Tornadoes · United States · Oklahoma · 1999

F0 tornado, Blackgum, Oklahoma, May 11, 1999

This F0 tornado ran 0.5 miles across Sequoyah County on May 11, 1999, touching down at 10:37 PM CDT and reaching 50 yards wide. It killed nobody, and did $1,000 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 mphthis tornado

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

  2. F173 to 112 mph

    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
F0
Date
May 11, 1999
Touchdown
10:37 PM CDT (03:37 UTC)
Path length
0.5 miles (0.8 km)
Path width
50 yards (46 m)
Deaths
0
Injuries
0
Property damage
$1,000
County
Sequoyah County
Touchdown point
35.620, -95.080
Lift-off point
35.620, -95.080
Record number
1099

What the Weather Service recorded

Summary of events for May 11 1999:Numerous severe thunderstorms formed along a slow-moving north-south cold front across eastern Oklahoma on the afternoon and evening of May 11. A cap of warm air in the mid-levels of the atmosphere kept thunderstorm activity suppressed until the first storm formed in Rogers and Mayes County late in the afternoon. This was the largest storm of the day as it showed strong rotation and produced very large hail to the size of baseballs.Many other severe thunderstorms formed and increased in coverage through the evening. With the slow movement of the cold front, many thunderstorms moved over areas already saturated by earlier thunderstorms, leading to localized flash flooding. Several brief tornadoes also formed through the evening, with the largest concentration in the Arkansas River valley between Muskogee and Sallisaw.

There was a brief tornado touchdown near Aqua Park, causing mainly tree damage.

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

The warnings

28 minutes of warning. NWS TSA issued a tornado warning for this county at 03:09 UTC, 28 minutes before the tornado touched down, in force until 04:00 UTC.

Issued (UTC)ExpiredProductOffice
02:25 UTC03:15 UTCTornado WarningTSA
03:09 UTC04:00 UTCTornado WarningTSA
03:54 UTC04:45 UTCSevere Thunderstorm WarningTSA

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 Slight Risk. The outlook in force was issued at 01:32 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.

Springfield, MO (KSGF), 00:00Z

3.6 hours before the tornado, 183 km from the path. May 12, 1999.

Skew-T log-pSkew-T log-p diagram, Springfield, MO at 00:00Z on 1999-05-12Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel lifted from the surface. Mixed-layer CAPE 1007 joules per kilogram, CIN -54, lifted condensation level 1168 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, Springfield, MO at 00:00Z on 1999-05-12The 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 33 square meters per square second, 0 to 6 km bulk shear 19 knots.193958Sfc1 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
1007 J/kg
Mixed-layer CIN
-54 J/kg
Surface CAPE
2481 J/kg
Mixed-layer LCL
1168 m above ground
0-6 km bulk shear
19 kt
0-1 km bulk shear
6 kt
0-1 km storm-relative helicity
33 m²/s²
0-3 km storm-relative helicity
83 m²/s²
Significant tornado parameter
0.0
Supercell composite
0.0
0-3 km lapse rate
6.9 K/km

Open this sounding in Storm Lab

Springfield, MO (KSGF), 12:00Z

8.4 hours after it, 183 km from the path. May 12, 1999.

Skew-T log-pSkew-T log-p diagram, Springfield, MO at 12:00Z on 1999-05-12Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel lifted from the surface. Mixed-layer CAPE 26 joules per kilogram, CIN -28, lifted condensation level 611 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, Springfield, MO at 12:00Z on 1999-05-12The 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 23 square meters per square second, 0 to 6 km bulk shear 30 knots.193958Sfc1 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
26 J/kg
Mixed-layer CIN
-28 J/kg
Surface CAPE
151 J/kg
Mixed-layer LCL
611 m above ground
0-6 km bulk shear
30 kt
0-1 km bulk shear
38 kt
0-1 km storm-relative helicity
23 m²/s²
0-3 km storm-relative helicity
-67 m²/s²
Significant tornado parameter
0.0
Supercell composite
0.1
0-3 km lapse rate
5.5 K/km

Open this sounding in Storm Lab

The surface observations

Muskogee/Davis Field (MKO), 26 km from the path, in the two hours either side.

Time (UTC)Temp (°F)Dew point (°F)WindGust (kt)Pressure (mb)Weather
01:38 UTC71.669.8340° 14 kt1010.8+RA BR
01:45 UTC69.869.8340° 10 kt1010.5+RA BR
01:53 UTC686820° 14 kt1010.8+RA BR
02:00 UTC66.266.230° 12 kt1010.8+RA BR
02:05 UTC686820° 11 kt1011.2RA BR
02:10 UTC66.266.240° 9 kt1011.2-RA BR
02:26 UTC66.264.430° 15 kt191011.2-RA
02:53 UTC686680° 11 kt1010.5-RA
03:05 UTC6866.280° 15 kt1010.8-RA
03:53 UTC6664120° 12 kt1010.8
04:39 UTC6864.4110° 6 kt1011.5+RA BR
04:53 UTC676690° 10 kt1011.2-RA BR
05:01 UTC66.264.4100° 13 kt161011.5-RA
05:25 UTC66.266.20° 0 kt1011.9

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

This ground now

The live radar over the trackEvery 1999 tornado in OklahomaEvery tornado in Oklahoma since 1950Warnings in force in Oklahoma nowThe forecast where you are

The other tornadoes of May 11, 1999

7 more were confirmed in the United States that day.

DateTimeRatingWherePath (mi)Width (yd)DeathsInjuriesDamage
May 114:18 PM CDTF0San Saba, TX0.250
May 115:57 PM CDTF0San Saba, TX0.220
May 116:05 PM CDTF4Loyal Valley tornado, TX7.01,40016$750,000
May 118:20 PM CDTF3Gillespie, TX3.5200$1.0 million
May 118:30 PM CDTF0Choctaw, OK1.050$1,000
May 119:00 PM CDTF0Muskogee, OK0.550$1,000
May 119:16 PM CDTF0Muskogee, OK0.550$1,000

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.