EF0 tornado, Ashton to Sibley Muni Arpt, Iowa, August 20, 2021

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

August 20, 2021

Touchdown

3:34 PM CDT (20:34 UTC)

Lifted

3:41 PM CDT (20:41 UTC)

On the ground

7 minutes

Path length

3.6 miles (5.9 km)

Path width

30 yards (27 m)

Deaths

0

Injuries

0

Property damage

$30,000

County

Osceola County

Touchdown point

43.334, -95.793

Lift-off point

43.379, -95.762

Record number

2108201434-01

4 damage points were recorded along this path by NWS FSD, in the Damage Assessment Toolkit the survey teams carry. The strongest indicator was rated EF0. The highest wind the damage implies is 85 mph.

• Small shed overturned and tossed into nearby tree. Poor anchoring. Several large tree branches also snapped. Image courtesy of Osceola County Emergency Management drone video.
• Start point of drone video provided by Osceola County Emergency Management. Sentinel-2 satellite imagery suggest additional path extended further south of path via drone video after crossing highway 60.
• Large branch broken off tree. Nearby RV camper was tossed into utility shed, causing walls to collapse. Photo provided by the Osceola County Sheriff.
• City of Sibley highway sign destroyed. Image courtesy of Osceola Emergency Management drone video.

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

Clusters of thunderstorms along an advancing cold front merged to produce linear structures in the presence of 500-1000 J/kg DCAPE. Short bowing segments with shear vectors weakly perpendicular to the boundary resulted in brief mesovortices.

A tornado developed one mile north of Ashton, just north of the intersection of Nuthatch Avenue and 220th Street. The tornado moved northeast across farmland with a track identified on Sentinel satellite imagery. The tornado then turned after around one mile to move largely parallel to State Highway 60, briefly crossing to the east side of the road where it overturned a small shed and snapped several branches on a large tree. As the tornado tracked between Highway 60 and the Sibley Municipal Airport, an RV was tossed into a utility shed which resulted in the collapse of the shed. The tornado then turned north and moved back across Highway 60, where it destroyed a road sign shortly before it lifted one-half mile north of 190th Street. The path along and east of Highway 60 was documented on drone video. The average path width was 30 yards. Property damage costs are a rough estimate.

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 0.05 chance of a tornado within 25 miles. The outlook in force was issued at 19:34 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 Sioux Falls radar (KFSD), 81 km away, was scanning. The volume nearest the touchdown began at 20:33 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 2021 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

6 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.