Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 170954
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PDT FRI APR 17 2015

.SYNOPSIS...A WEAK UPPER TROUGH WILL BRUSH WESTERN WASHINGTON TODAY
AS IT MOVES THROUGH BRITISH COLUMBIA. IT WILL BRING SOME CLOUDS...
BUT THE AREA SHOULD STILL HAVE A MOSTLY TO PARTLY SUNNY DAY. A
STRONG UPPER RIDGE WILL MOVE ACROSS THE REGION THIS WEEKEND AND
EARLY NEXT WEEK FOR SUNNY WARM WEATHER. AN UPPER TROUGH WILL REACH
THE AREA BY WEDNESDAY FOR COOLER SHOWERY WEATHER.

&&

.SHORT TERM...THE UPPER LEVEL RIDGE OVER THE PACIFIC NORTHWEST IS
WEAKENING AS IT MOVES EAST OF THE CASCADES EARLY THIS MORNING...AND
A WEAK UPPER LEVEL TROUGH OFFSHORE IS MOVING INTO BRITISH COLUMBIA.
SKIES ARE MAINLY CLEAR...EXCEPT FOR THE CLOUD COVER ASSOCIATED WITH
THE UPPER TROUGH OVER THE NORTHWEST CORNER OF THE OLYMPIC PENINSULA.
TEMPERATURES AT 2 AM WERE IN THE UPPER 30S AND 40S.

THE SOUTH END OF THE UPPER TROUGH WILL MOVE INTO WESTERN WASHINGTON
TODAY AND EAST OF THE CASCADES TONIGHT. ITS PRECIPITATION SHOULD
REMAIN NORTH OF OUR FORECAST AREA. THERE WILL BE SOME MID LEVEL
MOISTURE...AND THE TROUGH WILL TURN THE LOW LEVEL FLOW MORE STRONGLY
ONSHORE. STILL WE CAN EXPECT A MOSTLY SUNNY DAY FOR THE WESTERN
WASHINGTON INTERIOR LOWLANDS AND A PARTLY TO MOSTLY SUNNY DAY FOR
THE COAST AND MOUNTAINS. HIGHS WILL BE IN THE UPPER 50S TO 60S...
SIMILAR TO THURSDAY.

AN OFFSHORE UPPER RIDGE WILL BUILD AS IT MOVES INSIDE OF 130W ON
SATURDAY...WITH DRIER NORTHWEST FLOW ALOFT OVER WESTERN WASHINGTON.
ASIDE FROM BRIEF MORNING CLOUDS MAINLY ALONG THE COAST AND IN THE
LOWER CHEHALIS VALLEY...SATURDAY SHOULD BE A MOSTLY SUNNY DAY.
NORTHWEST PRESSURE GRADIENTS WILL KEEP THE LOW LEVEL FLOW WEAKLY
ONSHORE...BUT IT SHOULD BE A LITTLE WARMER WITH HIGHS MAINLY 60 TO
70.

THE UPPER RIDGE WILL PROGRESS TO THE PACIFIC NORTHWEST COAST ON
SUNDAY. AT MOST...LOW LEVEL FLOW WILL BE VERY WEAK...AND THERE COULD
BE A WEAK THERMALLY INDUCED SURFACE TROUGH OVER WESTERN WASHINGTON.
AT ANY RATE IT WILL BE A SUNNY DAY AND SLIGHTLY WARMER YET.
MCDONNAL

.LONG TERM...THE UPPER RIDGE WILL WEAKEN AS IT MOVES INLAND ON
MONDAY...BUT THE LOWER REACHES OF THE AIR MASS SHOULD REMAIN AS WARM
OR BE A LITTLE WARMER THAN SUNDAY.

THE RIDGE WILL MOVE EAST OF THE CASCADES ON TUESDAY. THEN AN
OFFSHORE UPPER TROUGH WILL BRING MOIST SOUTHERLY FLOW ALOFT OVER
WESTERN WASHINGTON...WITH INCREASING ONSHORE LOW LEVEL FLOW. THE
MODELS LEAVE LOTS OF UNCERTAINTY WITH REGARD TO WHEN THIS WILL
HAPPEN. THE MOST RECENT RUNS OF THE GFS...EUROPEAN MODEL...AND
CANADIAN MODEL ALL SHOW A FASTER PROGRESSION WITH STRONG ONSHORE
FLOW AND SOME SHOWERS OVER WESTERN WASHINGTON TUESDAY AFTERNOON.
UNTIL WE SEE THAT THIS IS A CLEAR TREND IN THE MODELS...WE WILL STAY
WITH A MOSTLY SUNNY FORECAST FOR TUESDAY. THE UPPER TROUGH SHOULD BE
OVER THE AREA WEDNESDAY AND THURSDAY FOR COOLER WEATHER AND
SCATTERED SHOWERS.   MCDONNAL

&&

.AVIATION...A WEAK UPPER LEVEL TROUGH AND SURFACE COLD FRONT WILL
CROSS WESTERN WA TODAY. HOWEVER THERE IS LITTLE MOISTURE ATTACHED
WITH THIS SYSTEM AND DRY WEATHER IS EXPECTED. THE FLOW ALOFT IS
WESTERLY. VFR CONDITIONS EXPECTED. HIGH PRESSURE WILL BUILD OVER THE
AREA TONIGHT FOR DRY AND STABLE WEATHER. 33

KSEA...FEW-SCT MID OR HIGH CLOUDS TODAY WITH N WINDS TO 10 KT. 33

&&

.MARINE...ONSHORE FLOW WILL INCREASE TODAY AS A COLD FRONT CROSSES
WESTERN WA. THE COASTAL WATERS WILL LIKELY SEE SMALL CRAFT ADVISORY
WINDS THIS EVENING...WITH GALES POSSIBLE THROUGH THE CENTRAL AND
EAST STRAIT. MEANWHILE...WEST SWELL 10-12 FT WILL SUBSIDE TO 9 FT BY
MID MORNING.

LOW PRESSURE WILL FORM ALONG THE COAST THIS WEEKEND FOR NORTHERLY
FLOW. OFFSHORE FLOW WILL CONTINUE ON MONDAY. THE NEXT FRONT WILL
ARRIVE NEXT TUESDAY. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COAST AND WEST ENTRANCE FOR HAZARDOUS SEAS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.
     SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT TODAY.
     GALE WATCH THIS EVENING CENTRAL AND EAST STRAIT.

&&

$$

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





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