Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 020350
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
850 PM PDT MON SEP 1 2014

.SYNOPSIS...A VIGOROUS FAST MOVING FRONTAL SYSTEM WILL BRING A
LITTLE RAIN TO WESTERN WASHINGTON ON TUESDAY. A CONVERGENCE ZONE
WILL FORM OVER PUGET SOUND BEHIND THE FRONT LATE TUESDAY AFTERNOON
AND EVENING. DRY NORTH FLOW ALOFT WILL BRING SUNNY WEATHER BY
WEDNESDAY AFTERNOON. AN UPPER LEVEL RIDGE WILL BUILD OVER THE REGION
THURSDAY FOR SUNSHINE AND A WARMING TREND THAT WILL LAST INTO NEXT
WEEKEND.

&&

.SHORT TERM...PARTLY TO MOSTLY CLOUDY SKIES OVER W WA THIS EVENING.
WITH NO PRECIP ON RADAR THE DRY FORECAST FOR TONIGHT LOOKS GOOD.

WATER VAPOR IMAGERY SHOWED A VIGOROUS UPPER LEVEL TROUGH OVER THE AK
PANHANDLE AT 03Z. MODELS REMAIN IN GOOD AGREEMENT IN BRINGING THE
TROUGH SE ALONG THE B.C. COAST TONIGHT...THEN DOWN OVER W WA TUE
AFTERNOON. SATELLITE IMAGERY SHOWS THE FRONT HAS NOT QUITE FORMED
YET BUT MODELS INDICATE THAT FRONTOGENESIS WILL OCCUR TONIGHT AS THE
TROUGH APPROACHES. GFS/NAM/ECMWF AGREE ON BRINGING THE FRONT DOWN TO
TOFINO (CYAZ) AT 18Z/11 AM AND THEN JUST E OF PUGET SOUND AT 00Z/5
PM. THE FRONT WILL BE NARROW AND THE SYSTEM FAST MOVING SO THIS
SHOULD BE A QUICK SHOT OF RAIN...STARTING ON THE N COAST JUST AHEAD
OF THE FRONT AROUND 18Z. MODELS SHOW A CONVERGENCE ZONE PATTERN
SETTING UP OVER PUGET SOUND BEHIND THE FRONT TUESDAY EVENING. MODELS
ALSO SHOW A SHOT OF RAIN OVER THE N HALF OF THE INTERIOR AND THE N
CASCADES TUESDAY EVENING AS THE UPPER LEVEL TROUGH AXIS MOVES
THROUGH.

MODELS ARE ALL OVER THE PLACE WITH PRECIP AMOUNTS. THE 18Z NAM
SHOWED VERY LITTLE OVER THE LOWLANDS OUTSIDE OF 0.2 TO 0.3 IN THE
CONVERGENCE ZONE. THE 00Z NAM DISCARDS THE CONVERGENCE ZONE AND ONLY
HAS A FEW HUNDREDTHS OVER THE LOWLANDS. THE 18Z GFS IS SIMILAR TO
THE 18Z NAM. THE 12Z ECMWF HAS 0.5 TO AN INCH OVER THE N HALF OF W
WA. ALL THE MODELS HIT THE N CASCADES WITH 0.5 TO AN INCH. THE
CURRENT FORECAST IS MORE REASONABLE WITH ABOUT A TENTH OF AN INCH
FROM PUGET SOUND NORTHWARD. QPF OVER THE CASCADES COULD PROBABLY BE
DOUBLED TO AT LEAST A HALF INCH...ASSUMING A DECENT HIT FROM THE
UPPER TROUGH AXIS TUESDAY NIGHT.

COLD AIR ALOFT TRAILING THE UPPER LEVEL TROUGH WILL LOWER SNOW
LEVELS DOWN TO A LITTLE ABOVE 6000 FEET LATE TUESDAY NIGHT OVER THE
NORTH AND CENTRAL CASCADES. BUT PRECIP WILL BE MOSTLY OVER BY THEN
SO AT BEST THERE MAY BE UP TO AN INCH OF SNOW OVER THE N CASCADES.

PRECIP WILL BE OVER FOR THE LOWLANDS BY LATE TUESDAY NIGHT...BUT
INCREASING SW ONSHORE FLOW AHEAD OF THE FRONT WILL ENSURE THAT THE
LOWER AIR MASS REMAINS MOIST THROUGH WEDNESDAY MORNING. RAPID DRYING
ALOFT BEHIND THE TROUGH WILL HELP SKIES CLEAR OUT FOR SUNNY
CONDITIONS WEDNESDAY AFTERNOON. THE AIR MASS WILL REMAIN COOL SO
HIGHS WILL ONLY WARM UP A LITTLE TO AROUND 70.

AN UPPER LEVEL RIDGE OFFSHORE WILL BUILD E OVER THE REGION ON
THURSDAY. ONSHORE FLOW WEDNESDAY NIGHT WILL BE NEGLIGIBLE OR EVEN
WEAKLY OFFSHORE...SO WITH A WARMING AIR MASS THURSDAY AND LITTLE OR
NO MORNING STRATUS TO GET IN THE WAY...MAX TEMPS WILL WARM INTO THE
LOWER 70S N TO UPPER 70S SW INTERIOR. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 PM AFD...FRIDAY AND
SATURDAY LOOK LIKE THE WARMEST DAYS OF THE WEEK WITH HIGH PRESSURE
ALOFT IN CONTROL. THE RIDGE WILL BEGIN TO WEAKEN ON SATURDAY BUT
THAT IS NOT EXPECTED TO HAVE MUCH IMPACT ON TEMPERATURES OR CLOUD
COVER.

IT LOOKS LIKE THERE WILL A MARINE PUSH SATURDAY NIGHT WITH AN UPPER
TROUGH MOVING OVER THE AREA ON SUNDAY AND MONDAY FOR COOLER AND
CLOUDIER WEATHER WITH A CHANCE OF SHOWERS AT TIMES. THE MODELS ARE
NOT IN GOOD AGREEMENT WITH THESE DETAILS HOWEVER. SCHNEIDER

&&

.AVIATION...LOW LEVEL ONSHORE FLOW CONTINUES ACROSS WESTERN WA
TONIGHT. LOW CLOUDS WILL FILL IN BY MORNING WITH MVFR CONDITIONS
EXPECTED. A COLD FRONT WILL BRING RAIN TO THE COAST AND NORTH
INTERIOR TUE MORNING...WITH SHOWERS SPREADING INTO THE INTERIOR
DURING THE AFTERNOON. THE TRAILING UPPER LEVEL TROUGH WILL BRING
MORE SHOWERS TO THE INTERIOR AND CASCADES TUE NIGHT. 33

KSEA...LOW CLOUDS DEVELOPING OVERNIGHT WITH MVFR CONDITIONS
EXPECTED. SURFACE WINDS ARE LIGHT NORTHERLY...BUT WILL FLIP BACK TO
SOUTHERLY BY 18Z TUE. EXPECT SHOWERS IN THE VICINITY TUE AFTERNOON
AND EVENING. 33

&&

.MARINE...ONSHORE FLOW WILL CONTINUE ACROSS WESTERN WA TONIGHT - A
SMALL CRAFT ADVISORY REMAINS IN EFFECT FOR THE CENTRAL/EAST STRAIT
OF JUAN DE FUCA. ONSHORE FLOW WILL INCREASE BEHIND A COLD FRONT TUE
AFTERNOON WITH GALE FORCE WINDS POSSIBLE OVER THE COASTAL WATERS AND
THROUGH THE STRAIT. ONSHORE FLOW WILL EASE LATE TUE NIGHT. HIGH PRES
OVER THE NE PAC WILL MAINTAIN ONSHORE FLOW THROUGH WED. NORTHERLY
FLOW WILL PREVAIL THU AND FRI AS LOW PRES FORMS ALONG THE COAST. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.
    .GALE WATCH COASTAL WATERS...CENTRAL AND EASTERN STRAIT OF JUAN
     DE FUCA TUESDAY AFTERNOON THROUGH WEDNESDAY MORNING.

&&

$$

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