Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 281708
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
908 AM PST SUN DEC 28 2014

.SYNOPSIS...AN UPPER LEVEL TROUGH OVER WESTERN WASHINGTON WILL BRING
A CHANCE OF SHOWERS TODAY AND TONIGHT. THE AIR MASS WILL COOL ENOUGH
TONIGHT FOR A CHANCE OF SNOW SHOWERS...MAINLY AFTER MIDNIGHT. 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...A STRONG HIGH AMPLITUDE UPPER LEVEL RIDGE JUST W OF
THE B.C. COAST WILL MAINTAIN N FLOW ALOFT ALONG THE B.C. COAST AND
NW-NNW FLOW ALOFT WITH A WEAK UPPER LEVEL TROUGH OVER W WA. THE
AIR MASS OVER W WA HAS DRIED AND COOLED QUITE A BIT. THE NW LOW
LEVEL FLOW WILL CONTINUE TO BRING SCATTERED SHOWERS TO MAINLY THE
CENTRAL COAST...THEN ACROSS THE SW INTERIOR. TERRAIN DRIVEN SHOWERS
COULD ALSO CONTINUE OVER MAINLY N PUGET SOUND...ORIGINATING OVER THE
EASTERN STRAIT OR THE NE OLYMPICS THEN TRAINING IN BANDS ACROSS SW
SNOHOMISH COUNTY AND N KING COUNTY. A SECONDARY BAND OVER CENTRAL
KING COUNTY DEVELOPED OVERNIGHT THEN FADED EARLY THIS MORNING...BUT
IT COULD RE-DEVELOP.

THE N FLOW ALOFT WILL SEND A MOSTLY DRY UPPER LEVEL SHORTWAVE TROUGH
S OVER W WA TONIGHT. THE MODELS HAVE BEEN CONSISTENT IN SHOWING THIS
SHORTWAVE BUT IT IS LOOKING INCREASINGLY DRY. IT COULD GENERATE A
FEW MORE SHOWERS OVER THE AREA TONIGHT BUT COVERAGE WILL BE SPARSE
ENOUGH TO KEEP POPS IN THE CHANCE RANGE. THE AIR MASS WILL BE
COOLING EVEN MORE TONIGHT. WITH 1000-850 MB THICKNESS FALLING BELOW
1300 METERS OVER THE N INTERIOR THIS EVENING AND OVER JUST ABOUT ALL
THE CWA AFTER 06Z/10 PM...ANY SHOWERS WILL LIKELY CONSIST OF SNOW.
ONLY SPOTTY AND LIGHT ACCUMULATIONS ARE EXPECTED.

A STRONG SURFACE HIGH WILL MOVE S OVER THE B.C. INTERIOR TONIGHT.
00Z ECMWF AND 12Z GFS SHOW A CENTRAL PRESSURE OF 1060 MB AROUND
12Z/4 AM MONDAY OVER W CENTRAL ALBERTA. THE BLI-YWL GRADIENT IS
FORECAST TO BE JUST ABOVE -20 MB ON THE MORE CONSERVATIVE ECMWF...SO
THERE IS MORE CONFIDENCE IN STRONGER FRASER VALLEY NE OUTFLOW WINDS.
THE 06Z CANADIAN 2.5KM MODEL SHOWS THE STRONGEST OUTFLOW WITH 35-40
KT NE WINDS ACROSS THE NORTHERN SAN JUANS AROUND 12Z MONDAY WITH
30-35 KT N WINDS THROUGH ROSARIO STRAIT. THE 00Z ARW-W MODEL IS JUST
A HALF STEP BEHIND...BUT DOES SHOW 36 KT WINDS OVER THE N COAST OF
SAN JUAN ISLAND. ALTHOUGH THESE MODELS ARE ON THE HIGH END OF THE
MODEL SPECTRUM AND THERE IS STILL SOME UNCERTAINTY REGARDING JUST
HOW STRONG THE WINDS WILL BE...IT IS ENOUGH TO ISSUE A HIGH WIND
WATCH LATER THIS MORNING. THIS WILL BE FOCUSED ON THE SAN JUAN
ISLANDS...COASTAL WHATCOM COUNTY...AND THE WESTERN ISLANDS IN SKAGIT
COUNTY AROUND ROSARIO STRAIT. THE MODELS HAVE ALSO TRENDED FASTER IN
INITIATING THE FRASER OUTFLOW...STARTING EARLY THIS EVENING. THE
STRONGEST WINDS WILL STILL BE EARLY MONDAY MORNING...THEN GRADUALLY
DIMINISH MONDAY AFTERNOON.

WHATEVER LOW LEVEL MOISTURE REMAINS ON MONDAY WILL DRY OUT
QUICKLY...WITH SUNNY SKIES EXPECTED BY AFTERNOON. MAX TEMPS WILL
REMAIN IN THE 30S FOR ALL AREAS EXCEPT THE COAST.

THE STRONG UPPER LEVEL RIDGE WILL CONTINUE TO DOMINATE THE REGIONS
WEATHER TUESDAY AND WEDNESDAY. COLD AIR AND CLEAR SKIES WITH GOOD
RADIATIONAL COOLING CONDITIONS WILL DROP TEMPERATURES INTO THE UPPER
TEENS AND 20S MONDAY NIGHT. HIGHS ON TUESDAY WILL REMAIN IN THE
30S...BUT WILL BE A LITTLE COLDER THAN MONDAY. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 AM AFD...AN UPPER RIDGE
WILL BE OVER THE AREA ON WEDNESDAY FOR A DRY BUT COOL DAY. THE RIDGE
WILL BEGIN TO WEAKEN ON THURSDAY AS SHORT WAVE ENERGY COMES OVER THE
TOP BUT IT LOOKS LIKE ANOTHER DRY DAY...PROBABLY WITH A FEW MORE
CLOUDS. TEMPERATURES SHOULD ALSO BEGIN TO MODERATE ON THURSDAY. A
WEATHER SYSTEM IN THE NORTHWEST FLOW ALOFT WILL SAG SOUTH OVER THE
AREA ON FRIDAY FOR A CHANCE OF RAIN. SNOW LEVELS WILL STILL BE LOW
AND IT IS NOT OUT OF THE QUESTION THAT THERE COULD BE SOME LOWLAND
SNOW...ESPECIALLY OVER THE NORTH INTERIOR. SCHNEIDER

&&

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

&&

.AVIATION...NORTHWEST FLOW ALOFT OVER THE PACIFIC NORTHWEST THIS
MORNING WILL VEER TO NORTHERLY TONIGHT AS AN UPPER LEVEL RIDGE
BUILDS INTO BRITISH COLUMBIA. AT THE SURFACE...NORTHERLY OFFSHORE
FLOW WILL DEVELOP TODAY AND INCREASE TONIGHT AS A STRONG HIGH MOVES
SOUTH THROUGH BRITISH COLUMBIA. THE AIR MASS WILL REMAIN STABLE AND
MOIST...WITH MOISTURE BECOME LIMITED TO LOWER LEVELS LATE TONIGHT.

PUGET SOUND CONVERGENCE ZONE FROM OVERNIGHT IS SLOWLY WINDING DOWN
AND WILL LIKELY DISSIPATE COMPLETELY WITHIN THE NEXT HOUR OR SO.

LOCALLY SUSTAINED NORTHEAST WINDS OF 25 KT WILL DEVELOP LATE TONIGHT
IN THE NORTH INTERIOR...ESPECIALLY NEAR THE MOUTH OF THE FRASER
RIVER.

VFR TO MVFR CONDITIONS OVER MUCH OF WESTERN WASHINGTON THIS
MORNING...THE ONLY EXCEPTION BEING KBLI DUE TO FOG. EXPECTING CIGS
TO IMPROVE THROUGHOUT THE DAY TODAY...LEADING TO WIDESPREAD VFR
CONDITIONS TONIGHT. LINGERING SHOWERS ALONG THE COAST THIS MORNING
AND AFTERNOON...HOWEVER...MAY IMPEDE THE LIFTING OF CIGS AT OR
AROUND KHQM.

KSEA...SOUTH WIND 4-8 KT...BECOMING NORTHEAST 6-12 KT LATE TODAY AND
TONIGHT. COLDER AIR WILL MOVE INTO THE PUGET SOUND REGION LATE
TONIGHT AS SHOWERS ARE GENERALLY ENDING. THIS WILL PROVIDE A VERY
NARROW WINDOW FOR MIXED PRECIPITATION AND POSSIBLY EVEN SNOW. GIVEN
THE TIMING OF THESE TWO ELEMENTS...THE ARRIVAL OF THE COLDER AIR AND
THE TAPERING OFF OF PRECIP...CHANCE FOR SNOW IS SMALL AT KSEA BUT
CANNOT BE RULED OUT ENTIRELY.   MCDONNAL/SMR

&&

.MARINE...STRONG HIGH PRESSURE WILL BEGIN MOVING SOUTH THROUGH
BRITISH COLUMBIA TODAY...WHILE A WEAK LOW TRACKS SOUTH ALONG THE
PACIFIC NORTHWEST COAST. AS A RESULT NORTHERLY OFFSHORE FLOW WILL
DEVELOP...THEN INCREASE TONIGHT AND MONDAY...PROBABLY REACHING ITS
PEAK LATE MONDAY OR MONDAY EVENING. A GALE WATCH IS IN EFFECT FOR
THE NORTHERN INLAND WATERS...THE CENTRAL AND WESTERN STRAIT OF JUAN
DE FUCA...AND THE NORTHERN COASTAL WATERS. AS MODEL DATA REMAINS
CONSISTENT WITH WIND SPEEDS...WILL LIKELY UPGRADE THIS WATCH INTO A
GALE WARNING WITH THE AFTERNOON PACKAGE. SMALL CRAFT ADVISORY WINDS
ARE LIKELY IN REMAINING COASTAL ZONES AND POSSIBLE FOR ADMIRALTY
INLET AND PUGET SOUND. WILL EVALUATE WITH 12Z MODEL RUNS BEFORE
ISSUING ANY ADVISORY...PARTICULARLY FOR THE LATTER TWO ZONES.

NORTHWEST SWELL OF 12 TO 15 FT THIS MORNING WILL SUBSIDE LATE TODAY
AND TONIGHT.

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...NONE.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS THROUGH TONIGHT.
     SMALL CRAFT ADVISORY WEST ENTRANCE STRAIT TODAY FOR HAZARDOUS
       SEAS.
     SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS GRAYS HARBOR
       THROUGH THIS EVENING.
     GALE WATCH NORTHERN INLAND WATERS...CENTRAL AND WESTERN STRAIT
       OF JUAN DE FUCA...NORTHERN COASTAL WATERS.

&&

$$

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










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