Tornadoes · United States · Iowa · 2004

F1 tornado, Harlan to Walnut, Iowa, May 8, 2004

From Harlan to Walnut, this F1 tornado ran 8.0 miles across Shelby and Pottawattamie counties on May 8, 2004, touching down at 8:55 PM CDT and reaching 150 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 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.

The record

Rating
F1
Date
May 8, 2004
Touchdown
8:55 PM CDT (01:55 UTC)
Path length
8.0 miles (12.9 km)
Path width
150 yards (137 m)
Deaths
0
Injuries
0
Counties
Shelby and Pottawattamie counties
Touchdown point
41.530, -95.270
Lift-off point
41.430, -95.170
Record number
931

What the Weather Service recorded

A tornado touched down in southern Shelby county around 9 miles south southeast of Harlan. The tornado tracked into Pottawattamie county on the west side of Walnut. There it strengthened to a f1 category, damaging buildings and roofs in town. One garage was destroyed and a radio tower was overturned. A grain bin was destroyed and carried for about a half of a mile. The tornado lifted around 4 miles south southeast of Walnut. The total path length was around 8 miles and it was on the ground for about 11 minutes.

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

The warnings

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

Issued (UTC)ExpiredProductOffice
01:32 UTC02:15 UTCSevere Thunderstorm WarningOAX

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, with a 0.02 chance of a tornado within 25 miles. The outlook in force was issued at 16:16 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.

Omaha, NE (KOAX), 00:00Z

1.9 hours before the tornado, 98 km from the path. May 9, 2004.

Skew-T log-pSkew-T log-p diagram, Omaha, NE at 00:00Z on 2004-05-09Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel lifted from the surface. Mixed-layer CAPE 2163 joules per kilogram, CIN -22, lifted condensation level 2150 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, Omaha, NE at 00:00Z on 2004-05-09The 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 20 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
2163 J/kg
Mixed-layer CIN
-22 J/kg
Surface CAPE
2655 J/kg
Mixed-layer LCL
2150 m above ground
0-6 km bulk shear
19 kt
0-1 km bulk shear
6 kt
0-1 km storm-relative helicity
20 m²/s²
0-3 km storm-relative helicity
104 m²/s²
Significant tornado parameter
0.0
Supercell composite
0.0
0-3 km lapse rate
8.2 K/km

Open this sounding in Storm Lab

Omaha, NE (KOAX), 12:00Z

10.1 hours after it, 98 km from the path. May 9, 2004.

Skew-T log-pSkew-T log-p diagram, Omaha, NE at 12:00Z on 2004-05-09Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel lifted from the surface. Mixed-layer CAPE 6 joules per kilogram, CIN -528, lifted condensation level 1348 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, Omaha, NE at 12:00Z on 2004-05-09The 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 44 square meters per square second, 0 to 6 km bulk shear 22 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
6 J/kg
Mixed-layer CIN
-528 J/kg
Surface CAPE
11 J/kg
Mixed-layer LCL
1348 m above ground
0-6 km bulk shear
22 kt
0-1 km bulk shear
23 kt
0-1 km storm-relative helicity
44 m²/s²
0-3 km storm-relative helicity
227 m²/s²
Significant tornado parameter
0.0
Supercell composite
0.0
0-3 km lapse rate
4.4 K/km

Open this sounding in Storm Lab

The radar

The Omaha radar (KOAX), 97 km away, was scanning. The volume nearest the touchdown began at 01:53 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
01:43 UTCHAIL 0.75EarlingShelby, IA
01:43 UTCHAIL 0.75EarlingShelby, IA
01:43 UTCHAIL 0.75EarlingShelby, IA
01:45 UTCHAIL 0.88IrwinShelby, IA
01:45 UTCHAIL 0.88IrwinShelby, IA
01:45 UTCHAIL 0.88IrwinShelby, IA
01:50 UTCHAIL 0.75HarlanShelby, IA
01:50 UTCHAIL 0.75HarlanShelby, IA
01:50 UTCHAIL 0.75HarlanShelby, IA
01:55 UTCTSTM WND GST 705 S HarlanShelby, IA
01:55 UTCTORNADO 09 Sse HarlanShelby, IAF0 TORNADO STARTED SOUTH SOUTHEAST OF HARLAN AND PRODUCED DAMAGE TO MAINLY TREES IN SHELBY COUNTY. TORNADO CONTINUED INTO POTTAWATTAMIE COUNTY.
02:00 UTCTSTM WND GST 701 N WalnutPottawattamie, IATRUCK BLOWN OVER ON I80. WIND SPEED ESTIMATED.
02:00 UTCHAIL 1AvocaPottawattamie, IA
02:00 UTCHAIL 1AvocaPottawattamie, IA
02:00 UTCHAIL 1AvocaPottawattamie, IA
02:06 UTCTORNADO 14 Sse WalnutPottawattamie, IATORNADO STARTED IN SHELBY COUNTY AT 855 PM AND CONTINUED TO MOVE SOUTH SOUTHEAST ACROSS POTTAWATTAMIE COUNTY INCREASING TO F1 STRENGTH NEAR AND SOUTH OF WALNUT BEFORE DISIPATING AT 906 PM. THERE WAS DAMAGE TO BUILDINGS A
02:08 UTCFUNNEL CLOUDWalnutPottawattamie, IA
02:08 UTCFUNNEL CLOUDWalnutPottawattamie, IA
02:08 UTCFUNNEL CLOUDWalnutPottawattamie, IA

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 8, 2004

3 more were confirmed in the United States that day.

DateTimeRatingWherePath (mi)Width (yd)DeathsInjuriesDamage
May 87:00 PM CDTF0Colfax, NE0.250
May 87:40 PM CDTF0Colfax, NE0.330
May 89:25 PM CDTF1Cass, IA7.050$100,000

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.