Tornadoes · United States · Iowa · 2004

F0 tornado, Grundy County, Iowa, May 21, 2004

This F0 tornado ran 0.5 miles across Grundy County on May 21, 2004, touching down at 3:15 PM CDT and reaching 40 yards wide. It killed nobody.

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 21, 2004
Touchdown
3:15 PM CDT (20:15 UTC)
Path length
0.5 miles (0.8 km)
Path width
40 yards (37 m)
Deaths
0
Injuries
0
County
Grundy County
Touchdown point
42.370, -92.970
Lift-off point
42.370, -92.970
Record number
900

The warnings

30 minutes of warning. NWS DMX issued a tornado warning for this county at 19:45 UTC, 30 minutes before the tornado touched down, in force until 20:30 UTC.

Issued (UTC)ExpiredProductOffice
18:55 UTC19:30 UTCSevere Thunderstorm WarningDMX
19:02 UTC19:45 UTCTornado WarningDMX
19:45 UTC20:30 UTCTornado WarningDMX
20:27 UTC21:15 UTCTornado WarningDMX
22:52 UTC23:45 UTCSevere Thunderstorm WarningDMX

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 Moderate Risk, with a 0.05 chance of a tornado within 25 miles. The outlook in force was issued at 20:02 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 surface observations

Marshalltown (MIW), 29 km from the path, in the two hours either side.

Time (UTC)Temp (°F)Dew point (°F)WindGust (kt)Pressure (mb)Weather
18:53 UTC8469160° 18 kt211008.8
19:53 UTC8368200° 16 kt261007.8
20:53 UTC8369220° 14 kt211007.8TS
21:53 UTC716370° 14 kt191007.5

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

The radar

The Des Moines radar (KDMX), 94 km away, was scanning. The volume nearest the touchdown began at 20:13 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.

What was reported nearby

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

Time (UTC)ReportWhereCountyRemark
17:35 UTCHAIL 0.753 W GreeneButler, IA
18:52 UTCHAIL 0.88Steamboat RockHardin, IAREPORTED BY TRAINED SPOTTER.
19:21 UTCHAIL 0.752 E DowsFranklin, IA
19:28 UTCTORNADO2 W HollandGrundy, IA
19:31 UTCHAIL 0.88Se PopejoyFranklin, IA
19:32 UTCTORNADO1 E HollandGrundy, IACORRECTION ON LOCATION...ORIGINAL REPORT HAD 3 WEST OF HOLLAND AND IT SHOULD BE 1 EAST OF HOLLAND.
19:32 UTCTORNADO3 W HollandGrundy, IA
19:38 UTCTORNADO3 N Grundy CenterGrundy, IATORNADO CROSSING HIGHWAY 14 WITH DEBRIS. STATE PATROL REPORT.
19:42 UTCHAIL 0.88DikeGrundy, IA
19:42 UTCTORNADO4 Ne Grundy CenterGrundy, IASTATE PATROL REPORT OF TWO SIMULATNEOUS TORNADOES.
19:49 UTCTORNADO7 Sw DikeGrundy, IAMOVING NORTHEAST TOWARD DIKE.
19:50 UTCHAIL 1AldenHardin, IA
19:53 UTCTSTM WND DMG9 E PopejoyFranklin, IAOUTBUILDINGS DAMAGED. TREES DOWN.
19:53 UTCHAIL 1.752 Nw DikeGrundy, IA
20:00 UTCHAIL 1AldenHardin, IA
20:06 UTCHAIL 0.88Grundy CenterGrundy, IA
20:15 UTCHAIL 1.25TraerTama, IA
20:15 UTCTORNADO10 W Grundy CenterGrundy, IAON STATE HIGHWAY 175
20:16 UTCHAIL 1TraerTama, IA
20:17 UTCHAIL 1DikeGrundy, IA
20:21 UTCHAIL 1WaterlooBlack Hawk, IANICKEL TO QUARTER SIZED HAIL REPORTED BY TRAINED SPOTTER.
20:21 UTCHAIL 1WaterlooBlack Hawk, IA
20:21 UTCTORNADO3 S WaterlooBlack Hawk, IA
20:22 UTCTORNADO5 Nw Grundy CenterGrundy, IA
20:30 UTCHAIL 1.75WaterlooBlack Hawk, IA
20:33 UTCTSTM WND GST 604 Nw Cedar FallsBlack Hawk, IA
20:35 UTCTSTM WND DMGEvansdaleBlack Hawk, IATREES DOWN ON LAFAYETTE ST.
20:41 UTCHAIL 0.88WaterlooBlack Hawk, IA
20:42 UTCHAIL 1.75Grundy CenterGrundy, IAHIGHWAY 175 WEST OF GRUNDY CENTER
20:45 UTCHAIL 0.884 Se WaterlooBlack Hawk, IAESTIMATED 4 MI SE OF WATERLOO, MM65, I380
21:04 UTCHAIL 0.75WaterlooBlack Hawk, IA
21:10 UTCTSTM WND DMG2 N TraerTama, IANUMEROUS TREES DOWN 1-2 MI NORTH OF TRAER
21:16 UTCHAIL 1TraerTama, IA
21:20 UTCHAIL 0.882 Nw DysartTama, IA
21:23 UTCTSTM WND GST 603 N ClutierTama, IA
23:00 UTCHAIL 0.751 Ssw Grundy CenterGrundy, IA
23:02 UTCHAIL 1.75Grundy CenterGrundy, IAWEST SIDE OF CITY.

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

This ground now

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

The other tornadoes of May 21, 2004

23 more were confirmed in the United States that day.

DateTimeRatingWherePath (mi)Width (yd)DeathsInjuriesDamage
May 212:28 PM CDTF0Grundy, IA3.050
May 212:38 PM CDTF1Grundy, IA3.5100$2,000
May 212:49 PM CDTF0Grundy, IA0.530
May 214:02 PM EDTF0Richland, OH0.00
May 213:21 PM CDTF0Black Hawk, IA0.330
May 213:22 PM CDTF0Grundy, IA0.540
May 214:05 PM CDTF0Benton, IA0.150
May 214:07 PM CDTF0Benton, IA0.150
May 214:13 PM CDTF1Benton, IA1.7150$50,000
May 214:22 PM CDTF3Linn, IA3.0200$250,000
May 214:44 PM CDTF0Benton, IA0.050
May 215:06 PM CDTF1Jones, IA6.0100$40,000
May 215:12 PM CDTF1Clay, IA1.0200$230,000
May 216:00 PM CDTF1Pocahontas, IA5.0300$30,000
May 216:08 PM CDTF2Pocahontas, IA12.588015$2.5 million
May 214:16 PM PDTF0Spokane, WA0.270
May 217:30 PM CDTF0Wright, IA0.225
May 219:14 PM EDTF0Berkeley, WV3.01
May 218:20 PM CDTF1Pierce, NE1.5150
May 218:44 PM CDTF0Brown, NE0.220
May 219:20 PM CDTF1Wayne, NE0.5150
May 219:20 PM CDTF0Yankton, SD0.150
May 2110:13 PM CDTF0Garfield, NE0.220

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.