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FOUS30 KWBC 270059
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
859 PM EDT SAT SEP 26 2026
DAY 1
VALID 01Z SUN SEP 27 2026 - 12Z SUN SEP 27 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
COASTAL NORTHEAST AND IOWA...
0100Z UPDATE: SOME ADJUSTMENTS WERE MADE TO THE SLIGHT RISK AREAS
ACROSS THE MIDWEST AND ALONG THE NORTHEAST COAST. THESE UPDATES
WERE LARGELY BASED ON OBSERVATIONAL TRENDS (RADAR, SATELLITE,
MESOANALYSIS), AND ALSO FROM MOST RECENT CAM GUIDANCE, INCLUDING
THE 18Z HREF AND REFS QPF EXCEEDANCE PROBABILITIES. BECAUSE OF
THIS, THE SLIGHT RISK AREA SHIFTED A LITTLE FARTHER SOUTH AND EAST
ACROSS IOWA AND NORTHERN MISSOURI. ACROSS THE NORTHEAST COAST,
RAINFALL RATES AND THUS THE FLASH FLOOD THREAT WILL REMAIN LIMITED
TO AREAS RIGHT ALONG THE COAST, ESPECIALLY IN URBAN AREAS, WHERE
SOME (ALBEIT WEAK) ELEVATED INSTABILITY WILL BE PRESENT. THIS WOULD
EXPLAIN THE NARROW CORRIDOR OF BOTH THE SLIGHT AND MARGINAL RISK
AREAS.
HURLEY
PREVIOUS DISCUSSION BELOW..
..COASTAL NORTHEAST
A LARGE NOR'EASTER CONTINUES TO SNAIL CRAWL OFF THE COAST OF
DELAWARE/SOUTHERN JERSEY THIS MORNING. THE COMBINATION OF THE
STRENGTH OF THE NOR'EASTER (986 MB AS OF THIS WRITING), ITS SNAIL
CRAWL MOVEMENT, AND A FULL MOON HAVE ALL WORKED TO WORSEN COASTAL
FLOODING CONCERNS. WHILE THE MOST RAIN FROM THE STORM WILL ALSO
FALL ALONG THE COAST, COASTAL AND TIDAL FLOODING ARE LIKELY TO BE
SUBSTANTIALLY MORE IMPACTFUL THAN ANY RAINFALL FLOODING.
NONETHELESS, AT HIGH TIDE, RUNOFF FROM THE RAINFALL TRYING TO DRAIN
INTO ALREADY FULL BAYS AND INLETS WILL WORSEN LOCAL FLOODING. THE
SLIGHT RISK IS LARGELY UNCHANGED, SAVE FOR SMALL EXPANSIONS OF THE
SLIGHT TO INCLUDE URBAN NORTHERN NEW JERSEY AS WELL AS URBAN
CENTRAL CONNECTICUT. OTHERWISE, THE INHERITED HIGHER END SLIGHT FOR
EASTERN MASS, RHODE ISLAND, AND EASTERN LONG ISLAND WAS DROPPED
WITH THIS UPDATE. ALL OF THE NORTHEAST IS ON THE COLD/NORTHWEST
SIDE OF THE STORM. THUS, INSTABILITY NEEDED TO GENERATE HEAVY RAIN
IS SORELY LACKING. THIS STORM HAS AND CONTINUES TO BE A PROLONGED
LIGHT TO MODERATE RAIN EVENT, NOT A HEAVY ONE. THUS, WHILE URBAN
AREAS MAY STILL STRUGGLE TO KEEP UP WITH THE COMBO OF PROLONGED
RAIN AND TIDES (WHERE APPLICABLE), THE FLOOD FROM RAINFALL THREAT
IS LOW WITH THIS STORM RELATIVE TO SEVERAL OTHER METEOROLOGICAL
THREATS, SUCH AS COASTAL/TIDAL FLOODING AND HIGH WINDS.
..IOWA
THE SLIGHT RISK FOR CENTRAL AND SOUTHWESTERN IOWA WAS LEFT
UNCHANGED WITH THIS UPDATE. SEVERAL OF THE HIGH-RESOLUTION
GUIDANCES SUGGEST SHOWERS AND STORMS WILL IMPACT THE AREA ON THE
NOSE OF A LOW LEVEL JET THIS EVENING AND TONIGHT. THE AREA HAS WET
SOILS FROM PREVIOUS RAINFALL, WHICH HAS LOWERED FFGS TO THE 1-1.5
IN/HR RANGE. INSTABILITY WILL BE THE PRIMARY LIMITING FACTOR, WITH
VALUES STRUGGLING TO EXCEED 500 J/KG. MOISTURE TRANSPORT WITH THE
LLJ SHOULD BE ABLE TO OVERCOME THIS, GIVEN THE LOWERED FFGS, SO A
LOW-END SLIGHT STILL LOOKS REASONABLE.
WEGMAN
DAY 2
VALID 12Z SUN SEP 27 2026 - 12Z MON SEP 28 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
COASTAL NORTHEAST...
..2030Z UPDATE
NO SIGNIFICANT CHANGES WERE MADE TO ANY OF THE RISK AREAS. THE
NOREASTER WILL CONTINUE SPINNING OFF THE COAST OF CAPE COD,
BRINGING SOAKING RAINS TO EASTERN NEW ENGLAND THROUGH NYC AND THE
JERSEY SHORE. WITH THE STORM'S CENTER EAST OF NEW ENGLAND, THE
NORTHERLY FLOW WILL CONTINUE TO KEEP INSTABILITY LOW, HAMPERING ANY
CHANCES OF HEAVY RAIN OCCURRING ANYWHERE. THUS, THIS REMAINS A
LONG-DURATION LIGHT TO MODERATE RAINFALL EVENT. MOST AREAS OUTSIDE
OF URBAN CENTERS CAN GENERALLY HANDLE A SLOW-FALLING RAIN. THUS,
THE SLIGHT REMAINS PRIMARILY FOR URBAN AND POOR-DRAINAGE AREAS.
A MARGINAL RISK REMAINS IN NORTH DAKOTA. IT'S A LOW-END MARGINAL,
AS ANY STORMS WILL OCCUR OVER A RELATIVELY FLAT, VERY RURAL, AND
GENERALLY SANDY PORTION OF THE PLAINS, ALL OF WHICH WORK AGAINST
ANY SIGNIFICANT CHANCES FOR FLOODING.
IN THE SOUTHWEST, THE MARGINAL RISK REMAINS LARGELY IN PLACE, OTHER
THAN CUTTING OUT A PORTION OF SOUTHEAST CALIFORNIA DUE TO AN
EASTWARD SHIFT WITH THE HEAVIEST RAIN. IT WILL VERY MUCH BE THE
CALM BEFORE THE STORM, AS THE MOISTURE PLUME WITH POLO/ODALYS WILL
BE MAKING ITS WAY NORTH UP THE BAJA CALIFORNIA PENINSULA.
WEGMAN
..PREVIOUS DISCUSSION
..NORTHEAST
THE POTENT NOR'EASTER WILL BEGIN TO WEAKEN AS IT LINGERS IN THE
VICINITY OF THE NORTHERN MID-ATLANTIC COAST. HOW FAR INLAND, IF AT
ALL, THE STORM DRIFTS WILL HELP DICTATE HOW FAR BEYOND THE COASTAL
REGIONS ITS ASSOCIATED HEAVY RAINS WILL REACH. HEAVY RAINFALL IS
EXPECTED TO LINGER OVERHEAD, LEADING TO AN ADDITIONAL 1-2.5 INCHES
OF RAINFALL OVER MUCH OF THE SAME AREA. THESE RAINS WILL OVERLAP
AREAS IMPACTED DURING THE DAY 1 PERIOD THUS MAINTAINING AN ELEVATED
THREAT FOR EXCESSIVE RAINFALL AND FLASH FLOODING CONCERNS. A SLIGHT
RISK REMAINS IN EFFECT FROM COASTAL NEW JERSEY TO EASTERN
MASSACHUESETTS WITH A MARGINAL FROM SOUTHERN NEW JERSEY TO
SOUTHERN MAINE.
..SOUTHWEST
INCREASING SOUTHWESTERLY FLOW AHEAD OF AN AMPLIFYING TROUGH
DIGGING SOUTH ACROSS CALIFORNIA WILL BEGIN TO TAP MOISTURE AND MID-
TO-UPPER LEVEL ENERGY, SUPPORTING AN INCREASE IN SHOWER AND
THUNDERSTORM ACTIVITY AND THE POTENTIAL FOR LOCALIZED AREAS OF
FLASH FLOODING FROM SOUTHERN CALIFORNIA INTO THE SOUTHWEST.
..NORTHERN PLAINS
A MARGINAL RISK AREA WAS RAISED FOR PORTIONS OF THE NORTHERN PLAINS
AS A NORTHERN STREAM TROUGH EXITS THE ROCKIES AND BEGINS TO TAP
INTO MOISTURE OVER THE PLAINS. RAINFALL RATES EXCEEDING 0.50
INCH/HOUR MAY LEAD TO ISOLATED INSTANCES OF FLASH FLOODING ACROSS
THE AREA.
CAMPBELL/PEREIRA
DAY 3
VALID 12Z MON SEP 28 2026 - 12Z TUE SEP 29 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE FOUR
CORNERS REGION...
..2030Z UPDATE
..SOUTHWEST
A PREDECESSOR RAINFALL EVENT (PRE) WILL IMPACT MUCH OF THE 4
CORNERS REGION ON MONDAY. THE RAIN WILL BE PRECEDING THE ARRIVAL
OF THE PRIMARY MOISTURE PLUME ASSOCIATED WITH EAST PACIFIC
HURRICANE POLO. SOME CONTRIBUTION FROM HURRICANE ODALYS CAN ALSO BE
INCLUDED, THOUGH IT'S LIKELY ODALYS WILL FUJIWHARA AROUND THE
STRONGER POLO, AND THEREFORE NOT FULLY MERGE WITH IT. SEE THE NHC
FORECAST FOR MORE DETAILS.
A LARGE UPPER LEVEL TROUGH AND LOW WILL GRADUALLY DRIFT
SOUTHEASTWARD THROUGH MONDAY. HOWEVER, THIS TROUGH WILL BE THE
CATALYST THAT DRAWS THE MOISTURE PLUME NORTHEASTWARD INTO THE
SOUTHWEST. IT WILL ALSO PROVIDE THE UPPER LEVEL COLD AIR THAT WILL
ALLOW FOR CONVECTIVE ACTIVITY TO IMPACT WESTERN PORTIONS OF THE
RISK AREA IN ARIZONA, NEVADA AND SOUTHWEST UTAH. PWAT VALUES WILL
BE 2-3 SIGMA ABOVE NORMAL, OR OVER AN INCH IN SOME CASES HIGHER
THAN CLIMATOLOGY. THIS MEANS THERE WILL BE AMPLE ATMOSPHERIC
MOISTURE IN PLACE TO BE WRUNG OUT AS HEAVY RAIN.
MUCH OF THE SOUTHWEST REMAINS IN A VERY WET PATTERN. WHILE
WONDERFUL FOR LONG-TERM DROUGHT IMPROVEMENT, THE SOILS ARE NOW
FULLY SATURATED OVER NEARLY THE WHOLE OF NEW MEXICO, SPILLING INTO
PORTIONS OF ALL THE NEIGHBORING STATES. THIS MEANS THAT THE SOILS
CANNOT TAKE ANYMORE RAINFALL. THUS, DESPITE BOTH TODAY BEING DRY
AND TOMORROW ONLY SEEING NEGLIGIBLE RAINFALL IN THIS AREA, THE
SOILS WILL NOT BE ABLE TO TAKE MUCH EVEN BY MONDAY BEFORE
CONVERTING THE REST TO RUNOFF. MUCH OF THE SOUTHWEST IS A VERY
FLASH-FLOOD-PRONE AREA DUE TO ROCKY SOILS, SLOT CANYONS, AND A
PLETHORA OF BURN SCARS. ADDING MUCH HEAVIER THAN NORMAL AMOUNTS OF
RAIN TO A LANDSCAPE LIKE THIS MEANS MAJOR IMPACTS ARE LIKELY IN THE
MOST PRONE AREAS.
INSTABILITY WILL ONCE AGAIN BE A LIMITING FACTOR AS FAR AS RAINFALL
RATES ARE CONCERNED. SHORTENING DAYLIGHT AND EXTENSIVE CLOUD COVER
WILL WORK AGAINST HEAVY RAINFALL. HOWEVER, GIVEN VERY DYNAMIC UPPER
LEVEL SUPPORT, WELL ABOVE NORMAL LEVELS OF ATMOSPHERIC MOISTURE,
NEARLY SATURATED SOILS TO BEGIN WITH, TOPOGRAPHIC EFFECTS AND ROCKY
SOILS, THIS SHOULD MORE THAN COMPENSATE FOR THE ABOVE-NOTED
LIMITATIONS.
THUS, IN COORDINATION WITH ALL THE FORECAST OFFICES FROM PHOENIX
AND VEGAS THROUGH SOUTHERN COLORADO AND NEW MEXICO, A LARGE
MODERATE RISK AREA WAS INTRODUCED TO COVER THE SLOT CANYONS AND
BURN SCARS OF SOUTHERN UTAH AND SOUTHWEST COLORADO, AND FOR THE
HEAVIER RAINS ACROSS ARIZONA AND NEW MEXICO. THE "MAIN EVENT" WILL
BE INTO DAY 4/WEDNESDAY.
..NORTH DAKOTA/NORTHWESTERN MINNESOTA
A SMALL SLIGHT RISK WAS INTRODUCED WITH THIS UPDATE IN COORDINATION
WITH FGF/FARGO-GRAND FORKS FORECAST OFFICE. THE NOSE OF THE LOW
LEVEL JET IN THAT AREA WILL BE UPLIFTED BY AN APPROACHING UPPER
LEVEL SHORTWAVE, THIS WILL ENHANCE LIFT AND PROMOTE TRAINING
THUNDERSTORMS ACROSS EASTERN NORTH DAKOTA AND NORTHWESTERN
MINNESOTA. THE AREA HAS SOME HISTORY OF FLASH FLOODING. WHILE THE
AREA WILL STRUGGLE FOR INSTABILITY, ONCE AGAIN GOOD FORCING FROM
ALOFT SHOULD WORK TO COMPENSATE FOR THAT.
..NEW ENGLAND
THE NOR'EASTER IMPACTING THE AREA WILL SLOWLY WEAKEN AND DRIFT
AWAY FROM NEW ENGLAND. DIMINISHING RAINFALL RATES ASSOCIATED WITH
THE NOR'EASTER MAY STILL CAUSE FLOODING ISSUES INTO INTERIOR NEW
ENGLAND ESPECIALLY, WHERE TOPOGRAPHY WILL ENHANCE ANY ONGOING
FLOODING STARTED IN THE PRIOR DAYS. A MARGINAL RISK REMAINS IN
PLACE, WITH A POSSIBILITY FOR A SMALL SLIGHT THAT MAY BE NEEDED,
ESPECIALLY ACROSS INTERIOR MASSACHUSETTS AND FAR SOUTHERN VERMONT &
NEW HAMPSHIRE, BUT THAT WILL LARGELY DEPEND ON HOW IMPACTFUL ANY
FLOODING IS FROM PRIOR DAYS, AND ANY UPGRADE WOULD LIKELY BE THE
CONTINUATION OF AN EARLIER SLIGHT FROM DAY 2/SUNDAY.
WEGMAN
..PREVIOUS DISCUSSION
..SOUTHWEST, SOUTHERN ROCKIES, AND ADJACENT HIGH PLAINS
THERE WILL BE A STRONG INFLUX OF TROPICAL MOISTURE FROM TROPICAL
CYCLONES ODALYS AND POLO DIRECTED INTO THE DESERT SOUTHWEST. THIS
ABUNDANT MOISTURE WHEN COUPLED WITH THE AFTERNOON DIURNAL
INSTABILITY MAX WILL TRIGGER SCATTERED TO WIDESPREAD CONVECTION
ACROSS THE REGION. AN INTERACTION BETWEEN THE EVENTUAL CLOSED UPPER
LOW OVER CALIFORNIA AND WHAT WILL BE POLO AS IT BEGINS MOVING
NORTHWARD TOWARDS BAJA OVER THE COURSE OF THE WEEKEND INTO EARLY
NEXT WEEK.
THE ENSEMBLE MEANS FOR THIS TIME PERIOD CONTINUE TO DEPICT A BROAD
EXPANSE OF HEAVIER RAINFALL INCREASINGLY LIKELY FOR THE SOUTHWEST
/FOUR CORNERS AREA DUE TO THE ASCENT PATTERN DOWNSTREAM OF THE
UPPER LOW SWINGING INTO THE REGION. PW ANOMALIES DURING THIS PERIOD
WILL BE BETWEEN 2-3.5 STANDARD DEVIATIONS ABOVE SEASONAL NORMAL
FOR LATE SEPTEMBER. A BROAD SLIGHT RISK FOR EXCESSIVE RAINFALL AND
FLASH FLOODING REMAINS IN EFFECT WHILE A HIGHER-END SLIGHT RISK
WAS RAISED FOR EAST-CENTRAL ARIZONA AND WESTERN NEW MEXICO.,
..NORTHERN AND CENTRAL PLAINS
PROGRESSIVE NORTHERN STREAM TROUGH COUPLED WITH PREVALENT MOISTURE
ADVECTION FROM THE SOUTHWEST WILL LEAD TO A PERIOD OF CONVECTIVE
CONCERNS AND ISOLATED FLASH FLOOD POTENTIAL. THERE IS A BROAD
MARGINAL RISK IN EFFECT FROM NORTHERN KANSAS TO NORTH
DAKOTA/NORTHWEST MINNESOTA. REMNANT ENERGY FROM WHAT IS CURRENTLY
TROPICAL CYCLONE ODALYS WILL GET DRAWN NORTHEASTWARD OUT OF THE
PACIFIC PRIOR TO THE PERIOD REACHING THE NORTHERN AND CENTRAL
PLAINS MONDAY INTO EARLY TUESDAY LEADING TO QUITE A LARGER COVERAGE
OF CONVECTION OVER THE ABOVE AREA. AREAL COVERAGE OF CONVECTION
WILL INCREASE TUESDAY AS THERE WILL BE A STRONGER PW SURGE WITH THE
REMNANT ENERGY FROM ODALYS. MEAN QPF IS CURRENTLY BETWEEN 1-2.5
INCHES WITH LOCALLY HIGHER AT 5-DAY LEADS, A TESTAMENT TO THE
THREAT IN QUESTION AS THERE WILL LIKELY BE MORE SIGNIFICANT
RAINFALL POTENTIAL WITH THIS SETUP.
..NEW ENGLAND
LINGERING RAINFALL EXPECTED AS THE COASTAL LOW BEGINS TO DRIFT
AWAY FROM THE REGION. THE CUMULATIVE EFFECT OF RAINFALL OVER THE
LONG PERIOD INTO THE URBAN CORRIDOR OF EASTERN MASSACHUSETTS WILL
CAUSE HYDROLOGIC CONCERNS, INCLUDING SOME FLOODING IN POOR DRAINAGE
LOCATIONS. A MARGINAL RISK AREA IS IN EFFECT FROM EASTERN
CONNECTICUT UP TO COAST TO SOUTHERN MAINE.
CAMPBELL/SANTORELLI
DAY 4 AND DAY 5
VALID 12Z TUE SEP 29 2026 - 12Z THU OCT 01 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHWEST AND SOUTHERN HIGH PLAINS TUESDAY AND TUESDAY NIGHT...
2030Z UPDATE: PER COLLABORATION WITH THE WFOS, THE BIGGEST CHANGE
WAS ADDING THE MODERATE RISK AREA ACROSS EASTERN AZ, MUCH OF NM,
AND INTO THE WESTERN PORTIONS OF THE SOUTHERN PLAINS. WHILE THE
GUIDANCE IS SLOWLY CONVERGING ON A SOLUTION, THERE IS STILL SPREAD
WITH THE LOCATION AND (ESPECIALLY) TIMING OF HEAVIEST RAIN AS
POLO'S REMNANT MOISTURE PLUME BECOMES ABSORBED BY THE DEEPENING SW
US UPPER TROUGH. TIMING ISSUES ARE MOST NOTABLE BY DAY 5 (12Z
WED-12Z THU), WITH MOST OF THE 12Z OPERATIONAL GLOBAL MODEL RUNS
AND BOTH AIGFS AND EC AIFS SLOWER (FARTHER WEST WITH THE HEAVIEST
QPF) THAN THE NBM AND ENSEMBLE MEAN QPFS. NOT WITHSTANDING THE
TIMING UNCERTAINTIES, THE LATEST ECMWF EFI CONTINUES TO SIGNAL A
POTENTIALLY SIGNIFICANT RAINFALL EVENT, PARTICULARLY ON TUESDAY
ACROSS EASTERN AZ AND WESTERN-CENTRAL NM.
HURLEY
PREVIOUS DISCUSSION BELOW...
THE EVENTUAL ABSORPTION OF TROPICAL CYCLONE POLO'S REMNANTS INTO
AN AMPLIFYING SOUTHWEST U.S. UPPER TROUGH HAVE LED TO A SLOWER AND
WETTER TREND ACROSS THE SOUTHWEST-FOUR CORNERS AND INTO THE HIGH
PLAINS BY MIDWEEK. THE LATEST SUITE OF GUIDANCE PERSISTS WITH THE
SPREADING OF TROPICALLY ENHANCED HEAVY RAINFALL FROM THE SOUTHWEST
INTO THE PLAINS OVERLAPPING SOME AREAS WITH HEAVY RAINFALL IN PRIOR
DAYS INCREASING THE THREAT FOR FLASH FLOODING PARTICULARLY OVER
SENSITIVE WASHES, BURN SCARS, SLOT CANYONS, AND AREAS OF HIGHER
TERRAIN. A BROAD SLIGHT RISK FOR TUESDAY SPANS FROM ARIZONA TO
TEXAS AND NORTHEAST TO IOWA/ILLINOIS AND FROM TEXAS TO
IOWA/ILLINOIS ON WEDNESDAY. THIS PART OF THE COUNTRY WILL CONTINUE
TO BE MONITORED FOR QPF/TRACK OF REMNANT ENERGIES TRENDS AS THEY
MAY IMPACT FUTURE UPGRADES OF THE RISK LEVELS.
CAMPBELL/HURLEY
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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