EF2 tornado, Kaycee Gosney Arpt, Wyoming, June 12, 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 mph

   Moderate damage. Roofs badly stripped, mobile homes overturned or badly damaged, exterior doors lost, windows broken.

3. EF2111 to 135 mphthis tornado

   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

EF2

Date

June 12, 2017

Touchdown

5:30 PM MDT (23:30 UTC)

Lifted

5:36 PM MDT (23:36 UTC)

On the ground

6 minutes

Path length

1.6 miles (2.5 km)

Path width

250 yards (229 m)

Deaths

0

Injuries

0

Property damage

$10,000

County

Johnson County

Touchdown point

43.692, -106.685

Lift-off point

43.714, -106.679

Record number

1706121730-01

8 damage points were recorded along this path by NWS RIW, in the Damage Assessment Toolkit the survey teams carry. The strongest indicator was rated EF2. The highest wind the damage implies is 120 mph.

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

A combination of an abnormally strong upper level low, strong jet dynamics and an approaching cold front produced one of the biggest severe weather outbreaks across western and central Wyoming in many years. A total of 4 tornadoes were confirmed, including an EF2 tornado which was the strongest tornado that occurred in the NWS Riverton county warning area since 2001 which completely destroyed a trailer and threw it's base over 200 yards. There were also multiple reports of large hail, especially in Johnson County where hail up to tennis ball size was reported. In the dry slot across central and southern Wyoming, strong mid level winds mixed to the surface. The strongest winds were in the Lander foothills where wind gusts up to 76 mph occurred. Many trees were uprooted with some damage to outbuildings. Wind gusts 58 mph or over were also reported in Sweetwater County.

A storm survey was conducted near Kaycee and confirmed a tornado. The tornado touched down on a hill just west of Interstate 25 and just north of the Middle Fork of the Powder River. The tornado downed many trees moving down the hill. The tornado then strengthened as it crossed a field. An abandoned trailer was destroyed with the base thrown over 200 yards. Trees in the field were snapped off above the bases with some ground scouring noted. The tornado then crossed Barnum Road and moved northward toward Pine Ridge. The tornado narrowly missed a house and a barn as it moved up the ridge. Numerous trees were downed along both sides of the ridgeline before the funnel lifted. Winds were estimated at 110 to 120 mph, a minimal EF2 tornado.

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

32 minutes of warning. NWS RIW issued a tornado warning for this county at 22:58 UTC, 32 minutes before the tornado touched down, in force until 23:30 UTC.

The county was under a severe thunderstorm watch, issued at 20:47 UTC and running to 03:29 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 Enhanced Risk, with a 0.02 chance of a tornado within 25 miles. The outlook in force was issued at 20:00 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.

Buffalo/Johnson County (BYG), 75 km from the path, in the two hours either side.

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

The Riverton/Lander radar (KRIW), 161 km away, was scanning. The volume nearest the touchdown began at 23:30 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 WyomingEvery tornado in Wyoming since 1950Warnings in force in Wyoming nowThe forecast where you are

20 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.