248
FOUS30 KWBC 021939
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
339 PM EDT THU OCT 2 2025
DAY 1
VALID 16Z THU OCT 02 2025 - 12Z FRI OCT 03 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SIERRA, GREAT BASIN AND EASTERN FLORIDA...
16Z UPDATE: THE PREVIOUS FORECAST HAS NOT CHANGED MUCH IN THE
OVERALL SYNOPTIC AND MESOSCALE EXPECTATIONS OVER THE TWO RISK AREAS
OUTLINED.
ACROSS FL, LOCALLY HEAVY RAINFALL WITHIN THE CONFINES OF
A SURFACE TROUGH AXIS ALIGNED PARALLEL, JUST INLAND FROM THE
COASTAL PLAIN WILL BE THE THREAT THROUGH THE PERIOD WITH URBAN
FLASH FLOODING THE MOST LIKELY CASE OF ANY POTENTIAL. HIGHEST 5"
PROBS VIA 12Z HREF NEIGHBORHOOD OUTPUT ARE STILL IN THAT 40-90%
RANGE, HOWEVER THE HIGHEST PROBS ARE CENTERED BETWEEN MELBOURNE TO
WEST PALM BEACH WITH A CENTROID NEAR PORT ST LUCIE. THIS IS HI-RES
INDICATIONS OF WHERE THEY ANTICIPATE THE STRONGEST LOW-LEVEL
CONVERGENCE REGIME WITH RELEVANT MOISTURE FLUX SITUATED JUST OFF
THE ATLANTIC COAST AT SIMILAR LATITUDE TO THE ELEVATED PROBS.
CONSIDERING SOME ERROR IN CONVECTIVE PLACEMENT, IT'S STILL
REASONABLE TO CONSIDER AREAS FROM MIAMI UP TO DAYTONA WITHIN THE
PROXY FOR LOCALLY ENHANCED RAINFALL RATES WITH HOURLY RATES ~3"/HR
AT PEAK INTENSITY WHEN YOU FACTOR PWATS HOVERING BETWEEN 1.75-2"
AS OF THE 10/12Z RAOB SOUNDINGS FROM CAPE CANAVERAL. THIS SIGNAL
IN THE MEANS AND INDIVIDUAL DETERMINISTIC IS MORE THAN SUFFICIENT
FOR A MAINTENANCE OF THE MRGL RISK IN PLACE WITH ONLY A MINOR
NORTHWARD EXPANSION IN THE AREA TO COVER FOR THE CURRENT RADAR
TRENDS.
ACROSS THE WEST, THE SETUP REMAINS CONSISTENT AMONG ALL THE CAMS
AND GLOBAL DETERMINISTIC WITH A RELATIVE MAX SITUATED OVER THE
NORTHERN SIERRA ALONG AND NEAR I-80 WHERE MODEST INSTABILITY AND
SUFFICIENT LARGE SCALE ASCENT SHOULD INITIATE A PERIOD OF MODERATE
TO HEAVY RAIN FOR MULTIPLE HOURS THAT COULD ENHANCE A LOCALIZED
FLOOD PROSPECT. AS WE MOVE FURTHER INLAND INTO THE GREAT BASIN, THE
SETUP SHIFTS TO MORE SCATTERED/WIDESPREAD CONVECTIVE SCHEMES THAT
ARE BETTER SUITED FOR THOSE HIGHLY LOCALIZED FLASH FLOOD CONCERNS
WITHIN TOPOGRAPHY THAT HISTORICALLY FAVORS FLASHIER OUTPUT DUE TO
THE SOIL COMPOSITION AND PREVALENCE OF DRY WASHES AND ARROYOS
LITTERED OVER THE HIGH DESERT. THIS THREAT EXPANDS INTO THE SNAKE
RIVER BASIN OF ID AND ADJACENT ZONES LATER THIS EVENING. TOTALS
BETWEEN 1-2" (LOCALLY HIGHER IN THE NORTHERN SIERRA) WILL BE MOST
COMMON IN THE MRGL RISK IN PLACE FROM THE CENTRAL VALLEY OF
CALIFORNIA UP INTO SAWTOOTH AND SALMON RIVER RANGES OF IDAHO.
KLEEBAUER
..PREVIOUS DISCUSSIONS..
..SIERRA/GREAT BASIN
AN UPPER LOW WILL DIG INTO THE WEST COAST WHILE SENDING STREAMS OF
VORTICITY INTO THE SIERRA NEVADA, GREAT BASIN AND NORTHERN
ROCKIES. DIFFLUENCE WILL SPREAD ACROSS THE AFOREMENTIONED AREAS
THROUGHOUT THE DAY ON FRIDAY. AT THE SURFACE, A LOW PRESSURE SYSTEM
WILL PROPAGATE ACROSS THE INTERIOR WEST, WHILE ANOMALOUS MOISTURE
(3-4 STNDV PWATS) STREAMS IN BEHIND THE PRECEDING COLD FRONT.
SCATTERED TO ISOLATED THUNDERSTORMS MAY DEVELOP AND PRODUCE
ISOLATED INSTANCES OF EXCESSIVE RAINFALL AND FLASH FLOODING.
UPSLOPE ENHANCEMENT ACROSS THE SIERRA COULD PRODUCE HEAVIER
RAINFALL TOTALS THAN WHAT ARE CURRENTLY FORECAST. EAS PROBABILITIES
OF 24 HOUR QPF EXCEEDING 1 INCH ARE OVER 40% IN THE SIERRA AND
BETWEEN 15-40% IN THE GREAT BASIN. THEREFORE, BURN SCARS OVER
CENTRAL AND SOUTHERN IDAHO ARE AT RISK OF FLASH FLOODING TODAY.
..EASTERN FLORIDA
MOIST EASTERLY FLOW INTO A SURFACE FRONT ORIENTED NORTH-SOUTH WILL
PRODUCE SCATTERED CONVECTION ACROSS FLORIDA TODAY. THE SURFACE
BOUNDARY DRAPED ALONG THE EASTERN COAST WILL FOCUS PWATS OF AROUND
2" IN THAT AREA WHERE MUCAPE BETWEEN 500-1000 J/KG WILL ALSO LIKELY
PRODUCE HIGH RAIN RATES. NEIGHBORHOOD PROBABILITIES OF EXCEEDING
5" IN 24 HOURS ARE BETWEEN 40-90% FROM NEAR MIAMI METRO UP THROUGH
MELBOURNE AND ORLANDO.
KEBEDE
DAY 2
VALID 12Z FRI OCT 03 2025 - 12Z SAT OCT 04 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE EAST-
CENTRAL FLORIDA COAST...
..EASTERN FLORIDA
20Z UPDATE: IN COORDINATION WITH THE MELBOURNE WFO, A SLGT RISK WAS
ADDED TO THE EAST-CENTRAL FLORIDA COAST SOUTH OF MELBOURNE. A
REMNANT MOISTURE FLUX TIED TO THE TRAILING LOW-LEVEL SHEAR AXIS
FROM IMELDA WILL SLOWLY PROPAGATE WESTWARD OUT OF THE BAHAMAS WITH
EMBEDDED MID-LEVEL VORTICITY TRYING TO DEVELOP INTO A MORE
CONCENTRIC CIRCULATION AS WE STEP INTO SATURDAY AM. THE PATTERN IS
A ONGOING EVOLUTION THAT BEGINS IN D1 AND JUST MOVES FORWARD IN
TIME WITH FRICTIONAL CONVERGENCE REGIME ALIGNED ALONG THE EASTERN
FL COAST, MAINLY IN LINE WITH A SURFACE TROUGH FORECAST PARALLEL TO
THE COASTAL PLAIN FOR THE FORESEEABLE FUTURE. SUFFICIENT LOW-LEVEL
BUOYANCY AIDED BY A THIN AXIS OF 1500-2000 J/KG OF MUCAPE ALONG THE
COAST WITH PWATS BETWEEN 1.8-2.2" SITUATED WITHIN THE SAME ZONE
SHOULD ALLOW FOR A HEIGHTENED THREAT OF LOCALLY HEAVY RAINFALL
WITHIN THE EAST-CENTRAL FL COASTAL PLAIN, NOTED BY SEVERAL CAMS AND
ENSEMBLE MEMBERS OF BOTH THE GEFS/ECENS. CONSIDERING THE HIGH
NEIGHBORHOOD PROBS FOR >5" (45-80%) FOCUSED NEAR AND JUST SOUTH OF
THE SPACE COAST, A SLGT RISK WAS ADDED FOR THE ELEVATED THREAT OF
FLASH FLOODING WITHIN THE URBAN ZONES THAT ALIGN THE IMMEDIATE
COAST TO JUST INLAND AROUND THE I-95 CORRIDOR. SOME ADJUSTMENTS IN
FUTURE UPDATES ARE PLAUSIBLE PENDING NEAR TERM CONVECTIVE
EVOLUTIONS, SO THIS IS A PERIOD TO MONITOR CLOSELY AS WE MOVE INTO
TOMORROW MORNING AND BEYOND.
..GREAT BASIN
20Z UPDATE: PREVIOUS FORECAST REMAINS STEADFAST WITH ONLY MINOR
CHANGES MADE TO THE MRGL RISK FORECAST ACROSS THE GREAT BASIN INTO
PORTIONS OF THE INTERIOR NORTHWEST. HEAVIEST PRECIP AXIS WILL BE
FOCUSED OVER CENTRAL NV INTO SOUTHERN ID AS THE UPPER TROUGH BEGINS
TO CLOSE OFF AS IT MIGRATES INLAND FROM THE PACIFIC LEADING TO
ENHANCED LARGE SCALE FORCING AND STEEPER LAPSE RATES FRIDAY
AFTERNOON AND EVENING IN THESE LOCATIONS. ANTICIPATING RATES TO
REMAIN <1"/HR FOR THE THREAT, HOWEVER SOME OF THESE LOCATIONS ARE
SUSCEPTIBLE TO EVEN 0.5"/HR RATES DUE TO HIGH RUN AND COMPLEX
TOPOGRAPHY THAT PROMOTES FUNNELING. AS A RESULT, THERE WASN'T MUCH
OF A REASON TO DEVIATE FROM THE LAST FORECAST ISSUANCE, SO ONLY
MINOR ADJUSTMENTS BASED ON UPDATED PROBS AND MEAN QPF DISTRIBUTION
WERE GENERATED IN THIS FORECAST CYCLE.
KLEEBAUER
..PREVIOUS DISCUSSION..
THE UPPER TROUGH PROPAGATES THROUGH THE GREAT BASIN AND BEGINS
LIFTING NORTH ON FRIDAY. THE EXCESSIVE RAINFALL THREAT BLEEDS INTO
FRIDAY FROM THURSDAY AS THUNDERSTORMS WEAKEN AND SPREAD FROM THE
GREAT BASIN INTO THE NORTHERN ROCKIES BY FRIDAY AFTERNOON. INSTABILITY
AND MOISTURE WILL BE RELATIVELY MODEST (100-200 J/KG MUCAPE)
(0.5-0.9"). ENSEMBLE EXCEEDANCE PROBABILITIES OF 1" ARE BETWEEN
10-20% OVER NORTHERN NEVADA WITH 2 DAY TOTALS POTENTIALLY ECLIPSING
2" IN SOME PLACES.
KEBEDE
DAY 3
VALID 12Z SAT OCT 04 2025 - 12Z SUN OCT 05 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE FLORIDA
SPACE COAST...
..EASTERN FLORIDA
20Z UPDATE: SURFACE EVOLUTION OF A WEAK SURFACE LOW OFF THE
EASTERN FL COAST WILL BE KEY IN DETERMINING THE EXTENT OF THE HEAVY
RAINFALL PERSISTENCE IN-OF THE FL COAST BY SATURDAY. AS OF NOW,
THE ENSEMBLE MEANS SUGGEST A WEAK SURFACE REFLECTION WILL FORM AND
MIGRATE SLOWLY INLAND OF THE PENINSULA BY SATURDAY AFTERNOON
LEADING TO A LARGER AREA OF THE FL COAST BEING SUBJECT TO ENHANCED
CONVERGENCE AND NORTHERN PUSH OF ELEVATED PWATS/INSTABILITY CAPABLE
OF FLASH FLOOD PROSPECTS. ENSEMBLE MEANS ARE MORE GENEROUS THAN
MOST DETERMINISTIC, HOWEVER THE THE SCHEME IS CERTAINLY FAVORABLE
FOR THE THREAT OF A REPEATED HEAVY RAIN SCENARIO IN-OF THE FL SPACE
COAST. IN COORDINATION WITH THE MELBOURNE WFO, ANOTHER SLGT RISK
WAS ADDED FOR MUCH OF THE CWA AS ENSEMBLE MEAN QPF OF >2" ALIGNS
BEST WITHIN THE ZONE THAT EXTENDS FROM DAYTONA BEACH DOWN TO PORT
ST LUCIE. IF THE D2 MATERIALIZES, THIS WILL END UP BEING A CASE
WHERE WET ANTECEDENT CONDITIONS WOULD EXACERBATE THE POTENTIAL
LEADING TO A GREATER CASE FOR SCATTERED FLASH FLOOD SIGNALS AS THE
PATTERN EVOLVES. WITH A BETTER LOW-LEVEL CONVERGENCE REGIME
GENERATING FROM THE SURFACE REFLECTION NEARBY, THE CHANCE FOR
HEAVIER PRECIP EXPANDING NORTH OF THE PREVIOUS PERIOD WOULD BE BE
HIGHER, THUS THE SLGT RISK HAVING A LARGER AREAL FOOTPRINT COMPARED
TO THE PREVIOUS D2. THIS REMAINS A PERIOD TO WATCH AS SMALL SHIFTS
IN THE SYNOPTIC SCHEME COULD SHIFT HEAVY RAIN PROXY AND SUBSEQUENT
MAGNITUDES.
..CENTRAL GULF COAST
20Z UPDATE: THIS SETUP REMAINS ON THE LOWER END OF THE MRGL RISK
THRESHOLD DUE TO THE DIFFERENCES IN PLACEMENT OF THE HEAVIEST
PRECIP ALIGNED WITH THE COASTAL CONVERGENCE REGIME FORMULATING OVER
THE NORTHERN GULF WATERS. THE LATITUDINAL PUSH OF THE CONVERGENT
AXIS WILL LIKELY DICTATE WHERE THE RISK IS EVEN NECESSARY AS THE
SOUTHERN PARISHES ARE MORE THAN CAPABLE TO ABSORB THE HEAVY RAIN
PROSPECTS PROMOTED BY SOME GUIDANCE. HOWEVER, THE URBAN CORRIDOR OF
NEW ORLEANS AND NEARBY SUBURBS ARE MORE SUSCEPTIBLE TO FLASH FLOOD
PROSPECTS AND WERE CLOSE ENOUGH TO THE HIGHEST MEAN QPF FORECAST
THAT THE MAIN SHIFT WAS TO INCLUDE THE METRO AND SHRINK THE OVERALL
RISK FOOTPRINT TO ALIGN WITH AREAS THAT WOULD SEE THE HEAVIEST
PRECIP POTENTIAL AND HAVE THE ABILITY TO ENTICE FLASH FLOOD
PROSPECTS.
KLEEBAUER
..PREVIOUS DISCUSSION..
MID- LEVEL VORTICITY SWIRLING OVER THE CENTRAL GULF
COAST WILL GENERATE INSTABILITY AND PULL AN INCREASING AMOUNT OF
GULF MOISTURE INTO THE REGION ON SATURDAY. CONVECTION SHOULD FIRE
ALONG A COASTAL CONVERGENCE ZONE THAT DEVELOPS AND DIURNAL HEATING
SHOULD ENHANCE AFTERNOON/EVENING STORMS ALONG THAT BOUNDARY. PWATS
WILL LIKELY BE OVER 2" WITH MUCAPE BETWEEN 500-750 J/KG. ENSEMBLE
EXCEEDANCE PROBABILITIES OF 2" ARE BETWEEN 15-30% FOR MUCH OF
SOUTHEAST LOUISIANA.
KEBEDE
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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