941
FXUS66 KSEW 201555
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
855 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/
NO CHANGES MADE TO THE
FORECAST THIS MORNING. THE PREVIOUS DISCUSSION CAN BE BE FOUND
BELOW ALONG WITH UPDATES TO THE AVIATION DISCUSSION:
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 BE 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
WESTERN WA SITS UNDER WEAK ZONAL FLOW (WEST-NORTHWEST).
THIS WILL WEAKEN GOING INTO TONIGHT/TUESDAY AS A WEAK RIDGE BUILDS
OVER SURFACE HIGH PRESSURE OVER THE REGION. THIS MORNING: AREAS OF
DRIZZLE CONTINUE THIS MORNING POST-FRONT (A CONVERGENCE AREA IS
PUSHING NORTH FROM KING INTO SNOHOMISH COUNTY). LOWER CIGS/VIS ARE
ASSOCIATED WITH THIS AREA OF DRIZZLE (WILL PRIMARILY MOVE UP AND
AFFECT KPAE THIS MORNING THROUGH 18-20Z). SOUTH OF THIS AREA IN
PUGET SOUND, CIGS WILL REMAIN MVFR THROUGH 17-19Z, FOG/LOW VIS
CONTINUES OVER THE KITSAP AREA THROUGH 17-18Z. ALL TERMINALS WILL
IMPROVE TO VFR LATE MORNING/AFTERNOON AS MID LEVEL CLOUDS MOVE IN.
COASTAL/SOUTH INTERIOR TERMINALS WILL HAVE BEST CHANCE OF SEEING
SHOWERS IN THE AFTERNOON (MAY SEE CIGS/VIS DIP BACK TO MVFR BRIEFLY
WITH THE PRECIP). THERE'S HIGH CONFIDENCE THAT IFR TO LIFR CIGS/VIS
WILL FILL BACK IN (FROM THE SOUTHWEST INTO MUCH OF THE INTERIOR)
TUESDAY MORNING (BEGINNING AS EARLY AS 10-13Z THROUGH 18-21Z). WINDS
TODAY (AT THE SURFACE) ARE AT THE SOUTHWEST 4-8 KT - WILL DROP TO
UNDER 5 KT TONIGHT (SOME AREAS WILL SEE LIGHT AND VARIABLE) INTO
TUESDAY MORNING BEFORE TURNING TO THE NORTH LATE TUESDAY AFTERNOON.
KSEA...MVFR STRATUS WILL CONTINUE THIS MORNING, WITH IMPROVEMENTS TO
VFR EXPECTED 17-19Z (CLOUDS WILL REMAIN BROKEN/OVERCAST IN THE
AFTERNOON/EVENING). NBM/HREF BOTH HAVE A ROUGHLY 40-50% CHANCE OF
IFR CIGS TUESDAY MORNING (WITH A 20-30% CHANCE OF LIFR CIGS) -
THERE'S SLIGHTLY LOWER CONFIDENCE INTO HOW FAR THE LOWER VIS WILL
MAKE IT FROM THE SOUTHWEST IN THE MORNING (BUT GIVEN THE LIGHT
WINDS/WEAK GRADIENTS, WILL BE FORECASTING ON THE LOWER SIDE TUESDAY
MORNING FOR THE 18Z PACKAGE). WINDS TODAY ARE OUT OF THE SOUTHWEST 4-
8 KT - WILL DROP BELOW 5 KT TONIGHT (POSSIBLY LIGHT AND VARIABLE)
WITH THE TIMEFRAME FOR LOW CIGS/VIS ROUGHLY 14-18Z. SHIFT TO THE
NORTH (UNDER 5 KT) IS EXPECTED ~18Z TUESDAY.
HPR
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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