Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 250402
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
902 PM PDT SUN MAY 24 2015

.SYNOPSIS...A RIDGE OFFSHORE WILL MAINTAIN LOW LEVEL ONSHORE FLOW
ACROSS WESTERN WASHINGTON THROUGH TUESDAY...KEEPING SKIES MOSTLY
CLOUDY. WEAKER ONSHORE FLOW FROM MID WEEK ONWARD ALONG WITH HIGHER
PRESSURE BUILDING OVER THE REGION WILL ALLOW MORE AFTERNOON SUNSHINE
AND A WARMING TREND. THE AIR MASS OVER THE CASCADES WILL CONTINUE TO
BECOME SLIGHTLY UNSTABLE LATE IN THE DAY FOR A CHANCE OF SHOWERS
NEAR THE CREST.

&&

.SHORT TERM...AN UPPER LEVEL LOW OVER SOUTHERN B.C. WILL SPIN OVER
WASHINGTON MONDAY AND TUESDAY FOR ISOLATED TO SCATTERED SHOWERS IN
THE CASCADES. THERE IS ALSO A CHANCE OF THUNDERSTORMS IN THE NORTH
CASCADES MONDAY AFTERNOON AND EVENING AS THE COLD POOL MOVES
OVERHEAD...HOWEVER MOST OF THE ACTIVITY WILL LIKELY BE EAST OF THE
CREST. MEANWHILE...LOW LEVEL ONSHORE FLOW WILL CONTINUE FOR MOSTLY
CLOUDY SKIES ACROSS THE LOWLANDS.

THE UPPER LOW WILL WEAKEN AS IT EXITS EAST ON WEDNESDAY WHILE A
RIDGE MOVES IN FROM THE WEST. THERE IS STILL ENOUGH MOISTURE AND
INSTABILITY FOR A CHANCE OF SHOWERS ALONG THE WEST SLOPES OF THE
CASCADES...WITH ISOLATED THUNDERSTORMS POSSIBLE. THE LOWLANDS WILL
REMAIN DRY WITH A FEW SUNBREAKS POSSIBLE IN THE AFTERNOON. 33

.LONG TERM...PREVIOUS DISCUSSION...THE OFFSHORE UPPER LEVEL RIDGE
AXIS WILL MOVE INLAND OVER W WA ON THURSDAY THEN SHIFT E OVER MT ON
FRIDAY. HOWEVER THE RIDGE WILL BE BROADENING AT THE SAME TIME SO
THAT W WA WILL REMAIN UNDER ITS INFLUENCE. A SURFACE RIDGE WILL
REMAIN ABOUT 500 NM OFFSHORE ALONG ROUGHLY 135W...BUT THE ONSHORE
PRESSURE GRADIENTS OVER W WA WILL BE WEAK AND PREDOMINANTLY
WESTERLY. THIS WILL LIMIT THE AMOUNT OF MARINE MOISTURE SPREADING
INLAND OVER W WA EACH EVENING...RESULTING IN AFTERNOON CLEARING
AFTER AREAS OF MORNING LOW CLOUDS. ADDITIONAL WARMING FROM THE RIDGE
WITH AFTERNOON SUNSHINE SHOULD KICK MAX TEMPS INTO THE 70S OVER THE
INTERIOR. ONSHORE FLOW WILL KEEP THE COAST IN THE 60S. KAM

&&

.AVIATION...A WEAK UPPER LEVEL LOW WILL SLOWLY MOVE SOUTHEAST
ACROSS MAINLY EASTERN WASHINGTON TONIGHT THROUGH TUESDAY. LIGHT NW
FLOW ALOFT. AIR MASS IS MOIST IN THE LOWER LEVELS AND SOMEWHAT MOIST
ALOFT. STABLE CONDITIONS EXCEPT SLIGHTLY UNSTABLE ALONG THE CASCADE
CREST. THE UPPER LOW WILL DEEPEN THE MARINE LAYER AND WEAK LIFT MAY
PRODUCE PATCHY DRIZZLE MONDAY MORNING. MVFR CEILINGS WILL BRIEFLY
SCATTER THIS EVENING BUT SHOULD FILL BACK IN OVERNIGHT. WIDESPREAD
MVFR IS EXPECTED MONDAY MORNING WITH PATCHY IFR CONDITIONS WHERE
DRIZZLE DEVELOPS.

KSEA...CEILING AROUND 3K FT WILL LOWER TO 1-2K FT OVERNIGHT. DRIZZLE
IS POSSIBLE MONDAY MORNING. CEILINGS WILL LIFT LATE MONDAY AFTERNOON
BUT ANY SCATTERING WILL BE SHORT LIVED AS THE DEEP MARINE LAYER
REMAINS INTACT. CHB

&&

.MARINE...HIGH PRES WILL REMAIN OFFSHORE WITH LOWER PRES INLAND.
WINDS OF 15 TO 25 KT WILL INCREASE TO 20 TO 30 KT DURING THE EVENING
THROUGH EARLY MONDAY MORNING AS ONSHORE FLOW INCREASES. SMALL CRAFT
ADVISORY STRENGTH WESTERLY WINDS ARE LIKELY EACH EVENING IN THE
STRAIT TONIGHT THROUGH AT LEAST TUESDAY NIGHT. ONSHORE FLOW WILL
WEAKEN WEDNESDAY THROUGH FRIDAY. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT.

&&

$$

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