Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 280008
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
408 PM PST THU NOV 27 2014

.SYNOPSIS...A COLD FRONT WILL MOVE SOUTH ACROSS WESTERN WA 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
WITH MODIFIED ARCTIC AIR 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.

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-PAE LINE 15Z/7AM...AND TO APPROXIMATELY AN AST-CLS LINE AROUND
19Z/11 AM. RAIN WILL INCREASE TONIGHT AND BE HEAVY IN SOME LOWLAND
AREAS AS WELL AS OVER THE MOUNTAINS. THE AIR MASS WILL REMAIN MOIST
AND MILD WITH LOWLAND TEMPERATURES IN THE 50S.

THE MAIN SHORT TERM PROBLEM IS HEAVY RAIN OVER THE MOUNTAINS WHICH
STILL POSES A FLOODING THREAT TO SOME AREA RIVERS. SEE THE HYDROLOGY
SECTION BELOW FOR MORE DETAIL. THE 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 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 WITH
SNOW LEVELS FALLING BELOW 1000 FEET...SO THE CONVERGENCE ZONE MAY
PRODUCE SOME SNOW FROM NORTH SEATTLE UP THROUGH ARLINGTON. SNOW
ACCUMULATIONS WILL PROBABLY BE AN INCH OR LESS BUT THERE COULD BE
LOCAL HIGHER ACCUMULATIONS DEPENDING UPON LOCATION AND DURATION OF
THE CONVERGENCE ZONE.

MODIFIED ARCTIC AIR WILL BEGIN SPILLING OUT OF THE FRASER VALLEY
FRIDAY EVENING. IT WILL BECOME VERY WINDY OVER PARTS OF THE N
INTERIOR FROM WHATCOM COUNTY ACROSS THE SAN JUAN ISLANDS. BEST GUESS
IS THAT WINDS WILL END UP IN THE WIND ADVISORY RANGE...BUT SOME
LOCAL HIGH WINDS ARE NOT OUT OF THE QUESTION. A HIGH WIND WATCH HAS
BEEN ISSUED FOR THIS. WITH AIR TEMPERATURES IN THE 20S...WIND CHILLS
WILL END UP IN THE TEENS AND SOME FREEZING SPRAY IS NOT OUT OF THE
QUESTION.

THE MODIFIED ARCTIC AIR WILL SPILL S ACROSS PUGET SOUND AS ANOTHER
COLD FRONT LATE FRIDAY NIGHT AND SATURDAY MORNING...AND SPILL WEST
THROUGH THE STRAIT OF JUAN DE FUCA. MORE WIDESPREAD LIGHT SNOW IS
LIKELY ACROSS THE GREATER PUGET SOUND REGION...AND OVER THE SW
INTERIOR AND OUT TOWARDS THE COAST. AGAIN THE MOST LIKELY OUTCOME IS
AN INCH OR LESS OF SNOW WITH LOCAL AMOUNTS OF A COUPLE OF INCHES.
HOWEVER UNCERTAINTY IS HIGH AND HEAVIER AMOUNTS AROUND 3 INCHES OR
SO ARE POSSIBLE.

OVER THE EASTERN STRAIT OF JUAN DE FUCA AREAS...NE OUTFLOW WINDS
WILL FLOW OVER THE N PART OF THE OLYMPICS...BRINGING UPSLOPE SNOW TO
AREAS ALONG THE STRAIT. THE MOST LIKELY SCENARIO IS FOR AN INCH OR
LESS NEAR THE IMMEDIATE COASTLINE AND POSSIBLY AN INCH OR TWO MORE
OVER THE HIGHER HILLS AWAY FROM THE COASTLINE. ONCE AGAIN
UNCERTAINTY IS HIGH AND AMOUNTS WILL DEPEND UPON HOW MOIST THE AIR
IS AND HOW MUCH UPSLOPE FLOW IS GENERATED.

COLD DRY MODIFIED ARCTIC AIR WILL PRETTY MUCH COVER W WA BY SATURDAY
EVENING WITH CLEARING SKIES. TEMPERATURES WILL DROP INTO THE UPPER
TEENS TO MID 20S BY SUNDAY MORNING. KAM

.LONG TERM...ONLY MINOR CHANGES WERE MADE TO THE LONG TERM
FORECAST...MONDAY THROUGH THURSDAY. THE SET-UP HAS A NORTHERN STREAM
RIDGE OFFSHORE OVER THE NE PACIFIC...WITH COOL NW FLOW ALOFT OVER W
WA. ALSO IN THE PICTURE IS A SOUTHERN STREAM CLOSED LOW W OF THE CA
COAST. WASHINGTON APPEARS TO BE CONTESTED TERRITORY BETWEEN THE
NORTHERN AND SOUTHERN STREAMS WHICH MEANS MODEL BATTLES TO DETERMINE
WHICH STREAM WILL WIN OUT. THE ECMWF HAS BEEN MORE OR LESS
CONSISTENTLY KEEPING THE AREA UNDER THE COOLER DRIER AND SUNNIER N
STREAM THROUGH MID NEXT WEEK. THE GFS HAS BEEN WOBBLING BETWEEN THE
ECMWF SOLUTION AND A WETTER SOUTHERN STREAM SOLUTION. FORECASTS
CONTINUE TO FOLLOW THE ECMWF...BUT CONFIDENCE IS ONLY MODERATE GIVEN
THE MODEL CONFLICTS AND THE OVERALL DIFFICULTY IN RESOLVING A
PATTERN LIKE THIS. MODELS DO AGREE THAT A SHORTWAVE TROUGH WILL MOVE
SE ACROSS THE AREA ON MONDAY. THIS LOOKS DRY AT THIS POINT BUT MORE
CLOUDS AND POSSIBLY SOME LIGHT PRECIP MAY NEED TO BE ADDED TO THE
FORECAST IF MODEL SOLUTIONS CHANGE. KAM

&&

.HYDROLOGY...THERE HAS BEEN A DISTINCT LULL IN RAINFALL TODAY WITH
WASHINGTON PRETTY CLEARLY BETWEEN SYSTEMS. RAINFALL AMOUNTS IN THE
MOUNTAINS HAVE BEEN LIGHTER THAN FORECAST. RIVERS HAVE BEEN STEADY
OR HAVE RISEN SLOWLY IN RESPONSE TO EARLIER RAIN.

THE FRONT THAT IS CURRENTLY IN SOUTHERN BRITISH COLUMBIA IS FORECAST
TO MOVE SOUTH TONIGHT AND TO CONTINUE SOUTH OUT OF THE STATE BY
FRIDAY EVENING. IT WILL BRING ANOTHER 2-4 INCHES OF RAIN TO THE
MOUNTAINS. CURRENT QPF REFLECTS A BLEND OF MESOSCALE MODELS. HEAVY
RAIN WILL REDEVELOP THIS EVENING AND SPREAD SOUTH OVERNIGHT. THE
RAINIEST PERIOD IS 12Z-18Z FRIDAY WHEN 1-1.5 INCHES OF RAIN IS
FORECAST OVER THE CASCADES OF SNOHOMISH AND KING COUNTIES...WITH UP
TO AN INCH OVER THE LOWLANDS OF THOSE COUNTIES. RAIN SHOULD TAPER IN
THE AFTERNOON AS THE AIR MASS COOLS AND DRIES FROM THE NORTH.

THE US ARMY CORPS OF ENGINEERS HAS TAKEN CONTROL OF DAM OPERATION ON
THE SKAGIT AND BAKER RIVERS. AS A RESULT THE SKAGIT RIVER SHOULD
HAVE ONLY MINOR FLOODING ON BOTH THE CONCRETE AND MOUNT VERNON
REACHES. A FLOOD WARNING HAS BEEN POSTED FOR THE SKAGIT RIVER.

THE NOOKSACK RIVER IS ALSO FORECAST TO REACH FLOOD STAGE AT NORTH
CEDARVILLE AND FERNDALE. HOWEVER THE LACK OF HEAVY RAIN TODAY WILL
DELAY ANY FLOODING UNTIL LATE TONIGHT OR FRIDAY.

HEAVY RAIN WILL REACH THE BASINS OF RIVERS FLOWING OFF THE CASCADES
OF SNOHOMISH AND KING COUNTIES LATE TONIGHT AND CONTINUE INTO THE
DAY FRIDAY. A SHEAF OF RIVERS FROM THE STILLAGUAMISH SOUTH TO THE
PUYALLUP NEAR ORTING...INCLUDING THE SKYKOMISH...SNOQUALMIE...
TOLT...AND SNOHOMISH...COULD ALL REACH FLOOD STAGE FROM THIS SHORT
BUT INTENSE BURST OF HEAVY RAIN. ANY FLOODING ON THESE RIVERS IS
EXPECTED TO BE MINOR.

THE SOUTHERN TIER OF RIVERS APPEAR TO BE AT LESS RISK OF FLOODING.
IN THE MODELS THE FRONT SEEMS A BIT SPENT AS IT MOVES INTO THE
SOUTHERN COUNTIES...PIERCE AND LEWIS...AND RAINFALL MAY NOT BE AS
HEAVY AS IN THE NORTHERN COUNTIES. A FLOOD WATCH REMAINS IN EFFECT
FOR ALL COUNTIES FROM WHATCOM TO LEWIS...AS WELL AS MASON COUNTY.

SPEAKING OF MASON COUNTY...THE SKOKOMISH RIVER IS ABOVE FLOOD STAGE
AND WILL LIKELY REMAIN NEAR OR ABOVE FLOOD STAGE THROUGH FRIDAY
NIGHT. THE FLOOD WARNING THERE REMAINS IN EFFECT. BURKE

&&

.AVIATION...THE FLOW ALOFT IS W/SW. THE AIR MASS IS MOIST AND
UNSTABLE. RAIN SHOWERS WILL CONTINUE 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. AFTER THE FRONTAL SYSTEM MOVES THROUGH...A PUGET SOUND
CONVERGENCE ZONE MAY FORM OVER PAE FRIDAY NIGHT INTO SATURDAY...AND
TEMPERATURES WILL BE COLD ENOUGH FOR PRECIPITATION TO FALL AS SNOW.
KCLM MAY ALSO BE AFFECTED BY UPSLOPE FLOW OVER THE OLYMPICS.

KSEA...EXPECT RAIN SHOWERS THROUGH 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. THIS MAY ALSO OCCUR AGAIN WHEN SURFACE WINDS SWITCH
TO SOUTH IN THE EARLY AFTERNOON FRIDAY. LATE FRIDAY NIGHT INTO
SATURDAY...THERE IS A CHANCE FOR SNOW AT SEA-TAC WITH MODIFIED
ARCTIC AIR IN THE AREA. AT THIS POINT WE ARE LOOKING AT
ACCUMULATIONS OF AN INCH OR LESS. 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 AND A GALE WATCH HAS BEEN
ISSUED FOR THE NORTHERN INLAND WATERS AND EAST ENTRANCE TO THE
STRAIT. OFFSHORE FLOW WILL PREVAIL EARLY NEXT WEEK WITH HIGH PRES
EAST OF THE CASCADES. JSMITH

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCHES AND WARNINGS IN EFFECT FOR PORTIONS OF
      WESTERN WASHINGTON.
     HIGH WIND WATCH NORTH INTERIOR FRIDAY NIGHT AND SATURDAY.
PZ...GALE WATCH NORTHERN INLAND WATERS AND EAST ENTRANCE STRAIT OF
      JUAN DE FUCA FOR FRIDAY NIGHT AND SATURDAY.
     SMALL CRAFT ADVISORY ALL WATERS.
     SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR.

&&

$$

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