Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 261207
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
405 AM PST FRI DEC 26 2014

.SYNOPSIS...THE WEATHER WILL BE MOSTLY DRY THROUGH FRIDAY EXCEPT FOR
ISOLATED SHOWERS ON THE COAST AND IN THE MOUNTAINS. A STRONGER AND
MORE ORGANIZED SYSTEM WILL MOVE THROUGH WESTERN WASHINGTON SATURDAY
AND SATURDAY NIGHT GIVING SIGNIFICANT SNOW TO THE CASCADES. MODIFIED
ARCTIC AIR WILL MOVE SOUTH ACROSS THE AREA SUNDAY NIGHT GIVING COLD
AND DRY WEATHER TO THE AREA FOR THE FIRST HALF OF NEXT WEEK.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS CLOUDY SKIES OVER WESTERN
WASHINGTON EARLY THIS MORNING. LANGLEY HILL RADAR PICKING UP A FEW
SHOWERS ALONG THE COAST. TEMPERATURES AT 3AM/11Z WERE IN THE MID 30S
TO LOWER 40S.

WEAK SYSTEM MOVING BY TO THE WEST TODAY WILL KEEP A CHANCE OF
SHOWERS IN THE FORECAST FOR THE COAST TODAY. OVER THE INTERIOR
STRATUS DECK RATHER LOW WITH CEILINGS BELOW 1000 FEET IN MANY
LOCATIONS. WINDS ALOFT AND SURFACE GRADIENTS ARE LIGHT EARLY THIS
MORNING WITH NO CHANGES IN THE LOWER LEVELS THROUGH MID AFTERNOON.
THESE CONDITIONS WILL MAKE IT TOUGH TO GET RID OF THE STRATUS TODAY.
WILL GO WITH A MOSTLY CLOUDY FORECAST. WITH ALL THE CLOUD COVER HIGH
TEMPERATURES WILL ONLY BE ABOUT 5 DEGREES WARMER THAN THE MORNING
LOWS...EXCEPT ALONG THE COAST...WITH HIGHS IN THE 40S.

A MUCH STRONGER AND MORE ORGANIZED SHORTWAVE WILL MOVE DOWN THE
BRITISH COLUMBIA COAST TONIGHT WITH THE PRECIPITATION OUT AHEAD OF
THE SYSTEM REACHING THE NORTHERN PORTION OF THE AREA AFTER MIDNIGHT.
WITH THE CONTINUED CLOUD COVER LOWS WILL ONLY DROP TO NEAR 40
DEGREES.

SYSTEM MOVING INTO WESTERN WASHINGTON ON SATURDAY MAKING FOR A WET
DAY ACROSS THE AREA. SNOW LEVELS RELATIVELY LOW...NEAR 2000 IN THE
NORTH TO NEAR 3000 FEET SOUTH OF MOUNT RAINIER. OROGRAPHICS
GOOD...NOT GREAT...WITH 850 MB WINDS WESTERLY AT 25 TO 35 KNOTS BUT
WITH PLENTY OF MOISTURE ASSOCIATED WITH THIS SYSTEM THE MOUNTAIN
WILL GET A SIGNIFICANT AMOUNT OF SNOW ON SATURDAY. HAVE UPGRADED THE
WINTER STORM WATCH THAT BEGINS LATE TONIGHT AND CONTINUES INTO
SATURDAY NIGHT TO A WINTER STORM WARNING. SATURDAY IS WHEN THE SNOW
RATES WILL PEAK WITH 8 TO 16 INCHES OF NEW SNOW FORECASTED.

SYSTEM MOVING OUT OF THE AREA SATURDAY NIGHT WITH THE PRECIPITATION
TAPERING OFF FROM NORTH TO SOUTH OVERNIGHT. SNOW WILL CONTINUE IN
THE MOUNTAINS WITH THE SNOW LEVELS REMAINING WELL BELOW PASS LEVELS.
BY LATE SATURDAY NIGHT NEW SNOW TOTALS EXPECTED TO BE IN THE 12 TO
20 INCHES RANGE. LOWS SATURDAY NIGHT IN THE MID 30S TO LOWER 40S.

SUNDAY TURNING INTO A TRANSITION DAY TO A COOLER AND DRIER AIR MASS.
ECMWF IS MUCH SLOWER IN KICKING THE SYSTEM OUT OF THE AREA WILL
PRECIPITATION LINGERING WELL INTO SUNDAY. THE NAM...GFS AND GFS
PARALLEL RUNS ARE MUCH QUICKER KICKING THE PRECIPITATION SOUTH OF
THE AREA. WITH THE COOLER AIR MASS FILTERING IN FORM THE NORTH SNOW
LEVELS WILL LOWER TO NEAR 500 FEET ESPECIALLY IN THE NORTH BUT THERE
WILL ALSO BE A DRYING TREND WITH JUST A CHANCE OF SHOWERS. HIGHS
IN THE LOWER 40S WILL BE COMMON. FELTON

.LONG TERM...MODIFIED ARCTIC FRONT MOVING THROUGH WESTERN WASHINGTON
SUNDAY NIGHT. SAME MODEL TRENDS CONTINUING WITH THE ECMWF LEAVING
MORE MOISTURE BEHIND OVER THE AREA VERSUS THE NAM...GFS AND GFS
PARALLEL RUNS. THE AIR MASS WILL BE COLD ENOUGH FOR THE
PRECIPITATION TO BE IN THE FORM OF SNOW BUT WILL GO WITH THE DRIER
MODEL SOLUTIONS SO THERE WILL NOT BE MUCH MOISTURE LEFT BY THIS
POINT. WILL CONFINE THE SHOWERS TO THE MOUNTAINS AND EXTREME
SOUTHERN PORTION OF THE AREA. LOW TEMPERATURES SUNDAY NIGHT WILL BE
BELOW FREEZING ACROSS THE ENTIRE AREA. NORTH NORTHEAST FLOW ALOFT
AND LOW LEVEL OFFSHORE FLOW MONDAY AND TUESDAY WITH A 1050 MB PLUS
HIGH OVER BRITISH COLUMBIA. COULD BE SOME OUTFLOW WINDS ON MONDAY
OVER THE NORTHWEST INTERIOR BUT WITH THE HIGH SPILLING SOUTH ON
TUESDAY AND THE GRADIENTS BECOMING MORE EASTERLY THE OUTFLOW WINDS
WILL WEAKEN BY MONDAY NIGHT. DRY AND COOL CONDITIONS THROUGH THE
MIDDLE OF THE WEEK WITH NORTHEASTERLY FLOW ALOFT AND EASTERLY
SURFACE GRADIENTS. THE PATTERN BEGINS TO BREAK DOWN ON THURSDAY AS
THE UPPER LEVEL RIDGE OFFSHORE RETROGRADES AND THE SURFACE HIGH
PRESSURE TO THE EAST WEAKENS. LOW TEMPERATURES BELOW FREEZING
THROUGH THE PERIOD WITH THE COLDER LOCATIONS NEAR 20 DEGREES MONDAY
AND TUESDAY NIGHT. HIGHS WILL MOSTLY BE IN THE 30S. FELTON

&&

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

&&

.AVIATION...NORTHERLY FLOW ALOFT OVER WESTERN WASHINGTON EARLY THIS
MORNING WILL CONTINUE THROUGH THIS EVENING...AS AN OFFSHORE UPPER
RIDGE MOVES SLOWLY EASTWARD TOWARD THE PACIFIC NORTHWEST. THE RIDGE
WILL MOVE INLAND LATE TONIGHT...AND THE LEADING EDGE OF A FRONTAL
SYSTEM WILL MOVE SOUTHEAST INTO THE AREA. AT THE SURFACE...WEAK HIGH
PRESSURE WILL REMAIN OVER THE REGION. THE AIR MASS BE STABLE AND
MOIST BELOW ABOUT 7000 FT.

AREAS OF LOW STRATUS AND FOG INCREASED OVERNIGHT AND WERE FAIRLY
EXTENSIVE ACROSS THE WESTERN WASHINGTON LOWLANDS BY 2 AM. WITH LIGHT
WINDS AND LOTS OF LOW LEVEL MOISTURE...THE FOG AND STRATUS WILL
INCREASE THROUGH DAY BREAK. THERE WILL BE GRADUAL IMPROVEMENT THIS
AFTERNOON...THEN CONDITIONS WILL DETERIORATE AGAIN TONIGHT WITH
PRECIPITATION BEGINNING LATE TONIGHT.

KSEA...SOUTH TO SOUTHEAST WIND 6 KT OR LESS. STRATUS AND FOG WITH
IFR CONDITIONS THIS MORNING SHOULD GIVE WAY TO MARGINAL IMPROVEMENT
LATER THIS AFTERNOON.   MCDONNAL

&&

.MARINE...WEAK HIGH PRESSURE WILL BE OVER THE REGION TODAY. A
FRONTAL SYSTEM WILL MOVE SOUTHEAST ALONG THE BRITISH COLUMBIA COAST
TONIGHT...AND THEN MOVE THROUGH THE COASTAL AND INLAND WATERS OF
WESTERN WASHINGTON SATURDAY. STRONG ONSHORE FLOW WILL FOLLOW THE
FRONT SATURDAY NIGHT. SMALL CRAFT ADVISORY WINDS ARE LIKELY ALL
WATERS AHEAD OF AND IN THE WAKE OF THE FRONT...AND WESTERLY GALES
ARE A GOOD POSSIBILITY IN THE CENTRAL STRAIT OF JUAN DE FUCA AND
EAST ENTRANCE STRAIT SATURDAY AFTERNOON AND EVENING. A GALE WATCH IS
BEING ISSUED WITH THE EARLY MORNING FORECAST.

NORTHERLY OFFSHORE FLOW WILL DEVELOP SUNDAY AND MONDAY AS HIGH
PRESSURE MOVES SOUTH THROUGH BRITISH COLUMBIA. SMALL CRAFT ADVISORY
NORTHEAST WINDS ARE LIKELY FOR MUCH OF THE FORECAST AREA...AND GALES
ARE POSSIBLE FOR AT LEAST THE NORTHERN INLAND WATERS AND WEST
ENTRANCE STRAIT LATER SUNDAY NIGHT AND MONDAY. WEAKER OFFSHORE FLOW
WILL PREVAIL TUESDAY AS THE HIGH MOVES INTO THE NORTHERN ROCKY
MOUNTAINS.

NORTHWEST SWELL WILL BUILD OVER THE COASTAL WATERS SATURDAY NIGHT
AND SUNDAY MORNING. A 12 FT SWELL WILL REACH THE COAST NEAR OCEAN
SHORES AND THE NORTH JETTY OF GRAYS HARBOR...WHERE THERE HAS BEEN
BEACH EROSION FROM PAST STORMS. EVEN WITH THE HEIGHTENED SENSITIVITY
IN THIS PARTICULAR SPOT...THE FORECAST SWELL IS NOT LARGE ENOUGH TO
MAINTAIN THE HIGH SURF ADVISORY.

SOME OF THE HIGHEST ASTRONOMICAL HIGH TIDES OF THE YEAR WILL OCCUR
THE NEXT FEW DAYS. WITH LIGHT WIND AND WEAK HIGH PRESSURE TODAY...
POSITIVE TIDAL ANOMALIES AND TIDAL OVERFLOW ARE UNLIKELY. HOWEVER
SMALL POSITIVE TIDAL ANOMALIES ARE POSSIBLE SATURDAY AND SUNDAY...
WHICH COULD PUSH THE ACTUAL TIDAL LEVELS CLOSE TO THE THRESHOLD FOR
MINOR TIDAL OVERFLOW FOR THE INLAND WATERS. OBVIOUSLY WE WILL
CONTINUE TO WATCH THIS SITUATION.   MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...WINTER STORM WARNING LATE TONIGHT THROUGH LATE SATURDAY NIGHT
       FOR THE CASCADES.
PZ...GALE WATCH SATURDAY AFTERNOON AND EVENING CENTRAL STRAIT OF
       JUAN DE FUCA AND EAST ENTRANCE STRAIT.

&&

$$

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








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