EF0 tornado, Knoop Arpt to Powersville, Iowa, August 27, 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 27, 2021

Touchdown

5:50 PM CDT (22:50 UTC)

Lifted

5:51 PM CDT (22:51 UTC)

On the ground

1 minutes

Path length

1.3 miles (2.1 km)

Path width

140 yards (128 m)

Deaths

0

Injuries

0

Crop damage

$2,000

County

Floyd County

Touchdown point

42.936, -92.997

Lift-off point

42.933, -92.972

Record number

2108271650-01

24 damage points were recorded along this path by NWS ARX, in the Damage Assessment Toolkit the survey teams carry. The strongest indicator was rated EF1. The highest wind the damage implies is 110 mph.

• Grain bin 75 yds off foundation. Empty. Propane tank moved.
• Corn affected, playset rolled
• Corn path. Looking se. nothing nw.

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

A complex of storms moved across northeast Iowa during the late afternoon and evening of August 27th. These storms produced two tornadoes in the vicinity of Marble Rock (Floyd County). The first tornado was an EF0 west of Marble Rock that only caused some minor crop and structural damage to a farm. The second tornado was rated as an EF1 that passed just to the south of Marble Rock as it moved southeast. Nearly all the buildings on one farm were destroyed and several other farm buildings and homes sustained some damage. The storms also produced heavy rain with totals ranging from 3 to 8 inches. Flash flooding from this rain closed several roads, including U.S. Highway 18 near New Hampton (Chickasaw County) and some homes sustained water damage. A train derailment southwest of New Hampton resulted from flooding along the Wapsipinicon River prompted officials to evacuate nearby residents when a tank car leaked anhydrous ammonia. The Chickasaw County Sheriff issued a no travel advisory for parts of the county because of the flooding. The Governor of Iowa issued a disaster proclamation for Allamakee, Chickasaw, Clayton, Floyd, Howard, Mitchell and Winneshiek Counties. A previous disaster proclamation was still in effect for Fayette County.

This very brief track was found in Sentinel satellite imagery and stayed in fields for its existence. Based on the location in the supercell, inflow wind trajectories along the left side of the track, and striation patterns in the fields, there is a high likelihood that this was an anti-cyclonic tornado.

An EF0 tornado formed west of Marble Rock and move to the east before dissipating. Damage was primarily limited to crops but one farm did sustain some damage to outbuildings.

An EF1 tornado developed just to the west of Marble Rock and passed south of town as it traveled southeast. Several farms sustained damage along the track with one farm having nearly every outbuilding destroyed and several large propane tanks were knocked over.

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

16 minutes of warning. NWS ARX issued a tornado warning for this county at 22:34 UTC, 16 minutes before the tornado touched down, in force until 23:15 UTC.

The county was under a tornado watch, issued at 22:15 UTC and running to 02:49 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 0.02 chance of a tornado within 25 miles. The outlook in force was issued at 19:41 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.

Hampton (HPT), 31 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), 147 km away, was scanning. The volume nearest the touchdown began at 22:51 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

4 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.