Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 290450
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
845 PM PST SUN DEC 28 2014

.SYNOPSIS...AN UPPER LEVEL TROUGH OVER WESTERN WASHINGTON WILL BRING
A CHANCE OF SHOWERS TONIGHT...WITH A CHANCE OF RAIN AND SNOW SHOWERS
AFTER MIDNIGHT AS SNOW LEVELS FALL BELOW 1000 FEET. A SURFACE HIGH
BUILDING OVER BRITISH COLUMBIA WILL PUSH COLD DRY AIR SOUTH OVER THE
AREA TONIGHT WITH WINDY CONDITIONS DEVELOPING OVER THE NORTH
INTERIOR. A STRONG UPPER LEVEL RIDGE WILL DOMINATE THE WEATHER
PATTERN THROUGH MID WEEK WITH SUNNY AND COLD WEATHER PREVAILING.

&&

.SHORT TERM...MODIFIED ARCTIC AIR MASS IS SET TO MOVE INTO WESTERN
WASHINGTON TONIGHT AND MONDAY. FROM THIS THERE ARE TWO WEATHER
PROBLEMS...WIND AND SNOW.

FIRST THE WIND...BEST RESOLUTION MESOSCALE MODELS SHOW FRASER
OUTFLOW REACHING GALE FORCE OVER THE NORTHERN INLAND WATERS BUT DO
NOT HAVE HIGH WIND OVER LAND AREAS. THIS IS REASONABLE SO WILL
CONVERT THE HIGH WIND WATCH FOR THREE NORTHWEST INTERIOR ZONES TO A
WIND ADVISORY FOR THE SAN JUANS AND WESTERN WHATCOM COUNTY. THE WIND
WILL LIKELY INCREASE AFTER 12Z TONIGHT AND CONTINUE MONDAY SO THE
ADVISORY WILL RUN 12Z TO 00Z. NORTHEAST WINDS 25-35 MPH WITH GUSTS
TO 50 MPH ARE POSSIBLE. THE WESTERN SKAGIT ZONE WILL BE A LITTLE
WINDY BUT WINDS SHOULD NOT BE ADVISORY LEVEL.

NEXT THE SNOW...THERE ARE SCATTERED SHOWERS AROUND...ALL RAIN SO FAR
AS TEMPERATURES ARE AROUND 40. AS COLD AIR SEEPS SOUTH THE AIR MASS
WILL DRY BUT THERE WILL BE A PERIOD OF SEVERAL HOURS LATE TONIGHT
AND EARLY MONDAY MORNING WHEN THERE COULD BE RAIN AND/OR SNOW
SHOWERS. THERE DOES NOT APPEAR TO BE A THREAT OF ANY SIGNIFICANT
ACCUMULATION AND ANY SNOW THAT DOES STICK WILL PROBABLY BE AT
ELEVATIONS ABOVE 500 FEET OR SO. THAT DOES INCLUDE THE NORTHERN
SLOPES OF THE OLYMPIC MOUNTAINS WHERE UPSLOPE NORTHERLY FLOW COULD
KEEP PRECIPITATION GOING A BIT LONGER MONDAY. BY MONDAY AFTERNOON
MOST AREAS WILL BE DONE WITH SHOWERS. SKIES WILL CLEAR MONDAY NIGHT.

MONDAY NIGHT THROUGH WEDNESDAY WILL BE COLD AND CLEAR WITH A BRISK
NORTHERLY BREEZE. HIGHS WILL BE IN THE 30S AND LOWS WILL BE IN THE
20S...WITH TEENS POSSIBLE OVER THE NORTHWEST INTERIOR. BURKE

.LONG TERM...PREVIOUS DISCUSSION...THE FORECAST BECOMES A NO
CONFIDENCE MESS VERY QUICKLY ON FRIDAY. THURSDAY HAS REASONABLE
CONFIDENCE AS THE UPPER LEVEL RIDGE OFFSHORE WEAKENS. INCREASING NW
FLOW ALOFT MAY BRING SOME CLOUDS OVER THE AREA BUT IT SHOULD REMAIN
DRY.

THE PROBLEM FOR FRIDAY ONWARD IS DUE TO A LARGE SPLIT IN FLOW OVER
THE NE PACIFIC. A LARGE RIDGE REMAINS ALONG ROUGHLY 150W POKING UP
OVER THE GULF OF AK. THIS FEATURE SETS UP THE NORTHERN BRANCH WITH
COLD N FLOW ALOFT OVER B.C.   A LARGE UPPER LEVEL LOW N OF
HAWAII...ALSO ALONG ROUGHLY 150W-160W...WILL BE SETTING UP THE WARM
MOIST SOUTHERN BRANCH THAT EVENTUALLY MERGES WITH THE NORTHERN
BRANCH SOMEWHERE OFFSHORE. THE GFS KEEPS W WA UNDER THE COLD N
BRANCH AND THE ECMWF STICKS WITH THE S BRANCH. THE MODELS ARE HAVING
TROUBLE RESOLVING WHICH BRANCH WILL DOMINATE W WA WEATHER THIS
WEEKEND AND I WOULD NOT EXPECT TOO MUCH RESOLUTION FOR THE NEXT
COUPLE OF DAYS. FOR NOW THE FORECAST HAS BEEN TILTED TOWARD THE
USUALLY MORE CONSISTENT ECMWF. KAM

&&

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

&&

.AVIATION...NORTHERLY FLOW ALOFT OVER WESTERN WASHINGTON THIS
EVENING WILL VEER TO NORTHEAST ON MONDAY...AS AN UPPER LEVEL RIDGE
BUILDS INTO WESTERN CANADA. AT THE SURFACE...NORTHERLY OFFSHORE FLOW
WILL INCREASE TONIGHT AND MONDAY...AS A STRONG HIGH MOVES SOUTH
THROUGH BRITISH COLUMBIA. THE AIR MASS WILL REMAIN STABLE AND MOIST
TONIGHT...THEN BECOME GENERALLY DRY EARLY MONDAY MORNING.

CEILINGS ARE MAINLY IN THE 2000 TO 3500 FT RANGE THIS EVENING WITH
SCATTERED SHOWERS ACROSS WESTERN WASHINGTON. SHOWERS WILL DECREASE
OVERNIGHT AND CEILINGS WILL IMPROVE...WITH ONLY LINGERING LIGHT
SPOTTY SHOWERS LATE TONIGHT AND EARLY MONDAY MORNING AS THE AIR MASS
BECOMES COLD ENOUGH FOR LOWLAND SNOW. LOCALLY SUSTAINED NORTHEAST
WINDS OF 25 KT WILL DEVELOP LATE TONIGHT IN THE NORTH INTERIOR...
ESPECIALLY NEAR THE MOUTH OF THE FRASER RIVER.

KSEA...MAINLY NORTHEAST WIND 4-10 KT TONIGHT...RISING TO 10-18 KT
GUSTING 27 KT ON MONDAY. COLDER AIR WILL MOVE INTO THE PUGET SOUND
REGION LATE TONIGHT AS SHOWERS ARE GENERALLY ENDING. THERE IS
ROUGHLY A 5 PERCENT CHANCE OF LIGHT SNOW AT KSEA BETWEEN 2 AM AND
6 AM. IF IT SNOWS...IT WOULD AMOUNT TO 0.2 INCH AT MOST.   MCDONNAL

&&

.MARINE...STRONG HIGH PRESSURE WILL MOVE SOUTH THROUGH BRITISH
COLUMBIA TONIGHT WHILE A WEAK LOW TRACKS SOUTH ALONG THE PACIFIC
NORTHWEST COAST. AS A RESULT NORTHERLY OFFSHORE FLOW WILL INCREASE
TONIGHT AND MONDAY...PROBABLY REACHING ITS PEAK MONDAY AFTERNOON OR
EVENING. THERE WILL LIKELY BE NORTHEAST GALES FOR THE NORTHERN
INLAND WATERS...STRAIT OF JUAN DE FUCA...AND NORTHERN COASTAL
WATERS. SMALL CRAFT ADVISORY NORTH WINDS ARE A GOOD BET FOR PUGET
SOUND AND HOOD CANAL.

WEST TO NORTHWEST SWELL 9 TO 11 FT THIS EVENING WILL SUBSIDE TO 7 TO
9 FT OVERNIGHT.

OFFSHORE FLOW WILL WEAKEN TUESDAY...WITH WINDS OF 20 KT OR LESS
WEDNESDAY AND THURSDAY.   MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...WIND ADVISORY SAN JUANS AND WESTERN WHATCOM COUNTY.
PZ...SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS GRAYS
      HARBOR THROUGH THIS EVENING.
     GALE WARNING NORTHERN INLAND WATERS...STRAIT OF JUAN DE
      FUCA AND NORTHERN COASTAL THROUGH LATE MONDAY NIGHT.
     SMALL CRAFT ADVISORY PUGET SOUND AND HOOD CANAL LATE TONIGHT
      THROUGH MONDAY EVENING.

&&

$$

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




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