302
FXUS02 KWBC 251945
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
345 PM EDT THU SEP 25 2025
VALID 12Z SUN SEP 28 2025 - 12Z THU OCT 02 2025
..HEAVY RAIN THREAT CONTINUES THROUGH SUNDAY FOR THE SOUTHWEST
...TROPICAL DISTURBANCE MAY BRING TROPICAL IMPACTS TO THE SOUTHEAST
AND MID-ATLANTIC NEXT WEEK...
..OVERVIEW
AN UPPER TROUGH/LOW WILL REMAIN STALLED OVER THE SOUTHWEST THROUGH
SUNDAY, BLOCKED BY STRONG RIDGING DOWNSTREAM. A DEEP UPPER LOW AND
TROUGH WILL MOVE TOWARDS THE WEST COAST EARLY NEXT WEEK, LIFTING
THE UPPER LOW OUT OF THE SOUTHWEST AND INCREASING PRECIPITATION
CHANCES ACROSS THE WEST. THE CENTRAL U.S. WILL REMAIN MOSTLY DRY
THROUGH THE PERIOD UNDER THE INFLUENCE OF HIGH PRESSURE RIDGING,
BUT THE EASTERN U.S. MAY BE ENTERING A WETTER PATTERN. A COLD FRONT
WILL MOVE ACROSS THE NORTHEAST SUNDAY INTO MONDAY WHILE A
STATIONARY FRONT LINGERS ALONG THE SOUTHEAST AND MID-ATLANTIC
COASTS. THERE MAY BE A CHANCE FOR HEAVY RAINFALL AND FLASH FLOODING
OVER PORTIONS OF THE SOUTHEAST AND MID-ATLANTIC MONDAY THROUGH
WEDNESDAY, BUT THAT WILL DEPEND ON THE EVOLUTION OF A LIKELY
TROPICAL LOW FORMING NEAR THE BAHAMAS OVER THE NEXT FEW DAYS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT ON THE LARGE SCALE
SYNOPTIC PATTERN THROUGH THE MEDIUM RANGE PERIOD, WITH SOME
DIFFERENCES OVER THE EAST AND WEST COASTS.
THE MOST IMPACTFUL ARE OF UNCERTAINTY WILL BE OVER THE EASTERN
U.S. WHERE A TROPICAL LOW MAY CREATE HAZARDOUS WEATHER FOR THE
CAROLINAS AND THE MID-ATLANTIC NEXT WEEK. MOST GUIDANCE HAS
CLUSTERED AROUND TWO SOLUTIONS, EITHER BRINGING THE TROPICAL LOW
NORTH INTO THE CAROLINAS OR STALLING OFFSHORE AND BEING SWEPT OUT
INTO THE ATLANTIC. THE EVOLUTION WILL ULTIMATELY BE DETERMINED BY
INTERACTIONS BETWEEN THE TROPICAL LOW, HURRICANE HUMBERTO TO THE
EAST, A FRONTAL BOUNDARY TO THE NORTH, AND AN UPPER TROUGH OVER THE
EAST. WPCS AFTERNOON FORECAST REFLECTS THE SOLUTION THAT BRINGS
THE LOW INTO THE CAROLINAS MONDAY INTO TUESDAY, WHICH IS IN LINE
WITH THE NATIONAL HURRICANE CENTER'S (NHC) APPROACH TO THE SYSTEM
AS WELL. THIS WAS A LARGE SHIFT FROM THE OVERNIGHT FORECAST,
RESULTING IN SIGNIFICANTLY HIGHER PRECIPITATION TOTALS OVER THE
MID-ATLANTIC AND SOUTHEAST. PLEASE REFER TO THE NHC FOR THE LATEST
INFORMATION ON THIS SYSTEM. THERE IS STILL A LARGE AMOUNT OF
UNCERTAINTY SURROUNDING THE EVOLUTION AND TRACK OF THIS LOW, AND
THE FORECAST WILL LIKELY CHANGE OVER THE NEXT FEW DAYS.
FOR THE WEST, THE UNCERTAINTY COMES FROM TIMING AND EVOLUTION
DIFFERENCES OF LOW PRESSURE SYSTEMS MOVING ONSHORE FROM THE
PACIFIC. THIS UNCERTAINTY WILL AFFECT PRECIPITATION LOCATION AND
AMOUNTS NEAR THE COAST, ESPECIALLY MONDAY INTO TUESDAY WHEN LOCALLY
HEAVY PRECIPITATION WILL BE POSSIBLE.
WPC'S AFTERNOON FORECAST FOR SUNDAY THROUGH TUESDAY WAS BASED ON A
BLEND OF THE 00Z ECMWF/CMC/UKMET AND 06Z GFS. GIVEN RELATIVELY GOOD
AGREEMENT, THIS PROVIDED A GOOD MIDDLE GROUND STARTING POINT. FOR
WEDNESDAY AND THURSDAY, ENSEMBLE MEANS FROM THE 06Z GEFS AND 00Z
ECENS WERE ADDED IN INCREASING AMOUNTS TO SMOOTH OUT MODEL
DIFFERENCES.
..WEATHER/HAZARDS HIGHLIGHTS
A FRONTAL BOUNDARY LINGERING ALONG THE EAST COAST WILL SUPPORT
DAILY SHOWERS AND STORMS, ESPECIALLY FOR THE MID-ATLANTIC STATES,
THROUGH EARLY NEXT WEEK. ANOMALOUS MOISTURE WITH A POSSIBLE
TROPICAL CONNECTION WILL MAINTAIN AT LEAST A LOCALIZED HEAVY
RAINFALL THREAT FOR THE MID-ATLANTIC/CAROLINAS AND PARTS OF THE
APPALACHIANS. THERE WILL BE A MARGINAL RISK OF EXCESSIVE RAINFALL
OVER EASTERN VIRGINIA, NORTH CAROLINA, AND SOUTH CAROLINA ON
SUNDAY, AND ANOTHER MARGINAL RISK FOR MOST OF THE MID-ATLANTIC AND
CAROLINAS ON MONDAY. WITH INCREASING POTENTIAL FOR TROPICAL
MOISTURE LEADING TO ENHANCED PRECIPITATION IN THE CAROLINAS ON
MONDAY, A SLIGHT RISK OF EXCESSIVE RAINFALL WAS INTRODUCED FOR
PORTIONS OF THE CAROLINA COASTS. THE THREAT MAY CONTINUE BEYOND
MONDAY DEPENDING ON THE EVOLUTION OF THE TROPICAL SYSTEMS OFF THE
SOUTHEAST COAST, AND THERE ARE PIECES OF GUIDANCE SUGGESTING
SIGNIFICANTLY HIGHER RAINFALL AMOUNTS WILL BE POSSIBLE. THE
RAINFALL FORECAST WILL LIKELY CHANGE OVER THE NEXT FEW DAYS.
REGARDLESS OF HOW THE RAINFALL FORECAST TURNS OUT, THE SOUTHEAST
COAST WILL LIKELY EXPERIENCE DANGEROUS SURF AND RIP CURRENT
CONDITIONS EARLY NEXT WEEK, AND SOME AREAS MAY EXPERIENCE INCREASED
WINDS.
TO THE WEST, PACIFIC MOISTURE AHEAD OF AN UPPER LOW OVER SOUTHERN
CALIFORNIA WILL RESULT IN RAINFALL CONTINUING INTO SUNDAY, THOUGH
WITH LESSER INTENSITY THAN THE SHORT RANGE. ANOMALOUS MOISTURE AND
SUFFICIENT INSTABILITY WILL CONTINUE TO SUPPORT AT LEAST A
LOCALIZED FLASH FLOOD THREAT SUNDAY INTO MONDAY, PARTICULARLY FOR
VULNERABLE BURN SCARS. THIS REGION REMAINS HIGHLIGHTED WITH A
MARGINAL RISK OF EXCESSIVE RAINFALL FOR BOTH SUNDAY AND MONDAY, WITH
A MORE TARGETED SLIGHT RISK ON SUNDAY ACROSS CENTRAL NEW MEXICO
WHERE QPF AMOUNTS ARE GREATEST.
PRECIPITATION WILL INCREASE BY NEXT WEEK ACROSS THE NORTHWEST U.S.
ASSOCIATED WITH AMPLIFIED TROUGHING OVER THE REGION. WIDESPREAD
RAIN IS EXPECTED ACROSS THE PACIFIC NORTHWEST AND INTO THE NORTHERN
GREAT BASIN/ROCKIES. THE HEAVIEST PRECIPITATION IS EXPECTED FROM
WESTERN WASHINGTON TO NORTHERN CALIFORNIA AND THE SIERRA NEVADA. A
MARGINAL RISK OF EXCESSIVE RAINFALL WAS ADDED FOR MONDAY FOR
NORTHERN CALIFORNIA. SOME SNOW MAY FALL IN THE HIGHEST ELEVATIONS
OF THE WEST AS WELL.
WARMER THAN AVERAGE TEMPERATURES WILL SHIFT INTO THE NORTHERN
PLAINS AND MAINTAIN AS UPPER RIDGING BUILDS OVER THE REGION NEXT
WEEK. DAYTIME HIGHS COULD BE 10-20 DEGREES ABOVE NORMAL FOR PARTS
OF THE UPPER MIDWEST. BELOW NORMAL TEMPERATURES OVER THE SOUTHWEST
WILL EXPAND NORTHWARD ALONG MUCH OF THE WEST COAST BY NEXT WEEK.
DOLAN/SANTORELLI
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,
QUANTITATIVE PRECIPITATION FORECAST (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=ERO
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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