The Boscobel, WI EF-3 Tornado

On the afternoon of August 7, 2021, a powerful tornado carved a 10+ mile path of destruction around the south side of the town of Boscobel and across large expanses of rural areas east of town. Damage throughout much of its path was consistent with EF1 to EF2 rating levels. However, in an area of valley-enhanced winds several miles east of Boscobel, one pocket of EF3 damage was found. In addition, the tornado will be remembered for its expansive damage track. The combination of inflow (light green areas on right image) and tornadic winds resulted in damage swaths that were over 1.5 miles wide at times. Note that the actual maximum tornado width was estimated at 1100 yards (0.6 miles).

Follow along as we take you along the damage track using radar imagery, data from the NWS post-event ground survey, and satellite imagery taken several days after the tornado.

The background satellite imagery used for this survey comes from the Sentinel-2 satellite network, whose 10-m imagery, acquired every few days, allows for a quick and modestly detailed assessment of the damage track. This particular pass was taken on August 14 and will be used for the background on all track maps. In the map on the right, use the slider bar to toggle before (right) and after (left) imagery of the track. Notice the tight track just south of town and the widespread tree damage (brown areas) to the east of town.

The maps on the following pages will allow you to view the damage survey results on top of the satellite imagery taken after the event. Use the map sliders to show/hide the damage survey information. Each map point represents a marker from the NWS survey and can be clicked for more information. Please note that the damage polygons in these maps are approximate and may not encompass the full extent of the damage.

For reference, the following wind speed ranges are associated with the EF Scale:

The supercell thunderstorm that would produce this tornado developed east of Prairie du Chien, along a warm front draped across southwest Wisconsin, between 3:30 and 4:00pm. The storm rapidly intensified within an airmass characterized by high instability and favorable low-level wind shear. A tornado warning was issued at 4:19pm CDT for the area that would soon experience this tornado.

The image to the right shows the radar reflectivity signature of the storm at the moment the tornado warning (purple polygon) was issued for northeastern Grant County. You can clearly pick out the hook echo to the west of Boscobel. The circulation of the mesocyclone (not shown) was also rapidly strengthening and contracting at this same time.

The tornado developed along the Wisconsin and Southern Railroad just southwest of Boscobel at 4:29pm CDT and was initially a narrow vortex as it tracked to the southeast and across State Highway 133. As it approached the True Value Hardware store on the very south side of town, the tornado made a turn to the east and began to steadily intensify as inflow winds fed into the tornado. These winds caused widespread tree limb and powerline damage across the southern side of Boscobel. After tracking over U.S. Highway 61, the tornado turned to the east-northeast and raced across open fields and into the town cemetery.

Tornado track through a corn field. — Damage path through corn on the southwest side of town (NWS Survey).

The tornado widened rapidly as it descended a hill on the south side of town with powerful inflow working further into the circulation. Winds within the inflow reached EF1 levels before working into the tornado circulation, with thousands of trees damaged or destroyed by both the tornado and inflow winds. Several houses along County Road MS sustained damage, with the highest damage rating of a low EF2 to one house.

House damaged by a tornado with walls and roof missing. — Low EF2 damage to a house and the surrounding trees. (NWS Storm Survey)

Velocity and correlation coefficient radar data showing the tornado and debris signature — A tornadic debris signature (TDS) develops on the La Crosse WSR-88D radar imagery between 4:34pm and 4:37pm CDT. This TDS shows up as a circular region of lower correlation coefficient values on the right pane of the animation (growing blue circle). This signal is seen when debris (e.g., leaves, corn, housing materials) are lofted into the air. The left pane features storm-relative velocity imagery.

Very few videos exist of the tornado itself, owing to the rugged topography of the region, low cloud bases, and the circulation being wrapped in rain at times. This video shows the tornado for a brief time, taken from a vantage point WSW of town along Wisconsin Highway 133 as the tornado was moving away from the photographer. Note the impressive wall cloud and low-level mesocyclone (seen as a more broad rotating cloud "ring") above the tornado.

Drawing over a video screenshot showing the tornado and rotating wall cloud

The half-mile wide tornado continued eastward through rural areas of Grant County. As the tornado passed over a north-south valley along Kiwanis Road, a narrow jet looks to have formed along the valley floor and parallel to the valley orientation. This extra burst of winds caused EF3 damage to a single house almost directly under the center of the circulation. Much of the debris from the house was deposited 100+ yards to the NNE. The only portion of the house to survive was, ironically, the attached garage. The garage faced to the northeast and was shielded by the southerly winds. The house sported a walkout basement that faced south, which was the likely point of failure with the structure.

House destroyed by a tornado east of Boscobel — EF3 damage to a house along Kiwanis Road. The only portion of the two story house still standing was the northeastward facing garage. The southern edge of the house foundation extends to the left edge of the photo frame. (NWS Storm Survey)

After crossing County Road T, the tornado underwent an occlusion, in which the circulation began to contract and separate from the parent mesocyclone in the supercell. This occlusion process caused the tornado to take a hard turn to the right (southeastward) and steadily weaken.

Radar loop — A reflectivity (top left), storm-relative velocity (top right), spectrum width (bottom left), and correlation coefficient (bottom right) image loop as the tornado occludes and dives southeastward. This is best seen in the storm-relative velocity data.

The tornado steadily narrowed and weakened as it tracked southeastward over the forests and farm fields of rural Grant County. The tornado dissipated as it skirted over Neff Road at 4:56pm CDT, almost a half hour after it first developed southwest of Boscobel. It stands as one of the stronger tornadoes in recent Wisconsin weather history, similar in strength to tornadoes that occurred by Elk Mound in 2019 and Chetek in 2017.