May 7, 2024 Severe Weather Event

A Busy Month Of Severe Weather Begins As Two Tornadoes Touch Down In Walworth County

NWS Milwaukee, WI

Photo of barn damage taken on the survey of the Boone-Walworth County tornado.

Overview

Multiple rounds of strong to severe thunderstorms impacted southern Wisconsin during the afternoon and evening hours on May 7, 2024. Developing in the wake of morning rain showers, the multiple episodes of thunderstorms would result in numerous reports of large hail and damaging winds, in addition to two confirmed tornadoes in Walworth County. This story takes a closer look at the meteorology of the May 7 severe event, the timeline of how it unfolded, and the damage that it caused.

Damage survey result graphic showing the Enhanced Fujita Scale rating, peak wind speed, path length, max width, and injuries/deaths affiliated with the Boone-Walworth County EF0 tornado. — Swipe showing damage survey graphics from the Lake Geneva EF0 tornado (left) and Darien EF1 tornado (right). Graphics made by NWS Milwaukee/Sullivan in the days following the May 7 severe weather event.

Damage survey results graphic showing the Enhanced Fujita Scale rating, estimated peak wind speed, path length, maximum width, and number of injuries & deaths affiliated with the Darien EF1 tornado. — Swipe showing damage survey graphics from the Lake Geneva EF0 tornado (left) and Darien EF1 tornado (right). Graphics made by NWS Milwaukee/Sullivan in the days following the May 7 severe weather event.

Event By The Numbers

Number of warnings issued by NWS Milwaukee/Sullivan during the event. This included 15 severe thunderstorm warnings (yellow), as well as 4 tornado warnings (red).

Total number of storm reports relayed from the public during the event. This included 18 large hail reports (green dots), 14 damaging wind reports (blue squares), as well as 2 confirmed tornadoes (light blue and light green lines).

The Meteorology

a.) Synoptic Environment

Wisconsin was positioned beneath an upper pattern favorable for the development of showers and storms on May 7. In the upper portions of the atmosphere, low pressure was centered over the Dakotas, with ridging apparent over the Great Lakes. A pocket of diffluence, or area of the atmosphere where air is fanning out in different directions, was present between the two features, lingering over the state into the evening hours. Air tends to rise within regions of diffluence, which can aid in the development of thunderstorms when other favorable ingredients are in place.

Animation showing 300 millibar heights (black lines), wind speeds (color fill), wind barbs, and divergence (yellow lines) throughout the day on May 7. Times shown are in UTC (CDT + 5 hours)

A similar pattern was evident in the middle levels of the atmosphere, with several areas of vorticity pivoting around the upper low in the Dakotas. Vorticity would pivot across the state during the afternoon and evening hours, providing additional support for large-scale lift regionally. Given the overlap of upper diffluence and positive vorticity advection in the mid levels, a broadly favorable environment for rising air and related shower/storm formation was coming together by the early afternoon hours on May 7.

Animation showing 500 millibar heights (black lines), vorticity (color fill), and differential positive vorticity advection (DPVA) in the 700-400 millibar layer (blue lines). Times shown are in UTC (CDT + 5 hours)

There can be an abundance of rising motion present in the mid to upper levels of the atmosphere, but thunderstorms will ultimately fail to form if there isn’t support for rising motion in the low levels as well. This support comes in the form of lift along a surface boundary. On May 7, there were several surface fronts in place across the region. An occluded front ran from north-central Minnesota into Wisconsin through the majority of the day. Cold and warm fronts branched out to the southwest and southeast of the occluded front respectively, steadily advancing eastward as the afternoon progressed. With several surface boundaries in place regionally, low level lift was sufficient to work alongside favorable mid-upper forcing to create an environment favorable for shower & thunderstorm development.

Animation showing the evolution of surface fronts throughout the day on May 7. Warm fronts shown in red, cold fronts in blue, stationary fronts in alternating red/blue, and occluded fronts in purple. High/low pressure centers are marked in H/L symbols respectively. Times shown are in UTC (CDT + 5 hours).

b.) Mesoscale Environment

In the presence of a broadly-favorable large scale setup for ascent, the local environment was also becoming conducive to the development of showers and storms moving into the afternoon of May 7. Bounded by the surface boundaries discussed above, a warm sector characterized by 750-1000 Joules per kilogram of mixed layer convective available potential energy (MLCAPE) built into portions of southern Wisconsin approaching the lunch hour on May 7.

This instability materialized thanks to surface dew points in the upper 50s and lower 60s, daytime heating, and steep mid-level lapse rates moving into the area from the Northern Plains.

Animation showing lapse rates in the 700-500 millibar layer (color fill) throughout the day on May 7. Lapse rates within this layer are often referred to generically as "mid-level lapse rates". Times shown are in UTC (CDT + 5 hours).

Given the upper dynamics in place aloft, wind shear was supportive for organized shower and storm activity, with many locations along and east of Interstate 39 seeing values at or above 30-35 knots through the majority of afternoon and evening hours.

Given the sufficient MLCAPE values in place across the area, and their placement within a synoptic environment primed for rising motion, conditions were thus primed for the development of showers and thunderstorms moving into the afternoon hours of May 7. With appreciable shear also in place, the potential for severe storms capable of large hail, damaging winds, and isolated tornadoes also existed.

Event Timeline

12:00 - 4:00 PM May 7: Round 1 Develops & Tracks Over Southeastern Wisconsin

01 / 05

12:00 - 1:30 PM

Showers and thunderstorms quickly begin to develop over north-central Illinois. Activity forms as an area of mid-level vorticity overspreads the surface warm sector and the instability within it. Given the shear in place across southern Wisconsin and northern Illinois, storms quickly begin to organize into clusters.

Animation showing radar imagery from Chicago, Illinois (KLOT), 500 millibar heights (black lines), and vorticity (color fill) during the late morning and early afternoon hours of May 7. Times shown are in UTC (CDT + 5 hours).

2:00 - 2:05 PM

An area of thunderstorms shows signs of rotation over northern Illinois. The first tornado of the day touches down at 2:05 PM over rural portions of northeastern Boone County, Illinois. The tornado stays on the ground for twenty minutes, tracking to the Lake Geneva area prior to dissipating. The tornado is rated EF0 on the Enhanced Fujita Scale, with peak wind speeds of 80 MPH.

Animation showing radar imagery from Milwaukee, WI (KMKX) between approximately 1:55 and 2:05 PM CDT May 7. Reflectivity imagery is shown in the left panel, with storm-relative velocity shown on the right. Rotation becomes evident near the Wisconsin-Illinois state line nearing the end of the loop.

2:10 PM

A tornado warning is issued for southern and southwestern Walworth County as tornadic storms cross the state line.

Tornado warning graphic posted to social media after the tornado warning for southwestern Walworth County was issued. Warning polygon shown on the right with major highways and towns overlaid, and once more in the lower left with the state of Wisconsin. Hazards & impacts highlighted in the warning text are shown in the top left.

2:15 PM

An additional area of radar-indicated rotation develops near Darien, with a second tornado touching down just southwest of town at 2:18 PM. The tornado remains on the ground for just three minutes, tracking just under a mile. The tornado is rated EF1 on the Enhanced Fujita Scale, with peak wind speeds of 90 MPH.

2:30 - 4:00 PM

Storms progress northeast toward the Lake Michigan shoreline. Additional warnings are issued, with a scattering of wind and hail reports being relayed by the public.

5:00 - 9:30 PM May 7: Round 2 Develops & Quickly Tracks Across Southern Wisconsin

01 / 07

4:30 - 6:00 PM

Additional storms begin to develop along the occluded front in the Wisconsin River Valley.

Animation showing radar reflectivity from Milwaukee, WI (KMKX), as well as RAP-derived wind-barbs and frontogenesis (abbreviated FGEN) in the 1000-925 millibar layer.

Nowcast graphic messaging the development of round two of thunderstorm activity during the evening hours of May 7. Still image of radar shown on the right, with approximate occluded front position and storm motions annotated. Key forecast messages included in the top left, with hazards highlighted on the bottom left.

6:30 - 7:00 PM

An area of thunderstorms quickly organizes as it encounters increasing wind shear values in Dane County. Damaging wind and hail potential appears to be rising, with additional warnings possible.

Animation showing radar imagery from Milwaukee, WI (KMKX), effective bulk shear magnitudes (blue contours), and effective bulk shear vectors (tan barbs) between approximately 6:30-7:00 PM CDT May 7. Storm organization becomes apparent as cells encounter higher shear over Dane County.

7:08 PM

A severe thunderstorm warning—the first warning of round two—is issued for portions of north-central portions of Dane County.

Graphic posted to social media following the issuance of the severe thunderstorm warning for central Dane County. Warning polygon overlaid atop major roads and cities on the right, as well as the state of Wisconsin in the lower left. Hazards highlighted in the warning text are summarized on the top left.

7:15 - 7:45 PM

Signs of rotation begin to appear on radar as the line of storms interact with increasing shear on their march east.

7:56 PM

With rotation continuing to strengthen on radar, a tornado warning is issued for central and western portions of Jefferson County. Fortunately, no tornado reports are received from the public.

8:00 - 8:20 PM

Additional tornado warnings are issued as rotation persists within the line of storms. A tornado warning is issued for eastern Jefferson and west-central Waukesha Counties at 8:15 PM, with an additional tornado warning for most of Walworth County being issued five minutes later at 8:20 PM. No tornado reports are received.

8:30 - 9:30 PM

Storms continue to move east, ultimately exiting over Lake Michigan. Additional severe thunderstorm warnings are issued, with several reports of straight line wind damage coming out of Brookfield and Menomonee Falls.