A closed upper low lifted out to the west of Iowa with the surface low tracking northeast to near the Iowa-South Dakota border.  A warm front extended southeast into Iowa and lifted north as the dry line pushed into western Iowa during the afternoon.  The synoptic situation was fairly classic.  The atmosphere was relatively unstable, though cloudiness limited that somewhat.  CAPE values rose to 1500 to 2000 J/kg with a lifted index around -7 at the storms initiation.  Downdraft CAPE was only 300 J/kg.  There was 45 to 50 kts of effective shear however.  CAPE in the -10 to -30 C layer was around 500 J/kg, with a freezing level around 12,000 feet.  The parameters were favorable for potential tornado development with the LCL only 750 meters and 50 to 100 J/kg of CAPE in the zero to 3 km layer with steep lapse rates.  Thunderstorms formed in an arc across western Iowa and quickly produced gusty winds and small hail.  The cells tracked north as the line lifted northeast. Numerous reports of tornadoes were received.  The reports were from the same tornado.  Debris was reported with the tornado in Carroll County.  Damage as done in the Lake City area of Calhoun County by a tornado, reported to be 1/4 mile wide by then.  The tornado touched down northwest of Lidderdale and tracked northeast, striking the west side of Lake City. The tornado continued passing through rural Calhoun County and hit some power poles on the west side of Rockwell City before lifting about 2 miles north of Rockwell City in rural Calhoun County. Much of the damage included leaning power poles, tree damage and uprooted trees. In the town of Lake City, the roofing material was removed from the High School, and the roof was removed from a double-wide manufactured home and a warehouse building north of town. The tornado was of EF1 strength with winds of 100 MPH.  The track was 23.5 miles long.  Additional thunderstorms produced hail and high winds.  An 80 MPH wind gust was reported west of Glidden, a machine shed was destroyed in Audubon County, and a weak shed was blown over in Cass County.  Winds were measured at 65 MPH in Pocahontas.  A strong storm moved through the Lake City area following the tornado and produced baseball size hail and destroyed a farm outbuilding west of Lidderdale.

Thunderstorm Wind in DALLAS, IOWA
From 10-MAY-15 20:34:00 to 10-MAY-15 20:34:00 (CST-6)
Location: VAN METER
Magnitude: 61.00 EG
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Power lines and poles were down just south of Van Meter near 360th and R Ave.

Tornado in CALHOUN, IOWA
From 10-MAY-15 18:22:00 to 10-MAY-15 19:00:00 (CST-6)
Location: LAKE CITY
Tornado F-scale: EF1
Property damage: 1.00M
Crop damage: 0.00K
Reported by: Trained Spotter

Tornado moved from Carroll County into Calhoun County.  It moved across the western side of Lake City, doing EF1 damage to houses and the high school.  It remained in rural areas of Calhoun County for the remainder of it's path, broadening as it went along.  The tornado lifted shortly before crossing Highway 20.

Hail in CALHOUN, IOWA
From 10-MAY-15 19:30:00 to 10-MAY-15 19:30:00 (CST-6)
Location: LAKE CITY
Magnitude: 2.75 
Property damage: 250.00K
Crop damage: 0.00K
Reported by: Public

Tornado in CARROLL, IOWA
From 10-MAY-15 18:10:00 to 10-MAY-15 18:22:00 (CST-6)
Location: MT CARMEL
Tornado F-scale: EF0
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Debris on the ground.  Tornado continued into Calhoun County.

Tornado in CALHOUN, IOWA
From 10-MAY-15 19:14:00 to 10-MAY-15 19:16:00 (CST-6)
Location: MANSON
Tornado F-scale: EF0
Property damage: 0.00K
Crop damage: 1.00K
Reported by: Trained Spotter

This was a broad and brief tornado that caused little to no damage. Reported by a spotter and likely track found in satellite data.

Thunderstorm Wind in CASS, IOWA
From 10-MAY-15 18:17:00 to 10-MAY-15 18:17:00 (CST-6)
Location: LEWIS
Magnitude: 50.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Broadcast Media

A small shed was blown over.

Thunderstorm Wind in AUDUBON, IOWA
From 10-MAY-15 18:50:00 to 10-MAY-15 18:50:00 (CST-6)
Location: VIOLA CENTER
Magnitude: 57.00 EG
Property damage: 35.00K
Crop damage: 0.00K
Reported by: Law Enforcement

A machine shed on a farm was destroyed.

Thunderstorm Wind in CARROLL, IOWA
From 10-MAY-15 19:15:00 to 10-MAY-15 19:15:00 (CST-6)
Location: CARROLL NEU ARPT
Magnitude: 70.00 MG
Property damage: 50.00K
Crop damage: 0.00K
Reported by: ASOS

Thunderstorm Wind in POCAHONTAS, IOWA
From 10-MAY-15 20:08:00 to 10-MAY-15 20:08:00 (CST-6)
Location: POCAHONTAS
Magnitude: 56.00 MG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A wind gust of 65 MPH as measured by a hand-held anemometer.