A nearly stationary frontal boundary remained draped across northwest Iowa.  The airmass became very unstable with dewpoints surging into the 65 to 70 degree range.  The wind flow at 850 mb was weak, only around 15 kts or so, but winds backed and were very convergent.  Moisture pooled over northwest Iowa with precipitable water values increasing to around 1.4 inches by mid afternoon.  The lifted index fell to between -5 and -10 C with CAPE rising to the 2000-3000 J/kg range.  Downdraft CAPE was quite high, in the 800 to 1200 J/kg range.  Ample CAPE was available in the -10 to -30 C layer of the atmosphere, between 400 and 800 J/kg.  Unlike the previous day, significant shear was available for any storm development with 40 to 50 kts of effective shear.  The freezing level was quite high for early May and was between 13,000 and 14,000 feet.  Though high enough to limit hail, other parameters were sufficiently strong to overcome this.  The LCL was fairly low with near storm environmental values around 750 meters.  A line of strong to severe thunderstorms formed near the frontal boundary during the mid and late afternoon hours.  The line moved and propagated east-southeast.  High winds were common with the storms with wind speeds of up to 65 MPH or more.  The storms also produced some quarter size hail.  A tornado occurred in Winnebago County just south of the Minnesota line.  It was about a mile long and was in open country.  A short time later, a funnel cloud was observed in Worth County.  After the main storms moved east, further thunderstorms developed behind the line.  One became severe and produced golf ball size hail in Pocahontas County at Pocahontas.

Tornado in WINNEBAGO, IOWA
From 04-MAY-12 16:52:00 to 04-MAY-12 16:54:00 (CST-6)
Location: VINJE
Tornado F-scale: EF1
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Storm Survey

Brief touchdown just south of the Minnesota border.  Damaged a farm building and some trees.

Hail in KOSSUTH, IOWA
From 04-MAY-12 16:45:00 to 04-MAY-12 16:45:00 (CST-6)
Location: ST BENEDICT
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Nickel to quarter size hail with ground covered.

Hail in POCAHONTAS, IOWA
From 04-MAY-12 21:55:00 to 04-MAY-12 21:55:00 (CST-6)
Location: POCAHONTAS
Magnitude: 1.75 
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Public

Golf ball size hail was reported on the north side of town with half dollar size hail reported at the Casey's location.

Hail in KOSSUTH, IOWA
From 04-MAY-12 16:50:00 to 04-MAY-12 16:50:00 (CST-6)
Location: HANNA
Magnitude: 1.00 
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Public

Thunderstorm Wind in EMMET, IOWA
From 04-MAY-12 15:12:00 to 04-MAY-12 15:12:00 (CST-6)
Location: DOLLIVER
Magnitude: 56.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Emergency Manager

The Emergency Manager estimated wind gusts of 65 MPH.

Thunderstorm Wind in EMMET, IOWA
From 04-MAY-12 15:45:00 to 04-MAY-12 15:45:00 (CST-6)
Location: ESTHERVILLE
Magnitude: 56.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Wind gusts of 65 MPH were reported by law enforcement.

Hail in EMMET, IOWA
From 04-MAY-12 16:02:00 to 04-MAY-12 16:02:00 (CST-6)
Location: ARMSTRONG
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Hail in KOSSUTH, IOWA
From 04-MAY-12 16:15:00 to 04-MAY-12 16:15:00 (CST-6)
Location: LONE ROCK
Magnitude: 1.00 
Property damage: 3.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Nickel to quarter size hail reported.

Thunderstorm Wind in WORTH, IOWA
From 04-MAY-12 17:11:00 to 04-MAY-12 17:11:00 (CST-6)
Location: BRISTOL
Magnitude: 56.00 MG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Funnel Cloud in WORTH, IOWA
From 04-MAY-12 17:20:00 to 04-MAY-12 17:20:00 (CST-6)
Location: MANLY
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Funnel cloud with two to three gustnadoes reported.  Brief appearance with rotating wall cloud.