F1 tornado, Udell, Iowa, September 20, 2001

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.

Rating

F1

Date

September 20, 2001

Touchdown

6:09 PM CDT (23:09 UTC)

Path length

0.5 miles (0.8 km)

Path width

50 yards (46 m)

Deaths

0

Injuries

0

Property damage

$10,000

County

Appanoose County

Touchdown point

40.830, -92.730

Lift-off point

40.830, -92.730

Record number

1099

A strong surge of moist and unstable air moved north into Iowa during the day on the 20th. A warm front lifted north across the state during the day as a strong upper level disturbance approached from the west and northwest. During the afternoon a cold front moved into northwest Iowa with a surface low forming along the front in that area. Ahead of the front the airmass became unstable as dew point temperatures rose into the mid 60s to low 70s. Thunderstorm erupted over north central Iowa during the early afternoon as colder temperatures moved in aloft, destabilizing the atmosphere. Surface based convection developed by mid to late afternoon over the central and southern counties of the state. The main severe weather feature with the storms was hail ranging from one to two inches in diameter. Two inch diameter hail broke out windows in cars north of Northwood in Worth County. Southeast of Northwood, near Silver Lake, hail nearly baseball in size caused considerable damage to property and crops. Hail was reported to be drifted 3 to 4 inches deep in places. Two inch diameter hail also fell in Hancock County at Garner. The storms over southern Iowa, being surface based, had a higher likelihood of becoming tornadic. Indeed one of the storms did produce a brief tornado touchdown in Appanoose County northwest of Udell. The tornado did little damage as it touched down in open country. A tornado also touched down in Lucas County north of Lucas. The tornado struck a farmstead causing considerable damage to several buildings. High winds about a mile north of the tornado damaged a machine shed and moved an outbuilding on a farmstead there. The severe weather event began to change into a wind event toward the end of its life cycle. Winds of 80 MPH were reported in Appanoose County in Udell for example. High winds also swept through areas north of Centerville, and also north of Moulton in Appanoose County.

Tornado touched down in open country, causing little damage.

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

4 minutes of warning. NWS DMX issued a tornado warning for this county at 23:05 UTC, 4 minutes before the tornado touched down, in force until 23:35 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 Slight Risk. The outlook in force was issued at 20:04 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 Des Moines radar (KDMX), 130 km away, was scanning. The volume nearest the touchdown began at 23:09 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.

The live radar over the trackEvery 2001 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

7 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.