Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 070426
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
826 PM PST FRI MAR 6 2015

.SYNOPSIS...THE STRONG UPPER LEVEL RIDGE OVER THE PACIFIC NORTHWEST
WILL BRING MOSTLY SUNNY AND MILD WEATHER TO WESTERN WASHINGTON
THROUGH TUESDAY. A WEAK FRONTAL SYSTEM WILL BRING SOME RAIN TO THE
AREA TUESDAY NIGHT THROUGH THURSDAY. ANOTHER UPPER LEVEL RIDGE WILL
BUILD OVER THE AREA LATE IN THE WEEK FOR MORE DRY WEATHER.

&&

.SHORT TERM...A STRONG UPPER LEVEL RIDGE WILL REMAIN PARKED OVER THE
PACIFIC NORTHWEST THROUGH TUESDAY. THIS WILL KEEP DAYS MOSTLY SUNNY
AND AFTERNOON TEMPERATURES APRIL-LIKE THROUGH TUESDAY. MAX TEMPS
WILL BE IN THE MID 50S TO LOWER 60S.

THE RIDGE WILL BUILD A LITTLE BIT W OF VANCOUVER ISLAND TONIGHT AND
SATURDAY WHICH WILL SHIFT THE ASSOCIATED SURFACE HIGH JUST OFFSHORE.
THE RESULT WILL BE SOME WEAK ONSHORE FLOW THIS WEEKEND WHICH MAY END
UP DEPRESSING MAX TEMPS ALONG THE COAST A LITTLE. THE FLOW ALOFT
WILL ALSO SHIFT MORE NW ON SATURDAY AS THE RIDGE SHARPENS SOME. THIS
COULD CUT BACK ON THE AMOUNT OF CIRRUS OVER THE AREA. MODELS SHOW
THE RIDGE BEGINNING TO WEAKEN SLOWLY STARTING SUNDAY.

7 PM TEMPS AND DEW POINTS WERE ONCE AGAIN A LITTLE HIGHER THAN 24
HOURS AGO...WITH THE BIGGEST INCREASE IN THE DEW POINTS. THIS MAY
RESULT IN A LITTLE MORE PATCHY FOG DURING THE MORNINGS THIS WEEKEND.
KAM

.LONG TERM...THE ECMWF AND GFS SHOW MORE CONSENSUS THIS EVENING THAN
24 HOURS AGO...AND EVEN THEN AGREEMENT WAS INCREASING. BOTH MODELS
HAVE THE UPPER LEVEL RIDGE WEAKENING MONDAY AND TUESDAY WITH THE
AXIS FINALLY SHIFTING INLAND AND EAST OF THE CASCADES BY TUESDAY
AFTERNOON. THIS WILL ALLOW THE FLOW ALOFT TO TURN SW ON TUESDAY. THE
LARGE UPPER LEVEL LOW THAT HAD BEEN W OF THE RIDGE WILL BE WEAKENING
AND EJECTING NE TOWARD THE CENTRAL B.C. COAST. BOTH MODELS SHOW THE
WEAKENING COLD FRONT ASSOCIATED WITH THE EJECTING LOW BRINGING SOME
LIGHT RAIN TO W WA LATE TUESDAY NIGHT OR WEDNESDAY MORNING. THERE
ARE SOME FINER DETAILS STILL TO BE RESOLVED...RELATED TO THE UPPER
LEVEL SHORTWAVE TROUGHS EMBEDDED IN THE SW FLOW...BUT FOR NOW THE
BIG PICTURE IS LIGHT RAIN RETURNING TUESDAY NIGHT.

MODELS ALSO AGREE ON BUILDING ANOTHER RIDGE OVER THE AREA ON
THURSDAY...BUT THE 18Z GFS IS STRONGER AND DRY THURSDAY AND
FRIDAY...WHILE THE ECMWF SHOWS A WEAKER SLOWER BUILDING RIDGE WITH
SOME RAIN OVER THE AREA. KAM

&&

.HYDROLOGY...FLOODING IS UNLIKELY FOR THE NEXT 7 DAYS.

&&

.AVIATION...HIGH PRESSURE AND LIGHT NW FLOW ALOFT OVER THE PAC NW
WILL CONTINUE. LIGHT NLY FLOW AT THE SURFACE. THE AIR MASS IS STABLE
WITH DECREASING HIGH LEVEL MOISTURE. DRY IN THE LOWER AND MID
LEVELS...EXCEPT SURFACE BASED MOISTURE IN PORTIONS OF THE SW
INTERIOR. SHELTERED LOW LYING AREAS OF THE SW INTERIOR WILL HAVE
PATCHY FOG DEVELOPMENT SATURDAY MORNING...INCLUDING AT KOLM. ANY FOG
WILL BURN OFF BY 17-18Z.

KSEA...HIGH CLOUDS. NE WIND 3-5 KT...BECOMING NW TO 5 KT AFTER 18Z
SATURDAY. FOG NOT EXPECTED. DTM

&&

.MARINE...NO UPDATES. WEAK HIGH PRESSURE OVER THE WATERS WILL SHIFT
OFFSHORE SATURDAY WITH A WEAK TROUGH ALONG THE WEST COAST. THE
TROUGH WILL SHIFT INTO EASTERN WASHINGTON SUNDAY AND MONDAY. THE
OFFSHORE HIGH WILL SHIFT INLAND TUESDAY AS A FRONT APPROACHES FROM
THE WEST. THE FRONT WILL MOVE THROUGH THE WATERS TUESDAY NIGHT AND
WEDNESDAY WITH SMALL CRAFT ADVISORY WINDS POSSIBLE MOST AREAS.
SCHNEIDER/DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML







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