Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 262320
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
320 PM PST FRI DEC 26 2014

.SYNOPSIS...A FRONTAL SYSTEM MOVING DOWN THE BRITISH COLUMBIA COAST
WILL BRING RAIN TO THE LOWLANDS AND HEAVY SNOW TO THE MOUNTAINS LATE
TONIGHT THROUGH SATURDAY NIGHT. STRONG HIGH PRESSURE BUILDING OVER
BRITISH COLUMBIA BEHIND THE FRONTAL SYSTEM WILL PUSH MODIFIED ARCTIC
AIR SOUTH ACROSS THE AREA SUNDAY NIGHT...BRINGING MOSTLY SUNNY AND
COLD WEATHER TO THE AREA FOR THE FIRST HALF OF NEXT WEEK.

&&

.SHORT TERM...VISIBLE SATELLITE IMAGERY AND OBS SHOWED THAT THE
GREATER SEATTLE AREA LUCKED OUT THIS AFTERNOON WITH A SMALL HOLE IN
THE CLOUD COVER FOR MOSTLY SUNNY SKIES. STRATUS CLOUDS KEPT THE REST
OF W WA MOSTLY CLOUDY THROUGH THE AFTERNOON.

THE FRONTAL SYSTEM HEADED SE ALONG THE B.C. COAST TOWARD W WA THIS
AFTERNOON HAS SPREAD THE LEADING EDGE CLOUDS ALMOST TO VANCOUVER
ISLAND AT 22Z/2 PM. MODELS REMAIN PRETTY MUCH ON TRACK...SPREADING
LIGHT RAIN S ACROSS THE LOWLANDS AFTER MIDNIGHT...AND LIGHT SNOW
ACROSS THE MOUNTAINS. THE COLD FRONT SHOULD REACH THE NW INTERIOR
ALONG A UIL-BLI LINE AROUND 15Z/7 AM THEN MOVE S OF CHEHALIS AFTER
21Z/1 PM. MODERATE NW LOW LEVEL FLOW WILL DEVELOP AS THE FRONT GOES
THROUGH THEN GRADUALLY DIMINISH SATURDAY NIGHT. THE RESULTING
OROGRAPHIC LIFT AS THE NW FLOW HITS THE CASCADES WILL PRODUCE HEAVY
SNOW INTO SATURDAY NIGHT. SNOW LEVELS WILL START OUT FROM 1500 FEET
N TO 3000 FEET S THIS EVENING...THEN RISE TO 2000 TO 3500 FEET ON
SATURDAY...THEN LOWER TO 1500 TO 2500 FEET SATURDAY NIGHT. A WINTER
STORM WARNING WILL REMAIN IN EFFECT FROM MIDNIGHT TONIGHT THROUGH
SATURDAY NIGHT...WITH 10 TO 20 INCHES OF NEW SNOW EXPECTED OVER THE
MAIN CASCADE HIGHWAYS.

ONCE THE UPPER LEVEL TROUGH MOVES S ACROSS THE AREA LATE SATURDAY
NIGHT...THE NW FLOW AND SNOW FALL WILL DIMINISH.

BY SUNDAY MORNING A HIGH AMPLITUDE SKINNY UPPER LEVEL RIDGE WILL BE
CENTERED JUST W OF THE B.C. COAST...WITH MODERATE N FLOW ALOFT
PARALLELING THE B.C. AND WASHINGTON COASTS. I WILL HANG ON TO CHANCE
POPS FOR SOME SHOWERS OVER THE LOWLANDS AND LIKELY POPS OVER THE
MOUNTAINS. ACCUMULATIONS WILL BE LIGHT.

BOTH THE GFS AND ECMWF BRING ANOTHER UPPER LEVEL SHORTWAVE TROUGH S
ACROSS THE AREA SUNDAY NIGHT. SNOW LEVELS WILL BE DROPPING SUNDAY
EVENING AND SHOULD BE BELOW 1000 FEET OVER THE N INTERIOR. 1000-850
MB THICKNESSES ARE BELOW THE 1300 METER THRESHOLD. WITH LIFT
PROVIDED BY THE PASSING SHORTWAVE TROUGH THERE WILL PROBABLY BE SOME
SNOW SHOWERS AROUND...ESPECIALLY AFTER MIDNIGHT. AT THE SAME
TIME...THE STRONG SURFACE HIGH MOVING S THROUGH INTERIOR B.C. WILL
BE FAR ENOUGH SOUTH TO BEGIN SQUIRTING MODIFIED ARCTIC AIR OUT
THROUGH THE FRASER VALLEY AND DOWN OVER W WA. THE ECMWF AND NAM HAVE
THE OUTFLOW RAMPING UP AFTER 06Z WHICH IS RIGHT WHEN THE PRECIP
FROM THE SHORTWAVE TROUGH SHOULD BE DIMINISHING. SO AT THIS POINT
I WILL HAVE A MENTION OF SNOW SHOWERS OVER THE N INTERIOR IN THE
FORECAST BUT ACCUMULATIONS WILL BE VERY LIGHT. THERE COULD BE SOME
CHUNKY RAIN FARTHER SOUTH OVER THE GREATER PUGET SOUND REGION SUNDAY
NIGHT...BUT FOR NOW WITH SNOW LEVELS STILL ABOVE 1000 FEET I WILL
LEAVE OUT THE MENTION OF SNOW SHOWERS.

COLD MODIFIED ARCTIC AIR WILL CONTINUE TO SPILL OUT THROUGH THE
FRASER VALLEY ON MONDAY...COMBINED WITH COLD DRY N FLOW ALOFT.
OVERALL SKIES WILL BE SUNNY WITH COLD AND DRY WEATHER. MAX TEMPS
WILL REMAIN IN THE 30S...EXCEPT FOR NEAR 40 ALONG THE IMMEDIATE WA
COAST. THERE COULD BE SOME UPSLOPE FLOW CLOUDS AND SNOW SHOWERS
ALONG THE STRAIT OF JUAN DE FUCA...CAUSED BY THE OLYMPICS...BUT THIS
CAN BE REFINED MORE WHEN THERE IS BETTER CONFIDENCE IN THE TIMING OF
THE OUTFLOW AND THE EXTENT OF THE REMAINING LOW LEVEL MOISTURE. KAM

.LONG TERM...GFS AND ECMWF KEEP A STRONG UPPER LEVEL RIDGE OFFSHORE
AND DRY COLD N-NE FLOW ALOFT OVER W WA THROUGH WEDNESDAY. SUNNY COLD
AND DRY WILL PREVAIL WITH MIN TEMPS IN THE TEENS AND 20S AND MAX
TEMPS IN THE 30S TO NEAR 40. THE UPPER LEVEL RIDGE WEAKENS ENOUGH
FOR A WEAK UPPER LEVEL SHORTWAVE TROUGH TO MOVE SE ACROSS THE AREA
THURSDAY NIGHT AND/OR FRIDAY. KAM

&&

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

&&

.AVIATION...NORTHERLY FLOW ALOFT OVER WESTERN WASHINGTON WILL
CONTINUE THROUGH THIS EVENING AS AN OFFSHORE UPPER RIDGE MOVES
SLOWLY EASTWARD TOWARD THE PACIFIC NORTHWEST. THE RIDGE WILL MOVE
INLAND LATE TONIGHT CAUSING TO THE FLOW TO SHIFT MORE NORTHWESTERLY
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 IN THE LOWER LEVELS.

LINGERING STRATUS OVER MUCH OF WESTERN WASHINGTON...ALTHOUGH CIGS
HAVE RECOVERED TO VFR TO MVFR IN MANY LOCATIONS. CIGS REMAIN IFR FOR
OLM AND HQM. SOME ADDITIONAL IMPROVEMENT CAN BE EXPECTED FOR THE
REMAINDER OF THE AFTERNOON AND INTO THE EARLY EVENING BEFORE CIGS
BEGIN TO LOWER ONCE MORE IN ADVANCE OF INCOMING FRONTAL SYSTEM. AS
PRECIP BEGINS TOMORROW MORNING...CAN EXPECT TO SEE CIGS DROP FURTHER
BEFORE MARGINAL IMPROVEMENT TOMORROW AFTERNOON.

KSEA...SOUTH TO SOUTHEAST WIND 6 KT OR LESS. LINGERING SCATTERED LOW
LEVEL CLOUDS THIS AFTERNOON AND EVENING BEFORE BKN CLOUDS BEGIN TO
RE-EMERGE IN MID LEVELS TONIGHT AS INCOMING FRONTAL SYSTEM
APPROACHES. CIGS FALLING TO MVFR TO IFR BY SATURDAY MORNING.
SMR

&&

.MARINE...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 IN EFFECT FOR THIS TIME PERIOD...ALTHOUGH CURRENT
MODEL TRENDS FALL JUST SHORT OF CRITERIA. WILL OPT TO LEAVE WATCH IN
PLACE AND ALLOW NEXT SHIFT TO MAKE DECISION BASED ON NEWEST MODEL
GUIDANCE.

NORTHERLY OFFSHORE FLOW WILL DEVELOP SUNDAY AND MONDAY 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. 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
THIS WEEKEND. HOWEVER...CURRENT GUIDANCE REMAINS ON A DOWNWARD TREND
FOR WESTERN WASHINGTON LOCATIONS. WILL CONTINUE TO MONITOR...BUT
THERE IS INCREASING CONFIDENCE THAT NO FURTHER COASTAL FLOOD
PRODUCTS WILL BE NEEDED IN THE SHORT TERM.  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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