765
FXUS63 KDMX 010004
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
704 PM CDT WED SEP 30 2026
...UPDATED FOR THE 00Z AVIATION DISCUSSION...
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.
AVIATION /00Z TAFS THROUGH 00Z FRIDAY/
ISSUED AT 646 PM CDT WED SEP 30 2026
VFR TO LIFR CONDITIONS ARE IN PLACE ACROSS THE AREA EARLY THIS
EVENING WITH RAIN ONGOING THROUGH MUCH OF CENTRAL AND SOUTHERN
IOWA. THIS RAIN IS EXPECTED TO CONTINUE, AND EVEN STRENGTHEN IN
INTENSITY TONIGHT THROUGH THE OVERNIGHT WHICH WILL DROP
VISIBILITIES AT TIMES, POTENTIALLY EVEN LOWER THAN ADVERTISED IN
THE CURRENT TAFS. CEILINGS WILL ALSO BECOME LARGELY IFR TO LIFR
TO GO ALONG WITH THE RAIN AND ANY POTENTIAL FOG. HAVE LIMITED
THUNDER MENTIONS IN THE SHORT TERM WITH THUNDER/LIGHTNING
POTENTIAL BACK CLOSER TO THE SURFACE LOW AND MAY ONLY AFFECT
SOUTHERN SITES KDSM, MAYBE KOTM LATE OVERNIGHT INTO THURSDAY
MORNING. TIMING IN ALL CASES (RAIN/VISBY/CIG/THUNDERSTORM) MAY
STILL NEED FURTHER ADJUSTMENTS AND WILL WATCH TRENDS CLOSELY.
RAIN GRADUALLY ENDS NORTHWEST TO SOUTHEAST AS A COLD FRONT MOVES
THROUGH THE AREA ON THURSDAY WITH GRADUALLY IMPROVING CEILINGS
BEHIND. ADDITIONALLY, WINDS OUT OF THE NORTH WILL INCREASE
BEHIND THE FRONT BRINGING BREEZY CONDITIONS ON THURSDAY.
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...05
HYDROLOGY...ZOGG
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