F1 tornado, Hartford to Swan, Iowa, May 18, 2000

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 18, 2000

Touchdown

12:01 PM CDT (17:01 UTC)

Path length

2.0 miles (3.2 km)

Path width

30 yards (27 m)

Deaths

0

Injuries

0

Property damage

$10,000

Counties

Warren and Marion counties

Touchdown point

41.470, -93.380

Lift-off point

41.470, -93.320

Record number

771

A very dynamic weather situation took shape over Iowa. The strength of the system was more typical for for mid April rather than mid May. An intense low pressure center developed over the central Rockies. A warm front extended east from the low across Iowa. To the south of the warm front, humid air was in place with dew point temperatures in the mid 60s to around 70. Strong winds above the surface lifted the moist and unstable air over the top of the front. Initially a round of thunderstorms fired north of the front. The line became severe quite quickly and produced hail of three quarters of an inch to one and one quarter inch in diameter. This east to west line of storms produced a strong outflow boundary that propagated south across the central and southern counties of the state. By late morning, a round of thunderstorms developed over southern Iowa as the upper level low pressure system advanced into southwest Iowa. The storms became severe as they moved north, then intensified even more as the line of storms from southern Iowa interacted with the outflow boundary from the storms to the north. The storms produced several tornadoes, however most of them were very small and could be best described as spinup tornadoes. The strongest one formed at the intersection of the of the line of storms and the outflow boundary in the Des Moines metro area. A tornado touched down in Warren County east of Hartford. This tornado was on the ground for a couple of miles, though did little damage as it was in open country moving from Warren into Marion County. Another small tornado touched down in Poweshiek County south of Deep Rock. It formed along the boundary with a strong updraft into the parent cumulonimbus cloud. The dirt it picked up made it visually apparent even though there was no condensation funnel visible. The greatest damage occurred from the hail. Many of the storms produced one inch or larger diameter hail. The storm that developed southeast of Des Moines and spawned the largest tornado also produced very large hail. Hail up to baseball size was reported in Jasper County in a few places, with one report of three inch diameter hail southeast of Prairie City. Some areas reported a hail depth of 3 inches, with drifts a few feet deep, after the storm had passed. The hail was still on the ground six hours after the storm moved through. Needless to say, the hail caused considerable damage to cars and roofs in the area. With the exception of the tornadoes, there was not a great deal of wind associated with the storms. Most of the wind gusts were under 40 MPH. One of the storms did produce a wind gust of around 60 MPH in the Waterloo area.

Moved from Warren County into Marrion County.

Moved in from Warren County.

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

10 minutes of warning. NWS DMX issued a tornado warning for this county at 16:51 UTC, 10 minutes before the tornado touched down, in force until 17:20 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 16:25 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.

Des Moines (DSM), 26 km from the path, in the two hours either side.

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

The Des Moines radar (KDMX), 42 km away, was scanning. The volume nearest the touchdown began at 17:00 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 2000 tornado in IowaEvery tornado in Iowa since 1950Warnings in force in Iowa nowThe forecast where you are

36 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.