F1 tornado, Grundy Center, Iowa, May 21, 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 21, 2004

Touchdown

2:38 PM CDT (19:38 UTC)

Path length

3.5 miles (5.6 km)

Path width

100 yards (91 m)

Deaths

0

Injuries

0

Property damage

$2,000

County

Grundy County

Touchdown point

42.420, -92.770

Lift-off point

42.400, -92.720

Record number

898

A very unstable airmass was over Iowa and helped kick off the seasons first severe weather outbreak. At the surface a warm frontal boundary extended nearly east to west across the state and provided the focus for thunderstorm development. By the late afternoon, lifted indices were approaching -10 C. with CAPE values around 5000 J/kg over western Iowa. Surface temperatures warmed into the upper 80s with dew points in the low to mid 70s. A southwest surface wind of 15 to 25 kts pushed over the front. Northeast of the frontal boundary winds were easterly around 10 to 15 kts. Actually, the surface boundary was further enhanced by outflow from convective complexes during the day, further sharpening the contrast. Thunderstorms continued to fire along and north of the boundary through the afternoon and into the evening. This resulted in widespread flash flooding as very heavy rains fell on areas that had had significant rainfall the previous night. There were reports in north central into northeast Iowa of 2 to 4 inches of rainfall in a little more than an hours time. Major flooding took place in the Mason City area where evacuations were taking place. The thunderstorms became most intense during the afternoon into the early evening. There were several tornadoes across northeast into north central Iowa. Most were relatively brief touchdowns and were in open areas. Reports of multi-vortex tornadoes were received from Grundy County. The days most significant tornado touched down in Pocahontas County east of Rolfe and tracked into Humboldt County through the Bradgate area. The tornado damaged or destroyed over 75% of the town of Bradgate (pop 120). Tornado damage in Bradgate was very extensive impacting most of the town. Hardly a building was not impacted. Outbuildings, light structures and garages were heavily damaged or destroyed. Two homes were destroyed. Several homes were damaged ranging from minor to extensive. The Survey Team found that the majority of the damage was F1, with a few cases of F2 damage. The F2 damage was to the snapping of very large trees just west of town and the structural damage to two buildings. Nearly all of the 53 homes there reported damage with several totally destroyed. Significant widespread property damage was reported in the tornadoes path. There were several injuries in the town, the most significant being a broken leg. A police officer was reportedly hit by debris from the tornado and suffered a broken leg. According to law officials, the damage path of the tornado was 2 miles wide and about 8 miles long. Storm chasers on the scene reported the tornado itself to be one half mile wide at one point. Reports of hail were numerous through the afternoon and evening. Most of the hail was in the three quarter inch to one inch diameter range. There were a few reports of golf ball size hail from the stronger storms. Winds were not a major problem with the activity. There were spotty reports of winds to around 60 MPH, but most reports were in the 40 to 50 MPH range. One of the stronger wind reports was a 64 MPH wind gust in Tripoli in Bremer County. This gust was measured by a mesonet station there. During the mid evening hours, thunderstorms rumbled across Grundy County. Lightning struck the Sheriff's office there and destroyed a significant amount of office equipment. Damage was reported around $100,000.

Tornado touched down in open fields northwest of Grundy Center.

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

36 minutes of warning. NWS DMX issued a tornado warning for this county at 19:02 UTC, 36 minutes before the tornado touched down, in force until 19:45 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 Moderate Risk, with a 0.05 chance of a tornado within 25 miles. The outlook in force was issued at 16:32 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.

Waterloo (ALO), 32 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), 111 km away, was scanning. The volume nearest the touchdown began at 19:38 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

23 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.