Tornadoes · United States · Montana · 2011
EF1 tornado, Terry, Montana, May 20, 2011
This EF1 tornado ran 0.1 miles across Prairie County on May 20, 2011, touching down at 8:27 PM MDT and reaching 66 yards wide. It killed nobody, and did $75,000 of property damage.
The rating scale
Tracks are drawn in the color of the rating. The Enhanced Fujita scale rates the damage rather than measuring the wind: the speeds are what that damage is taken to imply.
EF065 to 85 mph
Light damage. Surface peeled off some roofs, gutters and siding damaged, branches broken, shallow-rooted trees pushed over.
EF186 to 110 mphthis tornado
Moderate damage. Roofs badly stripped, mobile homes overturned or badly damaged, exterior doors lost, windows broken.
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.
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.
EF4166 to 200 mph
Devastating damage. Well-built houses leveled, structures with weak foundations moved some distance, cars thrown and large missiles generated.
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.
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.
The record
- Rating
- EF1
- Date
- May 20, 2011
- Touchdown
- 8:27 PM MDT (02:27 UTC)
- Lifted
- 8:28 PM MDT (02:28 UTC)
- On the ground
- 1 minutes
- Path length
- 0.1 miles (0.1 km)
- Path width
- 66 yards (60 m)
- Deaths
- 0
- Injuries
- 0
- Property damage
- $75,000
- County
- Prairie County
- Touchdown point
- 46.664, -105.300
- Lift-off point
- 46.664, -105.300
- Record number
- 1105202027-01
What the Weather Service recorded
A small EF-1 tornado occurred 8 miles south of Terry, MT. The NWS Glasgow office did a damage survey.
At approximately 8:27pm on May 20, a tornado struck a ranch 8 miles south of Terry on 10 Mile Road. The tornado was observed by a resident of the house on the ranch. The house itself received no damage, though it was about 150 feet away from observed damage. Significant damage occurred to 4 outbuildings on the ranch. There were no injuries and the horses on the ranch were uninjured as they were out in pasture at the time of the tornado. The damage path was about 500 feet and the width was 200 feet. Based on the damage, the wind speed was determined to be 90 mph or a low end EF-1 tornado.
From NOAA NCEI's Storm Events Database, the narrative the local forecast office wrote for Storm Data.
The warnings
No tornado warning for this county appears in the archive as in force at the moment the tornado touched down.
| Issued (UTC) | Expired | Product | Office |
|---|---|---|---|
| 16:06 UTC | 10:36 UTC | Flood Warning | GGW |
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 forecast that day
The Storm Prediction Center had this point in a General Thunderstorms Risk. The outlook in force was issued at 00:43 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 air that afternoon
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.
Glasgow Intl Airport, MT (KGGW), 00:00Z
2.5 hours before the tornado, 199 km from the path. May 21, 2011.
- Mixed-layer CAPE
- 3 J/kg
- Mixed-layer CIN
- -14 J/kg
- Surface CAPE
- 140 J/kg
- Mixed-layer LCL
- 807 m above ground
- 0-6 km bulk shear
- 40 kt
- 0-1 km bulk shear
- 15 kt
- 0-1 km storm-relative helicity
- 103 m²/s²
- 0-3 km storm-relative helicity
- 59 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.0
- 0-3 km lapse rate
- 6.0 K/km
Glasgow Intl Airport, MT (KGGW), 12:00Z
9.5 hours after it, 199 km from the path. May 21, 2011.
- Mixed-layer CAPE
- 31 J/kg
- Mixed-layer CIN
- -8 J/kg
- Surface CAPE
- 11 J/kg
- Mixed-layer LCL
- 522 m above ground
- 0-6 km bulk shear
- 31 kt
- 0-1 km bulk shear
- 15 kt
- 0-1 km storm-relative helicity
- 101 m²/s²
- 0-3 km storm-relative helicity
- 106 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.0
- 0-3 km lapse rate
- 5.1 K/km
The surface observations
Miles City (MLS), 52 km from the path, in the two hours either side.
| Time (UTC) | Temp (°F) | Dew point (°F) | Wind | Gust (kt) | Pressure (mb) | Weather |
|---|---|---|---|---|---|---|
| 00:53 UTC | 56 | 54 | 20° 14 kt | 1004.7 | -RA BR | |
| 01:03 UTC | 55.4 | 53.6 | 30° 20 kt | 24 | 1004.7 | |
| 01:53 UTC | 56 | 54 | 40° 15 kt | 1004.7 | ||
| 02:21 UTC | 55.4 | 53.6 | 30° 15 kt | 23 | 1004.7 | |
| 03:53 UTC | 54 | 52 | 10° 11 kt | 1005.1 |
From the Iowa Environmental Mesonet's archive of airport observations.
The radar
The Glasgow radar (KGGW), 198 km away, was scanning. The volume nearest the touchdown began at 02:25 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.
What was reported nearby
Local storm reports within 60 km of the path, in the three hours either side.
| Time (UTC) | Report | Where | County | Remark |
|---|---|---|---|---|
| 02:27 UTC | TSTM WND DMG | 8 S Terry | Prairie, MT | PRAIRIE COUNTY DES REPORTS 8 MILES SOUTH OF TERRY THUNDERSTORM WIND DAMAGE TO A SHED AND HORSE TRAILER. |
| 02:27 UTC | TSTM WND GST 50 | 16 Se Terry | Prairie, MT | THUNDERSTORM WIND GUST ESTIMATED AT 40-50 MPH. 4.00 INCHES OF RAINFALL REPORTED. |
From the Iowa Environmental Mesonet's archive of National Weather Service local storm reports.
This ground now
The live radar over the trackEvery 2011 tornado in MontanaEvery tornado in Montana since 1950Warnings in force in Montana nowThe forecast where you are
The other tornadoes of May 20, 2011
5 more were confirmed in the United States that day.
What this means
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.
Sources
NOAA Storm Prediction Center tornado database, 1950-2025, the official record kept by NOAA's Storm Prediction Center.