A strong upper level low pressure system lifted northeast out of the southwest U.S. during the early morning hours of the 17th.  The surface low was over South Dakota as a line of thunderstorms developed ahead of an advancing dry line that was south of the low.  The atmosphere was moderately unstable with about 2000 J/kg as the storms developed in the evening.  CAPE in the -10 to -30 C layer of the atmosphere was only about 400 J/kg with a freezing level around 13,000 feet.  The lifted index was in the -4 to -5 C range as the storms approached with 40 to 45 kts of shear available.  The LCL was quite low, around 750 meters.  CAPE in the zero to 3 km layer was 50 to 100 J/kg.  Precipitable water values were quite high, in the 1.25 to 1.5 inch range.  For the most part, the thunderstorms were not severe and produced mainly heavy rainfall.  Several reports of 2 to 3 inches of rainfall were received from southwest into north central Iowa.  As the line advanced eastward, a small mesoscale vorticity maximum was observed riding north along the line.  The thunderstorms became enhanced as it moved north with a line of damage extending from Clarke County north to Webster County.  Eighty two cars of a railroad train were derailed by the winds, with two of them blown off into the nearby field.  Numerous very large trees were downed and some farm buildings lost roofs.  Reports of trees and power lines being blown down continued north as the vorticity maximum lifted north.

Thunderstorm Wind in BOONE, IOWA
From 17-MAY-15 02:52:00 to 17-MAY-15 02:52:00 (CST-6)
Location: OGDEN
Magnitude: 52.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Strong winds caused tree and power line damage west of Ogden.

Thunderstorm Wind in DECATUR, IOWA
From 17-MAY-15 00:51:00 to 17-MAY-15 00:51:00 (CST-6)
Location: DECATUR CITY
Magnitude: 54.00 MG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Mesonet

Measured at the Leon road weather information system.

Heavy Rain in TAYLOR, IOWA
From 17-MAY-15 02:30:00 to 17-MAY-15 02:30:00 (CST-6)
Location: BLOCKTON
Property damage: 0.00K
Crop damage: 0.00K
Reported by: COOP Observer

Rainfall of 3.23 inches for the 24 hour period ending at 6 AM.  End time is based on Radar.

Tornado in MADISON, IOWA
From 17-MAY-15 01:20:00 to 17-MAY-15 01:31:00 (CST-6)
Location: TRURO
Tornado F-scale: EF0
Property damage: 25.00K
Crop damage: 5.00K
Reported by: NWS Storm Survey

This tornado affected several farmsteads along its path. Damage was mostly to trees and storage silos, with damage to outbuildings caused by flying debris from these sources. The tornado path became intermittent for the latter portion of its life as it tracked northward.

Heavy Rain in HUMBOLDT, IOWA
From 17-MAY-15 05:30:00 to 17-MAY-15 05:30:00 (CST-6)
Location: DAKOTA CITY
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Rainfall of 2.30 for the 24 hour period ending at 6 AM.  End time is based on Radar and relayed by KCCI TV.

Tornado in MADISON, IOWA
From 17-MAY-15 01:27:00 to 17-MAY-15 01:31:00 (CST-6)
Location: EAST PERU
Tornado F-scale: EF0
Property damage: 0.00K
Crop damage: 1.00K
Reported by: NWS Storm Survey

This short-lived tornado damaged a few trees along its path.

Heavy Rain in RINGGOLD, IOWA
From 17-MAY-15 03:00:00 to 17-MAY-15 03:00:00 (CST-6)
Location: BEACONSFIELD
Property damage: 0.00K
Crop damage: 0.00K
Reported by: COOP Observer

Rainfall of 1.93 inches for the 24 hour period ending at 6:30 AM.  End time is based on Radar.

Thunderstorm Wind in CLARKE, IOWA
From 17-MAY-15 01:05:00 to 17-MAY-15 01:05:00 (CST-6)
Location: OSCEOLA MUNI ARPT
Magnitude: 61.00 EG
Property damage: 75.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Trees and grain bins were blown down.

Tornado in MADISON, IOWA
From 17-MAY-15 01:40:00 to 17-MAY-15 01:42:00 (CST-6)
Location: PATTERSON
Tornado F-scale: EF1
Property damage: 50.00K
Crop damage: 5.00K
Reported by: NWS Storm Survey

This short-lived tornado tracked northwest and damaged several farmsteads. A horse trailer was thrown 40 yards into a pine tree, flipping a parked car along the way. Damage was otherwise contained to outbuilding roofs and trees.

Thunderstorm Wind in MADISON, IOWA
From 17-MAY-15 01:35:00 to 17-MAY-15 01:35:00 (CST-6)
Location: ST CHARLES
Magnitude: 61.00 EG
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Tree damage was reported.

Tornado in MADISON, IOWA
From 17-MAY-15 01:40:00 to 17-MAY-15 01:42:00 (CST-6)
Location: PATTERSON
Tornado F-scale: EF1
Property damage: 0.00K
Crop damage: 2.00K
Reported by: NWS Storm Survey

This intermittent tornado tracked northwest during its short life and damaged several groves of trees.

Tornado in DALLAS, IOWA
From 17-MAY-15 02:00:00 to 17-MAY-15 02:01:00 (CST-6)
Location: VAN METER
Tornado F-scale: EF0
Property damage: 0.00K
Crop damage: 0.00K
Reported by: NWS Storm Survey

This brief tornado only damaged trees near the North Raccoon River.

Thunderstorm Wind in DECATUR, IOWA
From 17-MAY-15 00:55:00 to 17-MAY-15 00:55:00 (CST-6)
Location: VAN WERT
Magnitude: 52.00 EG
Property damage: 50.00K
Crop damage: 0.00K
Reported by: Public

Trees were reported down on houses in town and power was out.  Relayed by KCCI TV.

Thunderstorm Wind in CLARKE, IOWA
From 17-MAY-15 01:15:00 to 17-MAY-15 01:15:00 (CST-6)
Location: OSCEOLA
Magnitude: 65.00 EG
Property damage: 50.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A wooden barn was completely destroyed along 180th Ave.

Thunderstorm Wind in CLARKE, IOWA
From 17-MAY-15 01:10:00 to 17-MAY-15 01:10:00 (CST-6)
Location: OSCEOLA
Magnitude: 75.00 EG
Property damage: 250.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Around 82 train cars were blown over.  Several very large trees were uprooted in a narrow corridor.  Many branches were snapped and several farm buildings lost roofs.

Thunderstorm Wind in DECATUR, IOWA
From 17-MAY-15 00:42:00 to 17-MAY-15 00:42:00 (CST-6)
Location: DECATUR CITY
Magnitude: 52.00 EG
Property damage: 100.00K
Crop damage: 0.00K
Reported by: Social Media

Many large branches were down in town and several car ports flipped.

Thunderstorm Wind in DALLAS, IOWA
From 17-MAY-15 02:28:00 to 17-MAY-15 02:28:00 (CST-6)
Location: BOUTON
Magnitude: 61.00 EG
Property damage: 50.00K
Crop damage: 0.00K
Reported by: Social Media

Multiple large trees over 12 inches in diameter were blown down throughout town.

Thunderstorm Wind in DALLAS, IOWA
From 17-MAY-15 02:12:00 to 17-MAY-15 02:12:00 (CST-6)
Location: DALLAS CENTER ARPT
Magnitude: 52.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Social Media

Minor tree damage was reported and shingles were torn from roofs.

Thunderstorm Wind in CLARKE, IOWA
From 17-MAY-15 01:10:00 to 17-MAY-15 01:10:00 (CST-6)
Location: MURRAY
Magnitude: 61.00 EG
Property damage: 100.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Farm buildings were damaged or destroyed in the 1675 to 1700 blocks of Highway 34.  A 2 X 10 was driven into the ground.

Thunderstorm Wind in MADISON, IOWA
From 17-MAY-15 01:50:00 to 17-MAY-15 01:50:00 (CST-6)
Location: WINTERSET ARPT
Magnitude: 65.00 EG
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Tree damage reported with tree tops snapped, a flag pole was bent and heavy outdoor objects were moved.

Thunderstorm Wind in DALLAS, IOWA
From 17-MAY-15 01:57:00 to 17-MAY-15 01:57:00 (CST-6)
Location: VAN METER
Magnitude: 52.00 EG
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Several 8 inch diameter tree limbs were damaged.  Minor tree damage was reported within a block or two radius of this location.