Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 270011
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
410 PM PST WED NOV 26 2014

.SYNOPSIS...A FRONT WILL MOVE EAST ACROSS WESTERN WASHINGTON TONIGHT
FOR MORE RAIN AND WINDY CONDITIONS. RAIN WILL CONTINUE ON
THANKSGIVING DAY AS A COLD FRONT APPROACHES FROM THE NORTH THEN
CROSSES THE AREA ON FRIDAY. A COLD UPPER LEVEL TROUGH WILL MOVE
OVER THE AREA FRIDAY NIGHT AND SATURDAY FOR A CHANCE OF
SHOWERS...POSSIBLY MIXED WITH SNOW. COOLER AND DRY NORTHWEST FLOW
ALOFT AND A SURFACE HIGH OVER BRITISH COLUMBIA WILL COMBINE FOR COOL
DRY WEATHER AND MORE SUNSHINE SUNDAY THROUGH EARLY NEXT WEEK.

&&

.SHORT TERM...IR IMAGERY SHOWS THE FRONT OFFSHORE ALONG 130W AT 23Z
MOVING STEADILY TOWARD THE COAST. MODELS SEEM TO HAVE A GOOD HANDLE
ON THIS SYSTEM...WITH FROPA ON THE COAST AROUND MIDNIGHT AND OVER
PUGET SOUND BEFORE BEFORE 12Z/4 AM. SE PRESSURE GRADIENTS WILL
INCREASE THIS EVENING AHEAD OF THE FRONT AND LOW END WIND ADVISORY
WINDS 20-35G45 MPH MAY DEVELOP OVER THE N COAST AND PARTS OF THE N
INTERIOR. WINDS WILL EASE BEHIND THE FRONT LATE TONIGHT.

THE FRONT WILL ALSO RAISE STRONG SW LOW LEVEL WINDS OF 35-45 KT
AROUND 850 MB WHICH SHOULD BRING HEAVY OROGRAPHIC RAIN TO THE
OLYMPICS AND N CASCADES. AFTER A BRIEF LULL BEHIND TONIGHTS
FRONT...A COLD FRONT WILL MOVE S ALONG THE B.C. COAST BRINGING RAIN
AT TIMES TO W WA AHEAD OF IT THROUGH THANKSGIVING. THE COLD FRONT
WILL MOVE SLOWLY ACROSS W WA ON FRIDAY. THIS COLD FRONT IS ACTUALLY
THE OLD WARM FRONT THAT STALLED OVER SOUTHERN B.C. TODAY. A STRONG
COLD UPPER LEVEL SHORTWAVE TROUGH MOVING DOWN THE B.C. COAST WILL
SHOVE THE BAROCLINIC BAND BACK SOUTHWARD AND ACROSS W WA AS A COLD
FRONT.

SW LOW LEVEL FLOW WILL BECOME STRONG AGAIN AS THE COLD FRONT
APPROACHES THE AREA ON THURSDAY...EXTENDING THE PERIOD OF HEAVY
OROGRAPHIC RAIN OVER THE MOUNTAINS INTO FRIDAY. THE HEAVY RAIN WILL
EASE OFF FROM THE NORTH AS THE FRONT MOVES THROUGH. 48 HOUR RAINFALL
AMOUNTS FROM 00Z THURSDAY/WED AFTERNOON THROUGH 00Z SATURDAY/FRIDAY
AFTERNOON...WILL BE AROUND 4 INCHES FOR THE OLYMPICS AND 3-4.5
INCHES FOR THE NORTH CASCADES...EXCEPT FOR A BULLSEYE AROUND 6
INCHES AROUND MT BAKER. THIS MAY BRING SOME OF THE RIVERS FLOWING
OFF THE N CASCADES AND POSSIBLY PARTS OF THE CENTRAL CASCADES TO
FLOOD STAGE THURSDAY NIGHT INTO FRIDAY. SEE THE HYDROLOGY SECTION
BELOW FOR MORE DETAILS.

THE MILD AIR MASS OVER THE AREA WILL RESULT IN TEMPERATURES IN THE
50S.

ONCE THE FRONT MOVES THROUGH ON FRIDAY...THE COLD UPPER LEVEL TROUGH
WILL MOVE OVERHEAD. THE AIR MASS WILL COOL ENOUGH SO THAT SNOW
LEVELS OVER THE LOWLAND LOWER TO 500-1000 FEET LATE FRIDAY NIGHT.
RESIDUAL MOISTURE FROM THE COLD FRONT WILL REMAIN OVER THE AREA AND
THE TROUGH WILL PROVIDE SOME INSTABILITY AND LIFT. A PUGET SOUND
CONVERGENCE ZONE WILL LIKELY SET UP FRIDAY NIGHT AND EXTEND INTO
SATURDAY MORNING. SOME LIGHT SNOW IS POSSIBLE LATE FRIDAY NIGHT OVER
THE N INTERIOR...OVER THE INTERIOR AREAS OF THE N COAST AND WESTERN
STRAIT...NEAR THE CASCADES AND POSSIBLY IN THE CONVERGENCE ZONE.
SNOW LEVELS WILL LOWER TO 500-1000 FEET IN THESE AREAS. SNOW LEVELS
WILL LOWER TO BELOW 500 FEET THROUGHOUT W WA SATURDAY MORNING. WITH
THE TROUGH STILL OVERHEAD THERE WILL BE A CHANCE OF SNOW SHOWERS.
SNOW ACCUMULATIONS WILL BE LESS THAN AN INCH...BUT THERE IS ALWAYS A
CHANCE FOR A LITTLE MORE IN A CONVERGENCE ZONE. KAM

.LONG TERM...A STRONG SURFACE HIGH SETTLING OVER B.C. ON FRIDAY WILL
ALLOW COLD FRASER RIVER VALLEY OUTFLOW TO DEVELOP...BRINGING MORE
COLD AIR DRY S OVER W WA. THE AIR MASS WILL DRY OUT SATURDAY NIGHT
WITH TEMPERATURES DROPPING INTO THE 20S SUNDAY MORNING. SKIES WILL
BE MOSTLY SUNNY. THE FRASER VALLEY OUTFLOW WILL PROBABLY GENERATE
ADVISORY LEVEL NE WINDS OVER WHATCOM AND SAN JUAN COUNTY FRIDAY
NIGHT INTO SATURDAY MORNING.

THE ECMWF HAS LEANED MORE CONSISTENTLY THAN THE GFS IN KEEPING A
COOL DRY PATTERN OVER W WA INTO EARLY NEXT WEEK. THE LATEST ECMWF
NOW KEEPS THE PATTERN DRY THROUGH WEDNESDAY ALTHOUGH THERE ARE
SYSTEM THAT GET CLOSE. THE GFS HAD BEEN MORE AGGRESSIVE IN BRINGING
RAIN UP OVER THE ARE FROM THE S...BUT HAS BACKED AWAY FROM THIS ON
THE 12Z AND 18Z RUNS. IT MAY HAVE BEEN PREMATURE...BUT I HAVE DRIED
OUT EARLY NEXT WEEK. CONFIDENCE IS SHAKY BUT HOPE IT STAYS THAT WAY.
KAM

&&

.HYDROLOGY...A WARM FRONT HELD HEAVIER RAINFALL NORTH OF THE
WA/BRITISH COLUMBIA BORDER TODAY. THIS IS ALLOWING MOST RIVERS TO
SLOWLY RECEDE ACROSS WRN WA. RAIN AMOUNTS UP TO 5 INCHES SINCE
MONDAY IN PLACES LIKE MOUNT RAINIER AND THE
SKYKOMISH-SNOQUALMIE-TOLT-SNOHOMISH BASIN WILL STILL CAUSE SOME
RESIDUAL FLOODING...MAINLY THROUGH THIS EVENING.

ANOTHER FRONT WILL BRING RENEWED RAINFALL TO WRN WA TONIGHT AND
THURSDAY...LOCALLY HEAVY IN THE MOUNTAINS. THE OLYMPICS AND NORTH
CASCADES WILL RECEIVE THE HEAVIEST RAINFALL WITH 2 TO 4 INCHES OF
RAIN EXPECTED. 1 TO 2 INCHES MAY FALL IN THE CENTRAL CASCADES. THE
SNOW LEVEL WILL BE NEAR 7000 FEET.

A SECONDARY SURGE OF MOISTURE WILL ARRIVE THURSDAY NIGHT INTO
FRIDAY. MODELS VARY MUCH MORE ON THE DISTRIBUTION AND POTENTIAL
RAINFALL AMOUNTS WITH THIS SECOND FRONTAL SYSTEM. IN
GENERAL...HEAVIER RAIN AMOUNTS THURSDAY NIGHT AND FRIDAY SHOULD
OCCUR ALONG S/SW FACING SLOPES OF THE OLYMPICS AND N
CASCADES...ALTHOUGH THE CENTRAL CASCADES COULD ALSO SEE HEAVY
PRECIPITATION BY FRIDAY. ADDITIONAL AMOUNTS OF 2 TO 3 INCHES ARE
POSSIBLE. THIS MEANS THE 36-48 HOUR RAINFALL TOTALS FOR THE OLYMPICS
AND CASCADES MAY REACH 4 TO 7 INCHES WITH LOCALLY HIGHER
AMOUNTS...ESPECIALLY IN THE N CASCADES.

THESE AMOUNTS WILL LIKELY DRIVE SEVERAL RIVERS TO FLOOD...ALTHOUGH
WIDESPREAD MAJOR FLOODING APPEARS UNLIKELY. AT HIGHEST RISK FOR
FLOODING IS THE SKOKOMISH RIVER IN THE OLYMPICS...THE NOOKSACK AND
SKAGIT RIVERS IN THE NORTH...AND THE
STILLAGUAMISH...SNOQUALMIE...AND TOLT ACROSS CENTRAL AREAS. IF THE
HEAVIER RAIN SHIFTS FAR ENOUGH SOUTH THEN FLOODING COULD BE POSSIBLE
ON THE DESCHUTES RIVER AND THE REACH OF THE SKOOKUMCHUCK RIVER NEAR
BUCODA IN THURSTON COUNTY.

A FLOOD WATCH REMAINS IN EFFECT THROUGH FRIDAY...WHICH INCLUDES ALL
THE COUNTIES WITH RIVERS FLOWING OFF THE WEST SLOPES OF THE CASCADES
AND MASON COUNTY FOR THE POSSIBILITY OF A FLOOD ON THE SKOKOMISH.

COLDER AND DRIER WEATHER IS LIKELY THIS WEEKEND...SO RIVERS SHOULD
RECEDE AND THE FLOOD THREAT WILL END FOR THE FORESEEABLE FUTURE.

MERCER

&&

.AVIATION...THE FLOW ALOFT IS STRONG AND FROM THE WEST SOUTHWEST.
THE AIR MASS IS RATHER MOIST AND RELATIVELY STABLE AFTER THE PASSAGE
OF A WARM FRONT. CIGS ARE CURRENTLY MVFR TO VFR ACROSS THE AREA. A
COLD FRONT IS TIMED TO MOVE THROUGH THE AREA OVERNIGHT TONIGHT AND
DESTABILIZE THINGS...CIGS MAY RISE A BIT BEFORE THE FRONT PASSES
THROUGH...BUT THEN CIGS WILL DETERIORATE AFTERWARDS TO IFR OR MVFR.
SLY SURFACE WINDS WILL BE STRONG AND GUSTY WITH THE FRONT AND INTO
THE DAY THURSDAY.

KSEA...SOUTH WIND INCREASING TO 12 TO 14 KT OVERNIGHT WITH GUSTS TO
20 KT DURING AND AFTER FRONTAL PASSAGE AROUND 7 TO 9Z. CIGS WILL
CHANGE FROM VFR TO MVFR SHORTLY AFTER THE FRONTAL PASSAGE AS A LOWER
DECK OF CLOUDS MOVES IN. JSMITH

&&

.MARINE...A COLD FRONT WILL MOVE ONSHORE TONIGHT. GALE WARNINGS HAVE
BEEN ISSUED TONIGHT FOR THE COAST...ENTRANCES TO THE STRAIT...AND
NORTHERN INLAND WATERS AS THEY ARE MOST PRONE TO STRONG SOUTHERLY
WINDS THAT THE FRONT WILL BRING. MODERATE TO STRONG ONSHORE AND
SOUTHERLY FLOW WILL PREVAIL BEHIND THE FRONT ON THURSDAY...AND SMALL
CRAFT ADVISORIES HAVE BEEN ISSUED FOR MOST OF THE WATERS THROUGH
THURSDAY NIGHT.

ANOTHER COLD FRONT WILL MOVE THROUGH THE WATERS EARLY FRIDAY. THIS
FRONT WILL BE FOLLOWED BY STRONG FRASER OUTFLOW IN THE NORTHERN
INLAND WATERS AND MODERATE NORTHERLY WINDS OVER THE OTHER WATERS
LATER FRIDAY INTO SATURDAY. MODERATE OFFSHORE FLOW IS LIKELY SUNDAY
INTO MONDAY WITH HIGHER SEA LEVEL PRESSURE INLAND AND LOWER PRESSURE
OVER THE SOUTHERN COASTAL WATERS. JSMITH

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCHES AND WARNINGS ARE IN EFFECT FOR PORTIONS OF
      WESTERN WASHINGTON.
     WIND ADVISORY N COAST AND PARTS OF N INTERIOR TONIGHT.
PZ...SMALL CRAFT ADVISORY ALL WATERS.
    .SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR.
    .GALE WARNING COASTAL WATERS...ENTRANCES STRAIT OF JUAN DE
     FUCA...AND NORTHERN INLAND WATERS.

&&

$$

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