A very warm and humid airmass was in place over Iowa on the 11th.  During the previous two days, strong capping was in place across the state.  A cold front began to move into the state on the 11th as upper level heights and temperatures began to fall.  The front stalled however, and the atmosphere became unstable due to the cooling aloft.  The freezing level was relatively low on the 11th as it fell to under 12,000 feet.  Most unstable CAPE was in the 1500 to 2500 J/kg range by the afternoon as lifted indices fell to -4 to -7 C.  The atmosphere exhibited weak shear with 25 kts or less of effective shear during the afternoon.  The potential was there for high winds, as well as hail, with downdraft CAPE in the 500-900 J/kg range, and CAPE in the -10 to -30 C layer of the atmosphere between 100 and 400 J/kg.  Helicity values in the 0-1 km layer increased sharply over southwest Iowa and were between 100 and 200 by mid to late afternoon.  In addition, considerable moisture pooling took place with precipitable water values increasing to 1.3 to 1.6 inches over the southwest half of Iowa by evening.  The LCL level was quite high initially, around 1500 meters, but lowered to between 500 and 1000 meters by mid to late afternoon as continued convection moistened the atmosphere.  An initial round of thunderstorms moved through the state during the morning of the 11th.  Thunderstorms continued to develop intermittently over southwest Iowa during the late morning into the afternoon.  The thunderstorms became more organized by mid to late afternoon as an upper level disturbance moved into the state from the south.  Thunderstorms became stronger and produced several tornadoes in Adams and Taylor Counties.  There were two brief touchdowns reported that caused no damage.  One was in Taylor County near Sharpsburg, the other near Kent in Adams County.  Two tornadoes moved through the town of Lenox in Taylor County.  The first tornado touched down in south central Lenox along South Maple Street, between West Platte Street and West Iowa Street Before moving north in a curving path until it reached Walnut Street. The  tornado then moved generally due north, along Walnut Street, eventually dissipating about one block north of West Van Buren Street around 1540 CST.  The damage path was generally diffuse with tree damage and shingles removed from buildings. The  greatest damage occurred along North Main Street, where two brick buildings received roof and upper facade damage.   A second tornado moved into Lenox a short time later.  The tornado touched down in the southwest part of Lenox near the intersection of State Highway 49 and West Platte Street, only a couple blocks southwest of the touchdown location of tornado number 1. The tornado traveled northward along State Highway 49, with the greatest damage occurring shortly after touchdown along Highway 49 between West Iowa Street and West Illinois Street. In that area, one house sustained damage to a second story dormer. Also in that area, a manufactured home on a foundation had approximately one half of its roof removed. The tornado then continued north along State Highway 49 generally causing tree damage and shingles to be removed from roofs. Eventually the tornado turned northwest just north of West Temple Street where an older home which housed a business had its roof removed. Shingle and tree damage continued as the tornado tracked northwest before finally exiting Lenox along Van Buren Street west of North Spear Street.  The tornado continued over open fields before dissipating about
one half mile northwest of Lenox around 1548 CST.  There was considerable damage done in the town of Lenox over a 35 square block area.  Damage was in the millions of dollars.  Reports indicated that at least 45% of the homes in the city of 1407 people were damaged by the tornadoes.  One hundred and sixty trees were downed as well.  The next day, Governor Terry Branstad proclaimed a Disaster Declaration for Taylor County.  In addition to the tornadoes, there were numerous reports of high winds and hail.  Most of the hail was around an inch in diameter with the largest report being around 2 inches in diameter in Adams County.  After the storms began to weaken and move out of southwest Iowa, they produced high winds with several reports of 60 to 65 MPH winds.  Heavy rainfall of 2 to 4 inches occurred in parts of southwest and south central Iowa with rainfall rates exceeding 2 inches per hour.  Flash flooding occurred in Union, Adams, and Taylor Counties.  Numerous roads were flooded and became impassible, along with a few state highways.  Soil erosion was severe, especially in Taylor County.  The heavy rainfall occurred during the worst possible time, right after planting.  The soil had been freshly plowed and much of it was loose.  Damage from soil erosion was many times more costly than the damage caused by the tornadoes.  As the storms moved through the Johnston area, lightning struck outside of a house.  The lightning entered the house via an invisible dog fence, causing a small fire inside of the house as the control unit was set on fire.  Lightning struck a house in Creston.  The lightning started fire, resulting in $40,000 in damage.

Flash Flood in UNION, IOWA
From 11-MAY-11 16:59:00 to 11-MAY-11 23:00:00 (CST-6)
Location: CRESTON ARPT
Flood cause: Heavy Rain
Property damage: 250.00K
Crop damage: 10.00K
Reported by: Amateur Radio

Four inches of rain with flash flooding in low lying areas.

Lightning in POLK, IOWA
From 11-MAY-11 18:30:00 to 11-MAY-11 18:30:00 (CST-6)
Location: JOHNSTON
Property damage: 2.00K
Crop damage: 0.00K
Reported by: NWS Employee

Lightning in UNION, IOWA
From 11-MAY-11 17:20:00 to 11-MAY-11 17:20:00 (CST-6)
Location: CRESTON
Property damage: 40.00K
Crop damage: 0.00K
Reported by: Newspaper

Flash Flood in ADAMS, IOWA
From 11-MAY-11 13:42:00 to 11-MAY-11 19:30:00 (CST-6)
Location: NODAWAY
Flood cause: Heavy Rain
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Water flowing over rural roads in several locations of southwest Adams County.  EM had to turn around one half mile south of Nodaway due to flash flooding.

Flash Flood in ADAMS, IOWA
From 11-MAY-11 15:30:00 to 11-MAY-11 19:30:00 (CST-6)
Location: NODAWAY
Flood cause: Heavy Rain
Property damage: 150.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Adams County Law Enforcement reported railroad tracks washed out between track mile posts 423 and 424.

Flash Flood in ADAMS, IOWA
From 11-MAY-11 15:30:00 to 11-MAY-11 19:30:00 (CST-6)
Location: NODAWAY
Flood cause: Heavy Rain
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Law Enforcement reported water over Chestnut Ave between 240th and 250th Street.

Flash Flood in ADAMS, IOWA
From 11-MAY-11 16:32:00 to 11-MAY-11 19:30:00 (CST-6)
Location: MERCER
Flood cause: Heavy Rain
Property damage: 20.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Adams County Law Enforcement reported the Adams-Taylor County Line Road was impassible between Poplar Ave and Orange Ave.

Flash Flood in TAYLOR, IOWA
From 11-MAY-11 17:15:00 to 11-MAY-11 20:30:00 (CST-6)
Location: GRAVITY
Flood cause: Heavy Rain
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Department of Highways

A bridge scour occurred on Highway 148 10 miles north of Bedford.

Tornado in TAYLOR, IOWA
From 11-MAY-11 15:38:00 to 11-MAY-11 15:41:00 (CST-6)
Location: LENOX
Tornado F-scale: EF1
Property damage: 5.00M
Crop damage: 0.00K
Reported by: NWS Storm Survey

This was the first of two tornadoes that moved through Lenox on this day. The tornado formed on south side of Lenox then proceeded to move through town.  Worst damage was on the south side of town where a roof was partially removed from a house.  Additional shingle damage and tree damage occurred as the tornado moved north northeast through the town.  Max wind speeds were around 105 MPH.

Tornado in TAYLOR, IOWA
From 11-MAY-11 15:45:00 to 11-MAY-11 15:50:00 (CST-6)
Location: LENOX
Tornado F-scale: EF1
Property damage: 10.00M
Crop damage: 0.00K
Reported by: NWS Storm Survey

This was the second tornado to move through Lenox on this day.  This tornado formed on the southwest edge of town and moved due north into town along State Highway 49.  It removed a roof from an old house and produced tree damage along its path. Shortly after removing the roof, the tornado began moving to the northwest and continued on this path prior to dissipating northwest of town. Maximum winds around 95 mph.

Tornado in ADAMS, IOWA
From 11-MAY-11 15:58:00 to 11-MAY-11 16:00:00 (CST-6)
Location: STRINGTOWN
Tornado F-scale: EF0
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Trained spotter observed a tornado just west of Kent heading slowly northward.

Funnel Cloud in ADAMS, IOWA
From 11-MAY-11 13:54:00 to 11-MAY-11 13:54:00 (CST-6)
Location: CORNING MUNI ARPT
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Funnel cloud on the southwest side of the town of Brooks.  Funnel dropped a little, then went back into cloud.

Funnel Cloud in ADAMS, IOWA
From 11-MAY-11 16:00:00 to 11-MAY-11 16:00:00 (CST-6)
Location: MERCER
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Funnel cloud approaching the ground, moving east.

Funnel Cloud in UNION, IOWA
From 11-MAY-11 16:08:00 to 11-MAY-11 16:08:00 (CST-6)
Location: KENT
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Spotter reported 2 funnels trying to descend.

Flash Flood in ADAMS, IOWA
From 11-MAY-11 15:30:00 to 11-MAY-11 19:30:00 (CST-6)
Location: NODAWAY
Flood cause: Heavy Rain
Property damage: 200.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Several places throughout Nodaway reported 6 inches of flowing water over roads and inundating homes and businesses.

Flash Flood in ADAMS, IOWA
From 11-MAY-11 16:00:00 to 11-MAY-11 19:30:00 (CST-6)
Location: NODAWAY
Flood cause: Heavy Rain
Property damage: 25.00K
Crop damage: 0.00K
Reported by: Public

Water reported up to the railroad tracks.

Flash Flood in UNION, IOWA
From 11-MAY-11 17:15:00 to 11-MAY-11 23:00:00 (CST-6)
Location: CRESTON
Flood cause: Heavy Rain
Property damage: 10.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Union County Law Enforcement reported the Osage Railroad bridge was impassible due to water over the road.  One car was stranded.

Thunderstorm Wind in WARREN, IOWA
From 11-MAY-11 17:54:00 to 11-MAY-11 17:54:00 (CST-6)
Location: CUMMING
Magnitude: 52.00 MG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Amateur Radio

Thunderstorm Wind in RINGGOLD, IOWA
From 11-MAY-11 17:04:00 to 11-MAY-11 17:04:00 (CST-6)
Location: KELLERTON
Magnitude: 52.00 EG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: CoCoRaHS

Observer reported wind gusts of at least 60 MPH with rain blowing horizontally.

Thunderstorm Wind in DECATUR, IOWA
From 11-MAY-11 17:08:00 to 11-MAY-11 17:08:00 (CST-6)
Location: WESTERVILLE
Magnitude: 52.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Spotter reported 60 plus MPH winds with horizontal rain and strong shaking of vehicles.

Thunderstorm Wind in MADISON, IOWA
From 11-MAY-11 17:31:00 to 11-MAY-11 17:31:00 (CST-6)
Location: WINTERSET
Magnitude: 56.00 EG
Property damage: 2.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Wind gusts estimated 60 to 70 MPH near Winterset.

Thunderstorm Wind in MADISON, IOWA
From 11-MAY-11 17:40:00 to 11-MAY-11 17:40:00 (CST-6)
Location: PATTERSON
Magnitude: 52.00 EG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Trained Spotter

A trained spotter experienced 60 MPH winds between Winterset and Patterson.

Thunderstorm Wind in DECATUR, IOWA
From 11-MAY-11 17:36:00 to 11-MAY-11 17:36:00 (CST-6)
Location: GARDEN GROVE
Magnitude: 52.00 EG
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Heavy rain blew horizontally under a 15 foot eave and into the spotters house.

Hail in ADAMS, IOWA
From 11-MAY-11 11:57:00 to 11-MAY-11 11:57:00 (CST-6)
Location: CARBON
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Area received nearly 3.5 inches of rain as well.

Hail in ADAMS, IOWA
From 11-MAY-11 13:36:00 to 11-MAY-11 13:36:00 (CST-6)
Location: NODAWAY
Magnitude: 1.25 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Hail in ADAMS, IOWA
From 11-MAY-11 13:42:00 to 11-MAY-11 13:42:00 (CST-6)
Location: NODAWAY
Magnitude: 1.00 
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Hail in ADAMS, IOWA
From 11-MAY-11 13:55:00 to 11-MAY-11 13:55:00 (CST-6)
Location: BROOKS
Magnitude: 2.00 
Property damage: 15.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Hail in ADAMS, IOWA
From 11-MAY-11 14:08:00 to 11-MAY-11 14:13:00 (CST-6)
Location: IVEYVILLE
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Hail lasted about 5 minutes.

Hail in UNION, IOWA
From 11-MAY-11 16:55:00 to 11-MAY-11 16:55:00 (CST-6)
Location: CRESTON
Magnitude: 1.00 
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Quarter size hail and 40-50 MPH winds at 160th and Highway 25.

Hail in UNION, IOWA
From 11-MAY-11 16:46:00 to 11-MAY-11 16:46:00 (CST-6)
Location: CROMWELL
Magnitude: 1.00 
Property damage: 1.00K
Crop damage: 0.00K
Reported by: Amateur Radio

Hail in WAPELLO, IOWA
From 11-MAY-11 20:45:00 to 11-MAY-11 20:45:00 (CST-6)
Location: OTTUMWA
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Tornado in TAYLOR, IOWA
From 11-MAY-11 15:20:00 to 11-MAY-11 15:21:00 (CST-6)
Location: SHARPSBURG
Tornado F-scale: EF0
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Brief tornado just northeast of Sharpsburg.  Wall cloud continues to rotate as it moves slowly northeast.