F1 tornado, Lewis, Iowa, May 8, 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

May 8, 2004

Touchdown

9:25 PM CDT (02:25 UTC)

Path length

7.0 miles (11.3 km)

Path width

50 yards (46 m)

Deaths

0

Injuries

0

Property damage

$100,000

County

Cass County

Touchdown point

41.300, -95.030

Lift-off point

41.250, -95.020

Record number

77

A warm, moist, and unstable airmass was in place over Iowa. The warm front that had produced the convection during the early morning hours of the 8th had become stationary over northern Iowa. With lifted indices below -10 C. over central Iowa and continued strong moisture advection, thunderstorms erupted over the central and northeast parts of the state during the late afternoon and evening hours. For the most part, the thunderstorms produced hail during this event. There were numerous reports of hail of one to two inches in diameter. Hail reportedly covered the ground in Guthrie County, into parts of Dallas County. During the later evening hours, the thunderstorms began to sink south and took on a bow echo type configuration, though the line was not a true bow echo. High winds began to be reported with Cass County being hit the worst. A swath of damage 7 miles long was reported from west of Lewis to southeast of Lewis. Embedded in the winds was a small tornado. The tornado touched down intermittently along the 7 mile path and destroyed or damaged several grain bins and downed numerous trees. A large corn crib was destroyed southeast of Atlantic and there were numerous reports of trees over highways. In addition to the hail and high winds, heavy rain occurred from central into parts of west central Iowa. Flash flooding occurred in Audubon, Guthrie, Madison, and Adair Counties. Rain fell as the thunderstorms trained and redeveloped repeatedly over the same areas. Rainfall rates were in the one to three inch per hour range with rainfall total reports of over 4 inches received in a few places in under two hours. Water flooded over highways, even some U.S. Highways, to as depth of one to two feet. This resulted in closure of some highways.

Intermittent path through rural areas.

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

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

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, 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 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.

Atlantic (AIO), 15 km from the path, in the two hours either side.

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

The Omaha radar (KOAX), 112 km away, was scanning. The volume nearest the touchdown began at 02:23 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.

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

3 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.