A strong cold front moved into Iowa during the afternoon and evening of the 15th into the morning hours of the 16th. The atmosphere was somewhat capped initially, however thunderstorms were able to develop after midnight.
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 strong cold front moved into Iowa during the afternoon and evening of the 15th into the morning hours of the 16th. The atmosphere was somewhat capped initially, however thunderstorms were able to develop after midnight. The available CAPE was in the 2000 to 3000 J/kg range. Downdraft CAPE was in the 800 to 1200 J/kg, with 700 J/kg available in the -10 to -30 C layer of the atmosphere. The freezing level was quite high, in excess of 15,000 feet. Deep moisture convergence took place during the evening of the 15th with precipitable water values around 1.6 inches. The atmosphere was weakly sheared with about 30 kts of effective shear present. The storms had strong cores present on radar, however the high freezing level and lack of tilt in the storms limited hail size. Hail up to ping pong ball size fell in Dallas County. Gusty winds of 45 to 55 MPH were reported across Guthrie, Dallas, and Polk Counties. A wind gust of 62 MPH was measured northwest of Des Moines in Polk County. Many of the storms produced winds in excess of 40 MPH. In addition to the wind, numerous reports of hail from pea to nickel size were received.
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 Aug 15, 9:18 PM CST, 4 hours 47 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DES MOINES IA 1018 PM CDT WED AUG 15 2012 .UPDATE 16/06Z AVIATION DISCUSSION... .SHORT TERM /TONIGHT/... HAVE REWORKED ZONES A BIT WITH SLOWER ONSET OP CONVECTION AND SO FAR...LITTLE CONVERGENCE ALONG THE FRONT OVER NORTHWEST IOWA. SO FAR MOST ACTIVITY IN COOLER H700 AIR IN MN. AS THE MID LEVELS COOL OVERNIGHT BETTER CHANCES FOR THUNDER TO GET GOING. HAVE TRIMMED BACK SOUTH UNTIL AFTER MIDNIGHT AND HIT THE NORTHEAST A BIT HARDER WITH EXPECTED COVERAGE TO BE HIGHER THERE COMPARED TO THE SOUTH AS WARM AIR ADVECTION AND H850 LOW LEVEL JET INCREASES TO 30-35KTS AFTER 06Z MAINLY EAST AND NORTHEAST. CONSEQUENTLY WITH LESSER CONVECTION... HAVE ALSO LOWERED QPF AMOUNTS SOUTH/WEST FRINGES. LOWS ON TRACK AS WELL. /REV .PREVIOUS SHORT TERM /TONIGHT/... POTENT UPPER LEVEL SYSTEM DROPPING SOUTHEAST INTO MONTANA AND SOUTH DAKOTA THIS AFTERNOON. ASSOCIATED LOW PRESSURE MOVING INTO WESTERN ONTARIO WITH A COLD FRONT EXTENDING THROUGH FAR NORTHWEST MINNESOTA AND SOUTH INTO CENTRAL NEBRASKA. SYSTEM WILL PROGRESS SLOWLY SOUTHEAST OVERNIGHT WITH THE KINEMATIC FORCING ARRIVING OVER THE NORTHEAST AROUND 03Z. MASS THETA-ADVECTION WILL SPREAD ACROSS CENTRAL IOWA IN ADVANCE OF THE UPPER LEVEL DYNAMICS WITH THE INSTABILITY AXIS SHIFTING EAST AND BEING CENTERED NEAR THE INTERSTATE 35 CORRIDOR BY 06Z. MUCAPE VALUES OF 1500 TO 2500 J/KG ARE EXPECTED WITH DCAPE VALUES INCREASING TO NEAR 1000 J/KG WITH WIND PROFILES BECOMING MORE PARALLEL. THIS SET UP SHOULD BE FAVORABLE FOR COLD POOL DEVELOPMENT AND BOWING SEGMENTS WITH WIND GUST POTENTIAL TO 70 MPH. WHILE A FEW STORMS LIKELY WILL PRODUCE LARGE HAIL...THE DAMAGING WINDS WILL BE THE PRIMARY THREAT. A FEW NON SEVERE STORMS ARE POSSIBLE THE REMAINDER OF THE AFTERNOON AS CURRENT WAVE OF THETA-E ADVECTION LIFTS THROUGH. THE FRONTAL BOUNDARY WILL BE OVER THE SOUTHEAST FORECAST AREA BY 12Z. SHOWERS AND ISOLATED THUNDERSTORMS WILL LINGER POST FRONTAL THOUGH SHOULD BE ENDING ACROSS THE NORTHWEST AS DRY AIR BEGINS TO ADVECT IN. OVERNIGHT LOW TEMPERATURES WILL BE DIFFICULT AND DEPENDENT ON STRENGTH OF CONVECTIVE COLD POOLS. .LONG TERM /THURSDAY THROUGH WEDNESDAY/... LONG WAVE TROUGH WILL PUSH DEEP COLD FRONT THROUGH IA OVERNIGHT WHICH SHOULD BE EXITING STATE BY THE BEGINNING OF THE PERIOD. POST FRONTAL FORCING WILL LINGER ACROSS SE HALF OF THE STATE HOWEVER RESULTING IN SCATTERED SHOWERS AND SOME THUNDER...MAINLY IN THE MORNING. 18Z NAM AS WELL AS PAST TWO HI RES WRF RUNS INDICATE BACK EDGE OF PRECIP ALONG DENISON-WEBSTER CITY-CHARLES CITY LINE BY 12Z AND JUST EXITING SE SECTIONS BY 18Z. LARGE HIGH PRESSURE AREA WILL THEN DROP INTO IA AND SURROUNDING STATES WITH AIRMASS CHANGE THE PRIMARY STORY. COOL AND DRY WEATHER IS ANTICIPATED INTO THE WEEKEND WITH THE NEXT MINOR PRECIP WINDOW AROUND SAT. SLIGHT CHANCES HAVE BEEN IN THE FORECAST AREA WIDE FOR WEAK NW FLOW SHORT WAVE BUT MODELS HAVE NOW PUSHED ITS TRACK FARTHER WEST INTO MO VALLEY AND BEYOND SO CHANCES ARE NOW ONLY RELEGATED TO WRN SECTIONS. THIS PATTERN WITH ONTARIO LONG WAVE TROUGH AND IA IN NW FLOW REMAINS IN PLACE THROUGH EARLY NEXT WEEK. THE STRENGTH OF THE ROCKIES RIDGE DOES BEGIN TO BREAK DOWN AROUND TUE OR WED HOWEVER ALLOWING SOME INCREASE IN WARM ADVECTION AND A FEW SHORT WAVES TOPPING THE RIDGE. THIS RESULTS IN SLIGHT CHANCES AROUND THAT TIME BUT EVEN IF IT DOES MATERIALIZE...NOTHING THAT WOULD HAVE ANY IMPACT ON OUR DROUGHT CONDITIONS. && .AVIATION...16/06Z MAIN CHALLENGE THIS PERIOD CONTINUES TO BE COVERAGE OF CONVECTION OVERNIGHT. FRONT ENTERING NORTHWEST IOWA NOW AND CONVERGENCE BEGINNING ALONG BOUNDARY IN NORTHERN IOWA. AFT 05Z CONDITIONS SHOULD BECOME MORE FAVORABLE FOR DEVELOPMENT AND AM CONTINUING TRENDS FROM EARLIER TONIGHT FOR CONVECTION. BETTER CHANCES FOR COVERAGE TO REMAIN INTACT AT THE NORTH TAF SITES KFOD...KMCW...AND KALO AS FRONT MOVES EAST ACROSS THE STATE OVERNIGHT DUE TO INCREASING LOW LEVEL JET MAKES ITS WAY INTO THE STATE. AT SOUTH SITES THE COVERAGE LOOKS TO BE MORE SCATTERED AND WILL CONTINUE VCTS THERE GIVEN UNCERTAINTY OF COVERAGE AFT 06Z. FOLLOWING THE FRONT...STRONG HIGH PRESSURE BUILDS IN FOR THURSDAY AND MIXING WINDS TO NEAR 30KTS EXPECTED NORTH AND TO NEAR OR ABOVE 25KTS SOUTH SITES. WINDS TO REMAIN WELL MIXED MOST OF AFTERNOON THOUGH MAIN PUSH OF COLD AIR ADVECTION AND STRONGER WINDS TO RELAX SLOWLY AFT 18Z THROUGH END OF TAF PERIOD FROM NORTHWEST TO SOUTHEAST. /REV && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && SHORT TERM...DONAVON/REV AVIATION...REV LONG TERM...SMALL
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE DES MOINES IA 1112 PM CDT WED AUG 15 2012 IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086- 092>097-161100- EMMET-KOSSUTH-WINNEBAGO-WORTH-PALO ALTO-HANCOCK-CERRO GORDO- POCAHONTAS-HUMBOLDT-WRIGHT-FRANKLIN-BUTLER-BREMER-SAC-CALHOUN- WEBSTER-HAMILTON-HARDIN-GRUNDY-BLACK HAWK-CRAWFORD-CARROLL-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- 1112 PM CDT WED AUG 15 2012 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...TONIGHT SCATTERED THUNDERSTORMS ARE EXPECTED TO DEVELOP ACROSS THE AREA OVERNIGHT. A FEW OF THESE STORMS COULD BE STRONG TO SEVERE...WITH DAMAGING WIND AND LARGE HAIL THE PRIMARY THREATS. .DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY ISOLATED THUNDERSTORMS WILL TAPER OFF TO THE SOUTHEAST ON THURSDAY. THERE IS ALSO A SLIGHT CHANCE OF THUNDERSTORMS WEST ON SATURDAY...AND AGAIN AREA WIDE AROUND TUESDAY. SEVERE WEATHER IS NOT ANTICIPATED THROUGH THIS PERIOD. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION IS POSSIBLE THIS EVENING AND EARLY TONIGHT. REV
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE DES MOINES IA 1254 AM CDT THU AUG 16 2012 IAZ044-160630- CRAWFORD- 1254 AM CDT THU AUG 16 2012 ...STRONG THUNDERSTORMS APPROACHING FROM THE WEST... AT 1249 AM CDT...STRONG THUNDERSTORMS ALONG A LINE EXTENDING FROM 16 MILES NORTHWEST OF DENISON TO 20 MILES SOUTH OF DENISON...WERE MOVING EAST AT 30 MPH. PENNY SIZE HAIL...WINDS GREATER THAN 30 MPH...BRIEF HEAVY DOWNPOURS...ARE POSSIBLE WITH THESE STORMS. GUSTY WINDS MAY CAUSE SMALL OBJECTS SUCH AS TRASH BINS TO BLOW AROUND. SEEK SHELTER IN A STURDY STRUCTURE UNTIL THESE STORMS HAVE PASSED.
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE DES MOINES IA 226 AM CDT THU AUG 16 2012 IAZ023-033>035-160800- CALHOUN-POCAHONTAS-SAC-WEBSTER- 226 AM CDT THU AUG 16 2012 ...LINE OF STRONG THUNDERSTORMS MOVING THROUGH THE AREA... AT 221 AM CDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED STRONG THUNDERSTORMS ALONG A LINE EXTENDING FROM SOUTHWESTERN POCAHONTAS COUNTY TO EXTREME EAST CENTRAL IDA COUNTY...MOVING EAST AT 40 MPH. NICKEL SIZE HAIL...WINDS GREATER THAN 40 MPH...BRIEF HEAVY DOWNPOURS...ARE POSSIBLE WITH THESE STORMS. LOCATIONS IMPACTED INCLUDE... PALMER... WALL LAKE... POMEROY... LYTTON... LAKE VIEW... JOLLEY... YETTER... MANSON... AUBURN... GUSTY WINDS MAY CAUSE SMALL OBJECTS SUCH AS TRASH BINS TO BLOW AROUND. SEEK SHELTER IN A STURDY STRUCTURE UNTIL THESE STORMS HAVE PASSED.
The Des Moines radar (KDMX), 53 km away, was scanning. The volume nearest the first event began at 08:06 UTC.
6 hail, 3 thunderstorm wind. Local standard time, as filed.
Numerous tree branches greater than 6 inches in diameter down.
Dallas County Emergency Management reported quarter to half dollar size hail falling in the Adel area along with 40 to 50 MPH winds.
Reported at the intersection of Highway 4 and 141.
Relayed by the Guthrie County Emergency Manager.
Gusts of 59 and 62 MPH were measured by a home weather station in the Urbandale Cross Creek neighborhood.
One foot diameter tree down in the 8300 block of Barnham Dr. in Johnston. Lawn furniture and small pools blown around the city. Report received from the Johnston Public Works Department.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Iowa in 2012Every state in 2012This day in weather historyWarnings in Iowa, August 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: 391735, 391539, 391470, 391540, 391471, 391543, 391545, 391736, 391476.