Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 281612
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
915 AM PDT THU AUG 28 2014

.SYNOPSIS...LOW LEVEL ONSHORE FLOW WILL HELP PRODUCE MARINE
CLOUDINESS AND COOLER TEMPERATURES FOR MUCH OF WESTERN WA TODAY. A
PACIFIC WEATHER SYSTEM IN THE NORTHEAST PACIFIC IS EXPECTED TO MOVE
ONSHORE FRIDAY NIGHT AND SATURDAY BRINGING SOME LIGHT SHOWERS AND
DAYTIME TEMPERATURES A BIT BELOW AVERAGE. ANOTHER SYSTEM WILL FOLLOW
ON SUNDAY. HIGHER PRESSURE IS FORECAST TO BUILD FOR A DRIER LABOR
DAY BEFORE ANOTHER WEAK WEATHER SYSTEM MOVES ONSHORE EARLY NEXT WEEK.

&&

.SHORT TERM...THE FLOW ACROSS THE NORTH PACIFIC IS MORE ZONAL NOW.
THE UPPER RIDGE FROM EARLIER THIS WEEK HAS FLATTENED AND MOVED INTO
THE NORTHERN ROCKIES. A QUITE WEAK POSITIVE TILTED SHORTWAVE NEAR
130W THIS MORNING SHOULD MOVE ONSHORE TODAY. SOME MIDDLE AND HIGH
CLOUDS ASSOCIATED WITH THIS SYSTEM AND KEEPING AN EYE ON DOPPLER
RADAR IN CASE THERE IS ANY LIGHT PRECIP.

THIS SHORTWAVE HELPED INDUCE STRONGER LOW LEVEL ONSHORE FLOW
OVERNIGHT. MUCH OF WESTERN WASHINGTON HAS A MARINE CLOUD LAYER THIS
MORNING. 12Z KUIL RAOB SHOWS TOPS NEAR 2700 FEET. TOPS REPORTS FROM
SEATTLE APPROACH AROUND 15Z WERE BETWEEN 2200 AND 2400 FEET. THE
COMBO OF THE INCOMING SHORTWAVE AND LOW LEVEL ONSHORE FLOW SHOULD
LIMIT THE BREAKOUT THIS AFTERNOON. FORECAST COOLER HIGH TEMPS THIS
AFTERNOON SHOULD BE AROUND 10 DEGREES COOLER THAN THURSDAY - GIVE OR
TAKE. MADE A FEW GRID ADJUSTMENTS FOR SKY...POP AND QPF INTO TONIGHT
TO REFLECT CURRENT AND SHORT-TERM FORECASTS.

SHORTWAVE RIDGING NEAR 135W THIS MORNING SHOULD MOVE ONSHORE THIS
EVENING. A STRONGER LOOKING SHORTWAVE CROSSING 145W THIS MORNING
WITH AN ASSOCIATED 120 KT JET MAX...12Z PROGS IN THUS FAR BRING THIS
ONSHORE FRIDAY NIGHT AND SATURDAY FOR A GREATER THREAT OF SHOWERS
MAINLY COAST AND MOUNTAINS. THE FLOW PATTERN MAY PRODUCE A BRIEF
PUGET SOUND CONVERGENCE LATE FRIDAY NIGHT/EARLY SATURDAY.

A FOLLOWING SHORTWAVE SUNDAY SHOULD KEEP THE THREAT OF SHOWERS
GOING. DAYTIME TEMPS WILL LIKELY BE A BIT BELOW SEASONAL AVERAGES AS
WELL. BUEHNER

.LONG TERM...FROM PREVIOUS DISCUSSION...500 MB HEIGHTS WILL REBOUND
EARLY IN THE PERIOD...PARTICULARLY ON LABOR DAY. IF THE MEDIUM RANGE
SOLUTIONS ARE CORRECT...DAYTIME TEMPS ON MON COULD END UP ABOUT
NORMAL. THUS IT IS POSSIBLE THAT WE MIGHT BE SLIGHTLY ON THE
COOL-SIDE FOR MON.

BEYOND MON...IT APPEARED THAT THE WEATHER PATTERN OVER THE PACIFIC
NORTHWEST WILL RETURN TO COOL AND UNSETTLED. IF THE ECMWF AND GFS
END UP BEING CORRECT...MAX TEMPS ACROSS THE LOWLANDS WILL ONLY BE
IN THE 50S NEXT WED. HOWEVER...THE CANADIAN MODEL WAS WARMER FOR
DAY 7 AND INDICATED HIGHS IN THE 60S. ALL THREE MODELS HAD AN
UPPER TROF OVER THE REGION...SO IT IS POSSIBLE THAT OUR FORECAST
MIGHT BE ON THE OPTIMISTIC SIDE FOR THE LATTER PART OF THE
EXTENDED PERIOD. WILL NEED TO SEE IF THE MEDIUM RANGE SOLUTIONS
CONTINUE WITH THE IDEA OF AN UPPER TROF OVER THE CWA LATE IN THE
PERIOD. LEFT THE INHERITED FORECAST UNCHANGED FOR THIS REASON. NC

&&

.AVIATION...A WEAK WEATHER SYSTEM PASSING THROUGH THE AREA THIS
MORNING WILL CONTINUE TO ENHANCE THE STRATUS LAYER OVER WESTERN
WASHINGTON TODAY. FLOW ALOFT WILL REMAIN WESTERLY AND LOW LEVEL FLOW
WILL REMAIN ONSHORE.

LOW LEVEL ONSHORE FLOW AND THE WEAK UPPER LEVEL SYSTEM MOVING
THROUGH WESTERN WASHINGTON THIS MORNING WILL HELP MAINTAIN THE
STRATUS LAYER CURRENTLY COVERING MOST OF THE WESTERN WASHINGTON
LOWLANDS THIS MORNING. CEILINGS TO RISE TO MVFR LEVELS AROUND
MIDDAY...THEN TO IFR LEVEL DURING THE AFTERNOON. DO NOT EXPECT SKIES
TO SCATTER OUT MUCH BEYOND BROKEN CONDITIONS BY LATE IN THE
AFTERNOON. WESTERLY FLOW ALOFT MAY HELP BREAK THINGS UP A BIT TO THE
LEE OF THE OLYMPICS. EXPECT CEILINGS TO FILL AND LOWER AGAIN OVER
THE LOWLANDS TO IFR LEVELS AGAIN FRIDAY MORNING.

KSEA...IFR CEILINGS WILL PERSIST THROUGH 18Z...RISING THROUGH THE
AFTERNOON TO MVFR AND THEN IFR LEVELS...BUT REMAINING OVERCAST TO
BROKEN.  LIGHT SLY WINDS 4 TO 6 KT WILL BECOME SWLY AND STRENGTHEN
SOMEWHAT TO 6 TO 8 KTS IN THE AFTERNOON. CEILINGS WILL FILL
OVERNIGHT AND LOWER AROUND DAYBREAK TO IFR LEVELS.

&&

.MARINE...A WEAK SYSTEM WILL AFFECT WESTERN WA THIS TODAY FOLLOWED
BY A SECOND WEAK SYSTEM WILL REACH THE AREA ON FRI. ONSHORE FLOW
WILL CONTINUE THROUGH THE WEEKEND. GRADIENTS HAVE RELAXED A BIT THIS
MORNING WITH WINDS THROUGH THE STRAIT COMING DOWN TO 20 TO 25 KT
THROUGH THE CENTRAL AND EAST STRAIT. GRADIENTS WILL TIGHTEN AGAIN
THIS EVENING BRINGING ANOTHER ROUND OF SMALL CRAFT ADVISORY WINDS
THROUGH THE STRAIT TONIGHT IN THE POST-FRONTAL ONSHORE FLOW.

&&

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

&&

$$

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