Gale Watch 13, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 AM PST WED DEC 10 2008

.SYNOPSIS...A FRONT WILL DISSIPATE AND SAG SOUTH OF WASHINGTON
TODAY. SURFACE HIGH PRES WILL WILL BUILD TONIGHT WITH DRY WEATHER
THROUGH THURSDAY. A MAJOR CHANGE IN THE PATTERN WILL BEGIN FRIDAY AS
A VERY STRONG LOW PRESSURE SYSTEM MOVES ACROSS THE AREA WITH
WIND...RAIN...AND MOUNTAIN SNOW. RAIN COULD BECOME MIXED WITH SNOW
DOWN TO SEA LEVEL BY FRIDAY NIGHT. ON SATURDAY...A REINFORCING SHOT
OF COLD AIR WILL DROP SOUTH OUT OF CANADA AND SETTLE OVER THE AREA.
SNOW SHOWERS COULD OCCUR DOWN TO SEA LEVEL AT TIMES OVER THE
WEEKEND. DRIER WEATHER AND COLD TEMPERATURES ARE EXPECTED EARLY NEXT
WEEK.

&&

.SHORT TERM...THE FRONTAL SYSTEM THAT PASSED THROUGH YESTERDAY HAS
MOVED TO THE SOUTH...LEAVING BEHIND A LOW DECK OF STRATUS OVER
WESTERN WASHINGTON. VISIBLE SATELLITE SHOWS THE HIGHER PEAKS OF THE
CASCADES AND THE BULK OF THE OLYMPICS MOUNTAINS POKING OUT OF THE
MUCK. ON AND OFF REPORTS OF DRIZZLE ARE BEING REPORTED THROUGHOUT
THE CWA. THE RISK DIMINISHES THROUGH THE AFTERNOON...AND WILL
CONSIDER ISSUING AN MORNING UPDATE TO REMOVE MENTION OF
PRECIPITATION. WEAK RIDGING INSIDE OF 130W WITH WNW FLOW ALOFT WILL BE
ACCOMPANIED BY A MOSTLY STABLE ATMOSPHERE AT THE LOWER LEVELS. WITH
FLAT GRADIENTS OVER THE AREA BECOMING LIGHT NORTHERLY THROUGH THE
SOUND WILL LIKELY NOT TO DO MUCH TO MIX US OUT.

LOW LEVEL MOISTURE WILL STAY WITH US OVERNIGHT...FOR AREAS OF LOW
CLOUDS AND FOG THURSDAY MORNING. NAMBUFR SOUNDING SHOWS THIN LAYER
OF LOW LEVEL MOISTURE EVENTUALLY MIXING OUT IN THE AFTERNOON
PRODUCING PARTLY TO MOSTLY SUNNY SKIES.

DEVELOPING WAVE INSIDE OF 150W AND AROUND 45N WILL PHASE WITH A
DIGGING COLD UPPER LEVEL TROUGH OF MARITIME PACIFIC ORIGINS TO
PRODUCE A MAJOR CHANGE IN WEATHER BEGINNING FRIDAY.

NOTES TO ADD THIS MORNING INCLUDE THE 12Z GFS ALSO FAVORS A MORE
NORTHERN TRACK OF THE RAPIDLY DEEPENING SURFACE LOW FRIDAY
AFTERNOON ACROSS THE NORTHERN SOUND/SAN JUANS. THIS TRACK ECHOES THE
INITIAL CONCERN OF THIS PATTERN ABOUT HIGH WINDS AND MOUNTAIN SNOW.

THE 12Z 12KM MM5 SURFACE WINDS RAISE CONFIDENCE IN A HIGH WIND WATCH
FOR COAST FOR NW 40-50 KT WINDS AS THE LOW MOVES INLAND. A STRONG NW
SURGE THROUGH THE STRAIT OF JUAN DE FUCA...AND EVENTUALLY DOWN THE
STRAIT OF GEORGIA WILL BE OTHER POTENTIAL AREAS OF CONCERN. NORTHERN
TRACK ALSO ENHANCES FOR AT LEAST WIND ADVISORY LEVELS FOR THE PUGET
SOUND FRIDAY AFTERNOON. A WATCH ADDRESSING WIND CONCERNS WILL COME
OUT SOMETIME THIS AFTERNOON.

STRONG EASTERLY GRADIENTS WILL LIKELY KEEP THE PASSES ALL SNOW FOR
THIS EVENT WITH A GOOD DUMP BEGINNING EARLY FRIDAY AFTERNOON AND
LASTING THROUGH SATURDAY MORNING. THE SNOW WILL BE ACCOMPANIED BY
925 WINDS OF 40+ KT TO HELP BLOW SNOW AROUND. THE EXTENT OF ONSHORE
FLOW AFTER THE SYSTEM PASSES THROUGH FRIDAY EVENING IS STILL IN DOUBT AS
DIFFERENT MODELS SUGGEST A SECONDARY SURFACE LOW OFF COAST HELPING
TO TURN THE FLOW MORE TO NORTHERLY ON SATURDAY...AND LIMITING THE
GOOD TO GREAT POTENTIAL SNOW TOTALS FOR THIS STORM.

WITH THE LATEST SOLUTION THERE SEEMS TO BE LESS OF A CONCERN FOR INITIAL
LOWLAND SNOW...SAVE ON THE NORTHERN FRINGE OF THE LOW NEAR THE
CANADIAN BORDER AND THE HOOD CANAL AREA. RATHER WITH THIS SOLUTION
THE GREATER THREAT FOR WET SNOW MIXING IN IS STILL FRIDAY NIGHT INTO
SATURDAY MORNING. STILL QUITE A FEW MODEL RUNS TO TRY AND PULL
EVERYTHING TOGETHER...SO STAY TUNED. DAMICO

.LONG TERM...FROM PREV DISCUSSION...ONCE THE LOW MOVES EAST FRIDAY
NIGHT THE UPPER TROUGH OVER B.C. WILL DIG SOUTH OVER THE AREA WITH
MUCH COLDER AIR BEGINNING TO POOL UP ALONG THE ROCKIES NORTH OF THE
WA BOARDER. 850 NE WINDS PICK UP TO AROUND 20-25 KT ON SATURDAY
WHICH SHOULD HELP PUSH THIS COLD AIR MASS OVER THE MOUNTAINS.
SURFACE GRADIENTS WILL ALSO BE SUFFICIENT TO DRAW COLD AIR IN
THROUGH THE FRASER.  MODELS GENERALLY AGREE AT DEVELOPING AN ARCTIC
FRONTAL BOUNDARY OVER THE FAR N INTERIOR SATURDAY MORNING AS THE LEE
SIDE SURFACE LOW FORMS. THIS WILL DROP SOUTH THROUGH THE DAY...SO
EXPECT COLDER AIR TO ALSO SLIDE SOUTH. THERE COULD BE A CONVERGENCE
BAND ASSOCIATED WITH THE FRONT WHICH WOULD LIKELY BRING PRECIP IN
THE FORM OF SNOW. HOWEVER...IT IS WAY TOO FAR OUT TO DISCUSS
POSSIBLE AMOUNTS AND EXACTLY WHERE. AREAS FAVORED BY UPSLOPE NLY
FLOW WILL ALSO HAVE A BETTER CHANCE AT ACCUMULATING SNOW.

MOISTURE COULD LINGER INTO SATURDAY NIGHT OR EVEN SUNDAY DEPENDING
WHICH MODEL YOU LOOK AT. SOME MODELS RELOAD THE TROUGH WITH UPPER
LEVEL ENERGY AND MAINTAIN A SURFACE LOW SOMEWHERE NEAR WRN WA WHICH
KEEPS SOME RISK OF SNOW SHOWERS OVER THE AREA. THE LOW WILL
EVENTUALLY DROP SOUTH WITH A PERIOD OF DRY BUT COLDER WEATHER BY
EARLY NEXT WEEK. IT IS VERY INTERESTING TO NOTE THAT ALL GLOBAL
MODELS MAINTAIN THIS LONG WAVE PATTERN INTO AT LEAST THE MIDDLE OF
NEXT WEEK WITH ADDITIONAL SYSTEMS AND REINFORCING SHOTS OF COLD AIR
POSSIBLE. IT SHOULD BE A VERY INTERESTING WEEK. STAY TUNED.  MERCER

&

.AVIATION...A DECAYING COLD FRONT WILL GRADUALLY SAG SOUTH THROUGH
WRN WA TODAY AND FINALLY DISSIPATE TONIGHT. IFR/MVFR CIGS WILL
PREVAIL ACROSS W WA THROUGH MOST OF THE DAY. EXPECT CIGS AND VIS TO
GRADUALLY LIFT TO ABOVE MVFR CONDITIONS BY 21-22Z...LATE THIS
AFTERNOON. HIGH PRESSURE WILL TRY TO BUILD IN...AND THE ACCOMPANYING
SUBSIDENCE SHOULD HELP MOST CLOUDS SCATTER OUT LATER THIS EVENING
AND OVERNIGHT. LINGERING LOW-LEVEL MOISTURE UNDER CLEARING SKIES
WILL THEN INCREASE THE POSSIBILITY OF WIDESPREAD FOG AND LOW CLOUDS
DEVELOPING OVERNIGHT.

KSEA...GRADUAL LIFTING OF CIGS THROUGHOUT THE DAY...TO ABOVE MVFR BY
21 OR 22Z. LIGHT AND VARIABLE WINDS EXPECTED TO SWITCH TO LIGHT
NORTHERLY AROUND SUNSET. PRANGE

&&

.MARINE...ONLY SMALL CRAFT CONDITIONS REMAINING ARE FOR HAZARDOUS
SEAS TODAY AND THU WITH SWELL HEIGHTS IN EXCESS OF 10 FEET ALONG THE
COAST AND WEST ENTRANCE AND FOR ROUGH BAR CONDITIONS.

THE BIG STORY IS THE CONTINUED MODEL TREND TOWARD A RAPIDLY
DEEPENING SURFACE LOW THAT MOVES ONSHORE ON FRI AFTN. THE LATEST
COMPUTER MODELS ARE IN EXCELLENT AGREEMENT TRACKING THIS FEATURE
ACROSS THE NRN OLYMPIC PENINSULA. STORM FORCE NORTHWESTERLIES ARE
EXPECTED BEHIND THE LOW CENTER OVER THE COASTAL WATERS WITH THE
LATEST COMPUTER MODELS SHOWING UP TO 55 KT SUSTAINED WINDS OFF THE
COAST ON FRI AFTN. HIGH WAVES IN THE SRN STRAIT OF GEORGIA WOULD
ALSO BE A CONCERN AS THE KBLI-CYZT GRADIENT ARND -16 MB WHIPS UP
STRONG NW WIND DOWN THE LENGTH OF THE STRAIT OF GEORGIA.
FORTUNATELY...THE TIMING OF THE LATEST MODELS STILL DOES NOT
COINCIDE WITH AN ASTRONOMICAL HIGH TIDE AT BLI...BUT A CHANGE IN THE
SPEED OF THE SYSTEM COULD INCREASE THE RISK OF COASTAL FLOODING TO
THE SANDY POINT AREA. STRONG FRASER OUTFLOW SHOULD DEVELOP SAT NIGHT
AND PERSIST INTO EARLY NEXT WEEK...BLOWING GALE FORCE AT TIMES.
WITH THE CURRENT MODEL TRACK OF THE LOW PRESSURE SYSTEM...WOULD
INCREASE THE RISK OF STORM FORCE WINDS IN THE STRAIT OF JUAN DE FUCA
AND INCREASE THE RISK OF A HIGH-END GALE FOR MOST OF PUGET SOUND.
PRANGE

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS AT THE GRAYS
        HARBOR ENTRANCE.
     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS ON THE COAST AND AT
        THE WEST ENTRANCE.

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