EF1 tornado, Thorntown, Indiana, May 20, 2017

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 20, 2017

Touchdown

7:40 PM EDT (23:40 UTC)

Lifted

7:41 PM EDT (23:41 UTC)

On the ground

1 minutes

Path length

0.1 miles (0.1 km)

Path width

40 yards (37 m)

Deaths

0

Injuries

0

Property damage

$20,000

County

Boone County

Touchdown point

40.134, -86.696

Lift-off point

40.134, -86.694

Record number

1705201740-01

1 damage points were recorded along this path by NWS IND, in the Damage Assessment Toolkit the survey teams carry. The strongest indicator was rated EF1. The highest wind the damage implies is 90 mph.

• Large section of roof removed and tossed several hundred yards into the farm field.

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

Scattered thunderstorms developed over west central and north central Indiana Saturday evening along a northward-moving warm front. A couple of the storms became severe with numerous wall clouds and funnel clouds. One storm produced a brief tornado in the town of Forest in northeast Clinton County, causing significant damage to a fire station and a pole barn. Another storm produced a brief tornado in Boone County. The tornadic threat transitioned to a heavy rain and flash flooding threat in the Howard County area. Widespread flash flooding was reported in the Kokomo area with many roads impassible and some water rescues.

A brief EF-1 tornado touched down west of Thorntown in Boone County. Winds estimated at 90 mph damaged a barn by removing a large section of roof and tossing it several hundred yards into a field. One of the barn's walls collapsed.

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

1 minute of warning. NWS IND issued a tornado warning for this county at 23:39 UTC, 1 minute before the tornado touched down, in force until 00:00 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 Marginal Risk, with a 0.02 chance of a tornado within 25 miles. The outlook in force was issued at 20:01 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 Indianapolis radar (KIND), 59 km away, was scanning. The volume nearest the touchdown began at 23:41 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 IndianaEvery tornado in Indiana since 1950Warnings in force in Indiana nowThe forecast where you are

3 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.