A cold front stretched from western Minnesota, across southeast South Dakota, into Nebraska during the evening of the 5th.  The front moved slowly southeast into Iowa during the overnight hours.  The airmass was very unstable in advance of the front with lifted indices of -3 to -6 and elevated CAPE of 2000 to 3000 J/kg.  A strong low level jet of 50 kts developed by the late evening hours with a broad area of deep moisture convergence producing precipitable water values of around 1.5 inches.  Downdraft CAPE was high, in the 600 to 1300 J/kg range.  Although the freezing level was around 14,500 feet, plenty of CAPE was available in the -10 to -30 C layer of the atmosphere with 500 to 700 J/kg present.  A strong line of thunderstorms formed along the cold frontal boundary.  The atmosphere was highly sheared as the storms progressed out of Nebraska and South Dakota with 45 to 55 kts of effective shear.  The LCL level just in advance of the line was between 750 and 1000 meters.  A line of thunderstorms moved across the state during the night.  There were two main clusters of storms.  A MCC moved across southern Minnesota, while a MCS pushed into Iowa during the late night hours.  One portion of the Iowa segment of the line became severe as the line interacted with a southward progressing outflow boundary from the Minnesota MCC.  Winds gusted to 69 MPH in Atlantic with the passage of the storms.  High winds progressed east-northeast with 65 MPH winds in Adair County west of Greenfield, and 70 MPH winds in Warren County south of the Des Moines International Airport.  The strongest measured wind gusts were in the central Iowa area.  A wind gust of 77 MPH was recorded east of Greenfield in Adair County.  In Dallas County, a wind gust of 71 MPH recorded east of Dallas Center.  There were several reports of one quarter to one half inch diameter hail, however the largest hail was about nickel size.  The storms produced very heavy rainfall with amounts of one half to one and one half inches being reported in less than an hour.  Local street flooding occurred, but no major flooding was reported.  The heaviest rainfall occurred in Audubon County.  Rainfall of 2.5 to 4.5 inches occurred with flash flooding reported west of Audubon.  The bridge on County Road N14 was under water.  About one half mile of the highway was under water.  Most of the damage was to roads that required regrading.  Logs were left on a bridge from the overflow of Indian Creek.

Flash Flood in AUDUBON, IOWA
From 06-MAY-12 07:00:00 to 06-MAY-12 09:00:00 (CST-6)
Location: ROSS
Flood cause: Heavy Rain
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Water covering the County Highway N14 bridge that crosses Indian Creek.  About one half mile of the road was under water. The worst area of flooding was 5 miles either side of a line from Kimballton to Audubon. A log was left on a bridge over the creek after the water had receded. Multiple locations required gravel grading and debris removal off of county roads.

Thunderstorm Wind in WARREN, IOWA
From 06-MAY-12 07:19:00 to 06-MAY-12 07:19:00 (CST-6)
Location: NORWALK
Magnitude: 61.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Wind gusts of up to 70 MPH blew shingles off of a roof.

Thunderstorm Wind in STORY, IOWA
From 06-MAY-12 08:00:00 to 06-MAY-12 08:00:00 (CST-6)
Location: NEVADA
Magnitude: 61.00 EG
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Amateur Radio

Two power line poles snapped off at the base.

Thunderstorm Wind in CASS, IOWA
From 06-MAY-12 05:31:00 to 06-MAY-12 05:31:00 (CST-6)
Location: ATLANTIC
Magnitude: 60.00 MG
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A few shingles were blown off of a house.  Some tree branches were blown down.

Thunderstorm Wind in ADAIR, IOWA
From 06-MAY-12 06:05:00 to 06-MAY-12 06:05:00 (CST-6)
Location: FONTANELLE
Magnitude: 56.00 MG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Mesonet

Wind gust was measured by a KCCI Schoolnet site at the Nodaway Valley School.

Thunderstorm Wind in CASS, IOWA
From 06-MAY-12 05:27:00 to 06-MAY-12 05:27:00 (CST-6)
Location: ATLANTIC ARPT
Magnitude: 52.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

A squad car was being rocked by winds.  Dime size hail was also reported.

Hail in AUDUBON, IOWA
From 06-MAY-12 05:10:00 to 06-MAY-12 05:10:00 (CST-6)
Location: SHARON
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail in POLK, IOWA
From 06-MAY-12 07:35:00 to 06-MAY-12 07:35:00 (CST-6)
Location: ALTOONA
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Amateur Radio

Hail in MARION, IOWA
From 06-MAY-12 08:15:00 to 06-MAY-12 08:15:00 (CST-6)
Location: LAKE RED ROCK
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Hail in MARION, IOWA
From 06-MAY-12 08:18:00 to 06-MAY-12 08:18:00 (CST-6)
Location: FIFIELD
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: County Official

Hail in AUDUBON, IOWA
From 06-MAY-12 05:13:00 to 06-MAY-12 05:13:00 (CST-6)
Location: SHARON
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Thunderstorm Wind in DALLAS, IOWA
From 06-MAY-12 07:10:00 to 06-MAY-12 07:15:00 (CST-6)
Location: DALLAS CENTER
Magnitude: 62.00 MG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: CoCoRaHS

Hail in AUDUBON, IOWA
From 06-MAY-12 05:20:00 to 06-MAY-12 05:20:00 (CST-6)
Location: AUDUBON
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Public

Heavy Rain in AUDUBON, IOWA
From 06-MAY-12 07:00:00 to 06-MAY-12 07:00:00 (CST-6)
Location: AUDUBON
Property damage: 0.00K
Crop damage: 0.00K
Reported by: COOP Observer

Total rainfall of 2.67 inches in the past 24 hours.

Thunderstorm Wind in CASS, IOWA
From 06-MAY-12 05:30:00 to 06-MAY-12 05:30:00 (CST-6)
Location: ATLANTIC
Magnitude: 57.00 EG
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Some shingles were blown off roofs.  About twelve to 14 one foot diameter trees were snapped.

Thunderstorm Wind in ADAIR, IOWA
From 06-MAY-12 06:23:00 to 06-MAY-12 06:23:00 (CST-6)
Location: ARBOR HILL
Magnitude: 67.00 MG
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Also measured a 73 MPH wind gust at 0626 CST.

Thunderstorm Wind in DALLAS, IOWA
From 06-MAY-12 06:40:00 to 06-MAY-12 06:40:00 (CST-6)
Location: LINDEN
Magnitude: 61.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Five to six 8 inch diameter tree limbs down across town.

Thunderstorm Wind in DALLAS, IOWA
From 06-MAY-12 06:55:00 to 06-MAY-12 06:55:00 (CST-6)
Location: PANTHER
Magnitude: 58.00 EG
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

One half of a tree was snapped off.  Section of the tree was one foot in diameter.  Winds were estimated at 55 to 60 MPH.

Heavy Rain in AUDUBON, IOWA
From 06-MAY-12 07:00:00 to 06-MAY-12 07:00:00 (CST-6)
Location: SHARON
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Rainfall of 4.5 inches recorded.