Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 281109
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
310 AM PST FRI NOV 28 2014

.SYNOPSIS...A COLD FRONT WILL MOVE SOUTHEASTWARD ACROSS THE AREA
THIS MORNING. LOCALLY HEAVY RAIN IS EXPECTED AHEAD OF THIS FRONT. A
CONVERGENCE ZONE WILL CONTINUE OVER NORTHERN PORTIONS OF THE PUGET
SOUND AREA TONIGHT WITH SNOW LEVELS LOWERING TO BELOW 300 FEET
OVERNIGHT. A MODIFIED ARCTIC COLD FRONT WILL MOVE SOUTH THROUGH THE
AREA LATE TONIGHT INTO SATURDAY MORNING. SOME SNOW SHOWERS CAN BE
EXPECTED AS THIS FRONT MOVES SOUTH ACROSS THE AREA SATURDAY MORNING.
WEAK HIGH PRESSURE ALOFT AND LOW LEVEL OFFSHORE FLOW WILL BRING
MAINLY COOL AND DRY WEATHER SUNDAY THROUGH EARLY NEXT WEEK.

&&

.SHORT TERM...AT 2 AM A PACIFIC COLD FRONT DROPPING SOUTHEAST FROM
THE GULF OF ALASKA EXTENDS FROM NEAR VANCOUVER BC TO AROUND
DESTRUCTION ISLAND ON THE NORTH COAST. TEMPERATURES ARE QUITE MILD
AHEAD OF THE FRONT WITH MID 50S IN MANY LOCATIONS. OBSERVATIONS OVER
VANCOUVER ISLAND AND THROUGH THE STRAIT OF GEORGIA SHOWED
TEMPERATURES FALLING FROM THE LOWER TO MID 50S DOWN INTO THE MID 40S
IN ONE HOUR AS THE FRONT PASSED. THE FRONT WILL MOVE THROUGH THE
SEATTLE AREA DURING THE MORNING HOURS THEN SOUTHEAST OF THE AREA
AFTER NOON. MAXIMUM TEMPERATURES ARE OCCURING THIS MORNING AND WILL
FALL INTO THE MID 40S AS THE FRONT PASSES. LOCALLY HEAVY RAIN IS
FALLING AHEAD OF AND WITH THIS FRONT. SEE THE HYDROLOGY SECTION
BELOW FOR DETAILS ON HOW THE RAIN WILL AFFECT LOCAL RIVERS.

A BRIEF LULL IN PRECIPITATION CAN BE EXPECTED BEHIND THE FRONT THIS
AFTERNOON. THE FORECAST THEN BECOMES UNCERTAIN THIS EVENING INTO
SATURDAY MORNING AS A DEEP UPPER LEVEL TROUGH DIGS OFFSHORE JUST
WEST OF VANCOUVER ISLAND THEN MOVES SOUTHEAST ACROSS THE AREA...AND
IS COMBINED WITH A SOUTHWARD MOVING MODIFIED ARCTIC FRONT COMING
DOWN FROM THE INTERIOR OF BRITISH COLUMBIA. THE 06Z GFS IS SOMEWHAT
DRY ACROSS MUCH OF THE AREA TONIGHT EXCEPT AT THE LEADING EDGE OF
THE FRASER OUTFLOW AND MODIFIED ARCTIC FRONT WHERE A BRIEF INCH OF
SNOW LOCALLY OCCURS IN THE INTERIOR. THE NAM12...CANADIAN...AND
ECMWF MODELS SHOW 1-3 INCHES OF SNOW LATE TONIGHT INTO SATURDAY
MORNING WITH A CONVERGENCE ZONE THEN THE MODIFIED ARCTIC FRONT. THE
MINOR DIFFERENCES BETWEEN THE WETTER/STRONGER ECMWF AND WEAKER/DRIER
GFS IS IN THE AMOUNT OF DIGGING OF THE UPPER LEVEL TROUGH TO THE
SOUTHWEST OVER THE COASTAL WATERS...WHICH WOULD HANG UP THE FRONTAL
BOUNDARY OVER THE CENTRAL PUGET SOUND AREA AROUND EVERETT TONIGHT
INTO EARLY SATURDAY AND PROVIDE MORE LOW LEVEL CONVERGENCE WITH THE
BOUNDARY. AT THIS TIME THE FORECAST WILL BE A BLEND OF ALL MODELS
THAT BRINGS POSSIBLE ADVISORY SNOW AMOUNTS TO THE EVERETT AND
VICINITY ZONE AND THE NORTH PORTION OF THE EAST PUGET SOUND
LOWLANDS. A MENTION OF UP TO AN INCH OF SNOW WILL BE MADE FOR THE
OTHER INTERIOR ZONES WITH THE EXCEPTION TO WHERE OUTFLOW BECOMES
STRONG AROUND THE SAN JUANS AND WESTERN WHATCOM COUNTY. WILL ALLOW
THE DAY SHIFT TO LOOK AT THE 12Z GUIDANCE BEFORE ISSUING ANY WINTER
WEATHER ADVISORIES FOR THE INTERIOR LOWLAND ZONES AND WILL FRESHEN
THE SPECIAL WEATHER STATEMENT.

CONCERNING WIND...THE BELLINGHAM-WILLIAMS LAKE PRESSURE GRADIENT HAS
FALLEN BELOW -8 MB AND CONTINUES TO RAPIDLY FALL. IT IS EXPECTED TO
REACH AROUND -20 MB MIDDAY AND CONTINUE QUITE STRONG THROUGH
SATURDAY. WITH A LAG BETWEEN THE GRADIENT AND THE WIND REACHING THE
NW INTERIOR...WINDS ARE EXPECTED TO RAPIDLY RAMP UP TO NE 25-35 MPH
WITH GUSTS TO 45-50 MPH THIS EVENING. HIGH WINDS ARE POSSIBLE IN THE
FRASER OUTFLOW AREAS...BUT ARE NOT CERTAIN YET...SO A HIGH WIND
WATCH WILL BE MAINTAINED FOR NOW. AFTER LOOKING AT THE 12Z GUIDANCE
THE NEXT SHIFT WILL HAVE A BETTER IDEA AS TO WHETHER THE WINDS WILL
BECOME DAMAGING...OR WILL REMAIN IN THE ADVISORY RANGE.

CONDITIONS WILL CLEAR AND DRY FROM THE NORTH ON SATURDAY AND IT WILL
BE QUITE COLD ACROSS THE AREA. TEMPERATURES WILL STRUGGLE TO HIT
FREEZING FROM SEATTLE SOUTHWARD AND WILL LIKELY REMAIN BELOW
FREEZING NORTH OF SEATTLE ON SATURDAY. AFTER A HARD FREEZE SATURDAY
NIGHT...TEMPERATURES WILL BEGIN A SLOW MODERATION STARTING ON
SUNDAY. ALBRECHT

.LONG TERM...NORTHERN STREAM SYSTEMS DROPPING SOUTHEAST FROM THE
GULF OF ALASKA...OR SOUTHERN STREAM MOISTURE WRAPPING NORTHWARD FROM
OFF CALIFORNIA. MODELS STILL CANNOT COME TO ANY AGREEMENT OVER THE
PACIFIC NORTHWEST WHERE FLOW WILL REMAIN SOMEWHAT CONFLUENT. THE
ECMWF...NAM12 AND CANADIAN SOLUTIONS WEAKEN NORTHERN STREAM SYSTEMS
BEFORE THEY MOVE INTO WASHINGTON DURING THE PERIOD. THE 06Z GFS IS
AGGRESSIVE IN BRINGING SOUTHERN STREAM MOISTURE INTO SW WASHINGTON
MON-THU RAIN THERE AND MORE CLOUDS TO THE NORTH. FORECASTS FOR NOW
WILL BE KEPT DRY WITH PARTLY CLOUDY CONDITIONS AND TEMPERATURES WILL
BE FORECAST TO SLOWLY MODERATE TOWARD NORMAL. ALBRECHT

&&

.HYDROLOGY...THERE WAS A RELATIVE LULL IN THE RAINFALL ON
THURSDAY...WITH UP TO 2 INCHES OF RAIN OVER THE OLYMPICS AND NORTH
CASCADES NEAR MOUNT BAKER AND GENERALLY LESS THAN 1.2 INCHES IN THE
CASCADES FURTHER SOUTH. THAT WAS LESS THAN WE HAD FORECAST.

A STRONG COLD FRONT IS MOVING INTO THE NORTH PART OF WESTERN
WASHINGTON FROM THE NORTH EARLY THIS MORNING. IT WILL MOVE RATHER
QUICKLY SOUTH THROUGH THE AREA TODAY...AND THEN SOUTHEAST OF
WASHINGTON THIS EVENING. THE FRONTAL BAND WILL BRING ABOUT A 12 HOUR
PERIOD OF HEAVY RAIN TO THE CASCADES AS IT MOVES THROUGH...WHICH HAS
ALREADY STARTED OVER THE NORTH CASCADES. THAT IS NOTHING EXCEPTIONAL
WHEN WE THINK OF SYSTEMS THAT COULD PRODUCE FLOODING...BUT MOST OF
THE RIVERS -- ESPECIALLY FROM THE SNOQUALMIE NORTHWARD -- ARE
ALREADY RUNNING HIGH. SO IT COULD BE ENOUGH TO DRIVE SOME OF THEM
ABOVE FLOOD STAGE. WHERE FLOODING DOES OCCUR...IT WILL PROBABLY BE
RELATIVELY MINOR.

IN THE CASCADES OF WHATCOM AND SKAGIT COUNTIES...RAIN INCREASED
OVERNIGHT WITH THE ARRIVAL OF THE COLD FRONT AND GENERALLY 0.5 TO
1.5 INCHES FELL IN THE PAST 6 HOURS. THIS AREA...WHICH AFFECTS THE
NOOKSACK AND SKAGIT RIVERS...WILL ONLY RECEIVE HYDROLOGICALLY
SIGNIFICANT RAIN FOR ANOTHER 6 HOURS OR SO...BUT IT SHOULD BE AS
HEAVY OR A LITTLE HEAVIER THAN THE LAST 6 HOURS. THE SKAGIT...WHERE
A FLOOD WARNING IS IN EFFECT...IS RISING AND CLOSE TO REACHING FLOOD
STAGE. THE NOOKSACK IS RISING SLUGGISHLY...BUT IT IS WITHIN A COUPLE
FEET OF FLOOD STAGE. BEING FURTHER NORTH...THE NOOKSACK BASIN MIGHT
NOT GET ENOUGH RAIN THIS MORNING TO PUSH THE RIVER ABOVE FLOOD
STAGE. FOR NOW IT REMAINS IN THE WATCH AREA...BUT A FLOOD WARNING
COULD BE ISSUED ANYTIME THIS MORNING.

OVER THE CASCADES OF SNOHOMISH AND KING COUNTIES THERE HAS BEEN 0.4
TO 1.2 INCHES OF RAIN IN THE PAST 6 HOURS. THE RIVERS HERE INCLUDE
THE STILLAGUAMISH...SKYKOMISH...SNOQUALMIE...TOLT...AND SNOHOMISH
RIVERS. THIS AREA SHOULD HAVE HEAVY RAIN THROUGH EARLY AFTERNOON...
SO ANOTHER 10 HOURS OR SO...WITH 1 TO 2 INCHES OF RAIN LIKELY. THE
RIVERS ARE ALL RUNNING FAIRLY HIGH AND AT LEAST THEIR UPPER REACHES
ARE RISING. THE SKYKOMISH LOOKS LIKE A GOOD BET TO FLOOD THIS
MORNING...AND ANY OF THE OTHERS COULD FLOOD AS WELL. FLOOD WARNINGS
COULD BE ISSUED FOR ANY OF THESE RIVERS THIS MORNING.

THE CASCADES OF PIERCE AND LEWIS COUNTIES...WHICH INCLUDES THE MOUNT
RAINIER AREA...WILL ALSO RECEIVE AROUND 1 TO 2 INCHES DURING THE
NEXT 12 HOURS. HOWEVER THIS REGION HAS HAD 1 INCH OR LESS OF RAIN
DURING THE PAST 24 HOURS...AND RIVERS HERE HAVE RECEDED MORE THAN
THE OTHERS. THE THREAT OF FLOODING ON THE RIVERS HERE LOOKS FAIRLY
LOW...BUT IT WILL REMAIN UNDER A WATCH FOR NOW.

THE OLYMPICS RECEIVED 0.5 TO 1.5 INCHES IN THE LAST 6 HOURS. IT WILL
PROBABLY RECEIVE A SIMILAR AMOUNT DURING THE NEXT 6 HOURS. THE
SKOKOMISH RIVER ROSE ABOVE FLOOD STAGE THURSDAY MORNING...AND IT
SHOULD CREST LATER THIS MORNING. IT REMAINS UNDER A FLOOD WARNING.

COLD AND RELATIVELY DRY WEATHER WILL FOLLOW THE COLD FRONT. RIVERS
WILL RECEDE AND THEN THE THREAT OF FLOODING WILL END FOR THE
FORESEEABLE FUTURE.

REGARDING THE POSSIBILITY OF LANDSLIDES...THE US GEOLOGICAL SURVEY
PROVIDES LANDSLIDE GUIDANCE THAT SUGGESTS THAT THE THREAT IS LOW.
THAT IS MAINLY DUE TO THE FACT THAT WE HAD ABOUT 10 DAYS WITH NO
PRECIPITATION BEFORE THIS WET PERIOD. BUT I THINK IT IS WORTH
MENTIONING THAT MODELS INDICATE THAT THE FRONT COULD PRODUCE 6 HOUR
RAINFALL AMOUNTS IN EXCESS OF 1 INCH OVER THE PUGET SOUND LOWLANDS
TODAY.   MCDONNAL

&&

.AVIATION...A COLD FRONT WILL SHIFT SOUTH THROUGH WESTERN WA THIS
MORNING BRINGING MODERATE RAIN AND GUSTY WINDS. THE FLOW ALOFT IS
WESTERLY. MVFR CONDITIONS ARE LIKELY. THIS FRONT WILL EXIT SOUTH
THIS AFTERNOON WITH STEADY RAIN TAPERING OFF.

HIGH PRES OVER INTERIOR B.C. WILL BRING STRONG NE FRASER RIVER
OUTFLOW WINDS TO THE NORTH INTERIOR TONIGHT. MEANWHILE A COOL AIR
MASS WILL SETTLE OVER THE REGION TONIGHT THROUGH SATURDAY AFTERNOON
AS AN UPPER LEVEL TROUGH TRACKS OVERHEAD. THIS SYSTEM WILL LIKELY
BRING LOW LEVEL SNOW TO THE AREA...ESPECIALLY THE STRAIT OF JUAN DE
FUCA AND INTERIOR. THE UPPER TROUGH WILL EXIT LATE SAT AFTERNOON OR
EVENING FOR DRY WEATHER ACROSS WESTERN WA. 33

KSEA...RAIN CONTINUES ACROSS THE REGION THIS MORNING AS A COLD FRONT
CROSSES FROM NORTH TO SOUTH. MVFR CONDITIONS WILL CONTINUE THROUGH
THIS MORNING. THIS FRONT WILL ALSO BRING A WIND SHIFT WITH WINDS
BECOMING NLY BY 18Z. WINDS WILL FLIP BACK TO SOUTHERLY AROUND 00Z
THIS EVENING. LATE TONIGHT INTO SATURDAY...THERE IS A CHANCE FOR
SNOW AT SEA-TAC WITH SURFACE TEMPS NEAR FREEZING. AT THIS POINT WE
ARE LOOKING AT ACCUMULATIONS OF AN INCH OR LESS FROM 12-18Z SAT. 33

&&

.MARINE...A STRONG COLD FRONT WILL SHIFT SOUTH THROUGH WESTERN WA
THIS MORNING. SMALL CRAFT ADVISORIES ARE IN EFFECT FOR ALL WATERS.
WINDS WILL FLIP FROM SOUTH TO NORTH AS THIS FRONT PUSHES THROUGH
PUGET SOUND. WINDS WILL THEN TURN BACK TO SOUTHERLY ACROSS THE
INTERIOR LATE THIS AFTERNOON. MODELS CONTINUE TO SHOW FRASER RIVER
OUTFLOW WINDS DEVELOPING THIS EVENING WITH NE GALES OVER THE
NORTHERN INLAND WATERS. GALES WILL CONTINUE THROUGH AT LEAST SAT
AFTERNOON...THEN EASE AS STRONG HIGH PRES OVER INTERIOR B.C. SHIFTS
EAST. OFFSHORE FLOW WILL PREVAIL SAT NIGHT THROUGH EARLY NEXT WEEK
WITH HIGH PRES EAST OF THE CASCADES. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCHES AND WARNINGS IN EFFECT FOR PORTIONS OF
      WESTERN WASHINGTON.
     HIGH WIND WATCH WESTERN WHATCOM...WESTERN SKAGIT AND THE
      SAN JUAN ISLANDS TONIGHT AND SATURDAY.

PZ...GALE WARNING NORTHERN INLAND WATERS.
     SMALL CRAFT ADVISORY REMAINING WATERS.
     SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR.

&&

$$

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