967
FXUS66 KSEW 200835
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
135 AM PDT MON OCT 20 2025
SYNOPSIS
A WEAK UPPER RIDGE WILL PRODUCE DRIER CONDITIONS FOR
MUCH OF WESTERN WASHINGTON INTO TUESDAY. ANOTHER FRONTAL SYSTEM
WILL REACH THE AREA ON WEDNESDAY WITH MAINLY LIGHT PRECIPITATION.
A DEEP UPPER TROUGH OVER THE GULF OF ALASKA WILL BRING A STRONGER
FRONTAL SYSTEM INTO THE REGION AT THE END OF THE WEEK WITH THE
POTENTIAL FOR SOME HEAVY RAINFALL. A TROUGH WILL SETTLE OVER THE
AREA BY THE WEEKEND FOR COOLER TEMPERATURES ALONG WITH PERIODS OF
RAIN SHOWERS AND MOUNTAIN SNOW.
SHORT TERM /TODAY THROUGH WEDNESDAY/
IT'S A QUIETER MORNING
ACROSS WESTERN WASHINGTON TODAY AS POST-FRONTAL ONSHORE FLOW
RELAXES AND A FEW RESIDUAL SHOWERS DISSIPATE OVER THE CASCADES.
A WEAK FRONT WILL BRUSH THE AREA LATE TODAY IN ZONAL FLOW ALOFT,
BUT MOST PRECIPITATION ASSOCIATED WITH IT WILL BE CONFINED TO THE
NORTHERN OLYMPIC PENINSULA AND AREAS NEAR THE CANADIAN BORDER.
SHORTWAVE UPPER RIDGING WILL PROVIDE DRY CONDITIONS AREAWIDE ON
TUESDAY WITH SOME AFTERNOON SUNSHINE POSSIBLE AFTER MORNING FOG
AND/OR STRATUS BREAKS UP. ANOTHER FRONT WILL REACH THE AREA ON
WEDNESDAY WITH SOME LIGHT PRECIP, BUT THIS WILL ONLY ACT AS A
PRECURSOR TO A CONSIDERABLY MORE ACTIVE PERIOD OF WEATHER FOR THE
SECOND HALF OF THE WEEK.
LONG TERM /THURSDAY THROUGH SUNDAY/
ENSEMBLE CLUSTER PLOTS AND
HEIGHT ANOMALIES ARE VIRTUALLY UNANIMOUS WITH A DEEP UPPER TROUGH
DIGGING SOUTHWARD OVER THE GULF OF ALASKA ON THURSDAY. THIS COULD
ACT TO TAP INTO SOME SUBTROPICAL MOISTURE AND FUNNEL SOME OF IT
TOWARD THE REGION. MODELS HAVE TRENDED A LITTLE SLOWER IN THE
LAST COUPLE RUNS WITH THE PERIOD OF HEAVIEST PRECIPITATION...PEGGING
IT ON FRIDAY WHICH LINES UP WITH WPC'S DAY 5 EXCESSIVE RAINFALL
OUTLOOK FOR THE OLYMPIC PENINSULA. IT'S WORTH NOTING THAT THE
MODELS REMAIN FAIRLY STEADFAST WITH BRINGING THE COOL UPPER
TROUGH ONSHORE BY SATURDAY. A LOOK AT MODEL INTEGRATED WATER VAPOR
TRANSPORT FROM BOTH THE GFS AND ECMWF SHOW THE HIGHEST VALUES
SHIFTING SOUTH OF THE CWA BY THE WEEKEND WHICH, AT LEAST FROM A
HYDROLOGIC STANDPOINT, SUGGESTS THAT THIS WILL A SHORT-DURATION
EVENT. DEEP UPPER TROUGHING IS EXPECTED TO SETTLE OVER WESTERN
WASHINGTON THIS WEEKEND WITH 500 MILLIBAR HEIGHTS FALLING TO
540-545 DECAMETERS AND SNOW LEVELS 3500 TO 4500 FEET. COMBINED
WITH STRONG WEST TO SOUTHWEST FLOW ALOFT, THERE IS POTENTIAL FOR
SOME GOOD EARLY SEASON SNOWFALL TOTALS IN THE CASCADES. 27
AVIATION
WEST TO NORTHWEST FLOW ALOFT WITH LARGELY ZONAL FLOW
OVER WESTERN WASHINGTON. MVFR CIGS ARE BEGINNING TO FILL IN ACROSS
PORTIONS OF THE INTERIOR. GUIDANCE DOESN'T SUGGEST CEILINGS WILL
LOWER TOO MUCH THROUGH THE NIGHT, THOUGH THERE COULD BE SOME BRIEF
PERIODS OF IFR BETWEEN 14-18Z THIS MORNING. FOG WILL BE POSSIBLE AT
PWT AND OLM FOR A BIT THIS MORNING, BUT CONFIDENCE IS LOWER WITH
CEILINGS ALREADY IN PLACE AT THESE LOCATIONS. WILL CONTINUE TO
MONITOR THE TRENDS IN CIGS THIS MORNING AND ADJUST APPROPRIATELY.
CEILINGS ARE EXPECTED TO LIFT AND SCATTER THIS MORNING AS MID AND
HIGH LEVEL CLOUDS FILL IN THIS AFTERNOON, ALLOWING CONDITIONS TO
RETURN TO VFR FOR THE REST OF TODAY AND INTO TONIGHT. WINDS SOUTH-
SOUTHEASTERLY 5 TO 10 MPH WILL REMAIN IN THAT RANGE THROUGH MOST OF
TODAY, THOUGH BECOMING MORE SOUTHWESTERLY THIS AFTERNOON. WINDS LOOK
TO BECOME LIGHT AND VARIABLE LATE TONIGHT.
KSEA...MVFR STRATUS HAS DEVELOPED THIS MORNING AND WILL REMAIN IN
PLACE THROUGH AROUND 18Z THIS MORNING. LOW (10-15%) CHANCE IN CIGS
LOWERING TO IFR AFTER 15Z. CIGS SHOULD LIFT AND SCATTER TO VFR WITH
HIGH CLOUDS FILLING IN THIS AFTERNOON THROUGH MUCH OF TONIGHT. SE
WINDS 4 TO 8 KT WILL BECOME SW THIS AFTERNOON BEFORE EASING LATE
TONIGHT. MODELS ARE BEGINNING TO HINT AT SOME LOW STRATUS/FOG
DEVELOPMENT EARLY TUESDAY MORNING.
62
MARINE
WINDS WILL CONTINUE TO EASE THIS MORNING ACROSS THE AREA
WATERS AS HIGH PRESSURE CONTINUES TO BUILD ACROSS THE NORTHEAST
PACIFIC. A FRONT WILL APPROACH THE COASTAL WATERS LATE MONDAY, BUT
WEAKEN AS IT TRAVERSES THE AREA MONDAY NIGHT INTO TUESDAY. WINDS
REMAIN LIGHT UNTIL WEDNESDAY WHEN A FRONT MOVES THROUGH THE AREA.
WINDS LOOK TO APPROACH SMALL CRAFT ADVISORY OVER THE COASTAL WATERS
WHILE THE INTERIOR WATERS LOOK TO REMAIN BELOW THRESHOLDS.
A MUCH STRONGER SYSTEM WILL BEGIN TO APPROACH THE AREA LATE WEEK.
SOUTH TO SOUTHEASTERLY WINDS WILL BEGIN TO INCREASE THURSDAY BEHIND
THE WARM FRONT, WITH THE STRONGEST WINDS THURSDAY NIGHT INTO FRIDAY
MORNING AS THE COLD FRONT TRAVERSES THE AREA. WINDS WILL LIKELY (60-
90% CHC) INCREASE TO GALE FORCE ACROSS THE COASTAL WATERS, AS WELL
AS THROUGH THE EAST STRAIT OF JUAN DE FUCA UP THROUGH THE NORTHERN
INLAND WATERS. LOCALIZED GUSTS UP TO STORM FORCE WILL BE POSSIBLE
LATE THURSDAY ACROSS THE COASTAL WATERS, WITH THE HIGHEST CONFIDENCE
IN STORM FORCE GUSTS BEING IN THE EAST STRAIT.
THE PATTERN GETS LESS CLEAR BEYOND FRIDAY INTO NEXT WEEKEND, BUT AN
EXTENDED PERIOD OF ELEVATED WINDS AND SEAS LOOKS PROBABLE DURING
THIS TIME WITH ADDITIONAL SYSTEMS MOVING THROUGH THE REGION.
SEAS STILL REMAIN IN THE 14-17 FT RANGE THIS MORNING BUT THE TREND
IN WAVE HEIGHTS SHOULD BE DOWNWARD TODAY WITH SEAS DROPPING BELOW 10
FT TONIGHT. LONG PERIOD SWELL FROM A SYSTEM IN THE GULF OF ALASKA
ARRIVES TO THE COASTAL WATERS ON TUESDAY, WITH SEAS RETURNING TO THE
14-17 FT RANGE WITH A DOMINANT PERIOD OF AROUND 17 SECONDS. SEAS
EASE TUESDAY NIGHT DOWN TO AROUND 9-11 FT BUT THERE WILL NOT BE MUCH
OF A BREAK UNTIL THE FRONTAL SYSTEM THURSDAY INTO FRIDAY BRINGS SEAS
BACK UP, PEAKING IN THE 18 TO 22 FT RANGE BY THURSDAY NIGHT. WAVES
WILL BE SLOW TO EASE, LIKELY REMAINING IN THE 12-18 FT RANGE THROUGH
NEXT WEEKEND, THOUGH CONSIDERABLE UNCERTAINTY EXISTS AFTER FRIDAY.
62
HYDROLOGY
A WET SYSTEM WILL ARRIVE ON FRIDAY. THIS SYSTEM
COULD TAP INTO SOME SUBTROPICAL MOISTURE BRINGING UP THE
POSSIBILITY OF HYDROLOGICALLY SIGNIFICANT RAINFALL OVER THE
OLYMPICS. QPF FORECASTS HAVE 2 TO 4 INCHES OF PRECIPITATION OVER
THE OLYMPICS 00Z FRIDAY TO 00Z SATURDAY WITH SNOW LEVELS FALLING
BELOW 6000 FEET FRIDAY AFTERNOON, KEEPING SOME OF THE WATER IN THE
OLYMPICS IN THE FORM OF SNOW INSTEAD OF RUNOFF. CURRENT FORECASTS
BRING THE SKOKOMISH RIVER NEAR ACTION STAGE BY LATE FRIDAY. 27
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...SMALL CRAFT ADVISORY UNTIL 6 PM PDT THIS EVENING FOR COASTAL
WATERS FROM CAPE FLATTERY TO JAMES ISLAND 10 TO 60 NM-
COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND OUT 10 NM-
COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE 10 TO
60 NM-COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE
OUT 10 NM-COASTAL WATERS FROM POINT GRENVILLE TO CAPE
SHOALWATER 10 TO 60 NM-COASTAL WATERS FROM POINT GRENVILLE
TO CAPE SHOALWATER OUT 10 NM-GRAYS HARBOR BAR-WEST ENTRANCE
U.S. WATERS STRAIT OF JUAN DE FUCA.
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