F1 tornado, Spring Green to Merrimac, Wisconsin, August 18, 2005

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

August 18, 2005

Touchdown

5:30 PM CDT (22:30 UTC)

Path length

1.9 miles (3.1 km)

Path width

75 yards (69 m)

Deaths

0

Injuries

0

Property damage

$2,000

County

Sauk County

Touchdown point

43.170, -90.080

Lift-off point

43.180, -90.050

Record number

1136

The largest single-day tornado outbreak in Wisconsin recorded history for south-central and southeast Wisconsin occurred on August 18, 2005. Sixteen tornadoes were documented on this day in south-central and southeast Wisconsin, a new single-day record. A line of supercells developed and pushed across south-central Wisconsin during the afternoon hours, and then pushed east across southeast Wisconsin during the evening hours. Most of the tornadoes were associated with two supercells. One main supercell tracked across Sauk County and then into Columbia, producing one F0 and one F2 tornado. The F2 tornado crossed into Columbia County. Another main supercell produced an F3 tornado that devastated the area north of city of Stoughton (Dane Co.), and a weaker F1 tornado just south of the city of Rockdale (Dane Co.). This supercell then tracked into southwestern Jefferson County a short time later, producing two F0 tornadoes and four F1 tornadoes. The sixteen (16) tornadoes contributed largely to both the new state record of 27 tornadoes on a single day, and 62 tornadoes for a calendar year. In additiion, there were scattered reports of funnel clouds, damaging straight-line, downburst winds, and large hail across south-central and southeast Wisconsin. The total direct damage estimate for the afternoon and evening tornado and severe weather outbreak in south-central and southeast Wisconsin on August 18th was about $36.6 M ($35.7 property and $948 K crop). Additional indirect costs totaled almost $1.9 M, resulting in a total direct and direct cost of about $38.5 M. Synoptically, a surface low pressure system was located over extreme southeast Minnesota early in the afternoon. A warm front extended east southeast from the low with dew points pooling in the lower 70s along it. The surface low moved east into east central Wisconsin by 2100CST that evening. Favorable wind shear associated with the warm front, combined with the strong instability supplied by the heat and humidity, helped to produce numerous tornadic supercells.

This tornado in Sauk County was a continuation of the "Clyde" tornado from Iowa County. It crossed into Sauk County at 1626CST at a point 4 miles west-southwest of Spring Green, or 1.0 miles southwest of the intersection of Kennedy and Dyke Roads. The tornado continued northeast 3.3 miles and dissipated at 1633CST about 2.3 miles NW of Spring Green, or 0.7 mile north-northwest of the intersection of Pearl Rd. and STH 14/60, just short of CTH G. Only tree, power-line, and some crop damage was noted, resulting in a F1 rating, with estimated wind speeds of 64-97 knots (73-112 mph). This tornado was well photographed. Average path width was about 50 yards.

This tornado spun up at 1630CST about 1.5 miles south-southwest of Spring Green, just inside the Sauk County line on the Wisconsin River. The tornado headed north-northeast for about 1.9 miles and dissipated at 1633CST about 0.5 mile east-northeast of Spring Green, on Rainbow Rd. just west STH 14. Minor roof damage in the village of Spring Green was noted on a couple buildings, and some large trees were uprooted, resulting in a rating of F1, and an estimated wind speed of 64-97 knots (73-112 mph). Average path width was about 50 yards.

A tornado spun up 1.8 miles west of Merrimac at 1700CST, along STH 78/113. The tornado headed southeast and dissipated 1.0 mile WSW of Merrimac at 1705 CST just short of the Sauk-Columbia County line in the Wisconsin River. Damage was confined to trees and power-lines. This tornado was rated an F0 with estimated wind speeds of 35-63 knots (40-72 mph). Average path width was about 50 yards.

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

53 minutes of warning. NWS MKX issued a tornado warning for this county at 21:37 UTC, 53 minutes before the tornado touched down, in force until 22:30 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, with a 0.05 chance of a tornado within 25 miles. The outlook in force was issued at 19:57 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 La Crosse radar (KARX), 116 km away, was scanning. The volume nearest the touchdown began at 22:27 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 2005 tornado in WisconsinEvery tornado in Wisconsin since 1950Warnings in force in Wisconsin nowThe forecast where you are

27 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.