185
FXUS66 KSEW 201633
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
933 AM PDT THU AUG 20 2026
SYNOPSIS
WEAK UPPER LEVEL RIDGE WITH LIGHT ONSHORE FLOW IN
THE LOWER LEVELS TODAY. UPPER LEVEL LOW APPROACHING FROM THE
SOUTHWEST FRIDAY WILL MOVE INTO THE COASTAL WATERS SATURDAY AND
INTO SOUTHERN BRITISH COLUMBIA EARLY SUNDAY MORNING. SYSTEM
SPINNING OUT OF LOW MOVING THROUGH SATURDAY WITH SHOWERS AND
POSSIBLE THUNDERSTORMS. UPPER LEVEL RIDGE REBUILDING SUNDAY WILL
MOVE EAST MONDAY. WEAK SYSTEM MOVING THROUGH LATE TUESDAY NIGHT
AS AN UPPER LEVEL TROUGH DEVELOPS WELL OFF THE COAST.
SHORT TERM /TODAY THROUGH SATURDAY/
THE CURRENT FORECAST
REMAINS ON TRACK WITH NO PLANNED UPDATES THIS MORNING. 33
PREVIOUS DISCUSSION...SATELLITE IMAGERY SHOWS STRATUS ALONG THE
COAST INTO THE LOWER CHEHALIS VALLEY AND ABOUT HALF WAY DOWN
THE STRAIT OF JUAN DE FUCA. CLEAR SKIES ELSEWHERE. TEMPERATURES
AT 3 AM/10Z WERE IN THE 50S TO MID 60S.
SLIGHT CHANGE IN THE PATTERN TODAY. A LITTLE BIT WEAKER ONSHORE
FLOW HAS RESULTED IN A SHALLOWER MARINE LAYER. STRATUS WAY
BEHIND SCHEDULE VERSUS THE OTHER MORNINGS IN THE LAST WEEK
GETTING INTO SHELTON ABOUT TWO HOURS LATER THAN THE LAST FEW
DAYS. WITH THE WEAKER ONSHORE GRADIENTS STRATUS NOT GETTING EAST
OF PUGET SOUND THIS MORNING. A LITTLE MORE SUNSHINE FOR THE
INTERIOR WILL PUSH HIGH TEMPERATURES UP A COUPLE OF DEGREES WITH
HIGHS IN THE MID 70S TO MID 80S. STRATUS RETREATING OFF THE
COASTLINE THIS AFTERNOON WARMING HIGHS ALONG THE COAST INTO THE
MID 60S TO MID 70S.
CHANGES IN THE PATTERN STARTING TONIGHT. UPPER LEVEL LOW WELL TO
THE SOUTHWEST BEGINNING TO MOVE NORTHEAST. THIS IN TURN WILL
BUILD THE UPPER LEVEL RIDGE OUT AHEAD OF THE LOW OVER THE
PACIFIC NORTHWEST. FLOW ALOFT GOING SOUTHERLY TONIGHT AND
REMAINING SOUTHERLY FRIDAY AS THE LOW CONTINUES TO SLOWLY MOVE
NORTHEAST. LIGHT FLOW IN THE LOWER LEVELS WILL KEEP THE STRATUS
WEST OF PUGET SOUND AGAIN FRIDAY MORNING BUT WITH THE SOUTHERLY
FLOW ALOFT THERE WILL BE PLENTY OF MIDDLE AND HIGH LEVEL CLOUDS.
LOWS TONIGHT IN THE 50S. CLOUD COVER THICK ENOUGH FRIDAY TO
LOWER THE HIGHS A LITTLE, 70S AND LOWER 80S INLAND AND MID 60S
TO LOWER 70S COAST.
UPPER LEVEL LOW CONTINUING TO MOVE NORTHEAST FRIDAY NIGHT AND
SATURDAY ENDING UP JUST WEST OF GRAYS HARBOR BY 00Z SUNDAY.
UPPER LEVEL DISTURBANCE SPINNING OUT OF THE LOW MOVING THROUGH
WESTERN WASHINGTON SATURDAY MORNING INTO THE AFTERNOON. WITH
SOME UPPER LEVEL DIFFLUENCE AIR MASS BECOMING SOMEWHAT UNSTABLE
AS THIS FEATURE MOVES THROUGH PRODUCING SHOWERS AND A CHANCE OF
THUNDERSTORMS EVEN FOR THE LOWLANDS. BREEZY WITH AND JUST
BEHIND THE SYSTEM SATURDAY. LOWS FRIDAY NIGHT IN THE MID 50S TO
LOWER 60S. HIGHS SATURDAY MUCH COOLER WITH THE CONVECTIVE
ACTIVITY, IN THE 60S AND LOWER 70S. FELTON
LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/
UPPER LEVEL LOW
MOVING INTO SOUTHERN BRITISH COLUMBIA BY 12Z SUNDAY WITH SHOWER
ACTIVITY JUST NEAR THE CANADIAN BORDER EARLY SUNDAY MORNING.
LOWS IN THE 50S.
UPPER LEVEL RIDGE BUILDING OVER WESTERN WASHINGTON SUNDAY MOVING
EAST MONDAY. ONSHORE FLOW IN THE LOWER LEVELS SUNDAY THROUGH
TUESDAY FOR AN AREAS MORNING LOW CLOUDS AND AFTERNOON SUNSHINE
FORECAST. HIGH TEMPERATURES NEAR OR JUST SLIGHTLY BELOW NORMAL.
DEEP UPPER LEVEL LOW IN THE GULF OF ALASKA MONDAY NIGHT THROUGH
WEDNESDAY. SOUTHWESTERLY FLOW ALOFT OVER THE AREA. WEAK SYSTEM
KICKING OUT OF THE LOW BRUSHING THE AREA TUESDAY NIGHT FOR A
SLIGHT CHANCE OF SHOWERS. INCREASING ONSHORE FLOW BEHIND THE
SYSTEM LOWERING HIGH TEMPERATURES BACK DOWN INTO THE MID 60S TO
MID 70S FOR WEDNESDAY. FELTON
AVIATION
MAJORITY OF TERMINALS VFR THIS MORNING WITH CLEAR
SKIES. LIFR CONTINUES ALONG THE COAST AND PORTIONS OF THE
NORTHERN INTERIOR. VFR WILL PREVAIL THIS AFTERNOON FOR INTERIOR
TERMINALS. BRIEF IMPROVEMENT TO VFR POSSIBLE ALONG THE COAST
THIS AFTERNOON, BUT IT IS POSSIBLE THAT CONDITIONS WILL ONLY
IMPROVE TO MVFR. ANOTHER WEAK PUSH OF MARINE STRATUS TONIGHT
INTO THE MORNING. LATEST GUIDANCE SUGGESTS THE STRATUS WILL STAY
WEST OF PUGET SOUND TERMINALS.
KSEA...VFR TODAY WITH MOSTLY CLEAR SKIES. NORTHWESTERLY WINDS THIS
MORNING WILL INCREASE THIS AFTERNOON TO 6-8 KT. LIGHT NORTHEASTERLY
WINDS TONIGHT INTO FRIDAY MORNING.
29
MARINE
BROAD HIGH PRESSURE OVER THE WATERS WILL WEAKEN
FRIDAY. A WEAK SURFACE LOW WILL MOVE INTO THE COASTAL WATERS
SATURDAY WITH THE ASSOCIATED SYSTEM MOVING THROUGH THE INLAND
WATERS SATURDAY MORNING INTO EARLY AFTERNOON. THE LOW WILL
DISSIPATE SATURDAY NIGHT. HIGH PRESSURE WILL REBUILD SUNDAY AND
REMAIN OVER THE AREA THROUGH MONDAY.
WINDS PICKING UP WITH AND BEHIND THE SYSTEM SATURDAY BUT AT THIS
POINT LOOKS LIKE THE WINDS WILL PEAK AROUND 20 KNOTS OVER THE
WATERS. DIURNAL WESTERLY PUSHES THROUGH THE STRAIT OF JUAN DE
FUCA ALSO PRODUCING UP TO 20 KNOT WINDS IN THE EVENING HOURS.
SEAS REMAINING MOSTLY IN THE 3 TO 5 FOOT RANGE FOR THE NEXT FEW
DAYS. FELTON
FIRE WEATHER
ABOVE NORMAL TEMPERATURES THE NEXT COUPLE OF
DAYS BUT MORNING LOWS DROPPING INTO THE 50S WITH GOOD RELATIVE
HUMIDITY RECOVERIES KEEPING FIRE WEATHER CONCERNS AT A MINIMUM.
UPPER LEVEL MOVING OFF THE COAST SATURDAY AND INTO SOUTHERN
BRITISH COLUMBIA SATURDAY NIGHT/EARLY SUNDAY MORNING. FIRE
WEATHER CONCERNS ELEVATED SATURDAY AS THIS FEATURE WILL BRING
CONVECTION TO THE AREA LATE FRIDAY NIGHT THROUGH SATURDAY
MORNING. THE HIGHEST CHANCE FOR CONVECTION IS OVER THE OLYMPICS
AND CASCADES BUT EVEN THE LOWLANDS WILL HAVE A THREAT OF
THUNDERSTORMS. RAINFALL AMOUNTS HAVE INCREASED FOR THIS SYSTEM,
AND LOCALIZED WETTING RAINS ARE POSSIBLE WITH ANY STORMS THAT
DEVELOP ALONGSIDE GUSTY AND ERRATIC OUTFLOW WINDS AND ABUNDANT
LIGHTNING. COOL DAY SATURDAY WITH THE CONVECTIVE ACTIVITY.
INCREASING ONSHORE FLOW BEHIND THE LOW WILL KEEP TEMPERATURES
COOL AND THE AIR MASS MOIST SUNDAY INTO MONDAY. ANY LIGHTNING
STRIKE MAY NOT HAVE LONG TO SMOLDER WITH MINIMUM RELATIVE
HUMIDITY VALUES BOTTOMING OUT IN THE 60-70% RANGE SATURDAY.
LOW LEVEL ONSHORE FLOW CONTINUING MONDAY INTO TUESDAY BRINGING
HIGH RELATIVE HUMIDITY RECOVERIES IN THE OVERNIGHT HOURS
REDUCING THE FIRE WEATHER CONCERNS. FELTON
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...NONE.
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