Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 271809 RRA
AFDSEW

AREA FORECAST DISCUSSION...UPDATED TO ADD NEW AVIATION SECTION
NATIONAL WEATHER SERVICE SEATTLE WA
1008 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 LATE TONIGHT
AND FRIDAY MORNING PRODUCING HEAVY RAIN OVER THE MOUNTAINS AND OVER
PARTS OF THE PUGET SOUND REGION. A CONVERGENCE ZONE WILL CONTINUE
OVER PUGET SOUND FRIDAY NIGHT...THEN ANOTHER COLD FRONT WILL MOVE
SOUTH THROUGH THE AREA LATE FRIDAY NIGHT INTO SATURDAY MORNING. SOME
SNOW OR RAIN AND SNOW MIXED IS LIKELY TO OCCUR WITH THE CONVERGENCE
ZONE AND ALONG THIS SECOND FRONT. COOL NORTHWEST FLOW ALOFT WILL
BRING MAINLY COOL AND DRY WEATHER SUNDAY THROUGH EARLY NEXT WEEK.

&&

.SHORT TERM...VERY ACTIVE AND CHANGEABLE WEATHER IS EXPECTED OVER
THE AREA THROUGH SATURDAY.

MOIST SW FLOW ALOFT AND ONSHORE FLOW WILL CONTINUE TODAY BRINGING
SOME LIGHT RAIN TO THE LOWLANDS AND HEAVY RAIN AT TIMES TO THE
MOUNTAINS. THE AIR MASS WILL REMAIN MOIST AND MILD WITH TEMPERATURES
IN THE 50S TODAY.

THE WARM FRONT FROM A COUPLE OF DAYS AGO IS STILL UP OVER B.C. AND
POISED TO MOVE BACK S OVER W WA AS A FAIRLY STRONG COLD FRONT
TONIGHT. RECENT MODELS ARE IN FAIRLY GOOD AGREEMENT WITH THE
TIMING...BRINGING THE FRONT TO A N COAST-BLI LINE AT 09Z/1 AM...TO A
HQM-SEA LINE 12Z-15Z/4AM-7AM...AND TO AN AST-CLS LINE AROUND 18Z/10
AM. HEAVY RAIN OVER THE MOUNTAINS WILL SHUT OFF BEHIND THE COLD
FRONT. SOME EARLIER MODELS SHOWED SOME RATHER HEAVY RAIN OVER OVER
THE GREATER PUGET SOUND REGION WITH THE FRONT...BUT THE NEWEST 12Z
RUNS HAVE TAKEN A STEP BACK. THE HIRES-ARW MODEL REMAINS THE
HEAVIEST PRECIP PRODUCER WITH AROUND AN INCH IN 3 HOURS WITH THE
FRONT. THE 12Z NAM12 AND HIRES-NMM MODELS ARE MUCH LIGHTER.
TEMPERATURES WILL REMAIN WARM ENOUGH TO KEEP THE PRECIP ALL RAIN.
TEMPERATURES WILL FALL INTO THE 30S TO LOWER 40S BY LATE FRIDAY
AFTERNOON.

BEHIND THE COLD FRONT A CONVERGENCE ZONE WILL FORM OVER CENTRAL
PUGET SOUND LATE FRIDAY AFTERNOON AND CONTINUE INTO FRIDAY NIGHT.
TEMPERATURES WILL FALL INTO THE LOWER TO MID 30S BY MID EVENING SO
THE CONVERGENCE ZONE MAY PRODUCE SOME SNOW FROM NORTH SEATTLE UP
THROUGH ARLINGTON. AT THE SAME TIME MODIFIED ARCTIC AIR WILL BEGIN
SPILLING OUT OF THE FRASER VALLEY. IT WILL BECOME VERY WINDY OVER
PARTS OF THE N INTERIOR FROM WHATCOM COUNTY ACROSS THE SAN JUAN
ISLANDS...BUT IT IS STILL UNCERTAIN WHETHER THE WINDS WILL END UP IN
THE WIND ADVISORY RANGE OR REACH HIGH WIND CRITERIA.

THE MOD-ARCTIC AIR WILL SPILL S ACROSS PUGET SOUND AS ANOTHER COLD
FRONT SATURDAY MORNING...AND SPILL WEST THROUGH THE STRAIT OF JUAN
DE FUCA. THERE IS POTENTIAL FOR A FEW INCHES OF SNOW OVER PUGET
SOUND SATURDAY MORNING AS THE FRONT MOVES S...AND OVER THE STRAIT AS
THE FLOW HITS THE NORTHERN OLYMPICS. THE SNOW WILL SHUT OFF IN THE
AFTERNOON AS THE AIR MASS DRIES OUT. KAM



.LONG TERM...PREVIOUS DISCUSSION FROM THE 330 AM AFD...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...THERE WAS A LULL IN HEAVY RAIN OVER THE CENTRAL
CASCADES LAST NIGHT WHICH ALLOWED THE SNOQUALMIE AND PUYALLUP RIVERS
TO FALL BELOW FLOOD STAGE. FLOOD WARNINGS ON THOSE RIVERS HAVE ENDED
FOR THE TIME BEING.

IN CONTRAST...HEAVY RAIN FELL LAST NIGHT OVER THE SOUTHWEST FACING
SLOPES OF THE OLYMPICS AND THE CASCADES FROM ABOUT THE STILLAGUAMISH
BASIN NORTHWARD. 12-HOUR AMOUNTS RANGE FROM ABOUT ONE INCH IN THE
SOUTH TO TWO INCHES IN THE NORTH. THE SKOKOMISH RIVER IS NOW ABOVE
FLOOD STAGE AND THE WARNING IS OUT. THE NORTH CASCADE RIVERS HAVE
STARTED RISING AGAIN WITH FLOODING ON AT LEAST SOME OF THEM LOOKING
LIKELY THIS EVENING OR TONIGHT.

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.

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. A FLOOD WATCH REMAINS IN EFFECT.

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

&&

.AVIATION...THE FLOW ALOFT IS W/SW. THE AIR MASS IS MOIST AND FAIRLY
UNSTABLE. RAIN SHOWERS WILL CONTINUE THROUGH THE DAY AND INTO
TONIGHT...WITH MVFR AND VFR CIGS AT MOST OF THE TERMINALS. A STRONG
COLD FRONT WILL MOVE SOUTH THROUGH THE AREA BETWEEN 10Z AND 17Z
EARLY FRIDAY MORNING...AND WINDS WILL SHARPLY SWITCH FROM S TO NW AT
THE TERMINALS. MODERATE RAIN AND DECREASED VISIBILITIES ARE POSSIBLE
WITH CONVERGENCE FROM THE FRONT.

KSEA...EXPECT RAIN SHOWERS THROUGH TODAY AND TONIGHT...WITH POSSIBLE
MODERATE RAIN EARLY FRIDAY JUST BEFORE THE FRONTAL PASSAGE. GUSTY
SOUTH WINDS WILL CHANGE TO N WITH THE PASSAGE OF THE COLD FRONT
AROUND 15Z. THERE MAY BE A BRIEF PERIOD FOR AN HOUR OR SO WHERE THE
WINDS AT BFI ARE FROM THE N AND WINDS AT SEATAC ARE FROM THE S JUST
BEFORE FRONTAL PASSAGE. JSMITH

&&

.MARINE...SMALL CRAFT ADVISORIES ARE IN EFFECT FOR ALL ZONES WITH
HIGHER PRESSURE IN THE SOUTH AND LOWER PRESSURE IN THE NORTH
PRODUCING HIGH END SMALL CRAFT ADVISORY LEVEL WINDS. PRESSURE
GRADIENTS WILL REMAIN TIGHT TONIGHT THROUGH FRI MORNING AS A WELL
DEVELOPED COLD FRONT SHIFTS SOUTH INTO WESTERN WA. WINDS WILL SWITCH
FROM THE SW TO THE N OR NW AS THE FRONT MOVES THROUGH.

STRONG HIGH PRES OVER INTERIOR B.C. WILL BRING N/NE FRASER RIVER
OUTFLOW WINDS LATE FRI NIGHT INTO SAT WITH GALES LIKELY OVER THE
NORTHERN INLAND WATERS. OFFSHORE FLOW WILL PREVAIL SUN AND MON WITH
HIGH PRES EAST OF THE CASCADES. JSMITH

&&

.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.

&&

$$

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