535
FOUS30 KWBC 292008
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
408 PM EDT FRI MAY 29 2026
DAY 1
VALID 16Z FRI MAY 29 2026 - 12Z SAT MAY 30 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST AND TENNESSEE VALLEY...
16Z UPDATE: MAIN CHANGE IS AN EXPANSION OF THE SLIGHT RISK ACROSS
TN, FAR SOUTHERN KY, NORTHERN AR AND SOUTHERN MO. OVERALL WOULD
GENERALLY CONSIDER THE FLASH FLOOD RISK ISOLATED, BUT A FEW
CLUSTERS OF MORE CONCENTRATED IMPACTS APPEAR PROBABLE. FFG
EXCEEDANCE PROBABILITIES FROM BOTH THE 12Z HREF AND REFS SHOW A
BROAD CORRIDOR OF 15-30% VALUES APPROXIMATELY ALIGNED WITH THE
ADJUSTED SLIGHT RISK AREA, WITH 3" NEIGHBORHOOD EXCEEDANCE
PROBABILITIES OF ~40-70%. SO WHILE COVERAGE OF THESE HIGHER AMOUNTS
SHOULD STAY LOCALIZED, WEAK FLOW AND BROAD LOWER LEVEL CONVERGENCE
WILL SUPPORT A FEW CONVECTIVE CLUSTERS CAPABLE OF 2" PER HOUR
RAINFALL AND TOTALS EXCEEDING 3".
MADE SOME TWEAKS TO CONFINE THE MARGINAL RISK OVER NEW ENGLAND TO
JUST NH INTO WESTERN MAINE. A WIDESPREAD 1-2" OF RAIN IS EXPECTED
AS A STRONG MID/UPPER LOW DIVES SOUTHEAST INTO THE REGION, WITH
BOTH THE HREF AND REFS SUPPORTING LOCALIZED TOTALS OVER 3". HOURLY
RAINFALL RATES WILL GENERALLY STAY BELOW 1", HOWEVER COULD LOCALLY
EXCEED 1" AS THE LOW DROPS SOUTH TONIGHT AND WE GET SOME ENHANCED
CONVERGENCE CLOSER TO THE NH AND ME COASTS. A FEW SPOTS COULD SEE
EVENT TOTAL RAINFALL AROUND OR JUST ABOVE 3", WHICH COMBINED WITH
1" PER HOUR RATES, COULD BE ENOUGH TO RESULT IN LOCALIZED FLASH
FLOOD IMPACTS.
A SMALL MARGINAL RISK WAS ADDED FOR DRYLINE CONVECTION ACROSS WEST
TX. STORM MOTIONS SHOULD BE QUICK ENOUGH TO LIMIT THE RISK,
HOWEVER, A FEW SUPERCELLS COULD BRIEFLY ANCHOR TO THE DRYLINE AND
LOCALLY ENHANCE RAINFALL TOTALS.
CHENARD
..PREVIOUS DISCUSSION
LARGELY MAINTAINED THE PREVIOUS SLIGHT RISK AREA THAT WAS INTRODUCED
IN THE SOUTHEAST US. A SHEARED 500MB VORTICITY MAXIMUM APPROACHING
FROM THE WEST WILL BE ENCOUNTERING A MOIST AIRMASS WITH INCREASING
AMOUNT OF INSTABILITY LATER DURING THE DAY AND EVENING. THE LATEST
NUMERICAL GUIDANCE KEPT EXPECTED PRECIPITABLE WATER VALUES IN THE
2 TO 2.25 INCH RANGE ALONG AND SOUTH OF A QUASI-STATIONARY FRONT
DRAPED ACROSS THE REGION FROM THE TN VALLEY TO THE SC COAST.
INSTABILITY WILL BE HIGHEST OVER AL ON EAST INTO CENTRAL AND
SOUTHERN GA WITH MUCAPE BETWEEN 1,000-2,000 J/KG MOST OFTEN. MUCAPE
UP TO 1,000 J/KG WILL BE AVAILABLE LATER TONIGHT FROM THE SAVANNAH
RIVER TO COASTAL SC. THIS COMBINATION SHOULD BE ENOUGH TO SUPPORT
AND SUSTAIN LOCALIZED RAINFALL RATES IN EXCESS OF 2 INCHES PER HOUR
WHERE WARM RAIN PROCESSES PREDOMINATE FROM EASTERN AL EASTWARD TO
THE SAVANNAH RIVER THIS AFTERNOON AND TONIGHT.
..MIDDLE MISSISSIPPI VALLEY TO THE SOUTHEAST
THE WESTERN PORTION OF THE FRONT AFFECTING THE SOUTHEAST US WILL
EXTEND INTO PORTIONS OF THE MIDDLE MISSISSIPPI VALLEY AND PROVIDE
THE FOCUS FOR SHOWERS AND THUNDERSTORMS...SOME OF WHICH MAY PRODUCE
LOCALLY HEAVY RAINFALL FROM THE TENNESSEE VALLEY INTO THE MIDDLE
MISSISSIPPI VALLEY INTO TONIGHT.
..EASTERN WASHINGTON TO NORTHERN ROCKIES
A MARGINAL RISK REMAINED IN PLACE ACROSS FAR NORTHEAST WASHINGTON
TO NORTHWEST MONTANA DUE TO AN INCREASE IN THE HEAVY RAINFALL
SIGNAL FOR THIS PERIOD AS AN UPPER LOW OVER THE WEST ON DAY 1
PLACES THIS REGION IN AN AREA WITH BETTER UPPER FORCING AND
ADVECTION OF MOISTURE. THE PRESENCE OF ANOMALOUS MOISTURE AND SOME
INSTABILITY MAY PRESENT A THREAT FOR LOCALIZED HEAVY RAINFALL AND
FLASH FLOODING, ESPECIALLY FOR BURN SCAR LOCATIONS WITHIN THIS
REGION.
BANN
DAY 2
VALID 12Z SAT MAY 30 2026 - 12Z SUN MAY 31 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
MONTANA...
20Z UPDATE: UPGRADED TO A SLIGHT RISK ACROSS PORTIONS OF MT WHERE
ISOLATED TO SCATTERED FLASH FLOODING IS POSSIBLE. THE EASTERN HALF
OF THE SLIGHT RISK AREA SHOULD SEE THE HIGHEST RAINFALL RATES,
WITH HREF NEIGHBORHOOD PROBABILITIES OF EXCEEDING 1"/HR IN THE
30-50% RANGE, AND 2"/HR AROUND 10%. THIS RESULTS IN HREF FFG
EXCEEDANCE PROBABILITIES BROADLY OVER 25%. THERE SHOULD BE ENOUGH
COVERAGE OF HIGHER RAINFALL RATES TO RESULT IN SOME FLASH FLOOD
RISK. INSTABILITY IS LESS THE FARTHER WEST YOU GO IN MT, BUT
RAINFALL COVERAGE WILL BE GREATER, RESULTING IN HIGHER AREAL
AVERAGED RAINFALL. THE FLOOD RISK SHOULD GRADUALLY TRANSITION FROM
FLASH FLOOD TO AREAL FLOOD/RIVER FLOOD AS YOU GO WEST ACROSS THE
STATE.
ELSEWHERE, THE MARGINAL RISK WAS BROADENED ACROSS THE NORTHERN
PLAINS INTO THE MS VALLEY ALONG AN ELONGATED MOISTURE AXIS.
CONVECTIVE DETAILS ARE UNCERTAIN AT THIS LEAD TIME, BUT THAT
ENTIRE CORRIDOR WILL HAVE ABOVE AVERAGE MOISTURE AND SOME
FORCING.THE MOST ORGANIZED CONVECTIVE THREAT MAY END UP OVER
PORTIONS OF NE AND SD SATURDAY NIGHT AS STRONGER FORCING EJECTS
INTO THE AREA. HOWEVER, THERE ARE ALSO SIGNALS OF ORGANIZED
CONVECTIVE CLUSTERS WITHIN THE MOISTURE AXIS ANYWHERE FROM AR TO
IA. WHILE CONVECTION IS UNLIKELY ALONG THIS ENTIRE AXIS...WHERE IT
DOES DEVELOP RAINFALL RATE POTENTIAL WILL BE HIGH ENOUGH FOR
LOCALIZED FLASH FLOODING.
CHENARD
..PREVIOUS DISCUSSION
..SOUTHEAST
LATEST NUMERICAL GUIDANCE HAS SHIFTED A MAJORITY OF THE RAINFALL
EASTWARD ENOUGH COMPARED WITH PREVIOUS RUNS TO PRECLUDE AN UPGRADE
TO A SLIGHT RISK. IN ADDITION...THE SPREAD OF MODEL
GUIDANCE BOTH IN TERMS OF PLACEMENT AND AMOUNTS PRECLUDED AN
UPGRADE. AS MENTIONED PREVIOUSLY...SOME OF THE REGION, ESPECIALLY
WESTERN PARTS OF SOUTH CAROLINA AND INTO THE APPALACHIANS, HAS
BEEN VERY WET AS OF LATE AND IS WELL PRIMED, SO MAY BY MORE
VULNERABLE TO FLOODING CONCERNS.
..NORTHERN ROCKIES TO HIGH PLAINS
THE UPPER LEVEL LOW OVER THE WEST DURING THE DAY 1 PERIOD WILL
WEAKEN AND SHIFT NORTHEASTWARD, MOVING OVER THE NORTHERN ROCKIES TO
HIGH PLAINS REGION ON SATURDAY. MOISTURE ANOMALIES WILL CONTINUE
TO BE IN THE 90-90TH PERCENTILE, SUPPORTING MODERATE TO HEAVY
RAINFALL, WITH WIDESPREAD QPF GREATER THAN 0.5-1" LIKELY ACROSS
WESTERN AND CENTRAL MONTANA. THERE MAY NOT BE A LOT OF INSTABILITY
TO WORK WITH, SO ANY FLOOD OR FLASH FLOOD THREATS SHOULD BE MAINLY
LOCALIZED WITH PARTICULAR ATTENTION TO BURN SCAR LOCATIONS. A
FAIRLY BROAD MARGINAL RISK REMAINED IN PLACE ACROSS MUCH OF
WESTERN AND CENTRAL MONTANA, INTO PARTS OF NORTHERN WYOMING AS
WELL.
BANN
DAY 3
VALID 12Z SUN MAY 31 2026 - 12Z MON JUN 01 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL STRETCHING FROM
MONTANA TO NORTHERN FLORIDA...
20Z UPDATE: EXPANDED THE MARGINAL RISK TO COVER MOST OF THE
ANOMALOUS MOISTURE PLUME THAT WILL EXTEND FROM MT INTO THE
SOUTHEAST U.S. IN BETWEEN TROUGHING OVER THE WEST AND NORTHEAST.
LARGE SCALE FORCING IS MORE SUBTLE OUTSIDE OF THE NORTHERN
PLAINS/ROCKIES...BUT THERE SHOULD BE SOME WEAK SHORTWAVES
EMBEDDED WITHIN THE WEAK FLOW. GLOBAL DETERMINISTIC MODELS,
ENSEMBLE MEANS AND AI MODELS SHOW A BROAD CONVECTIVE THREAT ALONG
THIS ENTIRE CORRIDOR, WITH NO CLEAR AREA OF HIGHER FOCUS. THUS AT
THE MOMENT A LARGE MARGINAL SEEMS TO HANDLE THE LOCALIZED FLASH
FLOOD RISK THAT SHOULD EXIST. SOME FINE TUNING OF THE MARGINAL
AND/OR FOCUSED SLIGHT RISK UPGRADES WILL BE POSSIBLE AS WE GET
CLOSER AND CONVECTIVE MODE/EVOLUTION BECOMES CLEARER.
CHENARD
..PREVIOUS DISCUSSION
..NORTHERN ROCKIES AND ADJACENT HIGH PLAINS
THE UPPER LEVEL LOW WHICH HAS BEEN SUPPORTING THE PROLONGED WET
PERIOD OVER THE NORTHERN ROCKIES WILL CONTINUE TO MOVE NORTHWARD
ACROSS MONTANA SLOWLY ON SUNDAY. THIS RESULTS IN SURFACE LOW
PRESSURE AND ASSOCIATED SURFACE FRONT TO MAKES ITS WAY OUT OVER THE
PLAINS AND GRADUALLY WEAKEN WHILE THE MID AND UPPER LEVEL
CIRCULATION KEEPS PRECIPITATION LINGERING OVER MONTANA AND FAR
NORTHERN WYOMING. MOISTURE REMAINS ANOMALOUSLY HIGH BUT INSTABILITY
STILL LOOKS TO BE DEPENDENT UPON CLOUD COVER. AS IN THE DAY 2
PERIOD...ANY FLOOD OR FLASH FLOOD THREATS SHOULD BE MAINLY
LOCALIZED WITH PARTICULAR ATTENTION TO BURN SCAR LOCATIONS.
..SOUTHEAST
UNTIL SURFACE HIGH PRESSURE BUILDS FAR ENOUGH SOUTHWARD INTO THE
SOUTHEAST STATES TO DECREASE COVERAGE OF LINGERING CONVECTION ON
SUNDAY EVENING...THE ANTECEDENT CONDITIONS KEEP A RISK OF FLOODING
CONCERNS OVER THE AREA.
BANN
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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