847
FXUS63 KDMX 291753
AFDDMX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DES MOINES IA
1253 PM CDT TUE JUL 29 2025
...UPDATED FOR THE 18Z AVIATION DISCUSSION...
KEY MESSAGES
- ONE MORE DAY OF HEAT & HUMIDITY THIS AFTERNOON ACROSS
SOUTHERN IOWA. HEAT ADVISORY WILL AGAIN BE IN EFFECT.
- NUMEROUS STORM CHANCES TUESDAY EVENING THROUGH WEDNESDAY.
LOCALLY HEAVY RAINFALL OF 2 TO 5 INCHES LEADING TO THE
POTENTIAL OF FLASH FLOODING AND RENEWED RIVER RISES EXISTS.
DAMAGING WIND GUSTS AND LARGE HAIL ARE THE MAIN SEVERE
HAZARDS.
- TURNING COOLER, LESS HUMID WITH NO WIDESPREAD RAIN AND STORM
CHANCES LATE THIS WEEK.
UPDATE
ISSUED AT 357 AM CDT TUE JUL 29 2025
FORECAST GENERALLY REMAINS ON TRACK TODAY. TEMPERATURES OVER
SOUTHERN AND SOUTH CENTRAL IOWA WILL REMAIN HOT AND HUMID, WHILE A
SURFACE BOUNDARY PULLED SOUTHWARD BY MORNING STORMS WILL KEEP
TEMPERATURES COOLER OVER NORTHERN IOWA. THE SOUTHERN EXTENT OF THIS
BOUNDARY, AND IT'S EVENTUAL RETREAT NORTH, WILL PLAY A KEY ROLE IN
TEMPERATURES, ESPECIALLY IN CENTRAL IOWA WHERE THE BOUNDARY IS
EXPECTED TO STALL. CURRENT CONVECTIVE TRENDS AS OF THE WRITING OF
THIS DISCUSSION INDICATE A SLIGHTLY MORE SOUTHERLY SOLUTION FOR
MORNING, SUGGESTING THERE COULD BE SOME BUST POTENTIAL ON THE
NORTHERN EDGE OF THE HEAT ADVISORY. EVEN THEN, STORMS MAY BE
COMPLETELY OUT OF THE AREA BY SUNRISE AND TUESDAY WILL REMAIN CLEAR
AND DRY THROUGH THE DAY, WHICH WOULD ALLOW THE BOUNDARY TO RETREAT
BACK NORTHWARD INTO THE ADVISORY. THEREFORE, GIVEN SOME UNCERTAINTY
IN MESOSCALE FEATURES, NO UPDATE TO HEAT HEADLINES AT THIS TIME.
AS WE GET LATER INTO THIS EVENING, ANOTHER WAVE PROPAGATING
AROUND THE UPPER RIDGING WILL BRING ADDITIONAL FORCING INTO THE
FORECAST AREA. THIS WILL RESULT IN WIDESPREAD SHOWER AND
THUNDERSTORM CHANCES, INCLUDING THE POTENTIAL FOR AN EMBEDDED
MCS/WIND THREAT AHEAD OF THE WAVE. THE ATMOSPHERE WILL CONTAIN
NO SHORTAGE OF INSTABILITY, AND DECENT DEEP LAYER WILL ALLOW FOR
ORGANIZED CONVECTION. LOW LEVEL WIND FIELDS WILL BE SOMEWHAT
MESSY GIVEN THE INCOMING CONVECTION AND REMNANT BOUNDARIES, BUT
MODELS SUGGEST A DECENT 0-3 KM SHEAR ENVIRONMENT (20 TO 30 KTS)
THAT WOULD HELP KEEP COLD POOLS IN BALANCE. FURTHER EMPHASIZING
THE WIND THREAT TUESDAY NIGHT, DCAPE VALUES WILL BE FAIRLY HIGH
ALONG AND SOUTH OF THE SURFACE BOUNDARY OVER CENTRAL IOWA,
ALLOWING FOR EFFICIENT DOWNBURST WIND POTENTIAL. THEREFORE,
WHILE THERE IS A CHANCE FOR SOME SMALL HAIL, ESPECIALLY WITH
DISCRETE CONVECTION ALONG THE BOUNDARY, DAMAGING WINDS WILL
AGAIN BE THE MAIN SEVERE THREAT.
SHOWERS AND STORMS TUESDAY NIGHT PERSIST OVER THE AREA INTO MID-DAY
WEDNESDAY, RESULTING IN A PROLONGED PERIOD OF HEAVY RAINFALL AS
MULTIPLE LOBES OF ENERGY EJECT ACROSS THE APEX OF THE SOUTHWARD
SINKING RIDGE. THE ENVIRONMENT WITH THIS PRECIPITATION WILL BE
QUITE FAVORABLE FOR HEAVY RAINFALL AND SOME SLOW MOVING TRAINING
STORMS, AS WAS EXCELLENTLY DESCRIBED IN THE BELOW DISCUSSION.
MODEL GUIDANCE SHOWS FAIRLY GOOD AGREEMENT IN WIDESPREAD
RAINFALL AMOUNTS OF 1 TO 3 INCHES, WITH SOME LOCALLY HIGHER
AMOUNTS OF 4 TO 6 INCHES EMBEDDED WITHIN. THE SAVING GRACE WILL
BE THE STEADILY SINKING BOUNDARY/RIDGE DIMINISHING THE AMOUNT OF
TIME RAINFALL FALLS OVER THE SAME AREA THROUGH WEDNESDAY,
MITIGATING EXTREMELY HIGH RAINFALL TOTALS. EVEN SO, THIS JULY
HAS BEEN UNUSUALLY WET, WITH RIVERS ALREADY HOLDING WATER FROM
PAST RAINFALLS AND MOIST SOILS. THEREFORE, PROLONGED, WIDESPREAD
RAINFALL COULD STILL LEAD TO FLOODING IN PORTIONS OF CENTRAL
AND NORTHERN IOWA WHERE SOME RIVERS ARE STILL IN ACTION STAGE.
LIKEWISE, ANY LOCALLY HIGHER AMOUNTS FROM TRAINING STORMS OVER
THE SAME AREA COULD CERTAINLY RESULT IN PONDING WATER AND FLASH
FLOODING, ESPECIALLY IN TOWNS AND URBAN AREAS.
DISCUSSION
ISSUED AT 328 PM CDT MON JUL 28 2025
MAIN CONCERN THIS FORECAST CYCLE IS THE ROUND OF STORMS THAT IS
FORECAST TO MOVE ACROSS SOUTH DAKOTA, SOUTHERN MINNESOTA, AND INTO
PORTIONS OF IOWA LATER THIS EVENING INTO THE PRE-DAWN HOURS OF
TUESDAY. DAMAGING WIND GUSTS POTENTIALLY OF 75 MPH OR A BIT HIGHER
ARE THE MAIN CONCERN WITH THIS SETUP BEING DETAILED SHORTLY BELOW.
MORNING VISIBLE SATELLITE IMAGERY SHOWED THE OVERNIGHT STORMS
AND THEIR ASSOCIATED OUTFLOW WELL SOUTH OF THE STATE, THOUGH A FEW
STORMS DID POP SEVERAL TIMES BETWEEN CRESTON, ATLANTIC AND CARROLL
THIS MORNING AND EARLY THIS AFTERNOON. IT IS JUST A TESTAMENT TO
THE JUICY AIRMASS GIVEN THE 700MB TEMPERATURES, WHICH KOAX RAOB
MEASURED 13.4C AT 18Z. OVERALL, TEMPERATURES HAVE MADE A FULL
RECOVERY INTO THE 80S TO NEAR 90 DEGREES AND RELIABLE DEWPOINTS
ARE BACK INTO THE 70S, WHICH IS A TAD LOWER THAN EXPECTED
YESTERDAY. WHILE HEAT INDEX VALUES MAY END UP BEING 5 OR SO
DEGREE LOWER, MESSAGING OF HEADLINES AND TAKING HEAT SAFETY
PRECAUTIONS WILL REMAIN UNCHANGED.
LOOKING AT THE SYNOPTIC PATTERN LATE THIS AFTERNOON INTO TONIGHT,
THERE REMAINS THE UPPER LEVEL RIDGE, WHICH IS NOW CENTERED OVER
THE LOWER MISSISSIPPI RIVER VALLEY. CONVECTION IS ONGOING OVER
NORTH DAKOTA AND IS ASSOCIATED WITH A SHORTWAVE TROUGH TOPPING
THE RIDGE. THE STORMS THAT ARE EXPECTED TO MAKE IT INTO IOWA
THIS EVENING MAY BE ATTEMPTING TO DEVELOP OVER WESTERN/CENTRAL
SOUTH DAKOTA THIS AFTERNOON. HOWEVER, GREATER DEVELOPMENT IS
FORECAST LATE THIS AFTERNOON WITH STORM CLUSTERS MERGING INTO A
PROPAGATING MESOSCALE CONVECTIVE SYSTEM (MCS). CONVECTIVE
ALLOWING MODELS (CAMS) SHOW A VARIETY OF SOLUTIONS RANGING FROM
THE MCS STAYING LARGELY NORTH OF THE STATE (LOW TO NO IOWA
IMPACT) TO THE MCS MOVING ACROSS NORTHERN AND SINK SOUTHEASTWARD
ACROSS THE STATE (HIGHER IMPACT TO IOWA). THERE IS HIGH
CONFIDENCE THAT STORMS WILL MOVE INTO THE REGION AND SEEMS AT
LEAST OVER NORTHERN IOWA AND OUR SERVICE AREA SOMETIME AFTER 8
OR 9PM THIS EVENING. THESE STORMS WILL BE MOVING QUICKLY AS WELL
AS THEY NAVIGATE THE TOP OF THE WARM 700MB TEMPERATURES, WHICH
MAKES IT UNCERTAIN HOW FAR SOUTH THE STORMS WILL MAKE IT OVER
IOWA. THE MAIN HAZARD WILL BE DAMAGING WIND GUST THAT WILL BE
OVER 60 MPH AND PERHAPS AT OR A BIT ABOVE 75 MPH. THE HIGHEST
WIND GUSTS ARE FORECAST OVER NORTHER IOWA, ROUGHLY NORTH OF
HIGHWAY 20. IN ADDITION, 0-3KM WIND SHEAR VALUES WILL HAVE
SUFFICIENT MAGNITUDE FOR MESOVORTEX GENERATION/EMBEDDED
CIRCULATIONS WITH THE BOWING SEGMENTS. THIS POTENTIAL OF LEADING
LINE TORNADOES IS HIGHEST FOR SEGMENTS THAT MOVE MORE EAST VS
SOUTHEAST OR SOUTH, WHICH WOULD RESULT IN THE SYSTEM BECOMING
OUT OF BALANCE. THE STORMS WILL BE TRUCKING AND SHOULD BE
EXITING OUR FORECAST AREA BY 3 OR 4AM. FINALLY, AS SPC
HIGHLIGHTED IN A DISCUSSION EARLIER TODAY, "ONE OR MORE
CORRIDORS OF WIDESPREAD/SIGNIFICANT WIND DAMAGE ARE EXPECTED,
POSSIBLY ACHIEVING DERECHO CRITERIA". OF COURSE, THERE IS A
RANGE OF MAGNITUDE OF DERECHOS RANGING FROM THE CRITERIA BASED
WIND GUSTS OF 75 MPH UP TO WHAT MANY IOWANS REMEMBER FROM THE
AUGUST 2020 OR SIOUXLAND DERECHOS OF SPRING 2022. AT THIS TIME,
THIS DOES NOT LOOK TO BE THE UPPER TIER, AUGUST 2020 TYPE
DERECHO. THE POTENTIAL THAT THIS MAY MEET THE CRITERIA AND
DEFINITION OF A DERECHO MAY EXTEND INTO AT LEAST NORTHERN IOWA,
BUT RECALL THAT WHETHER OR NOT AN EVENT IS DETERMINED TO MEET
THE CRITERIA OF A DERECHO IS DETERMINED AFTER THE EVENT IN
DISCUSSIONS WITH THE STORM PREDICTION CENTER.
MUCH OF THE DAYTIME TUESDAY WILL BE RAIN AND STORM FREE IN THE WAKE
OF THE NOCTURNAL MCS. A SURFACE BOUNDARY WILL BE DROPPING SLOWLY
THROUGH THE STATE. OVER SOUTHERN IOWA, ANOTHER DAY OF VERY WARM AND
HUMID CONDITIONS WILL LEAD TO A HEAT ADVISORY FOR THOSE AREAS
CENTERED AROUND THE AFTERNOON HOURS. AS WE MOVE INTO THE
EVENING, THE NEXT SHORTWAVE TROUGH WILL BE APPROACHING FROM
NEBRASKA. THIS FEATURE IN CONJUNCTION WITH A WEAKENING LOW
LEVEL JET (LLJ) AND PULSES OF LOW LEVEL THERMAL LIFT WILL HELP
TO INITIATE STORMS. THIS LOOKS TO BE AT THE EARLIEST LATE
AFTERNOON, BUT MORE LIKELY SOMETIME IN THE EVENING. FURTHER,
ANOTHER MCS MAY APPROACH FROM NEBRASKA SOMETIME LATE IN THE
EVENING IF NOT AFTER MIDNIGHT. THERE WILL BE PLENTY OF
INSTABILITY AND MARGINAL AMOUNTS OF DEEP LAYER SHEAR SUCH THAT
LARGE HAIL, AT LEAST INITIALLY IN THE AFTERNOON OR EVENING,
ALONG WITH DAMAGING WIND GUSTS THROUGHOUT THE EVENING INTO THE
NIGHT WILL BE THE MAIN SEVERE HAZARDS. STORMS WILL CONTINUE
INTO WEDNESDAY WITH SEVERE WEATHER ALSO POSSIBLE SOUTH OF THE
BOUNDARY SOMEWHERE OVER SOUTHERN IOWA. DAMAGING WIND GUSTS AND
HAIL ON WEDNESDAY WOULD BE THE MAIN CONCERN ONCE AGAIN.
IN ADDITION TO THE SEVERE WEATHER, FLASH FLOODING IS ALSO A CONCERN
TUESDAY NIGHT INTO WEDNESDAY. LOW LEVEL SPECIFIC HUMIDITY THROUGH
TUESDAY NIGHT IS FORECAST TO BE AT THE 99TH PERCENTILE WITH
PRECIPITABLE WATER VALUES TOPPING 2 INCHES ALONG THIS BOUNDARY. DEEP
WARM CLOUD DEPTHS WITH SLOW STORM MOTIONS SHOULD LEAD TO STORMS
REPEATING OVER THE SAME AREA. THOUGH AS MENTIONED YESTERDAY, THE
WEAKER LLJ MAY ALLOW THE SURFACE BOUNDARY TO CONTINUE TO SINK
SOUTHWARD AND THUS STORMS MAY NOT REPEAT OVER THE SAME AREAS.
RAINFALL HAS TICKED UPWARDS WITH TOTALS OF 3 TO 5 INCHES BY LATER
WEDNESDAY IN THE GLOBAL MODELS AND AVAILABLE CAMS. THIS
RAINFALL WILL BE FALLING OVER SOILS THAT NASA SPORT SHOW AT THE
70TH PERCENTILE OR HIGHER FROM AROUND HIGHWAY 20 SOUTHWARD, EAST
OF HIGHWAY 71, AND INTO SOUTHEAST IOWA IN THE 0-10CM AND
10-40CM LAYERS. THIS COULD SET THE STAGE FOR RAPID ONSET
FLOODING/FLASH FLOODING, PARTICULARLY IN URBAN AREAS OR THOSE
THAT HAVE HAD RECENT RAINFALL, WHICH WAS JUST MENTIONED BY MORE
SATURATED SOILS. THE NATIONAL WATER MODEL (NWM) RAPID ONSET
FLOODING FORCED BY THE GFS, WHICH LAYS OUT THE HEAVY RAINFALL
WEST TO EAST ACROSS IOWA, SHOWS HIGH PROBABILITIES OVER MANY
PORTIONS OF NORTH CENTRAL AND CENTRAL IOWA. FURTHER, THIS
RAINFALL WOULD CERTAINLY SLOW THE RECESSION AT THE VERY LEAST
WITH RENEWED RIVER RISES QUITE POSSIBLE. QPF ENSEMBLE
HYDROGRAPHS SHOW THAT WITH BEST ESTIMATE RAINFALL, A FEW RIVER
POINTS IN THE IOWA AND CEDAR BASINS COULD REACH MINOR OR EVEN
MODERATE FLOOD STAGE BY LATE IN THE WEEK OR THE WEEKEND.
EXAMINING THE NWM HIGH FLOW MAGNITUDE FORECAST FORCED BY THE
GFS SHOWS ANNUAL EXCEEDANCE PROBABILITIES (AEPS) WITH A LOW
PROBABILITY OF A HIGH IMPACT EVENT ON PARTS OF THE RACCOON, THE
UPPER DES MOINES, IOWA, AND CEDAR BASINS. OF COURSE, THIS CAN
EASILY CHANGE DEPENDING ON WHERE THE RAIN FALLS AND THE
MAGNITUDE AND DURATION OF SAID RAINFALL.
THE BOUNDARY FINALLY IS PUSHED SOUTH OF THE STATE THURSDAY USHERING
IN COOLER, LESS HUMID AIR ALONG WITH NO WIDESPREAD RAIN OR STORM
CHANCES THROUGH AT LEAST SATURDAY.
AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/
ISSUED AT 1253 PM CDT TUE JUL 29 2025
A FEW ISOLATED SHOWERS AND STORMS HAVE DEVELOPED OVER NORTHERN
IOWA THIS AFTERNOON, WHICH MAY TRACK EASTWARD OVER THE AREA
THROUGH THE EARLY AFTERNOON. MORE SCATTERED TO WIDESPREAD
CHANCES FOR THUNDERSTORMS ARE EXPECTED LATER THIS EVENING INTO
WEDNESDAY MORNING ACROSS MUCH OF THE STATE. PROB30S ARE
PRIMARILY USED WITH THIS UPDATE AS UNCERTAINTY IN EXACT TIMING
AND COVERAGE REMAINS AND WILL DEPEND ON HOW CONDITIONS SET UP.
CEILINGS ARE EXPECTED TO LOWER GIVEN THE STRATUS COVERAGE AFTER
09Z AND REMAIN THROUGH THE END OF THE TAF PERIOD. WINDS WILL
GRADUALLY SHIFT MORE NORTHEASTERLY INTO WEDNESDAY MORNING, WITH
GUSTS UP TO 15-20 KNOTS.
DMX WATCHES/WARNINGS/ADVISORIES
HEAT ADVISORY UNTIL 8 PM CDT THIS EVENING FOR IAZ057>062-
070>075-081>086-092>097.
UPDATE...DODSON
DISCUSSION...ANSORGE
AVIATION...BURY
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