Scattered thunderstorms developed ahead of a cold front, trailing from low pressure that moved from west central Wisconsin into western Ontario during the day on May 20th. One cluster of thunderstorms pulsed up to severe levels, producing damaging wind gusts over Walworth County that downed power lines and trees.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS MKX wrote it up.
Scattered thunderstorms developed ahead of a cold front, trailing from low pressure that moved from west central Wisconsin into western Ontario during the day on May 20th. One cluster of thunderstorms pulsed up to severe levels, producing damaging wind gusts over Walworth County that downed power lines and trees. Surface temperatures in the 80s and dew points around 60 degrees produced mixed-layer CAPE values around 1500 J/kg. Forcing with surface convergence along the front was aided by lift attributed to differential positive vorticity advection as an upper level shortwave lifted across western Wisconsin.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Milwaukee, WI wrote its last area forecast discussion before the first event at May 20, 2:58 PM CST, 35 minutes earlier.
AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE MILWAUKEE/SULLIVAN WI 358 PM CDT SUN MAY 20 2012 ...THIS AFTERNOON INTO EARLY EVENING...FORECAST CONFIDENCE MEDIUM. SURFACE CONVERGENCE RIGHT ALONG THE PRE-FRONTAL TROUGH LOCATED NEAR MADISON AND JANESVILLE AT 230 PM...IDENTIFIED BY A WIND SHIFT IN THE SURFACE OBS...WILL BE THE FOCUS FOR SOME INITIAL CONVECTION PRIOR TO THE MAIN FRONTAL BOUNDARY LOCATED IN THE FAR WESTERN FORECAST AREA /CWA/...WHERE THERE IS ANOTHER WIND SHIFT. A FEW QUICK-MOVING POP-UP SHOWERS WITH ISOLATED THUNDER ARE DEVELOPING JUST AHEAD OF THE PRE-FRONTAL TROUGH EARLY THIS AFTERNOON AS WELL. THESE ARE A RESULT AS GENERAL INSTABILITY WITH MIXED LAYER CAPE AROUND 1000 J/KG. STEEP LOW LEVEL LAPSE RATES WITH A WELL-MIXED LAYER UP TO 7000 FEET ARE ALLOWING FOR WINDS GUSTING UP TO 30 KNOTS. EXPECT HIGHER GUSTS BENEATH SHOWERS WITH VIRGA. EXPECT MORE THUNDERSTORM DEVELOPMENT OVER THE NEXT TWO HOURS IN THE CENTRAL CWA...ESPECIALLY BETWEEN THE PRE-FRONTAL TROUGH AND MAIN TROUGH WHERE THERE IS A PLUME OF HIGHER CAPE VALUES PER LATEST SPC MESOSCALE ANALYSIS. POTENTIAL FOR SEVERE STORMS IS HIGHEST AS THIS CORRIDOR SHIFTS INTO SOUTHEAST WI BETWEEN 21Z AND 00Z THIS AFTERNOON. VERY ISOLATED ONE-INCH HAIL AND DAMAGING WIND GUSTS TO 60 MPH WOULD BE THE SEVERE THREAT. THERE IS NOT ENOUGH 6KM BULK SHEAR OR LOW LEVEL SHEAR TO SUPPORT WELL-ORGANIZED SUPERCELLS OR LONG-LIVED STORMS THAT WOULD PRODUCE WIDESPREAD SEVERE STORMS. 17Z HRRR CONTINUES TO SHOW A N-S ORIENTED UNORGANIZED LINE OF THUNDERSTORMS THAT DEVELOP EAST OF MADISON AT 20Z AND TRACK DUE EAST TO THE SHORELINE AT 23Z. MODELS AGREE ON ALL TSTORMS OUT OF CWA BY 00Z. LEFT SMALL CHANCE POPS IN THE FAR EAST THROUGH 03Z TO ACCOUNT FOR POSSIBLE SLOWER MOVEMENT...THEN DRY WEATHER FOR THE REST OF THE NIGHT. .TONIGHT AND MONDAY...FORECAST CONFIDENCE HIGH. VISIBLE SATELLITE AND SURFACE OBSERVATIONS SHOW PLENTY OF CLOUDS BEHIND THE COLD FRONT THIS AFTERNOON...BUT THESE MIGHT SCATTER OUT THIS EVENING WITH THE LOSS OF DAYTIME HEATING AND THE STEEP LOW LEVEL LAPSE RATES...IN COMBINATION WITH SUBSIDENCE BEHIND THE FRONT. THIS MEANS THAT THERE IS AN OUTSIDE CHANCE THAT PEOPLE IN SOUTHERN WI COULD VIEW THE PARTIAL SOLAR ECLIPSE EARLY THIS EVENING... FILTERED THROUGH THE THINNER CLOUDS. A SECONDARY 500MB TROUGH WILL TRACK ACROSS WI BETWEEN MIDNIGHT TONIGHT AND NOON MONDAY. TOO DRY FOR PRECIP. A PERIOD OF LOW STRATUS IS EXPECTED WITH THIS FEATURE IN COMBINATION WITH COLD AIR ADVECTION...AND THEN SKIES SHOULD CLEAR FROM WEST TO EAST JUST AFTER SUNRISE MONDAY MORNING. EXPECT DIURNAL CUMULUS CLOUDS TO DEVELOP IN THE EARLY AFTERNOON. WINDS WILL BE GUSTY OUT OF THE NORTH MONDAY MORNING WITH THE COLD AIR ADVECTION AND STEEP LOW LEVEL LAPSE RATES. WINDS WILL TAPER OFF IN THE AFTERNOON AS HIGH PRESSURE MOVES OVER WISCONSIN. A LAKE BREEZE WILL DEVELOP WITH THE WEAKENING PRESSURE GRADIENT AND KEEP AREAS NEAR THE LAKE IN THE LOWER 60S/POSSIBLY UPPER 50S. INLAND TEMPS SHOULD MAKE IT UP TO AROUND 70. .MONDAY NIGHT THROUGH WEDNESDAY...FORECAST CONFIDENCE IS HIGH. HIGH PRESSURE MOVES OVERHEAD MONDAY NIGHT...SLIPPING SOUTHEAST OF WISCONSIN BY EARLY TUESDAY EVENING WITH RETURN FLOW BRINGING WARMER TEMPS TO THE AREA ON WEDNESDAY. DRY THROUGH THIS PERIOD. A LAKE BREEZE WILL BE FELT TUESDAY AND WEDNESDAY FOR COOLER READINGS ALONG THE LAKE MICHIGAN SHORE. DWPNTS SHOULD MIX OUT DURING THE DAY...SETTING UP SOME DECENT COOLING POTENTIAL BOTH MONDAY NIGHT AND TUESDAY NIGHT. .THURSDAY THROUGH SATURDAY...FORECAST CONFIDENCE LOW TO MEDIUM. THE ECMWF/GFS/CANADIAN NH ARE GETTING INTO BETTER AGREEMENT WITH THE LATE WEEK/MEMORIAL DAY WEEKEND PERIOD...AND IT IS THE ECMWF/CANADIAN THAT ARE COMING IN LINE WITH THE GFS. THERE ARE TIMING DIFFERENCES...BUT THEY ALL EVENTUALLY BRING A FRONT THROUGH THE AREA...PUTTING A DAMPER ON THE ONCE EXPECTED VERY WARM WEEKEND...AT LEAST FOR THE START OF IT. WILL HAVE A SMALL CHANCE OF SHOWERS AND STORMS REACHING THE FORECAST AREA ON THURSDAY...BASED ON THE FASTER GFS/CANADIAN WITH THE APPROACHING TROF/FRONT. THE ECMWF IS DRY FOR THURSDAY...THAT MAY WELL BE TRUE. THE ASSOCIATED LOW PRESSURE SYSTEM WITH THIS SYSTEM WILL LIFT NORTHEAST OF THE AREA...DRAGGING A COLD FRONT THROUGH SOUTHERN WISCONSIN PROBABLY BY FRIDAY MORNING. THIS WILL PUSH THE WARM/HOT TEMPS SOUTH OF THE BORDER...PUTTING US IN A COOLER NORTHERLY FLOW. THE BOUNDARY REMAINS TO THE SOUTH OF WISCONSIN THROUGH SATURDAY WITH A COOL EAST TO NORTHEAST SETTLING IN KEEPING THE STATIONARY SFC FRONT SOUTH OF WISCONSIN FOR THE FIRST PART OF THE WEEKEND. IN ADDITION...WITH THE BAROCLINIC ZONE IN THE VCNTY...WE COULD SEE SOME SHOWERS AND STORMS FROM TIME TO TIME THROUGH FRIDAY NIGHT. THE FRONT COULD LIFT NORTH BY SUNDAY AS SOUTHERLY WINDS INCREASE WITH LOW PRESSURE DEVELOPING WEST OF WISCONSIN. THIS WOULD BRING THE WARM SECTOR BACK IN FOR WARMER AND DRY CONDITIONS FOR SUNDAY AND MEMORIAL DAY. && .AVIATION/00Z TAFS/... CONVECTION AHEAD OF A COLD FRONT MOVING ACROSS WI THIS AFTERNOON. BULK OF THUNDERSTORMS WILL BE EAST OF MADISON BETWEEN 21Z AND 23Z...EXITING FAR EASTERN WI BY 00Z. THERE IS UNCERTAINTY ABOUT COVERAGE OF THESE THUNDERSTORMS...AS STRONGEST FORCING WILL BE NORTH AND SOUTH OF THE TAF SITES. SHORT TERM MODELS SUGGEST AN UNORGANIZED LINE OF MULTI-CELL THUNDERSTORMS ORIENTED NORTH-SOUTH TRACKING STRAIGHT EAST. WEAK SHEAR SHOULD INHIBIT STRONG AND LONG-LASTING THUNDERSTORM DEVELOPMENT THIS AFTERNOON...HOWEVER SHORT BURSTS OF MARGINALLY SEVERE HAIL AND DAMAGING WINDS ARE NOT OUT OF THE QUESTION WITH ANY STRONGER STORMS. WEST WINDS WILL DECREASE DURING THE EVENING AND BECOME NORTH BY MONDAY MORNING. THE NORTH WINDS WILL BE GUSTY DURING THE MORNING HOURS WITH COLD AIR ADVECTION BEHIND DEPARTING LOW PRESSURE SYSTEM. THEN WINDS WILL DECREASE IN THE AFTERNOON AS HIGH PRESSURE SETTLES INTO WI. CLOUDS SHOULD SCATTER OUT FOR A PERIOD OF TIME BEHIND THE FRONT EARLY THIS EVENING. THEN A PERIOD OF LOW STRATUS IN THE FUEL ALTERNATE CATEGORY IS EXPECTED WITH THIS FEATURE IN COMBINATION WITH COLD AIR ADVECTION...AND THEN SKIES SHOULD CLEAR FROM WEST TO EAST JUST AFTER SUNRISE MONDAY MORNING. EXPECT VFR DIURNAL CUMULUS CLOUDS TO DEVELOP IN THE EARLY AFTERNOON. && .MARINE... WINDS HAVE POTENTIAL TO FREQUENTLY GUST TO 25 KNOTS FOR A 4 TO 6 HOUR PERIOD OF TIME MONDAY MORNING IN THE WAKE OF THE DEPARTING LOW PRESSURE SYSTEM. HIGHEST RISK OF SMALL CRAFT ADVISORY CONDITIONS WILL BE FROM THE NORTH POINT LIGHT AND SOUTHWARD. TOO MARGINAL AT THIS TIME TO ISSUE A HEADLINE. && .MKX WATCHES/WARNINGS/ADVISORIES... WI...NONE. LM...NONE. && TONIGHT/MONDAY AND AVIATION/MARINE...MRC MONDAY NIGHT THROUGH SUNDAY...DAVIS
HAZARDOUS WEATHER OUTLOOK...UPDATED NATIONAL WEATHER SERVICE MILWAUKEE/SULLIVAN WI 400 PM CDT SUN MAY 20 2012 WIZ046-047-051-052-056>060-062>072-211000- MARQUETTE-GREEN LAKE-FOND DU LAC-SHEBOYGAN-SAUK-COLUMBIA-DODGE- WASHINGTON-OZAUKEE-IOWA-DANE-JEFFERSON-WAUKESHA-MILWAUKEE- LAFAYETTE-GREEN-ROCK-WALWORTH-RACINE-KENOSHA- 400 PM CDT SUN MAY 20 2012 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF EAST CENTRAL WISCONSIN...SOUTH CENTRAL WISCONSIN AND SOUTHEAST WISCONSIN. .DAY ONE...REST OF THIS AFTERNOON AND TONIGHT SCATTERED TO NUMEROUS THUNDERSTORMS HAVE DEVELOPED ALONG A COLD FRONT MOVING THROUGH SOUTHEAST WISCONSIN LATE THIS AFTERNOON. THERE IS A SMALL POTENTIAL FOR THE THUNDERSTORMS TO BECOME SEVERE WITH DAMAGING WIND GUSTS AND LARGE HAIL. THESE STORMS ARE EXPECTED TO MOVE EAST OUT OVER LAKE MICHIGAN BY 6 TO 7 PM THIS EVENING. .DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY THERE IS A CHANCE FOR THUNDERSTORMS FROM TIME TO TIME THURSDAY INTO SATURDAY AS A FRONTAL BOUNDARY STALLS NEAR THE AREA. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION MAY YET BE NEEDED LATE THIS AFTERNOON AND EVENING. LMZ643>646-211000- SHEBOYGAN TO PORT WASHINGTON WI- PORT WASHINGTON TO NORTH POINT LIGHT WI- NORTH POINT LIGHT TO WIND POINT WI- WIND POINT WI TO WINTHROP HARBOR IL- 400 PM CDT SUN MAY 20 2012 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF LAKE MICHIGAN. .DAY ONE...REST OF THIS AFTERNOON AND TONIGHT SCATTERED TO NUMEROUS THUNDERSTORMS HAVE DEVELOPED ALONG A COLD FRONT MOVING THROUGH SOUTHEAST WISCONSIN LATE THIS AFTERNOON. THERE IS A SMALL POTENTIAL FOR THE THUNDERSTORMS TO BECOME SEVERE WITH DAMAGING WIND GUSTS AND LARGE HAIL. THESE STORMS ARE EXPECTED TO MOVE EAST OF THE NEARSHORE WATERS OF LAKE MICHIGAN BY 7 OR 8 PM THIS EVENING GUSTY SOUTH WINDS WILL CONTINUE INTO THE EARLY EVENING HOURS...THEN SHIFT TO THE WEST THIS EVENING AS A COLD FRONT MOVES THROUGH. .DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY THERE IS POTENTIAL FOR AREAS FROM NORTH POINT LIGHT AND SOUTHWARD TO REACH SMALL CRAFT ADVISORY LEVELS BETWEEN 6 AM AND NOON MONDAY MORNING IN THE WAKE OF A DEPARTING LOW PRESSURE SYSTEM. BREEZY SOUTH WINDS AND INCREASING WAVE HEIGHTS MAY REACH SMALL CRAFT ADVISORY LEVELS FOR WEDNESDAY AND THURSDAY. THERE IS A CHANCE FOR THUNDERSTORMS FROM TIME TO TIME THURSDAY INTO SATURDAY AS A FRONTAL BOUNDARY STALLS NEAR THE AREA. WWW.WEATHER.GOV/MKX DAVIS/MRC
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE MILWAUKEE/SULLIVAN WI
420 PM CDT SUN MAY 20 2012
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0333 PM HAIL 2 E WHITEWATER 42.83N 88.70W
05/20/2012 E0.25 INCH WALWORTH WI TRAINED SPOTTER
PEA-SIZED HAIL FALLING FOR 1.5 MINUTES
&&
ETThe Milwaukee radar (KMKX), 51 km away, was scanning. The volume nearest the first event began at 21:33 UTC.
1 thunderstorm wind. Local standard time, as filed.
The Walworth County Emergency Manager reported numerous trees and power lines downed by thunderstorm wind gusts estimated at 65 mph (56 knots) in a swath from Bloomington Township, near Pell Lake, to Lyons.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wisconsin in 2012Every state in 2012This day in weather historyWarnings in Wisconsin, May 2012
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 386124.