Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 231552
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT SAT AUG 23 2014

.SYNOPSIS...HIGH PRESSURE ALOFT BUILDING OVER THE PACIFIC NORTHWEST
WILL GENERATE SUNSHINE AND WARMER TEMPERATURES FOR WESTERN
WASHINGTON THIS WEEKEND AND INTO NEXT WEEK. A WEAK WEATHER SYSTEM
MOVING SOUTHEAST THROUGH BRITISH COLUMBIA MAY PRODUCE A THREAT OF
SHOWERS NEAR THE CASCADE CREST LATE SUNDAY. A CHANGE TO MORE CLOUDS
AND COOLER CONDITIONS IS LIKELY NEAR THE END OF NEXT WEEK.

&&

.SHORT TERM...THE UPPER LOW IS OVER SOUTHERN IDAHO THIS MORNING AND
THE UPSTREAM UPPER RIDGE AXIS IS NEAR 150W. SATELLITE IMAGERY SHOWS
THE VERY WEAK SHORTWAVE THAT WAS CROSSING THE TOP OF THE UPPER RIDGE
FRIDAY AFTERNOON IS NOW SLIDING SE NEAR 140W IN THE NW FLOW ALOFT.

RAOBS SHOW THE AIR MASS CIRCULATING AROUND THE UPPER LOW REMAINS
UNSTABLE. ANY SHOWERS THAT POP UP OVER THE CASCADES DURING THE HEAT
OF THIS AFTERNOON SHOULD REMAIN EAST OF THE CREST WITH ONLY SOME
CLOUD COVER WEST OF THE CREST.

PRESSURE GRADIENTS WERE QUITE LIGHT THIS MORNING PERMITTING FOG AND
LOW STRATUS TO FORM ALONG THE COAST...THROUGH THE STRAIT TO THE
SOUTHERN SAN JUANS AND NORTHERN WHIDBEY ISLAND...INTO THE CHEHALIS
GAP AND IN LOCAL VALLEYS IN THE SOUND SUCH AS THE SNOHOMISH RIVER.
ALL BUT THE COAST AND PERHAPS PARTS OF THE STRAIT SHOULD DISSIPATE
WITH HIGHS IN THE INTERIOR WARMING ABOUT 5 DEGREES OR SO FROM
FRIDAYS.

THE INCOMING VERY WEAK SHORTWAVE WILL SWING THROUGH BC TONIGHT AND
SUNDAY. 12Z PROGS IN THUS FAR CONCUR WITH ESSENTIALLY A REPEAT OF
FRIDAY WITH LIMITED SHOWERS AND TSTMS IN THE CASCADES. THIS SYSTEM
SHOULD RESULT IN SOME LOW LEVEL ONSHORE FLOW AND COOL SUNDAY HIGHS A
BIT FROM TODAY.

THE SHORTWAVE WILL CONTINUE INLAND SUNDAY NIGHT AND MONDAY WITH THE
UPSTREAM UPPER RIDGE EDGING CLOSER. SO THE ASSOCIATED DRIER
NORTHERLY FLOW ALOFT MONDAY SHOULD PRODUCE ANOTHER SUNNY AND WARMER
DAY. BUEHNER

.LONG TERM...FROM THE PREVIOUS DISCUSSION...THE UPPER RIDGE WILL
PROGRESS INTO THE PACIFIC NORTHWEST TUESDAY AND WEDNESDAY...BUILDING
SLIGHTLY AS IT MOVES INLAND. THESE WILL BE SUNNY WARM DAYS WITH
LITTLE IF ANY MARINE STRATUS. UPPER HEIGHTS WILL BEGIN TO FALL ON
THURSDAY...AS THE UPPER RIDGE MOVES ACROSS THE ROCKY MOUNTAINS AND
AN OFFSHORE UPPER TROUGH STARTS APPROACHING THE REGION. IT WILL BE
ANOTHER DRY DAY...BUT WITH A STRONGER MARINE INTRUSION. ON FRIDAY
THE UPPER TROUGH WILL MOVE CLOSE ENOUGH FOR A CHANCE OF SHOWERS IN
THE NORTH AND OVER THE CASCADES.   MCDONNAL

&&

.AVIATION...NORTHERLY FLOW ALOFT WILL PREVAIL OVER W WA TODAY AND
TONIGHT...BETWEEN A LARGE UPPER LEVEL LONGWAVE RIDGE CENTERED WELL
OFFSHORE AND AN UPPER LEVEL LOW OVER THE NORTHERN ROCKIES. THE AIR
MASS OVER W WA IS DRY AND STABLE. LOW LEVEL STRATUS OVER THE LOWER
CHEHALIS VALLEY...THE WA COAST AND PARTS OF THE N INTERIOR IS
EXPECTED TO DISSIPATE BY 18Z FOR UNRESTRICTED VFR CONDITIONS THIS
AFTERNOON.

WEAK ONSHORE FLOW WILL BRING IN MORE SHALLOW MORNING STRATUS AND FOG
SUNDAY MORNING. IN ADDITION...PATCHY MAINLY VALLEY FOG WILL FORM IN
PLACE OVER THE TYPICAL AREAS OF W WA. LOCAL MVFR/IFR CONDITIONS ARE
EXPECTED SUNDAY MORNING.

AN UPPER LEVEL SHORTWAVE TROUGH OVER THE AK PANHANDLE AT 15Z/8 AM
TODAY WILL MOVE SE DOWN THE E SIDE OF THE OFFSHORE RIDGE...MOVING
OVER W WA SUNDAY MORNING. THE TROUGH WILL DESTABILIZE THE AIR MASS
FOR ISOLATED THUNDERSTORMS OVER THE CASCADES SUNDAY AFTERNOON AND
EVENING.

KSEA...MOSTLY CLEAR SKIES WITH UNRESTRICTED VFR CONDITIONS TODAY.
SURFACE WINDS WILL BE N-NW 5-10 KT THROUGH 03Z THEN SHIFT NE 3-6 KT.
KAM

&&

.MARINE...A LARGE SURFACE HIGH CENTERED NEAR 140W ALONG WITH LOWER
PRESSURE EAST OF THE CASCADES WILL MAINTAIN A WEAK ONSHORE FLOW
PATTERN INTO THE MIDDLE OF NEXT WEEK. ONSHORE FLOW WILL BE TYPICALLY
STRONGEST DURING THE LATE AFTERNOON AND EVENING HOURS AND WEAKEST
DURING THE MORNING. SMALL CRAFT ADVISORY WINDS ARE EXPECTED IN THE
CENTRAL AND EAST STRAIT EACH EVENING. AN UPPER LEVEL TROUGH WILL
MOVE SE OVER WESTERN WASHINGTON SUNDAY MORNING. SURFACE PRESSURE
GRADIENTS OVER THE COASTAL WATERS WILL INCREASE A LITTLE AHEAD OF
THE TROUGH...BUT BASED ON THE LATEST ARW-WEST AND NAM12 SURFACE WIND
FORECASTS...SURFACE WINDS OVER THE OUTER WATERS ARE EXPECTED TO
REMAIN JUST BELOW SCA LEVELS. KAM

&&

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

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML









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