774
FOUS30 KWBC 301554
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1154 AM EDT SAT MAY 30 2026
DAY 1
VALID 16Z SAT MAY 30 2026 - 12Z SUN MAY 31 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
MONTANA...
16Z UPDATE...
..TENNESSEE VALLEY
CURRENT RADAR AND RAP MESOANALYSIS SHOW A WEAK 925 MB CIRCULATION OVER
CENTRAL TN THAT IS CAUGHT BETWEEN THE FRONT TO THE NORTH AND THE LOW-
LEVEL WSWRLY OVER MS/AL. THE 12Z BMX SOUNDING SHOWS THE ATMOSPHERE
REMAINS WEAKLY CAPPED WITH A 1.9" PW, AND >500 MLCAPE, RH AT LOW
AND MID LEVELS >90%, AND A WARM CLOUD LAYER APPROACHING 12,000FT
IN DEPTH. A NEARBY OUTFLOW BOUNDARY IS ALSO MOVING SOUTH INTO
SOUTHERN TN THIS MORNING AND IS LIKELY TO TRIGGER ADDITIONAL HEAVY
THUNDERSTORMS THIS AFTERNOON. THERE IS NOT MUCH IN THE WAY OF
VERTICAL WIND SHEAR, SO STORMS WILL BE FAIRLY SHORT LIVED. BUT UP
TO 2.5"/HR RAINFALL RATES THIS AFTERNOON AND EVENING ATOP SOILS
THAT REMAIN QUITE SATURATED LED TO AN EXTENSION OF THE MARGINAL
RISK IN THE SOUTHEAST AS FAR NORTH AND WEST AS SOUTH-CENTRAL TN.
OTHERWISE, SOME MINOR TWEAKS WERE MADE TO THE MARGINAL IN THE
SOUTHEAST AND NORTHERN AND CENTRAL PLAINS, AS WELL AS THE SLIGHT
RISK IN MT BASED ON LATEST 12Z HREF AND 12Z CAMS GUIDANCE. THE
THREAT AREAS AND RATIONALE PROVIDED BY NIGHT SHIFT REMAIN IN GOOD
SHAPE AS OF THIS UPDATE.
MULLINAX
---PREVIOUS DISCUSSION---
..NORTHERN ROCKIES TO THE PLAINS
MAINTAINED THE SLIGHT RISK ACROSS PORTIONS OF MONTANA AS A MID-
AND UPPER-LEVEL SYSTEM MAKES ITS WAY NORTHWARD SLOWLY TODAY. THE
SYSTEM SHOULD FOSTER LOCALLY INTENSE RAINFALL RATES...ESPECIALLY IN
THE EASTERN PORTION OF THE SLIGHT RISK AREA...GIVEN MODERATELY
STEEP LAPSE RATES AND SOME POTENTIAL FOR ENOUGH BREAKS IN CLOUD
COVER. THE 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 PERCENT. THE RRFS SHOWS A
SIMILAR EVOLUTION IN ROUGHLY THE SAME AREA BUT WAS NOT QUITE AS
HIGH WITH ITS PROBABILITY OF EXCEEDANCE OR WITH THE PROBABILITY OF
1 INCH PER HOUR RAINFALL. FARTHER WEST;..THE INSTABILITY IS
EXPECTED TO BE WEAKER BUT THE GREATER COVERAGE OF RAINFALL LOOKS
TO OFFSET THE INSTABILITY SHORTFALL. AS MENTIONED PREVIOUSLY...THE
FLOOD RISK SHOULD GRADUALLY TRANSITION FROM FLASH FLOOD TO AREAL
FLOOD/RIVER FLOOD AS YOU GO WEST ACROSS THE STATE.
A MARGINAL RISK AREA EXTENDING EASTWARD FROM MONTANA AND THEN
SOUTHWARD FROM THE NORTHERN PLAINS AND MID-MISSISSIPPI VALLEY WAS
LARGELY LEFT IN PLACE GIVEN THE SIGNAL IN THE HREF FOR LOCALLY
HEAVY RAINFALL RATES WITHIN AN ELONGATED MOISTURE PLUME ORIGINATING
FROM THE SOUTHEAST US. IT WAS DONE SO DESPITE CONVECTIVE DETAILS
REMAINING UNCERTAIN. THE MOST ORGANIZED CONVECTIVE THREAT MAY END
UP OVER PORTIONS OF NE AND SD GIVEN A HIGHLY DIFLUENT FLOW ALOFT
AND LOW LEVEL CONVERGENT FLOW. HOWEVER, THERE ARE ALSO SIGNALS OF
ORGANIZED CONVECTIVE CLUSTERS WITHIN THE MOISTURE AXIS ANYWHERE
FROM ARKANSAS TO IOWA, WHILE CONVECTION IS UNLIKELY ALONG THIS
ENTIRE AXIS...WHERE IT DOES DEVELOP RAINFALL RATE POTENTIAL WILL BE
HIGH ENOUGH FOR LOCALIZED FLASH FLOODING.
..SOUTHEAST
LATEST NUMERICAL GUIDANCE CONTINUED TO SHOW LOCALLY HEAVY RAINFALL
TODAY AND TONIGHT ALONG AND SOUTH OF A BOUNDARY MOVING IN FROM THE
NORTH. THERE WAS A BIT OF A WESTWARD EXPANSION IN THE WESTERN
EXTENT SHOWN BY THE HREF AND RRFS OF THE 1-INCH PER HOUR
PROBABILITIES AND THE PROBABILITY OF EXCEEDANCE. AS A RESULT...THE
MARGINAL RISK WAS EXPANDED WESTWARD IN PARTS OF GEORGIA TO ALABAMA.
BANN
DAY 2
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...
ANY HEAVY TO EXCESSIVE RAINFALL SHOULD BE LOCATED IN OR NEAR A
PLUME OF ANOMALOUS MOISTURE PLUME THAT WILL EXTEND FROM MONTANA
INTO THE SOUTHEAST UNITED STATE. LARGE SCALE FORCING IS MORE
SUBTLE OUTSIDE OF THE NORTHERN PLAINS/ROCKIES...BUT THERE SHOULD BE
SOME WEAK SHORTWAVE ENERGY EMBEDDED WITHIN THE WEAK FLOW THAT WILL
MAKE THEM DIFFICULT OR IMPOSSIBLE TO TIME. AS THIS SHORTWAVE ENERGY
ENCOUNTERS A LATE DAY ATMOSPHERE WITH MUCAPE VALUES IN EXCESS OF
3000 J PER KG FROM THE PLAINS INTO THE MISSISSIPPI VALLEY AND PARTS
OF THE SOUTHEAST US...STORMS HAVE THE POTENTIAL TO PRODUCE VERY
INTENSE RAINFALL RATES THAT APPROACH OR EXCEED FLASH FLOOD
GUIDANCE ALONG THIS ENTIRE CORRIDOR. A LARGE AND GENERALLY
UNFOCUSED MARGINAL RISK AREA STILL SEEMS TO HANDLE THE LOCALIZED
NATURE OF FLASH FLOOD RISK THAT SHOULD EXIST.
THE MONTANA PORTION OF THE MARGINAL RISK HAS THE STRONGEST SYNOPTIC
FORCING AS A MID- AND UPPER-LEVEL SYSTEM GRADUALLY MOVES NORTH OF
THE BORDER...KEEPING THE WESTERN PART OF THE STATE IN LINGERING
WRAP-AROUND RAINFALL BEFORE GRADUALLY TAPERING OFF.
BANN
DAY 3
VALID 12Z MON JUN 01 2026 - 12Z TUE JUN 02 2026
THE PROBABILITY OF RAINFALL EXCEEDING FLASH FLOOD GUIDANCE IS LESS
THAN 5 PERCENT.
THERE ARE SOME HINTS IN THE GUIDANCE THAT THE OVERALL RISK OF
EXCESSIVE RAINFALL IS NON-ZERO OVER THE PLAINS FROM KANSAS INTO
COLORADO AS MODELS SHOW A LOW LEVEL AXIS OF MOISTURE AND
INSTABILITY MOVING NORTHWARD ON MONDAY AFTERNOON. THE ENSEMBLE AND
DETERMINISTIC QPF IS FAIRLY LOW...PERHAPS DUE TO A COMPONENT OF LOW
LEVEL FLOW OFF THE MOISTURE. IN ADDITION...LINGERING DEEP MOISTURE
MAY FUEL SOME LATE DAY THUNDERSTORMS ON MONDAY THAT ARE CAPABLE OF
LOCALLY HEAVY RAINFALL BUT THE MODEL SPREAD LIMITS CONFIDENCE.
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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