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000
FXUS66 KSEW 150420
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT TUE MAY 14 2013

.SYNOPSIS...AN UPPER LEVEL TROUGH WILL BRING SOME CLOUDS TO WESTERN
WASHINGTON TONIGHT AND SHOWERS ON WEDNESDAY. WEAK WESTERLY FLOW
ALOFT THURSDAY INTO EARLY NEXT WEEK WILL DIRECT A SERIES OF WEAK
UPPER LEVEL TROUGHS ACROSS THE AREA WHICH WILL MAINTAIN A CHANCE OF
SHOWERS EACH DAY ALONG WITH PARTLY TO MOSTLY CLOUDY SKIES AND MILD
CONDITIONS.

&&

.SHORT TERM...IR SATELLITE IMAGERY SHOWS A FRONTAL SYSTEM
APPROACHING THE PAC NW THIS EVENING. THERE ARE TWO DISTINCT
WAVES...ONE TRACKING NE INTO CENTRAL B.C. AND ANOTHER APPROACHING
130 W OFF THE OREGON COAST. MODELS INDICATE THE SRN WAVE WILL BECOME
THE DOMINATE FEATURE AS IT LIFTS NE INTO WA WEDNESDAY.

THE CURRENT RADAR LOOP IS INDICATING SOME SHOWER ACTIVITY JUST OFF
THE N WA COAST ASSOCIATED WITH THE NRN WAVE. BOTH THE NAM AND GFS
PRODUCE SOME SPOTTY LIGHT QPF OVER THE OLYMPIC PENINSULA TONIGHT.
HOWEVER...SATELLITE AND RADAR IMPLY THAT PRECIPITATION MAY EXTEND A
BIT FURTHER S. THE CURRENT FORECAST INDICATES A CHANCE OF SHOWERS
FROM THE S WA COAST NE ACROSS THE OLYMPIC PENINSULA AND INTO THE WRN
PORTION OF THE N INTERIOR WHICH LOOKS REASONABLE.

LIGHT RAIN WILL SPREAD INTO MOST OF WRN WA WEDNESDAY MORNING AND
CONTINUE THROUGH THE DAY. THE FLOW WILL BE LIGHT SO NOT MUCH
SHADOWING OFF THE OLYMPICS AND MOST PLACES WILL HAVE MEASURABLE
PRECIPITATION...GENERALLY LESS THAN A COUPLE TENTHS OF AN INCH.
MODEL TIME-HEIGHTS SHOW DEEPENING MOISTURE THROUGH THE DAY SO IT
WILL BE A CLOUDY AND COOLER DAY. HIGHS WILL REACH THE UPPER 50S TO
LOWER 60S.

THE FRONTAL WAVE WILL DISSIPATE AS IT MOVES E WEDNESDAY EVENING.
ONSHORE FLOW AND RESIDUAL MOISTURE WILL STILL PRODUCE SOME SHOWERS
ACROSS THE AREA. UPPER LEVEL LOW PRESSURE STALLS OVER THE AREA
THROUGH THURSDAY BUT THE BULK OF ENERGY HEADS S INTO NRN CA/OREGON.
COOLER AIR ALOFT WILL STILL CAUSE ENOUGH INSTABILITY FOR SOME
SHOWERS ACROSS WRN WA. BY FRIDAY...THE TROUGH BECOMES MORE SPLIT
WITH LESS UPPER LEVEL ENERGY OVER WA. BUT MODELS STILL SHOW SOME
SPOTTY LIGHT QPF OVER THE AREA MOSTLY PRODUCED BY DIURNAL
INSTABILITY. EXPECT PARTIAL SUNSHINE IN THE AFTERNOON BUT A CHANCE
OF SOME SHOWER ACTIVITY AS WELL. LITTLE CHANGE IN HIGH TEMPERATURES
RANGING FROM THE MID 50S NEAR THE COAST TO THE LOWER 60S ACROSS THE
PUGET SOUND REGION.   MERCER

.LONG TERM...FROM 300 PM DISCUSSION...THE GFS AND ECMWF ARE IN
DECENT AGREEMENT WITH THE LARGE SCALE PATTERN SATURDAY AND SUNDAY.
SOMEWHAT STRONGER WESTERLY FLOW ALOFT PREVAILS AND ANOTHER UPPER
LEVEL TROUGH CROSSES THE AREA. SATURDAY AND SATURDAY NIGHT LOOK A
LITTLE MORE SHOWERY THAN FRIDAY...AND INCREASING ONSHORE FLOW
SATURDAY NIGHT WILL HELP THE EFFORT. WEAK UPPER LEVEL RIDGING JUST
OFFSHORE ON SUNDAY MAY TRIM THE SHOWERS BACK BUT ONSHORE FLOW
CLOUDINESS WILL CONTINUE.

FOR MONDAY AND TUESDAY BOTH GFS/ECMWF BOTH SHOW AN UPPER LEVEL
TROUGH DIGGING SOUTHWARD ALONG 130W-140W...BUT DIFFER GREATLY ON
TIMING AND A LITTLE ON LOCATION. THE GFS WOULD BE FASTER AND WETTER
HERE WHILE THE ECMWF APPEARS TO SQUEEZE OUT A DRY MONDAY. THE
PRESENT BROAD BRUSH CHANCE OF SHOWERS FORECAST WILL CONTINUE FOR
NOW. KAM

&&

.AVIATION...FLOW ALOFT WILL TURN FROM THE WEST TO THE SOUTHWEST
TONIGHT AS AN UPPER TROUGH CENTERED NEAR 138W APPROACHES THE AREA
FROM THE WEST. A COLD FRONT ASSOCIATED WITH THIS UPPER TROUGH WILL
MOVE INTO THE COASTAL WATERS TONIGHT THEN WILL DISSIPATE AS IT
PUSHES INLAND ON WEDNESDAY. THE AIR MASS IS STABLE WITH MID AND
UPPER LEVEL MOISTURE. MOISTURE WILL THICKEN AND LOWER LATE TONIGHT
INTO WEDNESDAY AS THE FRONT MOVES ONSHORE.

CONDITIONS ACROSS THE AREA ARE VFR THIS EVENING WITH CIGS GENERALLY
ABOVE FL100. EXPECT LOW MVFR TO IFR CIGS TO DEVELOP ON THE COAST
AROUND 15Z WED AS LIGHT RA MOVES ONSHORE. CIGS WILL LIKELY LOWER TO
MVFR IN THE INTERIOR MIDDAY WEDNESDAY AS -RA MOVES INLAND. ALBRECHT

KSEA...DISCUSSION ABOVE APPLIES. CALM WINDS WILL BECOME LIGHT SLY AT
LESS THAN 6 KT OVERNIGHT THROUGH WEDNESDAY. ALBRECHT
&&

.MARINE...A COLD FRONT IS ENTERING THE OUTER PORTIONS OF THE COASTAL
WATERS THIS EVENING. THIS FRONT WILL SLOW AND WEAKEN AS IT MOVES
EAST...AND WILL PASS INTO THE INLAND WATERS AND DISSIPATE DURING THE
DAY WEDNESDAY. SMALL CRAFT ADVISORY SOUTH WINDS ARE EXPECTED TO
DEVELOP OVERNIGHT ACROSS THE OUTER COASTAL WATERS BEYOND 10 NM
STARTING LATER THIS EVENING AND CONTINUING THROUGH MUCH OF
WEDNESDAY. WITH THE FRONT WEAKENING AS IT MOVES EAST...EXPECT WINDS
IN THE INTERIOR TO REMAIN 20 KT OR LESS.

A WEAK TROUGH OF LOW PRESSURE WILL STALL OVER THE COASTAL WATERS ON
THURSDAY. THIS WEAK TROUGH WILL GIVE GENERALLY LIGHT WINDS TO THE
WATERS.

A DEVELOPING RIDGE OF HIGH PRESSURE OFFSHORE IS FORECAST TO MOVE
EAST TOWARD THE COASTAL WATERS ON FRIDAY. STRONG ONSHORE PRES
GRADIENTS WILL DEVELOP ON FRIDAY AND CONTINUE THROUGH FRIDAY NIGHT.
ONSHORE GRADIENTS WILL CONTINUE INTO NEXT WEEKEND. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY OUTER COASTAL WATERS.

&&

$$

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