368
FXUS66 KSEW 210255
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
755 PM PDT THU AUG 20 2026
SYNOPSIS
UPPER LEVEL LOW APPROACHING FROM THE SOUTHWEST FRIDAY WILL MOVE
INTO THE COASTAL WATERS SATURDAY AND INTO SOUTHERN BRITISH
COLUMBIA EARLY SUNDAY MORNING. THIS SYSTEM WILL BRING COOLER
WEATHER WITH A CHANCE OF SHOWERS AND THUNDERSTORMS. UPPER LEVEL
RIDGE REBUILDING SUNDAY WILL MOVE EAST MONDAY FOR DRY AND MILD
WEATHER. UPPER LOW SHIFTING INLAND ON WEDNESDAY WITH SHOWERS
RETURNING.
SHORT TERM /TONIGHT THROUGH SATURDAY NIGHT/
THE LATEST FORECAST REMAINS ON TRACK THIS EVENING WITH NO
UPDATES. WEAK HIGH PRESSURE IS OVER THE AREA FOR DRY AND MILD
CONDITIONS. THE AIR MASS WILL BE A LITTLE WARMER ON FRIDAY WITH
POCKETS OF MODERATE HEATRISK IN THE INTERIOR AND CASCADE VALLEYS
(HIGHS IN THE MID TO UPPER 80S).
THE NEXT WEATHER SYSTEM ROLLS IN LATE FRIDAY AND INTO SATURDAY.
THIS SYSTEM WILL BRING COOLER CONDITIONS, SHOWERS, AND A THREAT
OF THUNDERSTORMS TO WESTERN WASHINGTON. THE HIGHEST INSTABILITY
WILL BE OVER THE OLYMPICS AND NORTHERN CASCADES WHERE SCATTERED
THUNDERSTORMS MAY DEVELOP - LEADING TO ELEVATED TO CRITICAL
FIRE WEATHER CONDITIONS. THE HIGHEST PROBABILITY OF SEEING
WETTING RAINS WILL BE NEAR THE CANADIAN BORDER. THERE'S A 10-20
PERCENT CHANCE (ISOLATED COVERAGE) OF SEEING THUNDERSTORMS IN
THE LOWLANDS FROM SEATTLE NORTHWARD. WITH THE COOLER AIR MASS IN
PLACE, TEMPERATURES WILL TRACK COOLER THAN AVERAGE AND IN THE
60S TO LOWER 70S.
LONG TERM /SUNDAY THROUGH THURSDAY/
SHOWERS (MAINLY NORTH PART) WILL LINGER INTO SUNDAY AS THE LOW
TRACKS N/NE INTO B.C. DRY AND MILD WEATHER WILL RETURN EARLY
NEXT WEEK WITH LOW PRESSURE OFFSHORE AND WEAK HIGH PRESSURE
INLAND. TEMPERATURE RANGES WILL BE CLOSE TO AVERAGE EACH DAY.
LOW PRESSURE SHIFTS INLAND WEDNESDAY AND THURSDAY FOR A CHANCE
OF SHOWERS.
33
AVIATION
VFR THIS AFTERNOON WITH MOSTLY CLEAR SKIES. A WEAK PUSH OF
MARINE STRATUS WILL BRING IFR/LIFR CONDITIONS TO THE COAST THIS
EVENING INTO FRIDAY MORNING. THE STRATUS WILL ATTEMPT TO PUSH
INTO OLM, BUT DON'T EXPECT REACH PUGET SOUND TERMINALS.
REDUCTIONS IN VISIBILITY HIGHLY LIKELY WITH THE STRATUS.
ADDITIONALLY, PATCHY FOG POSSIBLE IN THE VALLEYS OF THE SOUTHERN
INTERIOR AND NORTHERN INTERIOR. MID-TO- HIGH CLOUDS WILL
INCREASE FRIDAY MORNING WITH CONDITIONS REMAINING VFR ACROSS
INTERIOR TERMINALS. MINIMAL IMPROVEMENT ALONG THE COAST ON
FRIDAY, WITH CONDITIONS LIKELY ONLY REBOUNDING TO MVFR.
GENERALLY NW WINDS THIS AFTERNOON, EASING TONIGHT.
KSEA...VFR WITH MOSTLY CLEAR SKIES TODAY AND INCREASING MID-TO-HIGH
CLOUDS FRIDAY MORNING. NW WINDS THIS AFTERNOON 6-8 KT WILL
TRANSITION TO NE THIS AFTERNOON, EASING TONIGHT TO 3-5 KT.
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. ADDITIONALLY, THERE IS A SLIGHT CHANCE (10-20%) FOR
THUNDERSTORMS ON SATURDAY FOR BOTH THE COASTAL WATERS AND
INTERIOR WATERS. HIGH PRESSURE WILL REBUILD SUNDAY AND REMAIN
OVER THE AREA THROUGH MONDAY. A COLD FRONT WILL MOVE OVER THE
WATERS ON TUESDAY.
WINDS WILL PICK UP BEHIND THE SYSTEM ON SATURDAY. GUIDANCE SUGGESTS
THERE IS A 40-60% WIND GUSTS MAY APPROACH NEAR 21 KT THROUGH THE
STRAIT OF JUAN DE FUCA, WITH WINDS ABOVE 21 KT STAYING ON THE
CANADIAN SIDE.
SEAS REMAINING MOSTLY IN THE 3 TO 5 FOOT RANGE FOR THE NEXT FEW
DAYS.
FELTON/29
FIRE WEATHER
WARM, DRY CONDITIONS CONTINUE INTO FRIDAY. MODERATE RH
RECOVERIES ARE EXPECTED TONIGHT FOR THE CASCADES. AN UPPER LOW
WILL MOVE NORTHWARDS TOWARDS WESTERN WASHINGTON LATE FRIDAY INTO
SATURDAY. THIS FEATURE WILL RESULT IN THE DEVELOPMENT OF
CONVECTION DURING THIS PERIOD. PROBABILITIES FOR CONVECTION ARE
HIGHEST OVER THE OLYMPICS AND NORTH CASCADES, PEAKING BETWEEN 20
TO 30 PERCENT. DUE TO THE POTENTIAL OF SCATTERED THUNDERSTORMS
FOR THESE AREAS, A FIRE WEATHER WATCH HAS BEEN ISSUED FOR ZONES
652, 658, AND 661. CONVECTION MAY EXIST ELSEWHERE, INCLUDING THE
SOUTHERN WASHINGTON CASCADES, HOWEVER, UNCERTAINTY EXISTS IN
THE COVERAGE FOR THESE REGIONS. SHOWERS AND THUNDERSTORMS MAY
BEGIN AS EARLY AS 11PM-2AM FRIDAY NIGHT OVER THE OLYMPIC
PENINSULA, WITH THE THREAT CONTINUING THROUGH LATE SATURDAY.
LIGHTNING MAY LEAD TO NEW FIRE STARTS, AS WELL AS RESULT IN
GUSTY AND ERRATIC WINDS.
ONSHORE FLOW INCREASES SUNDAY INTO EARLY NEXT WEEK, RESULTING IN
COOLER CONDITIONS AND INCREASED RH VALUES. RAIN MAY LINGER INTO
SUNDAY FOR THE NORTH CASCADES, WITH ANOTHER SLIGHT CHANCE OF SHOWERS
LATER TUESDAY INTO WEDNESDAY.
JD
SEW WATCHES/WARNINGS/ADVISORIES
WA...FIRE WEATHER WATCH FROM LATE FRIDAY NIGHT THROUGH SATURDAY
EVENING FOR WEST SLOPES OF THE NORTH CASCADES GENERALLY
ABOVE 1500 FEET.
FIRE WEATHER WATCH FROM LATE FRIDAY NIGHT THROUGH SATURDAY
EVENING FOR EAST PORTION OF THE OLYMPIC MOUNTAINS-WEST
PORTION OF THE OLYMPIC MOUNTAINS.
PZ...NONE.
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