807
FXUS66 KSEW 100308
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
808 PM PDT WED SEP 9 2026
SYNOPSIS
UPPER LEVEL RIDGING WILL FLATTEN INTO THURSDAY AS
TROUGHING DEVELOPS OVER THE PACIFIC NORTHWEST LATE THIS WEEK. AN
UPPER LOW PROGRESSES ACROSS THE REGION OVER THE WEEKEND BRINGING
RAIN POTENTIAL AND COOLER TEMPERATURES. DRIER CONDITIONS ARE
EXPECTED ON MONDAY WITH A SLOW WARMING TREND NEXT WEEK.
SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/
THE FORECAST IS ON TRACK WITH NO UPDATES PLANNED THIS EVENING.
PREVIOUS DISCUSSION FOLLOWS WITH UPDATED AVIATION PORTION. 27
MOSTLY SUNNY CONDITIONS THIS AFTERNOON WITH HIGH CLOUDS OVERHEAD. AN
UPPER RIDGE OVER THE REGION WILL FLATTEN TONIGHT WITH TROUGHING
DEVELOPING INTO THURSDAY. ONSHORE FLOW INCREASES INTO THURSDAY
RESULTING IN COOLER TEMPERATURES AND INCREASED CLOUDS. WEAK
CONVERGENCE THURSDAY EVENING INTO THURSDAY NIGHT MAY RESULT IN AREAS
OF DRIZZLE OR LIGHT RAIN ACROSS PORTIONS OF THE CENTRAL CASCADES.
TROUGHING FURTHER DEEPENS FRIDAY AS AN UPPER LOW DEVELOPS OFFSHORE
OF VANCOUVER ISLAND. DRIZZLE MAY ALSO EXIST FRIDAY MORNING
ASSOCIATED WITH WIDESPREAD STRATUS, HOWEVER, FRIDAY IS EXPECTED TO
REMAIN PREDOMINANTLY DRY WITH THE UPPER LOW WELL OFFSHORE.
TEMPERATURES WILL PEAK THIS AFTERNOON IN THE 70S TO LOW 80S, WITH
HIGHS ONLY REACHING THE MID 60S TO LOW 70S BY FRIDAY.
LONG TERM /SATURDAY THROUGH WEDNESDAY/
THE UPPER LOW OFFSHORE WILL MOVE INLAND OVER WASHINGTON AND OREGON
ON SATURDAY. THERE REMAINS SOME UNCERTAINTY IN ENSEMBLES REGARDING
THE EXACT TRACK OF THE UPPER LOW, LEADING TO SOME SPREAD IN THE QPF
AMOUNTS, AS WELL AS THE TIMING OF RAINFALL. AT THIS TIME, RAINFALL
AMOUNTS ARE HEAVIEST SOUTH OF SNOHOMISH COUNTY, PARTICULARLY IN THE
SOUTHERN CASCADES, WITH QPF AMOUNTS AROUND 0.50" OVER MT. RAINIER
AREA ON THE NBM. IN ADDITION, THERE IS ALSO A 15 TO 25% PROBABILITY
OF THUNDERSTORMS SOUTH OF SNOHOMISH COUNTY SATURDAY AFTERNOON. THE
UPPER LOW CONTINUES TO DRIFT EASTWARD ON SUNDAY, WITH A SLOW DRYING
TREND DURING THE DAY. THE EXCEPTION BEING THE CASCADES WHERE
LINGERING MOISTURE IS EXPECTED AT LEAST THROUGH SUNDAY AM. AN UPPER
RIDGE BEGINS TO BUILD WELL OFFSHORE MONDAY FOR A SLOW WARMING TREND.
PREDOMINANTLY DRY CONDITIONS ARE THEN FORECAST INTO THE MIDDLE OF
NEXT WEEK, BUT WILL NEED TO MONITOR THE EXISTENCE OF ANY WEAK
TROUGHING AROUND THE TUESDAY TIMEFRAME AS INDICATED BY A SMALL
SUBSET OF ENSEMBLES. EITHER WAY, TEMPERATURES TREND BACK TO NEAR
NORMAL EARLY NEXT WEEK. JD
AVIATION
NORTHWESTERLY TO WESTERLY FLOW ALOFT THIS EVENING AND THROUGHOUT THE
DAY THURSDAY AS AN UPPER LEVEL TROUGH CONTINUES TO DEEPEN OVER THE
PACIFIC NW. SURFACE WINDS MAINLY NORTHERLY WITH SOME VARIATIONS TO
THE NORTHEAST OR NORTHWEST DEPENDING ON LOCATION. SPEEDS GENERALLY 4-
8 KTS, ALTHOUGH SOME TERMINALS ARE ALREADY SHOWING LIGHT AND
VARIABLE WINDS WITH SPEEDS LESS THAN 5 KTS. NORTHERLY WINDS RANGING
5-10 KTS EXPECTED TO RESUME AT MOST TERMINALS THURSDAY AFTERNOON AND
EVENING.
WIDESPREAD VFR CONDITIONS THIS EVENING WITH HIGH CLOUDS PRESENT OVER
THE NORTHERN HALF OF THE CWA AND CLEAR SKIES OVER THE SOUTHERN HALF.
MID AND LOWER CLOUDS EXPECTED TO FILL IN LATER TONIGHT AND INTO
THURSDAY MORNING THANKS TO AN ONSHORE MARINE PUSH, LEADING TO
WIDESPREAD MVFR CEILINGS FOR MOST TERMINALS BY 12Z THURSDAY MORNING
AND POTENTIALLY STICKING THROUGH 21Z. VFR CONDITIONS EXPECTED TO
RESUME AFTERWARD. THE COAST HAS A 30-40% CHANCE OF IFR CONDITIONS
WITH THE INITIAL STRATUS ONSET, AND A 20% CHANCE OF LIFR.
KSEA...VFR THROUGH TONIGHT, THEN A 40-50% CHANCE OF MVFR STRATUS
FROM 12Z THURSDAY MORNING THROUGH AS LATE AS 21Z. NORTH WINDS 4-8 KT
THIS EVENING AND TONIGHT WITH LIGHT AND VARIABLE WINDS POSSIBLE
EARLY THURSDAY MORNING. NORTHERLY WINDS 5-10 KTS EXPECTED TO RESUME
BY 20Z THURSDAY AFTERNOON AND CONTINUE FOR MUCH OF THE REMAINDER OF
THE TAF PERIOD.
18/HPR
MARINE
HIGH PRESSURE REMAINS IN THE COASTAL WATERS THROUGH
THURSDAY. AN UNSETTLED PATTERN WILL SETUP LATE THURSDAY THROUGH
THE WEEKEND AS A COUPLE OF FRONTAL SYSTEMS MOVE THROUGH THE
WATERS. THE FIRST FRONT WILL BE MOSTLY DRY (EXCEPT FOR A FEW
SHOWERS), BUT WILL PRODUCE GUSTY NORTH-NORTHWEST WINDS IN THE
OUTER COASTAL WATERS OVER 20 KT THURSDAY, AND ALSO GUSTY WEST
WINDS IN THE EASTERN STRAIT OF JUAN TONIGHT/THURSDAY MORNING AS
ONSHORE FLOW INCREASES. THE WEEKEND SYSTEM WILL PRODUCE MORE
WIDESPREAD PRECIPITATION OVER THE WATERS (PARTICULARLY ON
SATURDAY), WITH PERHAPS THE FAR SOUTHERN PUGET SOUND WATERS
HAVING A SLIGHT CHANCE (15-20%) FOR THUNDER WITH THE SHOWER
ACTIVITY. WIND IMPACTS ARE NOT ANTICIPATED AT THIS TIME WITH
THIS SYSTEM. HIGH PRESSURE WILL REBUILD OVER THE WATERS NEXT
WEEK, WITH CONDITIONS REMAINING BENIGN.
SEAS AT 4-6 FT TODAY WILL BUILD ON THURSDAY, REACHING 8-11 FT IN THE
OUTER COASTAL WATERS AND 6-8 FT FOR THE INNER WATERS. SEAS DECREASE
TO 4-6 FT FRIDAY/SATURDAY, AND 3-5 FT THIS WEEKEND INTO NEXT WEEK.
HPR
FIRE WEATHER
A DRYING AND WARMING TREND PEAKS TODAY LEADING
TO BRIEF ELEVATED FIRE WEATHER CONCERNS FOR THE CASCADES. HOWEVER,
THIS WILL BE SHORT LIVED, WITH ONSHORE FLOW RETURNING ON
THURSDAY AS A WEAK AND MOSTLY DRY FRONTAL SYSTEM CROSSES TO THE
NORTH. COOLER AND MORE MOIST CONDITIONS WILL RETURN OVER THE
WEEKEND, WITH PRECIPITATION POTENTIAL ON SATURDAY INTO SUNDAY.
JD/15
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...SMALL CRAFT ADVISORY FROM 11 PM THIS EVENING TO 11 AM PDT
THURSDAY FOR CENTRAL U.S. WATERS STRAIT OF JUAN DE FUCA-
EAST ENTRANCE U.S. WATERS STRAIT OF JUAN DE FUCA.
SMALL CRAFT ADVISORY FROM 5 AM THURSDAY TO 5 AM PDT FRIDAY FOR
COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND 10 TO
60 NM-COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE
10 TO 60 NM-COASTAL WATERS FROM POINT GRENVILLE TO CAPE
SHOALWATER 10 TO 60 NM.
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