High Wind Watch 8, NWS Seattle, WA

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE SEATTLE WA
433 AM PST SAT NOV 21 2009

UPDATED THE AVIATION AND MARINE SEGMENTS.

.SYNOPSIS...A WEAK RIDGE WILL MOVE THROUGH WESTERN WASHINGTON THIS
MORNING...FOLLOWED QUICKLY BY A VIGOROUS FRONTAL SYSTEM. THIS SYSTEM
WILL BRING RAIN AND WINDY CONDITIONS TONIGHT AND SUNDAY. HEAVY SNOW
WILL FALL IN THE MOUNTAINS WITH THE SNOW LEVEL REMAINING AT OR BELOW
3000 FEET. A WEAKER FRONT WILL MOVE THROUGH THE AREA MONDAY AND
MONDAY NIGHT...AND ANOTHER WEAK FRONT WILL ARRIVE LATE IN THE WEEK.

&&

.SHORT TERM...WE HAVE NORTHWEST FLOW ALOFT OVER WESTERN WASHINGTON
BETWEEN AN UPPER TROUGH THAT MOVED THROUGH THE AREA OVERNIGHT AND A
WEAK UPPER RIDGE OFFSHORE ALONG 130W. THERE ARE STILL SCATTERED
SHOWERS ACROSS THE FORECAST AREA THIS MORNING...AND A PUGET SOUND
CONVERGENCE ZONE EXTENDING ACROSS ISLAND AND SNOHOMISH COUNTIES. IT
IS STILL BREEZY IN SOME AREAS...BUT THE WIND SHOULD EASE OVER THE
NEXT FEW HOURS AS THE SURFACE HIGH PRESSURE THAT HAS BEEN BUILDING
INTO THE AREA SHIFTS EAST OF THE CASCADES AHEAD OF THE NEXT FRONT.
THE FREEZING LEVEL HAS FALLEN TO AROUND 3000 FT AND TEMPERATURES
ACROSS THE WESTERN WASHINGTON LOWLANDS WERE IN THE UPPER 30S TO MID
40S AT 3 AM.

THE WEAK RIDGE OFFSHORE WILL MOVE QUICKLY ACROSS WESTERN WASHINGTON
THIS MORNING...AND THE CONVERGENCE ZONE WILL LIKELY WEAKEN AND
DISSIPATE. THE SOUTH END OF AN OCCLUDED FRONT SWINGING INTO BRITISH
COLUMBIA WILL QUICKLY FOLLOW THE RIDGE...WITH SIGNIFICANT
PRECIPITATION REACHING THE COAST AND OLYMPICS THIS AFTERNOON. A
FRONTAL WAVE BEGINNING TO DEVELOP NEAR 45N/145W WILL DEEPEN AS IT
MOVES RAPIDLY EASTWARD WITH THE HELP OF A 140 KT JET. IT WILL MOVE
THROUGH THE FORECAST AREA TONIGHT.

HEAVY SNOW IN THE MOUNTAINS LOOKS LIKE A SURE BET. THE SNOW LEVEL
WILL RISE ONLY TO AROUND 3000 FT AS THIS FEATURE MOVES THROUGH...AND
PRECIPITATION AMOUNTS WILL BE IMPRESSIVE. LIQUID AMOUNTS OF 3 TO 4
INCHES IN THE OLYMPICS ARE POSSIBLE THROUGH SUNDAY MORNING --
TRANSLATING ROUGHLY TO 30 TO 40 INCHES OF SNOW. EVEN THE RELATIVELY
DRIER HURRICANE RIDGE AREA SHOULD RECEIVE WINTER STORM WARNING
AMOUNTS...IE 12 INCHES IN 12 HOURS OR 18 INCHES IN 24 HOURS. WATER
EQUIVALENTS IN THE CASCADES WILL LIKELY BE IN THE 1 TO 3 INCH
RANGE...FOR SNOWFALL TOTALS OF 10 INCHES TO 2.5 FT. THE WINTER STORM
WATCH WILL BE UPGRADED TO A WARNING WITH THE EARLY MORNING FORECAST
PACKAGE.

ONE MATTER THAT IS SOMEWHAT UNCERTAIN IS THE TRACK OF THE SURFACE
LOW. THE MOST LIKELY SCENARIO IS THAT IT WILL REACH THE CENTRAL
WASHINGTON COAST AROUND MIDNIGHT TONIGHT...TRACK NORTHEAST ACROSS
THE OLYMPIC PENINSULA AND THE NORTH PART OF THE PUGET SOUND REGION
TO AROUND MT BAKER AT 4 AM...THEN HEAD RAPIDLY INTO BRITISH
COLUMBIA. IT WILL PRODUCE BREEZY TO WINDY CONDITIONS...BUT THERE
APPEARS TO BE LITTLE THREAT OF A WIND STORM.

POST-FRONTAL SHOWERS WILL TAPER OFF SUNDAY AFTERNOON AND EVENING...
AND AN UPPER RIDGE WILL MOVE INLAND SUNDAY NIGHT. THE RIDGE WILL
QUICKLY HEAD EAST OF THE CASCADES MONDAY AND ANOTHER FRONTAL SYSTEM
JUST UPSTREAM OF THE RIDGE WILL ARRIVE. THIS SYSTEM COULD INITIALLY
BRING DECENT PRECIPITATION AMOUNTS TO THE COAST AND OLYMPIC
PENINSULA MONDAY AFTERNOON...BUT IT WILL WEAKEN AS IT MOVES INLAND
MONDAY NIGHT. BY RECENT STANDARDS THIS WILL BE AN UNEVENTFUL SYSTEM
AND IT WILL MARK THE BEGINNING OF A PERIOD OF QUIETER...THOUGH NOT
NECESSARILY DRY...WEATHER. MCDONNAL

.LONG TERM...A WEAK UPPER RIDGE WILL FOLLOW THE FRONT THAT MOVES
THROUGH MONDAY AND MONDAY NIGHT. THERE IS JUST A CHANCE OF RAIN IN
THE FORECAST FOR TUESDAY AND WEDNESDAY...WITH A FAIRLY GOOD CHANCE
FOR SOME DRY WEATHER OVER THOSE TWO DAYS. A WEAK FRONT WILL SLOWLY
MOVE ACROSS WESTERN WASHINGTON WEDNESDAY NIGHT AND THURSDAY. ANOTHER
FRONTAL WAVE WILL PROBABLY KEEP THE AREA WET THURSDAY NIGHT AND
FRIDAY...THOUGH DISAGREEMENT BETWEEN MODELS DROPS OFF CONSIDERABLY
BY LATE IN THE WEEK. MCDONNAL

&&

.HYDROLOGY...THE SYSTEM COMING IN TODAY THROUGH MIDDAY SUNDAY ...
WHILE IT WILL PRODUCE IMPRESSIVE AMOUNTS OF PRECIPITATION...WILL
HAVE MINIMAL EFFECT ON RIVERS EXCEPT POSSIBLY FOR THE TWO RIVERS
THAT ARE ALREADY CLOSE TO OR ABOVE FLOOD STAGE -- THE SKOKOMISH AND
THE LOWER REACH OF THE CHEHALIS IN GRAYS HARBOR COUNTY. WITH THE
SNOW LEVEL ONLY RISING TO AROUND 3000 FT...IT WILL BE A MOUNTAIN
SNOW PRODUCER NOT A FLOODING RAIN PRODUCER.

THE SKOKOMISH RIVER IS FORECAST TO FALL BELOW FLOOD STAGE ABOUT NOON
TODAY. THERE WILL BE A PLANNED RELEASE OF 3000 CFS FROM CUSHMAN DAM
#2 INTO THE NORTH FORK SKOKOMISH RIVER TODAY. THIS RELEASE WILL KEEP
THE SKOKOMISH RIVER RUNNING HIGH. THE FORECAST CALLS FOR THE RIVER
TO REMAIN JUST BELOW FLOOD STAGE TONIGHT AND SUNDAY.

A FLOOD WATCH REMAINS IN EFFECT FOR THE LOWER REACH OF THE CHEHALIS
RIVER IN GRAYS HARBOR COUNTY...WHICH HAS BEEN WITHIN A FOOT OF FLOOD
STAGE FOR THE PAST TWO DAYS. IT IS CONCEIVABLE THAT THE
PRECIPITATION FROM THE INCOMING SYSTEM COULD HELP FINALLY NUDGE THE
LOWER CHEHALIS ABOVE FLOOD STAGE. IF FLOODING DOES OCCUR IT WILL BE
MINOR.

THE NEXT SYSTEM ON MONDAY SHOULD MOVE THROUGH QUICKLY AND BE TOO
WEAK TO CAUSE ANY FLOODING...EVEN WITH RISING SNOW LEVELS. FOR THE
REMAINDER OF THE WEEK...THE AIR MASS OVER THE NORTHWEST WILL BE
WARMER WITH HIGHER SNOW LEVELS...HOWEVER THE SYSTEMS THAT ARE
EXPECTED TO AFFECT THE AREA WILL BE FAIRLY WEAK AND ARE NOT EXPECTED
TO CAUSE ANY ADDITIONAL FLOODING CONCERNS.

NO FLOODING IS EXPECTED AT ANY TIME ON THE GREEN RIVER. MCDONNAL

&&

.AVIATION...A WEAK...UPPER RIDGE OVER THE REGION WILL MOVE E
TONIGHT...ALLOWING A STRONG...UPPER TROF TO MOVE OVER THE AREA.
INCREASING WLY FLOW ALOFT...SUBSIDING AND BECOMING NW LATE TONIGHT.

AT THE SFC...A FRONTAL SYSTEM OVER THE ERN PAC WILL RAPIDLY APPROACH
WRN WA DURING THE DAY TODAY...RESULTING IN INCREASED SLY WINDS. THE
OCCLUDED FRONT WILL MOVE ACROSS THE AREA THIS EVENING. A SECONDARY
SFC LOW IS ANTICIPATED TO DEVELOP OFF THE FAR SW WA OR FAR NW OREGON
COAST THIS EVENING...AND TRACK NEWD ACROSS THE AREA LATER TONIGHT.
THUS EXPECT THE WINDS TO BACK BEHIND THE OCCLUDED FRONT THIS
EVENING...AND THIS SHOULD RAISE THE POTENTIAL FOR LLWS SINCE WINDS
ABOVE THE SFC WILL REMAIN SLY. WINDS WILL PICK UP DRAMATICALLY FROM
THE W OR SW LATE TONIGHT OR EARLY SUNDAY ONCE THE SFC LOW MOVES E OF
THE AREA.

MEANWHILE...A PUGET SOUND CONVERGENCE ZONE /PSCZ/ WAS ROUGHLY
LOCATED BETWEEN KPAE AND KAWO AT THIS TIME. THIS FEATURE SHOULD
DRIFT N AND DISSIPATE BY MIDMORNING AS ITS SUPPORT WANES.

KSEA...WINDS WILL REMAIN SLY TODAY...BECOMING MORE ELY TONIGHT.
THERE WILL BE A POTENTIAL FOR LLWS BETWEEN 0000 AND 1200 UTC SUNDAY
DUE TO ELY WINDS AT THE SFC AND STRONG SLY WINDS NEAR 2K FT. HAVE
HELD OFF ON MENTIONING THIS IN THE TAF TIL WE SEE ANOTHER MODEL RUN.
THE CURRENT MODEL SOLUTIONS WERE QUITE DIFFERENT THAN THE PREVIOUS
RUN DUE TO A MORE VIGOROUS SYSTEM NOW EXPECTED TO AFFECT THE AREA.
WINDS WILL INCREASE DRAMATICALLY FROM THE SW SOMETIME BETWEEN 1000
UTC AND 1200 UTC SUNDAY. CONCERNING CIGS...ANTICIPATE CIGS TO
GRADUALLY LOWER DURING THE DAY...ALTHOUGH THERE WILL BE OCCASIONAL
CIGS NEAR 2500 FT THIS MORNING. BY LATE THIS EVENING...CIGS SHOULD
BE NEAR 2K FT...WITH RAIN AND FOG REDUCING VSBYS INTO THE 3-5SM
RANGE.

&&

.MARINE...THERE WAS A SIGNIFICANT CHANGE IN THE FORECAST FROM THE
PREVIOUS ONE DUE TO A MORE VIGOROUS SYSTEM EXPECTED TO AFFECT THE
AREA TODAY. SLY WINDS ARE STILL ANTICIPATED TO INCREASE DURING THE
DAY...AHEAD OF A FAST-MOVING FRONTAL SYSTEM STILL WELL OFFSHORE...
FOR GALE OR SMALL CRAFT ADVISORY WIND SPEEDS. THE OCCLUDED FRONT
WILL SWEEP ACROSS THE AREA THIS EVENING. DUE TO SOMEWHAT LOW
CONFIDENCE HAVE WENT WITH A SMALL CRAFT ADVISORY FOR THE NORTHERN
INLAND WATERS AND EAST ENTRANCE TO THE STRAIT OF JUAN DE FUCA TODAY
INSTEAD OF UPGRADING THE PREVIOUS GALE WATCH TO A WARNING.

ANOTHER SURFACE WAVE OR LOW IS EXPECTED TO DEVELOP OFF THE FAR SW
WASHINGTON OR FAR NW OREGON COAST TONIGHT...AND THIS WILL CAUSE THE
WINDS TO BACK AND DROP OFF...ESPECIALLY ACROSS THE INLAND WATERS.
THE LOW WILL MOVE NE ACROSS INTERIOR WESTERN WASHINGTON OVERNIGHT.
WINDS WILL KICK BACK UP LATE TONIGHT OR EARLY SUNDAY IN RESPONSE TO
STRONG ONSHORE OR WESTERLY FLOW DEVELOPING. CONFIDENCE WAS FAIRLY
HIGH THAT GALE FORCE WINDS WILL DEVELOP OVER THE STRAIT OF JUAN DE
FUCA EARLY SUNDAY...THUS HAVE ISSUED A GALE WATCH FOR THE CENTRAL
AND EASTERN STRAIT. THERE IS A POSSIBILITY THAT WINDS MAY BRIEFLY
REACH STORM FORCE...NEAR 50 KNOTS...OVER THE CENTRAL STRAIT SUNDAY
MORNING WITH THE INITIAL PUSH.

ANTICIPATE THE ONSHORE FLOW TO WEAKEN AND BECOME SOUTHERLY SUNDAY
NIGHT. THE NEXT FRONTAL SYSTEM WILL AFFECT THE WATERS ON MON.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNINGS AND/OR FLOOD WATCHES REMAIN IN EFFECT FOR A
     PORTION OF THE CWA.

     A WINTER STORM WARNING IS IN EFFECT FOR THE OLYMPICS AND
     CASCADES FROM THIS AFTERNOON THROUGH SUNDAY MORNING.

PZ...A GALE WARNING REMAINS IN EFFECT FOR THE COASTAL WATERS AND
     WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA THROUGH THIS
     EVENING.

     A SMALL CRAFT ADVISORY IS IN EFFECT FOR THE REMAINING
     WATERS.

     A SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR CONDITIONS
     IS IN EFFECT TIL NOON MON.

WWW.WEATHER.GOV/SEATTLE

FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATESTWEBAFD.HTML.