Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 171713
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PST WED DEC 17 2014

.SYNOPSIS...A WEAK PACIFIC WEATHER SYSTEM WILL BRING SOME AREAS OF
LIGHT RAIN TO WESTERN WASHINGTON THROUGH WEDNESDAY NIGHT. A STRONGER
PACIFIC FRONTAL SYSTEM IS EXPECTED TO MOVE ASHORE THURSDAY NIGHT
WITH MORE SIGNIFICANT RAIN. ANOTHER WET WEATHER SYSTEM IS FORECAST
FOR THIS WEEKEND WITH HEAVIER RAIN AMOUNTS ALONG THE COAST AND IN
THE MOUNTAINS.

&&

.SHORT TERM...MODELS ARE NOT HANDLING THE NRN STREAM PORTION OF THE
SPLIT TROUGH AFFECTING WRN WA ALL THAT WELL. THE ELONGATED 500 MB
TROUGH AXIS IS OFFSHORE JUST INSIDE 130 W AND THERE IS STILL SOME
SURFACE STRUCTURE LEFT ACCORDING THE HRRR WHICH MOST MODELS DO NOT
SHOW. THE HRRR FROM YESTERDAY AFTERNOON IS THE WINNER WHICH WAS
SHOWING SLY FLOW ENTRAINING SOME MOISTURE FROM THE SRN STREAM SYSTEM
INTO WRN WA EARLY THIS MORNING. RADAR NOW SHOWS LIGHT RAIN AROUND
MUCH OF WRN WA INCLUDING PUGET SOUND. SOME MODELS LIKE THE GFS40
WERE MOSTLY DRY AND STILL SHOW ONLY LIGHT QPF FIELDS W OF THE SOUND
AND ACROSS THE FAR N AND CASCADES. THE HRRR DOES DRY OUT THE
INTERIOR BY LATE MORNING BEFORE SPREADING SOME LIGHT RAIN BACK INTO
THE AREA EARLY THIS EVENING LIKE THE NAM12 SHOWS. THIS TIMING IS
QUESTIONABLE GIVEN THE BRIGHT CLOUDS ON IR IMAGERY APPROACHING THE
COAST COUPLED WITH THE LANGLEY HILL RADAR SHOWING MORE RAIN BANDS
ALONG THE COAST LIFTING NE. WILL MAINTAIN THE CURRENT LIKELY POPS
FOR TODAY AND TONIGHT.

THE 500 MB TROUGH AXIS FINALLY SHIFTS E OF THE AREA BY 12Z THURSDAY
MORNING. WITH LIGHT FLOW AND A SHORT WAVE RIDGE TRAVERSING THE
AREA...EXPECT LOW CLOUDS AND MAYBE SOME FOG AROUND THE AREA. THIS
WILL BREAK UP BY MIDDAY AS THE NEXT STRONGER FRONT APPROACHES
INCREASING THE FLOW OUT AHEAD OF IT. TIMING IS STILL TRICKY WITH THE
06Z GFS BRINGING RAIN AS FAR INLAND AS THE WRN PORTION OF PUGET
SOUND BY LATE THURSDAY AFTERNOON. THE NAM12/ECMWF ARE SIMILAR BUT
THE CANADIAN IS STILL RUNNING A FEW HOURS FASTER WITH RAIN
OVERSPREADING WRN WA EARLIER IN THE AFTERNOON. WILL LEAVE POPS
UNCHANGED FOR NOW BUT MAY CONSIDER REMOVING SOME OF THE INTERIOR
POPS FOR A DRY PERIOD THURSDAY MORNING. THIS WOULD PROBABLY RESULT
IN MORE FOG AROUND THE AREA.

THE WARM FRONT MOVES THROUGH QUICKLY WITH COLDER AIR ARRIVING WITH
THE MAIN FRONTAL BAND SLATED FOR THURSDAY NIGHT. THIS SYSTEM APPEARS
TO SPLIT LESS THAN PREVIOUS FRONTS AND THE SE GRADIENTS ARE TIGHT
OVER THE AREA SO EXPECT LOCALLY WINDY CONDITIONS NORTH OF PUGET
SOUND AND ALONG THE COAST. SNOW LEVELS WILL FALL FROM NEAR 5000 FEET
TO 3000 TO 3500 FEET BY LATER THURSDAY NIGHT AND FRIDAY MORNING.
THERE IS A 6 HOUR WINDOW OF STRONGER FLOW ALONG AND BEHIND THE FRONT
WHICH COULD SQUEEZE OUT SEVERAL INCHES OF SNOW EVEN DOWN TO SOME OF
THE PASSES. SKI RESORTS COULD POSSIBLY GET CLOSE TO 6 INCHES OF SNOW
WHICH IS BORDERLINE ADVISORY CRITERIA. MOUNT BAKER MAY GET THE
HIGHER AMOUNTS IN THIS PATTERN. PRECIPITATION WILL FALL AS JUST RAIN
IN THE LOWLANDS...CHANGING TO SHOWERS DURING THE AFTERNOON. ANY
BREAK IN PRECIPITATION WILL AGAIN BE SHORT LIVED AS A STRONG 180+ KT
JET ACROSS THE PACIFIC DRIVES WETTER STORMS TOWARD THE AREA BY
SATURDAY.  MERCER

.LONG TERM...NO UPDATES FROM PREVIOUS DISCUSSION...LOOKS LIKE A
SHORT BREAK IN THE WEATHER FRIDAY NIGHT BEFORE THE NEXT WETTER
SYSTEM ARRIVES OVER THE WEEKEND. THIS SYSTEM WAS CURRENTLY MOVING
ACROSS 150E THIS MORNING WITH A SUPPORTING 200 KT JET. THE PROGS ARE
STILL TRENDING A BIT DRIER WITH THIS SYSTEM GIVEN ITS MORE
PROGRESSIVE NATURE. EXPECT RAIN TO INCREASE ON SATURDAY WITH THE
WARM FRONT AND THE COLD FRONT MOVING ONSHORE SUNDAY NIGHT.

RIDGING ALOFT ALONG THE WEST COAST MEANS THE JET STREAM WILL BE AT
OUR LATITUDE WITH A WESTERLY COMPONENT INTO THE CASCADES AND
SHADOWING OFF THE OLYMPICS THIS WEEKEND. THE COAST AND MOUNTAINS
SHOULD GET THE BULK OF THE PRECIPITATION.

ANOTHER SHORT BREAK MONDAY BEFORE THE NEXT NOT AS WET SYSTEM ARRIVES
TUESDAY. THE PROGS CONTINUE TO SUGGEST A KONA LOW DEVELOPING BUT
THEY ARE INCONSISTENT IN ITS POSITION. THE FURTHER WEST USUALLY
RESULTS IN BUILDING AN UPPER RIDGE NEAR 150W OR 160W AND PRODUCING
NORTHERLY FLOW ALOFT INTO THE PACIFIC NW FOR A MUCH COOLER FLOW
PATTERN. FURTHER EAST WITH THE PATTERN AND THE PACIFIC NW GETS A
MILDER NORTHERLY FLOW ALOFT. STAY TUNED HERE. BUEHNER

&&

.HYDROLOGY...NO UPDATES FROM PREVIOUS DISCUSSION...RIVER FLOODING IS
NOT EXPECTED THROUGH SATURDAY. MOIST WESTERLY FLOW WILL DEVELOP
SATURDAY NIGHT AND CONTINUE THROUGH SUNDAY NIGHT...WITH HEAVY
MOUNTAIN RAIN AMOUNTS POSSIBLE. GUIDANCE IS NOW SHOWING A BIT
LIGHTER AMOUNTS GIVEN THE MORE PROGRESSIVE NATURE OF THIS SYSTEM.
PRIMARY CONCERN FOR ANY RIVER RISING ABOVE FLOOD STAGE IS THE
SKOKOMISH BUT ALSO NEED TO KEEP AN EYE ON OTHER CASCADE FLOOD PRONE
STREAMS. BUEHNER

&&

.AVIATION...A STRETCHED OUT AND WEAKENING UPPER LEVEL TROUGH ALONG
THE COAST IS MOVING ONSHORE THIS MORNING AND WILL BE FOLLOWED BY
ANOTHER ONE...CURRENTLY ALONG 132W...TONIGHT. AT THE SURFACE...HIGH
PRES E OF THE CASCADES COMBINED WITH LOWER PRES OVER THE COASTAL
WATERS IS GIVING LIGHT OFFSHORE FLOW. THE AIR MASS IS MOIST AND
STABLE WITH LIGHT S TO SW FLOW ALOFT.

PACIFIC NW RADARS SHOW BANDS OF LIGHT RAIN EXTENDING SW FROM WESTERN
WASHINGTON INTO THE OREGON COASTAL WATERS. THIS MOISTURE IS MOVING
NE AHEAD OF THE INCOMING WEAKENING TROUGHS OFFSHORE. MOST
TERMINALS IN THE INTERIOR ARE SEEING VFR CEILINGS AND VISIBILITIES
DESPITE THE LIGHT RAIN AS A RESULT OF THE OFFSHORE PRES GRADIENTS.
THE EXCEPTIONS ARE AT KCLM WHERE PRECIPITATION AND FLOW MOVING ONTO
THE NORTH SLOPES OF THE OLYMPICS IS PRODUCING A LOWER STRATUS DECK
AND AROUND KOLM WHERE PRECIPITATION IN THE MORE TRAPPED SOUTHWEST
INTERIOR IS PRODUCING LOW IFR STRATUS AND VIS 3-5SM BR.

THE OFFSHORE PRES GRADIENTS ARE EXPECTED TO SLOWLY RELAX THROUGH
TONIGHT AS THE STRETCHING AND DYING TROUGHS OFFSHORE MOVE INTO THE
AREA. THIS WILL RESULT IN GENERALLY DETERIORATING CEILINGS AND
POORER VISIBILITIES AS WE MOVE INTO THE TONIGHT AND THU MORNING
PERIODS. EXPECT AREAS OF IFR CONDITIONS 06Z-18Z THU AT MANY OF THE
TERMINALS IN WESTERN WASHINGTON. ALBRECHT

KSEA...EXPECT CIGS AROUND 040 THROUGH THIS EVENING WITH PERIODS OF
LIGHT RAIN. CEILINGS WILL FALL TO 008-012 AFTER 05Z THIS EVENING AND
CONTINUE UNTIL 18Z. EXPECT VIS TO GENERALLY REMAIN ABOVE 5SM IN OCNL
-RA. EAST WIND AROUND 5 KT BECOMING SE TONIGHT. ALBRECHT

&&

.MARINE...WEAK TROUGHS OF LOW PRES...ONE OVER THE COASTAL WATERS AND
ANOTHER OVER THE OFFSHORE WATERS...WILL STRETCH OUT AND DIE AS THEY
MOVE INTO THE WATERS THIS AFTERNOON THROUGH TONIGHT. EXPECT LIGHT
OFFSHORE FLOW. WITH SEAS OVER THE COASTAL WATERS REMAINING AROUND 10
FT...THE SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS WAS EXTENDED INTO
THE NEARSHORE COASTAL WATERS. SEAS WILL SUBSIDE LATE THIS AFTERNOON
AND TONIGHT.

A STRONG PACIFIC FRONT ASSOCIATED WITH A 960 MB LOW CENTERED NEAR
49N 159W WILL APPROACH THE OFFSHORE WATERS ON THU...THEN MOVE INLAND
THU NIGHT. A PERIOD OF GALE FORCE WINDS CAN BE EXPECTED OVER THE
COASTAL WATERS LATER THU AFTERNOON THRU THU NIGHT AHEAD OF AND WITH
THE FRONT. A GALE WATCH IS IN EFFECT FOR THE COASTAL WATERS FOR THU
AFTERNOON AND NIGHT. THESE WILL LIKELY BE UPGRADED TO WARNINGS THIS
AFTERNOON.

THE FETCH AREA TO THE SOUTH OF THE LOW NEAR 49N 159W IS MOVING WITH
THE WAVE FIELD IT IS PRODUCING. THE WAVEWATCH MODEL CONTINUES TO
FORECAST SEAS 40-45 FT AROUND 47N 150W BY LATE TONIGHT. THESE WAVES
ARE EXPECTED TO DECAY INTO AN ENERGETIC 20-22 FT AT 18-20 SEC SWELL
ON THE WASHINGTON COAST LATE FRI AFTERNOON AND FRI NIGHT. AS IT
TURNS OUT...TIDAL ANOMALIES ALONG THE COAST ARE JUST OVER 1 FT ABOVE
NORMAL AND HIGH TIDE OCCURS EARLY SAT MORNING. IF SWELL REMAINS HIGH
INTO SAT MORNING...SOME COASTAL FLOODING IS POSSIBLE ALONG PORTIONS
OF THE COASTAL WATERS. IT IS TOO EARLY TO TELL AT THIS POINT AND TOO
EARLY TO ISSUE ANY COASTAL FLOOD WATCHES...BUT INTERESTS ON THE
COAST ARE URGED TO KEEP UP WITH LATEST FORECASTS.

ANOTHER STRONG WARM FRONT WILL ARRIVE ON SAT AND WILL LIKELY RESULT
IN A PERIOD OF GALE FORCE WINDS OVER THE COASTAL WATERS...THE NORTH
INLAND WATERS...AND THE ENTRANCES TO THE STRAIT OF JUAN DE FUCA.
ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY TODAY FOR HAZARDOUS SEAS COASTAL
      WATERS.
     SMALL CRAFT ADVISORY TODAY FOR ROUGH GRAYS HARBOR BAR.
     GALE WATCH MIDDAY THURSDAY THROUGH LATE THURSDAY NIGHT FOR
      THE COASTAL WATERS.

&&

$$

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










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