Flood Watch 15, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
935 AM PST FRI NOV 30 2012

.SYNOPSIS...A SERIES OF DISTURBANCES ROTATING AROUND A DEEP LOW
SITTING OVER THE SOUTHERN GULF OF ALASKA WILL MOVE THROUGH THE AREA
THROUGH THE MIDDLE OF NEXT WEEK.

&&

.SHORT TERM...A DEEP AND VERTICALLY STACKED LOW IS CENTERED NEAR 48N
140W THIS MORNING. SATELLITE IMAGERY SHOWS A COMPLEX SERIES OF
DISTURBANCES ROTATING AROUND THIS LOW...ONE NOW OVER WESTERN
WASHINGTON...ONE EXTENDING SOUTHEAST OF THE LOW INTO THE SOUTHERN
OREGON OFFSHORE WATERS...AND OTHERS TO THE SOUTH AND SOUTHWEST OF
THE LOW. THE MAIN CORE OF THE 300 MB JET STREAM IS AIMED AT NORTHERN
CALIFORNIA AND SOUTHERN OREGON. THE AXIS OF THE JET IS EXPECTED TO
REMAIN SOUTH OF THE FORECAST AREA FOR THE NEXT SEVERAL DAYS.

WITH WESTERN WASHINGTON TO THE NORTH OF THE JET AXIS...THE AIR MASS
WILL GENERALLY REMAIN SHOWERY AND SOMEWHAT UNSTABLE. THE FIRST
IMPULSE LIFTING NORTHEAST ACROSS THE AREA THIS MORNING IS PRODUCING
HEAVY SHOWERS AND WINDY CONDITIONS ALONG THE COAST AND ON THE
OLYMPIC PENINSULA. LIGHTNING DETECTION DATA SHOWS ISOLATED
THUNDERSTORMS OVER THE COASTAL WATERS MOVING ONTO THE NORTH CENTRAL
COAST. THIS SOMEWHAT ORGANIZED SHOWER BAND WILL LIFT NORTHEAST INTO
SOUTHERN BRITISH COLUMBIA TODAY. THE COASTAL ZONES WERE UPDATED
EARLIER TO INCLUDE ISOLATED MORNING THUNDERSTORMS. THE AIR MASS
INLAND FROM THE COAST IS A BIT MORE STABLE AND CONVECTIVE
TEMPERATURES ARE HIGHER...SO THUNDER WAS LIMITED TO THE COAST.

AFTER A BRIEF DECREASE IN SHOWERS THIS AFTERNOON...PRECIPITATION
INCREASES AND THINGS REMAIN WINDY AGAIN THIS EVENING AS INCOMING
DISTURBANCE TO THE SOUTHEAST OF THE OFFSHORE LOW DEVELOPS
NORTHEASTWARD INTO THE AREA. WILL NEED TO CONSIDER A WINTER WEATHER
ADVISORY FOR THE NORTH CASCADES TONIGHT INTO SATURDAY MORNING AS
THIS RATHER HEAVY BURST OF SHOWER ACTIVITY IS ACCOMPANIED BY SNOW
LEVELS FALLING FROM AROUND 5000 FEET TO 3500 FEET SATURDAY MORNING.

SHOWERS CONTINUE ON SATURDAY. THEN ANOTHER ORGANIZED OCCLUSION MOVES
THROUGH THE AREA LATE SATURDAY NIGHT INTO SUNDAY FOR MORE RAIN.
WINDY CONDITIONS DEVELOP SUNDAY MORNING BEHIND THE OCCLUSION AS
PRESSURES RISE FROM THE SOUTHWEST.

TEMPERATURES WILL AVERAGE 1-4 DEGREES ABOVE NORMAL FOR LATE NOVEMBER
AND EARLY DECEMBER. ALBRECHT

.LONG TERM...FROM THE OVERNIGHT LONG TERM DISCUSSION...EXTENDED
MODELS IN GOOD AGREEMENT INITIALLY THIS MORNING WITH MONDAY BEING A
TRANSITION DAY WITH THE RECENT TROUGH OFF TO THE EAST AND YET
ANOTHER COOL LARGE TROUGH MAKING ITS WAY OUT OF THE GULF OF ALASKA.
PREVIOUS RUNS WERE INDICATING A POSSIBLE ATMOSPHERIC RIVER SCENARIO
SETTING UP OVER WESTERN WASHINGTON ON TUESDAY INTO WEDNESDAY. WHILE
THE MODELS ARE STILL WET THEY CONTINUE TO BACK OFF THE ON THE IDEA
THAT THE SYSTEM WILL TAP INTO A GOOD AMOUNT OF SUB TROPICAL
MOISTURE. IN ADDITION THE PATTERN IS MORE PROGRESSIVE WITH THE FRONT
ON THE 00Z MODELS THROUGH THE AREA BY WEDNESDAY MORNING. MODEL
SOLUTIONS START TO DIVERGE WEDNESDAY WITH THE GFS BRINGING ANOTHER
ORGANIZED SYSTEM INTO THE AREA WHILE THE ECMWF TRIES TO BUILD A WEAK
RIDGE. THE PREVIOUS GFS RUNS WERE MORE IN LINE WITH  THE ECMWF
SOLUTION SO WILL STAY WITH THE CHANCE OF SHOWERS FORECAST FOR DAYS 6
AND 7. FELTON

&&

.HYDROLOGY...HEAVY SHOWERS OVERNIGHT ARE GIVING GOOD RISES TO THE
SKOKOMISH RIVER IN MASON COUNTY. PRECIPITATION WILL LULL SOMEWHAT
THIS AFTERNOON AND WILL LIKELY RESULT IN THE RIVER STAYING BELOW
FLOOD STAGE. CONCERNS RISE TONIGHT THROUGH THE WEEKEND AS ADDITIONAL
PERIODS OF HEAVY SHOWERS ADD TO PRECIPITATION AMOUNTS OVER THE SOUTH
SLOPES OF THE OLYMPICS. RIVER FORECASTS ARE BEING MADE AT THIS TIME
AND WATCHES OR WARNINGS MAY BE REQUIRED FOR THE SKOKOMISH AT SOME
POINT LATER TODAY.

SHOWERY CONDITIONS WITH SNOW LEVELS 3000-5500 FEET TODAY THROUGH
MONDAY ARE EXPECTED TO KEEP OTHER RIVERS ACROSS WESTERN WASHINGTON
BELOW FLOOD STAGE.

MODELS CONTINUE TO BACK OFF THE ATMOSPHERIC RIVER SCENARIO FOR
TUESDAY INTO WEDNESDAY OF NEXT WEEK. IN ADDITION TO CUTTING OFF THE
SUB TROPICAL MOISTURE TAP THE 00Z RUNS ARE MORE PROGRESSIVE WITH THE
FRONT WITH THE CURRENT SOLUTIONS PUTTING THE FRONT EAST OF THE AREA
BY WEDNESDAY. WHILE THE THREAT HAS LESSENED WE STILL NEED TO REMAIN
IN A MODE OF WATCHFUL WAITING. ALBRECHT/FELTON

&&

.AVIATION...MOIST SOUTHWESTERLY FLOW ALOFT WILL CONTINUE THROUGH
SATURDAY. DEEP LOW SURFACE PRESSURE OFFSHORE WILL MOVE SLOWLY NORTH
WITH SOUTHEAST GRADIENTS...STRONG AT TIMES...OVER WASHINGTON. AIR
MASS MOIST AND SLIGHTLY UNSTABLE TODAY...STABILIZING TONIGHT.
CEILINGS LARGELY VFR WITH MVFR IN SHOWERS. CONDITIONS WILL
DETERIORATE TONIGHT. THE MOUNTAINS WILL REMAIN OBSCURED.

KSEA...SHOWERS OR RAIN AT TIMES...CONDITIONS WILL AVERAGE ABOUT
OVC040 WITH SOUTHEAST WIND 8-12 KT. CHB

&&

.MARINE...A DEEP LOW PRESSURE CENTER OFFSHORE WILL MOVE SLOWLY NORTH
TODAY THROUGH SATURDAY. DISTURBANCES ASSOCIATED WITH THE LOW WILL
MOVE THROUGH THE AREA AND AS EACH DISTURBANCE ROTATES THROUGH THE
AREA PRESSURE GRADIENTS SHOULD INCREASE ENOUGH FOR WINDS TO
OCCASIONALLY RISE TO 35KT GALES OVER THE COASTAL WATERS AND SOME
EXPOSED INLAND LOCATIONS. COASTAL FLOOD ADVISORIES FOR MINOR TIDAL
OVERFLOW REMAIN IN EFFECT FOR THE INLAND WATERS THROUGH THIS
MORNING. ENDED THE FLOOD WATCH FOR THE LA PUSH AREA BECAUSE SWELLS
ARE CONSIDERABLY LESS THAN WHAT IS CONSIDERED A PROBLEM.

A SERIES OF WEATHER SYSTEMS WILL MOVE THROUGH THE AREA SUNDAY
THROUGH EARLY NEXT WEEK. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...COASTAL FLOOD ADVISORY FOR MINOR TIDAL OVERFLOW AT HIGH
      TIDE FROM PUGET SOUND NORTH TO THE CANADIAN BORDER UNTIL 10
      AM PST.

PZ...GALE WARNING COAST...EAST ENTRANCE AND NORTH INLAND WATERS.

     SMALL CRAFT ADVISORY WEST ENTRANCE...CENTRAL STRAIT OF
     JUAN DE FUCA...ADMIRALTY INLET...AND PUGET SOUND AND
     HOOD CANAL.

     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

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