Tornadoes · United States · Iowa · 1964

F2 tornado, Delaware County, Iowa, May 4, 1964

This F2 tornado ran 4.7 miles across Delaware County on May 4, 1964, touching down at 7:15 PM GMT-5 and reaching 200 yards wide. It killed nobody and injured 2, and did $50,000 to $500,000 of property damage.

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 mph

    Moderate damage. Roofs badly stripped, mobile homes overturned or badly damaged, exterior doors lost, windows broken.

  3. F2113 to 157 mphthis tornado

    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
F2
Date
May 4, 1964
Touchdown
7:15 PM GMT-5 (00:15 UTC)
Path length
4.7 miles (7.6 km)
Path width
200 yards (183 m)
Deaths
0
Injuries
2
Property damage
$50,000 to $500,000
County
Delaware County
Touchdown point
42.330, -91.250
Lift-off point
42.380, -91.170
Record number
225

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.

Peoria Regional, IL (KPIA), 00:00Z

0.3 hours before the tornado, 230 km from the path. May 5, 1964.

Skew-T log-pSkew-T log-p diagram, Peoria Regional, IL at 00:00Z on 1964-05-05Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel lifted from the surface. Mixed-layer CAPE 575 joules per kilogram, CIN -112, lifted condensation level 1430 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, Peoria Regional, IL at 00:00Z on 1964-05-05The 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 43 square meters per square second, 0 to 6 km bulk shear 37 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
575 J/kg
Mixed-layer CIN
-112 J/kg
Surface CAPE
701 J/kg
Mixed-layer LCL
1430 m above ground
0-6 km bulk shear
37 kt
0-1 km bulk shear
15 kt
0-1 km storm-relative helicity
43 m²/s²
0-3 km storm-relative helicity
165 m²/s²
Significant tornado parameter
0.1
Supercell composite
0.4
0-3 km lapse rate
6.5 K/km

Open this sounding in Storm Lab

Peoria Regional, IL (KPIA), 12:00Z

11.7 hours after it, 230 km from the path. May 5, 1964.

Skew-T log-pSkew-T log-p diagram, Peoria Regional, IL at 12:00Z on 1964-05-05Temperature and dew point through the depth of the atmosphere, with the mixed-layer parcel lifted from the surface. Mixed-layer CAPE 23 joules per kilogram, CIN -407, lifted condensation level 1048 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, Peoria Regional, IL at 12:00Z on 1964-05-05The 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 102 square meters per square second, 0 to 6 km bulk shear 40 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
23 J/kg
Mixed-layer CIN
-407 J/kg
Surface CAPE
0 J/kg
Mixed-layer LCL
1048 m above ground
0-6 km bulk shear
40 kt
0-1 km bulk shear
17 kt
0-1 km storm-relative helicity
102 m²/s²
0-3 km storm-relative helicity
125 m²/s²
Significant tornado parameter
0.0
Supercell composite
0.0
0-3 km lapse rate
4.0 K/km

Open this sounding in Storm Lab

The surface observations

Dubuque (DBQ), 41 km from the path, in the two hours either side.

Time (UTC)Temp (°F)Dew point (°F)WindGust (kt)Pressure (mb)Weather
23:00 UTC7563180° 23 kt1008.1
00:00 UTC7563190° 25 kt1007.1-TSRA
01:00 UTC7264180° 22 kt1008.1-TSRA
02:00 UTC7064170° 17 kt1008.1TSRA

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

This ground now

The live radar over the trackEvery 1964 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

The other tornadoes of May 4, 1964

7 more were confirmed in the United States that day.

DateTimeRatingWherePath (mi)Width (yd)DeathsInjuriesDamage
May 45:40 PM GMT-5F2Jackson, WI9.72002$50,000 to $500,000
May 46:30 PM GMT-5F2Linn, IA2.32001$50,000 to $500,000
May 47:10 PM GMT-5F1Vilas, WI2.2127$5,000 to $50,000
May 47:10 PM GMT-5F2Allamakee, IA9.2200$50,000 to $500,000
May 47:15 PM GMT-5F2Crawford, WI12.62002$50,000 to $500,000
May 49:00 PM GMT-5F2Juneau, WI7.7100$5,000 to $50,000
May 49:30 PM GMT-5F1Juneau, WI1.027$500 to $5,000

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.