Small Craft Advisory 879, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PST THU DEC 29 2011

.SYNOPSIS...THE LAST IN THIS SERIES OF FRONTAL SYSTEMS WILL MOVE
THROUGH THE AREA TONIGHT AND FRIDAY BRINGING LOWLAND RAIN...MOUNTAIN
SNOW...AND LOCALLY WINDY CONDITIONS. STRONG WESTERLY FLOW ALOFT WILL
KEEP THE SNOW GOING IN THE MOUNTAINS BEHIND THE FRONT FRIDAY
AFTERNOON INTO FRIDAY EVENING. HIGH PRESSURE WILL BRING A QUIETER
WEATHER PATTERN THIS WEEKEND AND EARLY NEXT WEEK.

&&

.SHORT TERM...A STRONG WEST-SOUTHWEST JET WITH A 160 KT CORE EXTENDS
FROM THE PACIFIC NORTHWEST INTO THE NORTHEAST PACIFIC OCEAN. THE
LEADING EDGE OF THE HIGH CLOUD SHIELD ASSOCIATED WITH THE NEXT
FRONTAL SYSTEM IS MOVING INTO WESTERN WASHINGTON THIS MORNING. A
WEAK PUGET SOUND CONVERGENCE ZONE FORMED LAST NIGHT BEHIND THE FRONT
THAT MOVED THROUGH...AND SHOWERS ARE STILL FALLING IN A NARROW BAND
FROM THE NORTHEAST CORNER OF THE OLYMPIC PENINSULA ACROSS SNOHOMISH
COUNTY.

A WEAK RIDGE -- BOTH AT THE SURFACE AND ALOFT -- WILL MOVE QUICKLY
ACROSS WESTERN WASHINGTON WITHIN THE NEXT FEW HOURS...AND
PRECIPITATION WITH THE NEXT VIGOROUS FRONTAL SYSTEM WILL BEGIN
SPREADING INLAND RIGHT BEHIND IT. THE OCCLUDED FRONT WILL SWING
THROUGH THE FORECAST AREA FROM THE SOUTHWEST THIS EVENING. THEN
SURFACE HIGH PRESSURE WILL BUILD STRONGLY FROM THE SOUTHWEST IN THE
WAKE OF THE FRONT.

THIS SYSTEM WILL BRING ANOTHER ROUND OF SIGNIFICANT PRECIPITATION.
STORM TOTAL PRECIPITATION FOR THE 36-HOUR PERIOD THIS AFTERNOON
THROUGH FRIDAY EVENING WILL BE AROUND 0.50 TO 1.50 INCH FOR THE
WESTERN WASHINGTON LOWLANDS. PRECIPITATION IN THE MOUNTAINS WILL BE
1 TO 3 INCHES OF WATER EQUIVALENT...WITH THE SNOW LEVEL GRADUALLY
FALLING FROM AROUND 4000 FT WITH THE FRONT THIS EVENING TO AROUND
2000 FT FRIDAY. HEAVY SNOW -- 12 INCHES IN 12 HOURS OR 18 INCHES IN
24 HOURS -- STILL LOOKS LIKE A GOOD BET...ESPECIALLY AT MOUNT BAKER
AND PARADISE BUT POSSIBLY AT STEVENS PASS AS WELL. A WINTER STORM
WARNING IS ALREADY IN EFFECT FOR THIS.

THE FRONTAL SYSTEM WILL ALSO BRING ANOTHER ROUND OF WINDY
CONDITIONS...FIRST WITH THE OCCLUDED FRONT THIS EVENING AND
ESPECIALLY AS SURFACE HIGH PRESSURE BUILDS IN ITS WAKE LATER TONIGHT
THROUGH FRIDAY. AT THIS TIME IT LOOKS LIKE WINDS WILL REMAIN JUST
BELOW HIGH WIND WARNING CRITERIA -- IE SUSTAINED 40 MPH OR GUSTS 60
MPH -- AND WE ARE LEANING TOWARD A WIND ADVISORY FOR MUCH OF WESTERN
WASHINGTON. THE FINAL DECISION WILL BE MADE FOR THE AFTERNOON
FORECAST PACKAGE.

SHOWERS WILL DECREASE ACROSS WESTERN WASHINGTON FRIDAY NIGHT...AND A
HIGHER AMPLITUDE UPPER RIDGE WILL BUILD AS IT MOVES INTO THE PACIFIC
NORTHWEST ON SATURDAY. SURFACE HIGH PRESSURE BUILDING OVER THE
NORTHERN AND CANADIAN ROCKIES WILL PRODUCE OFFSHORE LOW LEVEL FLOW.
ASIDE FROM LINGERING LIGHT SHOWERS IN THE MORNING...SATURDAY WILL BE
A GENERALLY DRY DAY AND THE START OF A PERIOD OF LESS EVENTFUL
WEATHER.   MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE EARLY MORNING
ISSUANCE OF THE FORECAST DISCUSSION -- WHILE THE EXTENDED MODELS
THIS MORNING ARE SHOWING A MUCH LESS ACTIVE PERIOD OF WEATHER FOR
WESTERN WASHINGTON...THERE IS STILL A LACK OF CONSISTENCY IN THE
DETAILS. THE GFS CONTINUES WITH THE IDEA OF A WARM FRONT MOVING UP
FROM THE SOUTH SATURDAY NIGHT INTO SUNDAY MORNING WHILE THE ECMWF IS
WEAKER AND FURTHER WEST WITH THE FRONT. THERE IS SOME AGREEMENT ON A
WEAKENING FRONTAL SYSTEM MOVING THROUGH THE AREA ON MONDAY. AFTER A
BRIEF BREAK THERE IS ALSO SOME AGREEMENT ON AN ATMOSPHERIC RIVER
SCENARIO SETTING UP WITH THE HEAVIEST PRECIPITATION AIMED AT
VANCOUVER ISLAND WITH WESTERN WASHINGTON. FELTON

&&

.HYDROLOGY...A FLOOD WARNING REMAINS IN EFFECT FOR THE SKOKOMISH
RIVER. THE FRONTAL SYSTEM THAT MOVED THROUGH THE AREA LATE WEDNESDAY
THROUGH WEDNESDAY NIGHT BROUGHT ANOTHER 1 TO 3 INCHES OF
PRECIPITATION TO THE OLYMPICS...AND DROVE THE SKOKOMISH RIVER NEAR
POTLATCH TO A CREST JUST OVER 17 FT. PRECIPITATION WITH THE NEXT
SYSTEM WILL PROBABLY AMOUNT TO ANOTHER 1 TO 3 INCHES THIS AFTERNOON
THROUGH FRIDAY EVENING...AND IT IS A SAFE BET THAT THIS WILL KEEP
THE SKOKOMISH NEAR OR ABOVE FLOOD STAGE THROUGH FRIDAY.

THIS IS NOT THE TYPICAL FLOOD PRODUCING WEATHER PATTERN FOR WESTERN
WASHINGTON...HOWEVER THE ACCUMULATION OF RAINFALL OVER SUCCESSIVE
STORMS COULD PRODUCE MINOR FLOODING ON SOME OF THE MOST FLOOD-PRONE
RIVER REACHES...AND A FLOOD WATCH WAS ISSUED WEDNESDAY AFTERNOON FOR
THE PUYALLUP RIVER NEAR ORTING AND THE SNOQUALMIE RIVER NEAR
CARNATION. THE PUYALLUP NEAR ORTING CRESTED BELOW FLOOD STAGE LAST
NIGHT AND THE SNOQUALMIE NEAR CARNATION WILL CREST BELOW FLOOD STAGE
MIDDAY TODAY. WE WILL HANG ON TO THE FLOOD WATCH FOR NOW TO CONSIDER
THE POSSIBILITY THAT PRECIPITATION TONIGHT AND FRIDAY WILL CAUSE
MINOR FLOODING.

A DRIER WEATHER PATTERN WILL PREVAIL SATURDAY THROUGH NEXT
TUESDAY...AND THE THREAT OF FLOODING ACROSS THE ENTIRE FORECAST AREA
WILL EASE. BY WEDNESDAY A STRONG JET CARRYING LOTS OF MOISTURE WILL
EXTEND SOUTHWEST FROM VANCOUVER ISLAND INTO THE NORTHEAST PACIFIC
OCEAN. IF THIS FEATURE SHIFTS FROM VANCOUVER ISLAND INTO WESTERN
WASHINGTON THEN IT COULD RENEW A THREAT OF AT LEAST MINOR FLOODING
ON SOME OF THE MORE FLOOD-PRONE RIVERS.   MCDONNAL

&&

.AVIATION...STRONG WESTERLY JET WILL REMAIN OVER THE AREA THROUGH
FRIDAY. SURFACE PRESSURE GRADIENTS HAVE RELAXED CONSIDERABLY TODAY
IN A LULL BETWEEN WEATHER SYSTEMS. AIR MASS IS MOIST AND STABLE.
NEXT FRONT WILL ARRIVE TONIGHT WITH INCREASING WIND ALL TERMINALS.
MVFR CEILINGS SHOULD LIFT TO VFR LATE MORNING INTO AFTERNOON...THEN
MVFR CEILINGS WILL RETURN AFTER 03Z WITH RAIN SPREADING INLAND.

KSEA...DISCUSSION ABOVE APPLIES. SOUTHERLY WIND 8-12 KT TODAY WILL
INCREASE TO 10G20 KT TONIGHT...THEN BECOME SOUTHWEST AND RISE TO
15G30 KT FRIDAY. CHB

&&

.MARINE...SMALL CRAFT ADVISORY STRENGTH WINDS HAVE ENDED THIS
MORNING. UNDER BRIEF HIGH PRESSURE WINDS WILL REMAIN UNDER 20 KT
INTO THE AFTERNOON TODAY.

NEXT SYSTEM BARRELS ONTO THE COAST LATE IN THE AFTERNOON. THIS
SHOULD BRING WIDESPREAD SOUTHEASTERLY GALES OR NEAR GALES THIS
EVENING. STRONG ONSHORE FLOW FOLLOWS FRIDAY INTO FRIDAY NIGHT. THIS
COULD ALSO BRING GALES TO MOST IF NOT ALL THE WATERS.

HIGH PRESSURE AND OFFSHORE FLOW SETS UP OVER THE WEEKEND. ON
SATURDAY BIG SWELLS...CURRENTLY FORECAST TO BE IN THE 20-25 FOOT
RANGE...WILL ARRIVE ON THE COAST. ON SUNDAY A WEAK WARM FRONT WILL
MOVE SOUTH TO NORTH OVER WESTERN WASHINGTON. OFFSHORE FLOW COULD BE
STRONG ENOUGH TO BRING GALES TO THE WEST ENTRANCE. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING NORTH AND CENTRAL WASHINGTON CASCADES
     TONIGHT THROUGH FRIDAY EVENING.
     FLOOD WATCHES AND/OR WARNINGS FOR PORTIONS OF THE
     FORECAST AREA.
PZ...GALE WARNING COASTAL ALL WATERS EXCEPT PUGET SOUND TONIGHT.
     SMALL CRAFT ADVISORY STRAIT...ADMIRALTY INLET...AND
      NORTHERN INLAND WATERS THIS AFTERNOON AND EVENING.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

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