Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 181622
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
923 AM PDT THU SEP 18 2014

.SYNOPSIS...AN UPPER LEVEL TROUGH WILL CROSS WESTERN WASHINGTON
TODAY BRINGING MOSTLY CLOUDY SKIES AND SCATTERED SHOWERS. A SECOND
WEAKER UPPER LEVEL TROUGH WILL FOLLOW TONIGHT...PROLONGING THE
CHANCE OF SHOWERS THROUGH FRIDAY. A STRONG UPPER LEVEL RIDGE WILL
BUILD OVER THE REGION FRIDAY NIGHT AND SATURDAY...THEN SHIFT EAST ON
SUNDAY. THE RIDGE AND LOW LEVEL OFFSHORE FLOW WILL BRING SUNNY SKIES
AND HIGH TEMPERATURES IN THE 70S TO MID 80S. A SERIES OF UPPER LEVEL
TROUGHS WILL BRING A CHANCE OF RAIN AND COOLER WEATHER NEXT WEEK.

&&

.SHORT TERM...A WEAK UPPER LEVEL TROUGH AXIS WAS OVER WRN WA THIS
MORNING. THE TROUGH AXIS WILL MOVE EAST TO WRN MONTANA BY LATE THIS
EVNG. LARGE-SCALE LIFT IS FAIRLY WEAK...BUT THE AIR MASS IS VERY
MOIST. THE 12Z KUIL SOUNDING SHOWED PRECIPITABLE WATER OF 1.28
INCHES AND A MOIST LAYER ALL THE WAY UP TO NEAR 15 THOUSAND FT MSL.
SFC OBS ACROSS THE AREA SHOW DEWPOINTS IN THE UPPER 50S. WITH THIS
KIND OF MOISTURE...EVEN MINOR LIFT WILL CAUSE AT LEAST SOME SHOWERS
OR LIGHT RAIN. ONCE THE TROUGH AXIS PASSES EAST THIS AFTN...LIGHT
ONSHORE FLOW WILL CONTINUE TO PROVIDE THE MINOR LIFT NEEDED FOR SOME
SHOWERS THIS AFTN AND EVNG...BUT SHOWER ACTIVITY WILL BECOME MORE
FOCUSED ON TERRAIN AND IN AN ILL-DEFINED PUGET SOUND CONVERGENCE
ZONE THIS AFTN.

LATE TONIGHT...THE TAIL END OF A SHORTWAVE SCOOTING ACROSS NRN B.C.
WILL MAINTAIN ENOUGH WARM ADVECTION LIFT...DEEP MOISTURE AND ONSHORE
FLOW TO KEEP SHOWERS AND LIGHT RAIN GOING WELL INTO FRI MRNG.

BY FRI AFTN...UPPER HEIGHTS RISE QUICKLY AND CONSIDERABLY IN
RESPONSE TO A BUILDING UPPER RIDGE AXIS BETWEEN THE COAST AND 130W.
AS SUBSIDENCE ALOFT KICKS IN...THE SURFACE-BASED MOIST LAYER WILL
QUICKLY BECOME MORE SHALLOW OVER THE COURSE OF FRI AFTN AND FRI NGT.
WITH CLEARING SKIES ON FRI NGT...AND A SHIFT TO LIGHT N-NE SFC
WIND...AND LEFTOVER SFC MOISTURE...EXPECTING QUITE A BIT OF FOG
DEVELOPMENT OVER THE LOWLANDS DURING THE PRE-DAWN HOURS ON SAT MRNG.
AS NORTHERLY AND EASTERLY GRADIENTS INCRS ON SAT...LOW CLOUDS AND
MORNING FOG WILL READILY BURN OFF BY MID-DAY WITH AN OUTSTANDINGLY
SUNNY AND WARM AFTERNOON. THE LOW-LEVEL AIR MASS WILL ALSO DRY
OUT...WITH LOWLAND DEWPOINTS FALLING TO LESS MUGGY VALUES IN THE
UPPER 40S AND LOW 50S.       HANER

.LONG TERM...FROM PREVIOUS DISCUSSION...MODELS HAVE BEEN QUITE
CONSISTENT WITH THE UPPER LEVEL RIDGE AND THE THERMALLY INDUCED
SURFACE TROUGH. E-NE LOW LEVEL FLOW SATURDAY NIGHT WILL BRING THE
SURFACE TROUGH UP ALONG THE WA COAST ON SUNDAY. THIS WILL BE THE
WARMEST DAY WITH MAX TEMPS IN THE MID 70S TO MID 80S. MOS GUIDANCE
MAX TEMPS SEEM TO BE OVERLY COLD BUT MAY GET CLOSER TO THE EXPECTED
MID 80S MAXES AS SUNDAY DRAWS CLOSER. ALONG THE COAST THE E FLOW
WILL CONTINUE INTO SUNDAY AFTERNOON...SO EVEN THERE MAX TEMPS WILL
BE IN THE 70S TO LOWER 80S.

MODELS HAVE ALSO BEEN FAIRLY CONSISTENT IN BRINGING A WEAKENING
FRONT AND UPPER LEVEL TROUGH TO THE COAST LATE MONDAY OR TUESDAY.
TIMING IS STILL IN QUESTION...SO IT IS STILL UNCERTAIN JUST HOW MUCH
OF MONDAY WILL STAY MOSTLY SUNNY. THIS IS AN INTERESTING LITTLE
SYSTEM WITH AN 80-100 KT JET STREAK AND 500 MB VORTICITY LOBE
VERTICALLY STACKED OVER THE SURFACE FRONT. BOTH ECMWF AND GFS SHOW
THE SYSTEM WEAKENING RAPIDLY MONDAY EVENING AS IT NEARS THE COAST.
THE UPPER LEVEL TROUGH AND 500 MB VORTICITY LOBE SHEAR OFF FROM THE
SURFACE FRONT...PROBABLY BRINGING SOME SHOWERS TO THE CASCADES AND E
WA MONDAY NIGHT. 1000-850 MB RH PROGS SHOW THE SURFACE FRONT
STALLING ALONG THE COAST MONDAY NIGHT...BUT THE MOISTURE WILL
EVENTUALLY BE SHOVED INLAND BY LATE TUESDAY AS ANOTHER TROUGH
APPROACHES. KAM

&&

.AVIATION...AN UPPER LEVEL TROUGH IS OVER WESTERN WA THIS MORNING
AND WILL SHIFT EAST OF THE CASCADES THIS AFTERNOON. THE FLOW ALOFT
IS WESTERLY. THE AIR MASS IS MOIST WITH LIGHT RAIN MAINLY OVER THE
INTERIOR THIS MORNING. THERE ARE SEVERAL BROKEN LAYERS THIS MORNING
FROM IFR TO VFR. BUT IN GENERAL IT LOOKS LIKE CIGS WILL HOVER NEAR
3000 FT THIS AFTERNOON. CIGS WILL LOWER BACK INTO MVFR TO IFR RANGE
TONIGHT AS A WEAK WARM FRONT STALLS OVER THE REGION. THE COLD FRONT
WILL CROSS THROUGH WESTERN WA FRI MORNING FOR A FEW SHOWERS. 33

KSEA...CIGS NEAR 3000 FT TODAY. MODELS SHOW SPOTTY LIGHT SHOWERS IN
THE VICINITY THROUGH MUCH OF THE DAY. CIGS WILL LOWER BACK TO MVFR
RANGE OVERNIGHT. SURFACE WINDS WILL REMAIN LIGHT SLY. 33

&&

.MARINE...SURFACE WINDS WILL BE MAINLY LIGHT ACROSS THE WATERS TODAY
WITH WEAK HIGH PRES ALONG THE COAST. A WESTERLY ONSHORE PUSH WILL
BRING A FEW HOURS OF SMALL CRAFT ADVISORY WINDS THROUGH THE CENTRAL
AND EAST STRAIT OF JUAN DE FUCA THIS EVENING. A WEAK COLD FRONT WILL
CROSS THROUGH WESTERN WA ON FRIDAY. WINDS WILL FLIP TO OFFSHORE FRI
NIGHT THROUGH SAT AS A THERMALLY INDUCED TROUGH OF LOW PRES FORMS
ALONG THE COAST. THE WINDS WILL THEN TURN SOUTHERLY ON SUN AS A
SURFACE TROUGH DEVELOPS OVER THE OFFSHORE WATERS. THIS TROUGH WILL
CROSS THROUGH THE COASTAL WATERS ON MON. 33

&&

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

&&

$$

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