A strong cold front entered northwest Iowa during the afternoon of the 20th and advanced southeast into the state.  There was strong upper level support for the cold front as a negatively tilted trough of low pressure lifted northeast across the area.  The atmosphere became moderately unstable as highs ahead of the front were around 90 with dewpoints in the upper 60s to around 70.  CAPE increased to around 2000 J/kg with a lifted index of -6 C.  CAPE in the -10 to -30 C layer of the atmosphere was in the 500 to 700 J/kg range, however it was not fully realized as the freezing level was between 15,500 and 16,000 feet.  The LCL was around 750 meters near the developing squall line and 1250 meters just ahead of it.  The shear increased during the day, but was only in the 30 to 40 kt range near the squall line.  Precipitable water values were quite high, in the 1.7 to 2 inch range.  Initially two narrow lines of thunderstorms developed.  As the evening progressed it formed into more of a broad line of thunderstorms.  The thunderstorms produced brief heavy rainfall and spotty high winds.  The winds did not become widespread, in part due to the fact that the storms were not all discrete.  Wind gusts of around 60 MPH were reported in parts of central and northeast Iowa.  This resulted in tree damage in several locations.  One of the stronger storms blew a barn down in Butler County with winds estimated around 75 MPH.  One of the storms in central Iowa produced a 67 MPH wind gust south of Dawson in Dallas County.  Isolated damage was also reported in Marshall County south of State Center.  A metal roof was blown off of a large building and a small shed was blown 4 feet off of its foundation.

Thunderstorm Wind in BREMER, IOWA
From 20-JUN-12 19:35:00 to 20-JUN-12 19:35:00 (CST-6)
Location: WAVERLY MUNI ARPT
Magnitude: 52.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Trees were blown down.

Thunderstorm Wind in DALLAS, IOWA
From 20-JUN-12 19:03:00 to 20-JUN-12 19:03:00 (CST-6)
Location: DEXTER
Magnitude: 52.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Public

Small branches of around 1.5 inches in diameter were blown down.

Thunderstorm Wind in DALLAS, IOWA
From 20-JUN-12 19:18:00 to 20-JUN-12 19:18:00 (CST-6)
Location: KENNEDY
Magnitude: 52.00 EG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Public

Small limbs were blown down.

Thunderstorm Wind in DALLAS, IOWA
From 20-JUN-12 19:35:00 to 20-JUN-12 19:35:00 (CST-6)
Location: WAUKEE
Magnitude: 52.00 EG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Winds estimated by broadcast media of 60 MPH with minor damage.

Thunderstorm Wind in MARSHALL, IOWA
From 20-JUN-12 19:57:00 to 20-JUN-12 19:57:00 (CST-6)
Location: STATE CENTER
Magnitude: 61.00 EG
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Part of a metal roof was blown off part of a car wash and a portion of a gas station canopy was blown off. A storage shed was pushed four feet off the foundation by high winds.

Thunderstorm Wind in BUTLER, IOWA
From 20-JUN-12 19:21:00 to 20-JUN-12 19:21:00 (CST-6)
Location: ALLISON MUNI ARPT
Magnitude: 65.00 EG
Property damage: 75.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A barn was blown down by strong winds.  Branches of up to a foot in diameter were blown down as well.

Thunderstorm Wind in BUTLER, IOWA
From 20-JUN-12 19:27:00 to 20-JUN-12 19:27:00 (CST-6)
Location: SHELL ROCK
Magnitude: 52.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Broadcast Media

A tree was downed in a county park near Shell Rock.  Reported by KWWL-TV.

Thunderstorm Wind in MADISON, IOWA
From 20-JUN-12 19:30:00 to 20-JUN-12 19:30:00 (CST-6)
Location: EARLHAM
Magnitude: 52.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

One to two foot diameter branches blown down.  A few whole trees as well.