Small Craft Advisory 801, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
922 AM PDT WED AUG 17 2011

.SYNOPSIS...WESTERLY FLOW ALOFT WITH VARYING DEGREES OF LOW LEVEL
ONSHORE FLOW WILL CONTINUE THROUGH THE WEEK. THIS PATTERN WILL
BRING AREAS OF NIGHT AND MORNING CLOUDS MAINLY AT THE COAST...AND
GENERALLY SUNNY DAYS...ESPECIALLY INLAND WITH DRY WEATHER AND NEAR
NORMAL TEMPERATURES. A RIDGE OF HIGH PRESSURE WILL BUILD OVER THE
WEEKEND THEN A RETURN TO WESTERLY FLOW IS EXPECTED EARLY NEXT WEEK.

&&

.SHORT TERM...LOOK FOR A REPEAT IN TRANQUIL WEATHER ACROSS THE
REGION TODAY. WESTERN WA STILL LIES IN BETWEEN TWO SYSTEMS...AN
UPPER LEVEL LOW SPINNING OVER N B.C. AND A BAGGY TROUGH OFF THE CA
COAST. THE VISIBLE SATELLITE SHOWS LOW CLOUDS ONCE AGAIN HUGGING THE
NORTH COAST AND WEST STRAIT...WITH CLEAR SKIES
ELSEWHERE...PRACTICALLY THE SAME SCENARIO AS THIS TIME YESTERDAY.
SOME HIGH CLOUDS MAY BRUSH THE NORTHERN ZONES THIS AFTERNOON BUT
STILL LOOKS MOSTLY SUNNY. CURRENT FORECAST LOOKS GOOD.

MODELS SHOW THE WEAK TROUGH OFFSHORE MOVING INLAND TONIGHT INTO THU
BUT THE AIR MASS REMAINS DRY AND STABLE. HOWEVER...ONSHORE PRESSURE
GRADIENTS ARE STRONGER AND WE SHOULD SEE THE MARINE STRATUS LAYER
EXPAND AND PUSH FARTHER INLAND OVERNIGHT. EXPECT AREAS OF LOW CLOUDS
IN THE INTERIOR EARLY THU MORNING. LOW CLOUDS WILL BURN BACK TO THE
COAST THU AFTERNOON BUT THE PUSH WILL KNOCK OUR TEMPS DOWN A FEW
DEGREES COMPARED TO TODAY.

THE WEAK UPPER LEVEL TROUGH WILL EXIT ON FRI AS A STRONG RIDGE
BUILDS OFFSHORE. THIS PATTERN WILL KEEP W WA DRY AND SUNNY...WITH
SEASONAL CONDITIONS. BASED ON THE LATEST GUIDANCE...LITTLE CHANGES
ANTICIPATED FROM THE CURRENT FORECAST. 33

.LONG TERM...PREVIOUS DISCUSSION...MODELS CONTINUE TO WAVER ON THE
STRENGTH OF THE RIDGE OVER THE WEEKEND. LATEST RUNS SHOW STRONGER
RIDGING...ESPECIALLY THE GFS. THE ECMWF IS A BIT FLATTER WHICH COULD
IMPLY A BIT OF ONSHORE FLOW. IF THE 00Z GFS RUN IS CORRECT...HIGH
TEMPS MAY NEED TO BE BOOSTED A FEW DEGREES. LONG RANGE MODELS ALSO
DISAGREE ON WHETHER A FRONT WILL MOVE INTO THE AREA EARLY NEXT WEEK.
THE LATEST RUNS HAVE TRENDED DRIER BUT STILL SHOW SOME QPF ACROSS
THE NW PORTION OF THE FORECAST AREA. WITH 500 MB HEIGHTS NEAR 5800 M
AND THE MODELS TRENDS...IT SEEMS PRUDENT TO LEAVE THE FORECAST DRY
FOR NOW. MERCER

&&

.AVIATION...MODERATE WESTERLY FLOW ALOFT AND LOW LEVEL ONSHORE FLOW
WILL CONTINUE THROUGH THURSDAY. SATELLITE SHOWS LARGELY CLEAR SKIES.
SOME STRATUS IS ALONG THE COAST AND AT THE WEST ENTRANCE...AND A FEW
RIVER VALLEYS HAVE SOME FOG. THE FOG AND MOST OF THE STRATUS WILL
DISSIPATE BY 18Z. ANY CEILINGS ARE LIKELY TO BE IFR FOR A FEW HOURS
UNTIL THEN.

ONSHORE SURFACE GRADIENTS INCREASE TONIGHT AND STRATUS IS LIKELY TO
PENETRATE FARTHER INLAND. MOS GUIDANCE HAS SEVERAL HOURS OF MVFR
CEILINGS AT MOST TERMINALS THURSDAY MORNING.

KSEA...DISCUSSION ABOVE APPLIES. NORTHWEST WIND 5-9 KNOTS WILL
BECOME NORTHEAST BY 06Z. CHB

&&

.MARINE...ONSHORE SURFACE PRESSURE GRADIENTS ARE CONSIDERABLY HIGHER
THAN YESTERDAY. EXPECT AN EVEN STRONGER GRADIENT DOWN THE STRAIT
THIS EVENING. FORECAST IS FOR 20-30 KT...HIGH END SMALL CRAFT
ADVISORY STRENGTH...BUT A GALE IS NOT OUT OF THE QUESTION. WILL KEEP
THE ADVISORY GOING FOR NOW AND MONITOR.

ONSHORE FLOW OF VARYING STRENGTH WILL CONTINUE INTO THE WEEKEND WITH
STRONGER WINDS DURING THE AFTERNOON AND EVENING HOURS ESPECIALLY
THROUGH THE CENTRAL AND EAST STRAIT. SOMEWHAT WEAKER GRADIENTS ARE
FORECAST THURSDAY THROUGH SATURDAY...BUT WINDS WILL STILL BE AT THE
SMALL CRAFT ADVISORY LEVEL EACH AFTERNOON AND EVENING. LIGHTEST
WINDS ARE MOST LIKELY ON SUNDAY. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT.