404
FOUS30 KWBC 170758
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
358 AM EDT WED SEP 17 2025
DAY 1
VALID 12Z WED SEP 17 2025 - 12Z THU SEP 18 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR WESTERN AND
CENTRAL SOUTH DAKOTA...
..NORTHERN AND CENTRAL PLAINS
A BROAD MID-LATITUDE CYCLONE WILL CONTINUE TO IMPACT A LARGE
PORTION OF THE PLAINS OVER THE COURSE OF WEDNESDAY INTO THURSDAY
WITH MULTIPLE AREAS OF HEAVY RAIN PLAUSIBLE GIVEN A VARIETY OF
DYNAMICAL AND THERMODYNAMIC PROCESSES. THE AREA WITH THE GREATEST
CONCERN IS LIKELY WITHIN THE WELL-DEFINED AXIS OF DEFORMATION THAT
IS FORECAST TO MATERIALIZE ACROSS THE WESTERN HALF OF SD LEADING TO
SCATTERED FLASH FLOOD CONCERNS THANKS TO WET ANTECEDENT CONDITIONS
IN WAKE OF A PREVIOUS DISTURBANCE THAT DISTRIBUTED COPIOUS AMOUNTS
OF RAIN TO AREAS ALONG AND WEST OF THE MISSOURI RIVER. GUIDANCE IS
IN AGREEMENT ON THE THREAT FOR 1-3" OVER THE AREA EXTENDING FROM
THE RIVER OVER TO THE SD/WY STATE LINE, DELINEATING THE DEFORMATION
STRUCTURE FORECAST TO MATERIALIZE WITHIN THE NORTHERN AND WESTERN
HALF OF THE CYCLONE SITUATED OVER NE. FFG'S ON THE ORDER OF 0.5-2"
BETWEEN 1/3/6 HR. INTERVALS ARE RUNNING MUCH LOWER THAN CLIMO
LEADING TO A LOWER THRESHOLD TO BREACH THAT COULD VERY WELL CAUSE
ISSUES ACROSS THAT BROAD AREA OF WESTERN SD. 5 AND 10-YEAR ARI
EXCEEDANCE PROBS ARE RUNNING BETWEEN 50-80% AND 30-60%,
RESPECTIVELY BETWEEN THE TWO INTERVALS, A SOLID REPRESENTATION OF
AT LEAST SOME SCATTERED FLASH FLOOD CONCERNS WITH PERHAPS A BROADER
SCOPE OF IMPACT CONSIDERING THE AREAL FOOTPRINT OF THE
PROBABILITIES IN QUESTION. RATES WILL BE ~1"/HR AT PEAK INTENSITY,
BUT COULD VERY WELL LAST FOR MULTIPLE HOURS DUE TO THE ANTICIPATED SYNOPTIC
SCALE EVOLUTION OF THE SURFACE LOWS SLOWER FORWARD PROPAGATION
SPEED LEADING TO TRAINING OVER A LARGE AREA OF SD DOWN INTO
NORTHWEST NE. IN THIS CASE, THE PREVIOUS SLGT RISK WAS MAINTAINED
WITH SOME MINOR EXPANSION ON THE EDGES TO ACCOUNT FOR THE LATEST
PROBABILITIES AND HREF BLENDED MEAN QPF OUTPUT.
BY THE AFTERNOON AND EVENING, DEFINED WARM CONVEYOR BELT (WCB) WILL
BE SITUATED ACROSS MUCH OF THE MISSOURI RIVER BASIN EXTENDING FROM
OK/AR UP INTO EASTERN SD. A WARM FRONT WILL LIFT NORTH AND
TRANSITION TO A QUASI-STATIONARY BOUNDARY ACROSS EAST-CENTRAL SD
THROUGH SOUTH-CENTRAL MN LEADING TO A FOCUSED FRONTAL ALIGNMENT FOR
STORMS TO MATERIALIZE LATER IN THE PERIOD. THIS HAS BEEN A TREND
WITHIN THE RECENT CAMS AS CONVECTIVE INITIATION WILL LIKELY BE
SPURRED BY THE ARRIVAL OF A SHORTWAVE PIVOTING AROUND THE EASTERN
FLANK OF THE PRIMARY ULL, GENERATING A BETTER LARGE SCALE ASCENT
PATTERN FOCUSED DOWNSTREAM OF THE CENTER OF CIRCULATION. WITH DEW
POINTS EXPECTED TO BE IN THE 60S FOR LOCATIONS WITHIN THE DEFINED
WCB, THERE WILL BE ROOM FOR SCATTERED TO WIDESPREAD CONVECTIVE
DEVELOPMENT DURING AFTERNOON AND EVENING TIME FRAME AS THE
ENVIRONMENT RIPENS WITH THE TYPICAL DIURNAL HEAT FLUX. THE FRONTAL
POSITIONING OVER THE NORTHERN PLAINS AND UPPER MIDWEST WILL BE A
TARGET FOR TRAINING CONVECTION AS CELLS CAN BECOME ANCHORED TO THE
FRONT AND LEAD TO LOCALIZED AREAS OF HEAVY RAINFALL THAT COULD DROP
A QUICK 2-4" OVER A SPAN OF A FEW HRS. RATES BETWEEN 1-2"/HR IN
CONVECTION ARE LIKELY IN THIS SETUP FOR ANYWHERE WITHIN THE WCB
LEADING TO A GENERAL MAINTENANCE OF THE MRGL RISK ACROSS THE
PLAINS, BUT HAVE NOW EXPANDED THE MRGL RISK EASTWARD INTO MN TO
ACCOUNT FOR THE FRONTAL ALIGNMENT AND GROWING THREAT OF CONVECTION
SPAWNING NEAR AND WITHIN THE BOUNDARIES FORECAST LOCATION.
..SOUTHERN HIGH PLAINS
MULTI-ROUND CONVECTIVE EPISODE ANTICIPATED ACROSS THE SOUTHEAST CO
FRONT RANGE DOWN INTO THE OK/TX PANHANDLES AND ADJACENT NORTHEAST
NM BEGINNING LATER THIS AFTERNOON, CARRYING THROUGH THE EVENING
HOURS. LEE SIDE LOW OVER EASTERN CO/WESTERN KS WILL MATERIALIZE
WITHIN THE BROAD SYNOPTIC SCALE EVOLUTION OCCURRING OVER THE CENTER
OF THE CONUS. A TRAILING COLD FRONT WILL DRAG SOUTH AND SOUTHWEST
THROUGH THE ABOVE AREAS WITH AN INITIAL FLARE UP OF CONVECTIVE
ACTIVITY THIS AFTERNOON ALONG THE FRONT CREATING A MOMENT OF
PRIMING BEFORE A LARGER SHORTWAVE EJECTING OUT OF THE CENTRAL
ROCKIES BECOMES THE FOCUS FOR LATER. MODELS ARE IN AGREEMENT ON
QUITE A STRONG MID-LEVEL VORTICITY EJECTION INTO THE FRONT RANGE
LATE-AFTERNOON AND EARLY EVENING TODAY LEADING TO A BLOSSOMING
CONVECTIVE INITIATION IN-OF THE TERRAIN JUST WEST OF I-25 IN
SOUTHEAST CO. STORMS WILL GROW UPSCALE AND MIGRATE EASTWARD INTO
THE FRONT RANGE, CONTINUING TO SLIDE EAST-SOUTHEAST AS THEY DELIVER
LOCALLY HEAVY RAINFALL IN THEIR WAKE LEADING TO AN ISOLATED TO
SCATTERED FLASH FLOOD CONCERN THROUGH THE SECOND HALF OF THE
FORECAST. THIS SETUP HAS BEEN DOCUMENTED IN SOME WAY OVER THE PAST
SEVERAL FORECAST ITERATIONS, BUT THE THREAT IS NOW CLEAR IN THE
MAIN SHOW FOCUSED WITHIN THE STRONG MID-LEVEL ASCENT PATTERN
GENERATED BY THE SHORTWAVE ROUNDING THE TROUGH BASE. EXPECT
LOCALIZED TOTALS OF 2-3" IN THE HARDEST HIT LOCATIONS WITH A MAX OF
UP TO 4" ACROSS THE FAR SOUTHEAST CORNER OF CO DOWN INTO THE
NORTHERN TX PANHANDLE. THIS IS REFLECTED BY THE MODEST >3"
NEIGHBORHOOD PROBABILITIES IN THE LATEST 00Z HREF SUITE, A SOLID
INDICATOR FOR A MRGL RISK MAINTENANCE FROM PREVIOUS FORECASTS.
..SOUTHWEST
LITTLE CHANGE FROM THE PREVIOUS FORECAST AS GUIDANCE MAINTAINS A
SIGNAL FOR ANOTHER PERIOD OF ISOLATED TO SCATTERED CONVECTION
OVER THE TERRAIN ENCOMPASSING SOUTHERN AZ UP THROUGH THE CENTRAL
AND EASTERN PORTIONS OF NM WITH THE PRIMARY THREAT BEING HEAVY
RAINFALL SITUATED OVER MORE COMPLEX TERRAIN AND BURN SCAR REMNANTS.
ASSESSMENT OF FORECAST SOUNDINGS AND HODOGRAPHS INDICATE
RELATIVELY SLOWER CELL MOTIONS, A CLASSIC CHARACTERISTIC FOR FLASH
FLOOD PROSPECTS OVER THE DESERT SOUTHWEST. GUIDANCE REFLECTS LOCAL
1-2" TOTALS SCATTERED ACROSS THE REGION EXTENDING FROM SOUTHEAST AZ
INTO NM WITH SOME OF THE BETTER SIGNALS FOCUSED WITHIN THOSE
MOUNTAIN RANGES LIKE THE HUACHUCAS AND SACRAMENTO'S. THE PREVIOUS
MRGL RISK WAS RELATIVELY UNCHANGED CONSIDERING THE FAVORABLE RUN
TO RUN CONTINUITY.
..SOUTHEAST FLORIDA
BROAD CYCLONIC PATTERN ACROSS THE EASTERN CONUS WILL AID IN
ADVECTING A RELATIVELY STOUT MID-LEVEL SHORTWAVE CURRENTLY CENTERED
ALONG THE NORTHERN COAST OF CUBA. IR SATELLITE THIS EVENING SHOWS
THE DISTURBANCE IS VERY WELL-DEFINED WITH A CENTROID OF COOLER
CLOUD TOPS INDICATING SCATTERED TO WIDESPREAD CONVECTIVE COVERAGE
IN-OF THE WESTERN FLORIDA STRAITS AT THIS HOUR. CONSIDERING THE
SOUTH TO SOUTHWESTERLY FLOW BETWEEN THE 850-300MB LAYER, THE
EXPECTATION IS FOR CONVECTION TO MOVE GENERALLY NORTHEAST WITH THE
MEAN FLOW, USHERING IN A MORE FOCUSED AREA OF FORCING CAPABLE OF
IMPACTING ALL OF SOUTH FL, INCLUDING THE KEYS WITH HEAVY RAINFALL
AT ANY POINT. PWATS ARE ON THE RISE ACCORDING TO THE LATEST 00Z
RAOB'S OUT OF KMFL AND KEYW THIS EVENING WITH A BROAD EXPANSE OF
1.9-2.2" PWATS OVER THE REGION. FORECAST IS FOR PWATS TO CONTINUE
TO INCREASE WITH AN EXPECTATION OF INDICES TO SETTLE BETWEEN
2-2.3", A SOLID 1-1.5 DEVIATIONS ABOVE NORMAL WHEN ASSESSING THE
LATEST NAEFS OUTPUT. GUIDANCE IS KEYING IN ON THE FL KEYS TO BE THE
PRIMARY FOCUS FOR HEAVIEST QPF DURING THE FORECAST PERIOD, HOWEVER
THE ENVIRONMENT IS FAVORED FOR ANY AREA SITUATED SOUTH OF I-4,
ESPECIALLY OVER SOUTHEAST FL WHERE SEA BREEZE COMPONENTS AND DEEP
MOISTURE PRESENCE WILL LEND TO POCKETS OF VERY HEAVY RAINFALL FROM
CONVECTION THAT DEVELOPS OVER THE AREA. THESE CELLS WILL BE
SCATTERED IN NATURE, BUT THE PROSPECTS FOR 2-3"/HR RATES WITH
HIGHER INTRA-HOUR RATES ACROSS THE URBAN CORRIDOR FROM MELBOURNE
DOWN TO MIAMI WILL MAKE FOR AT LEAST A MODEST MRGL RISK FOR
EXCESSIVE RAINFALL OVER THE NEXT 24+ HOURS. THE PREVIOUS RISK WAS
EXPANDED NORTH TO INCLUDE KMLB AS HI-RES TRENDS ARE KEYING ON A
CONVERGENT AREA NORTH OF WHERE THE PREVIOUS RISK FORECAST(S) HAVE
BEEN MADE.
KLEEBAUER
DAY 2
VALID 12Z THU SEP 18 2025 - 12Z FRI SEP 19 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHERN PLAINS, MISSISSIPPI RIVER VALLEY, SOUTHWEST U.S.,
AND SOUTHEAST FLORIDA...
..CENTRAL U.S
BROAD OCCLUSION WILL BE TAKING SHAPE ACROSS THE CENTRAL CONUS WITH
LOCALLY HEAVY RAINFALL POTENTIAL FOCUSED WITHIN THE RESIDUAL AXIS
OF DEFORMATION ACROSS THE DAKOTAS, AS WELL AS ALONG THE PROGRESSION
OF THE COLD FRONT FROM MN DOWN INTO THE SOUTHERN PLAINS. GUIDANCE
IS ALL OVER THE PLACE WITH THE EXACT PLACEMENT OF RELEVANT QPF
MAXIMA, HOWEVER THE ENSEMBLE MEANS STILL EMPHASIZE THE POTENTIAL
WITHIN THESE ZONES REFERENCED ABOVE. EXPECTATION IS FOR MULTIPLE
STORM CLUSTERS TO AID IN THE THREAT WITH SOME PLACES LIKELY TO SEE
2-4" INCHES THANKS TO THE +1 TO +2 DEVIATION PWATS SITUATED FROM OK
UP THROUGH THE UPPER MIDWEST AND NORTHERN PLAINS. ENVIRONMENT
FAVORS RATE POTENTIAL OF 1-2"/HR IN ANY CELL WITHIN THE WCB AS
GENERAL BUOYANCY SHOULD SPUR SEVERAL THUNDERSTORMS ALONG AND AHEAD
OF THE FRONT AS IT MIGRATES INTO THE MISSISSIPPI VALLEY. AS WE GET
CLOSER IN TIME, IT'S PLAUSIBLE TO HAVE A TARGETED UPGRADE, OR TWO,
SO IT WILL BE A PERIOD TO MONITOR.
..SOUTHWEST U.S
REMNANT MOISTURE FROM POST-TROPICAL CYCLONE MARIO WILL ADVECT
POLEWARD WITH CALIFORNIA GENERALLY IN THE CROSS-HAIRS FOR ELEVATED
MOISTURE TO MOVE INTO THE SOUTHERN HALF OF THE STATE DURING THE END
OF THE D1 INTO D2 TIME FRAME. ENERGY FROM MARIO WILL LAG THE
INITIAL LOW TO MID- LEVEL MOISTURE SURGE, BUT EVENTUALLY WILL ENTER
INTO THE AREA BY THE LATE-MORNING HRS. THURSDAY. MODELS ARE PRETTY
CONSISTENT IN THE MERIDIONAL PUSH OF ELEVATED MOISTURE LEADING TO
SCATTERED/WIDESPREAD CONVECTION FORMING ACROSS MUCH OF SOCAL,
ESPECIALLY THE LOWER DESERTS AND TERRAIN FOCUSED AREAS FROM BIG SUR
DOWN INTO THE TRANSVERSE AND PENINSULAR RANGES. IT WILL BE
IMPORTANT TO MONITOR THE PROGRESSION OF THE REMNANT MID-LEVEL
ENERGY AS A MORE CONSOLIDATED VORTICITY MAXIMA WOULD LIKELY CAUSE A
PERIOD OF ENHANCED, FOCUSED RAINFALL IN-OF THE COASTAL TERRAIN
NORTH OF LOS ANGELES. AS OF NOW, THE SETUP FAVORS THE ENERGY
BECOMING INCREASINGLY SHEARED WITH A MORE SCATTERED CONVECTIVE
DEPICTION IN THE PRECIP FIELD. 00Z ECMWF SHOWS WHAT COULD TRANSPIRE
WITH A MORE CONSOLIDATED VORTICITY MAXIMA WITH A STRIPE OF HEAVY
RAINFALL ALIGNED SOUTH TO NORTH WHICH WOULD EASILY NECESSITATE A
RISK AREA HIGHER THAN THE CURRENT MRGL IN PLACE. WITH PWATS LIKELY
TO SURGE TO 1.7-2.0", RAINFALL RATES >1"/HR ARE CERTAINLY POSSIBLE,
A THRESHOLD THAT HISTORICALLY HAS CAUSED LOCALIZED FLOODING TO SOME
DEGREE, LEADING TO TOTALS OF 1-3" WITH ROOM FOR HIGHER IF
EVERYTHING BREAKS UNFAVORABLY FOR THE REGION. THIS IS SOMETHING TO
MONITOR AS WE STEP CLOSER IN TIME, BUT FOR NOW THE THREAT IS DEEMED
A MRGL WITH A CHANCE AT AN UPGRADE AS WE MOVE FURTHER INTO THE CAMS
WINDOW FOR A BETTER ASSESSMENT ON POTENTIAL PRECIP MAXIMA.
..SOUTHEAST FLORIDA
GENERAL PERSISTENCE IN THE PATTERN ACROSS SOUTHERN FL WITH ELEVATED
PWATS AND STREAMING MID-LEVEL VORTICITY PLAGUING THE REGION LEADING
TO SCATTERED BOUTS OF HEAVY RAINFALL WITHIN ANY CONVECTIVE
DEVELOPMENT. THE AREA OF INTEREST REMAINS CENTERED ON THE URBAN
CORRIDOR FROM MELBOURNE TO MIAMI JUST DUE TO THE REPEATED NATURE OF
CONVECTION AND THE LOWER FFG'S ALIGNED WITHIN THE URBANIZATION
FOOTPRINT. MODELS ARE STILL EVERYWHERE IN THE PLACEMENT OF THE
HEAVIEST PRECIP, SOME JUST OFFSHORE, AND OTHERS HITTING THE KEYS
AND SOUTHERN PENINSULA PRETTY HARD OVER THE COURSE OF D1 AND D2.
CONSIDERING THE ENVIRONMENTAL FACTORS AND CONTINUITY IN THE
PATTERN, MAINTAINING THE PREVIOUS MRGL RISK IS MORE THAN SUFFICIENT
FOR THE SETUP. LITTLE TO NO CHANGE WAS NECESSARY GIVEN THE 00Z NWP
OUTPUT.
KLEEBAUER
DAY 3
VALID 12Z FRI SEP 19 2025 - 12Z SAT SEP 20 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHERN PLAINS, MISSISSIPPI RIVER VALLEY, SOUTHWEST U.S.,
CENTRAL SIERRA NEVADA'S, AND SOUTHEAST FLORIDA...
..NORTHERN PLAINS TO MISSISSIPPI VALLEY
OUR OCCLUDED LOW WILL CONTINUE TO CHURN ACROSS THE NORTHERN CONUS
WITH ANOTHER ROUND OF CONVECTION FORECAST ON THE EASTERN FLANK OF
THE REMNANT CYCLONE. THE THREAT IS FORECAST TO WANE COMPARED TO THE
PREVIOUS PERIODS, BUT THERE IS STILL A GENERAL MRGL RISK FOR FLASH
FLOODING IN ANY CELLS THAT DEVELOP FROM THE MID TO UPPER-
MISSISSIPPI VALLEY. MODELS ARE STILL ALL OVER THE PLACE IN LOCATION
AND MAGNITUDE, BUT THE PREMISE OF THE ENVIRONMENT REMAINING
FAVORABLE FOR THE THREAT WARRANTED A CONTINUATION OF THE MRGL RISK
FROM PREVIOUS FORECASTS.
..WESTERN U.S
ELEVATED MOISTURE FROM REMNANTS OF MARIO WILL LINGER AND ALLOW FOR
SCATTERED TO WIDESPREAD CONVECTION TO MATERIALIZE OVER THE
SOUTHWESTERN U.S. AND OVER PORTIONS OF THE SIERRA'S OF CA. PENDING
THE MID-LEVEL EVOLUTION OF THE ENERGY FROM MARIO BLEEDING INTO D3,
A HEAVIER PRECIP THREAT IS POSSIBLE, BUT NOT ANTICIPATED AT THIS
TIME. THE SETUP SHOULD BE ABLE TO ADEQUATELY SHEAR ANY MID-LEVEL
VORTICITY LEADING TO A LESS PRONOUNCED SETUP REGIONALLY. STILL, THE
MOISTURE ANOMALIES AND EXPECTED INSTABILITY PRESENCE WILL ALLOW FOR
AT LEAST WIDELY SCATTERED FLASH FLOOD CONCERNS HEADING INTO THE END
OF THE WEEK AND EARLY WEEKEND. A MRGL RISK REMAINS FOR THE
SOUTHWEST AND THE CALIFORNIA SIERRA'S.
..SOUTHEAST FLORIDA
A CHANCE FOR LINGERING HEAVY RAINFALL ACROSS SOUTHEAST FL NEAR
MIAMI REMAINS WITHIN THE MEANS, ENOUGH TO WARRANT A MAINTENANCE
OF THE INHERITED MRGL RISK. IT WILL REALLY COME DOWN TO FINER
MESOSCALE DETAILS AND TIMING OF WHEN THE INCESSANT SHORTWAVE ENERGY
PROGRESSION VACATES THE AREA. FOR NOW, THERE'S ENOUGH TO KEEP WHAT
WAS FORECAST PRIOR, BUT WILL ASSESS AS WE GET CLOSER IN TIME. THE
AREA IS SMALL IN SPATIAL COVERAGE, SO THERE'S A CHANCE IT IS
REMOVED, OR EXPANDED ONCE MORE CAMS WINDOWS SHED LIGHT ON THE
SETUP.
KLEEBAUER
DAY 1 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/94EPOINTS.TXT
DAY 2 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/98EPOINTS.TXT
DAY 3 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/99EPOINTS.TXT
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