Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 252317
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
320 PM PST THU DEC 25 2014

.SYNOPSIS...WEAK WEATHER SYSTEMS MOVING SOUTHWARD ALONG THE COAST
WILL KEEP A CHANCE OF SHOWERS IN THE FORECAST TONIGHT. A CHANCE OF
SHOWERS WILL CONTINUE ALONG THE COAST ON FRIDAY. A STRONGER
AND MORE ORGANIZED SYSTEM WILL MOVE THROUGH WESTERN WASHINGTON
SATURDAY AND SATURDAY NIGHT GIVING WITH SIGNIFICANT SNOW TO THE
CASCADES. A MODIFIED ARCTIC FRONT WILL MOVE SOUTH ACROSS THE AREA
SUNDAY NIGHT. HIGH PRESSURE ALOFT AND OFFSHORE FLOW WILL GIVE COLD
AND DRY WEATHER TO THE AREA FOR THE FIRST HALF OF NEXT WEEK.

&&

.SHORT TERM...IN RESPONSE TO A DEEP AND VERTICALLY STACKED LOW NORTH
OF HAWAII AND A BROAD AND DEEP TROUGH OVER THE WEST CENTRAL
PACIFIC...A HIGH AMPLITUDE RIDGE CONTINUES TO BUILD AROUND 140-145W
THIS AFTERNOON. WHILE THIS TRANSITION HAD BEEN CAUSING SOME FORECAST
CHALLENGES FOR THE MODELS OVER THE PAST FEW DAYS...THE VARIOUS
SOLUTIONS APPEAR TO BE COMING INTO RATHER GOOD AGREEMENT THROUGH
EARLY NEXT WEEK. FORECASTS LEAN TOWARD A BLEND OF THE 12Z/00Z
ECMWF...THE 12Z/18Z GFS...AND THE 00Z/06Z/12Z/18Z GFS PARALLEL RUNS.

A WEAK DISTURBANCE IN THE NORTHWESTERLY FLOW ALOFT AHEAD OF THE
BUILDING OFFSHORE RIDGE WILL BRING A BIT OF AN INCREASE TO SHOWER
ACTIVITY THIS EVENING...MAINLY ON THE COAST. THE HRRR CONTINUES TO
SHOW SOME WEAK ENHANCEMENT TO THE SHOWER ACTIVITY THIS EVENING IN
WESTERN SKAGIT AND SNOHOMISH COUNTY AS THIS SYSTEM PASSES...AND THIS
IS STARTING TO TAKE FORM ON THE AREA RADARS. WHILE IT WILL BE MOSTLY
CLOUDY...SHORT WAVE RIDGING WILL LIKELY GIVE A DRIER DAY TO THE
INTERIOR ON FRIDAY. TEMPERATURES TONIGHT THROUGH FRIDAY WILL BE
SEASONABLE WITH LOWS IN THE LOWER TO MID 30S AND HIGHS IN THE LOWER
TO MID 40S.

A STRONG SHORT WAVE TROUGH WILL DROP SE ACROSS THE AREA ON SATURDAY.
THE 18Z GUIDANCE CONTINUES TO SHOW HEAVY SNOW FALLING IN THE
CASCADES ON SATURDAY WITH 36 HOUR TOTALS FROM LATE FRIDAY NIGHT
THROUGH SATURDAY EVENING ON THE ORDER OF 18-30 INCHES. SNOW LEVELS
IN THE MOUNTAINS WILL RANGE FROM 2000 FEET NORTH TO 3000 FEET SOUTH
ON SATURDAY. A WINTER STORM WATCH IS BEING ISSUED FOR THIS EVENT IN
THE CASCADES.

STRONG NW WINDS AT THE SURFACE ARE EXPECTED TO DEVELOPE OFF
VANCOUVER ISLAND ON SAT BEHIND THE SHORT WAVE TROUGH. WAVE WATCH
SHOWS A NW SWELL OF 14 TO 16 FT AT 13 SECONDS THAT MOVES ONTO THE
CENTRAL COAST SATURDAY NIGHT. THE HIGH TIDE EARLY SUNDAY MORNING ON
THE COAST WILL BE BELOW THE HIGH ASTRONOMICAL TIDE BY ABOUT 2
FEET...BUT THE COMBINATION OF THE HIGHER THAN NORMAL HIGH TIDE WITH
THE SWELL THIS WILL LIKELY GIVE SOME BEACH EROSION PROBLEMS TO A
PORTION OF SHORELINE IN SOUTHERN OCEAN SHORES WHERE BEACH PROTECTION
WAS DAMAGED BY RECENT STORMS. A HIGH SURF ADVISORY WILL BE ISSUED
FOR THE OCEAN SHORES AREA IN GRAYS HARBOR COUNTY FROM 4 AM TO 8 AM
PST ON SUNDAY.

ON SUNDAY THE AREA WILL BE UNDER POST FRONTAL ONSHORE FLOW. NW FLOW
ALOFT WILL RESULT IN CONVERGENCE ZONE ACTIVITY AND POST FRONTAL
SHOWERS AND 850 MB TEMPERATURES WILL BE AROUND -6C WITH 925 MB
TEMPERATURES AROUND -1C. THIS WILL RESULT IN SNOW LEVELS FALLING TO
AROUND OR POSSIBLY A BIT BELOW 1000 FEET ON SUNDAY.

SUNDAY NIGHT WILL BE A TRICKY FORECAST PROBLEM. THE HIGH RESOLUTION
ECMWF AND GFS13 PARALLEL SOLUTIONS DEVELOP LOW PRESSURE OFF
VANCOUVER ISLAND AND DOWN TO THE WASHINGTON COAST THAT SLOWLY MOVES
SOUTH TO AROUND PORTLAND OREGON MONDAY MORNING. THE GFS PARALLEL
SHOWS STRONG OUTFLOW SUNDAY NIGHT AND MONDAY AS PRESSURES OVER THE
INTERIOR OF BRITISH COLUMBIA RISE TO OVER 1055 MB. THERE IS A CHANCE
THAT THE PUGET SOUND LOWLANDS WILL SEE SOME SNOW AS THE MODIFIED
ARCTIC AIR DROPS SOUTH ACROSS THE AREA SUNDAY NIGHT OR MONDAY
MORNING. THESE ARE ALWAYS LOW SKILL FORECASTS...EVEN IN THE SHORT
TERM...SO STAY TUNED. ALBRECHT

.LONG TERM...AFTER MOISTURE PUSHES TO THE SOUTH OF THE AREA MONDAY
MORNING...THE BULK OF THE DAY MONDAY THROUGH NEW YEARS EVE OR NEW
YEARS DAY LOOK DRY AND COLD. TEMPERATURES WILL RISE INTO THE 30S
DURING THE DAY BUT FALL INTO THE 20S AT NIGHT. SOME LOCATIONS WILL
SEE TEENS ONCE WIND CALMS. THERE IS A CHANCE THAT ANOTHER SYSTEM
WILL MOVE SOUTH INTO THE AREA ON NEW YEARS DAY. ALBRECHT

&&

.HYDROLOGY...FLOODING IS NOT EXPECTED FOR THE NEXT 7 DAYS.

&&

.AVIATION...NORTH TO NORTHWEST FLOW ALOFT WILL PREVAIL OVER WESTERN
WASHINGTON INTO TOMORROW AS AN UPPER RIDGE REMAINS OVER THE
NORTHEAST PACIFIC. AT THE SURFACE...WEAK HIGH PRESSURE WILL BUILD
OVER THE PACIFIC. THE AIR MASS WILL BE MOIST AND GENERALLY STABLE.

STILL SOME SHOWERS LINGERING OFF THE COAST THIS AFTERNOON...JUST
WEST OF KHQM. THE REMAINDER OF WESTERN WASHINGTON CONTINUES TO SEE
IMPROVEMENT TO CIGS LEVELS AS MOST STATIONS REPORT MVFR OR VFR
LEVELS. ONLY OLM AND SURROUNDING AREA REMAIN IFR DUE TO EARLIER FOG
BEING SLOW TO BURN OFF. WILL SEE JUST A LITTLE MORE IMPROVEMENT
THROUGH LATE THIS EVENING BEFORE CIGS BEGIN TO LOWER ONCE MORE TO
MVFR LEVELS WITH ISOLATED IFR...AND FOG FOR KOLM...FOR OVERNIGHT AND
INTO TOMORROW.

KSEA...SOUTH TO SOUTHEAST WIND 6 KT OR LESS. VFR CIGS MAY LIFT A
LITTLE BIT MORE INTO THIS EVENING BEFORE DROPPING DOWN TO MVFR BY
EARLY FRIDAY MORNING.   SMR

&&

.MARINE...SMALL CRAFT ADVISORIES WILL BE ALLOWED TO EXPIRE FOR THE
COASTAL WATERS AND WEST ENTRANCE STRAIT OF JUAN DE FUCA FOR
HAZARDOUS SEAS AS WINDS AND SEAS CONTINUE TO EASE THIS AFTERNOON AND
INTO TONIGHT.

A DISSIPATING FRONT WILL MOVE SLOWLY EAST ACROSS THE COASTAL WATERS
LATE TONIGHT AND FRIDAY. ANOTHER FRONT WILL MOVE THROUGH FROM THE
NORTHWEST ON SATURDAY...FOLLOWED BY ONSHORE FLOW. SMALL CRAFT
ADVISORY WINDS SHOULD BE WIDESPREAD. HIGH SWELL MOVING SOUTH OFF
VANCOUVER ISLAND LATE SATURDAY NIGHT AND INTO SUNDAY MORNING WILL
ARRIVE AT AREAS NORTH OF THE GRAYS HARBOR BAR NORTH JETTY JUST IN
TIME FOR HIGH TIDE. THIS AREA HAS ALREADY SEEN SIGNIFICANT BEACH
EROSION WITH PAST STORMS. TO PROVIDE SUPPORT TO LOCAL
OFFICIALS...HAVE ISSUED A HIGH SURF ADVISORY TO CALL ATTENTION TO
THE EVENT. NORTHERLY OFFSHORE FLOW WILL DEVELOP SUNDAY AND MONDAY AS
HIGH PRESSURE MOVES SOUTH THROUGH BRITISH COLUMBIA.

SOME OF THE HIGHER ASTRONOMICAL HIGH TIDES OF THE YEAR WILL OCCUR
THE NEXT FEW DAYS. WITH LIGHT WIND AND WEAK HIGH PRESSURE TODAY AND
FRIDAY...POSITIVE TIDAL ANOMALIES AND TIDAL OVERFLOW ARE UNLIKELY.
WITH WEATHER SYSTEMS EXPECTED FOR THE WEEKEND THOUGH...THE SITUATION
WILL CONTINUE TO REQUIRE MONITORING.   SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...HIGH SURF ADVISORY FOR THE CITY OF OCEAN SHORES IN GRAYS
      HARBOR COUNTY 4 AM TO 8 AM PST SUNDAY.
     WINTER STORM WATCH FOR THE WEST SLOPES OF THE CASCADES 4 AM
      PST SATURDAY THROUGH SATURDAY EVENING.
PZ...SMALL CRAFT ADVISORY FOR ROUGH BAR 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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