May 21, 2024 Severe Weather Event

Multiple rounds of storms bring significant damaging winds, hail, and tornadoes to southern Wisconsin

NWS Milwaukee, WI

Two large and nearly empty grain bins that sustained extensive wind damage on May 21, 2024. Both bins have had their tops blown off and the side wall dented and blown inward. Both bins are still standing despite the damage.

Overview

A significant severe weather event unfolded across the NWS Milwaukee/Sullivan county warning area during the afternoon and evening hours of May 21, 2024. Multiple clusters of severe thunderstorms tracked across the region during this timeframe, bringing widespread wind damage and reports of hail. Multiple wind gusts between 70 and 100 MPH would be reported from the Madison metro southwest into Lafayette County. NWS damage surveys would confirm two EF1 tornadoes—one tracking through Mount Horeb and another near Lake Neshkoro in northern Marquette County. This story recaps the key numbers, meteorology, and forecast evolution leading into the May 21, 2024 severe weather event.

Event By The Numbers

Number of warnings issued by NWS Milwaukee/Sullivan during the event. 29 severe thunderstorm warnings (yellow) and 13 tornado warnings (red) would be issued. The warnings would cover every square mile of 13/20 southern Wisconsin counties, with portions of 6 more included in at least 1 polygon. Kenosha County was the only area county not touched by a warning polygon.

Number of storm reports received by NWS Milwaukee/Sullivan during the event. 74 damaging wind reports (blue squares), 9 large hail reports (green circles), as well as 2 confirmed tornadoes (lime green lines) contributed to this total.

The Meteorology

Synoptic Environment

The upper pattern was characterized by mean troughing over the Mountain West throughout the day on May 21, with neutral to rising heights apparent across the northeastern US, Ontario, and Quebec. Swinging through the base of the western trough, a cyclonically-curved jet streak progressed from the Central Plains into the Upper Mississippi Valley during the afternoon hours. Upper level divergence rapidly began to increase across the region as the jet approached, providing an abundance of large-scale ascent. Said ascent would aid in the development of thunderstorms, particularly during the late afternoon & evening phase of the event.

An animated gif of the RAP thirteen’s 300 millibar heights, winds and divergence. 300 millibar heights are the black lines. Winds are represented by the grey wind barbs, with values greater than 50 knots highlighted by blue polygons. The yellow lines represent Divergence. The gif shows an upper level trough advecting east from the Pacific northwest toward the Great Lakes Region with a building upper level jet shown by the growing blue polygon. As these features shift east, Divergence grows on the leading edge of the trough.

The mid-level pattern largely mirrored that of the upper levels on May 21. Troughing extended down the Front Range of the Rocky Mountains, with ridging apparent from the mid-Atlantic states into the Northeast. A number of impulses—smaller waves embedded within the prevailing flow—were evident along & ahead of the trough’s base. Two would track over the state on May 21, one during the early afternoon hours, and a second during the late afternoon and evening. Positive vorticity advection ahead of each feature provided additional large scale ascent, and thus additional support for thunderstorms, particularly during the late afternoon & early evening phase of the event.

An animated gif of the RAP thirteen’s 500 millibar heights and vorticity. 500 millibar heights are the black lines. Positive Vorticity values of 10 to 40 rotations per second are represented by polygons that are colored green for values near 10, yellow for values near 25 and red for values near 40. A trough and shortwave in the western Rocky Mountain range advects northeast toward Wisconsin with a large area of positive vorticity advection on the leading edge and center of the trough.

In the lower portion of the atmosphere, an expansive area of low pressure migrated from the Central Plains to the western Great Lakes throughout the day on May 21. The low rapidly deepened during the afternoon & evening hours as the previously discussed mid-upper level forcing mechanisms moved into the region. Southerly winds rapidly strengthened across the state as this deepening occurred, enhancing low level wind shear and drawing a moist, unstable air mass into the area. Said instability and wind shear will be explored further in the mesoscale environment section of this story.

An animated gif of the RAP thirteen’s 850 millibar heights and winds. 850 millibar heights are the black lines. Winds are represented by the grey wind barbs with values of 30 knots or greater highlighted by blue polygons. If winds are 50 knots or great the polygon colors change to purple. At 850 millibars, a low pressure center in Nebraska and Kansas advects northeast toward northern Wisconsin. The low level jet amplifies as the low pressure moves northeast. Values quickly climb above 50 knots.

At the surface, low pressure was positioned over central Minnesota by sunrise. A cold front extended southwest out of the low, with a warm front branching northeast toward Lake Superior. Tied to overnight showers and storms, an outflow boundary was evident from eastern Iowa into southwestern and south-central Wisconsin. The outflow would maintain definition through the morning hours, with its eastern segment moving north as a secondary warm front during the afternoon. This evolution placed southern Wisconsin within the warm sector through the second half of the day. With the cold front and a prefrontal trough approaching from Iowa, the stage was set for storms during the afternoon and evening.

Warm Sector Environment

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With winds turning southerly in the wake of the secondary warm front, a moist air mass characterized by high equivalent potential temperatures streamed north into the state during the afternoon.

An animated gif of the RAP thirteen’s surface pressure and equivalent potential temperature. Surface pressure is represented by black lines. The wind barbs are also black. The Equivalent Potential Temperatures are represented by polygons that are colored by strength. Values near 330 Kelvin are green, 340 values are blue, 350 values are purple, and 360 or greater are pink. The Equivalent Potential Temperatures values initially around 350 Kelvin have a brief period of strengthening before weakening below to near 340 by 0 Zulu (7 PM CDT).

With the help of daytime heating, between 1000-2000 joules per kilogram of instability built within this air mass by late afternoon.

An animated gif of the RAP thirteen’s surface wind barbs and mix Layer CAPE. Wind barbs are yellow and CAPE values are represented by orange and red lines. Orange lines represent weaker CAPE values less than 1000 Joules per kilogram. Red lines are used for CAPE values greater than 1000 Joules per kilogram. Mixed layer CAPE values slowly grow throughout the day from west to east with the max values ranging from 1000 to 2000 Joules per kilogram.

Thanks to the upper jet moving in from the Plains, an abundance wind shear overlapped with this instability.

An animated gif of the RAP thirteen’s Effective Bulk Shear Magnitude and Vectors. Blue to dark blue polygons designate the magnitude of the Effective Bulk shear with the yellow vectors. Effective bulk shear is growing from 18Z to 00Z with max values of 50-65 kts out of the southwest to west.

Given a strengthening low level jet, very high values of storm-relative helicity were in place as well. With large-scale forcing increasing moving deeper into the afternoon, the environment was quickly becoming primed for widespread, organized thunderstorms capable of all severe hazards.

Despite the favorable environment coming together within the warm sector, one limitation remained in the severe weather forecast picture across southern Wisconsin. Said limitation was tied to a cap in place across the region. The cap is a layer of warm air in the low to mid portions of the atmosphere, which acts to suppress thunderstorm development like a lid on a pot of boiling water.

The cap was in place thanks to an elevated mixed layer between 700 and 500 millibars, which was well-sampled in regional soundings during the afternoon hours.

Considerable amounts of convective inhibition (CIN) were present as a result of the elevated mixed layer & cap. With the bulk of mid-upper forcing remaining west of the region until early evening, said CIN would limit overall storm coverage through the first half of the afternoon hours.

Event Timeline

4:00 AM May 17 - 5:00 AM May 21: Consecutive runs of medium and short-range model guidance show the possibility of severe weather on May 21. This potential captures both national and local attention, with the Storm Prediction Center and NWS Milwaukee/Sullivan highlighting the emerging threat in their forecast products.

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4:00 AM May 17

The Storm Prediction Center introduces a 15% severe weather risk over much of the state in its May 21 severe weather outlook. Despite the long lead time, forecast confidence is high enough to introduce a threat area given the approach of an upper trough & surface front into an unstable air mass.

3:00 PM May 17

NWS Milwaukee/Sullivan begins to message the May 21 severe weather threat in its Area Forecast Discussion (AFD). With the forecast picture appearing largely similar to twelve hours prior, the key message remains largely unchanged relative to the morning outlook from the Storm Prediction Center.

A segment of the National Weather Service Milwaukee Sullivan’s Area forecast Discussion issued May 17th at 3 PM. This portion of the forecast discussion starts the messaging of the May 21st system. The track and strength of the shortwave trough are mentioned with a note that stronger thunderstorms are more likely to develop given the forecast soundings, shear, CAPE and lapse rate values.

4:00 AM May 18

With available model guidance continuing to show a near unanimous signal for severe weather on May 21, the Storm Prediction Center continues a 15% severe weather risk over southern Wisconsin in its updated May 21 severe weather outlook. An even more substantial 30% risk area is inserted over southwestern Wisconsin, where confidence in widespread severe storms is greatest.

3:00 PM May 18

NWS Milwaukee/Sullivan starts to highlight the May 21 severe weather potential in the Key Messages section of its AFD.

2:30 AM May 19

Areas along and west of Interstate 39 are placed within an enhanced (level 3/5) severe weather risk area in the May 21 severe weather outlook. Southeastern Wisconsin is included in a slight (level 2/5) risk area. The outlook messages the potential for multiple rounds of severe storms on May 21, with peak potential being during the evening hours.

4:45 AM May 19

NWS Milwaukee/Sullivan starts to ramp up messaging regarding the May 21 severe weather potential. Forecast graphics are disseminated over email and social media.

2:30 PM May 19

With additional, higher-resolution model guidance becoming available to forecasters, it becomes apparent that a long-duration & possibly significant severe weather event may occur on the afternoon of May 21. All hazards appear possible. NWS Milwaukee/Sullivan communicates this potential in its AFD and public-facing forecast graphics.

1:00 AM May 20

The Storm Prediction Center expands the enhanced (level 3/5) risk area into most of southern Wisconsin. Areas along the immediate Lake Michigan shoreline remain in the level 2/5 slight risk.

4:00 AM May 20

NWS Milwaukee/Sullivan continues to message a potentially widespread, all-hazards severe weather threat on the afternoon of May 21. Evolution of showers and storms during the predawn hours are likely to influence the precise severe weather threat during the afternoon hours.

3:30 PM May 20

NWS Milwaukee/Sullivan issues an updated forecast graphic. Expectations remain largely unchanged relative to twelve hours prior.

1:00 AM May 21

Developing in the wake of earlier day showers & storms, confidence is now high in widespread severe thunderstorms during the afternoon and evening hours of May 21. All severe hazards are possible. The Storm Prediction Center introduces a moderate (level 4/5) severe weather risk along & west of the Wisconsin River, with an enhanced (level 3/5) risk in place elsewhere.

5:00 AM May 21

NWS Milwaukee/Sullivan issues an updated forecast graphic conveying the even higher-confidence severe weather forecast for the upcoming afternoon and evening hours.

18Z May 21 Interactive Supercell Composite Map. Click icon in top right to enlarge and explore.

12:00 PM - 5:30 PM May 21: Forming in the vicinity of the northward-advancing warm front, storms attempt to develop along & south of the I-94 and US-18 Corridors. With the upper shortwave still well to the southwest of the area, and a stout cap in place across the region, most initiation attempts fail. Two storms—one near Stoughton and another over far southwestern Wisconsin—manage to maintain & reach severe levels. Thanks to the overlap of shear and instability in place locally, said storms form in an environment highly conducive for supercells, quickly taking on supercellular traits themselves.

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12:15 - 12:40 PM May 21

Aided by lift along the warm front, a supercell erupts on the Rock-Dane County border.

A map with a looping radar image from 17 to 18 Zulu overlaid with the 1000-925 millibar Frontogenesis and wind barbs. The highest low level frontogenesis is centered in Walworth county extending northwest toward Sauk City and south east toward Waukegan Illinois. Wind barbs are out of the south to southeast around 5 to 15 knots.

12:43 PM May 21

NWS Milwaukee/Sullivan issues a severe thunderstorm warning—the first of 42 warnings to be issued on May 21—in advance of the developing Rock-Dane County supercell.

Left is a map zoomed in on Dane County displaying radar with a storm over Stoughton, Wisconsin that has a severe thunderstorm warning on it. The warning extended to the northeast including portions of northwestern Jefferson County, far southeastern Columbia County and far southwestern Dodge County.

Right is the text product for the severe thunderstorm warning stating the storm is moving northeast at 35 mph with the main hazards being 60 mph wind gusts and quarter size hail. This was the first of many warnings issued on May 21st.

12:48 PM May 21

Reports of quarter-sized hail are relayed to NWS Milwaukee/Sullivan from the community of Stoughton. Said reports follow sightings of ping pong ball-sized hail just a few minutes prior.

Radar imagery from 12:48 PM showing a storm to the northwest of Stoughton. This storm has a severe thunderstorm warning out for 60 mph winds and quarter sized hail. The storm is moving northeast with the warning covering eastern Dane, northwest Jefferson, southwestern Dodge, and southeastern Columbia counties.

1:00 - 1:40 PM May 21

As quickly as it formed, the Stoughton supercell rapidly dissipates as it moves into a considerably more stable environment north of the warm front. Warnings are canceled at 1:11 PM.

2:25 - 2:50 PM May 21

Developing in the vicinity of a prefrontal trough, an area of moderate showers organizes into a supercell thunderstorm over eastern Iowa.

2:52 PM May 21

NWS Quad Cities issues a severe thunderstorm warning for portions of Dubuque, Jones, and Delaware Counties as the eastern Iowa supercell continues to strengthen.

3:25 - 3:35 PM May 21

Downstream severe thunderstorm warnings are issued for the northeastern Iowa supercell, which is quickly approaching the Iowa-Wisconsin state line. NWS Quad Cities issues a warning for portions of Dubuque County at 3:28 PM, with NWS La Crosse following with a severe thunderstorm warning for southern Grant County two minutes later.

3:57 PM May 21

Quarter size hail is reported in the community of Dickeyville in Grant County. A severe thunderstorm warning is issued for portions of Iowa and Lafayette Counties with the supercell quickly approaching.

4:00 PM May 21

An additional hail report is received from Dickeyville, this time confirming hail stones up to golf ball size. Given the ongoing activity, and widespread severe thunderstorm development anticipated by early evening, the Storm Prediction Center issues a tornado watch for portions of southwestern and south-central Wisconsin. The watch is later extended to include all NWS Milwaukee/Sullivan counties.

4:45 PM May 21

With the supercell continuing to push northeast, NWS Milwaukee/Sullivan issues a severe thunderstorm warning for central and northeastern Iowa County.

4:50 PM May 21

Quarter size hail is reported in Dodgeville.

5:10 PM May 21

The Iowa County supercell weakens below severe limits. The ongoing severe thunderstorm warning is allowed to expire five minutes later.

6:00 PM May 21 - 1:00 AM May 22: Numerous clusters of showers and storms push in from the southwest as the upper shortwave and surface cold front arrive into the area. Developing in a highly-conducive environment, storms quickly become severe. Numerous reports of damaging winds are received by NWS Milwaukee/Sullivan, in addition to a pair of tornadoes and large hail. Southwestern portions of the region are particularly hard hit, where multiple rounds of storms featuring the strongest winds of the event train across one another.

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6:00 - 6:40 PM May 21

Clusters of supercells move into the Wisconsin River Valley from the southwest. Multiple severe thunderstorm warnings are issued from Sauk County south into Lafayette County. Damaging wind reports are limited with storm organization still in-progress. Several reports of large hail are relayed to NWS Milwaukee/Sullivan.

Animated radar gif from 6:00 to 6:40 PM local time. Radar reflectivity is on the left with radar velocity on the right. Clusters of storms and supercells advect in from the southwest. As storms increase in strength and move into southern Wisconsin. Severe thunderstorm warnings are being issued. Storm motion is to the northeast.

6:45 - 7:25 PM May 21

Storms rapidly begin to organize into a mixture of supercells and bowing segments as they approach the I-39 Corridor. Signs of rotation become evident on radar, with multiple tornado warnings being issued. Impressive wind signals begin to appear in radar imagery, warranting additional severe thunderstorm warnings.

Animated radar image from 6:45 to 7:25 PM CDT centered over Dane County. Radar reflectivity is on the left with radar velocity on the right. The cluster of storms over western Wisconsin begin to congeal with a bowing line segment. There are some embedded supercells within this organizing convection. Rotation starts to develop during this time. Severe thunderstorm warnings and tornado warnings are issued across southwestern Wisconsin.

7:26 PM May 21

An 80 MPH wind gust is measured between Hollandale and Barneveld in Iowa County.

Radar reflectivity (left) and radar velocity (right) from 7:26 PM CDT centered over Barneveld, Wisconsin. Radar reflectivity is on the left with radar velocity on the right. A cluster of storms with high velocity signatures. These high readings on radar coincide with an 80 MPH wind gust measured on the ground.

7:30 - 7:45 PM May 21

Storms reach I-39 and the city of Madison. This will be the first of multiple rounds of severe storms to impact the city. With multiple areas of rotation persisting on radar, tornado warnings are extended to the east-northeast along the leading edge of the line of storms. Impressive wind signals continue to show up on radar, particularly from Madison southwest to the state line. Radar begins to estimate 90+ MPH wind speeds in storms approaching Lafayette and Green Counties from northwest Illinois.

7:49 PM May 21

The Springdale EF1 tornado touches down just to the southeast of Mount Horeb. The touchdown point overlaps with a notch in radar reflectivity along the leading edge of the storms. The tornado remains on the ground for five minutes.

7:50 PM May 21

An area of radar-estimated 90+ MPH winds bears down on Lafayette County.

7:53 PM May 21

A 70 MPH wind gust is measured to the southeast of Gratiot in Lafayette County.

7:56 PM May 21

A 100 MPH wind gust is measured to the west of Gratiot in Lafayette County.

7:59 PM May 21

The Lake Neshkoro tornado touches down to the west of Germania in Marquette County. The tornado remains on the ground for the next eight minutes.

8:02 PM May 21

Given the wind gust reports coming out of Lafayette County, a particularly dangerous situation (PDS) severe thunderstorm warning is issued for portions of Lafayette, Green, and Dane Counties.

8:05 - 8:35 PM May 21

Severe storms continue to march northeast across the area. Areas of rotation continue to persist over northern counties, where tornado warnings remain in effect. A second cluster of thunderstorms and embedded damaging winds spread into Dane County and the Madison metro.

8:39 PM May 21

A second PDS severe thunderstorm warning is issued for portions of Dane, Jefferson, Rock, Green, Dodge, and Waukesha Counties given continued radar-estimated wind speeds at and above 80 MPH.

8:45 - 9:30 PM May 21

Continuing to show signs of rotation on radar, tornado warnings remain in effect within the northern cluster of severe thunderstorms crossing Fond du Lac and Sheboygan Counties. Indications of damaging winds continue south to the Illinois border, with numerous severe thunderstorm warnings in effect.

9:36 PM May 21

A 74 MPH wind gust is measured in the community of Belleville in southern Dane County.

9:45 - 10:30 PM May 21

While storms further north show some signs of weakening, another cluster of severe storms moves into the greater Madison metro. The storms bring yet another round of strong to severe wind gusts to the area.

10:40 - 11:00 PM May 21

Storms over Jefferson, Rock, and Walworth Counties show signs of rotation as they push east of the Madison metro, prompting two tornado warnings.

11:15 PM May 21 - 12:45 AM May 22

Undercut by outflow, storms over north-central portions of the region remain below severe limits. Activity further south shows sporadic damaging wind potential, prompting additional severe thunderstorm warnings in southeastern Wisconsin.

Damage Photos

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Tree damage photo from Lafayette County that occurred south of Gratiot, Wisconsin. Tree is snapped half way up roughly 5-8 feet from base.

Tree Damage In Lafayette County

Estimated wind speeds of 70 MPH

Outbuilding in Gratiot, Wisconsin with the roof completely missing. Support beams remain in tack.

Outbuilding Damage In Lafayette County

Estimated wind speeds of 80 MPH

Outbuilding with significant damage from Gratiot, Wisconsin. Photo is of the damaged wood and structural beams down in a field. Sheet metal can be seen in the distance.

Outbuilding damage in Gratiot, Wisconsin. One exterior wall is leaning toward the camera with the top of each structural post broken at the top. There are no additional exterior walls standing.

Outbuilding Damage In Lafayette County

Estimated wind speeds between 95 & 100 MPH

Outbuilding Wall & Roof Damage In Lafayette County

Estimated wind speeds of 90 MPH

Toppled Tree In Lafayette County

Estimated wind speeds of 70 MPH

Tree Damage In Lafayette County

Estimated wind speeds between 70 and 75 MPH

Damage In Lafayette County

Estimated wind speeds of 80 MPH

Destroyed Barn In Lafayette County

Estimated wind speeds of 90 MPH

Barn Damage In Lafayette County

Estimated wind speeds of 70 MPH

Tree Damage In Lafayette County

Estimated wind speeds of 80 MPH

Tree Damage In Lafayette County

Estimated wind speeds between 70 and 75 MPH

Uprooted Tree In Lafayette County

Estimated wind speeds of 90 MPH

Barn Damage In Lafayette County

Estimated wind speeds of 85 MPH

Barn Damage & Scattered Sheet Metal In Lafayette County

Estimated wind speeds of 90 MPH

Outbuilding Damage In Lafayette County

Estimated wind speeds of 90 MPH

Snapped Tree In Lafayette County

Estimated wind speeds of 65 MPH

Tree Damage In Green County

Estimated wind speeds of 60 MPH

Damaged Grain Bins In Iowa County

Estimated wind speeds of 70 MPH

Snapped Trees In Dane County

Estimated wind speeds of 80 MPH

Snapped Trees In Dane County

Estimated wind speeds of 95 MPH

Tree Damage In Dane County

Estimated wind speeds of 85 MPH

Tree Damage Along The Mount Horeb Tornado Path

Estimated wind speeds of 80 MPH

Shed Damage Along The Mount Horeb Tornado Path

Estimated wind speeds of 85 MPH

Outbuilding Damage Along The Mount Horeb Tornado Path

Estimated wind speeds of 100 MPH

Tree Damage Along The Mount Horeb Tornado Path

Estimated wind speeds of 95 MPH

Toppled Tree Along The Mount Horeb Tornado Path

Estimated wind speeds of 95 MPH

House And Tree Damage Along The Mount Horeb Tornado Path

Estimated wind speeds of 100 MPH

Tree Damage Along The Mount Horeb Tornado Path

Estimated wind speeds of 90 MPH

Tree Damage Along The Mount Horeb Tornado Path

Estimated wind speeds of 90 MPH

Tree Damage Along The Wisconsin River

Tree Damage Along The Lake Neshkoro Tornado Path

Estimated wind speeds of 80 MPH

Tree Damage Along The Lake Neshkoro Tornado Path

Estimated wind speeds of 80 MPH

House Damage Along The Lake Neshkoro Tornado Path

Estimated wind speeds of 100 MPH

Tree Damage Along The Lake Neshkoro Tornado Path

Estimated wind speeds of 100 MPH

Tree Damage Along The Lake Neshkoro Tornado Path

Estimated wind speeds between 100 and 105 MPH

Tree Damage Along The Lake Neshkoro Tornado Path

Estimated wind speeds of 75 MPH