Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 271122
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PST THU NOV 27 2014

.SYNOPSIS...MOIST ONSHORE FLOW WILL GIVE PERIODS OF RAIN TO THE AREA
TODAY. A COLD FRONT WILL MOVE SOUTHEAST ACROSS THE AREA FRIDAY
MORNING GIVING FALLING TEMPERATURES DURING THE AFTERNOON. A STRONG
COLD FRONT WILL MOVE SOUTH THROUGH THE AREA FRIDAY NIGHT INTO
SATURDAY. SOME SNOW OR RAIN AND SNOW MIXED IS LIKELY TO OCCUR ALONG
THIS SECOND FRONT LATE FRIDAY NIGHT INTO SATURDAY. STRONG FRASER
OUTFLOW WILL GIVE WINDY CONDITIONS TO THE NORTH INTERIOR LATE FRIDAY
NIGHT INTO SATURDAY. COOL AND MAINLY DRY WEATHER IS EXPECTED SUNDAY
THROUGH EARLY NEXT WEEK.

&&

.SHORT TERM...VERY ACTIVE AND CHANGEABLE WEATHER IS EXPECTED OVER
THE AREA DURING THE SHORT TERM PERIOD.

A FRONTAL FEATURE MOVED INLAND THROUGH THE AREA EARLY THIS MORNING.
WHILE IT HAS BEEN WINDY ACROSS THE NORTH INTERIOR AND ALONG THE
NORTH COAST...WINDS HAVE BEEN BELOW WIND ADVISORY CRITERIA. WILL
ALLOW THE WIND ADVISORY TO EXPIRE AT 4 AM PST.

WHILE A WEAK FRONTAL SYSTEM IS MOVING ACROSS THE AREA EARLY THIS
MORNING...THE AIR MASS BEHIND THE FRONT IS ORIGINATING FROM THE
SUBTROPICAL CENTRAL PACIFIC. SO...WE CAN EXPECT A CONTINUATION OF
THE RATHER WARM AND MUGGY AIR MASS THAT WE HAVE SEEN OVER THE PAST
24 HOURS WITH TEMPERATURES REMAINING NEAR STEADY IN THE 50S THROUGH
TONIGHT. STRONG ONSHORE PRESSURE GRADIENTS AND THE HIGH MOISTURE
CONTENT OF THE LOW LEVEL AIR MASS - WITH DEWPOINTS AROUND 50 - IS
RESULTING IN GOOD OROGRAPHIC PRECIPITATION AMOUNTS ON THE WEST
SLOPES OF THE MOUNTAINS. LITTLE CHANGE IN THE OVERALL PATTERN IS
EXPECTED THROUGH TONIGHT...WITH POSSIBLY SOME INCREASE IN ONSHORE
FLOW TONIGHT AS A WAVE OF LOW PRESSURE MOVES BY TO THE NORTH OF THE
AREA. SEE THE HYDROLOGY SECTION BELOW FOR DETAILS ON FORECAST
PRECIPITATION IN THE AREA AND ITS IMPACT ON LOCAL RIVER.

BIG CHANGES BEGIN TO OCCUR FRIDAY MORNING. A WEAK UPPER LEVEL
TROUGH THAT IS PASSING SOUTHWARD THROUGH THE ALASKAN PANHANDLE THIS
MORNING IS FORECAST BY ALL MODELS TO SHARPEN AND DIG INTO WESTERN
WASHINGTON FRIDAY AFTERNOON. THIS WILL INTENSIFY THE FRONTAL SYSTEM
THAT NOW SITS OVER THE QUEEN CHARLOTTE SOUND AND SWEEP IT SOUTHEAST
ACROSS THE AREA FRIDAY MORNING. STRONG HEIGHT FALLS AT 500 MB BEGIN
FRIDAY MORNING AND CONTINUE THROUGH FRIDAY NIGHT...FALLING FROM NEAR
550 DAM TO AROUND 520 DAM PER THE 00Z ECMWF SOLUTION. THE GFS SHOWS
HEIGHT FALLS A BIT WEAKER THAN THE ECMWF...BUT IS STILL VERY
IMPRESSIVE AND IS RATHER CONSISTENT. SO A SHARP FRONT MOVES SE
ACROSS THE AREA DURING THE DAY WITH EARLY MORNING HIGH TEMPERATURES
IN MOST AREAS. PRECIPITATION AMOUNTS WITH THE FRONT PER MESOSCALE
MODELS LIKE THE UW WRFGFS ARE IMPRESSIVE FRIDAY MORNING. MOST AREAS
WILL BE BREEZY TO WINDY THROUGH FRIDAY.

A MODIFIED ARCTIC COLD FRONT WILL LIKELY REACH THE FAR NORTH
INTERIOR EARLY FRIDAY EVENING THEN TAKE ITS TIME MOVING SOUTH TO
AROUND EVERETT ABOUT MIDNIGHT FRIDAY NIGHT AND THROUGH THE SEATTLE
AREA SATURDAY MORNING. THIS FRONT WILL BE ACCOMPANIED BY A SHARP
UPPER LEVEL TROUGH AND POSSIBLY A LEE SIDE LOW THAT FORMS OFF
VANCOUVER ISLAND AND MOVES INLAND SOMEWHERE AROUND THE SOUTH
INTERIOR SATURDAY MORNING. A CONVERGENCE ZONE-LIKE FEATURE CAN BE
EXPECTED AHEAD OF THE MODIFIED ARCTIC FRONT. SNOW LEVELS WILL FALL
FRIDAY NIGHT...MOSTLY DUE TO THE COOLING ALOFT...THEN SHOULD BE NEAR
SEA LEVEL SATURDAY MORNING ALL AREAS. SNOWFALL AMOUNTS IN THE
LOWLANDS ARE HIGHLY UNCERTAIN AT THIS TIME. FORECAST MODELS RANGE
FROM ABOUT AN INCH IN THE GFS SOLUTION TO POSSIBLY 3 TO 5 INCHES
OVER PORTIONS OF THE NORTH INTERIOR DOWN TO THE NORTHERN PORTIONS OF
SEATTLE PER THE ECMWF...NAM12 AND CANADIAN SOLUTIONS. CURRENT
FORECASTS WILL BE A MODEL BLEND BETWEEN THESE AMOUNTS AND WILL
LIKELY NEED TO BE ADJUSTED AS WE GET CLOSER TO THE EVENT.

BEHIND THE MODIFIED ARCTIC FRONT...STRONG FRASER OUTFLOW WILL AFFECT
THE NORTH INTERIOR. MODELS GENERALLY AGREE THAT BELLINGHAM TO
WILLIAMS LAKE PRESSURE GRADIENTS WILL FALL TO -18 TO -23 MB. THIS
WOULD RESULT IN N TO NE WINDS 25 TO 40 MPH WITH GUSTS TO 55 MPH SAT
MORNING BELLINGHAM NORTHWARD AND IN THE SAN JUANS.

WILL ISSUE A SPECIAL WEATHER STATEMENT FOR THE RAPIDLY CHANGING
CONDITIONS...POSSIBLE LOWLAND SNOW...AND LOCALLY VERY WINDY
CONDITIONS FRIDAY THROUGH SATURDAY. ALBRECHT

.LONG TERM...AFTER A COLD MORNING ON SUNDAY...EXPECT GRADUALLY
MODERATING CONDITIONS LATER SUNDAY THROUGH EARLY NEXT WEEK. MODELS
VERY ON PRECIPITATION THREATS STARTING MONDAY WITH THE GFS BRINGING
SOUTHERN STREAM MOISTURE INTO OREGON AND FAR SOUTHERN WASHINGTON AND
THE ECMWF AND CANADIAN SOLUTIONS BRINGING A NORTHERN STREAM TROUGH
ACROSS THE AREA DURING THE SAME PERIOD. AT THIS TIME THE DRY
FORECAST WAS MAINTAINED BUT CLOUDS WERE INCREASED. ALBRECHT

&&

.HYDROLOGY...THE SNOQUALMIE RIVER NEAR CARNATION REMAINS ABOVE FLOOD
STAGE AS A RESULT OF THE HEAVY RAIN THAT FELL MONDAY AFTERNOON
THROUGH TUESDAY NIGHT...BUT IT IS FALLING AND WILL DROP BELOW FLOOD
STAGE IN A FEW HOURS.

RAIN BEGAN SPREADING INTO WESTERN WASHINGTON AHEAD OF THE NEXT
FRONTAL SYSTEM LATE WEDNESDAY. AS EXPECTED THE HEAVIEST RAIN HAS
BEEN OVER THE SOUTHWEST FACING SLOPES OF THE OLYMPICS...AND IN THE
CASCADES FROM ROUGHLY THE SNOHOMISH BASIN NORTHWARD. MANY STATIONS
THERE ARE REPORTING 0.6 TO 1.3 INCHES IN THE LAST 12 HOURS...AND
MOST OF THE RIVERS IN THE REGION HAVE STARTED RISING AGAIN.

SIGNIFICANT PRECIPITATION WILL CONTINUE THROUGH FRIDAY...WITH STRONG
OROGRAPHIC FORCING AS SOUTHWESTERLY LOW LEVEL WINDS OF 40 KT OR SO
CONTINUE. THE HEAVIEST PRECIPITATION WILL BE OVER THE OLYMPICS AND
NORTH CASCADES TODAY AND MOST OF TONIGHT...AND THEN THE FRONTAL BAND
AND HEAVIEST RAIN WILL GRADUALLY SHIFT SOUTHWARD ON FRIDAY. THE SNOW
LEVEL WILL BE 5500 TO 7000 FT THROUGH TONIGHT...AND IT SHOULD FALL
GRADUALLY ON FRIDAY AS THE COLD FRONT STARTS TO MOVE SOUTHEAST
THROUGH THE AREA.

ANOTHER 3.0 TO 5.5 INCHES OF PRECIPITATION IS LIKELY IN THE OLYMPICS
AND CASCADES FROM THE SNOHOMISH BASIN NORTHWARD THROUGH FRIDAY. THIS
COULD CAUSE THE SKOKOMISH RIVER TO FLOOD...AND IT COULD REACH FLOOD
STAGE THIS EVENING. IN THE NORTH CASCADES...THE NOOKSACK AND SKAGIT
RIVERS CAME CLOSE TO FLOODING ON WEDNESDAY...AND BOTH FACE A THREAT
OF FLOODING IN THIS EPISODE. IN ADDITION THE STILLAGUAMISH...
SKYKOMISH...TOLT...SNOQUALMIE...AND SNOHOMISH RIVERS -- ALL OF WHICH
FLOODED OR NEARLY FLOODED IN THE PAST COUPLE DAYS -- COULD FLOOD AS
WELL. FLOODING ON THESE CASCADE RIVERS WOULD PROBABLY BEGIN TONIGHT
OR FRIDAY. A MAJOR FLOODING EPISODE IS UNLIKELY...BUT IT WOULD BE NO
SURPRISE IF MANY OF THESE RIVERS HAVE MINOR TO MODERATE FLOODING.

THE CASCADES SOUTH OF THE SNOHOMISH BASIN WILL PROBABLY RECEIVE 2 TO
4 INCHES OF RAIN. MOST OF THAT COULD FALL ON FRIDAY AND SOME MODELS
-- ESPECIALLY THE UW WRF-GFS -- SHOW FAIRLY INTENSE RAINFALL IN THAT
AREA. SO THE RIVERS THERE FACE A THREAT OF FLOODING AS WELL. OF
COURSE SOME OF THEM RECENTLY FLOODED...AND THEY ARE ALL RUNNING
HIGH.

THE COLD FRONT SHOULD MOVE SOUTHEAST OF WESTERN WASHINGTON FRIDAY
NIGHT...WITH COLD AND DRIER WEATHER LIKELY THIS WEEKEND. RIVERS
SHOULD THEN RECEDE AND THE FLOOD THREAT WILL END FOR THE FORESEEABLE
FUTURE.   MCDONNAL

&&

.AVIATION...EXPECT RAIN AT TIMES THIS MORNING AS A COLD FRONT PUSHES
THROUGH WESTERN WA. THE FLOW ALOFT IS WESTERLY. MVFR CIGS ARE
POSSIBLE THIS MORNING IN -RA. GUSTY SOUTH WINDS ARE ALSO EXPECTED
WITH THIS FRONT. MOIST WESTERLY FLOW WILL CONTINUE THIS AFTERNOON
WITH SHOWERS MAINLY AFFECTING THE COAST AND MOUNTAINS. RAIN WILL
CONTINUE TONIGHT AS ANOTHER COLD FRONT SHIFTS S/SE INTO WESTERN WA.
33

KSEA...EXPECT LIGHT RAIN AT TIMES TODAY WITH CIGS NEAR 3000 FT.
GUSTY SOUTH WINDS 20015G25KT EXPECTED AT THE TERMINAL THROUGHOUT THE
DAY. 33

&&

.MARINE...A COLD FRONT IS PUSHING THROUGH WESTERN WA EARLY THIS
MORNING WITH MODERATE SOUTH/SOUTHWEST WINDS ACROSS THE WATERS -
SMALL CRAFT ADVISORIES ARE IN EFFECT FOR ALL ZONES. PRESSURE
GRADIENTS WILL REMAIN TIGHT TONIGHT THROUGH FRI MORNING AS ANOTHER
COLD FRONT SHIFTS SOUTH INTO WESTERN WA. STRONG HIGH PRES OVER
INTERIOR B.C. WILL BRING N/NE FRASER RIVER OUTFLOW WINDS LATE FRI
NIGHT INTO SAT WITH GALES POSSIBLE OVER THE NORTHERN INLAND WATERS.
OFFSHORE FLOW WILL PREVAIL SUN AND MON WITH HIGH PRES EAST OF THE
CASCADES. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCHES AND WARNINGS IN EFFECT FOR PORTIONS OF
      WESTERN WASHINGTON.

PZ...SMALL CRAFT ADVISORY ALL WATERS.
     SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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








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