Strong heating took place during the afternoon of the 4th.  Temperatures warmed into the 90s across the state with dewpoint readings in the mid 60s to low 70s.  CAPE increased to 3000 to 4000 J/kg by evening with the lifted index falling to -8 to -12 C.  Downdraft CAPE was in the 1500 to 1800 J/kg range.  The shear was decent with 30 to 50 kts of effective shear available.  CAPE in the -10 to -30 C layer of the atmosphere was 800 to 1100 J/kg.  The freezing level was quite high at 14,500 feet with precipitable water of 1.2 to 1.6 inches.  The LCL was very high, in the 1500 to 2000 meter range.  Strong dynamics were in place with a 90 kt jet to the north of the area.  A cold front moved toward the state as low pressure was located over northeast Nebraska with a weak frontal boundary extending east from the low.  Thunderstorms formed over eastern South Dakota into extreme northeast Nebraska  and advanced rapidly east across northern Iowa.  As the event progressed, part of the line took on a bow echo appearance as it pushed across northern Iowa.  South of the line a strong outflow boundary pushed south through central Iowa.  The bow echo produced high winds of 60 to 75 MPH as it plowed across the north.  Numerous reports of tree and power line damage were received.  Winds in the Mason City area topped out around 70 MPH, with winds farther south in Hamilton County approached 75 MPH.  There were few reports of hail with the storms.  Penny size hail was reported in the metro Des Moines area of Polk County.  This was the largest hail report received.

Thunderstorm Wind in PALO ALTO, IOWA
From 04-SEP-12 21:57:00 to 04-SEP-12 21:57:00 (CST-6)
Location: WEST BEND
Magnitude: 52.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Large trees were snapped.

Hail in POLK, IOWA
From 05-SEP-12 01:12:00 to 05-SEP-12 01:12:00 (CST-6)
Location: LOVINGTON
Magnitude: 0.75 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A 55 MPH wind gust was also reported.

Thunderstorm Wind in CERRO GORDO, IOWA
From 04-SEP-12 23:08:00 to 04-SEP-12 23:08:00 (CST-6)
Location: (MCW)MASON CITY MUNI
Magnitude: 61.00 MG
Property damage: 10.00K
Crop damage: 25.00K
Reported by: ASOS

Thunderstorm Wind in HAMILTON, IOWA
From 05-SEP-12 00:50:00 to 05-SEP-12 00:50:00 (CST-6)
Location: WILLIAMS
Magnitude: 64.00 MG
Property damage: 10.00K
Crop damage: 25.00K
Reported by: Mesonet

Thunderstorm Wind in POCAHONTAS, IOWA
From 04-SEP-12 22:02:00 to 04-SEP-12 22:02:00 (CST-6)
Location: ROLFE
Magnitude: 52.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Law Enforcement

A Large tree fell and toppled power lines in the town of Rolfe.

Thunderstorm Wind in HANCOCK, IOWA
From 04-SEP-12 22:45:00 to 04-SEP-12 22:45:00 (CST-6)
Location: MILLER
Magnitude: 51.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: AWOS

Thunderstorm Wind in CERRO GORDO, IOWA
From 04-SEP-12 23:01:00 to 04-SEP-12 23:01:00 (CST-6)
Location: (MCW)MASON CITY MUNI
Magnitude: 51.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: ASOS

Thunderstorm Wind in CERRO GORDO, IOWA
From 04-SEP-12 23:04:00 to 04-SEP-12 23:04:00 (CST-6)
Location: MASON CITY
Magnitude: 57.00 EG
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Large trees toppled power lines along State Street in downtown Mason City.

Thunderstorm Wind in KOSSUTH, IOWA
From 04-SEP-12 22:00:00 to 04-SEP-12 22:00:00 (CST-6)
Location: SWEA CITY
Magnitude: 52.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

An 8 inch diameter branch was down across the driveway.

Thunderstorm Wind in PALO ALTO, IOWA
From 04-SEP-12 22:01:00 to 04-SEP-12 22:01:00 (CST-6)
Location: WEST BEND
Magnitude: 52.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Large trees were snapped.

Thunderstorm Wind in CERRO GORDO, IOWA
From 04-SEP-12 23:05:00 to 04-SEP-12 23:05:00 (CST-6)
Location: MASON CITY
Magnitude: 57.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Several 6 inch diameter limbs were down.

Thunderstorm Wind in HUMBOLDT, IOWA
From 04-SEP-12 22:20:00 to 04-SEP-12 22:20:00 (CST-6)
Location: LIVERMORE
Magnitude: 52.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Strong winds broke several tree branches in Livermore.

Thunderstorm Wind in HANCOCK, IOWA
From 04-SEP-12 22:48:00 to 04-SEP-12 22:48:00 (CST-6)
Location: MILLER
Magnitude: 50.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Several branches downed.

Thunderstorm Wind in WEBSTER, IOWA
From 04-SEP-12 22:31:00 to 04-SEP-12 22:31:00 (CST-6)
Location: (FOD)FT DODGE MUNI A
Magnitude: 52.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Numerous small to medium branches down.  High winds accompanied by pea size hail.

Thunderstorm Wind in BLACK HAWK, IOWA
From 05-SEP-12 00:33:00 to 05-SEP-12 00:33:00 (CST-6)
Location: CASTLE HILL
Magnitude: 52.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A large linden tree was split in half due to strong winds.

Thunderstorm Wind in WEBSTER, IOWA
From 04-SEP-12 23:42:00 to 04-SEP-12 23:42:00 (CST-6)
Location: FT DODGE
Magnitude: 52.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Several six inch diameter tree limbs were downed in downtown Fort Dodge.

Thunderstorm Wind in HANCOCK, IOWA
From 04-SEP-12 22:37:00 to 04-SEP-12 22:37:00 (CST-6)
Location: MILLER
Magnitude: 52.00 EG
Property damage: 40.00K
Crop damage: 0.00K
Reported by: Public

A large metal shed which was bolted to a concrete pad was twisted and ripped from the foundation.