Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 201532
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT SUN JUL 20 2014

.SYNOPSIS...A WEAK UPPER TROUGH WILL MOVE THROUGH THE AREA TODAY
KEEPING A CHANCE OF SHOWERS IN THE FORECAST. AN UPPER LEVEL LOW WILL
BRING SHOWERS TUESDAY NIGHT AND WEDNESDAY. WEAK HIGH PRESSURE WILL
REBUILD LATE IN THE WEEK.

&&

.SHORT TERM...WE HAVE WESTERLY FLOW ALOFT OVER THE PACIFIC NORTHWEST
WITH MOIST ONSHORE FLOW INTO WESTERN WASHINGTON. A WEAK PUGET SOUND
CONVERGENCE ZONE DEVELOPED OVERNIGHT BUT IT IS PRODUCING VERY LITTLE
PRECIPITATION. THE FORECAST AREA WAS CLOUDY AT 8 AM WITH
TEMPERATURES IN THE MID 50S TO LOWER 60S.

A WEAK UPPER TROUGH WILL MOVE TO THE WASHINGTON COAST LATE TODAY AND
MAKE ITS WAY QUICKLY EAST OF THE CASCADES TONIGHT. THIS FEATURE WILL
MAINTAIN SOME LIGHT SHOWERS MAINLY ACROSS THE NORTH PART OF THE
FORECAST AREA...IN THE MOUNTAINS...AND IN THE CONVERGENCE ZONE.
COVERAGE WILL BE SCATTERED AND AMOUNTS WILL MOSTLY BE A FEW
HUNDREDTHS OF AN INCH. MUCH OF THE FORECAST AREA WILL REMAIN DRY AND
THERE COULD BE SOME SUN BREAKS IN THE AFTERNOON.

THE UPPER FLOW WILL BACK FROM WESTERLY TO SOUTHWEST MONDAY AS A DEEP
UPPER LOW STARTS DIGGING SOUTHEAST OFF THE COAST OF BRITISH
COLUMBIA. THE AIR MASS WILL DRY SOMEWHAT AS ONSHORE FLOW WEAKENS. WE
ARE LIKELY TO START THE DAY WITH CLOUDY SKIES...BUT THE AFTERNOON
SHOULD BE PARTLY TO MOSTLY SUNNY. THE UPPER LOW WILL TRACK SOUTHEAST
INTO THE AREA LATE TUESDAY AND SOUTHERLY FLOW ALOFT SHOULD RESULT IN
INSTABILITY AND SHOWERS DEVELOPING TUESDAY AFTERNOON OR EVENING.
MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE EARLY MORNING
FORECAST DISCUSSION -- RECENT GFS RUNS HAVE SLIGHTLY DELAYED THE
ARRIVAL OF THE UPPER LEVEL LOW...PROJECTING IT TO MOVE OVER WESTERN
WASHINGTON LATE WEDNESDAY EVENING INSTEAD OF WEDNESDAY AFTERNOON.
HOWEVER...THE FORECAST REMAINS THE SAME...HEAVIER AND MORE
WIDESPREAD SHOWERS AND PERHAPS A PERIOD OF CONTINUOUS SOAKING RAIN
IN THE LOWLANDS ON WEDNESDAY. THE SHOWERS WILL DECREASE WEDNESDAY
NIGHT AND THURSDAY MORNING AS THE TROUGH MOVES EAST OF THE CASCADES
AND AN UPPER RIDGE STARTS BUILDING OFFSHORE. HIGH PRESSURE ALOFT AND
WEAK ONSHORE FLOW WILL LIKELY PREVAIL FRIDAY AND SATURDAY FOR MOSTLY
SUNNY DAYS. JSMITH

&&

.AVIATION...AN UPPER LEVEL TROUGH WILL SWING THROUGH WESTERN
WASHINGTON THIS AFTERNOON AND EVENING WITH WESTERLY FLOW ALOFT. AT
THE SURFACE...HIGH PRESSURE WILL BE OFFSHORE WITH LOWER PRESSURE
INLAND. AIR MASS MOIST AND SLIGHTLY UNSTABLE.

MOSTLY 4000-7000 FOOT CEILINGS TODAY WHICH SHOULD SCATTER IN PLACES
DURING THE AFTERNOON. THERE ARE SOME LOWER CEILINGS IN A WEAK PUGET
SOUND CONVERGENCE ZONE OVER KING AND SNOHOMISH COUNTIES. THESE
SHOULD BREAK UP LATER TODAY AS THE CONVERGENCE ZONE FIZZLES. LOW
CLOUDS SHOULD FILL BACK IN ACROSS THE LOWLANDS MONDAY MORNING UNDER
CONTINUED ONSHORE FLOW.

KSEA...LOW CLOUDS WILL GRADUALLY LIFT AND BREAK UP TODAY. LIGHT WIND
BECOMING WESTERLY 4-8 KNOTS THIS AFTERNOON. SCHNEIDER

&&

.MARINE...HIGH PRESSURE WILL BE OFFSHORE WITH LOWER PRESSURE INLAND
FOR THE NEXT SEVERAL DAYS...RESULTING IN VARYING DEGREES OF ONSHORE
FLOW. THIS WILL GENERALLY PRODUCE SMALL CRAFT ADVISORY WEST WINDS
FOR THE CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA EACH EVENING INTO
THE EARLY MORNING HOURS. SCHNEIDER

&&

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