Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 261714
AFDSEW

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

.SYNOPSIS...A MOIST AIR MASS OVER WESTERN WASHINGTON WILL KEEP SKIES
MOSTLY CLOUDY TODAY. A WEAK UPPER LEVEL TROUGH WILL BRING SPOTTY
SHOWERS TO THE COAST. A STRONG UPPER LEVEL TROUGH WILL BRING RAIN TO
THE LOWLANDS AND HEAVY SNOW TO THE MOUNTAINS LATE TONIGHT THROUGH
SATURDAY NIGHT. STRONG HIGH PRESSURE BUILDING OVER BRITISH COLUMBIA
WILL PUSH MODIFIED ARCTIC AIR SOUTH ACROSS THE AREA SUNDAY NIGHT...
GIVING COLD AND DRY WEATHER TO THE AREA FOR THE FIRST HALF OF NEXT
WEEK.

&&

.SHORT TERM...A MOIST LOW LEVEL AIR MASS IS IN PLACE OVER W WA THIS
MORNING. CIGS IN THE STRATUS LAYER ARE IFR THROUGH PUGET SOUND AND
MVFR TO VFR OVER THE N INTERIOR. THE PUGET SOUND STRATUS IS LOW
ENOUGH SO THAT IT IS TOPPING THE HIGHER HILLS CAUSING LOCALIZED
DENSE FOG. CIGS WILL LIFT SLOWLY THIS MORNING...BUT WITH WEAK
SURFACE PRESSURE GRADIENTS ACROSS THE REGION MIXING OUT THE CLOUD
COVER WILL BE TOUGH. A SMALL WEAK UPPER LEVEL SHORTWAVE TROUGH
OFFSHORE...EMBEDDED IN THE N-NW FLOW OVER THE REGION WILL BRING SOME
SPOTTY SHOWERS TO COAST AND PART WAY INLAND TODAY.

RECENT MODELS HAVE BEEN CONSISTENT IN HANDLING THE STRONG SYSTEM
FORECAST TO BRING LOWLAND RAIN AND HEAVY MOUNTAIN SNOW ON SATURDAY.
THIS IS THE SYSTEM THAT WAS MOVING E ACROSS THE GULF OF AK AT
15Z...AROUND 53N/145W. PRECIP SHOULD REACH THE NORTHERN ZONES LATE
THIS EVENING THEN SPREAD S ACROSS ALL OF W WA BY MORNING. THE COLD
FRONT SHOULD CROSS W WA AROUND MIDDAY OR EARLY SATURDAY
AFTERNOON...ALLOWING LOWLAND RAIN TO DIMINISH.

HEAVY SNOW WILL DEVELOP OVER THE CASCADES SATURDAY MORNING AS THE
COLD FRONT APPROACHES...BUT WILL CONTINUE SATURDAY AFTERNOON AND
SATURDAY NIGHT IN THE MODERATE NW LOW LEVEL FLOW ALOFT BEHIND THE
FRONT. SNOW LEVELS WILL RANGE FROM 2000 FEET NORTH TO 3000 FEET
SOUTH WHEN THE SNOW RAMPS UP SATURDAY MORNING...THEN GRADUALLY LOWER
TO 1000 TO 2000 FEET BY SUNDAY MORNING. THE CURRENT ESTIMATE IS 12
TO 20 INCHES OF SNOW WILL FALL OVER THE CASCADES IN THE 24 HOUR
PERIOD FROM 12Z SATURDAY TO 12Z SUNDAY. NW FLOW ALOFT WILL WEAKEN
SUNDAY MORNING ALLOWING THE SNOW TO DIMINISH.

MODELS AGREE ON BUILDING A HIGH AMPLITUDE UPPER LEVEL RIDGE ALONG
THE B.C. COAST ON SUNDAY WITH COLD DRY N FLOW ALOFT DEVELOPING OVER
W WA. A COLD SURFACE HIGH WILL SLIDE S OVER THE INTERIOR OF B.C.
WITH COLD MODIFIED ARCTIC AIR SQUIRTING OUT THROUGH THE FRASER RIVER
VALLEY AND S OVER W WA SUNDAY NIGHT. SNOW LEVELS WILL LOWER CLOSE TO
SEA LEVEL WHILE THE AIR MASS IS RAPIDLY DRYING. SOME LIGHT LOWLAND
SNOW IS NOT OUT OF THE QUESTION AS THE LEADING EDGE OF MODIFIED
ARCTIC AIR PUSHES S ACROSS W WA. THE CURRENT FORECAST IS FOR DRY
CONDITIONS...BUT THIS IS ALWAYS TRICKY SO CONFIDENCE IS NOT THE
HIGHEST.

THE STRONG UPPER LEVEL RIDGE AND SURFACE HIGH WILL MAKE MONDAY
MOSTLY SUNNY COLD AND DRY. MAX TEMPS WILL REMAIN IN THE 30S. KAM

.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 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 PERSIST THIS MORNING AS CIGS OVER
WESTERN WASHINGTON ARE MVFR OR LOWER. PUGET SOUND AREA CIGS ARE
LOWEST IN THE IFR TO LIFR RANGE WITH FOG PRESENT. STRANGELY
ENOUGH...KOLM BEING THE EXCEPTION TO THIS WITH CIGS MVFR.
IMPROVEMENT WILL BE VERY GRADUAL THROUGH THE REMAINDER OF THE
MORNING AND INTO THE AFTERNOON...IF IT OCCURS AT ALL. BUT ANY
IMPROVEMENT WILL BE SHORT LIVED AS CONDITIONS DETERIORATE AGAIN
TONIGHT IN ADVANCE OF THE NEXT ROUND OF PRECIP.

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 BEFORE CIGS DROP AGAIN LATE TONIGHT.
MCDONNAL/SMR

&&

.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
UP FOR THIS...AND WILL DECIDE WHETHER OR NOT TO UPGRADE AS THE 12Z
MODELS COME IN.

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.

SOME OF THE HIGHEST ASTRONOMICAL HIGH TIDES OF THE YEAR WILL OCCUR
THE NEXT FEW DAYS. HOWEVER...CURRENT GUIDANCE IS TRENDING DOWNWARD
FROM PREVIOUS RUNS. WILL CONTINUE TO MONITOR...BUT SATURDAY LOOKS TO
BE THE HIGHEST DAY AND CURRENT GUIDANCE SUGGESTS VALUES JUST UNDER
HIGHEST ASTRONOMICAL TIDE FOR ONLY A COUPLE OF LOCATIONS. THAT
SAID...WILL OPT FOR NO COASTAL FLOOD ADVISORIES AT THIS TIME.
MCDONNAL/SMR

&&

.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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