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FOUS30 KWBC 090020
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
820 PM EDT WED JUL 8 2026
DAY 1
VALID 01Z THU JUL 09 2026 - 12Z THU JUL 09 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS THROUGH THE UPPER GREAT LAKES...
..01Z UPDATE
CHANGES INCLUDED TRIMMING THE NORTHERN PORTION OF THE MARGINAL AND
SLIGHT RISK AREAS IN THE NORTHERN PLAINS AS CONVECTION CONTINUES TO
MAKE ITS WAY SOUTH AND OUT OF THE AREA. THE LATEST HREF/RREFS
POINTED TO GREATER THAN 30 PCT PROBABILITY OF RAINFALL EXCEEDING 2
INCHES PER HOUR DEVELOPING AND MAKING ITS WAY INTO NORTHWEST
MISSOURI SO MADE AN EXPANSION OF THE SLIGHT RISK THERE. WITH
RESPECT TO THE EXCESSIVE RAINFALL POTENTIAL IN THE SOUTHEAST
US...TENDED TO FOLLOW SHORT-TERM TRENDS IN RADAR AND SATELLITE
IMAGERY TO CONFINE THE MARGINAL RISK AREA MAINLY IN THE
OHIO/TENNESSEE VALLEYS. THERE WAS AN EXPANSION OF THE MARGINAL RISK
AREA DUE TO A SIGNAL PRESENT IN BOTH THE HREF AND RREF FOR UPSCALE
GROWTH OF HEAVY-RAINFALL PRODUCING STORMS LATER TONIGHT.
BANN
PREVIOUS DISCUSSION...
..CENTRAL PLAINS TO THE UPPER GREAT LAKES
..16Z UPDATE
THE INHERITED SLIGHT RISK REMAINS ON TRACK WITH ONLY MINOR COSMETIC
ADJUSTMENTS AFTER EVALUATION OF THE MORNING 12Z GUIDANCE. THE
OVERLAP OF A SERIES OF WEAK MID-LEVEL IMPULSES WITH THE DIFFLUENT
RRQ OF A DEPARTING JET STREAK WILL PRODUCE ROBUST SYNOPTIC ASCENT
ATOP A WAVERING STATIONARY FRONT. THIS LIFT ACTING UPON ELEVATED
PWS IN THE PRESENCE OF 30-40 KTS OF BULK SHEAR WILL SUPPORT
CLUSTERS OF CONVECTION MOVING SLOWLY ALONG THE BOUNDARY, ESPECIALLY
THIS EVENING AS THE LLJ RAMPS UP TO 25-35 KTS. THIS IS AGREED UPON
BY THE MAJORITY OF CAMS, AND ALTHOUGH MEAN WINDS OF 20-25 KTS WILL
KEEP STORMS PROGRESSIVE, REPEATING ROUNDS OF 1-2"/HR RAIN RATES
WILL STILL SUPPORT SCATTERED FLASH FLOODING.
..PREVIOUS DISCUSSION
EMBEDDED UPPER-LEVEL SHORTWAVES ROUNDING AN UPPER-HIGH OVER THE
SOUTHWESTERN U.S. WILL LEAD TO INCREASINGLY NORTHWESTERLY FLOW OVER
A QUASI-STATIONARY FRONTAL BOUNDARY ACROSS THE UPPER GREAT
LAKES/MIDWEST. AN OVERNIGHT MCS MAY STILL BE ONGOING THROUGH THE
START OF THE DAY 1 PERIOD ACROSS THE MICHIGAN UP/NORTHERN WISCONSIN
WEST-SOUTHWEST THROUGH SOUTHERN MINNESOTA, WITH THE OUTFLOW FROM
THESE STORMS LIKELY HELPING TO DELINEATE THIS AS THE EVENTUAL
POSITION OF THE FRONTAL BOUNDARY LATER THIS AFTERNOON. THIS WILL
FAVOR RENEWED DEVELOPMENT OF STORMS ALONG AND JUST TO THE SOUTH OF
THE BOUNDARY FROM THE UP/NORTHERN LP OF MICHIGAN SOUTHWEST THROUGH
CENTRAL WISCONSIN AND INTO SOUTHERN MINNESOTA/NORTHERN IOWA.
SEASONABLY HIGH PWAT VALUES 1.5-2 STANDARD DEVIATIONS ABOVE THE
MEAN WILL SUPPORT RAIN RATES OF 1-2"+ PER HOUR. THE LATEST HI-RES
GUIDANCE SHOWS THE POTENTIAL FOR LOCALLY HEAVY RAINFALL TOTALS OF
3"+, BACKED BY MODERATE (40-60%) HREF PROBABILITIES OF TOTALS
EXCEEDING 3", SUPPORTING THE RISK FOR SCATTERED FLASH FLOODING.
THE SLIGHT RISK HAS BEEN EXTENDED FURTHER TO SOUTHWEST WHERE HEAVY
RAINFALL IS ALSO EXPECTED AHEAD OF A DEVELOPING SURFACE WAVE ALONG
THE COLD FRONT OVER NEBRASKA. THIS WILL HELP FOCUS CONVERGENCE
ALONG THE FRONT PROMOTING MORE WIDESPREAD STORM DEVELOPMENT WITH
STORM CLUSTERING/COLD POOL MERGERS IN THE PRESENCE OF NORTHWESTERLY
FLOW LEADING TO THE POTENTIAL FOR UPSCALE GROWTH INTO AN ORGANIZED
CONVECTIVE SYSTEM. SEASONABLY HIGH MOISTURE FLOWING NORTHWARD ON
THE WEST SIDE OF SURFACE HIGH PRESSURE TO THE EAST WILL POOL AHEAD
OF THE BOUNDARY WITH DAYTIME SURFACE HEATING LEADING TO MODERATE TO
STRONG INSTABILITY SUPPORTING HEAVY RAINFALL RATES OF 1-2"+. A
DEVELOPING LOW LEVEL JET OVER THE CENTRAL PLAINS WILL ALSO HELP TO
MAINTAIN STORMS INTO THE EVENING HOURS. SIMILAR TO THE UPPER GREAT
LAKES/MIDWEST, THE LATEST HI-RES GUIDANCE SUPPORTS THE POTENTIAL
FOR LOCALLY HEAVY RAINFALL OF 3"+ AND SCATTERED INSTANCES OF FLASH
FLOODING
THERE MAY BE A RELATIVE MINIMA BETWEEN THESE TWO REGIONS THAT
WOULD NOT QUITE RISE TO THE LEVEL OF A SLIGHT RISK. HOWEVER,
THERE'S ENOUGH AGREEMENT AMONGST THE GUIDANCE THAT THIS REGION
WOULD BE LIMITED IF PRESENT AT ALL SO FELT A FULL EXTENSION OF THE
SLIGHT RISK SOUTHWESTWARD WAS WARRANTED RATHER THAN INTRODUCING A
SEPARATE RISK AREA.
..CENTRAL/NORTHERN HIGH PLAINS
A COLD FRONT PUSHING INTO THE CENTRAL/NORTHERN HIGH PLAINS WILL
PROVIDE A FOCUS FOR CONVERGENCE AS WELL AS MOIST, POST-FRONTAL
UPSLOPE FLOW TO SUPPORT SCATTERED AFTERNOON THUNDERSTORMS OFF THE
HIGHER TERRAIN. LOCALLY HIGHER PWAT VALUES OF 1"+ WILL SUPPORT
HEAVIER DOWNPOURS WITH AN ISOLATED RISK FOR FLASH FLOODING.
..GULF COAST, TENNESSEE VALLEY, INTO THE MID-ATLANTIC
THE INHERITED MARGINAL RISK WAS MAINTAINED FROM THE OVERNIGHT
ISSUANCE. A POTENT AND SLOW MOVING SHORTWAVE WILL LIKELY HELP SPAWN
RAPID CONVECTIVE DEVELOPMENT THIS AFTN, WITH NUMEROUS THUNDERSTORMS
LIKELY, ESPECIALLY FROM KY/TN EAST TOWARDS VA. A LACK OF
MEANINGFUL SHEAR WILL KEEP STORMS OF THE PULSE VARIETY, BUT WITH
PWS 1.75-2.00 INCHES AND STORM MOTIONS GENERALLY JUST AROUND 10
KTS, RAINFALL OF 2-3" WITH LOCALLY HIGHER AMOUNTS IS POSSIBLE
THANKS TO RAINFALL RATES WHICH HAVE A 15-25% CHANCE OF EXCEEDING
2"/HR FOR SHORT DURATIONS. THE FFG EXCEEDANCE DOES PEAK ABOVE 20%
IN SOME LOCATIONS ACROSS THIS REGION DUE TO COMPROMISED FFG FROM
RECENT RAINFALL, BUT CAM SIMULATED REFLECTIVITY KEEPS STORMS VERY
SCATTERED, SUPPORTING THE MRGL RISK.
FARTHER SOUTH TOWARDS THE GULF COAST, AIRMASS TYPE THUNDERSTORMS
WILL LIKELY BECOME SCATTERED TO NUMEROUS WITH 2+"/HR RAINFALL
RATES, BUT HIGH FFG AND A LACK OF ORGANIZATION SHOULD PREVENT MORE
THAN ISOLATED FLASH FLOOD CONCERNS.
..ARIZONA
A FEW ISOLATED OR SCATTERED THUNDERSTORMS WILL DEVELOP WITHIN A
MODEST SURGE OF PW LIFTING INTO AZ THIS EVENING. THE UA WRF, HREF,
AND REFS ALL INDICATE LOCALLY MORE THAN 1" OF RAINFALL, BUT 1-HR
FFG EXCEEDANCE IS ONLY AROUND 5%. OPTED TO NOT INTRODUCE A MRGL
RISK TODAY DUE TO THE UNCERTAINTY IN COVERAGE AND GENERALLY PULSE
ENVIRONMENT, BUT SHOULD A VULNERABLE AREA EXPERIENCE A SLOW MOVING
CELL WITH HEAVY RAIN, SOME IMPACTS COULD OCCUR DUE TO FLASH
FLOODING.
PUTNAM/WEISS
DAY 2
VALID 12Z THU JUL 09 2026 - 12Z FRI JUL 10 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDDLE MISSISSIPPI AND OHIO VALLEYS, AS WELL AS THE MID-
ATLANTIC STATES...
..MIDDLE MISSISSIPPI AND LOWER OHIO VALLEYS
A SLOW MOVING COLD FRONT WILL DRAG SOUTHEAST FROM THE UPPER MIDWEST
INTO THE OHIO VALLEY AND MID-MISSISSIPPI VALLEY BY FRIDAY MORNING.
THIS EVOLUTION WILL OCCUR IN RESPONSE TO SHORTWAVES EMBEDDED WITHIN
GENERALLY NEAR-ZONAL FLOW, BUT LEADING TO SUBTLE AMPLIFICATION OF
THE MID-LEVEL TROUGH. THIS WILL CREATE INCREASING DEEP LAYER
ASCENT, WITH RRQ DIFFLUENCE OF A WEAKENING AND DEPARTING JET STREAK
ALSO CONTRIBUTING.
THE OVERLAP OF THIS ASCENT ON THE BAROCLINIC GRADIENT/FRONT WILL
LIKELY RESULT IN SCATTERED TO NUMEROUS THUNDERSTORMS TRACKING WEST
TO EAST ACROSS THE AREA. ALTHOUGH CONFIDENCE IN THE EXACT TIMING
AND PLACEMENT OF ANY CONVECTION IS MODEST, IT IS PROBABLE (AND
REFLECTED BY AVAILABLE HIGH-RES CAMS) THAT ROUNDS OF THUNDERSTORMS
WILL OCCUR GENERALLY BENEATH THE AFOREMENTIONED WEAK IMPULSES.
THESE THUNDERSTORMS WILL BLOSSOM WITHIN FAVORABLE THERMODYNAMICS
CHARACTERIZED BY PWS POTENTIALLY EXCEEDING 2 INCHES (APPROACHING
DAILY RECORDS AND ABOVE THE 90TH PERCENTILE OF THE SPC SOUNDING
CLIMATOLOGY) OVERLAPPING MUCAPE OF 2000-3000 J/KG. THIS WILL
SUPPORT HEAVY RAIN RATES OF 1-2"/HR, WHICH THROUGH MULTIPLE ROUNDS
HAS A 40-60% CHANCE OF CREATING STRIPES OF 3+" OF RAIN, DESPITE
GENERALLY FAST MOTION OF 20-25 KTS. DUE TO UNCERTAINTY IN THE EXACT
TIMING IN PLACEMENT, BUT THE LIKELIHOOD OF MULTIPLE ROUNDS OF
INTENSE RAINFALL, THE BROAD SLGT RISK WAS EXPANDED, AND ALSO PULLED
NORTH TO INCLUDE THE CHICAGO METRO AREA AFTER COORDINATION WITH
WFO LOT.
..MID-ATLANTIC
FARTHER EAST INTO THE MID-ATLANTIC STATES, ANOTHER AREA OF LOCALLY
HEAVIER RAINFALL HAS BECOME MORE LIKELY AS SUGGESTED BY MANY OF THE
AVAILABLE CAMS AND WITHIN THE 24-HR NEIGHBORHOOD PROBABILITIES FROM
BOTH THE HREF AND THE REFS, AND A SLGT RISK OF EXCESSIVE RAINFALL
HAS BEEN INTRODUCED FOR THE EASTERN MID-ATLANTIC.
A SHORTWAVE POSITIONED OVER KY IN THE MORNING WILL SPEED NORTHEAST
BRINGING HEIGHT FALLS AND PVA ACROSS THE AREA AND INTERACTING WITH
A WAVY STATIONARY FRONT ACROSS VA. THIS WILL LEAD TO A WEAK SURFACE
WAVE MOVING ALONG THE BOUNDARY, AND THIS LOW AND ITS ACCOMPANYING
LOCALLY ENHANCED ASCENT (THROUGH WAA AND ACCELERATED MOIST INFLOW)
WILL HELP SPAWN MORE ORGANIZED CONVECTION DURING THE AFTN AND EVE.
WHILE STORMS MAY MOVE STEADILY ENE ON 15-20 KTS OF 0-6KM MEAN
WINDS, SOME TRAINING ALONG THE WARM FRONT IS POSSIBLE AS REFLECTED
BY BOUNDARY PARALLEL MEAN FLOW AND PROPAGATION VECTORS. WITH HOURLY
RAINFALL OF 1-2" LIKELY (50-70% CHANCE), AND SHORT TERM TRAINING
COULD OVERWHELM SOILS WHICH HAVE RECENTLY BECOME MORE SATURATED DUE
TO RECENT RAINFALL. THE THREAT FOR EXCESSIVE RAINFALL IS ALSO
SUPPORTED BY MODEST (BUT NOTABLE FOR A CONVECTIVE ENVIRONMENT) EFI
OF 0.6 ACROSS NORTHERN VA AND MUCH OF MD. THE MOST LIKELY AREAS
FOR FLASH FLOODING WILL BE WHERE ANY OF THESE RATES TRAIN ACROSS
URBAN/MORE VULNERABLE SOILS, BUT WITH HREF PROBABILITIES FOR 5" OF
RAIN UP TO 30%, SCATTERED INSTANCES OF FLASH FLOODING ARE POSSIBLE
ANYWHERE WITHIN THE SLGT RISK REGION.
..CENTRAL HIGH PLAINS
A FAMILIAR SET-UP WILL REMAIN IN PLACE THURSDAY AS A FRONTAL
BOUNDARY BECOMES QUASI-STATIONARY THROUGH THE REGION. MOIST/POST-
FRONTAL UPSLOPE FLOW AND CONVERGENCE ALONG THE BOUNDARY WILL
CONTINUE TO SUPPORT STORMS WITH LOCALLY HEAVY DOWNPOURS AND AN
ISOLATED RISK FOR FLASH FLOODING.
..ARIZONA
A PLUME OF ELEVATED PWS POTENTIALLY EXCEEDING 1.25" WILL SQUEEZE
INTO SOUTHERN ARIZONA THURSDAY EVENING COINCIDENT WITH MODEST BUT
INCREASING SBCAPE OF 500-750 J/KG. THIS WILL RESULT IN SCATTERED
THUNDERSTORM DEVELOPMENT WHICH IS SUPPORTED BY ALL AVAILABLE CAMS.
THESE STORMS WILL MOVE SLOWLY AND CHAOTICALLY WITHIN 0-6KM MEAN
WINDS OF JUST 5-10 KTS, WHILE THE FAVORABLE THERMODYNAMICS PRODUCE
SHORT-TERM RAINFALL RATES LOCALLY EXCEEDING 0.75" (HREF AND UA WRF
SUPPORT). IF ANY OF THESE RATES LINGER ACROSS VULNERABLE SOILS OR
TERRAIN, ISOLATED INSTANCES OF FLASH FLOODING COULD RESULT, AND A
NARROW MRGL RISK WAS ADDED.
WEISS
DAY 3
VALID 12Z FRI JUL 10 2026 - 12Z SAT JUL 11 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDDLE MISSISSIPPI/OHIO/TENNESSEE VALLEYS AND CENTRAL
APPALACHIANS...
..MIDDLE MISSISSIPPI VALLEY TO THE CENTRAL APPALACHIANS
A SIMILAR SYNOPTIC PATTERN/SETUP WILL EXIST FRIDAY FROM THE MID-
MISSISSIPPI VALLEY THROUGH THE MID-ATLANTIC STATES AS A WAVERING
STATIONARY FRONT STALLS WEST TO EAST ACROSS THE REGION. THIS FRONT
WILL STALL IN RESPONSE TO GENERALLY ZONAL (WEAK TROUGHING) MID-
LEVEL FLOW, WITHIN WHICH RIPPLES OF SHORTWAVE ENERGY WILL TRAVERSE.
AHEAD OF THIS FRONT, MOISTURE WILL CONTINUE TO POOL AS PERSISTENT
W/SW 850MB FLOW OF 20-25 KTS SURGES GULF MOISTURE NORTHEASTWARD,
LEADING TO A PLUME OF PWS REACHING 1.75 TO 2.00+ INCHES. MOISTURE
TRANSPORT WITHIN THIS FLOW IS REFLECTED BY IMPRESSIVELY LONG
TRANSPORT VECTORS WHICH CONVERGE STRONGLY INTO THE BOUNDARY,
SUGGESTING ADDITIONALLY ENHANCED ASCENT FROM MISSOURI THROUGH WEST
VIRGINIA, WHICH WILL ALSO BE WITHIN A STEEP MUCAPE GRADIENT OF
1000-3000 J/KG. THIS PATTERN WILL SUPPORT ANOTHER DAY OF SCATTERED
TO NUMEROUS THUNDERSTORMS CONTAINING 1-2"/HR RAIN RATES. THE
TRAINING RISK ON FRIDAY APPEARS A BIT STRONGER THAN THURSDAY DUE TO
BOUNDARY-PARALLEL FLOW ALONG THE STEEP CAPE GRADIENT, WITH ENHANCED
BULK SHEAR OF 20-25 KTS SUPPORTING GREATER ORGANIZATION INTO
CLUSTERS. THE SLGT RISK HAS BEEN MAINTAINED AND COSMETICALLY
ADJUSTED FOR THE LATEST GUIDANCE (GENERALLY TO COVER THE ECENS/SREF
PROBABILITIES FOR 3"/24 HRS). IT IS POSSIBLE THAT AN
ENHANCED/SLIGHT RISK AREA MAY BE NEEDED WHERE RAINFALL D3 OVERLAPS
VULNERABLE SOILS FROM RAINFALL ON D2, BUT CONFIDENCE IS NOT HIGH
ENOUGH IN EITHER COVERAGE OR LOCATION TO INTRODUCE THIS YET.
..CENTRAL HIGH PLAINS
FARTHER WEST, THE TAIL OF THIS LINGERING FRONT WILL FINALLY PUSH
OUT OF THE HIGH PLAINS AND INTO THE CENTRAL PLAINS THROUGH FRIDAY.
CONVERGENCE ALONG THIS FRONT INTO FAVORABLE THERMODYNAMICS TO
SUPPORT HEAVY RAIN WILL CREATE AT LEAST AN ISOLATED RISK FOR FLASH
FLOODING DUE TO TRAINING OF 1-2"/HR RAIN RATES ON ALIGNED
CORFIDI/MEAN WIND VECTORS. THIS CONTINUES TO SUPPORT THE INHERITED
MRGL RISK WITH ONLY MODEST COSMETIC ADJUSTMENTS FOR RECENT
GUIDANCE.
WEISS
DAY 4 AND DAY 5
VALID 12Z SAT JUL 11 2026 - 12Z MON JUL 13 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE OHIO/MID-MISSISSIPPI VALLEYS AND CENTRAL APPALACHIANS ON
SATURDAY...
THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE
OHIO AND MID-MISSISSIPPI VALLEYS EASTWARD INTO THE CENTRAL PORTIONS
OF THE APPALACHIANS ON SATURDAY AS SHORTWAVE ENERGY ALOFT DROPS
INTO THE MID- MISSISSIPPI VALLEY ON THE EAST SIDE OF A BUILDING
UPPER-LEVEL HIGH OVER THE WESTERN US. A QUASI-STATIONARY FRONT AT
THE SURFACE SEPARATING COOLER/DRIER AIR TO THE NORTH OF THE FRONT
FROM A MOISTURE-RICH ATMOSPHERE TO THE SOUTH WILL BE ACTING TO
FOCUS THE CONVECTION. LOCALLY INTENSE RAINFALL RATES AND HEAVY TO
POTENTIALLY EXCESSIVE RAINFALL AMOUNTS ARE POSSIBLE IN AREAS WHERE
PRECIPITABLE WATER VALUES RANGE FROM 1.8 TO 2.2 INCHES AS THE
SHORTWAVE ALOFT TRACKS EASTWARD...REACHING THE CENTRAL APPALACHIANS
BY THE END OF THE DAY 4 PERIOD AT 12/12Z. CONDITIONS BECOME
SOMEWHAT LESS FAVORABLE FOR ORGANIZED HEAVY TO EXCESSIVE RAINFALL
ON SUNDAY AS THE UPPER FORCING PULLS AWAY BUT ENOUGH
MOISTURE/INSTABILITY REMAINS IN THE LOW LEVELS...ESPECIALLY AT TIME
OF MAXIMUM DAYTIME HEATING...TO SUPPORT A BROAD AND LARGELY
UNFOCUSED MARGINAL RISK IN THE AREA FROM PARTS OF THE MID-
MISSISSIPPI VALLEY ACROSS THE SOUTHEAST US.
INTRODUCED A MARGINAL RISK AREA ACROSS PARTS OF ARIZONA AND NEW
MEXICO ON DAY 5 AS MOISTURE CONTINUES TO GET DRAWN NORTHWARD.
PRECIPITABLE WATER VALUES ON THE ORDER OF 1.75 INCHES NOSE INTO
SOUTHWEST ARIZONA BY SUNDAY. THIS MARGINAL RISK AREA IS AN
EXTENSION OF THE MARGINAL RISK AREA ON THE DAY 4 TIME FRAME ALREADY
IN PLACE.
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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