Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 171615
AFDSEW

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

.SYNOPSIS...A WEAK LOW PRESSURE SYSTEM WILL MOVE ACROSS THE AREA
TODAY BUT WILL HAVE LITTLE IMPACT EXCEPT TO STRENGTHEN THE
ONSHORE FLOW. HIGH PRESSURE ALOFT WILL PREVAIL OVER THE REGION
THIS WEEKEND FOR DRY...WARM WEATHER.

&&

.SHORT TERM...
THE TAIL END OF A WEAK COLD FRONT WAS MOVING ACROSS THE CWA AT
THIS TIME. THERE WERE SOME SPRINKLES ASSOCIATED WITH IT. THE FRONT
SHOULD BE EAST OF THE CWA BY MIDDAY. OTHERWISE...THE MAIN IMPACT
OF THE FRONT WILL BE INCREASED ONSHORE FLOW. EXPECT THE UPPER
TROF TO MOVE ACROSS THE AREA THIS AFTERNOON. AN UPPER RIDGE WILL
BUILD OVER THE REGION FROM THE WEST TONIGHT.

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/05

.LONG TERM FROM THE PREV DISCUSSION...
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...AT 830 AM THERE WAS AN AREA OF LOW CLOUDS AND PATCHY
FOG FROM ABOUT KPAE NORTH THAT WILL BURN OFF LATE THIS MORNING.
THERE IS AN UPPER LEVEL TROUGH MOVING THROUGH B.C. TODAY AND AN
AREA OF MID LEVEL CLOUDS ASSOCIATED WITH THAT WAS OVER SOME OF WRN
WA THIS MORNING...A WEAK DRY COLD FRONT ALOFT. THE FLOW ALOFT IS
WSW AND WILL SWITCH TO NW AND INCREASE LATER THIS AFTERNOON AND
EVENING. SURFACE HIGH PRESSURE AND AN UPPER RIDGE WILL BUILD INTO
THE AREA FOR THE WEEKEND WITH THERMALLY INDUCED LOW PRESSURE OVER
CALIF EXTENDING SOMEWHAT NORTH ALONG THE OREGON COAST. A FEW AREAS
OF MORNING LOW CLOUDS AND FOG ON SATURDAY WILL GIVE WAY TO A SUNNY
DAY.

KSEA...SOME MID LEVEL CLOUDS FOR AWHILE TODAY AS AN UPPER TROUGH
MOVING THROUGH B.C. CLIPS WRN WA. THE NLY BREEZE SHOULD PICK UP THIS
AFTERNOON AND EVENING AS HIGH PRESSURE BUILDS IN. WE MAY SEE A FEW
HOURS OF LOW CLOUDS AND PATCHY FOG AROUND DAYBREAK SATURDAY.

&&

.MARINE...STRONG SURFACE HIGH PRESSURE WILL BUILD JUST OFFSHORE AND
INTO B.C. THROUGH TONIGHT AND SATURDAY. THIS SHOULD GIVE A PRETTY
DECENT NW BREEZE OVER THE COASTAL WATERS AND IN THE STRAIT THIS
AFTERNOON AND EVENING FOR SMALL CRAFT ADVISORY CONDITIONS THERE. A
DEVELOPING NW WIND IN THE STRAIT OF GEORGIA THIS AFTERNOON AND
EVENING MAY DRIVE SOUTH INTO THE SAN JUANS...SO I MAY EXTEND SMALL
CRAFT ADVISORIES TO THE NRN WATERS ON THE AFTERNOON FORECAST
DEPENDING ON WHAT THE WRFGFS AND HRRR SHOW TODAY...BUT AS OF 830 AM
THERE IS NOT YET MUCH WIND IN THE STRAIT OF GEORGIA DESPITE THE
STRONG PRESSURE RISES BETWEEN VRISL AND THE HAIDA GWAII. OVER THE
WEEKEND...THERMALLY INDUCED LOWER PRESSURE WILL DEVELOP OVER CALIF
AND EXTEND ALONG THE SRN OREGON COAST...THAT MAY CONTINUE TO DRIVE
A BRISK NW WIND ALONG THE COAST...ESPECIALLY IN THE AFTERNOON AND
EVENING HOURS. THE UPPER RIDGE WILL GIVE WAY MONDAY AND THE NEXT
FRONT WILL ARRIVE BY NEXT TUESDAY.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE COASTAL WATERS
     AND CENTRAL/EASTERN STRAIT OF JUAN DE FUCA.

     SMALL CRAFT ADVISORY FOR ROUGH BAR IN EFFECT TIL 5 AM SATURDAY
     FOR THE GRAYS HARBOR BAR.

&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
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