A broken line of thunderstorms developed behind a slow moving cold front in eastern Nebraska and western Iowa during the afternoon of June 20th. Instability across the region was modestly high, but wind shear was marginal and thus the storms mainly produced areas of heavy rain.
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 OAX wrote it up.
A broken line of thunderstorms developed behind a slow moving cold front in eastern Nebraska and western Iowa during the afternoon of June 20th. Instability across the region was modestly high, but wind shear was marginal and thus the storms mainly produced areas of heavy rain. However, one wet microburst occurred in the Omaha area causing tree and property damage, and training of heavy rain producing cells caused some flash flooding north of Omaha.
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 Omaha, NE wrote its last area forecast discussion before the first event at Jun 20, 2:02 PM CST, 2 hours 14 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE OMAHA/VALLEY NE 302 PM CDT WED JUN 20 2012 .DISCUSSION... A COLD FRONT STALLED OUT OVER EASTERN NEBRASKA AND WEST CENTRAL IOWA THIS MORNING...ALLOWING SOUTHERLY WINDS TO CONTINUE TO PUMP LOW LEVEL MOISTURE INTO OUR AREA. BREAKS IN CLOUD COVER PUSHED TEMPERATURES INTO THE MIDDLE 80S WITH SLOWLY COOLING TEMPERATURES ALOFT. THIS COMBINATION WAS YIELDING SURFACE-BASED AND MIXED-LAYER CAPE VALUES OVER 2000 J/KG. EFFECTIVE BULK SHEAR WAS MINIMAL SOUTH OF THE FRONT...BUT NEAR 40KTS NEAR AND BEHIND THE BOUNDARY. STILL HAVE A LITTLE CONVECTIVE INHIBITION TO OVERCOME SOUTH OF COLD FRONT...BUT SOME RADAR RETURNS WERE STARTING TO SHOW UP IN SOUTHEAST NEBRASKA. EXPECT STORMS TO FIRE SHORTLY SOUTH OF INTERSTATE 80 WHERE INSTABILITY IS HIGHEST...AND COULD SEE AN ISOLATED HAIL OR GUSTY WIND REPORT WITH THESE. BEHIND THE FRONT...ISENTROPIC UPGLIDE INDUCED BY SHORTWAVE ROTATING THROUGH THE NORTHERN PLAINS WILL AID SCATTERED SHOWERS AND THUNDERSTORMS ACROSS OUR NORTHERN AREAS INTO THE EVENING. THEN FOCUS WILL SHIFT SOUTH WITH FRONTAL BOUNDARY THAT WILL SLOWLY SAG SOUTH OF THE CWA AFTER MIDNIGHT. SO STORMS WILL BE MOVING OUT OF OUR SOUTHEAST CWA AFTER MIDNIGHT AND SHOULD BE WELL SOUTH OF THE AREA BY THURSDAY MORNING. HIGH PRESSURE SETTLING IN BEHIND THE FRONT WILL BRING NORTHWEST WINDS AND HELP DRY THINGS OUT FOR A TIME AS SURFACE DEW POINTS DROP INTO THE 50S. HOWEVER A QUICK TURN TO SOUTH OR SOUTHEAST WINDS COMES THURSDAY NIGHT AND FRIDAY ON BACK SIDE OF THE HIGH...PUSHING DEW POINTS BACK INTO THE 60S BY FRIDAY AFTERNOON OR EVENING. UPPER FLOW BECOMES NORTHWESTERLY ON FRIDAY AHEAD OF RIDGE MOVING INTO THE PLAINS. A SIGNIFICANT SHORTWAVE IS FORECAST TO RIDE SOUTHEAST OVER THE RIDGE AND INTO NEBRASKA/SOUTH DAKOTA AREA FRIDAY NIGHT. A MAJORITY OF MODEL AND ENSEMBLE FORECASTS INDICATE THIS SHORTWAVE...BUT EXACT TRACK AND RESULTANT AXIS OF PRECIPITATION IS STILL IN QUESTION. 12Z ECMWF IS FARTHER NORTH THAN ITS PREVIOUS FORECAST...PLACING PRECIPITATION ACROSS SOUTH DAKOTA MUCH OF FRIDAY NIGHT...THEN TRACKING IT SOUTHEAST THROUGH EASTERN NEBRASKA AND WESTERN IOWA LATE FRIDAY NIGHT AND SATURDAY. GFS IS MUCH MORE BEARISH IN TRACKING PRECIPITATION INTO EASTERN NEBRASKA. GIVEN STRONG WARM ADVECTION AHEAD OF THIS SYSTEM...HAVE OPTED TO INCREASE POPS IN OUR NORTH FRIDAY NIGHT...THEN SPREAD PRECIP SOUTHEAST THROUGH OUR EASTERN CWA SATURDAY MORNING AS PER ECMWF FORECAST. RIDGE BEGINS TO BUILD INTO THE PLAINS LATER SATURDAY THROUGH MUCH OF NEXT WEEK. MODELS ARE SHOWING VARIOUS SOLUTIONS ON STRENGTH OF THIS RIDGE...BUT MOST CONCLUDE AXIS WILL LIE JUST TO OUR WEST. WARM AND HUMID CONDITIONS LOOK LIKELY. PRECIP CHANCES WILL BE LOW...BUT WILL HAVE TO WATCH RIPPLES OF ENERGY WORKING THROUGH CYCLONIC FLOW AROUND UPPER LOW HOLDING OVER THE GREAT LAKES. DERGAN && .AVIATION... 18Z TAFS FOR KOMA...KLNK...AND KOFK. MVFR CIGS ARE AFFECTING ALL SITES THIS AFTERNOON. A FRONT HAS MOVED THROUGH KOFK...WHERE WINDS WILL SHIFT FROM NORTHEASTERLY TO NORTHWESTERLY THIS AFTERNOON. KEPT PRECIP MENTION OUT OF KOFK AS THE BULK OF THE PRECIP LIKELY WILL STAY WITH THE FRONT. THE FRONT WILL REACH KOMA/KLNK LATER THIS AFTERNOON...LIKELY ACCOMPANIED BY SHOWERS AND THUNDERSTORMS WITH VIS AND CIG REDUCTIONS. BEHIND THE PRECIPITATION...CIGS WILL LIFT TO VFR...WITH WINDS OUT OF THE NORTHWEST BUT AT OR BELOW 10KT THROUGH THE REST OF THE EVENING INTO TOMORROW MORNING. MAYES && .OAX WATCHES/WARNINGS/ADVISORIES... NE...NONE. IA...NONE. &&
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE OMAHA/VALLEY NE 434 PM CDT WED JUN 20 2012 IAZ043-055-056-069-079-080-090-091-NEZ011-012-015>018-030>034- 042>045-050>053-065>068-078-088>093-210800- ANTELOPE-BOONE-BURT-BUTLER-CASS-CEDAR-COLFAX-CUMING-DODGE-DOUGLAS- FREMONT IA-GAGE-HARRISON IA-JEFFERSON-JOHNSON-KNOX-LANCASTER-MADISON- MILLS IA-MONONA IA-MONTGOMERY IA-NEMAHA-OTOE-PAGE IA-PAWNEE-PIERCE- PLATTE-POTTAWATTAMIE IA-RICHARDSON-SALINE-SARPY-SAUNDERS-SEWARD- SHELBY IA-STANTON-THURSTON-WASHINGTON-WAYNE- 434 PM CDT WED JUN 20 2012 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF SOUTHWEST IOWA...WEST CENTRAL IOWA...EAST CENTRAL NEBRASKA...NORTHEAST NEBRASKA AND SOUTHEAST NEBRASKA. .DAY ONE...TONIGHT AND THURSDAY SCATTERED SHOWERS AND THUNDERSTORMS WILL MOVE ACROSS THE OUTLOOK AREA THIS EVENING...THEN GRADUALLY DISSIPATE AFTER MIDNIGHT. LOCALLY HEAVY RAINFALL WILL BE POSSIBLE WITH ANY SLOW MOVING STORMS. .DAYS TWO THROUGH SEVEN...THURSDAY NIGHT THROUGH WEDNESDAY THE NEXT CHANCE FOR THUNDERSTORMS WILL BE FRIDAY NIGHT INTO EARLY SUNDAY MORNING. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION IS NOT ANTICIPATED THROUGH THURSDAY.
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE OMAHA/VALLEY NEBRASKA
516 PM CDT WED JUN 20 2012
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0516 PM FLASH FLOOD 2 NW MAPLETON 42.19N 95.82W
06/20/2012 MONONA IA EMERGENCY MNGR
REPORT OF WATER OVER THE ROAD ON HWY 141 BETWEEN
SMITHLAND AND MAPLETON.
&&
DEEThe Omaha radar (KOAX), 107 km away, was scanning. The volume nearest the first event began at 22:15 UTC.
2 flash flood. Local standard time, as filed.
Rainfall estimated to have been at least 3 inches caused flash flooding across parts of Monona county, including across highway 141 northwest of Mapleton. The flooding caused no apparent damage.
Rainfall of 2 to 3 inches in a few hours caused flash flooding across several street in the Onawa area. However, damage was apparently minimal.
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, June 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: 378273, 378274.