Tornadoes · United States · Oklahoma · 1977

F1 tornado, Grant County, Oklahoma, May 20, 1977

This F1 tornado ran 0.1 miles across Grant County on May 20, 1977, touching down at 8:45 PM CDT and reaching 10 yards wide. It killed nobody, and did $50,000 to $500,000 of property damage.

The rating scale

Tracks are drawn in the color of the rating. The Fujita scale rates the damage rather than measuring the wind: the speeds are what that damage is taken to imply.

  1. F040 to 72 mph

    Light damage. Surface peeled off some roofs, gutters and siding damaged, branches broken, shallow-rooted trees pushed over.

  2. F173 to 112 mphthis tornado

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

  3. F2113 to 157 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. F3158 to 206 mph

    Severe damage. Whole stories of well-built houses destroyed, large buildings badly damaged, trains overturned, trees debarked, heavy cars lifted and thrown.

  5. F4207 to 260 mph

    Devastating damage. Well-built houses leveled, structures with weak foundations moved some distance, cars thrown and large missiles generated.

  6. F5261 to 318 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.

The record

Rating
F1, estimated from the damage record rather than surveyed
Date
May 20, 1977
Touchdown
8:45 PM CDT (01:45 UTC)
Path length
0.1 miles (0.2 km)
Path width
10 yards (9 m)
Deaths
0
Injuries
0
Property damage
$50,000 to $500,000
County
Grant County
Touchdown point
36.800, -97.700
Record number
345

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.

Ok Cty/Will Rgrs, OK (KOKC), 00:00Z

1.8 hours before the tornado, 156 km from the path. May 21, 1977.

Skew-T log-pSkew-T log-p diagram, Ok Cty/Will Rgrs, OK at 00:00Z on 1977-05-21Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel lifted from the surface. Mixed-layer CAPE 1430 joules per kilogram, CIN -9, lifted condensation level 818 meters above ground. Wind barbs in knots down the right edge.LCLLFCEL10008507005003002001001 km2 km3 km6 km9 km12 km15 km-30°-20°-10°0°10°20°30°40°
Temperature in red, dew point in green, the mixed-layer parcel dashed. The warm shading is its CAPE, the cool shading the inhibition it has to break through.
HodographHodograph, Ok Cty/Will Rgrs, OK at 00:00Z on 1977-05-21The wind through the lowest 10 kilometers as a curve, rings every 10 meters per second labeled in knots. Zero to 1 km storm-relative helicity 288 square meters per square second, 0 to 6 km bulk shear 60 knots.1939587897Sfc1 km3 km6 km9 kmRMLM
The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km. RM and LM are the right and left moving storms Bunkers' method gives; the shaded area is the 0 to 1 km storm-relative helicity.
Mixed-layer CAPE
1430 J/kg
Mixed-layer CIN
-9 J/kg
Surface CAPE
1267 J/kg
Mixed-layer LCL
818 m above ground
0-6 km bulk shear
60 kt
0-1 km bulk shear
34 kt
0-1 km storm-relative helicity
288 m²/s²
0-3 km storm-relative helicity
359 m²/s²
Significant tornado parameter
3.2
Supercell composite
13.2
0-3 km lapse rate
5.8 K/km

Open this sounding in Storm Lab

Ok Cty/Will Rgrs, OK (KOKC), 12:00Z

10.2 hours after it, 156 km from the path. May 21, 1977.

Skew-T log-pSkew-T log-p diagram, Ok Cty/Will Rgrs, OK at 12:00Z on 1977-05-21Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel lifted from the surface. Mixed-layer CAPE 0 joules per kilogram, CIN -9971, lifted condensation level 1931 meters above ground. Wind barbs in knots down the right edge.LCL10008507005003002001001 km2 km3 km6 km9 km12 km15 km-30°-20°-10°0°10°20°30°40°
Temperature in red, dew point in green, the mixed-layer parcel dashed. The warm shading is its CAPE, the cool shading the inhibition it has to break through.
HodographHodograph, Ok Cty/Will Rgrs, OK at 12:00Z on 1977-05-21The wind through the lowest 10 kilometers as a curve, rings every 10 meters per second labeled in knots. Zero to 1 km storm-relative helicity 194 square meters per square second, 0 to 6 km bulk shear 61 knots.1939587897117Sfc1 km3 km6 km9 kmRMLM
The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km. RM and LM are the right and left moving storms Bunkers' method gives; the shaded area is the 0 to 1 km storm-relative helicity.
Mixed-layer CAPE
0 J/kg
Mixed-layer CIN
-9971 J/kg
Surface CAPE
21 J/kg
Mixed-layer LCL
1931 m above ground
0-6 km bulk shear
61 kt
0-1 km bulk shear
33 kt
0-1 km storm-relative helicity
194 m²/s²
0-3 km storm-relative helicity
214 m²/s²
Significant tornado parameter
0.0
Supercell composite
0.0
0-3 km lapse rate
5.1 K/km

Open this sounding in Storm Lab

This ground now

The live radar over the trackEvery 1977 tornado in OklahomaEvery tornado in Oklahoma since 1950Warnings in force in Oklahoma nowThe forecast where you are

The other tornadoes of May 20, 1977

25 more were confirmed in the United States that day.

DateTimeRatingWherePath (mi)Width (yd)DeathsInjuriesDamage
May 2011:10 AM CDTF1Terry, TX2.030under $50
May 2011:30 AM CDTF0, SD0.110
May 2011:55 AM CDTF1Hockley, TX1.550under $50
May 201:09 PM CDTF1Hale, TX1.030under $50
May 202:45 PM CDTF1Hitchcock, NE0.120$5,000 to $50,000
May 202:58 PM CDTF1Briscoe, TX2.030under $50
May 203:20 PM CDTF3Jackson, OK10.48803$500,000 to $5 million
May 203:52 PM CDTF1Howard, TX0.520under $50
May 204:28 PM CDTF1Donley, TX2.030under $50
May 204:35 PM CDTF1Major, OK5.1100$5,000 to $50,000
May 204:43 PM CDTF1Grady, OK2.590$5,000 to $50,000
May 205:47 PM CDTF2Caddo, OK13.31,100$50,000 to $500,000
May 206:10 PM CDTF2Caddo, OK7.81,760$50,000 to $500,000
May 206:14 PM CDTF0Wheeler, TX1.020
May 206:38 PM CDTF3Tillman, OK1.5200$50,000 to $500,000
May 207:00 PM CDTF2Kiowa, OK3.6100$5,000 to $50,000
May 207:00 PM CDTF2Wilbarger, TX7.310$500 to $5,000
May 207:15 PM CDTF1Garfield, OK4.1100$5,000 to $50,000
May 207:40 PM CDTF2Oklahoma, OK14.9400$500,000 to $5 million
May 208:11 PM CDTF3Oklahoma, OK9.9400$50,000 to $500,000
May 208:20 PM CDTF2Logan, OK1.9100$50,000 to $500,000
May 208:30 PM CDTF1Logan, OK1.9100$5,000 to $50,000
May 2010:00 PM CDTF1Wichita, TX0.110
May 2010:10 PM CDTF1Taylor, TX0.110
May 2012:30 AM CDTF1Garvin, OK2.750$5,000 to $50,000

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.