314
FXUS66 KSEW 181015
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
315 AM PDT FRI JUL 18 2025
SYNOPSIS
WEAK SYSTEM MOVING INTO THE AREA FROM THE NORTHWEST
WILL SPREAD HIGH CLOUDS OVER THE AREA THIS AFTERNOON. BROAD UPPER
LEVEL TROUGH OVER WESTERN CANADA SATURDAY WILL DROP DOWN OVER
WESTERN WASHINGTON SUNDAY AND MONDAY FOR A CHANCE OF SHOWERS OVER
THE HIGHER TERRAIN. THE TROUGH WILL MOVE EAST TUESDAY WITH A WEAK
UPPER LEVEL RIDGE BUILDING INTO WEDNESDAY. ANOTHER TROUGH WILL
APPROACH FROM THE NORTHWEST THURSDAY.
SHORT TERM /TODAY THROUGH SUNDAY/
SATELLITE IMAGERY SHOWS
STRATUS ALONG THE COAST HAS MOVED DOWN THE STRAIT OF JUAN DE FUCA
AND THROUGH THE LOWER CHEHALIS VALLEY TO THE SOUTHERN END OF THE
HOOD CANAL AT 3 AM/10Z. TEMPERATURES WERE IN THE MID 50S TO MID
60S.
STRATUS MAKING A RUN AT GETTING EAST OF THE PUGET SOUND THIS
MORNING BUT EVEN IF IT MAKES IT INTO THE SEATTLE METRO AREA IT
WILL NOT LAST VERY LONG. STRATUS RETREATING TO THE COAST 18Z-20Z.
WEAK SHORTWAVE CURRENTLY AT THE NORTHERN TIP OF VANCOUVER ISLAND
MOVING ACROSS THE AREA THIS AFTERNOON. JUST MIDDLE AND HIGH LEVEL
CLOUDS WITH THIS FEATURE. ENOUGH TO WORD THE AFTERNOON FORECAST
MOSTLY CLOUDY/PARTLY SUNNY. LOW LEVEL ONSHORE FLOW CONTINUING
THROUGHOUT THE DAY WITH THE CROSS CASCADES GRADIENT UP TO PLUS 8
OR 9 MB BY LATE AFTERNOON. THIS WILL CONTINUE TO DRAW COOLER
MARINE AIR OVER WESTERN WASHINGTON. COMBINED WITH THE CLOUD COVER
IN THE AFTERNOON HIGHS WILL BE IN THE UPPER 60S TO MID 70S OVER
THE INTERIOR. HIGHS ON THE COAST IN THE LOWER TO MID 60S.
INCREASING ONSHORE FLOW TONIGHT BEHIND THE SHORTWAVE WITH A
POSSIBLE CONVERGENCE ZONE OVER SNOHOMISH COUNTY. NO PRECIPITATION
ASSOCIATED WITH THE CONVERGE ZONE, JUST SOME CLOUD COVER. STRATUS
ALONG THE COAST WILL SPREAD INLAND OVERNIGHT. WITH THE INCREASING
ONSHORE FLOW STRATUS LIKELY TO MAKE IT EAST OF PUGET SOUND BY
SUNRISE SATURDAY MORNING. LOWS IN THE 50S.
LOW CLOUDS SATURDAY MORNING A LITTLE SLOW TO DISSIPATE WITH THE
ONSHORE FLOW. BY EARLY TO MID AFTERNOON MOST PLACES IN THE
INTERIOR SHOULD BE IN THE SUNSHINE. LOW CLOUDS HANGING TOUGH
ALONG THE COAST ALL DAY. HIGHS PRETTY SIMILAR TO TODAY WITH UPPER
60S TO MID 70S FOR THE INTERIOR AND LOWER TO MID 60S FOR THE
COAST.
WEAK UPPER LEVEL TROUGH OVER WESTERN CANADA SATURDAY NIGHT SAGGING
OVER WESTERN WASHINGTON SUNDAY. LITTLE IN THE WAY OF WEATHER WITH
THE TROUGH WITH JUST A CHANCE OF LATE DAY SHOWERS IN THE NORTH
CASCADES. THE TROUGH OVERHEAD WILL KEEP THE LOW LEVEL ONSHORE FLOW
SCENARIO INTACT FOR ANOTHER ROUND OF LATE NIGHT AND MORNING CLOUDS
ADD SOME MIDDLE AND HIGH LEVEL CLOUDINESS WITH THE TROUGH MAKES
FOR A PARTLY SUNNY/MOSTLY CLOUDY SUNDAY. LIKE SATURDAY LOWS IN THE
50S AND HIGHS IN THE UPPER 60S TO MID 70S INTERIOR AND LOWER TO
MID 60S THE COAST.
LONG TERM /MONDAY THROUGH THURSDAY/
UPPER LEVEL TROUGH KICKING
OUT THE EAST MONDAY. SOMEWHAT UNSTABLE AIR MASS OVER THE CASCADES
IN THE AFTERNOON FOR A CHANCE OF SHOWERS AND A SLIGHT CHANCE OF
THUNDERSTORMS. HIGHS MOSTLY 70S FOR THE INTERIOR AND MID TO UPPER
60S COAST.
SMALL WARMING TREND TUESDAY AND WEDNESDAY WITH THE UPPER LEVEL
TROUGH TO THE EAST AND A WEAK UPPER LEVEL RIDGE NOSING INTO THE
AREA. THE RIDGE IS NOT VERY STRONG WITH 500 MB HEIGHTS ONLY IN THE
LOWER 580 DMS. IN THE LOWER LEVELS ONSHORE FLOW WEAKENS BUT
DOESN'T TOTALLY GO AWAY. TROUGH CLOSE ENOUGH TUESDAY FOR ANOTHER
CHANCE OF LATE DAY SHOWERS OR POSSIBLY A THUNDERSTORM IN THE
NORTH CASCADES. HIGHS WARMING INTO THE MID 70S TO MID 80S BY
WEDNESDAY FOR THE INTERIOR. ONSHORE FLOW WILL KEEP HIGHS IN THE
60S ON THE COAST.
UPPER LEVEL RIDGE WEAKENING THURSDAY AS ANOTHER UPPER LEVEL TROUGH
STARTS DROPPING DOWN THE BRITISH COLUMBIA COASTLINE. TROUGH STILL
TO THE NORTHWEST BY THE END OF THE DAY. INCREASING LOW LEVEL
ONSHORE FLOW KNOCKING A FEW DEGREES OFF THE HIGHS FOR THE INTERIOR
WITH 70S AND LOWER 80S FORECAST. HIGHS ON THE COAST REMAINING IN
THE 60S. FELTON
AVIATION
AN UPPER LEVEL TROUGH WILL DIG SOUTHWARD OVER BRITISH
COLUMBIA TODAY WITH INCREASING WEST TO NORTHWEST FLOW ALOFT OVER
WESTERN WASHINGTON. IFR OR LOW MVFR COASTAL STRATUS WILL SPREAD INTO
PARTS OF PUGET SOUND THIS MORNING BEFORE RETREATING BACK TOWARD THE
COAST THIS AFTERNOON. INCREASING HIGH AND MID LEVEL CLOUDS
ASSOCIATED WITH THE TROUGH WILL SPREAD ACROSS THE AREA BY THIS
AFTERNOON. PERSISTENT ONSHORE FLOW WILL LEAD TO A DEEP MARINE LAYER
ACROSS ALL OF THE LOWLANDS ON SATURDAY MORNING WITH WIDESPREAD MVFR
CONDITIONS.
KSEA...COASTAL STRATUS IS MAKING STEADY PROGRESS INLAND AS OF 09Z
WITH A PERIOD OF IFR IN LOW CLOUDS LIKELY BETWEEN 13Z AND 18Z BEFORE
LIFTING AND SCATTERING OUT TO INCREASING HIGHER LEVEL CLOUDS. MVFR
STRATUS WILL DEVELOP OVERNIGHT TONIGHT AND IS EXPECTED TO LINGER
INTO MIDDAY SATURDAY BEFORE LIFTING. SURFACE WINDS S/SW 6 TO 10
KNOTS RISING TO 9 TO 14 KNOTS THIS AFTERNOON.
27
MARINE
BROAD SURFACE RIDGING CENTERED WELL OFFSHORE AND LOWER
PRESSURE EAST OF THE CASCADES WILL LEAD TO VARYING DEGREES OF
ONSHORE FLOW INTO EARLY NEXT WEEK. AN UPPER LEVEL TROUGH MOVING
ACROSS THE AREA TODAY WILL ENHANCE THE ONSHORE FLOW LEADING TO HIGH
END SMALL CRAFT ADVISORY LEVEL WESTERLIES ACROSS THE CENTRAL AND
EAST STRAIT AS WELL AS SMALL CRAFT ADVISORY CONDITIONS IN THE FORM
OF GUSTY NORTHWEST WINDS AND STEEP SEAS OVER THE OUTER COASTAL
WATERS.
27
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...SMALL CRAFT ADVISORY UNTIL 5 AM PDT EARLY THIS MORNING FOR
CENTRAL U.S. WATERS STRAIT OF JUAN DE FUCA-EAST ENTRANCE
U.S. WATERS STRAIT OF JUAN DE FUCA.
SMALL CRAFT ADVISORY FROM 4 PM THIS AFTERNOON TO 3 AM PDT
SATURDAY FOR CENTRAL U.S. WATERS STRAIT OF JUAN DE FUCA-
EAST ENTRANCE U.S. WATERS STRAIT OF JUAN DE FUCA.
SMALL CRAFT ADVISORY UNTIL 2 PM PDT THIS AFTERNOON 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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