Small Craft Advisory 753, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
400 AM PDT WED JUN 29 2011

.SYNOPSIS...AN UPPER TROUGH MOVING DOWN THE BRITISH COLUMBIA COAST
WILL MOVE OVER WESTERN WASHINGTON LATER TODAY. THE UPPER LEVEL
TROUGH WILL REMAIN OVER WESTERN WASHINGTON THROUGH TOMORROW KEEPING
SHOWERS IN THE FORECAST. WARMER WEATHER AND MOSTLY SUNNY SKIES ARE
EXPECTED FRIDAY AND SATURDAY AS A WEAK RIDGE OF HIGH PRESSURE BUILDS
INTO THE AREA. ANOTHER UPPER LEVEL TROUGH WILL MOVE THROUGH ON
SUNDAY WITH THE RIDGE REBUILDING ON INDEPENDENCE DAY.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS PLENTY OF CLOUD COVER OVER
WESTERN WASHINGTON EARLY THIS MORNING. LEADING EDGE OF THE UPPER
LEVEL TROUGH MOVING DOWN THE BRITISH COLUMBIA COAST APPROACHING THE
NORTHERN TIP OF VANCOUVER ISLAND. AT THE LOWER LEVELS SURFACE
GRADIENTS ONSHORE THIS MORNING BUT NOT THAT IMPRESSIVE WITH
KOTH-KSEA BELOW PLUS 4 AND KHQM-KSEA PLUS 1 MB. THE ONSHORE FLOW
LOOKS BETTER TAKING A LOOK AT THE SANDPOINT PROFILER WHICH SHOWS
SOUTHWEST WINDS 15-25 KNOTS FROM JUST ABOVE THE SURFACE TO NEAR 850
MB. THIS WILL KEEP FEEDING MARINE AIR INTO WESTERN WASHINGTON THIS
MORNING. UPPER LEVEL TROUGH REACHING WESTERN WASHINGTON LATER THIS
MORNING INTO THE AFTERNOON HOURS. WITH THE LIFT PROVIDED BY THE
TROUGH SOME SHOWERS DEVELOPING OVER THE AREA AS THE TROUGH MOVES
THROUGH. BEHIND THE TROUGH LATE THIS AFTERNOON INTO THE EVENING
HOURS A PUGET SOUND CONVERGENCE ZONE FROM NEAR THE EAST ENTRANCE OF
THE STRAIT. GIVEN THE SURFACE PATTERN AT THIS POINT IT LOOKS LIKE
THE CONVERGENCE WILL STAY OVER SNOHOMISH COUNTY FOR THE MOST PART
TONIGHT. FOR THE REMAINDER OF THE LOWLANDS THE SHOWER ACTIVITY WILL
DECREASE OVERNIGHT INTO THURSDAY MORNING THEN PICK UP AGAIN MIDDAY
THURSDAY AS ANOTHER WEAK TROUGH MOVES THROUGH THE AREA IN THE WEST
NORTHWESTERLY FLOW ALOFT. THROUGH THE WHOLE PERIOD WITH THE LOW
LEVEL ONSHORE FLOW EXPECT PLENTY OF CLOUD COVER AND TEMPERATURE
STAYING IN A FAIRLY NARROW RANGE...50S AT NIGHT...LOWER TO MID 60S
BY LATE AFTERNOON. SOME IMPROVEMENT ON FRIDAY AS A SURFACE RIDGE
BUILDS INTO THE AREA AND A VERY WEAK UPPER LEVEL RIDGE STARTS TO
BUILD OVER WESTERN WASHINGTON. WITH PLENTY OF LOW LEVEL MOISTURE
REMAINING IN THE AREA EXPECT SOME MORNING CLOUDS ON FRIDAY BEFORE
THE SUN BURNS OFF THE MARINE LAYER. HIGH TEMPERATURES ON FRIDAY WILL
BE 5 TO 10 DEGREES WARMER THAN TODAY AND THURSDAY.

.LONG TERM...SOME CHANGES TO THE EXTENDED FORECAST THIS MORNING. THE
GFS SOLUTION ON THE 00Z RUN HAS NOW COME INTO LINE WITH WHAT THE
ECMWF HAS BEEN INDICATING ALL ALONG...A WEAK UPPER LEVEL TROUGH
MOVING BY TO THE NORTH ON SUNDAY. 48 HOURS AGO 500 MB HEIGHTS ON
THE GFS WERE NEAR 580 DMS. THE LATEST RUN THEY ARE CLOSER TO 570
DMS. THE CANADIAN MODEL IS ALSO STARTING TO TREND TOWARD THE ECMWF
SOLUTION. IN ADDITION...WHILE THE OPERATIONAL RUN OF THE GFS IS ONE
OF THE MORE PESSIMISTIC SOLUTIONS IN THE ENSEMBLES...ALMOST ALL OF
THE SOLUTIONS KEEP MOST OF WESTERN WASHINGTON BELOW 570 DMS. GIVEN
ALL THIS WILL UPDATE THE FORECAST FOR SUNDAY AND GO WITH COOLER
HIGHS AND ADD A CHANCE OF SHOWERS TO THE FORECAST. SATURDAY STILL
LOOKS LIKE A DRY DAY BUT WITH THE LOW LEVEL ONSHORE FLOW
TEMPERATURES WILL BE NEAR NORMAL FOR EARLY JULY. ALL OF THE MODELS
STILL STAY WITH THE IDEA OF THE UPPER LEVEL RIDGE REBUILDING ON
INDEPENDENCE DAY WITH THE SURFACE GRADIENTS GOING NORTHWESTERLY. AT
THIS POINT THE RIDGE LOOKS TO HOLD ON UNTIL AT LEAST THE MIDDLE OF
THE WEEK FOR A SPELL OF SUNNY WEATHER WITH NEAR OR SLIGHTLY ABOVE
NORMAL HIGHS. FELTON

&&

.AVIATION...A SERIES OF WEAK TROUGHS WILL BRUSH THE AREA PROVIDING
MOSTLY MID/HIGHER LEVEL MOISTURE AND SCATTERED SHOWERS. MODERATE WLY
FLOW ALOFT. ONSHORE FLOW WILL BRING LOWER MARINE MOISTURE AND
STRATUS INTO WRN WA. EXPECT WIDESPREAD MVFR CIGS THROUGH THE MORNING
WITH RISING BASES IN THE AFTERNOON...THOUGH SCATTERING IS UNLIKELY.
MAINLY STABLE CONDITIONS.

KSEA...SSW WIND 4-8 KT...RISING TO 10-13 KT DURING THE AFTERNOON. DTM

&&

.MARINE...VARYING DEGREES OF ONSHORE FLOW IS EXPECTED THROUGH THE
WEEKEND. MODEL FORECAST GRADIENTS VARY SO I WILL LEAVE THE GALE
WATCH UP FOR NOW. THE CURRENT FORECAST INDICATES WINDS TO 30 KT BUT
I COULD SEE RACE ROCKS HITTING GALE FOR A COUPLE HOURS...CLOSE CALL.

THE STRAIT WILL LIKELY EXPERIENCE MORE PERIODS OF SMALL CRAFT
ADVISORY WINDS THROUGH THE WEEK...ESPECIALLY DURING THE LATE
AFTERNOON AND EVENING HOURS. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT TONIGHT THROUGH 6
     AM PDT.
     GALE WATCH CENTRAL AND EAST STRAIT WEDNESDAY AFTERNOON AND
     EVENING.

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