564
FOUS30 KWBC 080057
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
857 PM EDT MON SEP 7 2026
DAY 1
VALID 01Z TUE SEP 08 2026 - 12Z TUE SEP 08 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
NORTHWESTERN MONTANA, THE SOUTHWEST, AND THE UPPER MIDWEST...
..NORTHWEST MONTANA
THE COMMA HEAD/DEFORMATION PRECIP BAND ROTATING AROUND SURFACE LOW
PRESSURE IN CANADA WILL CONTINUE TO WEAKEN AND PULL AWAY FROM THE
REGION OVERNIGHT. RECENT RAINFALL RATES HAVE BEEN MODEST, BUT FFG
VALUES REMAIN LOW AND RAIN HAS FALLEN MOST OF THE DAY. WE HAVE
DOWNGRADED THE SLIGHT RISK TO A MARGINAL FOR THE OVERNIGHT HOURS TO
COVER THE LOWER CHANCE OF SOME FLOOD ISSUES.
..UPPER MISSISSIPPI VALLEY
THE HEIGHT FALLS EJECTING NORTHEASTWARD FROM THE NORTHERN ROCKIES
INTO THE NORTHERN PLAINS WILL STRENGTHEN THE LOW LEVEL SOUTHERLY
FLOW ALONG AND AHEAD OF THE ASSOCIATED WARM FRONTAL BOUNDARY MOVING
THROUGH THE DAKOTAS, RAISING PW VALUES TO 1.75-2.00"+. OVERALL
FAVORABLE LARGE SCALE CONDITIONS WILL SUPPORT ORGANIZED PRE-
FRONTAL/FRONTAL CONVECTION OVER THE UPPER MS VALLEY THIS
EVENING/OVERNIGHT. CONVECTION SHOULD BE PROGRESSIVE WHICH WILL
LIMIT THE EXCESSIVE RAINFALL POTENTIAL -- THE MARGINAL RISK
SUFFICES.
..SOUTHEASTERN ARIZONA INTO WESTERN TO CENTRAL NEW MEXICO
ONGOING ISOLATED CONVECTION CONTINUES AT 01Z WITHIN THE RATHER
MOIST ENVIRONMENT WITH PW VALUES STILL ABOVE AVERAGE. CELLS WILL
REMAIN RATHER ISOLATED AND EVENTUALLY WANE LATER THIS EVENING, BUT
WILL MAINTAIN THE MARGINAL RISK FOR THE EARLIER HOURS.
..AREAS IN THE SOUTHEAST/GULF COAST
CONVECTION HAS DIMINISHED AND WE HAVE REMOVED THE MARGINAL RISK FOR
THE OVERNIGHT HOURS.
FRACASSO/ORAVEC
DAY 2
VALID 12Z TUE SEP 08 2026 - 12Z WED SEP 09 2026
..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER IOWA
2030Z UPDATE...
..FLORIDA PENINSULA
ANOMALOUS PWS UP TO 2.25" AND A STALLED FRONTAL BOUNDARY WILL
UTILIZE 1,000-2,000 J/KG OF MLCAPE TO TRIGGER MORE SCATTERED
THUNDERSTORMS CAPABLE OF PRODUCING 2-3"/HR RAINFALL RATES. THE
SETUP IS NOT TOO DIFFERENT THAN THE SETUP THAT IS SUPPORTING THE
MARGINAL RISK FOR DAY 1, BUT THIS TIME HREF/REFS MEMBERS SHOW
LOCALLY HEAVY RAINFALL RATES COULD RESIDE ALONG THE WESTERN COAST
OF FL. SLOW STORM MOTIONS WITH THE AFOREMENTIONED RATES MENTIONED
ABOVE SUGGEST THE POTENTIAL FOR A LOCALIZE FLASH FLOOD THREAT
TUESDAY AFTERNOON.
..MIDWEST & SOUTHWEST MARGINALS
THE MARGINAL RISKS IN THE MIDWEST AND THE SOUTHWEST WERE MERGED TO
ACCOUNT FOR AN INCREASE IN WPC QPF OVER EASTERN CO AND FAR WESTERN
KS. PWS ARE LIKELY TO SURPASS 1.25", WHICH IS ABOVE THE 97.5
CLIMATOLOGICAL PERCENTILE PER ECMWF GUIDANCE, AND A NEARBY FRONTAL
BOUNDARY WILL HELP TO GENERATE THUNDERSTORMS IN AN ENVIRONMENT
CONTAINING 500-1,000 J/KG MUCAPE. THE QPF ALSO INCREASED ALONG THE
COLD FRONT IN THE CENTRAL PLAINS AND MIDWEST, SUPPORTING AN
EXPANSION OF THE MARGINAL IN THOSE AREAS.
MULLINAX
---PREVIOUS DISCUSSION---
..UPPER MISSISSIPPI VALLEY INTO THE UPPER LAKES
THE SURFACE FRONT PUSHING EAST INTO THE NORTHERN PLAINS DAY 1 WILL
REMAIN VERY PROGRESSIVE DAY 2 INTO THE UPPER MISSISSIPPI
VALLEY/UPPER LAKES. THE PROGRESSIVE NATURE OF THE FRONT WILL
CONTINUE TO BE A DETRIMENT TO WIDESPREAD RUNOFF ISSUES. HOWEVER,
THE SOUTHWESTERN PORTION OF THIS FRONT WILL SLOW SOMEWHAT COMPARED
TO THE FARTHER NORTHERN PORTION IN THE VICINITY OF EASTERN NE
/NORTHEAST KANSAS, EAST INTO IOWA. ACROSS THESE REGIONS THERE IS A
BETTER CHANCE OF A PERIOD OF TRAINING OF CELLS LATE TUESDAY/EARLY
HOURS OF WEDNESDAY. THIS IS SHOWN IN THE LATEST GUIDANCE THAT HAS
CONSENSUS FOR A HEAVIER AXIS OF QPF HERE. A SMALL SLIGHT RISK AREA
WAS ADDED ACROSS PORTIONS OF CENTRAL TO SOUTHERN IOWA FOR THIS
TRAINING POTENTIAL. RRFS NEIGHBORHOOD PROBABILITIES ARE FAIRLY HIGH
FOR 1 AND 2"+ AMOUNTS ACROSS THE SLIGHT RISK AREA, WITH A SMALLER
50%+ PROBABILITY FOR 3"+ AMOUNTS OVER EAST CENTRAL IOWA.
MARGINAL RISK AREAS
.EASTERN ARIZONA INTO WESTERN TO CENTRAL NEW MEXICO
NO SIGNIFICANT CHANGES DAY 2 TO THE AXIS OF ABOVE AVERAGE PW
VALUES EXTENDING FROM EASTERN PORTIONS OF ARIZONA INTO WESTERN TO
CENTRAL NEW MEXICO. CONTINUED LOW CONFIDENCE WITH QPF DETAILS, BUT
POTENTIAL FOR ANOTHER ROUND OF WIDESPREAD SCATTERED CONVECTION
ACROSS THIS AREA. LOCALLY HEAVY RAIN AND ISOLATED RUNOFF ISSUES
AGAIN POSSIBLE. ANOMALOUS PW VALUES ALSO EXPECTED TO PUSH FARTHER
NORTH FROM SOUTHERN CA INTO CENTRAL CA. AT THE MOMENT, MODELS ARE
NOT SHOWING PRECIP DEVELOPING DAY 2 IN THIS ANOMALOUS AXIS.
THE PREVIOUS MARGINAL RISK AREA OVER NORTHEAST MONTANA WAS REMOVED
GIVEN LATEST MODEL QPF CONSENSUS. THE BROAD MARGINAL WAS ALSO
REMOVED ALONG THE GULF COAST INTO FL GIVEN LESS ORGANIZED PRECIP
THAN DAY 1
ORAVEC
DAY 3
VALID 12Z WED SEP 09 2026 - 12Z THU SEP 10 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SIERRA
INTO SOUTHEAST CALIFORNIA, THE SOUTHWEST, THE MID MISSISSIPPI
VALLEY INTO THE OHIO VALLEY AND NORTHERN NEW YORK STATE INTO
NORTHERN VERMONT...
2030Z UPDATE...
THE MARGINALS IN THE SOUTHWEST AND NORTHEAST CONTAINED MINOR
ADJUSTMENTS TO ACCOUNT FOR LATEST TRENDS IN GUIDANCE AND CHANGES
IN WPC QPF. NO UPGRADES WERE MADE IN THOSE REGIONS. WHILE NO SLIGHT
RISK WAS ISSUED THIS CYCLE, THERE WAS A NOTABLE BUMP IN QPF OVER
PARTS OF THE MID MISSISSIPPI AND OHIO VALLEYS THAT LED TO SOME
CONSIDERATION FOR A SLIGHT RISK, PARTICULARLY IN SOUTHERN MO.
ASSESSING MODEL GUIDANCE SHOWED THE BOTH QPF TOTALS AND THE
LOCATION OF QPF MAXIMA REMAINED UNCLEAR, AND SOILS IN SOUTHERN MO
COULD USE THE RAIN GIVEN WORSENING DROUGHT CONDITIONS. WE WILL
CONTINUE TO MONITOR MODEL TRENDS CLOSELY AS THE INGREDIENTS FOR
SCATTERED INSTANCES OF FLASH FLOODING (ANOMALOUS PWS, AMPLE
INSTABILITY, POTENTIAL TRAINING) WILL BE AVAILABLE TO DEVELOPING
THUNDERSTORMS ON THURSDAY.
MULLINAX
---PREVIOUS DISCUSSION---
..SIERRA, SOUTHEAST CALIFORNIA, SOUTHWEST
A VERY BROAD AREA OF ANOMALOUS PW VALUES, 2 TO 3+ STANDARD
DEVIATIONS ABOVE THE MEAN, WILL STRETCH DAY 3 FROM CENTRAL TO
SOUTHERN CALIFORNIA, ACROSS THE SOUTHERN GREAT BASIN AND INTO THE
SOUTHWEST. LATEST MODELS DO SHOW SCATTERED PRECIP IN THIS HIGH PW
AXIS FROM THE SIERRA INTO SOUTHEAST CALIFORNIA AND OVER THE
SOUTHWEST. THE HEAVIEST TOTALS CONTINUE ACROSS SIMILAR AREAS DAY 1
AND 2 FROM EASTERN ARIZONA INTO WESTERN TO CENTRAL NM WHERE A
MARGINAL RISK CONTINUES DAY 3. THE MARGINAL RISK WAS EXPANDED
NORTHWARD FROM DAY 2 INTO THE SIERRA TO CAPTURE ANY LOCALLY HEAVY
PRECIP AMOUNTS IN THE MUCH ABOVE AVERAGE PW AXIS.
..LOWER MISSOURI, MID MISSISSIPPI, OHIO VALLEY REGION
A SURFACE COLD FRONT EXPECTED TO SINK SOUTHWARD THROUGH THE MID
MS, LOWER MO AND INTO THE OH VALLEY REGION DAY 3. PW VALUES
EXPECTED TO REMAIN ANOMALOUS ALONG THIS FRONT WITH VALUES 2"+,
SUPPORTING POTENTIAL FOR WIDESPREAD MODERATE TO LOCALLY HEAVY
TOTALS. THE FRONT REMAINS VERY PROGRESSIVE TO THE SOUTH, WHICH
ALONG WITH RELATIVELY HIGH FFG VALUES, WILL LIMIT WIDESPREAD RUNOFF
POTENTIAL.
..NORTHERN NY STATE INTO NORTHERN VERMONT
THE NORTHEASTERN PORTION OF THE FRONT MOVING SOUTH INTO THE MID MS
VALLEY/OH VALLEY REGION WILL ALSO REMAIN VERY PROGRESSIVE TO THE
EAST ACROSS THE EASTERN LAKES, NORTHERN NY STATE AND NORTHERN NEW
ENGLAND. PW VALUES EXPECTED TO BE ANOMALOUS ALONG THIS PORTION OF
THE FRONT ALSO, WITH VALUES 1.75-2.00"+. NO CHANGES TO THE
MARGINAL RISK AREA FOR FAR NORTHERN NY STATE INTO NORTHERN VT WHERE
MODEL CONSENSUS IS FOR MODERATE TO HEAVY PRECIP TOTALS. THE
EXPECTED VERY PROGRESSIVE NATURE OF THE FRONT WILL BE A DETRIMENT
TO WIDESPREAD RUNOFF ISSUES, BUT THE ANOMALOUS PW VALUES AND
POTENTIAL FOR A BRIEF PERIOD OF TRAINING MAY PRODUCE ISOLATED
ISSUES.
ORAVEC
DAY 4 AND DAY 5
VALID 12Z THU SEP 10 2026 - 12Z SAT SEP 12 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL APPALACHIANS AND UPPER OHIO VALLEY ON THURSDAY...
..CENTRAL APPALACHIANS AND UPPER OHIO VALLEY
A FAST-MOVING COLD FRONT TRACKING EAST ACROSS THE MIDWEST WILL
PROVIDE THE FORCING, COINCIDENT WITH A STRONG UPPER LEVEL SHORTWAVE
TROUGH, FOR NUMEROUS SHOWERS AND THUNDERSTORMS TO IMPACT THE
CENTRAL APPALACHIANS AND UPPER OHIO VALLEY THURSDAY AFTERNOON AND
EVENING. AHEAD OF THIS FRONT, DEEP TROPICAL MOISTURE FROM OFF THE
GULF WILL ADVECT NORTHEASTWARD INTO THE REGION. PWATS ARE EXPECTED
TO EXCEED 1.75 INCHES, PERHAPS EXCEEDING 2 INCHES IN SPOTS, AS THE
THUNDERSTORMS RUMBLE THROUGH. THE FRONT ITSELF WILL BE ALIGNED
PARALLEL TO THE MOUNTAINS, MOVING SOUTHEASTWARD/PERPENDICULAR TO
THE MOUNTAINS. THUS, IDEAL UPLIFT WILL BE IN PLACE AS THE FRONT
HITS THE MOUNTAINS ALL AT ROUGHLY THE SAME TIME. THE NORTHWESTERLY
FLOW BEHIND THE FRONT RUNNING STRAIGHT INTO THE MOUNTAINS WILL
LOCALLY ENHANCE THE UPLIFT, AND THUS STRENGTHEN THE NUMEROUS STORMS
EXPECTED THURSDAY AFTERNOON. SOIL MOISTURE LEVELS ARE ABOUT NORMAL
FOR THIS TIME OF YEAR AND FFG VALUES ARE ONLY AROUND 1"/HR
(CURRENTLY). FORTUNATELY, NO SIGNIFICANT RAINFALL IS EXPECTED IN
THIS AREA UNTIL THURSDAY, WHICH SHOULD HELP MITIGATE SOME OF THE
RESULTANT FLASH FLOODING AS THE STORMS MOVE THROUGH. HOWEVER, WITH
IDEAL UPLIFT, THERE WILL LIKELY BE SOME TRAINING AND BACKBUILDING
AS CONTINUED UPLIFT THROUGH THE AFTERNOON AND EVENING ALLOWS REPEAT
THUNDERSTORMS TO DEVELOP ON THE WINDWARD SIDES OF THE MOUNTAINS,
THEN CONTINUE ADVECTING SOUTHEASTWARD.
GIVEN THE ABUNDANT MOISTURE AND FORCING PRESENT, AS WELL AS THE
FLASH-FLOOD-PRONE NATURE OF MANY OF THE MOUNTAIN AND RIVER VALLEYS
OF THE CENTRAL APPALACHIANS, AS WELL AS ANY URBAN CONCERNS FROM
PITTSBURGH SOUTH THROUGH CHARLESTON, THE SLIGHT RISK AREA WAS
MAINTAINED WITH THIS UPDATE. MODEL GUIDANCE IS IN GOOD AGREEMENT
ON THE POSITION OF THE FRONT DURING THIS PERIOD, WITH THE STORMS
MOVING THROUGH AT PEAK HEATING...MAXIMIZING THEIR RAINFALL OUTPUT.
..MARGINAL RISKS
ACROSS THE SOUTHEAST AND FLORIDA PENINSULA, ROUNDS OF AFTERNOON
SHOWERS AND THUNDERSTORMS ARE EXPECTED BOTH THURSDAY AND FRIDAY AS
MOISTURE LEVELS REMAIN ELEVATED AND WINDS ALOFT ARE LIGHT. TYPICAL
HEAVY RAINS ARE EXPECTED ALONG THE GULF COAST. WHILE ANY ONE STORM
IS UNLIKELY TO CAUSE FLASH FLOODING, THE WIDESPREAD COVERAGE OF
STORMS AND THE DECENTLY HIGH LIKELIHOOD THAT STORMS WILL IMPACT
MORE FLOOD- SENSITIVE URBAN AREAS LED TO THE CONTINUATION OF THE
MARGINAL RISK FOR THESE AREAS.
THE MONSOON IN THE SOUTHWEST WILL BE ONGOING BOTH THURSDAY AND
FRIDAY, BUT ITS COVERAGE AND INTENSITY SHOULD BE SUBDUED COMPARED
TO RECENT DAYS. THUS, THE MARGINAL RISKS ARE GENERALLY CONFINED TO
ARIZONA AND NEW MEXICO.
A STRONG SHORTWAVE WILL TRACK ALONG THE MONTANA/CANADA BORDER
THURSDAY NIGHT INTO FRIDAY, BUT MOST OF THE GUIDANCE INDICATES THAT
THE HEAVIEST RAIN WILL REMAIN IN CANADA. IN ADDITION, LITTLE TO NO
INSTABILITY SHOULD KEEP RATES IN CHECK DESPITE FFG VALUES
0.75-1.25"/HR. REMOVED THE MARGINAL RISK FOR D5 WITH THIS UPDATE.
FRACASSO/WEGMAN
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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