Small Craft Advisory 859, NWS Seattle, WA

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE SEATTLE WA
930 PM PST THU NOV 24 2011

CORRECTION TO THE HYDROLOGY SECTION.

.SYNOPSIS...SHOWERS WILL TAPER OFF TONIGHT...WHILE HEAVY SNOW
CONTINUES IN THE MOUNTAINS. FRIDAY WILL BE A PARTLY SUNNY DAY. A
WARM FRONT WILL CLIP WESTERN WASHINGTON FRIDAY NIGHT THROUGH
SATURDAY NIGHT BRING RAIN AT TIMES...THEN A COLD FRONT WILL MOVE
THROUGH THE AREA SUNDAY FOR ANOTHER WET DAY. HIGH PRESSURE WILL
BRING A SHORT PERIOD OF DRY WEATHER EARLY NEXT WEEK.

&&

.SHORT TERM...THE VIGOROUS OCCLUDED FRONT THAT MOVED INTO WESTERN
WASHINGTON EARLIER TODAY HAS QUICKLY MADE ITS WAY ACROSS THE
CASCADES. STRONG WESTERLY FLOW WILL CONTINUE TONIGHT AS AN UPPER
TROUGH MOVES THROUGH IN THE WAKE OF THE FRONT. THE STEADY RAIN HAS
CHANGED TO SHOWERS...AND WINDS ARE EASING. I WILL ISSUE AN EVENING
UPDATE THAT WILL END THE HIGH WIND WARNING FOR THE NORTHWEST
INTERIOR.

A FLAT PROGRESSIVE UPPER RIDGE WILL MOVE INTO THE PACIFIC NORTHWEST
FRIDAY FOR A RELATIVE BREAK IN THE WEATHER. SPOTTY SHOWERS COULD
LINGER OVER THE INTERIOR LOWLANDS AND THE CASCADES IN THE
MORNING...AND THE LEADING EDGE OF PRECIPITATION WITH THE NEXT
INCOMING SYSTEM COULD REACH THE COAST IN THE AFTERNOON. ALL IN ALL
THOUGH...IT WILL BE A PARTLY SUNNY DAY FOR MUCH OF WESTERN
WASHINGTON.

THE UPPER RIDGE AXIS WILL MOVE EAST OF THE CASCADES FRIDAY NIGHT...
AND THE WARM FRONT OF A FRONTAL SYSTEM IMMEDIATELY UPSTREAM WILL
SPREAD PRECIPITATION ACROSS MOST OR ALL OF THE FORECAST AREA. THE
CORE OF THIS FEATURE WILL BE FOCUSED ON VANCOUVER ISLAND FRIDAY
NIGHT THROUGH SATURDAY NIGHT...BUT IT WILL BRING RAIN AT TIMES TO
PARTS OF THE FORECAST AREA -- ESPECIALLY THE WEST AND NORTH PARTS --
SATURDAY AND SATURDAY NIGHT.

THE COLD FRONT OF THIS SYSTEM WILL THEN WEAKEN AND MOVE ACROSS THE
REGION SUNDAY AND SUNDAY NIGHT. THE COLD FRONT WILL BRING MORE
WIDESPREAD PRECIPITATION TO THE AREA AS IT MOVES THROUGH. THE SNOW
LEVEL AHEAD OF THE WARM FRONT WILL RISE TO 7000 FT OR HIGHER DURING
THE WEEKEND...THEN DROP TO 3500 FT OR SO BEHIND THE COLD FRONT
SUNDAY NIGHT. SEE THE HYDROLOGY SECTION BELOW FOR MORE DETAILS
REGARDING PRECIPITATION AMOUNTS AND POSSIBLE IMPACTS.   MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE AFTERNOON
ISSUANCE OF THE FORECAST DISCUSSION -- FORECAST MODELS CONTINUE TO
SHOW A DRYING TREND EARLY NEXT WEEK...MOST LIKELY OCCURING SUN NIGHT
THROUGH MON NIGHT. FLAT RIDGING WILL BRIEFLY STALL OUR NEXT
SYSTEM...A WELCOME RESPITE FROM OUR ACTIVE WEATHER PATTERN. BY TUE
WE WILL SEE A FEW SHOWERS AS AN UPPER LEVEL TROUGH PASSES THROUGH
WESTERN WA. THIS SHORT WAVE TROUGH WILL QUICKLY EXIT TUE NIGHT. A
STRONGER...HIGH AMPLITUDE RIDGE WILL BUILD INLAND MID WEEK
ONWARDS...PROVIDING ANOTHER BREAK IN THE ACTION. BOTH THE GFS AND
EURO MATCH WELL FOR THE MOMENT. THE RIDGE MAY BREAK DOWN TOWARD THE
END OF THE WEEK. 33

&&

.HYDROLOGY...THE FLOOD WARNING FOR THE CHEHALIS RIVER AT PORTER WILL
END THIS EVENING. IT WILL REMAIN JUST BELOW FLOOD STAGE THROUGH
FRIDAY...THEN FALL SLOWLY. IT COULD STILL LOCALLY SPILL OVER SOME
ROADWAYS NEAR THE RIVER...ESPECIALLY IN THE LOWER REACHES NEAR THE
TIMES OF HIGH TIDE.

TURNING OUR ATTENTION TO THE FUTURE -- NEW FLOODING IS POSSIBLE ON
THE SKOKOMISH AND NOOKSACK RIVERS DURING THE NEXT WEEK...BUT
UNLIKELY ON OTHER RIVERS.

THE FRONTAL SYSTEM THAT WILL MOVE THROUGH WESTERN WASHINGTON TONIGHT
WILL HAVE LITTLE EFFECT ON THE RIVERS. THE NEXT SYSTEM WILL MOVE
SLOWLY THROUGH THE REGION FRIDAY NIGHT THROUGH SUNDAY...AND IT IS
THE ONE THAT WARRANTS SOME CONCERN. THE CORE OF HEAVY RAIN
ASSOCIATED WITH ITS WARM FRONT WILL BE CENTERED OVER VANCOUVER
ISLAND...BUT IT WILL STILL BRING FAIRLY SIGNIFICANT AMOUNTS OF RAIN
TO THE OLYMPIC MOUNTAINS AND THE MOUNT BAKER AREA FRIDAY NIGHT
THROUGH SATURDAY NIGHT AS IT CLIPS NORTHWEST WASHINGTON. THEN THE
COLD FRONT WILL PROGRESS EASTWARD ACROSS WESTERN WASHINGTON ON
SUNDAY.

THE UW WRF-GFS SHOWS THE HEAVIEST PRECIPITATION AMOUNTS WITH 48-HOUR
STORM TOTAL BULLSEYES OF ALMOST 5 INCHES OVER THE OLYMPICS AND MOUNT
BAKER...AND AROUND 4 INCHES OF THAT WILL PROBABLY FALL DURING A
24-HOUR PERIOD BETWEEN SATURDAY EVENING AND SUNDAY EVENING. THE SNOW
LEVEL WILL BE 7000 FT OR HIGHER DURING THE HEAVIEST PERIOD OF
PRECIPITATION.

THE HYDROLOGIC MODELS FROM THE NWS NORTHWEST RIVER FORECAST CENTER
DO NOT SHOW THE SKOKOMISH OR NOOKSACK RIVER REACHING FLOOD STAGE
FROM THIS EVENT. GIVEN THE FORECAST RAINFALL AND SNOW LEVELS...ALONG
WITH THE ANTECEDENT CONDITIONS...WE CANNOT RULE OUT THE POSSIBILITY.
AT THIS POINT WE WILL CONTINUE TO EVALUATE THE POTENTIAL...AND ON
FRIDAY WE CAN DECIDE IF FURTHER ACTION -- LIKE THE ISSUANCE OF A
FLOOD WATCH OR FLOOD POTENTIAL OUTLOOK -- IS NEEDED.

A FINAL NOTE ABOUT LANDSLIDES...THE USGS LANDSLIDE GUIDANCE
INDICATES THAT LANDSLIDES WILL BE POSSIBLE IF MODERATE TO HEAVY
RAIN OCCURS. SPECIAL WEATHER STATEMENTS COULD BE ISSUED OVER THE
NEXT FEW DAYS TO ADDRESS THAT THREAT.   MCDONNAL

&&

.AVIATION...UPPER TROUGH EXITING TO THE EAST FOR THE REMAINDER OF
TONIGHT...THEN FLAT UPPER RIDGING BUILDING INTO WRN WA ON FRI.
MODERATE TO STRONG W-NW FLOW ALOFT. AIR MASS CURRENTLY MOIST AND
UNSTABLE...BECOMING MUCH MORE STABLE AND SOMEWHAT DRIER ON FRI.

ONSHORE FLOW AND THE MOIST...UNSTABLE AIR MASS WILL SUPPORT
CONVECTIVE SHOWERS INTO EARLY FRI MORNING AND PREVAILING MVFR CONDS.
ONSHORE FLOW WILL END ABRUPTLY LATE FRI MORNING WITH DRYING OF
LOW-MID LEVEL ATMOSPHERE. CONVECTIVE SHOWERS WILL END...AND
PREVAILING VFR CONDS EXPECTED ON FRI AFTN AND EARLY EVNG.

KSEA...CONVECTIVE SHOWERS WILL REMAIN A POSSIBILITY AT KSEA UNTIL
ABOUT 15Z...BUT KSEA WILL COME INCREASINGLY INTO THE OLYMPIC RAIN
SHADOW AS FLOW ALOFT BECOMES A LITTLE NORTH OF DUE WEST OVERNIGHT
AND EARLY FRI MORNING. A DRIER AND MORE STABLE LOW-MID LEVEL AIR
MASS IS EXPECTED AFT 20Z...LEADING TO CIGS ABV 050...BUT MID-HIGH
CLOUDS WILL START THICKENING AFT 20Z. SW WINDS UNTIL 20Z...BECOMING
SE ON FRI AFTN.     HANER

&&

.MARINE...STRONG ONSHORE FLOW AND SOUTHWESTERLY GRADIENTS ARE
UNDERWAY IN THE WAKE OF A STRONG COLD FRONT WHICH PASSED THE WRN WA
WATERS EARLIER TODAY. ALL GUIDANCE BRINGS WINDS SPEEDS DOWN QUICKLY
OVERNIGHT FOR ALL WATERS EXCEPT THE STRAIT...WHERE THE WESTERLY
SURGE OF GALE FORCE WINDS WILL CONTINUE THROUGH THE NIGHT. GIVEN
CURRENT OBS AND AN ANTICIPATED DOWNWARD TREND ON WIND SPEEDS...HAVE
CANCELLED ALL STORM WARNINGS...REPLACING THEM WITH GALES. HAVE
DOWNGRADED THE GALE FOR PUGET SOUND TO A SMALL CRAFT ADVISORY.

WEAK HIGH PRES WILL GIVE MUCH IMPROVED CONDITIONS TO THE AREA ON
FRIDAY. BUT SWELL FROM THURSDAYS SYSTEM OF 22 FT ON THE COAST WILL
WAIT TO SUBSIDE ON FRI AFTN...RESULTING IN HAZARDOUS SEAS.

A STRONG WARM FRONT WITH GALES...ESPECIALLY ALONG THE COAST...WILL
AFFECT THE COASTAL WATERS AND NORTHERN PORTIONS OF THE INLAND WATERS
FRIDAY NIGHT. GALES WILL LIKELY CONTINUE BEHIND THE WARM FRONT ALONG
THE COAST...AT THE ENTRANCES TO THE STRAIT...AND IN THE NORTHERN
INLAND WATERS IN THE WARM SECTOR SATURDAY. A STRONG COLD FRONT WILL
MOVE EAST ACROSS THE WATERS SATURDAY NIGHT OR EARLY SUN.

SEAS IN THE COASTAL WATERS 22-24 FT THROUGH EARLY FRI MORNING WILL
SUBSIDE MIDDAY FRI. SEAS NEAR 20 FT MAY REDEVELOP FRI NIGHT AND
CONTINUE AT TIMES THROUGH SUN MORNING WITH THE PERSISTENT SOUTHERLY
GALES AHEAD OF THE INCOMING FRONTAL SYSTEMS.

HIGH PRES WILL BUILD INTO THE AREA FOR MAINLY TRANQUIL CONDITIONS
MON INTO TUE.   HANER/ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING NORTH AND CENTRAL CASCADES UNTIL
       10 AM PST FRIDAY.
     WINTER WEATHER ADVISORY OLYMPICS UNTIL 4 AM PST FRIDAY.
     HIGH SURF ADVISORY FOR THE COAST.
PZ...GALE WARNING ALL WATERS EXCEPT PUGET SOUND.
     SMALL CRAFT ADVISORY FOR PUGET SOUND AND HOOD CANAL.
     SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR SEVERE BAR CONDITIONS.

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.

WWW.WEATHER.GOV/SEATTLE