430
FXUS63 KDMX 302031
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
331 PM CDT WED SEP 30 2026
KEY MESSAGES
- RAIN CONTINUES THROUGH MUCH OF THE DAY TODAY.
- A REINFORCING PUSH OF RAIN AND THUNDERSTORMS ARRIVES LATER
THIS AFTERNOON AND EVENING, BRINGING HEAVY RAIN AND FLASH
FLOOD POTENTIAL. THE PROLONGED RAINFALL AND ALREADY SATURATED CONDITIONS
MAY LEAD TO RIVER FLOODING AS WELL.
- RAIN AND STORMS DIMINISH FROM NORTHWEST TO SOUTHEAST THROUGH
THE DAY ON THURSDAY, GIVING WAY TO COOLER AND DRIER CONDITIONS
INTO THE WEEKEND.
SHORT TERM /THROUGH THURSDAY NIGHT/
ISSUED AT 328 PM CDT WED SEP 30 2026
THE CURRENT SURFACE ANALYSIS PLACES A STATIONARY BOUNDARY OVER THE
CENTRAL PLAINS, INCLUDING IOWA. THIS BOUNDARY HAS BEEN THE PRIMARY
FOCAL POINT FOR ONGOING PRECIPITATION ACROSS THE REGION. TO THE
SOUTHWEST, THE REMNANTS OF HURRICANE POLO HAVE MOVED ONSHORE AND
TRANSITIONED INTO AN EXTRATROPICAL LOW. THIS SURFACE LOW IS CENTERED
OVER THE HIGH PLAINS OF TEXAS, TRACKING ALONG THE STATIONARY
BOUNDARY. STATEWIDE SURFACE OBSERVATIONS SHOW WIDESPREAD RAINFALL
ACROSS THE SOUTHERN HALF OF IOWA, WITH SOUTHWESTERLY FLOW
MAINTAINING TEMPERATURES IN THE UPPER-60S TO LOWER-70S.
THROUGH THE REMAINDER OF THE AFTERNOON AND OVERNIGHT, THE UPPER-
LEVEL TROUGH ABSORBING THE REMNANTS OF POLO WILL CONTINUE TO EJECT
NORTHEASTWARD, SPREADING HEAVY RAINFALL ACROSS CENTRAL IOWA. RECENT
MODEL GUIDANCE SHIFTED THE HEAVIEST QPF AXIS SLIGHTLY NORTHWESTWARD,
BUT OVERALL IMPACTS REMAIN UNCHANGED AS FORECAST PARAMETERS MEET
CLIMATOLOGICAL MAXIMA. FROM A HYDROLOGICAL PERSPECTIVE, ALL FACTORS
ALIGN FOR A HEAVY RAIN EVENT THAT COULD CAUSE AREAL FLOODING AND
SUBSEQUENT RIVER FLOODING. GIVEN RECENT RAINFALL, RIVER LEVELS ARE
ALREADY RESPONDING RAPIDLY. ABUNDANT TROPICAL MOISTURE, PW VALUES
WELL ABOVE CLIMATOLOGICAL NORMALS, AND LOW-LEVEL FLOW PARALLEL TO
THE BOUNDARY COMBINE TO DRIVE THIS SEVERE HYDROLOGICAL EVENT. THIS
COULD POTENTIALLY MAKE SEPTEMBER 2026 THE WETTEST SEPTEMBER ON
RECORD FOR DES MOINES (DSM) WHILE PUSHING SEVERAL LOCAL RIVERS INTO
ACTION AND MINOR FLOOD STAGES.
AS THE CORE OF THE SURFACE LOW TRACKS THROUGH THE AREA OVERNIGHT
INTO THURSDAY MORNING, A SURFACE COLD FRONT WILL PUSH SOUTHWESTWARD
INTO THE REGION, SHUNTING THE TROPICAL LOW EAST OF THE AREA BY
THURSDAY AFTERNOON. SHOWERS WILL GRADUALLY DISSIPATE FROM NORTHWEST
TO SOUTHEAST THROUGHOUT THE DAY. BEHIND THE FRONT, SOUNDINGS
INDICATE STEEP LOW-LEVEL LAPSE RATES WITHOUT A CAP, AND HIGH BUFKIT
MOMENTUM TRANSFER VALUES ACROSS NORTHWESTERN IOWA FAVOR STRONG NORTH-
TO-NORTHWESTERLY POST-FRONTAL WINDS. SIGNIFICANTLY COOLER, FALL-LIKE
TEMPERATURES WILL FOLLOW IN THE WAKE OF THE FRONT.
A STATIONARY FRONT OVER THE CENTRAL PLAINS IS INTERACTING WITH
TROPICAL MOISTURE AND AN UPPER-LEVEL TROUGH ABSORBING THE
EXTRATROPICAL REMNANTS OF HURRICANE POLO, GENERATING WIDESPREAD
HEAVY RAINFALL AND POTENTIAL FLOODING ACROSS IOWA THROUGH THURSDAY
MORNING. AS THE SURFACE LOW MOVES OUT ON THURSDAY, A COLD FRONT WILL
CLEAR THE PRECIPITATION FROM NORTHWEST TO SOUTHEAST, BRINGING STRONG
GUSTY NORTHWESTERLY WINDS AND COOLER TEMPERATURES TO THE REGION.
LONG TERM /FRIDAY THROUGH WEDNESDAY/
ISSUED AT 328 PM CDT WED SEP 30 2026
THE EXTENDED FORECAST PERIOD IS EXPECTED TO MARK THE FIRST TRUE
TRANSITION TO FALL-LIKE WEATHER. BEHIND THURSDAYS COLD FRONT,
SURFACE HIGH PRESSURE WILL BUILD BACK INTO THE REGION, BRINGING
SEVERAL DAYS OF DRY WEATHER AND COOLER TEMPERATURES. PREVIOUS
FORECAST ITERATIONS DEPICTED A SHORTWAVE DIVING SOUTHEASTWARD TOWARD
THE UPPER MISSISSIPPI RIVER VALLEY, BUT THIS AFTERNOONS MODEL
GUIDANCE HAS LARGELY REMOVED THAT FEATURE, LEAVING THE AREA MOSTLY
DRY UNTIL PRECIPITATION CHANCES RETURN LATE IN THE PERIOD.
SURFACE HIGH PRESSURE FOLLOWING THURSDAYS COLD FRONT WILL BRING AN
EXTENDED STRETCH OF DRY, COOLER AUTUMN WEATHER TO THE REGION, AS
RECENT MODEL RUNS HAVE REMOVED A PREVIOUSLY FORECAST UPPER-LEVEL
SHORTWAVE AND DELAYED THE NEXT POTENTIAL RAIN CHANCES UNTIL LATE IN
THE PERIOD.
AVIATION /18Z TAFS THROUGH 18Z THURSDAY/
ISSUED AT 1233 PM CDT WED SEP 30 2026
LIGHT RAIN IS MOVING ACROSS MUCH OF SOUTHERN IOWA WITH THIS TAF
ISSUANCE WHICH IS LEAVING TERMINALS NORTH OF THE I-80 CORRIDOR
DRY. HOWEVER, MVFR TO IFR CIGS ARE PLAGUING EVERY SITE AT THE
START OF THE TAF WINDOW. THROUGH THE AFTERNOON, A BAND OF SHOWER
WITH EMBEDDED THUNDERSTORMS WILL MOVE NORTH INTO THE REGION AND
PROVIDE SEVERAL HOURS OF CONSTANT SHOWERS AND THUNDERSTORMS.
THIS WILL ALSO KEEP SITES UNDER CATEGORICAL MVFR TO LIFR THROUGH
MUCH OF THE TAF WINDOW. AS A COLD FRONT DIVES SOUTHWEST INTO
THE AREA, SHOWERS AND STORMS WILL COME TO AN END FROM NORTHWEST
TO SOUTHEAST. HOWEVER, BEHIND THE FRONT, INCREASING NORTHERLY
WINDS ARE FORECAST TO DEVELOP.
HYDROLOGY
ISSUED AT 330 PM CDT WED SEP 30 2026
ALL TYPES OF FLOODING, INCLUDING FLASH, AREAL AND RIVER, REMAIN
POSSIBLE AS ADDITIONAL MODERATE TO HEAVY RAIN FALLS ACROSS MUCH OF
THE CWA. RAIN ACROSS THE SOUTHERN CWA LAST NIGHT LED TO MORE
SIGNIFICANT RESPONSES THAN NORMAL FOR THE AMOUNT OF RAIN THAT FELL,
WHICH SHOWS THE DEGREE OF GROUND SATURATION.
RIVER FLOOD WARNINGS HAVE BEEN ISSUED FOR LOCATIONS WHERE WE HAVE
HIGH CONFIDENCE (AT LEAST 80%) OF REACHING AT LEAST FLOOD STAGE. WE
MENTIONED LARGER BASINS WHERE CONFIDENCE IS LOWER, IN OUR GOING
FLOOD WATCH INSTEAD OF ISSUING INDIVIDUAL RIVER FLOOD WATCHES FOR
THOSE LOCATIONS. REFER TO THE NATIONAL WATER PREDICTION SERVICE AT
WEATHER.GOV/DESMOINES/WATER FOR THE LATEST RIVER OBSERVATIONS AND
FORECASTS.
THE RACCOON RIVER BASIN WILL BE AMONG THE MORE TRICKY FORECASTS.
RECENT RAINFALL IN MUCH OF THAT BASIN WILL LEAD TO FASTER AND MORE
SIGNIFICANT RUNOFF. COMPETING FACTORS ARE NUDGING THE RIVER
FORECASTS HIGHER AND LOWER ESPECIALLY IN THAT BASIN. IF THE RAIN
FALLS FARTHER SOUTH THEN IT WILL TEND TO LOWER THE CRESTS ESPECIALLY
ON THAT STREAM.
FLASH FLOODING REMAINS POSSIBLE, ESPECIALLY IN URBAN AREAS INCLUDING
CREEKS AND SMALLER STREAMS. IT INCLUDES WALNUT AND FOURMILE CREEKS
IN THE DES MOINES AREA. SEASONALLY HIGH SOIL MOISTURE WILL LEAD TO
FASTER AND MORE SIGNIFICANT RUNOFF THAN NORMAL. GIVEN THE RELATIVELY
SMALL SIZE OF THOSE BASINS, THEY WILL BE MORE AFFECTED BY RAIN
LOCATION AND AMOUNTS. THUS ALTHOUGH REACHING FLOOD STAGE IS NOT A
CERTAINTY ON THOSE STREAMS, IT IS ALSO NONZERO, THUS WILL HAVE TO BE
MONITORED.
DMX WATCHES/WARNINGS/ADVISORIES
FLOOD WATCH THROUGH LATE THURSDAY NIGHT FOR IAZ024>028-033>039-
044>050-057>062-070>075-081>086-092>097.
SHORT TERM...VORST
LONG TERM...VORST
AVIATION...VORST
HYDROLOGY...ZOGG
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