around 50 MPH at the De Soto exit on I-80.

Hail in JASPER, IOWA
From 29-JUN-12 15:23:00 to 29-JUN-12 15:23:00 (CST-6)
Location: NEWTON
Magnitude: 2.50 
Property damage: 100.00K
Crop damage: 10.00K
Reported by: Law Enforcement

Hail in JASPER, IOWA
From 29-JUN-12 15:25:00 to 29-JUN-12 15:25:00 (CST-6)
Location: KELLOGG
Magnitude: 1.25 
Property damage: 10.00K
Crop damage: 5.00K
Reported by: Trained Spotter

Thunderstorm Wind in POWESHIEK, IOWA
From 29-JUN-12 15:39:00 to 29-JUN-12 15:39:00 (CST-6)
Location: GRINNELL
Magnitude: 52.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A trained spotter estimated winds of 60 MPH.

Thunderstorm Wind in JASPER, IOWA
From 29-JUN-12 15:08:00 to 29-JUN-12 15:08:00 (CST-6)
Location: METZ
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Storm Chaser

A storm chaser estimated winds of 60 MPH 3 miles east southeast of Metz.

Thunderstorm Wind in POWESHIEK, IOWA
From 29-JUN-12 15:35:00 to 29-JUN-12 15:35:00 (CST-6)
Location: GRINNELL ARPT
Magnitude: 63.00 MG
Property damage: 35.00K
Crop damage: 0.00K
Reported by: Mesonet

Measured wind by an Iowa DOT RWIS site on Interstate 80.  A semi-tractortrailer truck was reportedly jackknifed in the area.

Thunderstorm Wind in POWESHIEK, IOWA
From 29-JUN-12 15:45:00 to 29-JUN-12 15:45:00 (CST-6)
Location: BROOKLYN
Magnitude: 50.00 MG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Mesonet

Measured at the Brooklyn Schoolnet site.

Thunderstorm Wind in POWESHIEK, IOWA
From 29-JUN-12 15:33:00 to 29-JUN-12 15:33:00 (CST-6)
Location: GRINNELL ARPT
Magnitude: 57.00 EG
Property damage: 25.00K
Crop damage: 10.00K
Reported by: Trained Spotter

Several large road signs and large tree limbs down near Interstate 80, south of Grinnell.  Quarter size hail was also reported.

Hail in POWESHIEK, IOWA
From 29-JUN-12 15:33:00 to 29-JUN-12 15:33:00 (CST-6)
Location: GRINNELL ARPT
Magnitude: 1.25 
Property damage: 2.00K
Crop damage: 10.00K
Reported by: Trained Spotter

Thunderstorm Wind in POWESHIEK, IOWA
From 29-JUN-12 15:30:00 to 29-JUN-12 15:30:00 (CST-6)
Location: SEARSBORO
Magnitude: 61.00 EG
Property damage: 50.00K
Crop damage: 0.00K
Reported by: Public

Barn destroyed by high winds.

Thunderstorm Wind in POWESHIEK, IOWA
From 29-JUN-12 15:40:00 to 29-JUN-12 15:40:00 (CST-6)
Location: MONTEZUMA
Magnitude: 57.00 MG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: COOP Observer

Peak wind gust was measured by a weather station at the water treatment plant.