Small Craft Advisory 842, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 AM PDT THU OCT 27 2011

.SYNOPSIS...AN UPPER RIDGE WILL BRING DRY WEATHER TODAY. A FRONT
WILL BRING RAIN AND LOCALLY WINDY CONDITIONS FRIDAY. AN UPPER RIDGE
WITH DRIER WEATHER IS EXPECTED ON SATURDAY...THEN ANOTHER FRONT WILL
ARRIVE ON SUNDAY. AN UPPER RIDGE WILL BRING MORE DRY WEATHER
AROUND TUESDAY.

&&

.SHORT TERM...AREAS OF FOG AND LOW CLOUDS HAVE COVERED MUCH OF THE
LOWLANDS THIS MORNING. THE 12Z KUIL SOUNDING SHOWED A SHARP TOP OF
THE LOW-LEVEL MOIST LAYER NEAR 2500 FEET MSL. AS SYNOPTIC SCALE
SUBSIDENCE CONTINUES TODAY UNDER A TRANSIENT UPPER RIDGE AXIS...AND
AS LOW-LEVEL FLOW TURNS INCREASINGLY OFFSHORE...REMAINING LOW CLOUDS
AND FOG WILL DISSIPATE THROUGH THE DAY. STILL EXPECTING A MOSTLY
SUNNY AFTERNOON AREAWIDE...THOUGH HIGH CLOUDS WILL START TO INCREASE
AND THICKEN...FILTERING THE SUN IN THE HOUR OR TWO BEFORE SUNSET.

THE FLAT UPPER RIDGE AXIS WILL MOVE EASTWARD TOWARD THE NORTHERN
ROCKIES OVERNIGHT...OPENING THE DOOR TO INCREASING WEST-SOUTHWEST
FLOW ALOFT. ON FRIDAY...A FAST-MOVING UPPER TROUGH AND ACCOMPANYING
COLD FRONT WILL SPREAD ACROSS WRN WA. RAIN WILL REACH THE COAST
EARLY IN THE MORNING AND THEN SPREAD INTO THE INTERIOR AROUND
MID-DAY. GIVEN THE FAST MOVEMENT OF THIS FRONT...STEADY RAIN SHOULD
ONLY LAST A COUPLE OF HOURS BEFORE QUICKLY ZIPPING EAST.

LOW-LEVEL ONSHORE FLOW WILL LEAD TO A FEW POST-FRONTAL SHOWERS FROM
FRI AFTN THROUGH FRI NIGHT...BUT LACK OF INSTABILITY WILL NOT ADD
MUCH VIGOR TO SHOWER ELEMENTS...AND SHOWERS WILL MAINLY BE FOCUSED
ON THE TERRAIN AND IN A PUGET SOUND CONVERGENCE ZONE ON FRI EVNG.

ANOTHER FLAT UPPER RIDGE WILL TRY TO BUILD IN ON SATURDAY...BUT SOME
WARM ADVECTION LIFT ON THE PERIPHERY OF THE RIDGE AND SOME LOW-MID
LEVEL MOISTURE COULD LEAD TO A FEW DRIPS OF RAIN ON SAT MORNING. BY
SAT AFTN...THE UPPER RIDGE WILL BUILD FURTHER INTO WRN WA WITH WEAK
SYNOPTIC-SCALE SUBISDENCE AND MODEST DRYING OF THE ATMOSPHERE TAKING
PLACE. MOST CONFIDENTLY DRY PERIOD OF THE WEEKEND WILL BE ON SAT
EVNG.       HANER

.LONG TERM...FROM PREVIOUS DISCUSSION...BOTH THE GFS AND ECMWF BRING
ANOTHER FRONT INTO THE PACIFIC NORTHWEST ON SUNDAY. THE ECMWF IS A
LITTLE MORE AGGRESSIVE WITH SPREADING PRECIPITATION INLAND SUNDAY
AFTERNOON VERSUS THE MARGINALLY SLOWER GFS. THE MODELS REBUILD SOME
UPPER LEVEL RIDGING TUESDAY INTO WEDNESDAY...THOUGH THE ECMWF IS
CONSIDERABLY STRONGER WITH IT. BY LATE WEDNESDAY...THE GFS HAS
FLATTENED THE RIDGE WITH A SHARP UPPER LEVEL TROUGH APPROACHING FROM
THE NORTHWEST...WHILE THE ECMWF MAINTAINS SOME RIDGING WITH 500 MB
HEIGHTS SOME 160 DAM HIGHER. NO CHANGES WERE MADE TO THE EXTENDED
FORECAST OVERNIGHT. 27

&&

.HYDROLOGY...FLOODING IS UNLIKELY DURING THE NEXT 7 DAYS.

&&

.AVIATION...A WEAK UPPER LEVEL SHORTWAVE RIDGE WILL MOVE ACROSS W WA
TODAY...COINCIDING WITH A SURFACE HIGH OVER THE AREA. FLOW ALOFT IS
MODERATE NW...BECOMING W.

THE LOWER AIR MASS WITH AREAS OF IFR/MVFR FOG AND STRATUS MIXED IN
WITH VFR CONDITIONS. THIS IS EXPECTED TO CLEAR BY 19Z-20Z WITH JUST
CIRRUS FROM AN APPROACHING PACIFIC FRONTAL SYSTEM OVER THE AREA THIS
AFTERNOON.

MID LEVEL CLOUDS FROM THE APPROACHING FRONTAL SYSTEM WILL REACH THE
AREA TONIGHT...WHICH WILL LIMIT THE FORMATION OF FOG/STRATUS TO THE
TYPICALLY WORST AREAS.

THE APPROACHING FRONT WILL SPREAD RAIN AND MVFR CONDITIONS TO THE
COAST FRIDAY MORNING AND INLAND FRIDAY AFTERNOON.

KSEA...FOG IS SLOWLY LIFTING INTO A STRATUS DECK AT KSEA. VIDEO
CAMERAS SHOW AUBURN HAS ALREADY CLEARED OUT SO KSEA SHOULD CLEAR OUT
BY 18Z OR 19Z. NORTH WINDS 3-6 KT WILL CONTINUE THROUGH THIS
EVENING. WINDS SHOULD SHIFT S LATE TONIGHT AS THE FRIDAY COLD FRONT
APPROACHES. KAM

&&

.MARINE...A SURFACE HIGH OVER W WA WILL PRODUCE GENERALLY LIGHT
WINDS TODAY. THE HIGH WILL SHIFT EAST TONIGHT AS A MODERATELY STRONG
PACIFIC FRONTAL SYSTEM APPROACHES THE REGION. EASTERLY WINDS WILL
INCREASE TO SCA LEVELS TONIGHT OVER THE WEST ENTRANCE WITH SE WINDS
REACHING SCA LEVELS OVER THE OUTER COASTAL WATERS.

A 10-12 FOOT NW SWELL TRAIN WILL ARRIVE ALONG THE COAST THIS
AFTERNOON. THE OFFSHORE BUOYS...36 AND 5 ARE PICKING THIS SWELL
TRAIN UP AND BUOY 206 ALONG VANCOUVER ISLAND HAS REACHED 10 FT. WITH
SCA WINDS DEVELOPING OVER THE COAST AND W ENTRANCE TONIGHT HAVE
ROLLED THE HAZARDOUS SEAS SCA INTO A REGULAR SCA.

12Z NAM SURFACE WINDS REMAIN BELOW GALE FORCE FOR FRIDAYS COLD FRONT
BUT WILL AWAIT THE 12Z WRF-GFS BEFORE MAKING THE DECISION TO UPGRADE
TO A GALE WARNING IN THE AFTERNOON PACKAGE. FRONTAL TIMING LOOKS
ABOUT THE SAME...CROSSING THE COASTAL WATERS FRIDAY MORNING AND
MOVING INLAND FRIDAY AFTERNOON. 12Z NAM LOOKED A LITTLE SLOWER THAN
THE 06Z GFS.

SOME OF THE HIGHEST TIDES OF THE YEAR ARE OCCURRING THIS WEEK. THERE
IS SOME THREAT OF TIDAL OVERFLOW FRIDAY AFTERNOON WHEN THE TIME OF
HIGH TIDE AND THE TIME OF LOWEST PRESSURE ASSOCIATED WITH THIS STORM
MORE OR LESS COINCIDE. HOWEVER MINIMUM PRESSURES MAY ONLY REACH
AROUND 1011 MB WHICH IS NOT VERY LOW. WIND WAVE ACTION MAY BE MORE
OF A FACTOR DURING HIGH TIDE. KAM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COAST AND WEST STRAIT.
     GALE WATCH COASTAL WATERS...ENTRANCES STRAIT OF JUAN DE FUCA...
        AND NORTHERN INLAND WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

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