Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 110347
AFDSEW

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

.SYNOPSIS...AN UPPER LEVEL TROUGH WILL SPREAD CLOUDS ACROSS WESTERN
WASHINGTON TONIGHT AND FRIDAY...WITH A CHANCE OF SHOWERS OVER THE
COAST AND NORTH CASCADES ON FRIDAY. A STRONG UPPER LEVEL RIDGE WILL
BUILD OFFSHORE ON SATURDAY THEN MOVE INLAND SUNDAY FOR WARMER
TEMPERATURES AND MORE SUNSHINE THIS WEEKEND. WESTERLY FLOW ALOFT
WILL BRING A MORE UNSETTLED WEATHER PATTERN EARLY NEXT WEEK.

&&

.SHORT TERM...STILL JUST HIGH CLOUDS OVER W WA THIS EVENING EXCEPT
FOR THE COAST. WATER VAPOR IMAGERY SHOWS THE MAIN PART OF AN
APPROACHING UPPER LEVEL SHORTWAVE TROUGH HEADED TOWARD THE CENTRAL
B.C. COAST THIS EVENING. TO THE SOUTH OF THE TROUGH THERE IS A LARGE
MASS OF CLOUDS BETWEEN THE WEAK ASSOCIATED FRONT AND THE WA COAST
THAT WILL SPREAD INLAND TONIGHT AND FRIDAY. BASED ON UPSTREAM OBS
AND VISIBLE SATELLITE IMAGERY I WOULD GUESS THAT THE LOWEST MOISTURE
WAS IN THE BACK PART OF THE FRONTAL BAND BETWEEN VANCOUVER ISLAND
AND HAIDA GWAII. MODELS APPEAR TO BRING THIS AREA TO THE WA COAST
LATE TONIGHT AND INLAND BY 18Z. GFS/NAM TIME HEIGHT SECTION FOR KSEA
SHOWS THE MOISTURE ON FRIDAY AT LOW LEVELS FROM ABOUT 3000 FEET
THROUGH 8000 FEET. MODELS REMAIN CONSISTENT WITH THE
SHOWERS...SHOWING LIGHT QPF OF LESS THAN A TENTH OF AN INCH OVER THE
COAST AND W OLYMPICS...AND OVER THE CASCADES AND ADJACENT PARTS OF
THE N INTERIOR. OTHERWISE THE WEAK FRONTAL BAND WILL BE DRY FOR THE
REST OF W WA.

DRIER NW FLOW ALOFT WILL DEVELOP OVER W WA FRIDAY NIGHT AND SATURDAY
AS THE UPPER LEVEL TROUGH HEADS E AND A STRONG MODERATE AMPLITUDE
UPPER LEVEL RIDGE BUILDS OFFSHORE. ENOUGH LEFTOVER LOW LEVEL
MOISTURE FROM FRIDAY SHOULD KEEP SATURDAY MORNING MOSTLY
CLOUDY...BUT DRYING FROM THE UPPER LEVEL RIDGE SHOULD BRING MOSTLY
SUNNY SKIES SATURDAY AFTERNOON. BETWEEN THE MORNING CLOUDS AND
MODEST WARMING OF THE AIR MASS...MAX TEMPS SATURDAY WILL REMAIN IN
THE LOWER TO MID 60S.

MORE WARMING WILL TAKE PLACE SATURDAY NIGHT AND SUNDAY. AS THE UPPER
LEVEL RIDGE AXIS NEARS THE COAST THE INLAND SHIFT OF THE B.C.
SURFACE RIDGE WILL SHIFT THE LOW LEVEL FLOW OVER W WA TO OFFSHORE.
THIS ADDITIONAL WARMING SHOULD BOOST MAX TEMPS INTO THE MID TO UPPER
60S...PERHAPS EVEN NEAR 70 IN SOME SPOTS. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 259 PM AFD...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.

EXCEPT FOR A CHANCE OF LIGHT SHOWERS FRIDAY...THE WEATHER WILL BE
DRY THROUGH THIS WEEKEND AND PROBABLY INTO MONDAY. A SHIFT TO A
WESTERLY FLOW PATTERN EARLY NEXT WEEK WILL BRING A CHANCE OF RAIN
BACK TO THE AREA...BUT SO FAR NOTHING SIGNIFICANT HAS SHOWN UP IN
THE MODELS OUT THROUGH MID NEXT WEEK.

&&

.AVIATION...ONSHORE FLOW CONTINUES ACROSS WESTERN WA TONIGHT. THE
FLOW ALOFT IS WEST/NORTHWEST. A WEAK FRONT WILL DISSIPATE AS IT
MOVES THROUGH WESTERN WA FRIDAY MORNING. LIGHT RAIN IS POSSIBLE ON
THE COAST AND NORTH INTERIOR. VFR CONDITIONS ARE EXPECTED. 33

KSEA...VFR CONDITIONS EXPECTED. MODELS SHOW A BKN-OVC LAYER OF
3000-4000 FT FORMING IN THE INTERIOR FRIDAY MORNING...WHICH WILL
LIKELY PERSIST THROUGH THE AFTERNOON. SURFACE WINDS WILL BE LIGHT
N/NE. 33

&&

.MARINE...ONSHORE FLOW CONTINUES ACROSS WESTERN WA TONIGHT. A WEAK
COLD FRONT WILL DISSIPATE AS IT MOVES INLAND FRI MORNING. EXPECT
SMALL CRAFT ADVISORY WINDS THROUGH THE CENTRAL/EAST STRAIT OF JUAN
DE FUCA. OFFSHORE FLOW WILL DEVELOP ON SATURDAY AS AN INVERTED
SURFACE TROUGH FORMS ALONG THE COAST. OFFSHORE FLOW WILL INCREASE ON
SUNDAY AS THE TROUGH STRENGTHENS. THE FLOW WILL THEN FLIP BACK TO
ONSHORE MON/MON NIGHT AS THE SURFACE TROUGH SHIFTS INLAND. 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.

&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT
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