Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 182254 AAA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PST THU DEC 18 2014

.SYNOPSIS...A FRONT WILL BRING RAIN AND LOCALLY WINDY CONDITIONS TO
WESTERN WASHINGTON THROUGH TONIGHT. AN UPPER LEVEL TROUGH WILL BRING
SCATTERED SHOWERS ON FRIDAY. STRONG MOIST WEST FLOW ALOFT OVER THE
NORTHEAST PACIFIC WILL BRING WIND AND RAIN INTO WESTERN WASHINGTON
THIS WEEKEND. A BROAD UPPER LEVEL RIDGE WILL REMAIN OVER THE REGION
EARLY NEXT WEEK FOR NOT QUITE AS WET WEATHER. COOLER TEMPERATURES
ARE POSSIBLE BY MID WEEK.

&&

.SHORT TERM...WARM ADVECTION RAIN IS FILLING IN ACROSS WRN WA THIS
AFTERNOON. VISIBLE SATELLITE IMAGERY IMPLIES THE SURFACE FRONT IS
NOW JUST INSIDE 130 W HEADING TOWARD WRN WA. IR/WV IMAGERY SHOWS A
WAVE FORMING ON THE SRN EXTENT OF THE FRONTAL BAND AS IT BEGINS TO
SPLIT. THIS WILL SLOW THE PROGRESS OF THE SYSTEM EWD BUT ALSO RESULT
IN WEAKER FLOW ALOFT SO OROGRAPHICS WILL NOT PLAY AS BIG A ROLE AS
USUAL. MODELS STILL SHOW 12 HOUR QPF TOTALS RANGING FROM ROUGHLY ONE
HALF TO ONE INCH LIQUID WITH LOCALLY HIGHER AMOUNTS. PLACES WHERE
THIS FALLS AS ALL SNOW COULD SEE UP TO 1 FOOT. SOME OF THE HIGHER
SKI RESORTS AND S/SW FACING SLOPES ARE MOST LIKELY TO SEE THESE TYPE
OF SNOW TOTALS. THE SNOW LEVEL IS DIFFICULT TO PINPOINT WITH THIS
SYSTEM WITH SOME FLUCTUATION LIKELY BETWEEN 3500 AND 5000 FEET.
TEMPERATURES IN MOST PASSES ARE STILL IN THE UPPER 20S TO AROUND 30
DEGREES...SO PRECIPITATION WILL LIKELY START AS SNOW EVEN AT
SNOQUALMIE PASS. A COUPLE INCHES MAY FALL...THEN SNOW LEVELS SHOULD
GENERALLY START TO RISE AS WARMER AIR STARTS TO WORK INLAND. THE
MAIN SNOW LEVEL SHOULD HOVER AROUND 4500 TO 5000 FEET TONIGHT WHICH
PUTS STEVENS PASS ON THE EDGE. UP TO 6 INCHES STILL SEEMS POSSIBLE
DEPENDING ON HOW MUCH RAIN MIXES IN. SNOW LEVELS WILL FALL BACK DOWN
TO AROUND 3000 FEET BEHIND THE FRONT BUT ONLY MINOR ADDITIONAL
ACCUMULATIONS EXPECTED FRIDAY MORNING.

SE PRESSURE GRADIENTS WILL CONTINUE TO TIGHTEN THIS EVENING. SE
WINDS IN THE ADMIRALTY INLET AREA AND SAN JUANS ARE CURRENTLY
GUSTING INTO THE LOW 30 MPH RANGE AND SHOULD PICK UP CONSIDERABLY
THIS EVENING. THE WIND ADVISORY WILL REMAIN IN PLACE FOR GUSTS TO 45
MPH ACROSS THE N INTERIOR...EXCLUDING WRN SKAGIT COUNTY. STRONGEST
WINDS WILL BE ALONG THE WATER AND AT PLACES EXPOSED TO SE WINDS. THE
WIND ADVISORY RUNS THROUGH MIDNIGHT TONIGHT.

IT WILL BE COOL AND SHOWERY ON FRIDAY AS THE FRONT MOVES E AND AN
UPPER TROUGH REACHES THE AREA. COLD TOPPED CU OFFSHORE INDICATES
UNSTABLE CONDITIONS BEHIND THE TROUGH. MODEL LI`S ARE NEGATIVE OVER
THE COASTAL WATERS BUT NOT OVER THE LAND AREAS. BUFFER FORECAST
SOUNDINGS SHOW THE UNSTABLE LAYER CAPPED AT 700 MB SO THUNDERSTORMS
WOULD SEEM UNLIKELY.

STRONG AND MOIST WLY FLOW DRIVEN BY A STRONG 180+ KT JET WILL STEER
A WARM FRONT INTO WRN WA ON SATURDAY. TIMING IS A LITTLE FASTER WITH
MOST MODELS DEVELOPING WIDESPREAD RAIN ACROSS WRN WA BY LATE FRIDAY
NIGHT OR EARLY SATURDAY MORNING. INITIALLY THE FLOW WILL BE OUT OF
THE S/SW AND STRONG AT 40 TO 50 KT. THIS WILL PRODUCE HEFTY
OROGRAPHICALLY ENHANCED RAINFALL AMOUNTS ALONG S/SW FACING SLOPES OF
THE OLYMPICS AND CASCADES. THIS WILL ALMOST CERTAINLY CAUSE FLOODING
ON THE SKOKOMISH RIVER BY LATER SATURDAY NIGHT. THE FLOW TURNS MORE
WLY SATURDAY NIGHT AND SUNDAY WHICH IS A FAVORABLE DIRECTION FOR
HEAVY RAIN IN MORE BASINS ACROSS THE W SLOPES OF THE CASCADES. THIS
WILL CAUSE ADDITIONAL FLOOD CONCERNS. REFER TO THE EXTENDED FLOOD
OUTLOOK STATEMENT AND HYDRO DISCUSSION BELOW FOR FURTHER DETAILS.

THIS NEXT SYSTEM WILL ALSO HAVE STRONGER SE PRESSURE GRADIENTS AND
STRONGER FLOW ALOFT AHEAD OF THE COLD FRONT ON SATURDAY. MESO MODELS
ARE SHOWING HIGH END ADVISORY LEVEL TYPE WINDS BUT HIGH WIND IN A
COUPLE SPOTS CANNOT BE RULED OUT EITHER. THIS IS STILL A COUPLE DAYS
OUT AND WILL OPT TO MAINTAIN A HIGH END ADVISORY WIND FORECAST FOR
THE COAST/N INTERIOR. PUGET SOUND WILL ALSO BECOME
BREEZY...ESPECIALLY AS GRADIENTS BECOME MORE SLY SATURDAY NIGHT AND
SUNDAY. BUT FORECAST GRADIENTS DO NOT SEEM TO SUPPORT EVEN ADVISORY
LEVEL WINDS FOR PUGET SOUND AT THIS TIME.

THE BAROCLINIC BOUNDARY WILL START TO DRIFT SWD SUNDAY AFTERNOON
DIRECTING THE HEAVIEST PRECIPITATION INTO THE WA CASCADES FROM
ABOUT KING COUNTY SWD INTO THE N/CENTRAL OREGON CASCADES. THIS WILL
HELP MITIGATE FLOODING CONCERNS ACROSS THE FAR N ALTHOUGH DETAILS ON
WHERE THIS BOUNDARY SETS UP REMAINS UNCERTAIN. THERE DOES SEEM TO BE
PRETTY GOOD CONSENSUS EVOLVING AMONG THE MODELS THAT THE HEAVY
MOUNTAIN RAIN WILL TAPER OFF FROM N TO S ON SUNDAY AND MOSTLY SHUT
OFF BY SUNDAY NIGHT AS THE RIDGE BEGINS TO BUILD.

.LONG TERM...THE EXTENDED FORECAST REMAINS CHALLENGING WITH A WIDE
RANGE OF SOLUTIONS AMONG THE MODELS. THERE IS SOME AGREEMENT THAT
MONDAY WILL GENERALLY FEATURE SHOWERS OR A LULL IN RAINFALL AS
HIGHER PRESSURE BUILDS ALOFT. MODELS DO NOT AGREE ON THE STRENGTH OR
TRACK OF THE NEXT SYSTEM APPROACHING FROM THE W ON TUESDAY. THE
LATEST 12/18Z GFS BRINGS THE LOW IN FURTHER N WHICH WOULD STILL
PRODUCE WARM ADVECTION RAINFALL ACROSS THE AREA...THOUGH NOT AS MUCH
AS WAS SHOWN BY PREVIOUS SOLUTIONS.

MOST MODELS DIG A COLD UPPER TROUGH SWD OUT OF B.C. THROUGH WRN WA
ON WEDNESDAY. THIS COULD USHER IN COOLER AIR BUT MODELS DISAGREE ON
THE EXTENT OF RIDGING OFFSHORE AND WHERE IT SETS UP. THE ECMWF
CONTINUES TO BE ONE OF THE COLDER SOLUTIONS AND HAS THE KONA LOW CUT
OFF FURTHER W...THUS A FURTHER W DEVELOPMENT OF THE UPPER RIDGE. THE
KONA LOW IS FURTHER E ON THE GFS40 WITH A RIDGE NEAR 135-140 WHICH
WILL STILL USHER IN COOLER NLY FLOW BUT PERHAPS LESS CHANCE OF
PRECIPITATION. IT WILL BE INTERESTING TO SEE HOW THIS PANS OUT BUT A
SWITCH TO COOLER OR COLD TEMPERATURES IS A POSSIBILITY BY THURSDAY.
POPS WERE TRIMMED WAY BACK AND MAY NEED TO BE REMOVED ALTOGETHER IF
MODEL TRENDS CONTINUE. SNOW LEVELS WILL ALSO BE MUCH
LOWER...POSSIBLY TO SEA LEVEL...BUT IT IS VERY UNCLEAR AS TO HOW
MUCH MOISTURE WILL REMAIN BY THAT TIME.

&&

.HYDROLOGY...THOUGH RAIN WILL FALL AT TIMES...FLOODING IS UNLIKELY
ON ANY RIVER THROUGH SATURDAY.

HEAVY RAIN COULD CAUSE FLOODING BY LATE THIS WEEKEND. A WEATHER
SYSTEM WITH STRONG WESTERLY FLOW ALOFT WILL BRING HEAVY RAIN AT
TIMES TO THE COAST AND OLYMPICS ON SATURDAY. HEAVY RAIN WILL SPREAD
INTO THE CASCADES SATURDAY AFTERNOON.

MODELS CURRENTLY TAPER RAIN OVER THE OLYMPICS ON SUNDAY BUT KEEP
HEAVY RAIN GOING IN THE CASCADES...ESPECIALLY OVER THE CASCADES FROM
ABOUT KING COUNTY SOUTH. THE TOTAL MODEL RAINFALL OVER ABOUT 48
HOURS FROM SATURDAY MORNING THROUGH SUNDAY NIGHT AMOUNTS TO AROUND
FOUR TO SIX INCHES OVER THE OLYMPICS AND CENTRAL CASCADES WITH A
TEN-INCH BULLSEYE OVER MOUNT RAINIER. THE SNOW LEVEL WILL BE ABOVE
5000 FEET ON SATURDAY AND SUNDAY BEFORE FALLING TO 3500 FEET OR SO
SUNDAY NIGHT.

THESE RAINFALL AMOUNTS ARE ENOUGH TO CAUSE FLOODING. AT THIS TIME
THE RIVERS MOST LIKELY TO FLOOD ARE THE ONES FLOWING OFF THE
CASCADES OF KING...PIERCE...AND LEWIS COUNTIES...AS WELL AS THE
UNUSUALLY FLOOD-PRONE SKOKOMISH RIVER IN MASON COUNTY. ANY FLOODING
WOULD LIKELY BEGIN AS EARLY AS SATURDAY NIGHT ON THE SKOKOMISH
RIVER. THE CASCADE RIVERS COULD FLOOD SUNDAY WITH CRESTS SUNDAY
NIGHT OR MONDAY.

A COOLER AND DRIER AIR MASS IS FORECAST TO BE OVER THE AREA NEXT
WEEK. AFTER MONDAY THE RIVERS WILL RECEDE AND ANY FURTHER FLOODING
IS UNLIKELY. BURKE

&&

.AVIATION...A STRONG PACIFIC FRONTAL SYSTEM OVER THE OFFSHORE WATERS
WILL MOVE ACROSS THE AREA TONIGHT. SOUTHWESTERLY FLOW ALOFT WILL
INCREASE AND BECOMING STRONG THIS EVENING THEN BECOMING MODERATE ON
FRIDAY. THE AIR MASS IS MOIST AND STABLE...IT WILL BECOME SOMEWHAT
UNSTABLE LATE TONIGHT AND FRIDAY BEHIND THE FRONT.

AT 2230Z A STRONG FRONT WAS MOVING THROUGH THE OFFSHORE WATERS. IT
WILL PUSH ACROSS THE AREA REACHING THE INTERIOR VALLEYS ABOUT 08Z
FRIDAY THEN EAST OF THE CASCADES BY 12Z. LOW END VFR CONDITIONS THIS
AFTERNOON WILL DETERIORATE TO MVFR SHORTLY AFTER 00Z THIS EVENING AS
PRECIPITATION INCREASES WITH THE APPROACH OF THE FRONT.

THERE IS LIKELY TO BE A PERIOD OF LOW LEVEL WIND SHEAR IN THE
INTERIOR FROM ABOUT KPAE SOUTHWARD AND ALONG THE COAST FOR A FEW
HOURS AHEAD OF THE FRONT AS WINDS AT 2000 FT INCREASE TO SSW 40-50KT
AND SURFACE WINDS REMAIN SELY.

POST FRONTAL ONSHORE FLOW WITH SCATTERED SHOWERS ARE EXPECTED BEHIND
THE FRONT LATE TONIGHT AND FRIDAY WITH MVFR CONDITIONS AND SOME
CLEARING AT TIMES IN THE LEE OF THE OLYMPICS. ISOLATED THUNDERSTORMS
ARE NOT OUT OF THE QUESTION ALONG THE COAST ON FRIDAY AS COLD AIR
ALOFT ARRIVES WITH AN UPPER LEVEL TROUGH. ALBRECHT

KSEA...EXPECT BKN-OVC035-040 WITH PERIODS OF LIGHT RAIN...WITH
CEILINGS DETERIORATING TO OVC015 BY 06Z. SE WIND 7-11 KT WILL SHIFT
TO S 11-17 KT WITH GUSTS TO 25 KT BEHIND THE FRONT 08Z-10Z. THERE
WILL BE 2-4 HOURS OF WIND 19035-40KT AT FL020 AHEAD OF THE INCOMING
FRONT ABOUT 05Z-08Z TIME PERIOD. ALBRECHT

&&

.MARINE...A FRONT ASSOCIATED WITH A 970 MB LOW WELL WEST OF HAIDA
GWAII WILL MOVE THROUGH THE WATERS TONIGHT. GALE FORCE WINDS CAN BE
EXPECTED OVER THE COASTAL WATERS AND AT THE EAST ENTRANCE TO THE
STRAIT UNTIL THE FRONT PASSES WITH SMALL CRAFT ADVISORY CONDITIONS
FOR THE STRAIT...NORTHERN INLAND WATERS...ADMIRALTY INLET AND PUGET
SOUND AND HOOD CANAL.

EXPECT MODERATE TO STRONG ONSHORE FLOW BEHIND THE FRONT AND WITH A
SURFACE TROUGH AS IT MOVES ACROSS THE AREA ON FRIDAY. SMALL CRAFT
ADVISORY CONDITIONS WILL CONTINUE OVER THE COASTAL WATERS AND
POSSIBLY SOME OF THE INTERIOR ZONES. ISOLATED THUNDERSTORMS CANNOT
BE RULED OUT OVER THE COASTAL WATERS ON FRIDAY IN A RATHER UNSTABLE
AIR MASS.

30-45 FOOT WAVES OVER A LARGE AREA OF THE NORTH CENTRAL PACIFIC WILL
DECAY TO A LONG PERIOD 22-25 FT SWELL THAT WILL REACH THE COAST
MIDDAY FRIDAY AND CONTINUE TO PUSH ONSHORE THROUGH SATURDAY. TIDAL
ANOMALIES ALONG THE COAST ARE JUST OVER 1 FOOT ABOVE NORMAL AND HIGH
TIDE OCCURS SATURDAY MORNING. SOME COASTAL FLOODING IS POSSIBLE
ALONG THE COAST AROUND THE HIGH TIDE ON SATURDAY...AND A COASTAL
FLOOD WATCH IS IN EFFECT. A HIGH SURF ADVISORY WAS ISSUED FOR THE
COAST FROM MIDDAY FRIDAY THROUGH FRIDAY NIGHT.

ANOTHER STRONG FRONT WILL ARRIVE ON SATURDAY WITH GALE
FORCE WINDS ONCE AGAIN LIKELY OVER THE COASTAL WATERS...THE NORTHERN
INLAND WATERS...AND THE STRAIT ENTRANCES. STRONG ONSHORE FLOW
SATURDAY NIGHT WILL SLOWLY DIMINISH ON SUNDAY. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...WIND ADVISORY THROUGH TONIGHT FOR THE SAN JUAN ISLANDS...
     WESTERN WHATCOM COUNTY...AND THE ADMIRALTY INLET AREA.
     WINTER WEATHER ADVISORY THROUGH TONIGHT FOR THE OLYMPICS.
     WINTER WEATHER ADVISORY FOR THE WEST SLOPES OF THE CASCADES
     4 PM PST THIS AFTERNOON THROUGH TONIGHT.
     HIGH SURF ADVISORY FOR THE NORTH AND CENTRAL COAST FROM LATE
     FRIDAY MORNING THROUGH EARLY SATURDAY MORNING.
     COASTAL FLOOD WATCH FOR THE NORTH AND CENTRAL COAST FROM
     SATURDAY MORNING THROUGH SATURDAY AFTERNOON.

PZ...GALE WARNING THROUGH TONIGHT COASTAL WATERS AND EAST
     ENTRANCE.
     SMALL CRAFT ADVISORY ALL REMAINING WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR TONIGHT THROUGH
     FRIDAY.
     GALE WATCH COAST...ENTRANCES TO THE STRAIT OF JUAN DE FUCA...
     ADMIRALTY INLET...AND NORTHERN INLAND WATERS FRIDAY NIGHT
     THROUGH SATURDAY.




&&

$$

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
















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