Gale Warning 78, NWS Seattle, WA

AREA FORECAST DISCUSSION ...UPDATED TO ADD AVIATION MARINE SECTIONS
NATIONAL WEATHER SERVICE SEATTLE WA
920 AM PST FRI DEC 19 2008

.SYNOPSIS...HIGH PRESSURE AND A COLD AIR MASS WILL PRODUCE
SUB-FREEZING TEMPERATURES TODAY AND FRIDAY. A STRONG FRONTAL SYSTEM
WILL AFFECT THE AREA SATURDAY AND SUNDAY...BRINGING STRONG WINDS AND
SIGNIFICANT SNOW TO VARIOUS PARTS OF WESTERN WASHINGTON. A COLD
UPPER LEVEL LOW TRAILING THE FRONTAL SYSTEM WILL MAINTAIN THE COLD
AIR MASS AND SNOW SHOWERS ON MONDAY. ANOTHER STRONG AND COLD WEATHER
SYSTEM WILL AFFECT THE REGION TUESDAY AFTERNOON THROUGH THURSDAY.

&&

.SHORT TERM...MUCH TO CONSIDER THIS MORNING WITH NEW COMPUTER
MODELS JUST NOW ARRIVING. MODELS ARE ALL IN GOOD AGREEMENT THAT A
MAJOR WINTER STORM WILL AFFECT MUCH OF THE WESTERN WASHINGTON
LOWLANDS OVER THE WEEKEND WITH A HIGH WIND EVENT FOR THE CASCADE
FOOTHILLS AND EAST PUGET SOUND LOWLANDS EAST OF SEATTLE.

FIRST OFF...THE 12Z GFS/NAM AND ECMWF ARE A LITTLE WEAKER WITH THE
MOUNTAIN WAVE EVENT STARTING SATURDAY MORNING AND CONTINUING THROUGH
SUNDAY AFTERNOON. MODELS SHOW A LOT OF DEEP SLY FLOW ALL THE WAY
EAST TO THE FOOTHILLS THROUGH MOST THE PERIOD WITH LIGHT TO MODERATE
850 MB ELY FLOW FROM THE CREST EWD. THIS IS GENERALLY NOT FAVORABLE
FOR A STRONG DOWNSLOPE WIND EVENT W OF THE CREST. HOWEVER...WITH ALL
THE VERY COLD AIR DAMMED UP ON THE EAST SIDE...THERE WILL BE AN
ACCELERATION THROUGH THE GAPS AND POSSIBLY SPILL OVER DOWNSLOPING
BUT MAYBE NOT QUITE AS STRONG AS PREVIOUSLY THOUGHT. THE 12Z MM5GFS
IS THE FLY IN THE OINTMENT WHICH STILL INSISTS ON 60 KT DOWNSLOPE
WINDS...MUCH STRONGER THAN ALL THE OTHER MODELS.  THE WATCH STILL
LOOKS GOOD BUT WHEN UPGRADED TO A WARNING LATER TODAY...WILL
CONSIDER WORDING THE STRONGEST WINDS AT GAP LOCATIONS SUCH AS NORTH
BEND...CUMBERLAND...GOLD BAR...SNOQUALMIE FOR EXAMPLE.

THE SECOND MAJOR PROBLEM CONCERNS SNOW AMOUNTS AND PERHAPS SOME
ISSUES WITH MIXED PRECIP TYPES. THE NAM-12 IS THE COLDEST OF THE
MODELS. VIRTUALLY ALL SNOW EVERYWHERE. THIS MODEL SUGGESTS 6 INCHES
TO A FOOT OF SNOW IS POSSIBLE FROM NEAR OLYMPIA SOUTH AND WEST AND
LOCALLY TO 18 INCHES ALONG HOOD CANAL AND THE KITSAP PENINSULA.
HOWEVER...THE WARMER GFS/ECMWF SUGGEST SOME CHANGE OVER TO FREEZING
RAIN/SLEET HALF WAY THROUGH THE EVENT WHICH WOULD CUT THOSE AMOUNTS
IN HALF BUT WOULD END UP WITH POSSIBLE SIGNIFICANT ICING ON TOP OF
THE SNOW. PRECIP WOULD LIKELY STAY M0STY SNOW IN THE KITSAP/HOOD
CANAL AREA DUE TO POOLING OF COLD AIR AND THE COAST WOULD MOST
LIKELY CHANGE OVER TO ALL RAIN AT SOME POINT BY SUNDAY MORNING.

FOR THE I-5 CORRIDOR NORTH OF OLYMPIA THROUGH
SEATTLE...EVERETT...AND INCLUDING BELLINGHAM...A SIGNIFICANT SNOW
EVENT IS POSSIBLE. GIVEN THE AMOUNT OF OVERRUNNING MOISTURE AND HIGH
QPF TO SNOW RATIOS...I FELT IT PRUDENT TO ADD THE EVERETT/BELLINGHAM
ZONES TO THE WATCH. THE EAST PUGET SOUND LOWLANDS ARE STILL NOT
INCLUDED BECAUSE OF EAST WIND DRYING EFFECTS IN THE LOW LEVELS. IF
EAST WINDS ARE LIGHTER THERE COULD BE SOME SNOW ACCUMULATION
THERE...HOWEVER CONFIDENCE IS LOW AND AMOUNTS WOULD LIKELY BE MUCH
LIGHTER THAN AREAS WEST OF LAKE WASHINGTON. THE NEW NAM-12/GFS ARE
SHOWING RATHER IMPRESSIVE QPF AMOUNTS FOR THE METRO AREAS WITH THE
QPF TO SNOW RATIOS INCREASING AS YOU GO NORTH. AMOUNTS OVER 4 INCHES
ARE LIKELY AND POSSIBLY IN EXCESS OF 6 INCHES SO THE WATCH LOOKS
GOOD. THERE COULD ALSO BE SOME BLOWING AROUND OF SNOW WITH SOME
EASTERLY WIND SURFACING AT TIMES. REALLY MESSY. THERE IS SOME
POSSIBILITY OF A CHANGE OVER THE FREEZING RAIN AS FAR NORTH AS
SEATTLE LATE IN THE EVENT BUT MOST PRECIP WOULD HAVE ALREADY FALLEN
AS SNOW. PRECIP SHOULD LIGHTEN UP SUNDAY AFTERNOON.

THIS IS A RARE AND VERY COMPLEX WEATHER SITUATION. WE HAVE VERY COLD
AIR TRAPPED AT THE SURFACE...VERY MOIST OVERRUNNING AND SOME WARMING
ALOFT LEADING TO POSSIBLE ICING FOR SPOTS. WE WILL BE POURING OVER
THE LATEST MODELS DATA TODAY WITH SOME WARNINGS LIKELY TO BE ISSUED
FOR BOTH HIGH WIND AND SNOW LATER THIS AFTERNOON WITH MUCH MORE
DETAIL REGARDING SNOW AND ICE AMOUNTS...AND WIND.

MERCER

.LONG TERM...NO CHANGES.  THE UPPER LEVEL LOW REMAINS OVER THE AREA
ON MONDAY. SHOULD JUST BE A SNOW SHOWER DAY. NO IDEA ON AMOUNTS
THOUGH. MAIN EFFORT WAS DEALING WITH THE WEEKEND EVENT. ONE KEY
FACTOR IS THAT THE COLDER NE FLOW CONTINUES...SO THERE WONT BE MUCH
OF A WARM UP. GFS SHOWS THE NEXT FRONTAL SYSTEM BOMBING OFFSHORE AS
IT APPROACHES THE COAST. ECMWF IS MUCH WEAKER. BOTH MODELS MAINTAIN
THE COLD E-NE LOW LEVEL FLOW. GFS SOLUTION WOULD MEAN ANOTHER WIND
EVENT. SNOW IS MORE QUESTIONABLE. KAM

&&

.AVIATION...SOME STEAM FOG RISING OFF LAKE WASHINGTON IS PUSHING
WEST INTO THE SEATTLE AND BOEING FIELD TERMINALS THIS MORNING. A
PLUME OF MOISTURE OFF THE SOUND IS ALSO PUSHING INTO OLYMPIA THESE
WILL DECREASE FROM NORTH TO SOUTH 20-23Z. EXPECT CLEAR SKIES TO
DOMINATE TONIGHT. MID AND HIGH CLOUDS ASSOCIATED WITH THE NEXT
STRONG WEATHER SYSTEM WILL MOVE INTO THE AREA DURING THE AFTERNOON
HOURS SAT WITH SNOW SPREADING FROM SW TO NE AROUND 00Z SUN.

.SEA...LIGHT NE WIND TODAY. E WIND PICKING UP GENERALLY AFTER THE
18Z TIME FRAME BUT NOT SIGNIFICANTLY INCREASING UNTIL AROUND THE END
OF THE 30 HOUR PERIOD 00Z SUN. ALBRECHT

&&

.MARINE...IN THE SHORT TIME STILL HAVE SOME SMALL CRAFT ADVISORY
CONDITIONS AROUND THE WEST ENTRANCE TO THE STRAIT AND THE NORTHERN
INLAND WATERS. EXPECT GALE CONDITIONS TO DEVELOP SAT AFTERNOON AND
SAT NIGHT AND SPREAD RAPIDLY E COVERING ALL BUT THE PUGET SOUND
ZONE. WILL LIKELY ISSUE GALE WARNINGS FOR ALL WATERS EXCEPT PUGET
SOUND WITH THE AFTERNOON PACKAGE. ANOTHER SYSTEM WILL MOVE THROUGH
THE WATERS LATE TUE INTO WED. ALBRECHT

&&

SEW WATCHES/WARNINGS/ADVISORIES...
WA...HIGH WIND WATCH IN EFFECT FOR THE EAST PUGET SOUND
LOWLANDS...WEST STRAIT...AND THE CASCADES SATURDAY THROUGH SUNDAY
AFTERNOON.
.WINTER STORM WATCH FOR ALL OF WESTERN WASHINGTON...EXCLUDING THE
EAST PUGET SOUND LOWLANDS AND THE CASCADES LATE SATURDAY AFTERNOON
THROUGH LATE SUNDAY NIGHT.

PZ...SMALL CRAFT ADVISORY WEST ENTRANCE TO THE STRAIT AND NORTH
INLAND
      WATERS.
    .GALE WATCH COASTAL WATERS MIDDAY SAT THROUGH MIDDAY SUN.
    .SMALL CRAFT ADVISORY GRAYS HARBOR FOR ROUGH BAR CONDITIONS.

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