Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 102159
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
259 PM PDT THU APR 10 2014

.SYNOPSIS...A WEAK DISTURBANCE MOVING ACROSS BRITISH COLUMBIA WILL
CLIP THE AREA LATE TONIGHT AND FRIDAY. LOOK FOR HIGH PRESSURE
ALOFT TO BUILD STRONGLY OVER THE REGION THIS WEEKEND. THIS
COMBINED WITH OFFSHORE FLOW WILL RESULT IN ABOVE NORMAL
TEMPERATURES...ESPECIALLY ON SUNDAY. MONDAY WILL BE A TRANSITION
DAY FROM DRY...WARM WEATHER TO A COOLER AND MORE UNSETTLED
PATTERN.

&&

.SHORT TERM...
A SHORT WAVE TROF MOVING ACROSS BRITISH COLUMBIA LATE TONIGHT AND
FRI WILL CLIP NORTHWEST WA...RESULTING IN A THREAT OF LIGHT
SHOWERS ACROSS MAINLY THE NORTHWEST QUARTER OF THE CWA ON FRI. THE
MAIN IMPACT THIS FEATURE WILL HAVE ON OUR WEATHER WILL BE TO
STRENGTHEN THE ONSHORE GRADIENT...RESULTING IN INCREASED CLOUD
COVER.

AN UPPER RIDGE WILL AMPLIFY STRONGLY OVER THE PACIFIC NORTHWEST
THIS WEEKEND. THIS COMBINED WITH INCREASINGLY LOW LEVEL OFFSHORE
FLOW WILL RESULT IN DAYTIME TEMPS ABOVE NORMAL...ESPECIALLY ON
SUNDAY. THE PRESENCE OF NORTHERLY BREEZES ON SATURDAY WILL LIKELY
KEEP THE INTERIOR LOCATIONS ADJACENT TO THE MAJOR WATER BODIES
FROM WARMING TOO MUCH. THE NORTH COAST MIGHT END UP WARMER THAN
PREDICTED ON SATURDAY DEPENDING ON THE WHETHER THERE IS AN EASTERLY
COMPONENT TO THE WIND.

THE LOW LEVEL FLOW WILL BE DECIDEDLY OFFSHORE ON SUNDAY...THUS MOST
INTERIOR PLACES...ESPECIALLY THE PUGET SOUND REGION SOUTHWARD...
WILL BE ALMOST AS WARM AS THEY WERE THIS PAST MON. THE COAST IS
EXPECTED TO BE JUST AS WARM AS THE INTERIOR...IF NOT WARMER.

.LONG TERM...
MON IS ANTICIPATED TO BE A TRANSITION DAY. THE UPPER RIDGE AXIS IS
EXPECTED TO MOVE EAST OF THE AREA...ALLOWING ONSHORE FLOW TO
RETURN. THIS WILL BE A TRICKY FORECAST AS FAR AS TEMPS ARE
CONCERNED. IF THE GFS SOLUTION OF A FARTHER SOUTHWARD TRACK OF THE
CLOSED UPPER LOW OFFSHORE ENDS UP VERIFYING...THE INTERIOR COULD
EXPERIENCE ADDITIONAL WARMING ON MON. IF THE ECMWF SOLUTION...
WHICH HAS A FARTHER NORTHWARD TRACK OF THE UPPER LOW...ENDS UP
CORRECT...EXPECT COOLER CONDITIONS ACROSS MUCH OF THE CWA. DECIDED
TO COMPROMISE BY COOLING OFF THE COAST ON MON WHILE SHOWING LITTLE
CHANGE ACROSS THE INTERIOR.

ALSO...IF THE GFS SOLUTION IS CORRECT...THERE COULD BE A RISK OF
TSTMS OVER THE CASCADES MON NIGHT THROUGH TUE MORNING.

BEYOND MON...CONFIDENCE IN THE FORECAST WAS LOW DUE TO CONTINUED
DIFFERENCES AND LACK OF CONTINUITY IN THE MEDIUM RANGE SOLUTIONS
AS FAR AS THE DETAILS WERE CONCERNED. THE ONE IDEA THEY AGREED
ON WAS THAT THE PATTERN OVER THE PACIFIC NORTHWEST WILL BECOME
COOLER AND MORE UNSETTLED...WHICH IS WHAT THE FORECAST INDICATES.

&&

.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.

EXPECT THE WEATHER PATTERN TO BECOME INCREASINGLY UNSETTLED...
BEGINNING ABOUT THE MIDDLE OF NEXT WEEK. HOWEVER CONFIDENCE IN THE
DETAILS OF THE FORECAST BEYOND NEXT MON WAS LOW. FOR EXAMPLE...ONE
MODEL SOLUTION INDICATED THAT ABOUT AN INCH OF RAIN COULD FALL ON
THE SLIDE AREA NEXT THU WHILE OTHER MODELS WERE DRIER...SHOWING
LESS THAN A QUARTER OF AN INCH. FOR NOW...LEANING TOWARD THE DRIER
SOLUTIONS.

&&

.AVIATION...WEST TO NORTHWEST FLOW ALOFT WILL CONTINUE TONIGHT AND
FRIDAY. AT THE SURFACE...HIGH PRESSURE WILL BE OFFSHORE WITH LOWER
PRESSURE INLAND. A WEAK SURFACE TROUGH WILL REACH THE COASTAL WATERS
LATE TONIGHT THEN DISSIPATE. THE AIR MASS IS STABLE WITH MOSTLY HIGH
LEVEL MOISTURE. MID AND LOW LEVEL MOISTURE WILL INCREASE AFTER
MIDNIGHT TONIGHT.

KSEA...HIGH CLOUDS THIS EVENING. SOME MID CLOUDS SHOULD DEVELOP
AFTER MIDNIGHT...THEN LOW CLOUDS FRIDAY MORNING. NORTHERLY WIND 4-8
KNOTS WILL BECOME FRIDAY MORNING. SCHNEIDER

&&

.MARINE...ONSHORE FLOW WILL BRING ANOTHER ROUND OF SMALL CRAFT
ADVISORY WEST WINDS TO THE CENTRAL AND EASTERN STRAIT THROUGH THE
EARLY MORNING HOURS TONIGHT.

A WEAK TROUGH WILL MOVE INTO THE COASTAL WATERS LATE TONIGHT AND
WASH OUT AS IT MOVES ONSHORE FRIDAY. THE TROUGH WILL NOT HAVE MUCH
EFFECT ON THE SURFACE PATTERN...ONSHORE FLOW WILL CONTINUE AND THERE
SHOULD BE ANOTHER ROUND OF SMALL CRAFT ADVISORY WEST WINDS FOR THE
CENTRAL AND EAST STRAIT FRIDAY EVENING.

OFFSHORE FLOW WILL DEVELOP SATURDAY AND STRENGTHEN ON SUNDAY AS A
THERMAL TROUGH FORMS ALONG THE COAST AND HIGHER PRESSURE BUILDS INTO
INTERIOR BRITISH COLUMBIA. SMALL CRAFT ADVISORY EAST WINDS ARE
POSSIBLE AT THE WEST ENTRANCE SATURDAY NIGHT AND SUNDAY MORNING.

THE TROUGH ALONG THE COAST IS EXPECTED TO SHIFT INLAND ON MONDAY
WITH MODERATE ONSHORE FLOW RETURNING TO THE WATERS ALTHOUGH THE
MODELS ARE HAVING A HARD TIME WITH THE DETAILS AND CONFIDENCE IN THE
FORECAST IS NOT HIGH. SCHNEIDER

&&

.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 ADVISORIES ARE IN EFFECT FOR THE CENTRAL/
     EASTERN TRAIT OF JUAN DE FUCA AND ADMIRALTY INLET.

&&

$$

WWW.WEATHER.GOV/SEATTLE

FOR THE GRAPHICAL AFD...VISIT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML




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