F0 tornado, Mason City, Iowa, April 17, 2004

1. F040 to 72 mphthis tornado

   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 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

F0

Date

April 17, 2004

Touchdown

9:40 PM CDT (02:40 UTC)

Path length

0.2 miles (0.3 km)

Path width

25 yards (23 m)

Deaths

0

Injuries

0

County

Cerro Gordo County

Touchdown point

43.150, -93.130

Lift-off point

43.150, -93.130

Record number

21

The cold front that had moved southeast across the state the previous night became stationary across southern Iowa by the morning of the 17th. A sharp contrast in dew point temperatures was found across the front. Dew points were in the upper 50s to mid 60s south of the front, with dew points in the 30s north of it. During the late afternoon the front began to return north as a warm front. By evening a strengthening southerly jet in the low levels impinged on the front with wind speeds around 50 kts. The overall low levels of the atmosphere were relatively dry and the airmass was capped initially. The LCL height was around 750 mb. There atmosphere was quite unstable with Lifted Indices in the -6 to -10 C range. CAPE values were in the 2500 to 3500 J/kg range. Thunderstorms erupted over northern Iowa shortly after sunset. The storms formed into discrete supercells. A tornado touched down east of Mason City from one of the stronger cells. It was in open area and did not cause any damage during its brief touchdown. The dominant severe weather mode for the event was hail. Baseball size hail fell in Palo Alto County in Mallard. There were numerous reports of hail three quarters of an inch to one and one quarter of an inch in diameter. The largest hail was around two inches in diameter in Palo Alto County. Hail reportedly covered the ground as the storms passed. During the late stages of the event, a thunderstorms produced high winds as it swept across Interstate 35 west of Northwood in Worth County. Two vehicles and one semi-tractortrailer truck were blown off of the interstate near mile post 214. In addition, one telephone was blown down by the high wind.

Brief touchdown in open area east of Mason City.

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

3 minutes of warning. NWS DMX issued a tornado warning for this county at 02:37 UTC, 3 minutes before the tornado touched down, in force until 03:15 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, with a 0.05 chance of a tornado within 25 miles. The outlook in force was issued at 01:07 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), 165 km away, was scanning. The volume nearest the touchdown began at 02:40 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

1 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.