875
FOUS30 KWBC 070057
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
857 PM EDT SUN SEP 6 2026
DAY 1
VALID 01Z MON SEP 07 2026 - 12Z MON SEP 07 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
IOWA...
..CORN BELT/UPPER MIDWEST
AFTERNOON CONVECTION PERSISTS THIS EVENING OVER IOWA NORTHWARD INTO
MINNESOTA, WITH 1-1.5"/HR RATES BUT OVER SOME AREAS THAT HAVE SEEN
SEVERAL INCHES OF RAIN THIS AFTERNOON. REMOVED THE TARGETED
MODERATE RISK GIVEN THE OVERALL DOWNWARD TREND IN
COVERAGE/INTENSITY BUT MAINTAINED A SLIGHT RISK AS THE SLOW
MOVEMENT COULD ALLOW FOR HIGHER AMOUNTS OVER SATURATED AREAS.
LATEST GUIDANCE SHOWS RAINFALL LIGHTENING AFTER A FEW MORE HOURS
AND WE HAVE KEPT THE MARGINAL IN TACT OVER THE BROADER REGION
GIVEN THE POTENTIAL FOR SOME HEAVIER RAIN, ESPECIALLY EARLY, TO
MOVE OVER AREAS THAT SAW HEAVIER RAIN EARLIER IN THE DAY, INCLUDING
CENTRAL MINNESOTA.
..NORTHERN MT
MAINTAINED THE MARGINAL RISK FOR MOST OF MONTANA IN ADVANCE OF THE
UPPER TROUGH OVER WA/OR. WELL ABOVE NORMAL MOISTURE REMAINS IN
PLACE, AND STILL EXPECT AN UPTICK IN RAIN COVERAGE OVERNIGHT AS THE
UPPER TROUGH APPROACHES AND UPPER DIFFLUENCE EXPANDS ATOP SURFACE
LOW PRESSURE IN SOUTHEASTERN MT AFTER 06Z.
..GREAT BASIN & NEW MEXICO
GENERALLY LIGHTER RAIN WITH LITTLE CONVECTION REMAINS OVER UTAH.
REMOVED THE SLIGHT RISK GIVEN LOWER RATES BUT WILL MAINTAIN THE
MARGINAL RISK FOR AT LEAST THE NEAR TERM AS SOME CONVECTION HAS
FIRED OVER THE REGION AS THE UPPER JET STREAK PUSHES THROUGH. OVER
NEW MEXICO, CONVECTION IS ON THE DOWNTURN BUT WILL MAINTAIN THE
MARGINAL RISK FOR THE REMAINING POTENTIAL.
..SOUTHEAST
REMOVED THE SLIGHT RISK OVER THE SOUTHEAST AS CONVECTION HAS
WEAKENED FROM EARLIER THIS AFTERNOON, BUT STILL PERSISTS AND SINKS
SOUTHWARD. SOME 1-2.5"/HR RATES HAVE BEEN OBSERVED BUT SHOULD
GENERALLY BE BELOW FFG VALUES THAT ARE >=2.5"/HR.
FRACASSO
DAY 2
VALID 12Z MON SEP 07 2026 - 12Z TUE SEP 08 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
NORTHERN MONTANA...
1930Z UPDATE...
NO UPGRADES WERE MADE THIS UPDATE WITH ADJUSTMENTS MADE PRIMARILY
THROUGH REVIEWING THE LATEST WPC QPF AND THE 12Z HREF/REFS
PROBABILISTIC GUIDANCE. THE BIGGEST CHANGE WAS TO EXPAND THE
MARGINAL RISK FROM CENTRAL MN ALL THE WAY TO THE MO RIVER AND
NORTHEAST NE. THIS INCLUDES PORTIONS OF NORTHWEST IA, WHERE SOILS
WILL BE MORE SENSITIVE FOLLOWING THE EXCESSIVE RAINFALL THAT
UNFOLDED ON SUNDAY. LATEST GUIDANCE SHOWS A FRONTAL BOUNDARY
DRIFTING EAST TOWARDS WESTERN IA COULD BE THE FOCUS FOR ADDITIONAL
THUNDERSTORMS ON LABOR DAY. 12Z HREF PROBABILITIES CURRENTLY SHOW
LOW CHANCES (15-30%) FOR POCKETS OF RAINFALL TOTALS >3", WHICH ON
TOP OF SATURATED SOILS COULD POSE A LOCALIZED FLASH FLOOD THREAT.
ELSEWHERE, THE MARGINAL RISKS IN CO AND THE INTERIOR SOUTHEAST WERE
SCALED BACK GIVEN RECENT DRIER TRENDS IN MODEL GUIDANCE. THE
INHERITED SLIGHT RISK IN NORTHERN MT WAS ADJUSTED TO ACCOUNT FOR
THE LATEST WPC QPF, BUT NO RISK UPGRADES WERE MADE.
MULLINAX
---PREVIOUS DISCUSSION---
..NORTHERN MONTANA
A DYNAMIC SETUP METEOROLOGICALLY WILL BE IN PLACE ACROSS MONTANA ON
MONDAY. A PLAINS LOW TRACKING NORTHWARD WILL SUPPORT ADVECTING
PLENTIFUL GULF MOISTURE WESTWARD AHEAD OF THE LOW INTO WESTERN
MONTANA. THIS WILL START SUNDAY NIGHT, BUT AS THE PLAINS LOW
STRENGTHENS, SO TOO WILL THE MOISTURE ADVECTION INTO MONDAY
MORNING. MEANWHILE A STRONG PAIR OF UPPER LEVEL SHORTWAVES WILL
ADVECT EAST ACROSS THE MOUNTAINS. THE FIRST WILL BE OVER WESTERN
MONTANA AT THE START OF THE PERIOD, THEN THE SECOND WILL FOLLOW
MONDAY NIGHT. THE FIRST WAVE INTERACTING WITH MOISTURE THAT WILL BE
OVER 3 SIGMA ABOVE NORMAL FOR THIS TIME OF YEAR WILL PROVIDE THE
BULK OF THE RAINFALL INTO WESTERN MONTANA. THIS FIRST SHORTWAVE
WILL MOVE NORTHEAST WITH THE PLAINS LOW, BUT WITH THE SECOND ONE
OFF TO THE WEST, THIS SECOND ONE WILL PROLONG LIGHT TO MODERATE
RAIN IN THIS REGION THROUGH THE REST OF THE DAY MONDAY AND INTO
MONDAY NIGHT. WHEN ADDED TO ANY RAINFALL THAT FALLS IN THIS AREA
SUNDAY NIGHT AT THE START OF THE STORM, MONDAY'S FORECAST OF A
BROAD 1-2 INCHES OF RAIN OVER THESE NORMALLY DRY AND FLOOD-PRONE
AREAS OF WESTERN MONTANA AND WHERE TERRAIN WILL LOCALLY ENHANCE THE
SEVERITY OF ANY FLASH FLOODING ALL POINTS TO THE SLIGHT RISK
UPGRADE BEING NEEDED FOR THIS DAY 2 PERIOD.
..MARGINAL RISKS
THE PLUME OF MOISTURE TO THE EAST OF THE LOW CENTER OVER NORTHERN
MINNESOTA COULD RESULT IN LOCALIZED FLASH FLOODING. HOWEVER, THE
HIGHLY PROGRESSIVE NATURE OF THE STORMS TOWARDS THE EAST SHOULD
LIMIT THE FLASH FLOODING POTENTIAL.
MONSOONAL MOISTURE CONTINUES INTO PORTIONS OF THE SOUTHWEST,
SPARKING A NEW ROUND OF SHOWERS AND STORMS MONDAY AFTERNOON,
PRIMARILY FOCUSED INTO WESTERN NEW MEXICO. ISOLATED INSTANCES OF
FLASH FLOODING ARE PROBABLE.
AFTERNOON SHOWERS AND STORMS ARE ALSO EXPECTED INTO PORTIONS OF THE
SOUTHEAST AGAIN ON MONDAY AFTERNOON. ONSHORE FLOW SHOULD SOMEWHAT
LIMIT THE INSTABILITY AT THE COAST. THE STALLED OUT FRONT SOME
50-100 MILES NORTH OF THE GULF COAST WILL PROVIDE FORCING FOR
STORMS, BUT ABUNDANT CLOUD COVER SHOULD LIMIT STORM INTENSITY. WITH
NORMALLY HIGH FFGS IN THIS AREA, A MARGINAL RISK APPEARED A GOOD
BALANCE OF THESE COMPETING FACTORS.
WEGMAN
DAY 3
VALID 12Z TUE SEP 08 2026 - 12Z WED SEP 09 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR NORTHERN
MONTANA, UPPER MISSISSIPPI VALLEY INTO THE UPPER GREAT LAKES, THE
SOUTHWEST, AND PORTIONS OF THE SOUTHEAST...
1930Z UPDATE...
AFTER REVIEWING 12Z GUIDANCE AND THE LATEST WPC QPF, ONLY MINOR
ADJUSTMENTS WERE MADE TO THE MARGINAL RISKS TO ACCOUNT FOR SMALL
CHANGES TO THE QPF FOOTPRINTS. NO UPGRADES WERE MADE THIS FORECAST
CYCLE.
MULLINAX
---PREVIOUS DISCUSSION---
..MARGINAL RISKS
LINGERING MOISTURE FROM THE DEPARTING LOW INTO CANADA COULD ALLOW
FOR A CONTINUATION OF FLOODING THAT LIKELY STARTS ON MONDAY INTO
THE DAY ON TUESDAY. ANY NEW INSTANCES OF FLASH FLOODING ON TUESDAY
SHOULD REMAIN ISOLATED.
MONSOONAL MOISTURE IN THE SOUTHWEST MAY DIMINISH SOMEWHAT FROM
PREVIOUS DAYS, BUT ENOUGH SHOULD STILL BE AROUND FOR WIDELY
SCATTERED SHOWER AND THUNDERSTORM DEVELOPMENT FROM SOUTHERN ARIZONA
INTO SOUTHWESTERN COLORADO. AN ISOLATED INSTANCE OF FLASH FLOODING
OR 2 ARE POSSIBLE.
THE TRAILING COLD FRONT BEHIND THE LOW MOVING INTO CANADA WILL SLOW
ITS FORWARD MOVEMENT FROM EASTERN NEBRASKA INTO THE UPPER GREAT
LAKES. AFTERNOON AND OVERNIGHT THUNDERSTORMS AHEAD OF THE FRONT MAY
BACKBUILD AND TRAIN IN SOME AREAS. ISOLATED FLASH FLOODING IS
POSSIBLE WHERE BACKBUILDING AND TRAINING OCCUR OVER FLOOD-PRONE
AREAS.
NUMEROUS SHOWERS AND THUNDERSTORMS WILL IMPACT THE GULF COAST AND
THE FLORIDA PENINSULA ON TUESDAY. DUE TO HIGH FFGS AND GENERALLY
LOW STORM ORGANIZATION, ONCE AGAIN ISOLATED INSTANCES OF FLASH
FLOODING WILL BE POSSIBLE, BUT NOTHING OUT OF THE ORDINARY IS
EXPECTED.
WEGMAN
DAY 4 AND DAY 5
VALID 12Z WED SEP 09 2026 - 12Z FRI SEP 11 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS CENTRAL
TO SOUTHERN CALIFORNIA, THE SOUTHWEST, THE CENTRAL PLAINS TO THE
LOWER MISSISSIPPI VALLEY, THE UPPER OH VALLEY/CENTRAL APPALACHIANS
AND NORTHERN NY STATE INTO NORTHERN VT...
..SOUTHWEST
PW VALUES WILL REMAIN VERY ANOMALOUS BOTH DAY 4 AND 5 ACROSS
CENTRAL TO SOUTHERN CALIFORNIA, THE SOUTHERN GREAT BASIN AND
SOUTHWEST FROM TROPICAL MOISTURE BEING DRAWN NORTHEASTWARD BY THE
CLOSED LOW AMPLIFYING DAYS 2 AND 3 WELL OFF THE CA COAST. LOW
CONFIDENCE IN ANY PRECIP IN THIS ANOMALOUS PW AXIS, 2-3+ STANDARD
DEVIATIONS ABOVE THE MEAN, BUT CONTINUED POTENTIAL FOR LOCALIZED
HEAVY RAINS AND ISOLATED RUNOFF ISSUES ACROSS THESE REGIONS.
TRIMMED A BIT ON THE NORTHERN EDGE TO REMOVE SOUTHERN NV PER A LACK
OF ANY QPF IN NEARLY ALL GUIDANCE.
..CENTRAL PLAINS
LOWER MISSOURI VALLEY INTO THE MID MISSISSIPPI
VALLEY...
A SURFACE FRONT IS EXPECTED TO MOVE SLOWLY SOUTHWARD DAY 4 THROUGH
THE CENTRAL PLAINS, LOWER MO VALLEY, MID MS VALLEY REGION,
ORIENTED IN A GENERAL WEST TO EAST DIRECTION. FAVORABLE RIGHT
ENTRANCE REGION JET DYNAMICS MAY ENHANCE UVVS ALONG THIS FRONT IN
AN AXIS OF 1.75-2.00" PW VALUES, 2-2.5+ STANDARD DEVIATIONS ABOVE
THE MEAN, SUPPORTING THE POTENTIAL FOR ORGANIZED FRONTAL/PRE-
FRONTAL CONVECTION. THERE MAY BE A PERIOD OF TRAINING OF CELLS
PARALLEL TO THE FRONT AS THE H85 FLOW BECOME INCREASINGLY WESTERLY
AND PARALLEL TO THE BOUNDARY. THIS COULD EXTEND INTO OHIO (WHERE
FFG VALUES DECREASE) PER SOME 12Z GUIDANCE.
..NORTHERN NEW YORK STATE INTO NORTHERN NEW ENGLAND
THE NORTHERN AND EASTERN EXTENSION OF THE FRONT OVER THE CENTRAL
PLAINS TO MID MS VALLEY REGION IS EXPECTED TO REMAIN PROGRESSIVE
DAY 4 AS IT PUSHES FROM THE GREAT LAKES INTO THE NORTHEAST. THERE
IS THE POTENTIAL FOR ORGANIZED FRONTAL CONVECTION IN AN AXIS OF PW
VALUES 1.75-2" MOVING EASTWARD ACROSS NY STATE INTO NEW ENGLAND. THE
VERY PROGRESSIVE NATURE OF THE FRONT WILL BE A DETRIMENT TO ANY
WIDESPREAD RUNOFF ISSUES, BUT WITH SATURATED/MOIST ADIABATIC
SOUNDINGS IN A DEEP LAYER OF THE ATMOSPHERE, CANNOT RULE OUT AN
ISOLATED RUNOFF ISSUE.
..UPPER OHIO VALLEY INTO THE CENTRAL APPALACHIANS
A MARGINAL RISK AREA IS DEPICTED DAY 5 FOR POTENTIAL HEAVY RAINS
ALONG THE FRONT DROPPING SOUTH THROUGH THE UPPER OH VALLEY/CENTRAL
APPALACHIANS AREA THURSDAY. WEST SOUTHWESTERLY H85 FLOW IN AN AXIS
OF 1.75-2.00" PW VALUES WILL SUPPORT LOCALLY HEAVY RAINS ALONG THE
FRONT, WITH SOME OROGRAPHIC ENHANCEMENT OF THE UVVS INTO THE
CENTRAL APPALACHIANS. WITH A BROADER FOOTPRINT OF HIGHER QPF IN THE
12Z GUIDANCE, EXPANDED THE MARGINAL RISK TO THE SOUTHWEST. NOTE
THAT LOWER FFG VALUES (~1"/HR) EXIST CURRENTLY OVER MOST OF
WV/SOUTHEAST OH/SOUTHWEST PA.
FRACASSO/ORAVEC
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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