EF1 tornado, Sharpsburg to Lenox Arpt, Iowa, May 11, 2011

1. EF065 to 85 mph

   Light damage. Surface peeled off some roofs, gutters and siding damaged, branches broken, shallow-rooted trees pushed over.

2. EF186 to 110 mphthis tornado

   Moderate damage. Roofs badly stripped, mobile homes overturned or badly damaged, exterior doors lost, windows broken.

3. EF2111 to 135 mph

   Considerable damage. Roofs torn off well-built houses, frame homes shifted on their foundations, mobile homes destroyed, large trees snapped or uprooted, cars lifted off the ground.

4. EF3136 to 165 mph

   Severe damage. Whole stories of well-built houses destroyed, large buildings badly damaged, trains overturned, trees debarked, heavy cars lifted and thrown.

5. EF4166 to 200 mph

   Devastating damage. Well-built houses leveled, structures with weak foundations moved some distance, cars thrown and large missiles generated.

6. EF5over 200 mph

   Incredible damage. Strong frame houses swept off their foundations, car-sized missiles thrown more than 100 meters, steel-reinforced concrete badly damaged, high-rise buildings deformed.

7. Unrated

   The survey recorded a tornado but could not put a number on it, most often because it crossed open country and damaged nothing a rating can be read from.

Rating

EF1

Date

May 11, 2011

Touchdown

4:45 PM CDT (21:45 UTC)

Lifted

4:50 PM CDT (21:50 UTC)

On the ground

5 minutes

Path length

1.0 miles (1.6 km)

Path width

400 yards (366 m)

Deaths

0

Injuries

0

Property damage

$10 million

County

Taylor County

Touchdown point

40.878, -94.565

Lift-off point

40.890, -94.576

Record number

1105111545-01

3 damage points were recorded along this path by NWS DMX, in the Damage Assessment Toolkit the survey teams carry. The strongest indicator was rated EF1. The highest wind the damage implies is 104 mph.

• Roof complete off a small older house that had been converted to a realty business. Tornado moved northwest.

Survey points are preliminary and are the raw material the published rating was built from. Source: NWS Damage Assessment Toolkit.

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.

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

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.

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.

From NOAA NCEI's Storm Events Database, the narrative the local forecast office wrote for Storm Data.

34 minutes of warning. NWS DMX issued a tornado warning for this county at 21:11 UTC, 34 minutes before the tornado touched down, in force until 22:00 UTC.

The county was under a tornado watch, issued at 19:56 UTC and running to 23:53 UTC.

Every warning and watch covering the touchdown point that day, from the Iowa Environmental Mesonet's archive of National Weather Service products. Lead time is measured from the issue of the warning in force to the touchdown time in the database, both of which were recorded to the minute by different people.

The Storm Prediction Center had this point in a Slight Risk, with a 0.05 chance of a tornado within 25 miles. The outlook in force was issued at 19:55 UTC.

SPC's Day 1 convective outlook as it stood when the tornado began, read at the touchdown point. The same archive is on the radar archive.

The radiosonde launches on either side of the tornado, from the nearest station that was launching then, drawn and analyzed the way Storm Lab does: the same diagram, the same parcel and shear definitions SPC's mesoanalysis uses.

The Des Moines radar (KDMX), 118 km away, was scanning. The volume nearest the touchdown began at 21:47 UTC.

Open the radar archive at this moment, over the track

Reflectivity and velocity, decoded from the Level II archive in the browser. Volumes take a few seconds to open the first time.

Local storm reports within 60 km of the path, in the three hours either side.

From the Iowa Environmental Mesonet's archive of National Weather Service local storm reports.

The live radar over the trackEvery 2011 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

15 more were confirmed in the United States that day.

Ratings are the survey's, read off the damage rather than measured; the rating scale is above. Tornado counts rise through the record as reporting improved, so a year in the 1950s is not comparable with one today except for the strongest tornadoes, which were always noticed.

NOAA Storm Prediction Center tornado database, 1950-2025, the official record kept by NOAA's Storm Prediction Center.