Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 192326
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 PM PST FRI DEC 19 2014

.SYNOPSIS...AN UPPER LEVEL TROUGH WILL BRING SCATTERED SHOWERS
TODAY. A PAIR OF STRONG WARM FRONTS WILL BRING RAIN AND WIND
TO WESTERN WASHINGTON TONIGHT THROUGH THIS WEEKEND. AN UPPER LEVEL
RIDGE WILL MOVE BRIEFLY OVER THE REGION MONDAY FOR DRIER WEATHER.
ADDITIONAL FRONTAL SYSTEMS WILL REACH THE REGION NEXT WEEK.

&&

.SHORT TERM...AN UPPER LEVEL TROUGH IS BRINGING SCATTERED SHOWERS TO
WESTERN WA THIS AFTERNOON. THERE WERE A FEW LIGHTNING STRIKES OVER
THE COAST THIS MORNING BUT ACTIVITY HAS SINCE DIMINISHED. THE
TROUGH AXIS WILL EXIT THIS EVENING WITH SHOWERS TAPERING OFF.

ANOTHER ROUND OF WET AND BLUSTERY WEATHER IS ANTICIPATED LATE
TONIGHT THROUGH SATURDAY AS OUR NEXT SYSTEM ARRIVES. THIS SYSTEM MAY
BRING HIGH WIND TO THE COAST AND NORTH INTERIOR A WARM FRONT MOVES
INLAND SATURDAY MORNING. A HIGH WIND WARNING IS IN EFFECT FOR THESE
ZONES. WINDS WILL PEAK AROUND 12-15Z ALONG THE COAST...AND 18-20Z
FOR THE NORTH INTERIOR.

MEANWHILE...A PLUME OF MOISTURE IS BEING TRANSPORTED EAST ALONG A
STRONG 175 KT JET AIMED AT WASHINGTON...WHICH WILL LIKELY RESULT IN
HEAVY PRECIP ALONG THE COAST AND IN THE MOUNTAINS. INITIALLY SNOW
LEVELS WILL HOVER AROUND 3000-4000 FT SATURDAY MORNING BUT WILL RISE
TO 5000-7000 FT SATURDAY AFTERNOON AS WARMER AIR MOVES IN. THE WEST
SLOPES CASCADES COULD SEE BRIEF HEAVY SNOW BEFORE CHANGING OVER TO
RAIN SATURDAY AFTERNOON AND EVENING. THE SNOW LEVELS WILL REMAIN
HIGH SAT NIGHT THROUGH SUN WITH MAINLY RAIN IN THE MOUNTAINS...HEAVY
AT TIMES. THIS MAY RESULT IN RIVER FLOODING AND A FLOOD WATCH WAS
ISSUED. HEAVY RAIN WILL CONTINUE IN THE MOUNTAINS SAT NIGHT INTO SUN
MORNING WITH STRONG WEST WINDS ALOFT. AN UPPER LEVEL RIDGE WILL THEN
BUILD OVER THE AREA SUN NIGHT. THIS RIDGE WILL CUT OFF THE FLOW
WHICH WILL BRING AN END TO THE STEADY/HEAVY PRECIP IN THE MOUNTAINS.
THIS UPPER LEVEL RIDGE WILL REMAIN OVER THE AREA ON MONDAY FOR DRIER
WEATHER. 33

.LONG TERM...MODELS ARE WEAKENING THE RIDGE OVER THE PAC NW ON TUE
WITH WESTERLY FLOW ALOFT OVER THE REGION. A BAROCLINIC BAND WILL
STALL OVER THE AREA FOR MORE RAIN. SNOW LEVELS ARE STILL HIGH AROUND
5000-6000 FT. THERE ARE SOME SLIGHT DIFFERENCES IN THE MODELS WITH
THE ECMWF ABOUT 6 HOURS FASTER THAN THE GFS. AN UPPER LEVEL TROUGH
WILL THEN MOVE INLAND ON WED...SHIFTING THE BAROCLINIC BAND SOUTH OF
THE CWA. NORTHERLY FLOW WILL PREVAIL ON CHRISTMAS DAY FOR MAINLY DRY
WEATHER. THIS PATTERN CONTINUES ON FRIDAY. 33

&&

.HYDROLOGY...HEAVY PRECIPITATION IS FORECAST SATURDAY AND SUNDAY IN
THE MOUNTAINS AND A FLOOD WATCH IS IN EFFECT FOR THE RIVERS FLOWING
OFF THE OLYMPICS AND THE NORTH AND CENTRAL CASCADES.

PRECIPITATION WILL INITIALLY FALL AS SNOW IN THE MOUNTAINS WITH SNOW
LEVELS AROUND 3000-4000 FT. WARMER AIR WILL THEN MOVE INLAND
SATURDAY AFTERNOON AND EVENING WITH SNOW LEVELS RISING TO 6000-7000
FT FOR MAINLY RAIN IN THE MOUNTAINS. PRECIPITATION AMOUNTS IN THE
OLYMPICS AND CASCADES ARE GENERALLY AROUND 3-5 INCHES...WITH 6-9
INCHES POSSIBLE AT MOUNT RAINIER.

ANY FLOODING WOULD LIKELY BEGIN AS EARLY AS SATURDAY NIGHT ON THE
SKOKOMISH RIVER. THE CASCADE RIVERS COULD FLOOD SATURDAY NIGHT OR
SUNDAY. MODELS SHOW HEAVY RAIN ENDING SUNDAY NIGHT.

&&

.AVIATION...SCATTERED SHOWERS REMAIN OVER WESTERN WASHINGTON THIS
AFTERNOON. THERE HAVE BEEN ISOLATED THUNDERSTORMS ON THE COAST. A
STRONG FRONTAL SYSTEM WILL REACH THE AREA LATER TONIGHT AND SATURDAY
WITH WESTERLY FLOW ALOFT. THE SHOWERS WILL END THIS EVENING AND RAIN
WILL DEVELOP OVERNIGHT. AT THE SURFACE STRONG SOUTHEAST WINDS WILL
DEVELOP TONIGHT. THE FLOW WILL TURN ONSHORE SATURDAY NIGHT.

CEILINGS ARE GENERALLY VFR WITH SOME MVFR IN SHOWERS. CLOUDS WILL
THICKEN AND LOWER TONIGHT WITH SATURDAY BEING MOSTLY MVFR WITH RAIN.

KSEA...DISCUSSION ABOVE APPLIES. WIND WILL BE SOUTHERLY 10-15 KT
OVERNIGHT INCREASING TO 15G25KT SATURDAY. CHB

&&

.MARINE...A STRONG SYSTEM WILL APPROACH TONIGHT WITH GALES LIKELY
OVER THE COAST...ENTRANCES...AND NORTHERN INLAND WATERS. AFTER A
BRIEF LULL IN WINDS SATURDAY AFTERNOON MODELS SHOW A PERIOD OF
STRONG ONSHORE FLOW SATURDAY NIGHT THAT MAY ALSO HAVE NEAR GALE
SOUTHWEST WINDS OVER MOST WATERS.

A WEAKER SYSTEM ARRIVES MONDAY NIGHT OR TUESDAY.

SWELL ON THE COAST WILL REACH 20 FEET OR SO TONIGHT AND SATURDAY.
COUPLED WITH HIGH TIDES AROUND NOON SATURDAY THERE COULD BE SOME
MINOR TIDAL OVERFLOW AT SPOTS. SO FAR BUOY 5...ABOUT 300 MILES OFF
THE COAST...SHOWS ONLY 21 FOOT SWELLS. THESE WILL DECAY A BIT BEFORE
ARRIVING AT THE COAST. HAVE ISSUED A HIGH SURF ADVISORY FOR THE
WAVES ON THE BEACHES TONIGHT AND SATURDAY AND A COASTAL FLOOD
ADVISORY FOR MINOR FLOODING OF LOW AREAS NEAR THE BEACHES DURING THE
TIME OF HIGH TIDE SATURDAY MORNING.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...HIGH WIND WARNING COAST...WESTERN WHATCOM...SAN
     JUANS...ADMIRALTY INLET.
     WIND ADVISORY WESTERN SKAGIT COUNTY.
     WINTER WEATHER ADVISORY CASCADES.
     HIGH SURF ADVISORY NORTH AND CENTRAL COAST.
     COASTAL FLOOD ADVISORY NORTH AND CENTRAL COAST SATURDAY MORNING.

PZ...GALE WARNING COAST...ENTRANCES...NORTHERN INLAND WATERS.
     SMALL CRAFT ADVISORY CENTRAL STRAIT...ADMIRALTY INLET...AND
     PUGET SOUND AND HOOD CANAL.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

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









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