Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 200354
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT TUE AUG 19 2014

.SYNOPSIS...AN UPPER LEVEL TROUGH OVER THE PACIFIC NORTHWEST WILL
MAINTAIN NORTHWEST FLOW ALOFT AND MODERATE LOW LEVEL ONSHORE FLOW
OVER WESTERN WASHINGTON THIS WEEK. SMALL WEAK UPPER LEVEL TROUGHS
BRUSHING THE AREA MAY BRING A FEW SHOWERS TO MAINLY THE CASCADES AT
TIMES. THE ONSHORE FLOW WILL BRING MOSTLY CLOUDY SKIES TO THE
LOWLANDS WITH PARTIAL AFTERNOON CLEARING. THE UPPER LEVEL TROUGH
WILL SHIFT EAST THIS WEEKEND ALLOWING AN UPPER LEVEL RIDGE TO BUILD
INLAND FOR A SMALL WARMING AND DRYING TREND.

&&

.SHORT TERM...A WEAK UPPER LEVEL TROUGH BROUGHT A FEW SHOWERS AND
THUNDERSTORMS TO EASTERN WA TODAY...BUT NOT MUCH ACTIVITY ON THE
WESTSIDE. THERE IS LOW LEVEL CONVERGENCE OVER KING COUNTY JUST LIKE
THE MESOSCALE MODELS SHOWED BUT NO SHOWERS ON THE RADAR. ALL THAT IS
SHOWING UP IS A FLAT MID LEVEL DECK OVER THE NORTH AND CENTRAL
SOUND. SO WITH THAT IN MIND...UPDATED THE FORECAST TO REMOVE THE
MENTION OF SHOWERS IN THE INTERIOR AND CASCADES. ALSO ABSENT IS THE
MARINE LAYER OFFSHORE...BUT THIS MAY FORM IN PLACE OVERNIGHT THEN
SPREAD INLAND.

NORTHWEST FLOW WILL PREVAIL ON WED AS AN UPPER LEVEL RIDGE BUILDS
OVER THE NE PAC. A WEAK DISTURBANCE OVER B.C. MAY TRIGGER A FEW
SHOWERS IN THE NORTH CASCADES. LOW CLOUDS MAY LINGER IN THE INTERIOR
THROUGH WED AFTERNOON WHICH WILL KEEP TEMPS A FEW DEGREES COOLER
THAN NORMAL.

THE UPPER LEVEL RIDGE WILL CONTINUE TO BUILD OFFSHORE ALONG 140W ON
THURSDAY WITH THE FLOW ALOFT TURNING MORE NORTHERLY. THIS WILL ALLOW
AN UPPER LOW TO DRIFT SOUTH AND SPIN OVER THE PAC NW THU NIGHT AND
FRIDAY. TEMPS WILL REMAIN A FEW DEGREES BELOW NORMAL DURING THE
PERIOD...WITH SHOWERS MAINLY IN THE CASCADES. 33

.LONG TERM...PREVIOUS DISCUSSION...THE UPPER LEVEL LOW OVER THE
PACIFIC NW ON FRIDAY WILL SHIFT E OVER ID BY SATURDAY
MORNING...ALLOWING AN OFFSHORE UPPER LEVEL RIDGE TO BUILD EASTWARD
OVER THE AREA. WITH WEAK ONSHORE FLOW FRIDAY AND SATURDAY NIGHTS AND
DRIER NW FLOW ALOFT ON SATURDAY...SKIES WILL BE MOSTLY SUNNY WITH
MAX TEMPS WARMING BACK TO THE 70S TO LOWER 80S. BOTH THE GFS AND
ECMWF BRING AN UPPER LEVEL SHORTWAVE TROUGH DOWN OVER W WA ON
SUNDAY. AT THIS POINT THE SHORTWAVE LOOKS MAINLY DRY WITH JUST AN
INCREASE IN MID/HI CLOUD COVER EXPECTED. ONSHORE FLOW WILL PROBABLY
INCREASE SUNDAY NIGHT AND MONDAY. KAM

&&

.AVIATION...NORTHWEST FLOW ALOFT WILL PREVAIL THROUGH WEDNESDAY WITH
AN UPPER TROUGH OVER THE PACIFIC NORTHWEST. AT THE SURFACE...ONSHORE
FLOW WILL CONTINUE WITH HIGH PRESSURE OFFSHORE AND LOWER PRESSURE
INLAND. AIR MASS GENERALLY STABLE.

THERE ARE AREAS OF MID LEVEL CLOUDS OVER THE AREA THIS EVENING...
MOSTLY OVER THE CASCADES AND IN A CONVERGENCE ZONE OVER CENTRAL
PUGET SOUND. LOW CLOUDS ARE EXPECTED TO FORM AT THE COAST AND SPREAD
INLAND WEDNESDAY MORNING UNDER MODERATE ONSHORE FLOW.

KSEA...MID CLOUDS THIS EVENING WITH A WEAK PUGET SOUND CONVERGENCE
ZONE TO THE NORTH. LOW CLOUDS ARE EXPECTED TO DEVELOP LATE
TONIGHT...THEN BURN OFF BY EARLY AFTERNOON WEDNESDAY. SOUTHERLY WIND
4-8 KNOTS. THERE IS A SMALL CHANCE THE WIND COULD SWITCH TO NORTH
FOR A COUPLE OF HOURS THIS EVENING IF THE CONVERGENCE ZONE SAGS A
LITTLE FARTHER SOUTH. SCHNEIDER

&&

.MARINE...ONSHORE FLOW WILL PREVAIL FOR THE NEXT SEVERAL DAYS WITH
HIGH PRESSURE OFFSHORE AND LOWER PRESSURE INLAND. THIS WILL PRODUCE
SMALL CRAFT ADVISORY WEST WINDS IN THE CENTRAL AND EASTERN STRAIT OF
JUAN DE FUCA MOST EVENINGS.

ONSHORE FLOW WILL BE STRONGEST THIS EVENING IN THE WAKE OF AN UPPER
LEVEL TROUGH. MARGINAL GALE FORCE WEST WINDS ARE POSSIBLE THIS
EVENING IN THE CENTRAL AND EASTERN STRAIT. THERE WILL ALSO BE SOME
SMALL CRAFT ADVISORY NORTHWEST WINDS OVER THE OUTER COASTAL WATERS
THROUGH WEDNESDAY MORNING. SCHNEIDER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.
    .SMALL CRAFT ADVISORY OUTER COASTAL WATERS...ADMIRALTY INLET...
     NORTHERN INLAND WATERS.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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





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