Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 271733
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
932 AM PST SAT DEC 27 2014

.SYNOPSIS...A COLD FRONT OVER WESTERN WASHINGTON THIS MORNING WILL
MOVE SOUTH THIS AFTERNOON. MODERATE WEST FLOW ALOFT BEHIND THE FRONT
WILL ALLOW SNOW TO CONTINUE OVER THE CASCADES THROUGH TONIGHT. AN
UPPER LEVEL TROUGH WILL CROSS THE AREA TONIGHT FOLLOWED BY ANOTHER
TROUGH SUNDAY EVENING FOR A CONTINUING CHANCE OF SHOWERS OVER THE
LOWLANDS. STRONG HIGH PRESSURE BUILDING OVER BRITISH COLUMBIA SUNDAY
WILL PUSH MODIFIED ARCTIC AIR SOUTH ACROSS WESTERN WASHINGTON SUNDAY
NIGHT...BRINGING MOSTLY SUNNY AND COLD WEATHER TO THE AREA FOR THE
FIRST HALF OF NEXT WEEK.

&&

.SHORT TERM...IR SATELLITE IMAGERY AND PARTICULARLY RADAR SHOWS THE
PRECIP PRODUCING PART OF THE COLD FRONT EXTENDING FROM THE CENTRAL
COAST ACROSS ABOUT WHIDBEY ISLAND AT 1630Z/830 AM THIS MORNING. W
FLOW ALOFT ACROSS THE OLYMPICS HAS SET UP A LARGE RAIN SHADOW FROM
SEATTLE UP ACROSS WHIDBEY ISLAND. BASED ON IR IMAGERY THE ACTUAL
BACK EDGE OF THE COLD FRONT IS STILL OVER CENTRAL VANCOUVER
ISLAND...BUT THIS FEATURE IS NOT THAT SIGNIFICANT. THE PRECIP
PRODUCING PART OF THE COLD FRONT WILL BE S OF THE CWA BEFORE MIDDAY
SO AN UPDATE WAS ISSUED TO LOWER POPS FOR THE LOWLANDS THIS
AFTERNOON.

A WINTER STORM WARNING WILL CONTINUE FOR THE CASCADES TODAY AND
TONIGHT. MOUNTAIN OBS SHOW THE SNOW WAS JUST STARTING TO DEVELOP
EARLY THIS MORNING...AND LOW LEVEL WESTERLY FLOW SHOULD BE IN THE
PROCESS OF CRANKING UP THIS MORNING WITH AND BEHIND THE FRONT. THE
15Z HRRR 12 HOUR PRECIP THROUGH 06Z STILL SHOWS 1-1.5 INCHES OF
LIQUID OVER THE CASCADES...SO A FOOT PLUS OF SNOW THROUGH TONIGHT IS
STILL ON TRACK.

THE UPPER LEVEL TROUGH TRAILING THE FRONT WAS OVER THE CENTRAL B.C.
COAST AT 16Z/8 AM. MODELS BRING THIS FEATURE DOWN OVER W WA EARLY
THIS EVENING FOR A SMALL INCREASE IN SHOWERS OVER MAINLY THE
INTERIOR LOWLANDS...AS WELL AS CONTINUING THE OROGRAPHIC FLOW OVER
THE CASCADES. LIKELY POPS TONIGHT OVER ALL THE LOWLANDS IS IN THE
BALL PARK...BUT COULD BE SLIGHTLY HIGH FOR SOME AREAS.

THE HIGH AMPLITUDE UPPER LEVEL RIDGE THAT HAS SET UP OFFSHORE WILL
MAINTAIN MODERATE N FLOW ALOFT PARALLELING THE B.C AN WA COASTS ON
SUNDAY. THERE COULD BE A COUPLE OF TINY SHORTWAVE TROUGHS EMBEDDED
THE FLOW ALOFT...SO LOW CHANCE POPS WILL BE KEPT FOR MOST AREAS.
BASED ON THE NAM IT LOOKS LIKE THE BEST AREA FOR SHOWERS WILL BE
DOWNWIND OF THE OLYMPICS...IN THIS CASE CAUSED BY OROGRAPHIC FLOW
ACROSS THE OLYMPICS CREATING SHOWERS THAT DRIFT DOWNWIND OVER THE SW
INTERIOR IN THE MOUNTAINS WAKE.

ALL MODELS CONTINUE TO SHOW ANOTHER UPPER LEVEL SHORTWAVE TROUGH
DROPPING S THROUGH B.C. AND OVER W WA SUNDAY NIGHT. THIS FEATURE IS
ON A DRY TRAJECTORY AND WILL PROBABLY JUST BRING A CHANCE OF PRECIP
TO A WIDER AREA. AT THE SAME TIME THE STRONG SURFACE HIGH MOVING
OVER THE B.C. INTERIOR WILL BEGIN PUSHING COLD DRY MODIFIED ARCTIC
AIR THROUGH THE FRASER RIVER VALLEY AND OUT OVER NW WA. THE MODIFIED
ARCTIC OUTFLOW COULD CREATE A SMALL SURFACE FRONT WHICH ALONG WITH
THE UPPER LEVEL TROUGH COULD GENERATE ENOUGH UPWARD MOTION FOR
SCATTERED SNOW SHOWERS SUNDAY NIGHT. 1000-850 MB THICKNESSES WILL BE
BELOW 1300 METERS OVER THE N HALF OF THE CWA SUNDAY EVENING AND OVER
ALL THE CWA AFTER 06Z...SO THE AIR WILL CERTAINLY BE COLD ENOUGH FOR
SNOW SHOWERS. THE AIR MASS WILL BE PRETTY DRY SO ACCUMULATIONS WILL
PROBABLY AMOUNT TO A TRACE IN MOST AREAS.

THE FRASER VALLEY OUTFLOW WILL PRODUCE AT LEAST WINDY CONDITIONS
OVER THE N INTERIOR OF W WA SUNDAY NIGHT AND MONDAY. THE GFS STILL
CRANKS THE BLI-YWL PRESSURE GRADIENT UP TO -20 MB 12Z MONDAY...BUT
THE ECMWF IS A LITTLE WEAKER AT AROUND -18 MB. THE 12Z ARW-W RUN HAS
SUSTAINED SURFACE WINDS 25-30 KT OVER COASTAL WHATCOM COUNTY AND THE
SAN JUANS...SO AT THIS POINT IT LOOKS LIKE A LOW END WIND ADVISORY
WILL BE NEEDED.

WITH COLD DRY AIR FLOWING OVER W WA THROUGH MONDAY...SKIES ARE
EXPECTED TO CLEAR BY AFTERNOON. MAX TEMPS WILL REMAIN IN THE 30S IN
MOST AREAS EXCEPT FOR THE LOWER 40S ALONG THE COAST. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 405 AM AFD...EXTENDED
MODELS IN GOOD AGREEMENT THROUGH THURSDAY NIGHT. HIGH AMPLITUDE
RIDGE OFF THE COAST THROUGH WEDNESDAY WITH NORTHEASTERLY FLOW ALOFT.
SURFACE HIGH SLIDING SOUTH ON TUESDAY BRINGING ABOUT AN END TO THE
OUTFLOW WINDS OVER THE NORTHWEST INTERIOR. WITH THE COLD AIR MASS
OVER THE AREA OVERNIGHT LOWS WILL BE BELOW FREEZING WITH THE COLDEST
LOCATIONS GETTING INTO THE UPPER TEENS. HIGHS TUESDAY AND WEDNESDAY
WILL BE MOSTLY IN THE 30S. UPPER RIDGE BEGINS TO FLATTEN ON THURSDAY
WITH THE SURFACE GRADIENTS GOING FLAT TO LIGHTLY ONSHORE. SOME
DIFFERENCES IN THE MODELS BEGIN TO SHOW UP ON FRIDAY WITH THE
CANADIAN AND GFS PARALLEL RUN BRINGING SOME PRECIPITATION INTO THE
AREA WHILE THE OPERATIONAL GFS AND ECMWF HOLD OFF ON THE
PRECIPITATION UNTIL THE WEEKEND. WILL STAY WITH THE CHANCE POPS ON
FRIDAY. WITH THE LOW LEVEL FLOW LIGHT ONSHORE THE PRECIPITATION TYPE
WILL BE RAIN IN THE LOWLANDS. FELTON

&&

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

&&

.AVIATION...NORTHWEST FLOW ALOFT OVER THE PACIFIC NORTHWEST WILL
CONTINUE THROUGH TONIGHT. THE FRONT HAS ALREADY STARTED TO MAKE ITS
WAY THROUGH WESTERN WASHINGTON...AT THE TIME OF THIS WRITING
STRETCHING FROM CLEARWATER TO ARLINGTON...AND WILL CONTINUE ALONG
ITS SOUTHEASTWARD PATH. MOIST ONSHORE FLOW WILL FOLLOW THE FRONT
LATER THIS AFTERNOON AND INTO TONIGHT. THE AIR MASS WILL REMAIN
MOIST AND STABLE.

CIGS GENERALLY MVFR TO IFR OVER WESTERN WASHINGTON THIS MORNING. AS
THE FRONT CONTINUES TO MOVE ALONG ITS PATH...WILL START TO SEE
MARGINAL IMPROVEMENTS IN CIGS AND VISIBILITIES AS THE DAY
PROGRESSES. MOST TERMINALS WILL SEE CIGS CLIMB BACK UP TO MVFR
CONDITIONS BY THIS EVENING EXCEPT FOR KPAE. MOIST ONSHORE FLOW WILL
BE CONDUCIVE TO THE DEVELOPMENT OF A PUGET SOUND CONVERGENCE
ZONE...LIKELY KEEPING CIGS AND THE PROSPECT FOR RAIN OVER THE NORTH
PUGET SOUND REGION.

KSEA...SOUTH TO SOUTHEAST WIND 4-8 KT...BECOMING SOUTH TO SOUTHWEST
10-16 KT GUSTING 24 KT BY NOON...THEN EASING TO 6-12 KT LATER
TONIGHT. IFR CIGS EXPECTED TO CLIMB BACK TO MVFR BY THIS EVENING.
MCDONNAL/SMR

&&

.MARINE...A FRONTAL SYSTEM...CURRENTLY EXTENDING FROM CLEARWATER TO
ARLINGTON...CONTINUES TO MAKE ITS WAY SOUTHEAST THE COASTAL AND
INLAND WATERS OF WESTERN WASHINGTON. MODERATELY STRONG ONSHORE FLOW
WILL FOLLOW THE FRONT BY LATE THIS AFTERNOON AND INTO TONIGHT.
SMALL CRAFT ADVISORY WINDS ARE LIKELY ALL WATERS AHEAD OF AND IN THE
WAKE OF THE FRONT...AND AT LEAST BRIEF MARGINAL WESTERLY GALES ARE
EXPECTED IN THE CENTRAL STRAIT OF JUAN DE FUCA AND EAST ENTRANCE
STRAIT THIS AFTERNOON AND EVENING. A GALE WARNING REMAINS IN EFFECT
FOR THAT. THE SWELL WILL ALSO BUILD TO 12 TO 15 FT LATE TONIGHT AND
SUNDAY MORNING. THIS MAY WARRANT A SMALL CRAFT FOR ROUGH BAR
CONDITIONS SUNDAY FOR THE GRAYS HARBOR BAR...BUT WILL WAIT TO
EVALUATE THAT FOR THE AFTERNOON FORECAST PACKAGE.

NORTHERLY OFFSHORE FLOW WILL DEVELOP SUNDAY AND PROBABLY REACH ITS
PEAK MONDAY OR MONDAY NIGHT...AS HIGH PRESSURE MOVES SOUTH THROUGH
BRITISH COLUMBIA. SMALL CRAFT ADVISORY NORTHEAST WINDS ARE LIKELY
FOR THE COAST AND PORTIONS OF THE STRAIT...WHILE GALES ARE POSSIBLE
FOR AT LEAST THE NORTHERN INLAND WATERS AND WEST ENTRANCE STRAIT
LATER SUNDAY NIGHT AND MONDAY. WILL LIKELY PUT OUT A GALE WATCH WITH
THE AFTERNOON FORECAST PACKAGE FOR THESE AREAS. OFFSHORE FLOW WILL
EASE TUESDAY AND WEDNESDAY AS THE HIGH MOVES INTO THE NORTHERN ROCKY
MOUNTAINS.     MCDONNAL/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...WINTER STORM WARNING THROUGH LATE TONIGHT FOR THE CASCADES.
PZ...GALE WARNING SATURDAY AFTERNOON AND EVENING CENTRAL STRAIT
       OF JUAN DE FUCA AND EAST ENTRANCE STRAIT.
     SMALL CRAFT ADVISORY ALL OTHER MARINE ZONES.

&&

$$

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










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