Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 200548
AFDSEW

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

.SYNOPSIS...A STRONG LOW PRESSURE SYSTEM WILL BRING LOCALLY HEAVY
PRECIPITATION AND STRONG WINDS TO THE AREA LATE TONIGHT THROUGH
SATURDAY. STRONG AND MOIST WESTERLY FLOW ALOFT WILL PREVAIL
SATURDAY NIGHT THROUGH SUNDAY FOR HEAVY RAIN AT TIMES IN THE
MOUNTAINS. HIGH PRESSURE ALOFT WILL BUILD INTO THE REGION SUNDAY
NIGHT AND THEN PREVAIL OVER THE AREA ON MONDAY FOR A DRYING TREND.

&&

.SHORT TERM...
THE LEADING EDGE OF THE PRECIP SHIELD ASSOCIATED WITH THE NEXT
PACIFIC STORM WAS APPROACHING THE COAST AT THIS TIME. LOOK FOR THE
PRECIP TO OVERSPREAD THE CWA OVERNIGHT. IN ADDITION TO THE PRECIP...
THE SOUTHEAST TO NORTHWEST SURFACE PRESSURE GRADIENT WILL TIGHTEN
OVERNIGHT AHEAD OF A WEAKENING WARM FRONT. THIS WILL RESULT IN THE
DEVELOPMENT OF LOCALLY GUSTY WINDS OVER THOSE AREAS PRONE TO
STRONG SOUTHEAST WINDS.

THE LATEST GFS AND NAM MOS GUIDANCE INDICATED THAT THE STRONGEST
WINDS WILL HOLD OFF OVER THE COAST AND NORTH INTERIOR UNTIL LATE
SATURDAY MORNING OR SATURDAY AFTERNOON. HOWEVER...THE HRRR
INDICATED THE POTENTIAL FOR HIGH WINDS OVER THE COAST LATE TONIGHT
OR EARLY SATURDAY MORNING...AND OVER THE NORTH INTERIOR SATURDAY
MORNING. THERE IS A POSSIBILITY THAT THERE MAY BE TWO PERIODS OF
VERY STRONG WINDS OVER THE COAST AND NORTH INTERIOR...ONE WITH THE
WEAKENING WARM FRONT LATE TONIGHT/SATURDAY MORNING AND ONE WITH
THE OCCLUDED FRONT SATURDAY AFTERNOON. IT IS POSSIBLE THAT THE
HIGH WIND WARNING MAY NEED TO BE EXTENDED.

CONCERNING THE HIGH SURF AND THE POTENTIAL FOR MINOR COASTAL
FLOODING...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. THERE IS A HIGH SURF ADVISORY
FOR THE COASTAL BEACHES THROUGH SATURDAY AND A COASTAL FLOOD
ADVISORY FOR MINOR FLOODING OF LOW AREAS NEAR THE BEACHES DURING
THE TIME OF HIGH TIDE SATURDAY MORNING.

MEANWHILE...A PLUME OF MOISTURE WAS BEING TRANSPORTED EAST ALONG A
STRONG 180-KNOT JET AIMED AT WESTERN WASHINGTON. THIS WILL RESULT
IN HEAVY PRECIP ON THE COAST AND MOUNTAINS. INITIALLY...SNOW
LEVELS WILL BE IN THE 3000 TO 4000 FOOT RANGE SATURDAY MORNING BUT
WILL RISE INTO THE 6000 TO 7000 FOOT RANGE SATURDAY AFTERNOON AS A
MILDER AIR MASS MOVES IN.

THE CASCADES COULD SEE BRIEF HEAVY SNOW BEFORE THE PRECIP CHANGES
OVER TO RAIN SATURDAY AFTERNOON AND EARLY EVENING. THE SNOW LEVELS
WILL REMAIN RELATIVELY HIGH SATURDAY NIGHT THROUGH SUNDAY WITH
MAINLY RAIN...HEAVY AT TIMES...FALLING IN THE MOUNTAINS. THIS
COULD LEAD TO RIVER FLOODING...HENCE THE FLOOD WATCH.

PERIODS OF HEAVY RAIN WILL CONTINUE IN THE MOUNTAINS SATURDAY NIGHT
THROUGH SUNDAY MORNING DUE TO STRONG WESTERLY WINDS ALOFT. THE
PRECIP WILL TAPER OFF TO SHOWERS SUNDAY NIGHT AS AN UPPER RIDGE
BUILDS IN OVER THE AREA FROM THE WEST.

AN UPPER RIDGE WILL PREVAIL OVER THE REGION ON MONDAY FOR DRIER
CONDITIONS. IN FACT...MUCH OF THE CWA MIGHT END UP DRY ON MON.
33/05/19

.LONG TERM FROM THE PREVIOUS DISCUSSION...
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 RAIN IS FORECAST SATURDAY AFTERNOON THROUGH SUNDAY IN THE
MOUNTAINS...THUS A FLOOD WATCH IS IN EFFECT FOR THE RIVERS FLOWING
OFF THE OLYMPICS AND CASCADES.

PRECIPITATION WILL INITIALLY FALL AS SNOW IN THE MOUNTAINS WITH SNOW
LEVELS IN THE 3000 TO 4000 FOOT RANGE. A MILDER AIR MASS WILL MOVE
INTO THE CWA SATURDAY AFTERNOON AND EVENING...CAUSING THE SNOW
LEVELS TO RISE INTO THE 6000 TO 7000 FOOT RANGE. THIS MEANS THAT
THE PRECIPITATION WILL FALL AS RAIN IN THE MOUNTAINS BELOW AT
LEAST THE 6000 FOOT LEVEL. AMOUNTS IN THE OLYMPICS AND CASCADES
ARE FORECAST TO BE GENERALLY IN THE 3 TO 5 INCH RANGE...WITH
LOCALLY UP TO 8 INCHES POSSIBLE ON MOUNT RAINIER.

FLOODING WOULD LIKELY BEGIN SATURDAY NIGHT ON THE OLYMPIC RANGE
WHILES FLOODING ON THE CASCADES WOULD PROBABLY HOLD OFF UNTIL
SUNDAY. THE RIVERS WOULD CREST SUNDAY NIGHT OR MONDAY.

THE HEAVY RAIN IS ANTICIPATED TO END SUNDAY NIGHT. BEYOND MONDAY...
FLOODING IS NOT EXPECTED THROUGH FRIDAY.

&&

.AVIATION...A STRONG FRONTAL SYSTEM WILL MOVE INTO WRN WA OVERNIGHT
THROUGH SATURDAY EVENING. THERE WILL BE STRONG WESTERLY FLOW ALOFT.
THE AIR WILL BE MOIST AND STABLE WITH RAIN...LOWERING CIGS TONIGHT
AND SATURDAY MORNING...PERHAPS A LITTLE IMPROVEMENT MIDDAY...THEN
LOWERING AGAIN WITH A SECOND FRONTAL WAVE LATER IN THE AFTERNOON AND
EVENING. THE MOUNTAINS AND FOOTHILLS WILL BE OBSCURED THROUGH THE
WEEKEND.

KSEA...RAIN DEVELOPING OVERNIGHT AND AN INCREASING SOUTHERLY BREEZE
THROUGH SATURDAY...PROBABLY BECOMING QUITE BLUSTERY BY SATURDAY
EVENING WITH THE PASSAGE OF THE SECOND FRONTAL WAVE.

&&

.MARINE...A STRONG SYSTEM WILL APPROACH TONIGHT WITH GALES LIKELY
OVER THE COAST...ENTRANCES...AND NORTHERN INLAND WATERS. AFTER A
BRIEF LULL IN WINDS MIDDAY SATURDAY IT SHOULD BECOME QUITE BLUSTERY
WITH MORE GALES LATER IN THE AFTERNOON AND EVENING. A WEAKER SYSTEM
ARRIVES MONDAY NIGHT OR TUESDAY.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...HIGH WIND WARNINGS ARE IN EFFECT FOR THE COAST AND PARTS OF THE
     NORTH INTERIOR.

     HIGH SURF ADVISORY IN EFFECT TIL 4 PM SATURDAY FOR THE COAST.

     WIND ADVISORY IN EFFECT FROM MIDNIGHT TONIGHT TIL NOON
     SATURDAY FOR WESTERN SKAGIT COUNTY.

     WINTER WEATHER ADVISORY IN EFFECT FROM 4 AM TIL 4 PM SATURDAY
     FOR THE CASCADES.

     COASTAL FLOOD ADVISORY IN EFFECT FROM 8 AM TIL 1 PM SATURDAY
     FOR THE COAST.

     FLOOD WATCHES ARE IN EFFECT FOR MUCH OF THE CWA FROM SATURDAY
     AFTERNOON THROUGH LATE SUNDAY NIGHT.

PZ...GALE WARNINGS ARE IN EFFECT FOR THE COASTAL WATERS...ENTRANCES
     TO THE STRAIT OF JUAN DE FUCA...AND THE NORTHERN INLAND WATERS.

     SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE REMAINING WATERS.

     SMALL CRAFT ADVISORY FOR ROUGH BAR IN EFFECT TIL 9 PM
     SATURDAY FOR THE GRAYS HARBOR BAR.&&

$$

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