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FOUS30 KWBC 211942
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
342 PM EDT FRI AUG 21 2026
DAY 1
VALID 16Z FRI AUG 21 2026 - 12Z SAT AUG 22 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
VIRGINIA AND THE CAROLINAS...
..VIRGINIA TO CAROLINAS
FOR THE SOUTHERN MID-ATLANTIC, THE LATEST CAMS REMAIN CONSISTENT
IN SHOWING SOLID COVERAGE 2-4" AMOUNTS ACROSS CENTRAL NC INTO
ADJACENT NORTH-CENTRAL SC AND FAR SOUTHERN VA, WITH LOCALIZED
MAXIMUM AMOUNTS IN EXCESS OF 6 INCHES. HREF NEIGHBORHOOD
PROBABILITIES FOR >5" ARE 35-45%, AND REFS NEIGHBORHOOD
PROBABILITIES FOR >5" ARE 40-60%. THIS INCLUDES THE SANDHILLS OF
THE CAROLINAS WHERE FFGS ARE HIGHER, BUT ALSO EXTENDS INTO THE
PIEDMONT WHERE TERRAIN IS MORE AT RISK FOR FLASH FLOODING. HREF/REFS
SHOW SIMILAR PLACEMENT OF THE PROBABILITIES OF 2 INCH ACCUMULATION
PER HOUR LATER THIS AFTERNOON, AND SHOW PROBABILITIES UP TO 50-70%.
ENVIRONMENTAL CONDITIONS WILL REMAIN VERY BUOYANT WITH PWATS
BETWEEN 2-2.3" FOR THE ENTIRE AREA ALONG AND SOUTH OF A QUASI-
STATIONARY FRONT. GLOBAL-SCALE MODELS REMAINED CONSISTENT WITH
THEIR SIGNATURE OF 1-2" OF RAINFALL ON AN AREAL AVERAGE BASIS WITH
SOME OF THE DETERMINISTIC OUTPUTS APPROACHING UPWARDS OF 3-5" IN
THE CAROLINAS. OVERALL, MODELS ARE SUPPORTIVE OF THE PREVIOUSLY
ISSUED SLIGHT RISK AREA, INCLUDING THE URBANIZED ZONES ALONG I-95
AND POINTS EAST. THE SLIGHT RISK AREA WAS EXPANDED SOUTHWARDS WITH
THE 16Z UPDATE TO INCLUDE THE AUGUSTA, GEORGIA METRO AREA, AND
HIGH-END SLIGHT IMPACTS CONTINUE TO LOOK POSSIBLE, WITH LOCALLY
SIGNIFICANT IMPACTS IF CONDITIONS ALIGN THIS AFTERNOON. A BROADER
MARGINAL RISK SURROUNDS THE SLIGHT, STRETCHING FROM THE MID-
ATLANTIC TO THE CENTRAL GULF COAST WHERE LOCALLY HEAVY RAINFALL
ALONG THE FRONTAL BOUNDARY WILL BE POSSIBLE. PORTIONS OF THE
CENTRAL APPALACHIANS AND CENTRAL GULF COAST DOWN TO NEW ORLEANS
WERE ADDED WITH THE 16Z UPDATE TO ACCOUNT FOR INCREASING QPF TRENDS
IN A FAVORABLE ENVIRONMENT AND INCREASED SENSITIVITIES IN
URBANIZED AREAS.
..SOUTHWEST
MONSOONAL ACTIVITY WILL CONTINUE FOR YET ANOTHER DAY AS THE
SOUTHWEST CONUS LIES UNDERNEATH A BROAD RIDGE CENTERED OVER THE
FOUR CORNERS WITH MULTIPLE MID-LEVEL VORTICITY MAXIMA PIVOTING
UNDER THE RIDGE AXIS. SEASONAL TO SLIGHTLY ELEVATED PWATS WILL
EXTEND THROUGHOUT THE SOUTHWEST, EVEN REACHING A BIT FARTHER NORTH
INTO THE GREAT BASIN AND INTO PORTIONS OF NV AND CENTRAL UT.
THIS MOISTURE WILL LEAD TO SOME LOCALLY HEAVY RAINFALL THAT COULD
IMPACT SOME AREAS THAT SAW CONVECTIVE ACTIVITY YESTERDAY. LOCAL
TOTALS OF 1-2" ARE FORECAST, BUT AREAL QPF AVERAGE IS PRETTY MUCH
BETWEEN 0.25-0.5", WHICH IS TYPICAL FOR THESE TYPES OF SETUPS. A
BROAD MARGINAL RISK AREA REMAINS IN PLACE FOR MUCH OF THE DESERT
SOUTHWEST UP INTO THE SOUTHERN ROCKIES AND WAS EXPANDED WITH THE
16Z UPDATE TO INCLUDE MORE OF THE GREAT BASIN.
..SOUTHERN WISCONSIN TO NORTHERN ILLINOIS
A MARGINAL RISK AREA WAS INTRODUCED WITH THE 16Z UPDATE THAT
COVERS PORTIONS OF SOUTHERN WISCONSIN AND NORTHERN ILLINOIS TO
COVER HEAVY RAIN THREATS WITH PRE-FRONTAL CONVECTION. A LINEAR
CONVECTIVE COMPLEX WAS MOVING SOUTH ACROSS SOUTHERN WISCONSIN LATE
THIS MORNING, AND PRODUCED RAINFALL RATES IN EXCESS OF 1.5 IN/HR,
WHICH EXCEEDS THE FFGS FOR THIS AREA. CAMS SUGGEST CONVECTION WILL
CONTINUE TO BE SUPPORTED THROUGH THE AFTERNOON AND GRADUALLY PUSH
SOUTH INTO NORTHERN ILLINOIS. THE ENVIRONMENT WILL CONTINUE TO BE
SUPPORTIVE OF LOCALLY HEAVY RAINFALL WITH PWAT VALUES UP TO 1.6"
AND DECENT LIFT AND INSTABILITY IN THE VICINITY OF THE ADVANCING
COLD FRONT.
..SOUTHERN NEBRASKA TO NORTHERN KANSAS
A SMALL MARGINAL RISK AREA REMAINS IN PLACE FOR PORTIONS OF
NEBRASKA SOUTHEAST OF THE SANDHILLS AND FAR NORTHERN KANSAS. AN
UPPER LEVEL SHORTWAVE IS FORECAST TO MOVE SOUTH ACROSS THIS REGION
AS A COLD FRONT SINKS SOUTH ACROSS THE PLAINS, WHICH IS EXPECTED
TO TRIGGER A BURST OF CONVECTIVE ACTIVITY OVERNIGHT TONIGHT. THIS
WAVE SHOULD BE FAIRLY PROGRESSIVE, WHICH WILL LIMIT THE FLOOD
POTENTIAL, BUT THERE IS SOME CHANCE FOR REPEATING/TRAINING STORMS.
CAMS SUGGEST SOME LOCALLY HEAVY RAINFALL WITH RATES OF 1-2 IN/HR
WILL BE POSSIBLE. THE CURRENT 1 HR FFGS FOR THIS AREA ARE 1.5
IN/HR, WHICH HAVE A CHANCE OF BEING EXCEEDED.
DOLAN/BANN
DAY 2
VALID 12Z SAT AUG 22 2026 - 12Z SUN AUG 23 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MID-
ATLANTIC...
..20Z UPDATE
THE SLIGHT RISK AREA OVER THE MID-ATLANTIC WAS TRIMMED UP A BIT TO
ACCOUNT FOR QPF TRENDS/PLACEMENT IN THE MOST RECENT CAM RUNS. THIS
AREA ALSO ALIGNS WELL WITH WHERE THE HREF AND REFS HAVE THE HIGHEST
PROBABILITIES (50-60%) OF 2" IN 6 HRS. LOCALIZED AREAS WITHIN THE
SLIGHT THAT RECEIVED HEAVY RAINFALL YESTERDAY AND TODAY MAY SEE
HIGH-END SLIGHT IMPACTS. ELSEWHERE, THE SURROUNDING MARGINAL RISK
AREA WAS MAINTAINED AND EXPANDED TO INCLUDE PORTIONS OF THE OHIO
VALLEY AND EASTERN GREAT LAKES WHERE QPF HAS TRENDED UP IN THE
VICINITY OF A STRENGTHENING LOW PRESSURE SYSTEM. THIS PATTERN MAY
ALLOW FOR SOME LAKE-ENHANCED PRECIPITATION ACROSS PORTIONS OF OHIO,
PENNSYLVANIA, AND NEW YORK. GIVEN RECENT HEAVY RAINS IN THE OHIO
VALLEY, THE MARGINAL WAS EXPANDED TO INCLUDE PORTIONS OF OHIO AND
WEST VIRGINIA THAT MAY BE SENSITIVE TO FLOODING WITH ADDITIONAL
PRECIPITATION ON SATURDAY. THE MARGINAL RISK AREA ACROSS THE
INTERMOUNTAIN WEST WAS ALSO MAINTAINED AND EXPANDED TO INCLUDE MORE
OF THE NORTHERN ROCKIES AS ANOMALOUS MOISTURE PUSHES NORTH ACROSS
THE REGION.
DOLAN
..PREVIOUS DISCUSSION
..EAST COAST
LARGE SCALE PATTERN WILL CONTINUE ITS EVOLUTION INTO A MUCH MORE
AMPLIFIED SETUP WITH A DEEPENING MID-LATITUDE TROUGH EXTENDING FROM
CANADA ALL THE WAY INTO THE EASTERN CONUS. THIS PLACES THE ENTRANCE
REGION OF AN UPPER LEVEL JET IN FAVORABLE POSITION OVER AN AIRMASS
WITH PRECIPITABLE WATER VALUES OF 2 TO 2.25 INCHES AS A FEW ROBUST
DISTURBANCES ROTATE AROUND THE LEADING EDGE OF THE TROUGH WHICH
WILL SUPPORT HEAVY RAINFALL ALONG THE EASTERN SEABOARD. CURRENT
MODEL CONSENSUS IS A MODERATE TO HEAVY QPF FOOTPRINT EXTENDING FROM
COASTAL SC ALL THE WAY UP INTO ME. PRIMARY CONCERN FOR EXCESSIVE
RAINFALL IS ALONG AND EAST OF I-95 IN THE MID-ATLANTIC DUE TO THE
HEAVY/EXCESSIVE RAIN THAT OCCURRED ON THURSDAY AS MUCH AS THE WITH
EXPECTED RAINFALL AMOUNTS IN THE DAY 2 PERIOD.
..WEST
PERSISTENT RIDGING ACROSS THE WESTERN U.S. WILL FINALLY BEGIN TO
SHIFT BACK OVER THE PLAINS AS A ROBUST TROUGH OVER THE PACIFIC
OCEAN MAKES HEADWAY INTO THE PACIFIC NORTHWEST AND BEGINS PUSHING
THE PATTERN DOWNSTREAM AND FLATTENING THE RIDGE UP TOP. THE
PROGRESSION OF THE TROUGH WILL LEAD TO AN INFLUX OF BOTH REGIONAL
MOISTURE AND ASCENT AS THE TROUGH MAKES PROGRESS INTO OR/WA AS WE
MOVE INTO THE MIDDLE PART OF THE WEEKEND. PWAT ANOMALIES ARE
ANTICIPATED TO SURGE UPWARDS OF 2-3 STANDARD DEVIATIONS ABOVE
NORMAL FOR THIS TIME OF YEAR AS FAR NORTH AS IDAHO AND WESTERN
MONTANA WITH THE REST OF THE WEST FIRMLY ENTRENCHED WITHIN A +1
STANDARD DEVIATION PWAT ANOMALY, GOOD ENOUGH TO MAINTAIN A
PERSISTENT MONSOONAL TYPE PATTERN OVER THE DESERT SOUTHWEST AND
GREAT BASIN INTO THE CENTRAL ROCKIES. A BROAD MRGL RISK OVER MUCH
OF THE WESTERN U.S, EAST OF THE PACIFIC COAST STATES WILL SUFFICE.
BANN
DAY 3
VALID 12Z SUN AUG 23 2026 - 12Z MON AUG 24 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
CENTRAL US AND THE SOUTHWEST...
..20Z UPDATE
THE PREVIOUS FORECAST REMAINS ON TRACK. SMALL ADJUSTMENTS WERE MADE
TO THE EXISTING MARGINAL RISK AREAS OVER THE WEST AND GREAT PLAINS.
ANOMALOUS MOISTURE WILL CONTINUE TO SUPPORT LOCALLY HEAVY RAINFALL
IN THE WEST, WITH SOME HEAVY RAINFALL POTENTIAL SPILLING OVER INTO
THE CENTRAL HIGH PLAINS AS WELL. THE MARGINAL OVER THE GREAT PLAINS
AND MIDWEST REMAINS IN PLACE, BUT WAS ADJUSTED TO MATCH THE QPF
FOOTPRINTS FROM THE LATEST MODEL RUNS AND WPC FORECAST. THE
MARGINAL RISK IN THE NORTHEAST WAS REMOVED WITH THIS ISSUANCE GIVEN
A MORE PROGRESSIVE TREND WITH THE COLD FRONT, RESULTING IN LOWER
QPF VALUES ACROSS THE REGION.
DOLAN
..PREVIOUS DISCUSSION
..NORTHEAST
CYCLOGENESIS NORTH OF THE GREAT LAKES WILL HELP PUSH A COLD FRONT
EASTWARD ACROSS NEW ENGLAND ON SUNDAY. UNTIL IT CLEARS THE
AREA...SOUTHERLY FLOW AHEAD OF THE FRONT WILL CONTINUE TO DRAW AN
AIRMASS WITH PRECIPITABLE WATER VALUES OF 1.8 TO 2.2 INCHES ACROSS
THE COASTAL REGION. THIS SHOULD SUPPORT A LOW-END RISK OF EXCESSIVE
RAINFALL MAINLY WHERE LOW LEVEL FLOW INTERACTS WITH THE TERRAIN.
NOTE THAT THE AREAL COVERAGE OF THE MARGINAL RISK AREA HAS
DECREASED IN RESPONSE TO THE QUICKER FORWARD SPEED COMPARED WITH
PREVIOUS RUNS.
..MIDWEST
ENERGY APPROACHING FROM THE WEST BEGINS TO FLATTEN THE NORTHERN
PORTION OF THE RIDGE...ALLOWING FOR SHOWERS AND THUNDERSTORMS TO
DEVELOP ACROSS THE NORTHERN PLAINS WHICH CAN BUILD SOUTHEAST INTO
THE OZARKS. GIVEN THE POTENTIAL FOR REPEAT CONVECTION...MAINTAINED
THE MARGINAL RISK AREA EVEN THOUGH INDIVIDUAL CELLS SHOULD BE
PROGRESSIVE.
..WESTERN US
THE OVERALL PATTERN IS EXPECTED TO REMAIN RELATIVELY UNCHANGED
WITH THE UPPER RIDGE REMAINED ANCHORED OVER THE SOUTHWEST AND
SOUTHERN PLAINS. MOISTURE AND ENERGY MOVING ALONG THE WESTERN AND
NORTHERN PERIPHERY OF THE RIDGE WILL CONTRIBUTE TO DAILY SHOWER AND
THUNDERSTORM ACTIVITY AND THE POTENTIAL FOR ISOLATED FLASH
FLOODING EXTENDING FROM THE SOUTHWEST TO THE NORTHERN INTERMOUNTAIN
WEST AND ROCKIES.
BANN
DAY 4 AND DAY 5
VALID 12Z MON AUG 24 2026 - 12Z WED AUG 26 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, INTERMOUNTAIN WEST, AND ROCKIES DAYS 4/5 AND THE
CENTRAL PLAINS DAY 5...
AN UPPER RIDGE WILL REMAIN ANCHORED OVER THE SOUTHWEST-SOUTHERN
PLAINS INTO MIDWEEK, WITH MOISTURE AND ENERGY ON THE BACKSIDE OF
THE HIGH CONTRIBUTING TO DIURNAL SHOWERS AND STORMS THAT WILL RAISE
THE POTENTIAL FOR ISOLATED FLASH FLOODING BOTH DAYS FROM PARTS OF
THE SOUTHWEST TO THE NORTHERN ROCKIES.
AREAS OF HEAVY RAINFALL LEADING TO POTENTIAL FLASH FLOODING ARE
EXPECTED TO DEVELOP ACROSS PORTIONS OF THE CENTRAL PLAINS TUESDAY
INTO EARLY WEDNESDAY AS SHORTWAVES EMANATING FROM THE WEST SPILL
OFF OF THE TOP OF THE RIDGE. UNCERTAINTY IN THE TIMING AND
PLACEMENT OF THESE FEATURES LIMITED CONFIDENCE IN THE LOCATION OF
THESE HEAVIER AMOUNTS. HOWEVER, A CORRIDOR OF DEEPENING MOISTURE
AND INCREASING INSTABILITY ALONG A COLD FRONT DROPPING SOUTH INTO
THE REGION, COMBINED WITH THE POTENTIAL FOR TRAINING, PROMPTED THE
INTRODUCTION OF THE MARGINAL RISK AREA EXTENDING EAST OF THE
ROCKIES ACROSS PORTIONS OF THE CENTRAL PLAINS ON DAY 5.
PEREIRA
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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