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000
FXUS66 KSEW 161615
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT THU MAY 16 2013

.SYNOPSIS...A WEAK UPPER TROUGH OVER THE PACIFIC NORTHWEST WILL
RESULT IN SUN BREAKS AND SPOTTY SHOWERS TODAY AND FRIDAY. A STRONGER
UPPER TROUGH WILL BRING WIDESPREAD SHOWERS TO WESTERN WASHINGTON
SATURDAY. SUNDAY AND MONDAY WILL SEE A RETURN TO PARTLY SUNNY SKIES
AND JUST SPOTTY SHOWERS. AN UPPER LOW WILL MOVE SOUTH OVER THE
REGION TUESDAY AND WEDNESDAY FOR MORE SHOWERS AND COOLER WEATHER.

&&

.SHORT TERM...A LARGE UPPER LEVEL TROUGH COMPLEX WILL REMAIN STALLED
OVER THE PAC NW TODAY THROUGH FRIDAY. THE BULK OF MOISTURE AND WEAK
DYNAMICS ARE AT THE BASE OF THE TROUGH OVER NRN CA/OREGON. BUT THERE
IS PLENTY OF LOW/MID LEVEL RESIDUAL MOISTURE OVER WRN WA SO ANY
WEAK UPPER LEVEL DISTURBANCES OR AREAS OF LOW LEVEL CONVERGENCE WILL
TRIGGER SHOWERS AROUND THE AREA. SHOWER ACTIVITY WILL BE MOST
PREVALENT DURING THE AFTERNOON/EARLY EVENING DUE TO ADDED DIURNAL
INSTABILITY. CONVECTION LOOKS QUITE SHALLOW SO THUNDERSTORMS ARE NOT
EXPECTED. THE LOW LEVEL FLOW IS ONLY WEAKLY ONSHORE SO HIGH
TEMPERATURES WILL BE CLOSE TO THE SEASONAL AVERAGE IN THE MID 60S.
THE CURRENT FORECAST SHOWS HIGH TEMPERATURES COMING DOWN 2-3 DEGREES
ON FRIDAY BUT WITH LITTLE CHANGE IN UPPER HEIGHTS OR STRENGTH OF
ONSHORE FLOW MAKES THIS UNCERTAIN. MAKE NEED TO TWEAK HIGH
TEMPERATURES UP A COUPLE DEGREES IF IT LOOKS LIKE THERE WILL BE SOME
PARTIAL AFTERNOON SUNSHINE OR SUNBREAKS.

THERE WILL BE A BREAK BETWEEN UPPER TROUGHS FRIDAY NIGHT SO MOSTLY
DRY CONDITIONS ARE EXPECTED. MODELS ARE IN FAIR AGREEMENT ON
BRINGING A WEAK BUT PROGRESSIVE FRONT THROUGH WRN WA ON SATURDAY.
LIGHT RAIN SHOULD REACH THE COAST EARLY IN THE MORNING AND SPREAD
INLAND DURING THE LATE MORNING AND AFTERNOON. THE 500 MB TROUGH AXIS
SHIFT E OF THE PUGET SOUND REGION BY AROUND 00Z SATURDAY AFTERNOON
SO RAIN WILL BECOME SHOWERY AND BEGIN TO DECREASE THEREAFTER. 500 MB
HEIGHTS LOWER TO NEAR 5600M SO HIGH TEMPERATURES ACROSS THE INTERIOR
SHOULD MODERATE CLOSER TO 60 DEGREES.  MERCER

.LONG TERM...FROM 300 AM DISCUSSION...THE TROUGH TALKED ABOUT JUST
ABOVE WILL MOVE EAST OF THE CASCADES SATURDAY NIGHT...AND A MINOR
UPPER RIDGE WILL MOVE OVER THE AREA SUNDAY. THERE SHOULD BE MORE
SUNSHINE AND FEWER SHOWERS. ANOTHER DEEPER UPPER TROUGH WILL START
DIGGING SOUTHEAST TOWARD THE REGION FROM SOUTHEAST ALASKA ON
MONDAY...AND IT LOOKS LIKE ANOTHER DAY OF PARTLY SUNNY SKIES AND
ONLY SPOTTY SHOWERS.

THE UPPER TROUGH WILL DIG OVER THE PACIFIC NORTHWEST TUESDAY AND
WEDNESDAY FOR MORE CLOUD COVER...MORE WIDESPREAD SHOWERS...AND
COOLER WEATHER. I NUDGED OUR POPS UPWARD FROM NEAR CLIMATOLOGY
CLOSER TO MOS GUIDANCE. OUR SNOW LEVEL IS ALSO LIKELY TOO HIGH FOR
THOSE PERIODS. TAKING THE GFS AND EUROPEAN SUGGEST THE SNOW LEVEL
COULD BE AROUND 4000 FT. THOSE ARE DAYS 6 AND 7 OF THE FORECAST AND
I WILL LEAVE IT FOR THE DAY SHIFT TO CONSIDER MORE SIGNIFICANT
CHANGES TO THE FORECAST.   MCDONNAL

&&

.AVIATION...A WEAK UPPER LEVEL TROUGH OVER THE PAC NW WILL MAINTAIN
THE CHANCE FOR ISOLATED TO SCATTERED SHOWERS ACROSS WESTERN WA
TODAY. THE FLOW ALOFT IS WESTERLY. CIGS CONTINUE TO BE VARIED OVER
WESTERN WA...WITH MVFR TO IFR CONDITIONS ALONG THE COAST AND WEST OF
THE SOUND...MVFR EAST OF THE SOUND. STILL EXPECTING CIGS TO LIFT AS
AFTERNOON APPROACHES FOR MAINLY VFR CONDITIONS. THOSE VFR CONDITIONS
LIKELY CARRY ON THROUGH THE NIGHT...THOUGH SOME LOCATIONS MAY SEE
CIGS FALL INTO MVFR. 33/SMR

KSEA...ISOLATED TO SCATTERED SHOWERS WILL AFFECT THE PUGET SOUND
AREA TODAY. MVFR CIGS THIS MORNING WITH IMPROVING CONDITIONS THIS
AFTERNOON. VFR CONDITIONS THIS AFTERNOON AND EVENING. LIGHT SURFACE
WINDS. 33/SMR

&&

.MARINE...A WEAK 1013 LOW WILL CONTINUE TO DISSIPATE OVER THE
OFFSHORE WATERS TODAY. PRES GRADIENTS AND WINDS ACROSS WESTERN WA
WILL REMAIN LIGHT. ONSHORE FLOW WILL PREVAIL TONIGHT AND FRI WITH
HIGHEST WINDS THROUGH THE STRAIT OF JUAN DE FUCA. WIND SPEEDS
MAY APPROACH SMALL CRAFT ADVISORY SPEEDS IN THE STRAIT LATE
TONIGHT AND INTO FRIDAY. WILL EVALUATE 12Z DATA AND MAKE A DECISION
FOR AFTERNOON FORECAST. A COLD FRONT WILL CROSS THE AREA ON SAT FOR
INCREASING WINDS. HIGH PRES OVER THE NE PAC WILL MAINTAIN ONSHORE
FLOW THROUGH THE WEEKEND. 33/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

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