EF0 tornado, Thayer to Lorimor, Iowa, May 6, 2007

1. EF065 to 85 mphthis tornado

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

2. EF186 to 110 mph

   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

EF0

Date

May 6, 2007

Touchdown

4:45 AM CDT (09:45 UTC)

Lifted

4:50 AM CDT (09:50 UTC)

On the ground

5 minutes

Path length

4.2 miles (6.8 km)

Path width

50 yards (46 m)

Deaths

0

Injuries

0

Property damage

$75,000

County

Union County

Touchdown point

41.061, -94.091

Lift-off point

41.110, -94.043

Record number

0705060345-01

A very strong upper level low pressure trough had carved out over the western U.S. Iowa was in the strong southwest flow on the front side of this system. A surface warm front lifted north into southwest Iowa during the evening of the 5th. The airmass became quite unstable with plenty of deep moisture advection into the areas. Precipitable water values were in the 1.5 to 1.75 inch range by midnight. The lifted index fell to the -6 to -8 C. There was a reasonable amount of CAPE available with about 2500 J/kg at the onset of the event. The LCL was around 1000 meters and the freezing level was a relatively high 13,500 feet. With the warm front in the vicinity, there was a considerable amount of helicity present with values in the 300 to 550 range. During the evening of the 5th, most of the significant convection remained to the west of the DMX CWA. After midnight, the low level jet increased to 50 to 70 kts and began to veer. This helped push the convection east during the overnight hours. Strong cells developed in southwest Iowa near the warm frontal boundary. Two tornadoes occurred with this initial set of convection. An EF2 tornado touched down in Cass County. The tornado first developed right on the southern Cass County line, from highway 71 to three quarters of a mile west. Four farmsteads were damaged along county road G66 within the mile west of highway 71, producing damage up to EF1. The tornado narrowed slightly and intensified in the next mile, downing two sets of metal electrical transmission poles. The most severe damage occurred at a farm service and grain elevator business on highway 71. This was 2 miles into Cass County or 3 miles south of the town of Lyman. EF2 damage occurred at this site with winds estimated at 120-130 mph. The average path width was 100 to 200 yards, with a maximum width of about one half mile. Empty semi trailers were flipped, two tall grain bins were flattened. A large metal clad truck service building was flattened on the south end and much of the roof was off the north end. The west part of the roof of an office building was torn off and the wall collapsed. The house at the north end of the site had windows blown out and part of the roof damaged. The tornado continued north along and just east of highway 71 for another two miles causing some damage to homes with some outbuildings unroofed or destroyed. The last apparent damage along the tornado path occurred two miles northeast of Lyman where power poles were snapped. Several areas between Cumberland and Anita were also surveyed. Damage along this path was consistent with straight line winds of 70 to 90 mph. Damage included downed trees and sheet metal torn off sheds and outbuildings. A storage building in Anita was significantly damaged. A total of 115 to 120 homes were damaged in Cass County by the storm. As the night progressed, a bow echo advanced into the state from the southwest. There were several reports of high winds and hail with this line. One of the storm in the line produced a small EF0 tornado in Union County. Some damage occurred at several farmsteads along the path. This same cell produced very large hail with hail up to baseball size reported in the Afton area. The hail was wind driven, damaging skylights, windows, and cars. Hail stones were driven through car windshields and also through windows in houses. Some damage even occurred inside of two homes as the hail stones continued inside of the house. This was the most significant hail from the storms. The hail reportedly piled up several inches deep and lasted for at least 4 hours after the storm. In the survey conducted by the Emergency Manager it was found that in Afton, 236 primary buildings and 174 out buildings and garages had moderate hail damage consisting of roof damage...primarily to the slopes facing south and west. All roof vents and eves, either aluminum or plastic, were damaged. Almost 100 percent of the buildings with south-facing windows had damage to screens, broken glass storm windows, interior single and double pane windows, and in cases of plastic framed windows even the sashes and stools were damaged. All siding was damaged on those sides no matter of the material. In the rural areas around Afton, 30 of the 38 residences received moderate damage. In Arispe, 5 miles south of Afton, hail damage occurred, but was not as severe with 40 residences reporting minor damage and 30 out buildings being damaged. All together, 346 vehicles were hail damaged and many had Windows broken completely out. Twenty three campers suffered vent and sky lite damages as well as exterior damages. Hardly any of the skylights withstood the hail, either in campers or houses. Though many of the storms produced hail, much of it was under one half inch in diameter. Another brief tornado touched town in Guthrie County south of Bayard. The tornado caused EF1 damage. A roof was blown off of a building there, and blown westward. Wind was the primary threat with the storms. There were several reports of trees downed by the high winds, barns, and outbuildings being blown over or destroyed. A semi-tractortrailer was blown off of Interstate 80 west of Adair. The event turned into a significant event both in areal coverage, as well as for its variety of weather features. Flooding became a significant problem later in the event with several reports of 3 to 5 inch rainfall received. Flash flooding occurred over parts of southwest into west central Iowa. Cass, Audubon, and Carroll Counties were the hardest hit. Areal flood warnings were issued for these counties. Widespread flooding too place, with Cass County hard hit. The Governor of Iowa, Chet Culver, declared a state disaster for several counties across southwest and south central Iowa due to the flash flooding. This included Decatur County in the Des Moines CWA. Toward dawn, high winds became a problem over parts of northwest Iowa. The winds affected a 6 county area with wind gusts in the 50 to 65 MPH range for several hours.

Tornado formed on the back side of a bow echo. Damage was limited to EF0, but evidence suggested weak circulation.

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

No tornado warning for this county appears in the archive as in force at the moment the tornado touched down.

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 0.02 chance of a tornado within 25 miles. The outlook in force was issued at 05:58 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.

Creston (CSQ), 26 km from the path, in the two hours either side.

From the Iowa Environmental Mesonet's archive of airport observations.

The Des Moines radar (KDMX), 77 km away, was scanning. The volume nearest the touchdown began at 09:45 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 2007 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.