810
FOUS30 KWBC 241607
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1207 PM EDT SUN SEP 24 2023
DAY 1
VALID 16Z SUN SEP 24 2023 - 12Z MON SEP 25 2023
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHEAST AND ARKLATEX...
16Z UPDATE:
THE INHERITED RISK AREAS WERE GENERALLY IN GOOD SHAPE, SO ONLY
MADE SOME MINOR ADJUSTMENTS BASED ON THE LATEST
OBSERVATIONAL/MODEL TRENDS.
OVER THE NORTHEAST THE 12Z HREF GUIDANCE CONTINUES TO SUPPORT
AREAS OF HEAVY RAINFALL TODAY INTO TONIGHT. NEIGHBORHOOD
PROBABILITIES OF EXCEEDING 3" ARE NOW OVER 70%, WITH EVEN SOME LOW
END 5" PROBABILITIES. INSTABILITY REMAINS A LIMITING FACTOR FOR
DEEP CONVECTION AND INTENSE RAINFALL RATES. HOWEVER STRONG LOW
LEVEL CONVERGENCE AND VERY WEAK INSTABILITY SHOULD HELP DRIVE SOME
SHALLOW CONVECTIVE ELEMENTS EMBEDDED WITHIN THE STRATIFORM RAIN
SHIELD. THE SHALLOW NATURE OF ACTIVITY WILL MEAN ITS MAINLY WARM
CLOUD PROCESSES IN PLAY, AND THUS EFFICIENT RAINFALL PRODUCTION IS
PROBABLE. SO HOURLY RAINFALL AROUND 0.5"/HR ON A LOCALIZED BASIS
SEEMS REASONABLE (AND IS SUPPORTED BY THE 12Z HREF)...BUT ANYTHING
1"/HR OR HIGHER SEEMS MUCH LESS LIKELY. OVERALL, GENERALLY
THINKING MOST AREAS WILL SEE BETWEEN 1 AND 3 INCHES OF RAIN...BUT
LOCALIZED AREAS OF 3-5" SEEM PROBABLE. WHERE THESE HIGHER TOTALS
FALL SOME FLASH FLOOD/FLOOD RISK SEEMS POSSIBLE TODAY INTO
TONIGHT, MAINLY FROM NORTHEAST PA INTO NORTHERN NJ, SOUTHEAST NY
AND PORTIONS OF CT...ESPECIALLY IF THERE IS OVERLAP WITH ANY MORE
SENSITIVE URBAN AREAS.
OVER THE SOUTHERN PLAINS INTO THE LOWER MS VALLEY THE SLIGHT RISK
STILL LOOK GOOD, WITH ANOTHER ROUND OF CONVECTION DEVELOPING LATER
TODAY INTO TONIGHT. THERE MAY BE SOME OVERLAP WITH AREAS THAT SAW
HEAVY RAINFALL OVER THE PAST 24 HOURS...BUT THE HEAVIEST OF THE
ADDITIONAL RAINFALL SHOULD END UP SOUTHWEST OF THE HARDEST HIT
AREAS. CONVECTION LATER TODAY AND TONIGHT SHOULD TRAIN/BACKBUILD
TO SOME EXTENT...BUT SHOULD ALSO EXHIBIT A PROGRESSION OFF TO THE
SOUTH AND EAST AS WELL. THUS WHILE AT LEAST ISOLATED FLASH
FLOODING APPEARS LIKELY INTO TONIGHT...THINK THE
COVERAGE/ORGANIZATION OF THIS RISK REMAINS BELOW MDT RISK
THRESHOLDS. WE'LL CONTINUE TO MONITOR, BUT FOR NOW A SLIGHT TO
HIGHER END SLIGHT RISK SEEMS TO BEST PORTRAY THE THREAT.
CHENARD
..PREVIOUS DISCUSSION
..NORTHEAST
POST-T.C. OPHELIA'S REMNANT 850MB CIRCULATION WILL CONTINUE TO
GRADUALLY WEAKEN BUT REMAIN INTACT AS IT TRACKS OVER THE
DC/BALTIMORE METRO AREAS SUNDAY AFTERNOON AND SLOWLY TRACKS EAST
TOWARDS THE NORTHERN DELMARVA PENINSULA SUNDAY NIGHT. THERE
REMAINS NO SHORTAGE OF MOISTURE WITH PWS OF 1.75" LIKELY TO BE
MEASURED FROM NORTHERN MD AND THE LOWER SUSQUEHANNA VALLEY OF PA
ON EAST ACROSS THE DELAWARE VALLEY AND INTO SOUTHERN NJ. THESE
AREAS IN PARTICULAR RESIDE WITHIN A NARROW RIBBON OF EASTERLY
925-850MB THETA-E ADVECTION WITH MUCAPE UP TO 500 J/KG AND UPSLOPE
ENHANCEMENT INTO HIGHER TERRAIN (PARTICULARLY EASTERN PA AND
NORTHERN NJ). SAMPLED SOUNDINGS ALREADY SHOWED WARM CLOUD LAYERS
UP TO 13,000FT DEEP, BUT GIVEN THE >90% RH VALUES AND LINGERING
BUOYANCY FROM BOTH AVAILABLE INSTABILITY AND FORCED-TOPOGRAPHIC
ASCENT, HOURLY RAINFALL RATES COULD TOP OUT AS HIGH AS 0.75"/HR.
THE 850MB LOW WILL BE IN NO RUSH TO EXIT OFF THE COAST, SO A
PROLONGED PERIOD OF MODERATE-TO-HEAVY RAINFALL WILL OCCUR FOR MUCH
OF THE DAY. GIVEN THESE FACTORS, AS WELL AS SOILS BECOMING A
LITTLE MORE SATURATED FOLLOWING SATURDAY'S RAINFALL, HAVE
MAINTAINED THE SLIGHT RISK FROM THE MASON-DIXON LINE ON EAST TO
THE SOUTHERN COAST OF LONG ISLAND. THERE ARE STILL LIKELY TO BE
EFFICIENT WARM RAIN PROCESSES INTO SOUTHERN NEW ENGLAND, BUT GIVEN
THE LACK OF INSTABILITY NORTH OF THE COAST, HAVE TRIMMED DOWN THE
NORTHERN EXTENT OF THE SLIGHT WHILE MAINTAINING MOST OF THE
INHERITED MARGINAL RISK AREA.
..SOUTHERN PLAINS TO THE LOWER MISSISSIPPI VALLEY
A SIMILAR SETUP THAT RESULTED IN WIDESPREAD THUNDERSTORMS ON
SATURDAY WILL CONTINUE TO MAKE ITS WAY SOUTH INTO CENTRAL TX AND
AS FAR EAST AS THE LOWER MS VALLEY. LATEST GUIDANCE STILL SHOWS A
GOOD SIGNAL FOR EXCESSIVE RAINFALL RATES ACROSS THE ARKLATEX, BUT
MORE RECENT CAMS HAVE PUT THE AXIS A LITTLE FARTHER SOUTH INTO
EASTERN TEXAS. AN 850MB LOW BORDERING THE RED RIVER WILL DIRECT A
CONTINUOUS FETCH OF 850MB MOISTURE FLUX AT A STALLED FRONT
POSITIONED SOMEWHERE NEAR THE ARKLATEX, RESULTING IN PWS TOPPING
2" AND AS MUCH AS 2,000-3,000 J/KG OF MLCAPE. SIMILAR TO SATURDAY,
THE STEADY STREAM OF LOW LEVEL MOISTURE BECOMES ORIENTED
QUASI-PARALLEL TO THE STATIONARY FRONT, PROVIDING A SUITABLE SETUP
FOR POTENTIAL BACKBUILDING AND TRAINING STORMS. STORMS WILL BE
MORE THAN CAPABLE OF PRODUCING 2-3"/HR RAINFALL RATES, AND GIVEN
THE POTENTIAL TRAINING OF THUNDERSTORMS, LOCALIZED AMOUNTS COULD
REACH 4-6". THIS IS SUPPORTED BY THE 00Z HREF SHOWING 20-25%
PROBABILITIES OF 24-HR QPF > 5". DID INCREASE THE FOOTPRINT OF THE
SLIGHT RISK AREA TO INCLUDE MORE OF EASTERN TX AND NORTHERN LA
WHERE RECENT CAMS SHOWED THE POTENTIAL FOR STORMS TO INCH MORE TO
THE SOUTH. HAVE HELD OFF ON A MODERATE RISK UPGRADE GIVEN THE BULK
OF THE SLIGHT RISK AREA FEATURES HIGHER FFGS COMPARED TO THEIR
NEIGHBORS FARTHER NORTH ACROSS EASTERN OK AND SOUTHWEST MO. SHOULD
CONFIDENCE GROW IN A SWATH OF 5+" AMOUNTS IN NEW GUIDANCE OVER THE
NEXT 12 HOURS, THERE IS A CHANCE A MODERATE RISK MAY BE NEEDED FOR
PARTS OF THE ARKLATEX OR EAST TEXAS IN FUTURE FORECAST CYCLES.
..SOUTHWEST OREGON
THE LEADING EDGE OF ONE OF THE UPCOMING COLD SEASON'S FIRST
ATMOSPHERIC RIVERS ARRIVES SUNDAY AFTERNOON AND CONTINUES TO
DELIVER PERIODS OF RAIN TO THE PACIFIC NORTHWEST INTO SUNDAY
NIGHT. A NARROW SWATH OF HEAVY RAINFALL WILL BE DIRECTED AT THE
OREGON COAST SUNDAY NIGHT THANKS TO A ROBUST 1000 KG/M/S IVT THAT
PEAKS AT JUST OVER 8 STANDARD DEVIATIONS ABOVE NORMAL FOR LATE
SEPTEMBER. RAINFALL TOTALS COULD RANGE BETWEEN 1-2" WITH LOCALLY
HIGHER AMOUNTS CLOSER TO 3", BUT THE BULK OF THIS IS FALLING
WITHIN A 6-12 HOUR SPAN. DUE TO THESE POSSIBLE RAINFALL RATES,
HAVE KEPT THE MARGINAL RISK THAT WAS IN PLACE WITH ANY NEARBY BURN
SCARS MOST AT-RISK FOR POSSIBLE FLASH FLOODING.
MULLINAX
DAY 2
VALID 12Z MON SEP 25 2023 - 12Z TUE SEP 26 2023
...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE PACIFIC NORTHWEST, SOUTHERN PLAINS, LOWER AND UPPER
MISSISSIPPI VALLEY...
..PACIFIC NORTHWEST
A POWERFUL UPPER LOW APPROACHING THE PACIFIC NORTHWEST WILL DIRECT
AN ATMOSPHERIC RIVER AT THE WEST COAST ON MONDAY, DELIVERING A
SOAKING RAIN TO RESIDENTS FROM NORTHERN CALIFORNIA TO WESTERN
OREGON AND WASHINGTON. THE UPPER LOW OFF THE COAST IS VERY
IMPRESSIVE FOR LATE SEPTEMBER WITH HEIGHTS WITHIN THE 500-850MB
LAYER 3-5 STANDARD DEVIATIONS BELOW NORMAL ACCORDING TO NAEFS.
THIS POWERFUL UPPER TROUGH WILL DIRECT A ROBUST 500-750 KG/MS/S
IVT (PEAKING AROUND 8 STANDARD DEVIATIONS ABOVE NORMAL) AT
NORTHERN CA AND WESTERN OR WHERE PWS WILL BE AS HIGH AS 2-3
STANDARD DEVIATIONS ABOVE NORMAL. THIS IS EFFECTIVELY THE FIRST
BONAFIDE ATMOSPHERIC RIVER OF THE SEASON FOR THE PACIFIC
NORTHWEST. FORTUNATELY, PARTS OF FAR NORTHERN CALIFORNIA AND
SOUTHERN OREGON COULD USE THE RAIN AS PARTS OF THE REGION ARE IN
D0-D2 DROUGHT. THE PRIMARY CONCERNS ARE RESIDUAL BURN SCARS WHERE
SOILS WILL BE MORE SUSCEPTIBLE TO POSSIBLE DEBRIS FLOWS. THE
MARGINAL RISK REMAINS IN PLACE AS RATES DO NOT APPEAR TO POSE A
CONSIDERABLE FLOOD THREAT, BUT DETRIMENTAL IMPACTS COULD BE FELT
IN BURN SCARS.
..SOUTHERN PLAINS & LOWER MISSISSIPPI VALLEY
A FRONTAL BOUNDARY STRETCHING FROM WEST TEXAS TO THE OZARKS IS
FORECAST TO WEAKEN AND PUSH SOUTH THROUGHOUT THE DAY WHILE AT THE
SAME TIME, SOUTHERLY 850MB MOISTURE FLUX IS EXPECTED TO WAIN.
THESE FEATURES WILL STILL BE PRESENT AT A TIME WHEN A WEAK 500MB
DISTURBANCE APPROACHES FROM THE NORTH, PROVIDING SOME MODEST
VERTICAL ASCENT ATOP THE ATMOSPHERE. PWS ARE CURRENTLY FORECAST TO
REACH AS HIGH AS 1.75" (1-2 STANDARD DEVIATIONS ABOVE NORMAL
ACCORDING TO NAEFS) AND MLCAPE BETWEEN 1,000-2,000 J/KG FROM
CENTRAL TX TO THE LOWER MISSISSIPPI VALLEY. THE AVAILABLE PWS AND
INSTABILITY SHOULD SUPPORT >2"/HR RAINFALL RATES WITHIN THE MORE
INTENSE CELLS, AND SOILS SHOULD BE A LITTLE MORE SATURATED
FOLLOWING SUNDAY'S RAINFALL. THE MARGINAL RISK REMAINS IN PLACE
WITH ANY CONSIDERATION OF A SLIGHT RISK LIKELY TO BE DISCUSSED IN
FUTURE FORECAST CYCLES WHEN MORE CAM GUIDANCE BECOMES AVAILABLE TO
KEY IN ON THE MORE AT-RISK AREAS.
..UPPER MISSISSIPPI VALLEY
A MEANDERING CLOSED LOW OVER THE UPPER MIDWEST PROVIDES A SUITABLE
ENVIRONMENT FOR DIVERGENT FLOW ATOP THE ATMOSPHERIC COLUMN ON
MONDAY. PWS WILL BE AS HIGH AS 1.25" AND MLCAPE IS LIKELY TO RANGE
BETWEEN 250-500 J/KG. A CONVEYOR BELT OF 850MB MOISTURE FLUX AHEAD
OF AN OCCLUDED FRONT SHOULD HELP TO SUPPLY DEVELOPING STORMS WITH
ENOUGH MOISTURE AND LIFT TO GENERATE STORMS CONTAINING RAINFALL
RATES >1.5"/HR. THIS ROUND OF STORMS IS ALSO COMING ON THE HEELS
OF A 2-DAY STRETCH WHERE AS MUCH AS 2-3" OR RAINFALL IS FORECAST
THROUGH SUNDAY NIGHT, SO SOILS WILL BE A LITTLE MORE SATURATED
COMPARED TO EARLIER IN THE WEEK. A MARGINAL RISK REMAINS IN PLACE
GIVEN THAT FLASH FLOODING HAS A CHANCE TO OCCUR IN POOR DRAINAGE
AREAS, URBANIZED COMMUNITIES, AND WHERE SOILS ARE MOST SENSITIVE.
..NORTHEAST
WHILE IT STILL LOOKS TO BE A WET DAY ALONG THE SOUTHERN NEW
ENGLAND COAST, THERE WILL BE LITTLE IN THE WAY OF INSTABILITY TO
WORK WITH. HELD OFF ON INTRODUCING A MARGINAL RISK FOR THE TIME
BEING, BUT SHOULD QPF INCREASE AND THE POTENTIAL FOR HEAVIER
RAINFALL RATES ARISE, A MARGINAL RISK MAY BE NEEDED IN FUTURE
FORECAST CYCLES.
MULLINAX
DAY 3
VALID 12Z TUE SEP 26 2023 - 12Z WED SEP 27 2023
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
FLORIDA...
..FLORIDA
THERE IS A LARGE RESERVOIR OF 2-2.25" PWS ACROSS THE SUNSHINE
STATE WITH A STATIONARY FRONT DRAPED OVER THE REGION. AT UPPER
LEVELS, A LARGE TROUGH OVER THE GULF OF MEXICO IS PROVIDING SOME
SUFFICIENT DIVERGENCE ALOFT WHILE A LARGE DOME OF HIGH PRESSURE IN
THE CARIBBEAN WILL DIRECT A UPPER LEVEL DISTURBANCE TO APPROACH
FLORIDA FROM THE SOUTH. NAEFS SHOWS THESE PWS A LITTLE OVER 1
STANDARD DEVIATION ABOVE NORMAL AND MLCAPE VALUES COULD REACH UP
TO 2,000 J/KG. THESE PARAMETERS SHOULD BE ABLE TO SUPPORT HOURLY
RAINFALL RATES UP TO 3"/HR IN THE MOST INTENSE STORMS. THE OTHER
FACTOR WORTH NOTING IS VERTICAL WIND SHEAR, WINDS WITHING THE
1000-850MB LEVEL WILL BE SOUTHEASTERLY, BUT VEER AND STRENGTHEN
WITH HEIGHT UP TO 200MB, ALLOWING FOR SURFACE-6KM SHEAR VALUES TO
RANGE BETWEEN 20-30 KNOTS. THIS COULD HELP KEEP SOME STORMS AROUND
A LITTLE LONGER THAN THE USUAL SUMMER-TIME PULSE STORMS. GIVEN
THESE FACTORS, CHOSE TO INTRODUCE A MARGINAL RISK FOR FLORIDA.
THIS AREA WILL LIKELY BE TWEAKED DEPENDING UPON QPF TRENDS IN
GUIDANCE OVER THE NEXT COUPLE OF DAYS.
MULLINAX
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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