Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 242359
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
400 PM PST MON NOV 24 2014

.SYNOPSIS...A WARM FRONT WILL SPREAD RAIN ACROSS WESTERN WASHINGTON
THROUGH THIS EVENING...THEN SETTLE OVER WESTERN WASHINGTON TONIGHT
THROUGH WEDNESDAY MORNING ALLOWING RAIN...MILD TEMPERATURES AND
OCCASIONALLY WINDY WEATHER TO CONTINUE. A WEAK COLD FRONT WILL MOVE
ACROSS THE AREA WEDNESDAY NIGHT. ANOTHER WARM FRONT WILL BRING A
LITTLE MORE RAIN ON THANKSGIVING. A COLD FRONT WILL MOVE SLOWLY
SOUTH ACROSS THE AREA FRIDAY AND FRIDAY NIGHT. AN UPPER LEVEL RIDGE
MAY BRING A COOLER DRIER PERIOD THIS WEEKEND.

&&

.SHORT TERM...A BROAD LOW AMPLITUDE RIDGE IS IN PLACE OFFSHORE WITH
THE AXIS ALONG ROUGHLY 130W. MODELS AND IR IMAGERY SHOW HEALTHY WARM
ADVECTION MOVING THROUGH THE RIDGE WHICH HAS SPREAD RAIN INLAND OVER
W WA THIS AFTERNOON. COASTAL RADAR SHOWS A BACK EDGE TO THE PRECIP
OFFSHORE WHICH SHOULD BE THE INITIAL WARM FRONT. STRONG LOW LEVEL W
FLOW ALOFT WITH HEALTHY WARM ADVECTION WILL CONTINUE LATE TONIGHT
AND TUESDAY MORNING...FOCUSED MORE ON THE SOUTH HALF OF THE AREA.
THIS WILL RESULT IN HEAVY RAIN OVER THE CENTRAL CASCADES FROM ABOUT
SNOQUALMIE PASS SOUTHWARD. THE OFFSHORE RIDGE WILL BUILD TUESDAY
AFTERNOON AND TUESDAY NIGHT WHICH WILL SLOWLY SHIFT THE MAIN WARM
ADVECTION AREA OVER THE N CASCADES. THE QPF FORECAST IS A BLEND OF
THE MESOSCALE MODELS AND DROPS 3 TO 4.5 INCHES OVER MUCH OF THE
CASCADES...WITH BULLSEYES OF 5-7 INCHES OVER MT BAKER AND MT
RAINIER...OVER THE 48 HOUR PERIOD FROM 18Z TODAY THROUGH 18Z
WEDNESDAY. THAT MAY BE ENOUGH FOR SOME OF THE RIVERS FLOWING OFF THE
CASCADES TO REACH FLOOD STAGE...SO A FLOOD WATCH WAS ISSUED EARLIER
THIS AFTERNOON. OVER THE LOWLANDS RAINFALL AMOUNTS WILL PROBABLY END
UP RANGING FROM ABOUT A THIRD OF AN INCH TO A LITTLE OVER AN INCH.
THE GREATER PUGET SOUND REGION WILL PROBABLY BE PARTIALLY SHADOWED
BY THE OLYMPICS IN THIS W FLOW PATTERN.

ONCE THE WARM FRONT MOVES N OVER B.C. WEDNESDAY MORNING THERE WILL
BE A LULL IN PRECIP OVER THE AREA. THE MOIST AIR MASS COULD STILL
DROP A LITTLE LIGHT RAIN...SO POPS ARE IN THE CHANCE RANGE. A WEAK
COLD FRONT WILL MOVE E ACROSS THE AREA WEDNESDAY EVENING...BRINGING
A LITTLE MORE LIGHT RAIN TO MAINLY THE MOUNTAINS.

W FLOW ALOFT WILL INCREASE AGAIN BEHIND THE COLD FRONT AS ANOTHER
BATCH OF WARM ADVECTION MOVES OVER THE AREA. THIS LOOKS LIKE IT
BRING ANOTHER ROUND OF MODERATE TO HEAVY RAIN TO THE OLYMPICS AND N
CASCADES THURSDAY THROUGH THURSDAY EVENING.

INCREASING SOUTHERLY SURFACE FLOW BEHIND TUESDAYS WARM FRONT WILL
BRING BREEZY TO WINDY CONDITIONS TO MANY AREAS THROUGH WEDNESDAY.
BREEZY SE WINDS WILL DEVELOP WEDNESDAY NIGHT AND THURSDAY ASSOCIATED
WITH THE COLD FRONT. ALL THE WARM ADVECTION WILL KEEP TEMPERATURES
RATHER MILD...WITH LOWS IN THE 45-50 RANGE AND HIGHS IN THE MID TO
UPPER 50S. KAM

.LONG TERM...ON FRIDAY AND FRIDAY NIGHT THE ECMWF AND GFS AGREE ON
MOVING A COLD FRONT SOUTHWARD ACROSS THE AREA. THIS FEATURE IS
ACTUALLY THE LEFTOVER OCCLUDED FRONT FROM THE MAIN LOW OFFSHORE.
COOLER DRIER N FLOW ALOFT AND AT LOW LEVELS WILL SPREAD S BEHIND THE
FRONT FRIDAY NIGHT AND SATURDAY MORNING AS AN UPPER LEVEL RIDGE
BUILDS OFFSHORE AND A SURFACE HIGH SHIFTS S OVER B.C.  COULD GET
SOME BREEZY NE OUTFLOW OVER THE N INTERIOR FRIDAY NIGHT THROUGH
SUNDAY.

BOTH THE 12Z/18Z GFS AND 12Z ECMWF KEEP THE AREA DRY ON SUNDAY AS
WELL BUT RUN-TO-RUN CONSISTENCY IS NOT GOOD YET...SO CONFIDENCE IS
LOW. LOW CHANCE POPS ARE STILL IN THE FORECAST FOR BOTH SATURDAY AND
SUNDAY...BUT IF THE MODELS STICK WITH THIS SOLUTION SATURDAY AND
POSSIBLY EVEN SUNDAY COULD BE DRIED OUT AND SUNSHINE INCREASED. THE
COOL DRY FRASER RIVER VALLEY OUTFLOW AIR WILL LOWER SNOW LEVELS
CLOSE TO THE SURFACE...BUT MENTION OF SNOW HAS BEEN DROPPED FROM THE
FORECAST SINCE CONFIDENCE IS STILL LOW.

IF MODELS DO BRING PRECIP BACK OVER THE AREA THIS WEEKEND IT WILL
PROBABLY BE FROM AN UPPER LEVEL LOW W OF THE OREGON COAST SPREADING
WARM MOIST AIR NORTHWARD OVER THE AREA. KAM

&&

.HYDROLOGY...STRONG WARM ADVECTION COMING THROUGH THE BROAD FLAT
UPPER LEVEL RIDGE OFFSHORE WILL BRING HEAVY RAIN AT TIMES TO THE
CASCADES TONIGHT THROUGH TUESDAY NIGHT. THE WHOLE CASCADES WILL GET
A BRIEF SHOT OF RAIN THIS EVENING FROM AN INITIAL WARM FRONT MOVING
THROUGH THE AREA. STRONG WARM ADVECTION WILL RE-DEVELOP LATE TONIGHT
AND TUESDAY MORNING...FOCUSED ON THE CENTRAL CASCADES FROM ABOUT
SNOQUALMIE PASS SOUTHWARD. THIS WILL AFFECT MAINLY THE PUYALLUP AND
COWLITZ RIVERS.

THE WARM FRONT WILL SHIFT SLOWLY N TUESDAY AFTERNOON AND NIGHT
SHIFTING THE HEAVIEST RAIN TO THE NORTH CASCADES. THIS WILL AFFECT
MULTIPLE RIVER BASINS INCLUDING THE SNOQUALMIE...STILLAGUAMISH...
SKAGIT...AND NOOKSACK.

THE WARM FRONT WILL LIKELY SHIFT OVER B.C. AROUND 12Z WEDNESDAY. THE
QPF FORECAST IS A BLEND OF THE MESOSCALE MODELS AND HAS 3 TO 4.5
INCHES SPREAD GENERALLY ACROSS THE CASCADES IN THE 48 HOUR PERIOD
ENDING 18Z WEDNESDAY. BULLSEYES OF 5 TO 7 INCHES ARE PRESENT OVER MT
BAKER AND MT RAINIER RESPECTIVELY. THERE IS STILL A LOT OF
UNCERTAINTY REGARDING THIS HEAVY RAIN EVENT...WITH INTENSITY OF THE
RAIN AND EXACTLY WHERE AND HOW LONG THE HEAVIEST PRECIP WILL FALL. A
FLOOD WATCH WAS ISSUED EARLIER THIS AFTERNOON COVERING THE CASCADES.

MODELS SHOW THE OLYMPICS GETTING LOWER AMOUNTS OF RAIN...BUT THERE
IS GREATER UNCERTAINTY WHETHER THIS W FLOW EVENT MIGHT DRIVE THE
BOGACHIEL RIVER TO FLOOD STAGE. THE EARLIER HYDROLOGIC OUTLOOK
COVERS THE HIGHER UNCERTAINTY OVER THE OLYMPICS.

AFTER THE WARM FRONT MOVES N ON WEDNESDAY MORNING THERE WILL BE A
SHORT LULL IN THE PRECIP. A WEAK COLD FRONT WILL MOVE THROUGH THE
AREA WEDNESDAY EVENING BRINGING A QUICK SHOT OF MODERATE RAIN.
ANOTHER BATCH OF WARM ADVECTION DEVELOPS BEHIND THE COLD FRONT FOR A
SECOND PERIOD OF MODERATE TO HEAVY RAIN FOR THE OLYMPICS AND N
CASCADES. KAM

&&

.AVIATION...MODERATE NORTHWESTERLY FLOW ALOFT WILL CONTINUE TONIGHT
AND TUESDAY. A WARM FRONT WILL REMAIN OVER THE AREA THROUGH TUESDAY.
MODERATE SOUTHERLY GRADIENTS WILL GIVE BREEZY TO WINDY CONDITIONS AT
TIMES.

CONDITIONS ARE MAINLY VFR THIS AFTERNOON EXCEPT FOR SOME MVFR
CEILINGS OVER THE NORTH INTERIOR. CLOUDS WILL CONTINUE TO THICKEN
AND LOWER THIS EVENING GIVING 3-5K FT CEILINGS THEN MVFR 1-2K FT
CEILINGS TONIGHT AS RAIN INCREASES.

KSEA...DISCUSSION ABOVE APPLIES. SOUTHEAST WIND 5-8 KT WILL BECOME
SOUTHERLY AND INCREASE TO S10G20 KT TONIGHT. CHB

&&

.MARINE...SOUTHERLY WINDS OF 15-30 KT ARE LIKELY OVER MOST WATERS
TONIGHT AS A WARM FRONT MOVES OVER THE AREA. THE WARM FRONT IS
FORECAST TO MOVE NORTH INTO SOUTHERN BRITISH COLUMBIA TUESDAY
AFTERNOON AND EVENING. SMALL CRAFT ADVISORIES ARE IN EFFECT OVER ALL
WATERS INCLUDING THE CENTRAL STRAIT WHERE A BRIEF WESTERLY PUSH IS
LIKELY THIS EVENING BEHIND THE WARM FRONT.

ANOTHER FRONT WILL ARRIVE THURSDAY FOR ANOTHER ROUND OF SOUTHERLY
WINDS. ON FRIDAY A COLD HIGH PRES SYSTEM WILL DEVELOP OVER THE
INTERIOR OF BRITISH COLUMBIA. BY LATER FRIDAY INTO THE WEEKEND
STRONG NORTHEAST FLOW IS POSSIBLE OVER THE AREA WITH FRASER OUTFLOW
OVER THE NORTHERN INLAND WATERS. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCH FOR COUNTIES WITH RIVERS FLOWING OFF THE CASCADES.
PZ...SMALL CRAFT ADVISORY ALL WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

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










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