A nearly stationary frontal boundary remained draped across northwest Iowa. The airmass became very unstable with dewpoints surging into the 65 to 70 degree range.
Every event with a place to put it. Tap one for its entry below.
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 DMX wrote it up.
A nearly stationary frontal boundary remained draped across northwest Iowa. The airmass became very unstable with dewpoints surging into the 65 to 70 degree range. The wind flow at 850 mb was weak, only around 15 kts or so, but winds backed and were very convergent. Moisture pooled over northwest Iowa with precipitable water values increasing to around 1.4 inches by mid afternoon. The lifted index fell to between -5 and -10 C with CAPE rising to the 2000-3000 J/kg range. Downdraft CAPE was quite high, in the 800 to 1200 J/kg range. Ample CAPE was available in the -10 to -30 C layer of the atmosphere, between 400 and 800 J/kg. Unlike the previous day, significant shear was available for any storm development with 40 to 50 kts of effective shear. The freezing level was quite high for early May and was between 13,000 and 14,000 feet. Though high enough to limit hail, other parameters were sufficiently strong to overcome this. The LCL was fairly low with near storm environmental values around 750 meters. A line of strong to severe thunderstorms formed near the frontal boundary during the mid and late afternoon hours. The line moved and propagated east-southeast. High winds were common with the storms with wind speeds of up to 65 MPH or more. The storms also produced some quarter size hail. A tornado occurred in Winnebago County just south of the Minnesota line. It was about a mile long and was in open country. A short time later, a funnel cloud was observed in Worth County. After the main storms moved east, further thunderstorms developed behind the line. One became severe and produced golf ball size hail in Pocahontas County at Pocahontas.
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 Des Moines, IA wrote its last area forecast discussion before the first event at May 4, 2:53 PM CST, 19 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DES MOINES IA 353 PM CDT FRI MAY 4 2012 .SHORT TERM /TONIGHT/... THUNDERSTORMS ENTERING FORECAST AREA AS THIS IS WRITTEN...AND QUESTION WILL BE EVOLUTION INTO THE NIGHT. SOUTHEAST FLOW AT THE SURFACE WILL CONTINUE WITH A SHIFT TO EASTERLY WINDS ALONG THE MINNESOTA BORDER. QUITE UNSTABLE SOUTH OF THIS BOUNDARY AND EVEN RICHER DEWPOINTS ARE WAITING IN MISSOURI. THE STORM COMPLEX FROM SOUTH DAKOTA HAS DEVELOPED A SIZABLE COLD POOL ALREADY...AND THIS IS THE MAIN QUESTION FOR TONIGHT. WILL STORMS CONTINUE ON CURRENT EASTERLY TRACK OR TURN MORE SOUTHEAST INTO INSTABILITY? HI-RES NMM/ARW AND SOMEWHAT THE GFS MODEL FAVOR A SURGE SOUTHEAST SO HAVE INCLUDED ABOUT 50-50 CHANCE OF THUNDERSTORMS FOR SOUTHEAST THIRD OF FORECAST AREA. BY FAR HIGHEST CHANCE WILL BE ACROSS THE NORTH AND NORTHEAST INTO WATERLOO. TORNADO THREAT HAS BEEN DISCUSSED WITH REPORTS IN SOUTHWEST MN. BELIEVE THIS THREAT COULD EXTEND INTO THE AREA NORTH OF FORT DODGE AND WEST OF MASON CITY UNTIL AROUND 7 PM...BUT THE WEAK STORM RELATIVE FLOW IN MID LEVELS INDICATES ROTATING STORMS AND ANY TORNADO THAT WOULD DEVELOP WOULD BE SHORT LIVED IN DMX FORECAST AREA. HEAVY RAIN IS ALSO POSSIBLE IN LOCAL AREAS IF THE COLD POOL STALLS AND CELLS REDEVELOP ON BACK SIDE. DIFFICULT TO FAVOR AN AREA AT THIS POINT BUT IT COULD BE ACROSS CENTRAL IOWA. THIS WOULD NOT BE SAME LOCATIONS AS HAD HEAVY RAIN LAST NIGHT...SO WILL NOT GO WITH A FLASH FLOOD WATCH AT THIS TIME. IF LINE ORIENTATION SHOWS ITS HAND...THIS COULD BE ADDED LATER. WEAK MOIST FLOW WILL LIKELY ALLOW SOME FOG TO DEVELOP BY MORNING. .LONG TERM /SATURDAY THROUGH FRIDAY/... NO MAJOR CHANGES IN LARGE SCALE PATTERN FOR THE NEXT COUPLE OF DAYS. AFTER ANOTHER ROUND OF CONVECTION TONIGHT...SHOULD SEE A BREAK FOR A PORTION OF SATURDAY UNTIL INSTABILITY RETURNS IN THE LATE AFTERNOON AND EVENING. THEN...STORMS WILL FIRE IN THE AREAS OF BEST SFC CONVERGENCE AND OVER NORTHWEST AS UPSTREAM WAVE FINALLY PUSHES EAST INTO THE AREA LATE SATURDAY NIGHT AND SUNDAY. OVERNIGHT SATURDAY NIGHT BEST AREAS FOR CONVECTION SHOULD REMAIN NORTHWEST OR NORTH GIVEN THE PROXIMITY OF THE SYNOPTIC SCALE BOUNDARIES. HAVE RAISED HIGHS ON SATURDAY INTO THE MID 80S AND SHOULD BE A RATHER WARM AND HUMID OVERNIGHT SATURDAY NIGHT. SUNDAY WILL BE WARM AND HUMID BUT HAVE CUT BACK ON PM HIGHS DUE TO EXPECTED DEBRIS CLOUDS AND SCT STORMS ON SUNDAY. MEANWHILE BY AFTERNOON LI VALUES WILL DECREASE TO -8 TO -11 ACROSS THE REGION WITH MLCAPES APPROACHING 3000 J/KG ONCE AGAIN. WITH INCREASING DYNAMICS AND WIND SHEAR...SEVERE STORMS AGAIN LOOK MORE LIKELY WITH EXPECTATION OF MORE WIDESPREAD COVERAGE THAN ON SATURDAY. WITH EXPECTED STORM MOTIONS TO BE RATHER MEAGER ...THREAT FOR ADDITIONAL HEAVY RAINFALL ALSO POSSIBLE AS SFC CONVERGENCE INCREASES ALONG THE COLD FRONT PASSING THROUGH THE STATE. WITH MOST OF THE REGION STILL ABLE TO HANDLE SOME MODERATE TO HEAVY RAINS...WILL NOT TRY TO EXACT AREA OF GREATEST THREAT AT THIS TIME DUE TO SOME EVENTUAL ALTERATIONS BASED ON THE MESOSCALE EVOLUTION OF THE STORMS THIS WEEKEND. MODELS HAVE BEEN MORE RAINFALL OVER THE NORTH HALF OF THE FORECAST AREA...AND WILL LEAN IN THAT DIRECTION FOR NOW. AFTER THIS SYSTEM PROGRESSES EAST ON SUNDAY NIGHT OUR WEATHER WILL QUIET DOWN FOR THE EARLY PART OF NEXT WEEK. AT LEAST ON MONDAY SOME SUNSHINE WILL RETURN PRIOR TO AN UPPER LEVEL TROUGH DROPPING SOUTH INTO THE AREA TUESDAY. THE 12Z GEM/GFS HAVE BOTH TRENDED TOWARD A MORE EASTERLY SOLUTION FOR THE H500 LOW DROPPING SOUTH INTO THE GREAT LAKES RATHER THAN THE CENTRAL PLAINS. THIS WOULD FAVOR LESS CLOUDS AND NO PRECIP RATHER THAN UPPER LEVEL SUPPORT FOR ISO AND SCT THUNDER ACROSS THE AREA. BY MID WEEK THROUGH THE END OF THE WEEK A RIDGE WILL REBUILD INTO THE AREA AND ALLOW TEMPERATURES TO REBOUND INTO THE 70S TO NEAR 80 THROUGH THE PERIOD. VERY LATE IN THE EXTENDED...A TROUGH WILL APPROACH AND ALLOW FOR A RETURN CHANCE OF SHOWERS AND THUNDERSTORMS ACROSS THE REGION BY THURSDAY NIGHT OR FRIDAY. && .AVIATION...04/18Z MOIST SOUTHEASTERLY FLOW WILL CONTINUE THROUGH THE 24 HOUR TAF PERIOD...BUT NOT A LOT OF STRONG FORCING FOR ORGANIZED INTENSE THUNDERSTORMS. CONVECTION HAS DISRUPTED WARM FRONT LEAVING A BOUNDARY IN NORTH IOWA AND NORTH MISSOURI. THUNDERSTORMS MOST LIKELY TO WORK EAST ACROSS NORTHERN IOWA THIS EVENING...SO HAVE INCLUDED THEM IN TAFS...FOR KDMX AND KOTM...CHANCES ARE TOO SMALL TO INCLUDE AT THIS TIME. GIVEN HIGH MOISTURE EXPECT FOG AND/OR LOW STRATUS TO DEVELOP AGAIN OVERNIGHT ACROSS STATE...AND THIS WILL TAKE SOME TIME TO BURN OFF SATURDAY MORNING. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE && SHORT TERM/AVIATION...JUNGBLUTH LONG TERM...REV
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE DES MOINES IA 303 PM CDT FRI MAY 4 2012 IAZ004>007-015>017-023>026-033>036-044-045-052015- EMMET-KOSSUTH-WINNEBAGO-WORTH-PALO ALTO-HANCOCK-CERRO GORDO- POCAHONTAS-HUMBOLDT-WRIGHT-FRANKLIN-SAC-CALHOUN-WEBSTER-HAMILTON- CRAWFORD-CARROLL- 303 PM CDT FRI MAY 4 2012 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...THIS AFTERNOON AND TONIGHT SCATTERED SEVERE THUNDERSTORMS ARE POSSIBLE THIS AFTERNOON AND EVENING. THE MAIN THREAT WILL BE FOR LARGE HAIL AND DAMAGING THUNDERSTORM WINDS. ISOLATED TORNADOES ARE ALSO POSSIBLE. THE THREAT WILL CONTINUE UNTIL 9 PM THIS EVENING ACROSS THE OUTLOOK AREA. THESE STORMS COULD ALSO BRING HEAVY RAINS ALONG WITH THE THREAT FOR SEVERE WEATHER. LOCALLY HEAVY RAINS AND FLASH FLOODING ARE ALSO POSSIBLE IN SOME OF THE SLOWER MOVING STORMS AS MOISTURE LEVELS REMAIN HIGH ACROSS THE REGION. .DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY OUR RECENT ACTIVE WEATHER PATTERN WILL CHANGE LITTLE THROUGH THE WEEKEND WITH OFF AND ON CHANCES FOR THUNDERSTORMS. A FEW OF THESE STORMS MAY STILL BE SEVERE WITH LARGE HAIL AND DAMAGING WINDS THE PRIMARY THREATS. LOCALLY HEAVY RAINS AND FLASH FLOODING REMAIN POSSIBLE AS WELL. THE BEST CHANCES FOR WIDESPREAD THUNDERSTORMS WILL BE SATURDAY NIGHT AND SUNDAY ACROSS THE REGION. OTHER LESSER THUNDERSTORM CHANCES ARE IN THE FORECAST EARLY NEXT WEEK...BUT THE POTENTIAL FOR SEVERE WEATHER APPEARS LOW AT THIS TIME. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION MAY BE NEEDED THIS EVENING AND TONIGHT. IAZ027-028-037>039-046>050-057>062-070>075-081>086-092>097-052015- BUTLER-BREMER-HARDIN-GRUNDY-BLACK HAWK-GREENE-BOONE-STORY-MARSHALL- TAMA-AUDUBON-GUTHRIE-DALLAS-POLK-JASPER-POWESHIEK-CASS-ADAIR-MADISON- WARREN-MARION-MAHASKA-ADAMS-UNION-CLARKE-LUCAS-MONROE-WAPELLO-TAYLOR- RINGGOLD-DECATUR-WAYNE-APPANOOSE-DAVIS- 303 PM CDT FRI MAY 4 2012 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...THIS AFTERNOON AND TONIGHT STRONGER THUNDERSTORMS ARE EXPECTED TO REDEVELOP LATE THIS AFTERNOON OR EVENING AND CONTINUE THROUGH THE NIGHT. THESE STORMS COULD BRING HEAVY RAINS ALONG WITH THE THREAT FOR SEVERE WEATHER. LARGE HAIL AND DAMAGING WINDS ARE THE MAIN THREATS THOUGH AN ISOLATED TORNADO IS STILL POSSIBLE. LOCALLY HEAVY RAINS AND FLASH FLOODING ARE ALSO POSSIBLE IN SOME OF THE SLOWER MOVING STORMS AS MOISTURE LEVELS REMAIN HIGH ACROSS THE REGION. .DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY OUR RECENT ACTIVE WEATHER PATTERN WILL CHANGE LITTLE THROUGH THE WEEKEND WITH OFF AND ON CHANCES FOR THUNDERSTORMS. A FEW OF THESE STORMS MAY STILL BE SEVERE WITH LARGE HAIL AND DAMAGING WINDS THE PRIMARY THREATS. LOCALLY HEAVY RAINS AND FLASH FLOODING REMAIN POSSIBLE AS WELL. THE BEST CHANCES FOR WIDESPREAD THUNDERSTORMS WILL BE SATURDAY NIGHT AND SUNDAY ACROSS THE REGION. OTHER LESSER THUNDERSTORM CHANCES ARE IN THE FORECAST EARLY NEXT WEEK...BUT THE POTENTIAL FOR SEVERE WEATHER APPEARS LOW AT THIS TIME. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION MAY BE NEEDED THIS EVENING AND TONIGHT. REV
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
1046 AM CDT FRI MAY 04 2012
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0538 AM HAIL NNW LENOX 40.88N 94.56W
05/04/2012 E1.00 INCH TAYLOR IA TRAINED SPOTTER
TIME ESTIMATED FROM RADAR.
&&
KDPRELIMINARY LOCAL STORM REPORT...CORRECTED
NATIONAL WEATHER SERVICE DES MOINES IA
1049 AM CDT FRI MAY 04 2012
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0338 AM HAIL 5 NNW LENOX 40.88N 94.56W
05/04/2012 E1.00 INCH TAYLOR IA TRAINED SPOTTER
TIME ESTIMATED FROM RADAR.
&&
KDPRELIMINARY LOCAL STORM REPORT...CORRECTED
NATIONAL WEATHER SERVICE DES MOINES IA
1055 AM CDT FRI MAY 04 2012
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0338 AM HAIL NNW LENOX 40.88N 94.56W
05/04/2012 E1.00 INCH TAYLOR IA TRAINED SPOTTER
TIME ESTIMATED FROM RADAR.
&&
KDPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
124 PM CDT FRI MAY 04 2012
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0303 AM HAIL 5 ESE CORNING 40.97N 94.65W
05/04/2012 E1.25 INCH ADAMS IA PUBLIC
TIME ESTIMATED FROM RADAR.
&&
KSPRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE DES MOINES IA
132 PM CDT FRI MAY 04 2012
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0234 AM HAIL ENE PRESCOTT 41.02N 94.61W
05/04/2012 E1.00 INCH ADAMS IA TRAINED SPOTTER
TIME ESTIMATED FROM RADAR.
0300 AM HAIL PRESCOTT 41.02N 94.61W
05/04/2012 E0.88 INCH ADAMS IA EMERGENCY MNGR
TIME ESTIMATED.
0303 AM HAIL 5 ESE CORNING 40.97N 94.65W
05/04/2012 E1.25 INCH ADAMS IA PUBLIC
TIME ESTIMATED FROM RADAR.
0338 AM HAIL NNW LENOX 40.88N 94.56W
05/04/2012 E1.00 INCH TAYLOR IA TRAINED SPOTTER
TIME ESTIMATED FROM RADAR.
&&
KSPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
412 PM CDT FRI MAY 04 2012
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0412 PM TSTM WND GST DOLLIVER 43.46N 94.61W
05/04/2012 E65 MPH EMMET IA EMERGENCY MNGR
&&
JJOHNSONThe Sioux Falls radar (KFSD), 171 km away, was scanning. The volume nearest the first event began at 21:11 UTC.
5 hail, 3 thunderstorm wind, 1 tornado, 1 funnel cloud. Local standard time, as filed.
The Emergency Manager estimated wind gusts of 65 MPH.
Wind gusts of 65 MPH were reported by law enforcement.
Nickel to quarter size hail reported.
Nickel to quarter size hail with ground covered.
Brief touchdown just south of the Minnesota border. Damaged a farm building and some trees.
Funnel cloud with two to three gustnadoes reported. Brief appearance with rotating wall cloud.
Golf ball size hail was reported on the north side of town with half dollar size hail reported at the Casey's location.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Iowa in 2012Every state in 2012This day in weather historyThe tornado archive for Iowa, 2012Warnings in Iowa, 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: 368941, 368942, 368943, 368946, 368999, 368998, 370091, 368954, 368955, 368969.