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FOUS30 KWBC 301553
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1153 AM EDT WED JUL 30 2025
DAY 1
VALID 12Z WED JUL 30 2025 - 12Z THU JUL 31 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDDLE MISSISSIPPI VALLEY AND FOR PORTIONS OF THE FRONT RANGE
AND ADJACENT HIGH PLAINS...
..MID MS VALLEY
AS ANTICIPATED FROM OVERNIGHT, THE MCS HELD TOGETHER AND HAS
REMAINED PROGRESSIVE AS IT TRACKED ACROSS THE MISSISSIPPI RIVER
THIS MORNING. WITH THE MCS AND COLD FRONT HAVING TRACKED THROUGH
WI/IA, HAVE TRIMMED THE SLIGHT RISK OUT OF THE REGION BUT EXTENDED
IT A LITTLE FARTHER EAST INTO SOUTHERN MICHIGAN BASED ON 12Z HREF
GUIDANCE FOCUSING LOW CHANCE PROBABILITIES (20-30%) FOR LOCALIZED
RAINFALL TOTALS >5". OTHERWISE, HAVE MAINTAINED THE MARGINAL RISK
BACK THROUGH THE IOWA AND EXTENDING IT AS FAR WEST AS SOUTHEAST
SD. THIS IS DUE TO THE APPROACH OF A REMNANT MCV THAT IS CATCHING
THE EYES OF SOME CAMS (HRRR/ARW) WITH LOCALLY HEAVY AMOUNTS THIS
EVENING AND INTO TONIGHT. IT IS POSSIBLE THIS MCV WORKS ITS WAY
EAST INTO IOWA BY EARLY TOMORROW MORNING, POSING A LOW CHANCE FOR
ADDITIONAL HEAVY RAINFALL AND FLASH FLOODING LATE TONIGHT AND INTO
EARLY THURSDAY MORNING.
..SOUTHERN APPALACHIANS
A SLIGHT RISK WAS INTRODUCED THIS CYCLE GIVEN THE REGION IS MIRED
IN HIGHLY ANOMALOUS PWATS AND VISIBLE SATELLITE SHOWS STRONG
SURFACE BASED HEATING IS UNDERWAY. THE 12Z RNK SOUNDING, LOCATED
JUST NORTH OF THE SLIGHT RISK AREA, MEASURED A 1.80" PWAT AND
MLCAPE >600 J/KG WITH LOW-MID LEVEL RH VALUES AVERAGING CLOSE TO
85%. THERE WAS ALSO VERY LITTLE CIN PRESENT AND THE CONVECTIVE TEMP
WAS 80F, WHICH IS LIKELY TO BE MET BEFORE 16Z. PWATS ARE LIKELY TO
RANGE BETWEEN 1.7-1.9" THIS AFTERNOON, WHICH IS ABOVE THE 97.5
CLIMATOLOGICAL PERCENTILE, AND RAP FORECASTS SUGGEST AS MUCH AS
1,000 J/KG OF MLCAPE COULD BE AVAILABLE THIS AFTERNOON. STEERING
WINDS ARE LIGHT AND VERTICAL WIND SHEAR IS WEAK, BUT TORRENTIAL
DOWNPOURS WITH MAXIMUM RAINFALL RATES UP TO 2.5"/HR ARE POSSIBLE IN
COMPLEX TERRAIN SPORTING SATURATED SOILS.
OUTSIDE OF MINOR TWEAKS TO THE INHERITED MARGINAL AND SLIGHT RISKS
AREAS, THE OVERNIGHT FORECAST DISCUSSION DETAILS LARGELY REMAIN ON
TRACK.
MULLINAX
---PREVIOUS DISCUSSION---
..MID MS VALLEY
AN EASTWARD MOVING SQUALL LINE WILL BE ONGOING AT 12Z THIS MORNING
ACROSS PORTIONS OF EASTERN IA INTO NORTHERN MO,AND GIVEN THE
DOWNSTREAM INSTABILITY AXIS DO EXPECT THIS TO GENERALLY SURVIVE
ACROSS MUCH OF CENTRAL AND NORTHERN IL INTO EARLY THIS
AFTERNOON. THE PROGRESSIVE NATURE OF THIS CONVECTION SHOULD LIMIT
THE EXTENT AND MAGNITUDE OF FLASH FLOODING, HOWEVER HOURLY RAINFALL
OF 1-2" WILL REMAIN LIKELY. THIS WILL BE ENOUGH TO DRIVE AT LEAST
AN ISOLATED FLASH FLOOD RISK AS CONVECTION MOVES EASTWARD. THIS
MCS/MCV WILL HELP DRIVE A COLD FRONT SOUTH ACROSS MO AND KS, AND DO
EXPECT CONVECTION TO EXPAND OVER THESE AREAS BY AFTERNOON. A BIT
UNCLEAR WHETHER IT TAKES THE FORM OF A PROGRESSIVE SQUALL LINE
(LIMITING THE FLASH FLOOD RISK), OR IF MORE DISCRETE CELL FORMATION
ALLOWS FOR SOME TRAINING/CELL MERGING.
THE OVERALL ENVIRONMENT...A SLOWLY SOUTHWARD SHIFTING COLD FRONT
HELPING FOCUS CONVERGENCE, THE RIGHT ENTRANCE REGION OF THE UPPER
JET ADDING SOME DIVERGENCE ALOFT, PLENTIFUL CAPE NEAR THE FRONT AND
PWS LOCALLY OVER 2"...CERTAINLY SUPPORTS AREAS OF EXCESSIVE
RAINFALL. AT A MINIMUM, ISOLATED INSTANCES OF FLASH FLOODING CAN
BE EXPECTED ACROSS PORTIONS OF KS/MO/IA/IL/IN. THE MAGNITUDE AND
COVERAGE OF THIS THREAT WILL COME DOWN TO CONVECTIVE MODE AND
EVOLUTION TODAY/TONIGHT. AS MENTIONED ABOVE, CONVECTIVE MODE
GENERALLY FAVORING MORE PROGRESSIVE SQUALL LINES MAY END UP
KEEPING THE RISK ISOLATED. HOWEVER IF WE ARE ABLE TO GET ANY
RECOVERY AFTER THIS INITIAL ROUND OF CONVECTION THE MCV AND
FAVORABLE SYNOPTIC AND MOISTURE INGREDIENTS WILL REMAIN IN PLACE.
THUS, HIGHER END IMPACTS ARE STILL POSSIBLE IN THIS SETUP, BUT FAR
FROM GUARANTEED GIVEN THE QUESTIONS THAT REMAIN REGARDING
CONVECTIVE MODE/EVOLUTION AND INSTABILITY PERSISTENCE.
..SOUTHWEST AND HIGH PLAINS
ISOLATED TO SCATTERED CONVECTION WILL LIKELY DEVELOP NEAR AREAS OF
TERRAIN FROM NM INTO THE ROCKIES. EASTERLY LOW LEVEL FLOW INTO THE
FRONT RANGE OF CO WILL LIKELY RESULT IN SLOW CELL MOTIONS AND THE
POSSIBILITY OF AT LEAST ISOLATED FLASH FLOODING. INCREASED
CONVERGENCE NEAR THE FRONT COULD SUPPORT A BIT MORE CONVECTIVE
COVERAGE AND EVENTUAL ORGANIZATION FROM PORTIONS OF EASTERN NM INTO THE
OK AND TX PANHANDLES AS WELL. ONLY CHANGE TO THE INHERITED SLIGHT
RISK WAS TO EXPAND IT NORTHWARD INTO WY WHERE BOTH THE HREF AND
REFS SHOW AN UPTICK IN 1" PER HOUR PROBABILITIES BY THIS EVENING AS
CONVECTIVE COVERAGE INCREASES AND A FEW CELL MERGERS BECOME
LIKELY.
..ELSEWHERE
A BROAD MARGINAL RISK EXTENDS FROM THE CENTRAL GULF COAST ALL THE
WAY INTO PORTIONS OF SOUTHERN NY. HIGH PWS AND CAPE AND CONVECTIVE
COVERAGE WILL DRIVE THE RISK FROM THE GULF COAST INTO THE TN
VALLEY. A LACK OF ORGANIZATION MAY KEEP FFG EXCEEDANCE COVERAGE
DOWN, BUT HIGH RAINFALL RATES WILL LIKELY LOCALLY APPROACH OR
EXCEED FFG. A COLD FRONT WILL DRIVE THE RISK FORM PORTIONS OF WV
INTO SOUTHERN NY. CONVECTIVE COVERAGE AND ORGANIZATION SHOULD BE
LACKING...HOWEVER WITH THE FRONT SLOWING CELLS SHOULD BE SLOW
MOVING AND COULD DROP A QUICK 1-2" OF RAIN. LIKELY ENOUGH TO DRIVE
AT LEAST A LOCALIZED URBAN FLASH FLOOD RISK.
CHENARD
DAY 2
VALID 12Z THU JUL 31 2025 - 12Z FRI AUG 01 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID-ATLANTIC INTO SOUTHERN NEW ENGLAND...
..MID-ATLANTIC INTO NEW ENGLAND
HEAVY RAINFALL AND AT LEAST SOME FLASH FLOODING APPEARS LIKELY
THURSDAY ACROSS PORTIONS OF THE MID-ATLANTIC AND POSSIBLY INTO
SOUTHERN NEW ENGLAND. CONVECTION IS LIKELY TO FOCUS ALONG A SLOW
MOVING COLD FRONT, WITH BROAD MID LEVEL TROUGHING AND HEIGHT FALLS
PROVIDING LARGE SCALE ASCENT. THE UPPER LEVEL JET IS DISPLACED
PRETTY FAR NORTH, HOWEVER STILL LIKELY TO GET SOME ENHANCED UPPER
LEVEL DIVERGENCE INTO THE MID-ATLANTIC FROM THIS FEATURE.
STILL QUITE A BIT OF UNCERTAINTY WITH THE DETAILS OF THIS
CONVECTIVE RISK. SOME OF THIS UNCERTAINTY IS TIED TO THE EVOLUTION
OF THE SHORTWAVE/MCV ASSOCIATED WITH THE ONGOING CONVECTION OVER
IA/IL THIS MORNING. THE STRENGTH OF THAT FEATURE AND HOW IT
INTERACTS WITH THE FRONT AND DEVELOPING LONGWAVE TROUGHING WILL
LIKELY IMPACT THE FRONTAL TIMING AND BOTH THE AXIS AND MAGNITUDE
OF THE FLASH FLOOD RISK. THE FRONT SHOULD BECOME MORE PROGRESSIVE
BY THURSDAY EVENING, BUT HOW THINGS EVOLVE BEFORE THEN ARE
UNCLEAR. THERE IS A CONCERN THAT WE COULD SEE CELL DEVELOPMENT
THURSDAY AFTERNOON JUST AHEAD OF THE FRONT AS SOUTHERLY FLOW
INCREASES WHICH COULD SET THE STAGE FOR A FAIR AMOUNT OF CELL
MERGER ACTIVITY DRIVING UP RAINFALL TOTALS. ALSO IT LOOKS LIKE AN
AREA OF LOW PRESSURE TRIES TO SPIN UP ALONG THE FRONT THURSDAY
NIGHT WITH AN INVERTED TROUGH AXIS EXTENDING NORTHWARD. THE ECMWF
AND 3KM NAM KEEP THIS TROUGHING INLAND, WHICH COULD BE A PRETTY
IDEAL SETUP FOR LOW TOPPED WARM RAIN CONVECTION THROUGH THE
OVERNIGHT. HOWEVER, MOST OTHER MODELS PUSH THIS TROUGHING OFFSHORE
AND CLEAR THINGS OUT BY THE OVERNIGHT.
THE ENVIRONMENT (SLOW MOVING FRONT, INCREASING LARGE SCALE
FORCING, PWS OVER 2" AND PLENTIFUL INSTABILITY) SUPPORTS EXCESSIVE
RAINFALL...ITS JUST GOING TO COME DOWN TO LOCATION AND DURATION,
BOTH OF WHICH REMAIN UNCERTAIN. THE CURRENT CONSENSUS AMONG THE
MODELS STILL SUPPORTS A CORRIDOR FROM DC TO PHL TO NYC (INCLUDING
MUCH OF EASTERN PA AND NJ) AS THE AREAS MOST LIKELY TO BE
IMPACTED. THE 00Z EXPERIMENTAL REFS ENSEMBLE NEIGHBORHOOD
PROBABILITIES OF EXCEEDING 5" ARE 40-70% ACROSS THIS CORRIDOR. THE
REFS CAN RUN HIGH WITH ITS QPF OUTPUT, AND MAY BE UNDER
DISPERSIVE...BUT NONETHELESS STILL A SIGNAL WORTH WATCHING. WE GAVE
SOME CONSIDERATION TO A MDT RISK UPGRADE THIS CYCLE, BUT AFTER
COLLABORATION WITH LOCAL WFOS WE OPTED TO MAINTAIN A SLIGHT RISK
GIVEN SOME OF THE LINGERING QUESTIONS ON HOW THE DETAILS OF THE
EVENT WILL EVOLVE. AS THE EVENT COMES MORE INTO RANGE OF THE HREF
TODAY AND TONIGHT WE WILL HOPEFULLY GAIN MORE CONFIDENCE ON THESE
DETAILS, AND A MDT RISK UPGRADE IS STILL A POSSIBILITY WITH LATER
UPDATES. FOR NOW THIS IS STILL CONSIDERED A HIGHER END SLIGHT RISK
FROM DC TO EASTERN PA, NJ AND NYC, AND LOCALLY SIGNIFICANT IMPACTS
COULD EVOLVE.
..GULF COAST INTO OH VALLEY
ANOTHER DAY OF SLOW MOVING PULSE THUNDERSTORMS IS EXPECTED OVER A
LARGE SWATH OF THE REGION TO THE SOUTH OF THE COLD FRONT. SIMILAR
TO WEDNESDAY, HIGH PWS AND CAPE AND CONVECTIVE COVERAGE WILL DRIVE
A LOCALIZED FLASH FLOOD RISK. A LACK OF ORGANIZATION MAY KEEP FFG
EXCEEDANCE COVERAGE DOWN, BUT HIGH RAINFALL RATES WILL LIKELY
LOCALLY APPROACH OR EXCEED FFG.
..ROCKIES INTO THE PLAINS
SCATTERED CONVECTION IS LIKELY FROM EASTERN AZ INTO NM NORTHWARD
ALL THE WAY INTO MT. CURRENTLY NOT SEEING A STRONG INDICATION OF
ORGANIZATION THAT WOULD LEAD TO SLIGHT RISK LEVEL FLASH FLOOD
COVERAGE. HOWEVER LOCALIZED FLASH FLOODING WILL AGAIN BE POSSIBLE
OVER THIS BROAD REGION. THERE ARE SOME HEIGHT FALLS BEGINNING TO
APPROACH FROM THE WEST, SO THAT COULD PERHAPS END UP DRIVING A
MORE ORGANIZED RISK SOMEWHERE WITHIN THIS BROAD MARGINAL RISK. BUT
AT THE MOMENT NOT SEEING ENOUGH OF A SIGNAL IN THE GLOBAL MODELS
OR CAMS TO GO WITH ANY SLIGHT RISK AREAS.
CHENARD
DAY 3
VALID 12Z FRI AUG 01 2025 - 12Z SAT AUG 02 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CAROLINAS...
..SOUTHEAST AND MID-ATLANTIC
A SLIGHT RISK WAS MAINTAINED ACROSS MUCH OF THE CAROLINAS. LIKELY
TO HAVE A WELL DEFINED COLD FRONT DROPPING SOUTH OVER NC THAT WILL
HELP TRIGGER A MORE ORGANIZED CORRIDOR OF CONVECTION FRIDAY
AFTERNOON. MEANWHILE BOTH TERRAIN DRIVEN AND COASTAL CONVERGENCE
DRIVEN CONVECTION ARE ALSO LIKELY BY AFTERNOON. THIS COMBINATION
WILL RESULT IN GOOD CONVECTIVE COVERAGE, WHICH WILL ULTIMATELY
LIKELY LEAD TO A DECENT DEGREE OF CELL MERGER ACTIVITY. PWS ARE
FORECAST AS HIGH AS 2.3"-2.5", WITH PLENTIFUL INSTABILITY. VERY
HEAVY RAINFALL RATES ARE EXPECTED IN THIS THERMODYNAMIC
ENVIRONMENT, WHICH COMBINED WITH THE LIKELIHOOD OF CELL MERGERS,
SHOULD DRIVE AN ISOLATED TO SCATTERED FLASH FLOOD RISK.
THERE IS STILL SOME DEGREE OF UNCERTAINTY WITH THE DETAILS BY
FRIDAY. THE 00Z ECMWF IS AN OUTLIER REALLY HANGING THE FRONT AND
BETTER UPPER LEVEL SUPPORT WELL TO THE NORTH. THUS IT KEEPS A HEAVY
RAINFALL RISK OVER NJ INTO NYC AND LESS OF A CONCENTRATED RISK OVER
THE CAROLINAS. THIS IS NOT SUPPORTED BY THE GFS/UKMET/CMC...AND
THE 00Z ECMWF AIFS ENSEMBLE ALSO LOOKS MORE IN LINE WITH THE
GFS/GEFS. THUS WE WILL CONTINUE TO LEAN THE EROS TO BE MORE IN LINE
WITH THE NON ECMWF SOLUTION. HOWEVER WE WILL NEED TO CONTINUE TO
MONITOR TRENDS GOING FORWARD.
..LOWER MS VALLEY AND VICINITY
A COLD FRONT SHOULD MAKE IT ALL THE WAY INTO PORTIONS OF TX/LA AND
MS BY FRIDAY AFTERNOON. THIS SHOULD ACT AS A FOCUS FOR CONVECTIVE
DEVELOPMENT, AND WITH A LARGE POOL OF PWS OVER 2", HEAVY RAINFALL
RATES ARE LIKELY. LARGE SCALE FORCING AND MOISTURE TRANSPORT IS
WEAKER HERE, SO NOT EXPECTING WIDESPREAD HEAVY RAINFALL...BUT
LOCALIZED HEAVY RAINFALL RATES SHOULD BE ABLE TO DRIVE AN ISOLATED
FLASH FLOOD RISK.
..ROCKIES AND PLAINS
CONVECTION IS LIKELY FROM NM INTO MT FRIDAY. THE BEST ENVIRONMENT
FOR HEAVY RAINFALL WILL PROBABLY BE ACROSS THE NORTHERN END OF
THIS RISK AREA, CENTERED ACROSS MT AND VICINITY. MORE IN THE WAY
OF MID LEVEL TROUGHING WILL BE OVER THIS CORRIDOR, TO GO ALONG
WITH THE HIGHEST PW ANOMALIES IN THE CONUS...NEAR EARLY AUGUST
PEAK VALUES. COMBINE THIS WITH CAPE FORECAST AROUND 2000 J/KG AND
THE ENVIRONMENT APPEARS CONDUCIVE TO AREAS OF FLASH FLOODING.
HOWEVER NONE OF THE 00Z GLOBAL MODELS ARE ALL THAT AGGRESSIVE WITH
QPF OUTPUT, AND THE ONE CAM THAT GOES THIS FAR (THE 00Z RRFS)
SUGGESTS A MORE PROGRESSIVE CONVECTIVE MODE. THUS WE WILL STICK
WITH A MARGINAL RISK AND CONTINUE TO MONITOR TRENDS.
CHENARD
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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