F0 tornado, Gravity, Iowa, September 7, 1999

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.

Rating

F0

Date

September 7, 1999

Touchdown

7:30 PM CDT (00:30 UTC)

Path length

0.7 miles (1.1 km)

Path width

30 yards (27 m)

Deaths

0

Injuries

0

County

Taylor County

Touchdown point

40.800, -94.770

Lift-off point

40.800, -94.770

Record number

557

The seasons first dynamic cold front moved southeast into Iowa during the evening hours of the 7th. Temperatures warmed rapidly during the day on a gusty south wind. Readings topped out in the mid 80s to low 90s across the state. Dew point temperatures climbed into the mid 60s to low 70s across the region by the late afternoon. These conditions combined to produce surface based CAPE values of 3500 to 4500 J/kg ahead of the front. A narrow line of thunderstorms developed just ahead of the cold front during the late afternoon hours. The environment became favourable for severe thunderstorm development as mid level winds were increasing and had increased to 50 to 60 kts before sunset. The storms became severe and produced hail at several locations from north central into southwest Iowa. The largest hail was golf ball in size and fell in the north central part of the state. Gusty winds were also common with the storms, though most of the storms only produced winds around 50 MPH. One of the storms produced very high winds in Webster County from what appeared to be a downburst. Winds around 90 MPH flattened crops in a 5 square mile area. One farmstead was hit with damage reported there including a grain dryer toppled. In addition, a grain dryer was toppled at the cooperative, costing between $300,000 and $400,000. The storms weakened a few hours after sunset with no severe weather reported by the late evening hours. During the early evening as the line of storms moved through southwest Iowa, hail nearly golf ball in size fell in Adams County. A tornado touched down in Taylor County, briefly, northwest of Gravity. The only damage was crop damage as it was over open country.

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

No tornado warning for this county appears in the archive as in force at the moment the tornado touched down.

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 Slight Risk. The outlook in force was issued at 19:59 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 live radar over the trackEvery 1999 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

2 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.