170
FXUS02 KWBC 250650
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
250 AM 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,
WITH VERY UNCERTAIN TROPICAL IMPACTS ACROSS PARTS OF THE SOUTHEAST
NEXT WEEK...
..OVERVIEW
AN UPPER TROUGH/LOW OVER THE SOUTHEAST WILL BE SLOW TO DISLODGE
THROUGH MUCH OF THE PERIOD DUE TO A LARGE BERMUDA HIGH ANCHORED
OVER THE ATLANTIC. NORTHERN STREAM SHORTWAVES NEAR THE CANADIAN
BORDER WILL BE MUCH MORE PROGRESSIVE. A PERSISTENT UPPER LOW NEAR
THE SOUTHWEST WILL FINALLY LIFT NORTH AND WEAKEN SUNDAY AND BEYOND
IN RESPONSE TO AMPLIFIED TROUGHING ACROSS THE WEST. HEAVY RAINFALL
THREATS FOR THE MEDIUM RANGE PERIOD WILL INITIALLY FOCUS ACROSS THE
SOUTHWEST AND SOUTHERN MID-ATLANTIC, BUT INCREASING PRECIPITATION
IS EXPECTED BY NEXT WEEK ACROSS THE WEST, WITH SOME SNOW IN THE
HIGHER ELEVATIONS. LATEST GUIDANCE CONTINUES TO SHOW SIGNIFICANT
UNCERTAINTY REGARDING THE TRACKS OF NEWLY FORMED HUMBERTO AND
ANOTHER LIKELY TROPICAL LOW TO ITS WEST NEAR THE BAHAMAS AND WHAT
IMPACTS THIS WILL HAVE ON PARTS OF THE SOUTHEAST OR MID-ATLANTIC
COASTS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE LATEST GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT ON THE LARGE
SCALE SYNOPTIC SET UP THROUGH AT LEAST THE FIRST HALF OF THE
PERIOD, BUT STILL WITH SOME UNCERTAINTY IN THE DETAILS. THE BIGGEST
AND MOST IMPACTFUL AREA OF UNCERTAINTY REMAINS THE SOUTHEAST AND
WESTERN ATLANTIC AS NEWLY FORMED HUMBERTO AND ANOTHER LIKELY
TROPICAL SYSTEM TO ITS WEST ROUND THE WEST SIDE OF THE BERMUDA
HIGH. GUIDANCE SHOWS GOOD AGREEMENT THAT HUMBERTO SHOULD CURVE AWAY
FROM THE EAST COAST, BUT IT'S THE SECOND LIKELY TROPICAL THAT
FORMS TO ITS WEST NEAR THE BAHAMAS THAT NEEDS TO BE CLOSELY
MONITORED. WITH THIS SYSTEM, GUIDANCE HAS SHOWN SIGNIFICANT
UNCERTAINTY ON HOW IT DOES OR DOES NOT INTERACT WITH HUMBERTO AND
HOW CLOSE IT COMES TO THE COAST. THERE IS A LOT OF RUN TO RUN
INCONSISTENCIES IN THE GUIDANCE RANGING FROM A RECURVATURE AWAY
FROM THE COAST TO A LANDFALL IN THE CAROLINAS. THE WPC FORECAST
TONIGHT PREFERRED TO STAY CLOSE TO CONTINUITY UNTIL THERE IS A
BETTER CONSENSUS ON THIS SYSTEM. REFER TO THE NATIONAL HURRICANE
CENTER FOR THE LATEST ON BOTH OF THESE SYSTEMS.
OUT WEST, THE GUIDANCE SHOWS GOOD AGREEMENT THAT AN UPPER LOW OVER
THE SOUTHWEST WILL WEAKEN AND LIFT TO THE NORTHEAST AS IT'S
ABSORBED INTO THE LARGER SCALE TROUGHING MOVING INTO THE WEST
COAST. THERE REMAINS SOME TIMING UNCERTAINTIES ON HOW QUICKLY THIS
HAPPENS. THE MODELS SHOW GOOD AGREEMENT ON AN AMPLIFIED TROUGH
MOVING INTO THE WEST COAST AROUND TUESDAY OF NEXT WEEK, BUT A LOT
OF UNCERTAINTY STILL ON THE TIMING AND EJECTION OF ENERGY OUT OF
THIS TROUGH INTO THE INTERIOR WEST WHICH HAS SIGNIFICANT
IMPLICATIONS FOR DISTRIBUTION OF QPF ACROSS THE REGION.
THE WPC FORECASTS TONIGHT WERE BASED ON A NON-UKMET BLEND (USING
YESTERDAYS 12Z/18Z GUIDANCE) THROUGH DAY 5 (THE UKMET WAS SLOWER
WITH THE POSSIBLE TROPICAL NORTH OF THE BAHAMAS). AFTER DAY 5,
INCREASED WEIGHTING OF THE ENSEMBLE MEANS TO HELP WITH LATE PERIOD
UNCERTAINTY ACROSS THE WEST AND ALSO IN THE SOUTHEAST WITH THE
LINGER UPPER LOW/ENERGY AND INTERACTIONS WITH THE TROPICAL SYSTEMS.
THIS OVERALL MAINTAINS GOOD CONTINUITY WITH THE PREVIOUS SHIFT.
..WEATHER/HAZARDS HIGHLIGHTS
A FRONTAL BOUNDARY LINGERING ALONG THE EAST COAST WILL SUPPORT
DAILY SHOWERS AND STORMS ESPECIALLY FOR THE MID-ATLANTIC STATES
INTO NEXT WEEK. ANOMALOUS MOISTURE WITH A POSSIBLE TROPICAL
CONNECTION WILL MAINTAIN AT LEAST A LOCALIZED HEAVY RAINFALL THREAT
FOR THE SOUTHERN MID-ATLANTIC/CAROLINAS AND PARTS OF THE
APPALACHIANS. MARGINAL RISK AREAS ARE MAINTAINED ON THE DAYS 4 AND
5 (SUNDAY AND MONDAY) EXCESSIVE RAINFALL OUTLOOKS FOR THIS REGION
FOR NOW GIVEN SIGNIFICANT UNCERTAINTY IN QPF AMOUNTS. 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 SIGNIFICANT RAINFALL AMOUNTS SO ADJUSTMENTS TO
THE CURRENT QPF FORECAST WILL BE NEEDED. AT THE VERY LEAST, MUCH OF
THE EAST COAST CAN EXPECT AN INCREASED RISK OF RIP CURRENTS AND
HIGH SURF ASSOCIATED WITH THESE SYSTEMS NEXT WEEK.
MEANWHILE, 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 FOR BOTH SUNDAY AND MONDAY EROS WITH A MORE TARGETED
SLIGHT RISK ON DAY 4 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 WAS ADDED TO THE DAY 5/MONDAY ERO TONIGHT 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.
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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