Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 170333
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
830 PM PDT SAT MAY 16 2015

.SYNOPSIS...MOIST ONSHORE FLOW WILL KEEP WESTERN WASHINGTON CLOUDY
THOUGH SUNDAY MORNING...WITH PARTIAL CLEARING SUNDAY AFTERNOON. WEAK
LOW PRESSURE ALOFT WILL BRING A CHANCE OF SHOWERS MAINLY TO THE
CASCADES...WITH A CHANCE OF CASCADE THUNDERSTORMS SUNDAY AFTERNOON
AND EVENING. THIS PATTERN WILL CONTINUE NEXT WEEK WITH PARTLY SUNNY
AND MILD CONDITIONS AND A CHANCE OF MAINLY MOUNTAIN SHOWERS.

&&

.SHORT TERM...STRATUS COVERS WESTERN WASHINGTON EXCEPT FOR A BIT OF
CLEARING IN THE FAR NORTHWEST INTERIOR. A COUPLE OF THUNDERSTORMS
HAVE BROKEN OUT THIS EVENING IN THE CASCADES OF OKANOGAN COUNTY BUT
THESE ARE UNLIKELY TO CROSS OVER THE CREST. ONSHORE FLOW IS PROBABLY
STRONG ENOUGH TO KEEP CLOUDS OVER MOST OF THE AREA ALL NIGHT.

ONSHORE PRESSURE GRADIENTS WILL BECOME FLAT OVERNIGHT AND WILL NOT
INCREASE MUCH SUNDAY AFTERNOON. THERE COULD EVEN BE SOME LIGHT
OFFSHORE FLOW SUNDAY MORNING. THIS WILL ALLOW THE MARINE LAYER TO
BREAK UP FOR AT LEAST PARTLY SUNNY CONDITIONS SUNDAY AFTERNOON. THE
INCREASED SUNSHINE WITH A LITTLE WARMING FROM THE OFFSHORE FLOW
SHOULD BE ENOUGH TO BOOST MAX TEMPS INTO THE MID TO UPPER 60S OVER
THE INTERIOR LOWLANDS. THE COAST WILL WARM A FEW DEGREES BUT NOT AS
MUCH AS THE INTERIOR.

MODELS STILL SHOW A VERY WEAK SHORTWAVE TROUGH AROUND 700 MB THAT
MOVES WEST OVER THE CASCADES SUNDAY AFTERNOON. THE AIR MASS WILL
BECOME UNSTABLE OVER THE CASCADES SUNDAY AFTERNOON WITH LIFTED
INDICES RANGING FROM -1 TO -4. THIS COULD PROVIDE ENOUGH LIFT TO
TRIGGER SOME AFTERNOON AND EVENING THUNDERSTORMS OVER THE CASCADES.
WITH THE SE 700 MB FLOW IT IS POSSIBLE THAT DECAYING CELLS COULD
DRIFT WEST OVER THE LOWLANDS. THUS THERE IS A CHANCE OF SHOWERS OVER
MUCH OF THE INTERIOR LOWLANDS SUNDAY AFTERNOON.

VERY WEAK PRESSURE GRADIENTS AND ANY MID LEVEL CLOUDINESS SUNDAY
NIGHT WILL NOT PRODUCE MUCH STRATUS MONDAY MORNING. MONDAY SHOULD BE
PARTLY TO MOSTLY SUNNY WITH HIGH TEMPERATURES IN THE MID 70S OVER
THE INTERIOR. THE AIR MASS WILL REMAIN SLIGHTLY UNSTABLE FOR A
CHANCE OF SHOWERS AND A CHANCE OF THUNDERSTORMS IN THE CASCADES. ON
TUESDAY TYPICAL AFTERNOON SHOWERS SHOULD DEVELOP OVER THE MOUNTAINS.
ONSHORE FLOW WILL REMAIN WEAK MONDAY NIGHT SO JUST PATCHY STRATUS
MAY BE AROUND. SKIES SHOULD BE PARTLY TO MOSTLY SUNNY. KAM/BURKE

.LONG TERM...PREVIOUS DISCUSSION...MODELS SHOW SW ONSHORE FLOW
INCREASING AGAIN TUESDAY NIGHT...SO A FRESH LAYER OF STRATUS SHOULD
INUNDATE W WA ON WEDNESDAY. GFS SHOWS PRECIP FROM THE POSSIBLE
SHORTWAVE OVER OREGON MOVING N OVER W WA TUESDAY NIGHT...BUT THE NAM
AND ECMWF DO NOT. THE LARGE SCALE PATTERN DOES NOT CHANGE...WITH A
BROAD LONGWAVE TROUGH CENTERED OVER THE SW U.S. AND A BROAD RIDGE TO
THE N OVER WESTERN CANADA. MORE OF THE SAME FOR W WA THROUGH
SATURDAY...WITH PARTLY SUNNY DAYS WITH AN UNSTABLE AIR MASS FOR A
CHANCE OF AFTERNOON SHOWERS OVER THE MOUNTAINS. KAM

&&

.AVIATION...LIGHT NORTHERLY FLOW ALOFT TONIGHT WILL BACK TO LIGHT
SOUTHEAST FLOW ON SUNDAY...AS A VAGUELY DEFINED UPPER TROUGH REMAINS
OVER THE REGION. WEAK LOW LEVEL ONSHORE FLOW WILL WEAKEN FURTHER
TONIGHT WITH LIGHT PRESSURE GRADIENTS SUNDAY. THE AIR MASS IS MOIST
BELOW ABOUT 7000 FT...BUT IT WILL DRY SOMEWHAT SUNDAY AFTERNOON AS
THE AIR MASS BECOMES WEAKLY UNSTABLE.

THE FORECAST AREA IS ALMOST ENTIRELY COVERED BY OVERCAST MARINE
STRATOCUMULUS WITH BASES 2500 TO 3500 FT. THE CLOUD COVER SHOULD
LOWER OVERNIGHT WITH WIDESPREAD MVFR CEILINGS LIKELY LATER TONIGHT
AND SUNDAY MORNING. CONDITIONS SHOULD IMPROVE LATE SUNDAY MORNING
AND AFTERNOON...WITH WIDESPREAD VFR CONDITIONS EXPECTED. ISOLATED
THUNDERSTORMS ARE POSSIBLE OVER THE CASCADES SUNDAY AFTERNOON.

KSEA...SOUTH WIND 3-7 KT...BECOMING NORTHWEST SUNDAY AFTERNOON.
CEILINGS WILL PROBABLY LOWER TO 1000-2000 FT LATE TONIGHT...THEN
CONDITIONS SHOULD IMPROVE 18Z-20Z SUNDAY.   MCDONNAL

&&

.MARINE...ONSHORE FLOW WILL WEAKEN TONIGHT. SMALL CRAFT ADVISORY
WESTERLIES WILL CONTINUE IN THE CENTRAL AND EASTERN STRAIT OF JUAN
DE FUCA UNTIL LATE TONIGHT.

LIGHT ONSHORE FLOW WILL PREVAIL SUNDAY AND MONDAY. A STRONGER
ONSHORE PUSH MAY DEVELOP ON TUESDAY WITH SMALL CRAFT ADVISORY WINDS
LIKELY IN THE STRAIT OF JUAN DE FUCA. ONSHORE FLOW WILL CONTINUE
WEDNESDAY AND THURSDAY.   MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EASTERN STRAIT OF JUAN DE
       FUCA.

&&

$$

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





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