Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 120314
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
814 PM PDT FRI APR 11 2014

.SYNOPSIS...THE UPPER LEVEL TROUGH BRUSHING BY THE REGION TONIGHT
WILL KEEP SKIES PARTLY TO MOSTLY CLOUDY WITH A CHANCE OF SHOWERS
OVER THE NORTH CASCADES. A STRONG UPPER LEVEL RIDGE WILL BUILD
OFFSHORE THIS WEEKEND BRINGING WARMER TEMPERATURES AND SUNSHINE
THROUGH MONDAY. THE RIDGE WILL SHIFT INLAND AND WEAKEN MONDAY
AFTERNOON...ALLOWING A SERIES OF SYSTEMS TO PASS THROUGH THE PACIFIC
NORTHWEST BRINGING COOLER AND UNSETTLED THROUGH THE REMAINDER OF THE
NEXT WEEK.

&&

.SHORT TERM...WATER VAPOR IMAGERY SHOWS THE UPPER LEVEL SHORTWAVE
TROUGH CURRENTLY OVER SW B.C. ON ITS WAY TO THE SE. IR AND VISIBLE
IMAGERY SHOW DECREASING MOISTURE BEHIND THE TROUGH WITH MAINLY LOW
CLOUDS OFFSHORE. SKIES ARE PARTLY TO MOSTLY CLOUDY ACROSS W WA THIS
EVENING AND CLOUDS WILL PROBABLY DECREASE IN COVERAGE A LITTLE
THROUGH THIS EVENING. COOLING LATE TONIGHT MAY CAUSE A MINOR
INCREASE IN CLOUDS AGAIN TOWARD MORNING...BUT EVERYTHING WILL BURN
OFF BY LATE SATURDAY MORNING.

THE STRONG UPPER LEVEL RIDGE BUILDING OFFSHORE ALONG 140W THIS
EVENING WILL BE THE DOMINANT FEATURE THROUGH MONDAY...BRINGING IN
SUNNY AND WARMER WEATHER. MODELS PROGRESS THE RIDGE SLOWLY EASTWARD
THROUGH THE WEEKEND. ON SATURDAY THE RIDGE AXIS WILL REMAIN FAR
ENOUGH OFFSHORE SO THAT COOLER N-NW LOW LEVEL FLOW WILL LIMIT MAX
TEMPS TO THE LOWER TO MID 60S. THE RIDGE AXIS WILL BE APPROACHING
130W SATURDAY NIGHT...CLOSE ENOUGH SO THAT LOW LEVEL EASTERLY
OFFSHORE FLOW WILL DEVELOP. THIS WARMING BOOST SHOULD BE ENOUGH TO
PUT SUNDAYS MAX TEMPS IN THE MID TO UPPER 60S. I STILL THINK THERE
COULD BE SOME NEAR 70 READINGS OVER THE SW INTERIOR AND NEAR THE
CASCADES.

ON MONDAY THE UPPER LEVEL RIDGE WEAKENS AND SLIDES QUICKLY INLAND AS
STRONGER WESTERLY FLOW DEVELOPS UPSTREAM OVER THE NE PACIFIC.
GFS/ECMWF HAD BEEN LACKING CONSISTENCY WITH AN UPPER LEVEL LOW
APPROACHING THE REGION ON MONDAY BUT NOW AGREE THAT THE LOW WILL
CURVE NE...REACHING THE CENTRAL B.C. COAST MONDAY AFTERNOON. IN
RESPONSE TO THE RETREATING RIDGE AND THE APPROACHING LOW...ONSHORE
FLOW REDEVELOPS OVER W WA AND THE REMAINS OF A WEAK FRONT REACH THE
COAST SOMETIME IN THE AFTERNOON. THE MAIN MODEL DIFFERENCE IS THAT
THE GFS IS FASTER THAN THE ECMWF WHICH WOULD MEAN MORE COOLING ON
MONDAY. KAM

.LONG TERM...BOTH THE GFS AND ECMWF AGREE ON THE LARGER SCALE
FEATURES IN THE CHANGE TO A MORE ACTIVE WETTER WESTERLY FLOW PATTERN
EARLY NEXT WEEK. THE BIGGEST DIFFERENCES ARE IN STRENGTH...TIMING
AND HOW WET THE SYSTEMS ARE. THE GFS IS A LITTLE FASTER AND STRONGER
WITH THE NEXT WEAK UPPER LEVEL SHORTWAVE TROUGH MONDAY NIGHT...BUT
BOTH MODELS SUGGEST SHOWERS MAINLY OVER THE NORTH PART. THE ECMWF
THEN BRINGS A WETTER SYSTEM IN TUESDAY NIGHT OR WEDNESDAY...WHILE
THE GFS HOLDS OFF UNTIL WEDNESDAY AFTERNOON. THE EXISTING BROADBRUSH
CHANCE OF RAIN FORECAST WILL SUFFICE UNTIL MODEL CONSISTENCY
IMPROVES. KAM

&&

.HYDROLOGY...RIVER FLOODING IS NOT EXPECTED ON AREA RIVERS...EXCEPT
THE STILLAGUAMISH...THE NEXT 7 DAYS.

A FLOOD WARNING REMAINS IN EFFECT UNTIL FURTHER NOTICE FOR THE
NORTH FORK OF THE STILLAGUAMISH RIVER IN SNOHOMISH COUNTY JUST
UPSTREAM OF THE SR 530 LANDSLIDE. FLOODING CONTINUED AS A RESULT
OF WATER POOLED BEHIND THE DEBRIS DAM THAT PARTIALLY BLOCKS THE
RIVER.

THE NORTH FORK OF THE STILLAGUAMISH NEAR ARLINGTON AND THE POOL
UPSTREAM OF THE SR 530 SLIDE WILL CONTINUE FALLING THROUGH EARLY
NEXT WEEK.

A STRONG UPPER LEVEL RIDGE WILL BRING SUNNY AND WARMER WEATHER TO W
WA THIS WEEKEND. DRYNESS WILL CONTINUE THROUGH MONDAY. A SHIFT TO A
WESTERLY FLOW PATTERN EARLY NEXT WEEK WILL BRING A CHANCE OF RAIN
BACK TO THE AREA LATE MONDAY NIGHT AND TUESDAY WITH A WETTER SYSTEM
LIKELY TUESDAY NIGHT OR WEDNESDAY. HOWEVER AT THIS POINT THESE
SYSTEMS TO NOT APPEAR TO BE WET ENOUGH TO BE HYDROLOGIALLY
SIGNIFICANT. KAM

&&

.AVIATION...CLOUDS HAVE NOT FILLED IN SOUTH OF TACOMA AND THE
CLOUDIEST AREAS THIS EVENING ARE THE CENTRAL PUGET SOUND AREA AND
WEST SLOPES OF THE CASCADES. WITH ONSHORE/WLY FLOW THIS EVENING SOME
AREAS OF MID LEVEL CLOUDS BETWEEN 4.5 AND 7K MAY INCREASE LATER
TONIGHT...BUT THEN SCATTER OUT BY LATE SATURDAY MORNING. DRY
NORTHERLY FLOW WILL GIVE MOSTLY SUNNY SKIES SATURDAY AFTERNOON.


KSEA...A BROKEN DECK OF MID LEVEL CLOUDS AROUND 6K THIS EVENING MAY
SCATTER OUT FROM TIME TO TIME...BUT WILL PROBABLY REMAIN AND LOWER
OVERNIGHT. THE LOWER CLOUD DECK SHOULD THEN SCATTER OUT AGAIN AROUND
MID TO LATE MORNING ON SATURDAY. WINDS EXPECTED TO REMAIN NORTHERLY
OVERNIGHT. 13

&&

.MARINE...ONSHORE FLOW THIS EVENING...THERE IS A SMALL CRAFT
ADVISORY FOR WESTERLIES IN THE CENTRAL AND EASTERN STRAIT OF JUAN DE
FUCA...AS WINDS THERE ARE IN THE 20 TO 30 KNOT RANGE. ADMIRALTY
INLET ALSO HAS SCA BUT THOSE WINDS ARE CLOSER TO 20 KNOTS.  MORE
NORTHERLY FLOW WILL DEVELOP ON SATURDAY AND THEN TURN EASTERLY BY
SUNDAY AS A THERMALLY INDUCED SURFACE TROUGH FORMS ALONG THE COAST.
THE FLOW WILL THEN FLIP BACK TO ONSHORE MONDAY AND MONDAY NIGHT AS
THE SURFACE TROUGH SHIFTS INLAND AND EAST OF THE CASCADES. 13

&&

.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
      AND ADMIRALTY INLET.

&&

$$

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