F1 tornado, Delmar, Iowa, April 20, 2004

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

April 20, 2004

Touchdown

7:08 PM CDT (00:08 UTC)

Path length

1.3 miles (2.1 km)

Path width

100 yards (91 m)

Deaths

0

Injuries

0

Property damage

$30,000

County

Clinton County

Touchdown point

41.950, -90.680

Lift-off point

41.970, -90.670

Record number

70

A low topped supercell developed in East Central Iowa and interacted with a warm front across Clinton County. The increased wind shear produced a low end F1 Tornado. The tornado developed just east of 220th Avenue and south of 150th street, southwest of Delmar, Iowa and dissipated just west of the junction of 150th Street and U.S. 61. A brand new 42 x 80 foot machine shed was destroyed as well as a 100 year old barn, and a hog building. The farm house sustained very minor damage with some siding and a few shingles removed while a water tower had its wooden roof removed. According to the owners, the 100 year old barn was on its last legs.

Rated low end F1;winds estimated at 75 mph.

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

3 minutes of warning. NWS DVN issued a tornado warning for this county at 00:05 UTC, 3 minutes before the tornado touched down, in force until 01:00 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 General Thunderstorms Risk. The outlook in force was issued at 19:58 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.

Clinton (CWI), 32 km from the path, in the two hours either side.

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

Local storm reports within 60 km of the path, in the three hours either side.

From the Iowa Environmental Mesonet's archive of National Weather Service local storm reports.

The live radar over the trackEvery 2004 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

31 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.