EF0 tornado, Napier to Midvale, Iowa, August 21, 2017

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 21, 2017

Touchdown

5:52 PM CDT (22:52 UTC)

Lifted

5:55 PM CDT (22:55 UTC)

On the ground

3 minutes

Path length

3.3 miles (5.3 km)

Path width

80 yards (73 m)

Deaths

0

Injuries

0

Property damage

$5,000

Crop damage

$9,000

Counties

Boone and Story counties

Touchdown point

41.949, -93.726

Lift-off point

41.922, -93.674

Record number

1708211652-01

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

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

With the help of a couple weak boundaries in the area, storms initiated on the nose of the low level jet and generally remained over the same areas overnight. The primary result of the storms was heavy rainfall, though a few initial storms were able to produce damaging winds and severe hail within 2000+ J/kg CAPE and borderline effective bulk shear around 30 kts. Other than the few initial severe storm reports, total rainfall reports exceeded 5 inches in some locations. During the afternoon and evening on the 21st, ahead of a cold front, storms once again initiated and produced primarily sub-severe winds and hail, though two weak tornadoes and one severe wind gust were reported.

This tornado was found in hi-resolution satellite imagery and crossed mostly open farm fields. The tornado did damage trees across the corner of one farmstead but otherwise did little other damage. This tornado continued into Story county.

Tornado found in hi-resolution satellite imagery. This tornado entered from Boone county and stayed in open row crops, mainly corn where the path was very visible in the imagery.

Tornado captured by hi-resolution satellite imagery to the southeast of Ames. Started just southeast of the water treatment plant and moved quickly through open farm fields with a distinctive path noted in both bean and corn fields. Little damage noted other than to the crops.

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. The outlook in force was issued at 19:51 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), 23 km away, was scanning. The volume nearest the touchdown began at 22:49 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 2017 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

1 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.