379
FXUS66 KSEW 152133
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
233 PM PDT TUE SEP 15 2026
SYNOPSIS
CLOUDY SKIES WILL CONTINUE TO SCATTER TODAY AS HIGH PRESSURE
PROMOTES A WARMING AND DRYING TREND THROUGH THURSDAY. NEAR
NORMAL TEMPERATURES AND PARTLY CLOUDY CONDITIONS ARE EXPECTED
TO RETURN THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK WITH
CONTINUED LOW LEVEL ONSHORE FLOW.
SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/
DRY NORTHERLY FLOW WILL CONTINUE TO FILL INTO WESTERN WASHINGTON
THIS AFTERNOON AS A POSITIVELY TILTED TROUGH SHIFTS SOUTH OF THE
AREA. THIS WILL ALLOW CLOUDS TO CONTINUE TO SCATTER OUT FOR MOST
LOWLAND AREAS, REVEALING CLEAR SKIES BY THIS EVENING.
TEMPERATURES ON THE COOLER SIDE WILL CONTINUE TODAY WITH HIGHS
GENERALLY IN THE MID 60S TO LOWER 70S.
A HIGH PRESSURE RIDGE WILL SLOWLY BUILD INTO THE REGION TONIGHT
THROUGH THURSDAY, BRINGING A WARMING AND DRYING TREND TO
WESTERN WASHINGTON. CLEAR SKIES TONIGHT COMBINED WITH RECENT
LIGHT RAIN AND LIGHT WINDS WILL ALLOW FOG TO FORM IN FOG PRONE
AREAS LIKE THE SOUTHERN INTERIOR EARLY WEDNESDAY MORNING,
BURNING OFF BY THE AFTERNOON. TEMPERATURES WILL WARM UP TO
NEAR-NORMAL WEDNESDAY UNDER MOSTLY CLEAR SKIES, WITH HIGHS
GENERALLY IN THE MID 70S. A THERMAL TROUGH BUILDING ALONG THE
COAST WILL SHIFT INLAND ON THURSDAY, INDUCING COOLING ALONG THE
COAST WITH EVEN WARMER TEMPERATURES FURTHER INLAND. TEMPERATURES
THURSDAY WILL BE THE WARMEST OF THE WEEK, WITH HIGHS IN THE MID
TO UPPER 70S AND AROUND 80 DEGREES FOR SOME AREAS ALONG THE
CASCADE FOOTHILLS AND VALLEYS AND OVER THE SOUTHERN INTERIOR.
LONG TERM /FRIDAY THROUGH TUESDAY/
HIGH PRESSURE OFFSHORE WILL CONTINUE TO INFLUENCE THE WEATHER
PATTERN OVER THE WEEKEND AND INTO EARLY NEXT WEEK, MAINTAINING
SEASONABLE TEMPERATURES AND DRY CONDITIONS. ONSHORE FLOW WILL
CONTINUE TO PROMOTE MORNING CLOUD COVER WITH PLENTY OF SUN
BREAKS IN THE AFTERNOONS. FORECAST MODELS CONTINUE TO HINT AT A
POTENTIAL PATTERN CHANGE EARLY NEXT WEEK, THOUGH DETERMINISTIC
MODELS CONTINUE TO PUSH BACK THE TIMING OF AN APPROACHING
TROUGH AS THE PACIFIC RIDGE HOLDS ON.
15
AVIATION
FLOW ALOFT WILL TURN NORTHERLY AS A TROUGH CONTINUES TO DIG
SOUTHWARD TONIGHT INTO WEDNESDAY. CEILINGS ARE BEGINNING TO LIFT AND
SCATTER THIS AFTERNOON AS SHOWER ACTIVITY MOVES EAST OF THE
CASCADES. REMAINING MVFR CEILING DECKS WILL LIFT UP TO VFR THROUGH
THE AFTERNOON AND EVENING AS CLOUDS CLEAR OUT. LIGHT WINDS OUT OF
THE NORTH AT 5 TO 10 KT WILL DECREASE TO UNDER 5 KT TONIGHT (CALM OR
LIGHT AND VARIABLE IN A FEW SPOTS). THERE REMAINS A 20-30% OF
IFR/LIFR CONDITIONS FOR A MAJORITY OF CENTRAL, AND SOUTH INTERIOR
TERMINALS AS WELL AS THE PACIFIC COAST WEDNESDAY MORNING BETWEEN 12
AND 18Z. THE FORECAST SOUNDINGS IN THE MORNING DEPICT WIDENING DEW
POINT DEPRESSIONS JUST ABOVE THE SURFACE. THE DRIER AIR ALOFT WILL
LIKELY LEAD TO AREAS OF PATCHY FOG IN THE MORNING (VERSUS WIDESPREAD
ADVECTION FOG), AND BE SHALLOW IN NATURE. CLEAR SKIES WILL TAKEOVER
THE IFR/LIFR CONDITIONS LATE WEDNESDAY MORNING/AFTERNOON. WINDS
WEDNESDAY WILL PICK UP OUT THE NORTH 5 TO 10 KT.
KSEA...VFR THROUGH THE EVENING/EARLY TONIGHT WITH NORTH WINDS 5 TO
10 KT DECREASING TO 5 KT OVERNIGHT. THERE REMAINS A 20-30% CHANCE OF
IFR CONDITIONS IN THE MORNING. MODEL GUIDANCE HAS 10-15% LIFR
PROBABILITIES OF VICINITY FOG TO THE NORTH AND SOUTH OF THE
TERMINAL. TIME PERIOD FOR FORMATION IS FROM 12-18Z, WITH CLEAR SKIES
WEDNESDAY AFTERNOON. WINDS INCREASE OUT OF THE NORTH WEDNESDAY 5-10
KT.
HPR
MARINE
A THERMAL TROUGH WILL BUILD LOCAL LOW PRESSURE ALONG THE COAST
WEDNESDAY INTO THURSDAY. THIS WILL CAUSE THE WINDS TO BRIEFLY TURN
OFFSHORE. ONCE THE THERMAL TROUGH MOVES INLAND ON FRIDAY, THE
ONSHORE FLOW WILL RETURN WITH WEAK SYSTEMS PASSING TO THE NORTH THIS
WEEKEND. THE LIKELIHOOD OF HAZARDOUS WINDS DURING THE NEXT SEVEN
DAYS OVER THE WATERS IS LOW, BUT A FEW PORTIONS OF THE COASTAL
WATERS AND THE STRAIT OF JUAN DE FUCA MAY SEE WIND GUSTS APPROACH 20
KT WITH THE WEAK SYSTEM PASSING THROUGH EARLY NEXT WEEK. SEAS WILL
HOLD AROUND 3-5 OR 4-6 FT THROUGH THE FORECAST PERIOD.
HPR
FIRE WEATHER
A BRIEF WARMING AND DRYING TREND WILL KICK OFF TODAY AND
CONTINUE INTO THURSDAY AS HIGH PRESSURE BUILDS OFFSHORE. SURFACE
WINDS WILL BRIEFLY TURN OFFSHORE ON WEDNESDAY AS A THERMAL
TROUGH BUILDS ALONG THE PACIFIC COAST. WEAK OFFSHORE FLOW WILL
CONTINUE THROUGH THURSDAY, CAUSING ADDITIONAL DRYING WITH
RELATIVE HUMIDITY VALUES BOTTOMING OUT IN THE MID 20S TO LOW
30S IN THE NORTH CASCADES AND LOWLANDS SOUTH OF THE PUGET
SOUND. THE THERMAL TROUGH WILL SHIFT INLAND ON THURSDAY AHEAD OF
THE RETURN OF ONSHORE FLOW, WHICH WILL MINIMIZE FIRE WEATHER
CONCERNS THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK.
15
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...NONE.
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