Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 151010
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PDT FRI AUG 15 2014

.SYNOPSIS...AN UPPER LEVEL LOW PRESSURE SYSTEM WILL BRING MORE
SHOWERS TO WESTERN WASHINGTON TODAY...ESPECIALLY AT THE COAST AND IN
THE MOUNTAINS. PRECIPITATION WILL DECREASE TONIGHT AS THE LOW MOVES
TO NORTHWEST MONTANA...AND DRY AND WARMER WEATHER IS EXPECTED THIS
WEEKEND AND EARLY NEXT WEEK. ANOTHER UPPER TROUGH WILL PROBABLY
ARRIVE BY WEDNESDAY WITH MORE SHOWERS.

&&

.SHORT TERM...THE UPPER LOW THAT HAS BEEN BRINGING UNSETTLED WEATHER
TO THE REGION SINCE MONDAY EVENING IS ALONG THE BORDER OF WASHINGTON
AND OREGON EARLY THIS MORNING...BETWEEN THE DALLES AND THE TRI
CITIES. BANDS OF PRECIPITATION WRAPPING AROUND THE LOW CONTINUED TO
BRING SHOWERS TO PARTS OF WESTERN WASHINGTON OVERNIGHT...MAINLY IN
THE MOUNTAINS AND ALONG THE COAST. SINCE LATE THURSDAY AFTERNOON
THOUGH...ALL THE THUNDERSTORM ACTIVITY HAS REMAINED TO THE NORTH AND
EAST OF OUR FORECAST AREA.

THE UPPER LOW WILL TRACK SLOWLY NORTHEAST INTO EASTERN WASHINGTON
TODAY...AND THE AIR MASS OVER WESTERN WASHINGTON WILL REMAIN
SOMEWHAT MOIST AND UNSTABLE. WRAP AROUND SHOWERS WILL AFFECT THE
FORECAST AREA TODAY...ESPECIALLY OVER THE MOUNTAINS AND AT THE
COAST. THERE SHOULD BE SOME AFTERNOON CLEARING OVER THE INTERIOR
LOWLANDS...AND MUCH OF THAT AREA -- INCLUDING SEATTLE -- SHOULD
REMAIN DRY OR NEARLY DRY THROUGH THE DAY. SOME THUNDERSTORMS COULD
DEVELOP OVER THE CASCADES AGAIN THIS AFTERNOON.

THE UPPER LOW WILL BEGIN TO OPEN AND FILL AS IT HEADS NORTHEAST
TONIGHT AND INTO SOUTHERN ALBERTA LATE SATURDAY. THE AIR MASS OVER
WESTERN WASHINGTON WILL GRADUALLY STABILIZE AND DRY AS A WEAK UPPER
RIDGE MOVES OVER THE AREA. SHOWERS SHOULD BECOME MAINLY LIMITED TO
THE MOUNTAINS...ESPECIALLY THE NORTH CASCADES AND OLYMPICS. FOR MOST
OF WESTERN WASHINGTON SATURDAY WILL BE PARTLY SUNNY AND A LITTLE
WARMER.

A POORLY DEFINED UPPER TROUGH WILL MOVE ONTO THE WEST COAST OF THE
UNITED STATES SATURDAY NIGHT AND SUNDAY...PROBABLY CENTERED OFF
SOUTHERN OREGON. THERE WILL BE LITTLE WEATHER ASSOCIATED WITH IT...
AND THE AIR MASS OVER WESTERN WASHINGTON SHOULD CONTINUE TO DRY AS
ONSHORE FLOW REMAINS QUITE WEAK. THE MORNING CLOUD COVER SHOULD BE
MAINLY ALONG THE COAST...AND MOSTLY SUNNY SKIES ARE LIKELY.
MCDONNAL

.LONG TERM...WEAK WESTERLY FLOW ALOFT AND LIGHT ONSHORE FLOW SHOULD
CONTINUE MONDAY AND TUESDAY...FOR A COUPLE MORE MOSTLY SUNNY DAYS.
OVER THE LAST 24 TO 36 HOURS...MODELS HAVE ALL TRENDED RATHER
SHARPLY TOWARD THE IDEA OF AN UPPER TROUGH MOVING INTO THE REGION BY
WEDNESDAY. THEY DIFFER ON HOW DEEP THE TROUGH WILL BE AND OTHER
DETAILS...BUT AT THIS POINT WE CAN BE FAIRLY CONFIDENT THAT THERE
WILL BE SOME SHOWERS OVER THE FORECAST AREA WEDNESDAY AND
THURSDAY.   MCDONNAL

&&

.AVIATION...AN UPPER LOW OVER CENTRAL WA WILL MOVE INTO NRN ID THIS
EVENING. A WEAK UPPER RIDGE WILL MOVE OVER THE AREA IN ITS WAKE.
CONTD NLY FLOW ALOFT AND LOW LEVEL ONSHORE FLOW. THE AIR MASS WILL
BECOME SLIGHTLY UNSTABLE OVER THE CASCADES THIS AFTERNOON FOR A RISK
OF ISOLD TSTMS.

MEANWHILE...PARTIAL CLEARING HAS TAKEN PLACE OVER PARTS OF THE
FORECAST AREA AND THIS HAS RESULT IN AREAS OF FOG REDUCING VSBYS TO
1SM OR LESS. THERE WERE ALSO AREAS OF CIGS AOB 500 FT. EXPECT THE
FOG AND LIFR CIGS TO LIFT BY LATE MORNING.

KSEA...CIGS ARE EXPECTED TO LOWER INTO THE 2-3K FT RANGE EARLY THIS
MORNING. EXPECT VFR CONDITIONS TO RETURN BY MIDDAY. CANNOT
COMPLETELY RULE OUT FOG AT THE TERMINAL EARLY MORNING...ESPECIALLY
IF THERE IS ANY CLEARING. THUS...CONFIDENCE IN THE FORECAST WAS NOT
HIGH.

&&

.MARINE...HIGH PRES OFFSHORE AND LOWER PRES E OF THE CASCADES WILL
RESULT IN ONSHORE FLOW OF VARYING STRENGTH THROUGH THE MIDDLE OF
NEXT WEEK. SOME STRENGTHENING OF THE FLOW WILL OCCUR THIS WEEKEND
THROUGH TUE. THERE IS A POSSIBILITY OF WINDS REACHING GALE FORCE
OVER PARTS OF THE STRAIT OF JUAN DE FUCA SOMETIME EARLY NEXT WEEK.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY IN EFFECT FROM 3 PM THIS AFTERNOON TIL
     2 AM SATURDAY FOR THE CENTRAL STRAIT OF JUAN DE FUCA.

&&

$$

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




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