Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 180950
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
250 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...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...THE MOIST UPPER LEVEL TROUGH ALONG THE COAST THIS
MORNING WILL MOVE OVER THE CASCADES BY 18Z/11 AM AND TO THE ID
BORDER BY 00Z/5 PM THIS AFTERNOON. RADAR IMAGERY INDICATES THE MAIN
SHOWER ACTIVITY HAS SHIFTED OVER THE N INTERIOR AHEAD OF THE TROUGH
AXIS...WITH DIMINISHING SHOWERS ALONG THE COAST. ALTHOUGH THE TROUGH
WILL BE EAST OF THE AREA THIS AFTERNOON...THE LOWER AIR MASS WILL
BECOME UNSTABLE UP TO ABOUT 700 MB/10000 FEET THIS AFTERNOON...SO A
CHANCE OF SHOWERS WILL CONTINUE.

THIS EVENING SHOULD QUALIFY AS A MINOR LULL WITH W FLOW ALOFT
MAINTAINING A CHANCE OF MOUNTAIN SHOWERS...BUT ONLY ABOUT A 20
PERCENT CHANCE OF SHOWERS OVER THE LOWLANDS.

W FLOW ALOFT OVER THE NE PACIFIC WILL DRIVE THE NEXT UPPER LEVEL
SHORTWAVE TROUGH OVER THE REGION LATER TONIGHT AND ON FRIDAY. THIS
IS THE SYSTEM AROUND 50N AND BETWEEN 130W AND 135W AT 09Z/2 AM. THIS
SYSTEM WILL BRING SHOWERS TO PRIMARILY THE COAST AND N INTERIOR
THROUGH FRIDAY MORNING...WITH SHOWER ACTIVITY DIMINISHING FRIDAY
AFTERNOON AS THE BUILDING RIDGE OFFSHORE WEAKENS THE SYSTEM. MODELS
HAVE BEEN PRETTY DRY FOR FRIDAY NIGHT SO THE CHANCE OF SHOWERS WAS
DROPPED. CLOUDS WILL DECREASE OVERNIGHT...SO WITH IMPROVING
RADIATIONAL COOLING CONDITIONS PATCHY FOG WAS ADDED TO THE FORECAST
FOR SATURDAY MORNING.

THE MODELS REMAIN ON COURSE IN BUILDING A STRONG UPPER LEVEL RIDGE
OFFSHORE FRIDAY NIGHT THEN MOVING IT OVER W WA ON SATURDAY. THE AIR
MASS WILL DRY RAPIDLY FOR SUNNY SKIES ON SATURDAY. E-NE LOW LEVEL
OFFSHORE FLOW WILL CAUSE A THERMALLY INDUCED SURFACE TROUGH TO FORM
ALONG THE OREGON COAST. THE SURFACE TROUGH WILL INCREASE THE LOW
LEVEL N-NE FLOW THROUGH PUGET SOUND SATURDAY AFTERNOON. THIS SHOULD
CAP MAX TEMPS IN THE 70S AROUND PUGET SOUND. KAM

.LONG TERM...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...A WEAK UPPER LEVEL TROUGH WILL MOVE THROUGH THE AREA
TODAY WITH INCREASING WESTERLY FLOW ALOFT TONIGHT. AT THE SURFACE...
HIGH PRESSURE WILL BE OVER THE COAST WITH LOWER PRESSURE INLAND. AIR
MASS MOIST AND UNSTABLE.

CEILINGS WILL GRADUALLY LOWER EARLY THIS MORNING AS A BATCH OF
SHOWERS MOVING NORTH THROUGH THE AREA MOISTENS THE MID AND LOW
LEVELS. LOW CLOUDS ARE EXPECTED TO BE FAIRLY WIDESPREAD ACROSS THE
AREA FOR MUCH OF THE DAY WITH SOME IMPROVEMENT IN THE AFTERNOON.

KSEA...MID CLOUDS EARLY THIS MORNING WITH A BATCH OF SHOWERS PUSHING
NORTH THROUGH THE AREA. LOW CLOUDS ARE EXPECTED TO DEVELOP BY
DAYBREAK AND PERSIST THROUGH THE MORNING...LIFTING A BIT IN THE
AFTERNOON. SOUTHERLY WIND 4-8 KNOTS. SCHNEIDER

&&

.MARINE...WEAK HIGH PRESSURE WILL BE OVER THE COAST THROUGH TONIGHT.
THERE COULD BE A FEW HOURS OF SMALL CRAFT ADVISORY WEST WINDS IN THE
CENTRAL AND EAST STRAIT THIS EVENING AS ONSHORE FLOW INCREASES A BIT
BUT THIS LOOKS PRETTY MARGINAL.

A WEAK FRONT WILL CROSS THE WATERS FRIDAY. OFFSHORE FLOW WILL
DEVELOP FRIDAY NIGHT AND CONTINUE THROUGH SATURDAY NIGHT WITH A
TROUGH JUST OFFSHORE. SOUTHERLY WINDS WILL DEVELOP SUNDAY AND
CONTINUE INTO MONDAY AS A TROUGH APPROACHES FROM THE WEST. SCHNEIDER

&&

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

&&

$$

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