F1 tornado, Martensdale to Norwalk, Iowa, June 18, 1998

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

June 18, 1998

Touchdown

2:04 PM CDT (19:04 UTC)

Path length

13.0 miles (20.9 km)

Path width

40 yards (37 m)

Deaths

0

Injuries

0

Property damage

$10,000

County

Warren County

Touchdown point

41.380, -93.750

Lift-off point

41.520, -93.630

Record number

363

A warm front extended from a low pressure area over northeast Nebraska, southeast through central into southeast Iowa. A cold front extended south from the low into Kansas. South of the warm front, dew point temperatures were in the low 70s. A speed max passed to the northwest of the state, placing Iowa in the right rear entrance region of the speed max. This set the stage for thunderstorm development. A strong low level jet was in place with speeds in excess of 50 knots. Initially, a small like of strong, but not severe, thunderstorms formed from southwest into north central Iowa. As the morning progressed, the storms initially produced hail of three quarters inch up to golf ball in size. The hail event quickly turned into a wind event as the storms accelerated along the line. Wind damage was widespread. A roof was taken off of a house in Des Moines as one of the cells moved through. A later storm took a roof off of a building in West Des Moines later in the morning. Wind damage was also significant in Webster and Hamilton Counties as the storms moved through. Numerous reports of sheds blown over were received. Some of the pieces from the sheds were found over a mile downwind. A school roof was 40 percent removed in Blairsburg by high winds. Damage to the building was placed at $150,000. The next problem was with the storms training over the same areas. The cells moved northeast along the line with little eastward progress of the line itself. This caused urban and small stream flooding over a several county area as widespread rainfall of one to two inches occurred in several central Iowa counties. The situation became more serious in a several county area. Rainfall of three to five inches was recorded in parts of Dallas, Madison, Polk, Story, and Warren Counties. This resulted in flash flooding as all creeks and streams swelled over their banks. The situation became very serious in the larger cities. In Des Moines, a state of emergency was declared as water of 6 feet or greater in depth choked many of the city intersections. Hundreds of cars were stalled in the streets and many basements were flooded. Between the high winds and water, electricity was knocked out to 15000 homes in Des Moines during the height of the storm. Damage estimates were in the hundreds of thousands of dollars in all of the flash flood counties, and over one million dollars in Polk County, where the Governor declared a state disaster. A disaster declaration was also made for Dallas County. Reports indicated that in the Des Moines metropolitan area, up to 10,000 homes had water in the basement. The city of Des Moines reported that infrastructure losses alone around $1 million. Crop damage estimates from this event, combined with the event of just a few days earlier, totalled around $100 million. During the early afternoon hours, the cold front over Nebraska began moving east, resulting in a change in the thunderstorm configuration. Several bow echo elements formed along the line which caused another round of high winds and also began shifting the line east. From Des Moines east, numerous places reported winds of 60 to 75 MPH, along with considerable damage. Seventy two MPH winds buffeted Grinnell in Poweshiek County, breaking out windows in homes there. Winds approached 85 MPH west of Montezuma causing extensive damage on several farms there. Two semi-tractortrailor trucks were blown off of I-380 southeast of Waterloo in Black Hawk County. The highest wind reported was in Tama County, at Montour where 90 MPH winds struck the city, resulting in widespread damage. One tornado touched down south of Des Moines. The tornado was small and rain wrapped and produced an intermittent track of minor damage from near Martensdale to northeast of Norwalk in Warren County. Scattered thunderstorms formed ahead of the main area of thunderstorms during the late morning hours. Initially, they were not severe, but one did produce a lightning stroke in Grinnell that touched off a minor fire in a house there.

Intermittent track

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

8 minutes of warning. NWS DMX issued a tornado warning for this county at 18:56 UTC, 8 minutes before the tornado touched down, in force until 19:30 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 Slight Risk. The outlook in force was issued at 14:31 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.

The Des Moines radar (KDMX), 31 km away, was scanning. The volume nearest the touchdown began at 19:04 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.

The live radar over the trackEvery 1998 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

21 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.