Winter Storm Warning 25, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 PM PST TUE NOV 17 2009

.SYNOPSIS...SHOWERS WILL CONTINUE ACROSS WESTERN WASHINGTON TONIGHT
AS AN UPPER LEVEL TROUGH PASSES THROUGH THE AREA. ANOTHER STRONG
PACIFIC FRONTAL SYSTEM WILL ARRIVE LATE WEDNESDAY AFTERNOON. WET
WEATHER WILL CONTINUE THROUGH FRIDAY MORNING AS THE SURFACE FRONT
REMAINS NEARLY STATIONARY. COOLER AND SHOWERY WEATHER IS ANTICIPATED
OVER THE WEEKEND.

&&

.SHORT TERM...SHOWERS ARE TRAVERSING THE AREA FROM SOUTHWEST TO
NORTHEAST. A MORE ORGANIZED BAND OF SHOWERS IS MOVING ASHORE THIS
EVENING AND SHOULD SPREAD INLAND THROUGH THE NIGHT. POPS ARE QUITE
HIGH IN ALL GUIDANCE FOR ALL AREAS...BASICALLY CALLING FOR
CATEGORICAL SHOWERS OR RAIN THROUGH THE NEXT FIVE PERIODS. NO REASON
TO DOUBT THIS AS THE PATTERN IS A WET ONE. HOWEVER THE HEAVIEST RAIN
APPEARS TO BE CONFINED TO THE OLYMPICS AND PERHAPS WILLAPA HILLS.
MORE ON THIS BELOW IN HYDROLOGY SECTION.

WIND IS A CONCERN WEDNESDAY AFTERNOON AND NIGHT. LATEST GUIDANCE HAS
A SWATH OF 45+ KT WINDS OVER THE WATERS OFF THE COAST...TOUCHING THE
COAST IN PLACES. THERE ARE SOME 35+ KT WINDS OVER THE NORTH
INTERIOR. HIGH WIND WATCH IS IN EFFECT FOR THE COAST AND NORTH
INTERIOR. AS THINGS NOW STAND THE SITUATION IS REMARKABLY SIMILAR TO
THE LAST TWO NIGHTS...WHICH PRODUCED SPOTTY HIGH WIND ON THE COAST
AND OVER THE NORTHWEST INTERIOR...WITH WIND ADVISORY TYPE WINDS
ELSEWHERE INCLUDING THE SEATTLE METRO AREA.

YET ANOTHER CONCERN IS SNOW IN THE CASCADES. THE FIRST IMPULSE OF
PRECIPITATION ON WEDNESDAY NIGHT WILL BE RAIN IN THE OLYMPICS BUT
SNOW IN THE CASCADES...AND THE NORTH CASCADES COULD PUSH HEAVY SNOW
WARNING AMOUNTS. A WINTER STORM WATCH IS IN EFFECT.

AFTER ANOTHER SHOT OF HEAVY MOUNTAIN RAIN ON THURSDAY AFTERNOON AND
NIGHT...WHICH WILL INCLUDE THE NORTH CASCADES THIS TIME...A COLD
FRONT SWEEPS THROUGH EARLY FRIDAY. THIS WILL CHANGE THE REGIME TO A
COOL SHOWERY ONE...PERHAPS EVEN WITH A LITTLE SUN. BURKE

.LONG TERM...PREVIOUS DISCUSSION...COOLER AND SHOWERY WEATHER
EXPECTED FRI NIGHT AS THE LONG WAVE UPPER LEVEL TROUGH SHIFTS
INLAND. THIS WILL SHUT DOWN ANY HEAVY PRECIP ACROSS THE REGION WITH
SNOW LEVELS LOWERING BACK DOWN BELOW 2000 FT. WEAK RIDGING WILL
DEVELOP BEHIND THE TROUGH ON SAT...THEN ANOTHER SYSTEM DIVE SE INTO
THE AREA SAT NIGHT. THIS SYSTEM LOOKS RATHER PROGRESSIVE BUT MAY
STILL NEED TO WATCH SNOW AMOUNTS IN THE MOUNTAINS. MODELS AGREE ON
BROADER SCALE RIDGING EARLY NEXT WEEK...ALTHOUGH THE FLOW ALOFT OVER
WA REMAINS NEARLY ZONAL...WITH ANOTHER SYSTEM CLIPPING THE AREA BY
MON. 33

&&

.HYDROLOGY...ONLY REMAINING FLOOD WARNING IS FOR THE SKOKOMISH
RIVER...WHICH IS STILL AT MAJOR FLOOD STAGE AND WHICH PROBABLY WILL
NOT FALL BELOW FLOOD STAGE UNTIL WEDNESDAY AFTERNOON. A FLOOD WATCH
IS STILL IN EFFECT FOR THE LOWER CHEHALIS. IF THE UPPER CHEHALIS
CONTINUES TO RESPOND TO THE RAIN YESTERDAY LESS THAN FORECAST...THIS
WATCH COULD BE DROPPED AT SOME POINT.

THOUGH SHOWERS WILL CONTINUE TONIGHT AND WEDNESDAY MORNING...AMOUNTS
ARE TOO LIGHT TO CAUSE ANY HYDROLOGICAL PROBLEMS.

ANOTHER STRONG PACIFIC FRONTAL SYSTEM WILL REACH THE COAST AND
OLYMPICS WEDNESDAY AFTERNOON. HEIGHTS WILL RISE WEDNESDAY NIGHT AND
HEAVY RAIN WILL FALL OVER THE OLYMPICS AND WILLAPA HILLS. THIS FIRST
BAROCLINIC BAND WILL SAG SOUTH SOMEWHAT ON THURSDAY MORNING...BUT
ANOTHER IMPULSE MOVING UP THE BAND WILL SHIFT THE FOCUS OF RAIN BACK
TO THE OLYMPICS THURSDAY AFTERNOON AND NIGHT. A COLD FRONT WILL PUSH
THE RAIN BAND EAST ON FRIDAY MORNING...USHERING IN COLDER AIR AND
ENDING THE HYDROLOGICALLY SIGNIFICANT PRECIPITATION.

DURING THE FIRST HEAVY RAIN PERIOD ON WEDNESDAY NIGHT THE NORTH
CASCADES WILL BE IN COLD AIR AND MUCH OF THE PRECIPITATION THERE
WILL BE IN THE FORM OF SNOW. DURING THE SECOND PERIOD ON THURSDAY
NIGHT THE NORTH CASCADES WILL BE WARMER AND IT WILL RAIN THERE TOO.

THE UPSHOT FOR THE RIVERS IS PROBABLY ANOTHER ROUND OF FLOODING FOR
RIVERS FLOWING OFF THE OLYMPICS...A THREAT OF FLOODING FOR THE
CHEHALIS RIVER...AND A LESSER THREAT FOR THE NORTH CASCADES RIVERS.
A FLOOD WATCH WILL LIKELY GO OUT FOR RIVERS ON THE OLYMPIC PENINSULA
SOMETIME WEDNESDAY MORNING. THE CURRENT WATCH ON THE CHEHALIS RIVER
MAY NOT HAVE TIME TO EXPIRE BEFORE A NEW WATCH GOES UP. THE WATCH
WILL LIKELY INCLUDE NORTH CASCADES RIVERS. A FLOOD POTENTIAL OUTLOOK
IS IN EFFECT RIGHT NOW.

FLOODING IS NOT ANTICIPATED ON THE GREEN RIVER...OR ON ANY RIVERS
FLOWING OUT OF THE CENTRAL CASCADES.

MORE UPPER LEVEL DISTURBANCES WILL ROTATE INTO THE PACIFIC NORTHWEST
OVER THE WEEKEND AND EARLY NEXT WEEK...BUT SO FAR SIGNIFICANT
PRECIPITATION IS NOT EXPECTED IN THE MOUNTAINS. BROAD SCALE RIDGING
AMPLIFIES ACROSS THE WEST MID WEEK DEFLECTING SYSTEMS INTO SOUTHERN
BRITISH COLUMBIA. BURKE

&&

.AVIATION...SOUTHWEST FLOW ALOFT WILL CONTINUE TONIGHT AS A WEAK
UPPER TROUGH MOVES THROUGH THE AREA. SOUTHEASTERLY SURFACE PRESSURE
GRADIENTS WILL INCREASE WEDNESDAY...AND ANOTHER VIGOROUS PACIFIC
FRONTAL SYSTEM WILL REACH WESTERN WASHINGTON WEDNESDAY AFTERNOON.
THE AIR MASS WILL REMAIN UNSTABLE TONIGHT...THEN STABILIZE WEDNESDAY.

WE WILL CONTINUE TO HAVE A MIX OF MOSTLY LOW-END VFR AND HIGH-END
MVFR CONDITIONS TONIGHT WITH SHOWERS OFF AND ON...AND A CHANCE OF
THUNDERSTORMS THIS EVENING. WINDS WILL INCREASE AS THE NEXT FRONTAL
SYSTEM APPROACHES WEDNESDAY...WITH WINDS AT 3000 FT RISING TO 50 KT
LATE AFTERNOON.

KSEA...SOUTHERLY WIND 8-12 KT RISING TO 12-18 KT GUSTING 28 KT LATE
WEDNESDAY. MCDONNAL

&&

.MARINE...A WEAK SURFACE TROUGH OVER THE OFFSHORE WATERS WILL CROSS
THE FORECAST AREA TONIGHT. WHILE THERE WILL BE SOME SMALL CRAFT
ADVISORY WINDS ASSOCIATED WITH THIS FEATURE...IT IS MORE OR LESS A
LULL BETWEEN THE VIGOROUS FRONTAL SYSTEM MONDAY AND THE ONE DUE TO
ARRIVE WEDNESDAY.

THE WEDNESDAY SYSTEM WILL PRODUCE ANOTHER ROUND OF HIGH-END TO STORM
FORCE WINDS OVER THE FORECAST AREA WEDNESDAY AND WEDNESDAY NIGHT.
THESE WARNINGS HAVE ALREADY BEEN POSTED WITH THE PACKAGE THIS
AFTERNOON. A TRAILING FRONTAL WAVE WILL PROLONG GALES OVER MUCH OF
THE AREA THURSDAY.

PRESSURE GRADIENTS WILL WEAKEN FRIDAY AS A WEAK SURFACE LOW CROSSES
THE AREA. YET ANOTHER FRONTAL SYSTEM WILL AFFECT THE AREA THIS
WEEKEND.

THE LARGE WESTERLY SWELLS THAT ARRIVED WITH THE LAST SYSTEM ARE
CURRENTLY SUBSIDING. HOWEVER SWELLS WILL LIKELY BUILD TO JUST OVER
20 FT WITH THE NEXT SYSTEM...SO WE CAN EXPECT ANOTHER ROUND OF HIGH
SURF ADVISORIES WEDNESDAY NIGHT. MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCHES AND WARNING REMAIN IN EFFECT FOR PORTIONS OF
     WESTERN WA.
     .HIGH WIND WATCH IN EFFECT FOR COAST AND NORTH INTERIOR
     WED AFTERNOON THROUGH LATE WED EVENING.
     .WINTER STORM WATCH FOR THE CASCADES WED NIGHT THROUGH FRI
      MORNING.

PZ...SMALL CRAFT ADVISORY ALL WATERS.
    .SMALL CRAFT ADVISORY GRAYS HARBOR FOR ROUGH TO SEVERE BAR
     CONDITIONS.
    .GALE WATCH CENTRAL STRAIT...PUGET SOUND AND HOOD CANAL.
    .STORM WARNING COASTAL WATERS AND WEST ENTRANCE STRAIT.
    .GALE WARNING EAST ENTRANCE STRAIT...NORTHERN INLAND WATERS...AND
     ADMIRALTY INLET.

WWW.WEATHER.GOV/SEATTLE

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