Strong warm air advection took place over Iowa during the night of the 22nd into the morning of the 23rd. A low level jet of over 40 kts developed, transporting moisture into the state.
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.
Strong warm air advection took place over Iowa during the night of the 22nd into the morning of the 23rd. A low level jet of over 40 kts developed, transporting moisture into the state. Precipitable water values increased to between 1.7 and 2 inches over northwest into central Iowa. CAPE was in the 2000 to 4000 J/kg range to the southwest of the precipitable water maximum, with lifted indices of -6 to -10. Thunderstorms formed over South Dakota and moved into northwest and north central Iowa. The storms produced both hail and high winds as they advanced into the state. Downdraft CAPE was quite high with 1200 to 1400 J/kg available. CAPE in the -10 to -30 C layer of the atmosphere was between 500 and 1000 J/kg, however a very high freezing level limited the size of the hail as the freezing level was around 15,000 feet. The atmosphere was moderately sheared with 35 to 45 kts of shear available. By far the predominant mode of severe weather was the high winds. Trees were uprooted in Palo Alto County at Emmetsburg. Several other reports of winds of 60 to 70 MPH were received in Palo Alto and Kossuth County. A mesonet station northeast of Whittemore recorded a 75 MPH wind gust, with 68 MPH winds recorded at the Algona Airport for example. In Webster County, one inch diameter hail fell north of the Fort Dodge Airport. As the thunderstorms progressed eastward, the intensity diminished. There were spotty reports of high winds and hail across north central and parts of northeast Iowa. Scattered thunderstorms developed behind the main area before sunrise. For the most part they were not severe, but did produce high winds and some nickel to quarter size hail.
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 22, 10:35 PM CST, 2 hours 39 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DES MOINES IA 1135 PM CDT MON AUG 22 2011 .SHORT TERM /TONIGHT/... WARM FRONT OVER NW TO CENTRAL IOWA WILL CONTINUE ADVANCING NORTH ACROSS THE REGION NOW...WITH A FEW MINOR CONVECTIVE ELEMENTS STILL BUBBLING UP ALONG THE BOUNDARY THIS AFTERNOON. NORTHERN STREAM SHORTWAVE NEARING FSD AREA NOW...AND SHOULD BEGIN TO INCREASE STORM COVERAGE ACROSS THE NORTHWEST THIS AFTERNOON/EVENING AS IT APPROACHES WARM FRONT AND RIBBON OF HIGHER INSTABILITY. STILL SOME UNCERTAINTIES IN THE FORECAST TONIGHT WITH REGARD TO THE LOCATION OF EVENTUAL MCS EXPECTED TO DEVELOP. CURRENT SUBJECTIVE ANALYSIS AND SPC RUC ANALYSIS SUGGESTS THAT STORMS SHOULD FIRE NW SECTIONS AND HEAD EAST SOUTHEAST WITH TIME THIS EVENING AND OVERNIGHT. 4KM WRF APPEARS TOO FAR WEST WITH EVENTUAL EVOLUTION WHILE GFS/NAM/EURO HAVE BETTER IDEA OF EVOLUTION THROUGH MORNING HOURS...TRACKING MAINLY ALONG THE WARM FRONT AND TAPPING INTO INSTABILITY AXIS WHICH SHOULD BE DRIFTING NORTHEAST TONIGHT AS LOW LEVEL FLOW AND UPPER LEVEL FLOW RESPOND TO LOW OVER SOUTHERN CANADA. OF THE THREE MODELS...THE EURO APPEARS TO BE SLIGHTLY FASTER WITH UPPER LEVEL SUPPORT AND KEEPS THE MCS FARTHER NORTH AND EAST THAN THE GFS/NAM. AM LEANING MORE TOWARD THE GFS/NAM SOLNS ATTM. BULK SHEAR AND INSTABILITY APPEARS ENOUGH TO SUPPORT SOME SCT SEVERE STORMS NORTHWEST THIS EVENING AND THEN OVER THE NORTHEAST TOWARD SUNRISE. MAIN THREAT WILL REMAIN WIND AND HAIL. CORFIDI VECTORS SUGGEST FORWARD PROPAGATION WITH SOME SLIGHT SLOWING/TURNING TOWARD MORNING. WILL HAVE TO WATCH FOR SOME HEAVY RAINFALL NORTHEAST SECTIONS AFT 08Z IF STORMS BEGIN TO TRAIN SOUTHEAST. WITH GENERATION ZONE UPSTREAM...AND PROPAGATION SOUTHEAST...CURRENT PWATS OF 1.75 TO NEAR 2 INCHES FORECAST OVERNIGHT SUGGEST SOME POTENTIAL FOR LOCAL AMTS TO EXCEED 2-3 INCHES...IF TRAINING STORMS OVER THE NORTH AND NORTHEAST OCCURS TOWARD MORNING. CURRENT FFG IS RATHER HIGH WITH 1 AND 3 HOUR FLASH FLOOD GUIDANCE VALUES IN THE 3 TO 4 INCH RANGE NORTH AND NORTHEAST. ONLY CONCERNS WOULD BE FOR URBAN LOCATIONS...IF HEAVY RAIN OCCURS...THUS WILL NOT ISSUE A FLASH FLOOD WATCH. LOWS WILL REMAIN RATHER WARM WITH MID 60S TO LOWER 70S AND DEW POINTS CLIMBING BACK INTO UNCOMFORTABLE RANGES ONCE AGAIN. .LONG TERM /TUESDAY THROUGH MONDAY/... WITH THE SURFACE HIGH THAT HAS DOMINATED OUR AREA DEPARTING TO THE EAST OVERNIGHT WE WILL SEE WINDS TURN TO SOUTHERLY...THEN TO SOUTHWESTERLY ON TUESDAY WHICH WILL BRING SIGNIFICANTLY WARMER AND MORE HUMID AIR INTO OUR AREA. UPSTREAM 850 MB TEMPS ARE AROUND 25C OVER KS...AND EXPECT OUR SFC TEMPS TO GET INTO THE LOW TO POSSIBLY MID 90S AT A FEW LOCATIONS SOUTH OF I-80. COMBINED WITH DEW POINTS IN THE LOW TO MID 70S THIS WILL RESULT IN HEAT INDICES AROUND 100 TO 105...AND A HEAT ADVISORY MAY BE WARRANTED. FOR NOW THE THREAT OF REACHING ADVISORY CRITERIA IS TOO MINIMAL AND UNCERTAIN SO WILL LEAVE THIS DECISION TO SUBSEQUENT SHIFTS. MEANWHILE THE 500 MB SHORTWAVE RESPONSIBLE FOR OVERNIGHT STORMS AS DISCUSSED IN THE SHORT TERM SECTION ABOVE WILL BE SKIRTING BY JUST TO OUR NORTHEAST ON TUESDAY MORNING AND AWAY EASTWARD THEREAFTER. THIS WILL MAINTAIN SOME THUNDERSTORM CHANCES MAINLY ACROSS OUR NORTHEASTERN COUNTIES ON TUESDAY MORNING. ON TUESDAY AFTERNOON AND EVENING AROUND PEAK HEATING TIME THE GUIDANCE ENVELOPE CONTINUES TO SHIFT ADDITIONAL THUNDERSTORM DEVELOPMENT EASTWARD IN CONJUNCTION WITH THE FORCING ASSOCIATED WITH THE SHORTWAVE. HOWEVER...IF THE WAVE LINGERS JUST A BIT OR AFTERNOON CONVECTION DEVELOPS EARLIER THAN EXPECTED OR ALONG ANY SOUTH/SOUTHWESTWARD MOVING BOUNDARIES PRODUCED BY MORNING CONVECTION...WE COULD STILL SEE SOME STORMS IN OUR EAST. HAVE THUS TRIMMED BACK WESTERN PORTION OF AFTERNOON POPS BUT MAINTAINED THEM IN OUR EASTERN ZONES. AFTER 00Z WEDNESDAY IT APPEARS THAT ANY STORMS WILL HAVE CLEARED THE AREA AND WE WILL HAVE A BRIEF QUIET RIDGING PERIOD BEFORE A COOL FRONT APPROACHES FROM THE NORTHWEST ON WEDNESDAY MORNING. AS THE FRONT SWEEPS ACROSS OUR CWA ON WEDNESDAY WE WILL USHER IN A COUPLE OF DAYS OF QUIET WEATHER WITH CLEAR SKIES AND LOWER DEW POINTS. HIGH TEMPERATURES ON WEDNESDAY WILL BE PROBLEMATIC...AS ANY MODERATELY COOLER AIR POOLED BEHIND THE FRONT WILL LIKELY BE DELAYED WELL BEHIND THE INITIAL BOUNDARY...AND EVEN AS IT PASSES THROUGH WE WILL SEE SOME 500 MB RIDGING ON WEDNESDAY AND A DECENT AMOUNT OF MIXING WHICH SHOULD COMBINE WITH DAYTIME HEATING AND SLIGHTLY LOWER DEW POINTS FOR MORE TEMPERATURES IN THE 90S ESPECIALLY ACROSS OUR SOUTH AND SOUTHEAST. BY WEDNESDAY NIGHT HOWEVER WINDS WILL DROP OFF SHARPLY AND COOLER/DRIER AIR WILL WORK INTO THE AREA...GIVING US A GORGEOUS LATE SUMMER NIGHT. THE QUIET PERIOD WILL BEGIN TO END AROUND FRIDAY AS HURRICANE IRENE APPROACHES THE EAST COAST AND SEVERS THE EASTERN EXTENSION OF THE LARGE SOUTHERN U.S. RIDGE. AS THE RIDGE IS THUS SHUNTED OVER THE SOUTHWESTERN STATES WE WILL SEE A RETURN OF MORE NORTHWESTERLY 500 MB FLOW BRINGING A SLOWLY COOLER AND MORE ACTIVE PATTERN THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK. DETAILS OF THIS TRANSITION ARE STILL UNCERTAIN AT THIS TIME AND SOMEWHAT DEPENDENT ON THE PRECISE TRACK AND INTENSITY OF IRENE AT THAT TIME...HOWEVER IT APPEARS LIKELY THAT THERE WILL BE A LEADING SHORTWAVE COMING AROUND THE PERIPHERY OF THE HIGH AS IT RETREATS AT THE END OF THE WEEK...AND HAVE MAINTAINED SOME 20 TO 30 POPS DURING THAT PERIOD. LONGER RANGE MODELS ALSO REMAIN FAIRLY CONSISTENT IN BRINGING A MORE ROBUST LONG WAVE TROUGH DOWN THE NORTHWESTERLY FLOW AROUND SUNDAY OR SUNDAY NIGHT...WHICH SHOULD RESULT IN PRECIPITATION AND COOLER TEMPERATURES INTO EARLY NEXT WEEK. HAVE INCREASED POPS TO 30 TO 40 ON SUNDAY AND SUNDAY NIGHT BUT CANNOT GO HIGHER DUE TO DISCREPANCIES IN THE TIMING AND SOUTHWARD EXTENT OF THE TROUGH. && .AVIATION...23/06Z STILL EXPECT GREATEST CHANCES FOR TSRA ACROSS THE AREA TONIGHT NEAR KMCW AND KALO...POSSIBLY KFOD. LEFT IT DRY AT KOTM AND KDSM. OTHERWISE REDEVELOPMENT POSSIBLE TUESDAY AFTERNOON AT KALO...WITH THE REMAINING SITES DRY WITH ANY ACTIVITY EXPECTED TO BE EAST OF THE AREA FOR TUESDAY AFTERNOON/EVENING. SOUTHEASTERLY WINDS TO BECOME SOUTHERLY TO SOUTHWESTERLY THROUGH THE PERIOD. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && SHORT TERM...REV AVIATION...MJB LONG TERM...LEE
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE DES MOINES IA 1043 PM CDT MON AUG 22 2011 IAZ004-005-015-023>025-033>037-044>049-057>061-070>075-081>086- 092>097-231015- EMMET-KOSSUTH-PALO ALTO-POCAHONTAS-HUMBOLDT-WRIGHT-SAC-CALHOUN- WEBSTER-HAMILTON-HARDIN-CRAWFORD-CARROLL-GREENE-BOONE-STORY-MARSHALL- AUDUBON-GUTHRIE-DALLAS-POLK-JASPER-CASS-ADAIR-MADISON-WARREN-MARION- MAHASKA-ADAMS-UNION-CLARKE-LUCAS-MONROE-WAPELLO-TAYLOR-RINGGOLD- DECATUR-WAYNE-APPANOOSE-DAVIS- 1043 PM CDT MON AUG 22 2011 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...TONIGHT A ROUND OF THUNDERSTORMS IS FORECAST TO DEVELOP LATE TONIGHT INTO EARLY TUESDAY MORNING. THERE IS A SLIGHT RISK OF SEVERE WEATHER MAINLY ALONG AND NORTH OF HIGHWAY 30. LARGE HAIL AND DAMAGING WINDS WOULD BE THE MAIN SEVERE THREATS...ALONG WITH THE POTENTIAL FOR HEAVY RAINFALL. .DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY SCATTERED THUNDERSTORMS WILL PERSIST DURING THE DAY TUESDAY. AN ISOLATED SEVERE THUNDERSTORM IS POSSIBLE...GENERALLY EAST OF INTERSTATE 35. THE MAIN THREAT WILL BE LARGE HAIL AND LOCALLY HEAVY RAIN. A SLIGHT CHANCE OF THUNDERSTORMS ARE FORECAST FROM FRIDAY THROUGH SUNDAY...BUT THE SEVERE PROBABILITY REMAINS LOW AT THIS TIME. HEAT INDICES MAY APPROACH 100 TO 105 DEGREES OVER THE SOUTHERN HALF OF THE OUTLOOK AREA TUESDAY THROUGH THE AFTERNOON AND EVENING. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION MAY BE NEEDED LATE TONIGHT AND TUESDAY. IAZ006-007-016-017-026>028-038-039-050-062-231015- WINNEBAGO-WORTH-HANCOCK-CERRO GORDO-FRANKLIN-BUTLER-BREMER-GRUNDY- BLACK HAWK-TAMA-POWESHIEK- 1043 PM CDT MON AUG 22 2011 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...TONIGHT A ROUND OF THUNDERSTORMS IS FORECAST TO DEVELOP LATE TONIGHT INTO EARLY TUESDAY MORNING. THERE IS A SLIGHT RISK OF SEVERE WEATHER MAINLY ALONG AND NORTH OF HIGHWAY 30. LARGE HAIL AND DAMAGING WINDS WOULD BE THE MAIN SEVERE THREATS...ALONG WITH THE POTENTIAL FOR HEAVY RAINFALL. .DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY SCATTERED THUNDERSTORMS WILL PERSIST DURING THE DAY TUESDAY. AN ISOLATED SEVERE THUNDERSTORM IS POSSIBLE...GENERALLY EAST OF INTERSTATE 35. THE MAIN THREAT WILL BE LARGE HAIL AND LOCALLY HEAVY RAIN. A SLIGHT CHANCE OF THUNDERSTORMS ARE FORECAST FROM FRIDAY THROUGH SUNDAY...BUT THE SEVERE PROBABILITY REMAINS LOW AT THIS TIME. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION MAY BE NEEDED LATE TONIGHT AND TUESDAY. MJB
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE DES MOINES IA 119 AM CDT TUE AUG 23 2011 IAZ004-005-015-023-024-230800- EMMET-KOSSUTH-PALO ALTO-POCAHONTAS-HUMBOLDT- INCLUDING THE CITIES OF...ESTHERVILLE...ALGONA...EMMETSBURG... POCAHONTAS...HUMBOLDT 119 AM CDT TUE AUG 23 2011 ...STRONG TO SEVERE THUNDERSTORMS APPROACHING NORTH CENTRAL IOWA... AT 115 AM...METEOROLOGISTS AT THE NATIONAL WEATHER SERVICE IN DES MOINES WERE TRACKING A CLUSTER OF STRONG TO SEVERE THUNDERSTORMS OVER O'BRIEN AND OSCEOLA COUNTIES IN NORTHWEST IOWA. CURRENTLY...THESE STORMS ARE TRACKING TO THE EAST SOUTHEAST AT 30 MPH AND ARE FORECAST TO ENTER EMMET AND PALO ALTO COUNTIES SHORTLY AFTER 200 AM. THE STORMS COULD ALSO TAKE A SOUTHWARD TURN AND SPREAD INTO POCAHONTAS...CALHOUN...AND SAC COUNTIES. THE PRIMARY THREATS WITH THESE STORMS WOULD BE DAMAGING WINDS AND LARGE HAIL. IF THESE STORMS MAINTAIN THEIR INTENSITY...ADDITIONAL STATEMENTS AND WARNINGS WILL BE NECESSARY. STAY TUNED TO THE NATIONAL WEATHER SERVICE AND NOAA WEATHER RADIO FOR THE LATEST INFORMATION. KS
The Des Moines radar (KDMX), 172 km away, was scanning. The volume nearest the first event began at 07:14 UTC.
10 thunderstorm wind, 4 hail. Local standard time, as filed.
Numerous trees uprooted in town. Some minor flooding was also noted.
Patio furniture was blown around and branches were down in the yard.
Power lines were down around town.
Considerable tree damage in town. Many roads impassable.
High winds blew down trees and power lines.
Extensive tree damage throughout the town of Webster City. Multiple tree limbs down, each about a foot in diameter.
Trees down partially blocking the road near the intersection of Willow and Main Street.
A few 4 to 6 inch diameter limbs down. Extensive corn and bean crop damage.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Iowa in 2011Every state in 2011This day in weather historyWarnings in Iowa, August 2011
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: 325024, 325025, 325026, 325027, 325028, 325029, 333651, 325030, 325466, 325464, 325675, 325465, 325469, 325470.