Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 090952
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PDT WED APR 9 2014

.SYNOPSIS...DRIER WESTERLY FLOW ALOFT WILL PREVAIL TODAY THROUGH
THURSDAY AS A FLAT UPPER RIDGE BUILDS OVER WESTERN WASHINGTON...
RESULTING IN MOSTLY SUNNY DAYS. THE SOUTH END OF A WEAK UPPER TROUGH
MOVING THROUGH BRITISH COLUMBIA WILL BRING A CHANCE OF LIGHT SHOWERS
FRIDAY. A STRONGER UPPER RIDGE WILL PRODUCE SUNNY WARMER DAYS THIS
WEEKEND...WITH A CHANCE OF SHOWERS RETURNING EARLY NEXT WEEK.

&&

.SHORT TERM...THE COLD FRONT THAT MOVED THROUGH WESTERN WASHINGTON
LATE TUESDAY IS EAST OF THE CASCADES NOW...AND THE UPPER TROUGH
FOLLOWING THE FRONT IS MOVING ACROSS THE FORECAST AREA. A WEAK PUGET
SOUND CONVERGENCE ZONE FORMED TUESDAY EVENING BUT WAS SHORT LIVED...
AND BY 2 AM ONLY LIGHT SPOTTY SHOWERS CONTINUED IN THE MOUNTAINS.
THE MOIST ONSHORE FLOW THAT DEVELOPED IN THE WAKE OF THE FRONT IS
KEEPING SKIES MAINLY CLOUDY EARLY THIS MORNING.

THE UPPER TROUGH WILL MOVE EAST THIS MORNING...AND DRIER WESTERLY
FLOW ALOFT WILL ENSUE AS A FLAT UPPER RIDGE STARTS TO BUILD. THE FEW
SHOWERS LINGERING IN THE CASCADES WILL END THIS MORNING...AND THE
CLOUD COVER OVER WESTERN WASHINGTON SHOULD GIVE WAY TO MOSTLY SUNNY
SKIES BY MIDDAY. LOW LEVEL FLOW WILL REMAIN ONSHORE THOUGH...KEEPING
HIGH TEMPERATURES IN THE MID 50S TO LOWER 60S.

THE FLAT UPPER RIDGE WILL BUILD SLIGHTLY TONIGHT AND THURSDAY...BUT
LOW LEVEL ONSHORE FLOW WILL CONTINUE. CLOUD COVER WILL LIKELY
INCREASE TONIGHT IN THE LOWER LAYERS...BUT IT SHOULD GIVE WAY TO
MOSTLY SUNNY SKIES AGAIN BY MIDDAY THURSDAY AS THE AIR MASS DRIES
FURTHER. TEMPERATURES THURSDAY SHOULD BE JUST SLIGHTLY HIGHER THAN
TODAY.

THE FLAT RIDGE WILL MOVE EAST THURSDAY NIGHT...AND A WEAK UPPER
SHORTWAVE WILL MOVE INTO THE REGION FRIDAY. THIS FEATURE LOOKS
INCREASINGLY UNIMPRESSIVE...BASICALLY GIVING WESTERN WASHINGTON A
GLANCING BLOW AS IT MOVES THROUGH BRITISH COLUMBIA. SHOWERS
ASSOCIATED WITH IT WILL PROBABLY AMOUNT TO 0.1 INCH OR LESS -- EVEN
IN THE MOUNTAINS -- AND COVERAGE WILL PROBABLY BE SCATTERED AT MOST.
THE SNOW LEVEL WILL BE AROUND 4500 FEET OVER THE NORTH CASCADES...
WHERE PRECIPITATION IS MOST LIKELY...AND 5000 TO 6000 FT FURTHER
SOUTH IN THE CASCADES AND IN THE OLYMPICS.   MCDONNAL

.LONG TERM...THE WEAK UPPER TROUGH WILL MOVE EAST FRIDAY NIGHT. AN
UPPER RIDGE WILL PROGRESS INTO THE OFFSHORE WATERS ALONG 130W
SATURDAY AND THEN INLAND SUNDAY FOR SUNNY WARMER DAYS THIS WEEKEND.
THE LOW LEVEL FLOW SHOULD TREND OFFSHORE SUNDAY AND BOOST
TEMPERATURES INTO THE MID TO UPPER 60S OVER MUCH OF THE AREA.

MODELS AGREE THAT THE RIDGE WILL MOVE EAST SUNDAY NIGHT...WITH A
MORE PROGRESSIVE WESTERLY FLOW EARLY NEXT WEEK. BUT THEY ARE SHOWING
LITTLE CONSISTENCY ON TIMING AND CHARACTERS OF THE WEAK SYSTEMS THAT
COULD AFFECT WESTERN WASHINGTON. THE EUROPEAN MODEL SHOWS DRY
WEATHER MONDAY...BUT THE MOST RECENT GFS CERTAINLY MAKES OUR CURRENT
DRY FORECAST LOOK OPTIMISTIC. WITH MODELS CHANGING CONSIDERABLY FROM
RUN TO RUN...CONFIDENCE IS TOO LOW TO WARRANT A CHANGE IN THE
FORECAST NOW.   MCDONNAL

&&

.HYDROLOGY...ASIDE FROM THE NORTH FORK STILLAGUAMISH RIVER...AS
DISCUSSED BELOW...FLOODING IS NOT EXPECTED ON AREA RIVERS DURING THE
NEXT 7 DAYS.

A FLOOD WARNING REMAINS IN EFFECT FOR THE NORTH FORK OF THE
STILLAGUAMISH RIVER IN SNOHOMISH COUNTY JUST UPSTREAM OF THE SR530
LANDSLIDE. FLOODING CONTINUES AS A RESULT OF WATER POOLED BEHIND THE
DEBRIS DAM THAT PARTIALLY BLOCKS THE RIVER. THE FLOOD WARNING WILL
BE IN EFFECT UNTIL FURTHER NOTICE.

THE COLD FRONT PRODUCED ROUGHLY 0.3 TO 0.6 INCH OF RAIN IN THE NORTH
FORK STILLAGUAMISH BASIN...WHICH IS CAUSING THE NORTH FORK
STILLAGUAMISH RIVER TO RISE EARLY THIS MORNING. ANY ADDITIONAL
RAINFALL THROUGH AT LEAST SUNDAY WILL BE TOO LITTLE TO AFFECT THE
RIVER. THEREFORE THE RIVER WILL RISE ANOTHER FOOT OR SO BEFORE
CRESTING THIS MORNING...THEN FALL GRADUALLY THROUGH AT LEAST THIS
WEEKEND.

THE POOL UPSTREAM OF THE SR 530 SLIDE WILL RISE ANOTHER HALF FOOT TO
FOOT THIS MORNING...BUT REMAIN 2-3 FEET BELOW THE HIGH LEVEL OF
MARCH 30...THEN FALL GRADUALLY THROUGH AT LEAST THIS WEEKEND.

&&

.AVIATION...ONSHORE FLOW WILL CONTINUE ACROSS WESTERN WASHINGTON
TODAY. THE FLOW ALOFT IS WEST/NORTHWEST. SHOWERS ARE CONFINED TO THE
PUGET SOUND CONVERGENCE ZONE CURRENTLY OVER KING COUNTY. THE PSCZ
WILL DISSIPATE THIS MORNING WITH LIGHT SHOWERS ENDING.
OTHERWISE...THERE IS NOT MUCH CLOUD COVERAGE ACROSS THE CWA EXCEPT
FOR A BKN025 LAYER OVER THE NORTH COAST. LOW CLOUDS OVER THE CENTRAL
SOUND AND COAST WILL LIFT/SCATTER THIS AFTERNOON FOR VFR CONDITIONS.
33

KSEA...THE PSCZ WILL REMAIN IN THE VICINITY THIS MORNING WITH CIGS
NEAR 3000 FT. LOW CLOUDS WILL SCATTER THIS AFTERNOON WITH VFR
CONDITIONS EXPECTED. THE SURFACE WINDS WILL REMAIN LIGHT N/NE. 33

&&

.MARINE...HIGH PRES OVER THE NE PAC WILL MAINTAIN ONSHORE FLOW
ACROSS WESTERN WA THROUGH FRI. THE CENTRAL/EAST STRAIT OF JUAN DE
FUCA IS CURRENTLY SEEING WEST WINDS OF 15 TO 30 KT. WINDS IN THE
STRAIT WILL GRADUALLY EASE THIS MORNING...THEN PICK BACK UP AGAIN
LATE THIS AFTERNOON AND EVENING. THE CENTRAL/EAST STRAIT WILL LIKELY
SEE SMALL CRAFT ADVISORY WINDS THU AND FRI EVENING AS WELL. THE FLOW
WILL FLIP TO OFFSHORE DURING THE WEEKEND AS LOW PRES FORMS ALONG THE
COAST. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNING REMAINS IN EFFECT FOR THE NORTH FORK OF THE
     STILLAGUAMISH RIVER EAST OF THE SR 530 LANDSLIDE.

PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST 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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