Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 281623
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
915 AM PDT SAT MAR 28 2015

.SYNOPSIS...SHOWERS WILL GENERALLY DECREASE TODAY BEHIND A FRONT.
WEAK HIGH PRESSURE WILL KEEP WEATHER SYSTEMS MAINLY NORTH OF THE
AREA TONIGHT THROUGH EARLY MONDAY. A COLD FRONT WILL REACH WESTERN
WASHINGTON LATE MONDAY WITH RAIN. UPPER LEVEL LOW PRESSURE WILL BE
OVER THE AREA FOR MUCH OF THE REST OF THE WEEK.

&&

.SHORT TERM...POST FRONTAL SHOWERS ARE STREAMING INTO WRN WA THIS
MORNING WITH BREEZY WINDS. GALES OCCURRED IN THE STRAIT EARLIER AND
THIS OUTFLOW WILL MEET UP WITH THE S-SW FLOW IN PUGET SOUND
PRODUCING A CONVERGENCE ZONE TODAY. WESTERLY FLOW AT 850 MPH SHOULD
KEEP MOST THE PRECIPITATION FOCUSED ALONG THE W SLOPES OF THE
CASCADES AND FOOTHILLS OF FAR NRN KING AND SNOHOMISH COUNTIES.
THE NAM-12 HOLDS ONTO THE CONVERGENCE ZONE THROUGH THE DAY WHILE THE
HRRR IS LESS ENTHUSIASTIC. HOWEVER...THE OVERALL PATTERN SUGGESTS IT
WILL HANG AROUND INTO THE AFTERNOON HOURS TODAY. JUST A CHANCE OF
SHOWERS ELSEWHERE. SNOW LEVELS FELL OVER THE PAST FEW HOURS...DOWN
TO ABOUT 4500 FEET NORTH AND 5000 FEET SOUTH. MOUNT BAKER PICKED UP
A COUPLE INCHES OF SNOW AS DID HURRICANE RIDGE.  NOT
SURPRISINGLY...IT REMAINED TOO WARM IN THE PASSES FOR ACCUMULATING
SNOW. IT WAS ALSO BREEZY BEHIND THE FRONTAL PASSAGE LAST NIGHT WITH
GUSTS OVER 30 MPH INTO THIS MORNING. GRADIENTS ARE EASING WITH
KPDX-KBLI DOWN TO +6 MB FROM ABOUT +8 MB AT THE PEAK EARLIER. WINDS
WILL CONTINUE TO GRADUALLY EASE THROUGH THE DAY.

THE TROUGH PULLS AWAY FROM THE AREA TONIGHT WITH NEXT SYSTEM
DIRECTING MOIST W-SW FLOW INTO MAINLY BRITISH COLUMBIA. 06Z/12Z
MODELS HAVE TRENDED FURTHER N WITH THE STALLED FRONTAL BOUNDARY
TONIGHT THROUGH MONDAY MORNING. NEWEST MODELS KEEP PRECIPITATION
CONFINED TO MAINLY THE N COAST/OLYMPICS AND FAR N INTERIOR. PUGET
SOUND SWD LOOKS TO STAY MOSTLY DRY AND A TOUCH WARMER WITH HIGHS IN
THE LOW 60S.

A BROAD TROUGH WILL DIG OFFSHORE DRAGGING A FRONT ACROSS THE AREA
LATE MONDAY/MONDAY NIGHT. THIS STILL APPEARS THE MOST LIKELY PERIOD
FOR MORE WIDESPREAD LIGHT RAINFALL. MERCER

.LONG TERM...FROM PREVIOUS DISCUSSION...A BROAD UPPER TROUGH WILL BE
OVER THE REGION TUESDAY THROUGH FRIDAY FOR SHOWERY WEATHER AT TIMES.
MODELS SUGGEST AN UPPER RIDGE WILL BRING DRIER WEATHER THURSDAY.
MODELS ARE IN POOR AGREEMENT WITH HOW THINGS WILL PROGRESS AFTER
THAT. SCHNEIDER

&&

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

&&

.AVIATION...A FLAT UPPER LEVEL RIDGE OVER W WA WILL SLIDE OVER E WA
TONIGHT. MODERATE W FLOW ALOFT WILL CONTINUE OVER THE AREA. THE
PUGET SOUND CONVERGENCE THAT FORMED OVERNIGHT HAS MOVED OVER THE
CASCADES AND IS DISSIPATING.

WEAKENING W ONSHORE FLOW HAS LEFT THE LOWER AIR MASS OVER W WA
SOMEWHAT MOIST...BUT THE MODERATE W FLOW HAS ALSO CAUSED CLEARING IN
THE LEE OF THE OLYMPICS AND VANCOUVER ISLAND. THE W FLOW IS ALSO
PRODUCING CLOUDS AND SCATTERED SHOWERS OVER THE W SLOPES OF THE
CASCADES AND OLYMPICS. THE LOWER AIR MASS IS ALSO WEAKLY UNSTABLE
AND AREAS OF CUMULUS OR STRATOCUMULUS WITH BASES 020-040 WILL BE
PRESENT. LOW LEVEL WINDS WILL REMAIN GUSTY THROUGH 22Z.

A WARM FRONT APPROACHING VANCOUVER ISLAND WILL SPREAD MID LEVEL
MOISTURE OVER MAINLY THE N TWO THIRDS OF W WA BEGINNING AFTER 22Z.

KSEA...MVFR CIGS BKN018-030 ARE EXPECTED THROUGH 21Z...LIFTING TO
SCT-BKN040 OR BETTER AFTER 21Z. THE CONVERGENCE ZONE IS NO LONGER A
FACTOR. SURFACE WINDS WILL REMAIN S 14G24KT THROUGH ABOUT 00Z WITH A
GRADUAL WEAKENING TREND AS 00Z APPROACHES. KAM

&&

.MARINE...STRONG ONSHORE FLOW EARLIER THIS MORNING IS WEAKENING AS A
SURFACE RIDGE BUILDS OVER W WA...BUT PER RACE ROCKS THE FLOW IS NOT
WEAKENING QUITE AS QUICKLY AS EXPECTED IN THE STRAIT. THE GALE WILL
BE EXTENDED FOR A COUPLE MORE HOURS UNTIL 11 AM FOLLOWED BY A
TRANSITION SCA EARLY THROUGH EARLY THIS AFTERNOON. WINDS OVER THE
REMAINDER OF THE INLAND WATERS HAVE EASED BELOW SCA LEVELS THIS
MORNING BUT GUSTY WINDS TODAY MAY COME CLOSE TO SCA LEVELS.

MODELS SEEM TO BE CONSISTENT IN THEIR HANDLING OF THE WARM ADVECTION
APPROACHING VANCOUVER ISLAND TODAY. PRESSURE FALLS CAUSED BY THE
WARM FRONT REACHING VANCOUVER ISLAND LATE TODAY WILL INCREASE THE S
WINDS OVER THE COASTAL WATERS LATE THIS AFTERNOON AND TONIGHT...AND
OVER THE INLAND WATERS ON SUNDAY. AN SCA IS ALREADY IN EFFECT FOR
THE COAST. SCA WINDS ARE ALREADY IN THE FORECAST FOR THE INTERIOR.

S FLOW SHOULD EASE LATE SUNDAY AS THE WARM FRONT MOVES N. THE
TRAILING COLD FRONT WILL CROSS W WA LATE MONDAY. STRONG ONSHORE FLOW
BEHIND THE COLD FRONT WILL PROBABLY PRODUCE GALES IN THE STRAIT
AGAIN MONDAY NIGHT AND TUESDAY. KAM

&&

$$

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










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