Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 201200
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
400 AM PST SAT DEC 20 2014

.SYNOPSIS...TWO STRONG WARM FRONTS WILL MOVE ACROSS WESTERN
WASHINGTON EARLY THIS MORNING AND LATE THIS AFTERNOON. STRONG MOIST
WEST FLOW ALOFT WILL FOLLOW TONIGHT AND SUNDAY. THIS WILL BRING WET
AND WINDY CONDITIONS TO THE LOWLANDS AND HEAVY RAIN OVER THE
MOUNTAINS. AN UPPER LEVEL RIDGE WILL BUILD OVER THE REGION MONDAY
FOR DRIER WEATHER. THE RIDGE WILL WEAKEN MONDAY NIGHT ALLOWING A
COUPLE OF WEATHER SYSTEMS TO REACH THE AREA THROUGH MID NEXT WEEK.

&&

.SHORT TERM...STRONG ZONAL FLOW ACROSS THE NE PACIFIC PUMPED UP BY A
170 KT JETSTREAK WILL DRIVE A PLUME OF WARM MOIST AIR OVER W WA THIS
WEEKEND. MODELS HAVE BEEN GENERALLY CONSISTENT. A STRONG WARM FRONT
WILL MOVE ACROSS W WA EARLY THIS MORNING...WITH STRONG S FLOW ALOFT
AHEAD OF THE WARM FRONT CRANKING UP HEAVY RAIN OVER THE OLYMPICS.
RADAR SHOWS HEAVY RAIN BEGINNING TO REACH THE COAST AND OLYMPICS AT
11Z WITH LIGHT RAIN OVER PUGET SOUND. SURFACE WINDS ARE RISING ALONG
THE COAST AND OVER THE N INTERIOR.

THE WARM FRONT WILL BRING THE STRONGEST WINDS TO THE COAST EARLY
THIS MORNING...AND TO THE N INTERIOR THROUGH MID MORNING...THEN
SHIFT TO SW-S AND EASE BEHIND THE WARM FRONT.

MODELS STILL HAVE A SECOND STRONG WARM FRONT REACHING THE AREA THIS
AFTERNOON...BUT THE 00Z-06Z RUNS HAVE TRENDED A LITTLE WEAKER WITH
THE SURFACE WINDS. EARLIER MODELS WITH STRONGER SOLUTIONS SHOWED A
SMALL SURFACE LOW TRACKING ACROSS VANCOUVER ISLAND BEHIND THE WARM
FRONT...BUT THE GFS NOW ONLY SHOWS AN OPEN SURFACE TROUGH AND THE
ECMWF AND NAM HAVE WEAKER SURFACE LOWS. IT NOW LOOKS LIKE SURFACE
WINDS WILL REMAIN JUST BELOW ADVISORY LEVELS THIS AFTERNOON AND
EVENING. ONE CONCERN IS THAT STRONG SW FLOW ALOFT ACROSS THE
OLYMPICS MAY ALLOW A SMALL MESO-LOW TO FORM OVER THE NE CORNER OF
THE OLYMPICS BRINGING STRONGER WINDS TO THE HOOD CANAL AND ADMIRALTY
INLET AREA. HOWEVER ONLY THE 06Z NAM SHOWS THIS. FOR NOW I WILL
STICK WITH THE PLAN...ALLOWING THE HIGH WIND WARNINGS FOR THE COAST
AND N INTERIOR TO EXPIRE AT 9 AM AND NOON RESPECTIVELY.

HEAVY PRECIPITATION IS EXPECTED TO DEVELOP OVER THE CASCADES THIS
MORNING AS LOW LEVEL WINDS SHIFT TO SW BEHIND THE WARM FRONT.
CASCADE SNOW LEVELS ARE STARTING OUT AROUND 3000 FEET...SO SOME
BRIEF HEAVY SNOW COULD DEVELOP THIS MORNING BEFORE THE WARM AIR
BEHIND THE WARM FRONT LIFTS SNOW LEVELS ABOVE CRITICAL PASS LEVELS.
PARADISE COULD SEE 6-11 INCHES OF SNOW BEFORE SNOW LEVELS RISE TO
6500 FEET AROUND MIDDAY. STEVENS PASS HAS A CHANCE AT REACHING 3-8
INCHES BY EARLY AFTERNOON...AND MOUNT BAKER COULD SEE UP TO 9 INCHES
THROUGH 4 PM. THE SNOW IS GOING TO HAVE TO RAMP UP PRETTY FAST THIS
MORNING TO MEET THESE ACCUMULATIONS...SO CONFIDENCE IN THESE SNOW
AMOUNTS IS NOT ALL THAT HIGH. THERE IS A GOOD CHANCE THAT SNOW
TOTALS WILL BE LOWER.

THE THIRD PROBLEM IS HEAVY RAIN OVER THE OLYMPICS AND CASCADES
THROUGH SUNDAY AFTERNOON. S FLOW WITH THE FIRST WARM FRONT WILL NOT
BRING MUCH SIGNIFICANT RAIN TO THE CASCADES...BUT IT WILL BRING
HEAVY RAIN TO THE OLYMPICS. LOW LEVEL WINDS WILL SHIFT TO WESTERLY
THIS AFTERNOON THEN STRENGTHEN TO 40-50 KT AT 850 MB THIS
EVENING...THEN SLOWLY EASE THROUGH SUNDAY. HEAVY RAIN WILL DEVELOP
OVER THE CASCADES...AND RAIN WILL CONTINUE TO FALL OVER THE
OLYMPICS. SNOW LEVELS WILL BE HIGH THIS AFTERNOON THROUGH SUNDAY.
THE HEAVIEST RAIN OVER THE CASCADES STILL LOOKS LIKE IT WILL BE
FROM ABOUT SNOQUALMIE PASS SOUTHWARD. THE LATEST QPF FORECAST STILL
HAS 3-5 INCHES OVER MUCH OF THE CASCADES WITH A BULLSEYE OF 7 INCHES
OVER MT RAINIER. THE OLYMPICS GET 3-4.5 INCHES THROUGH 12Z SUNDAY.

STRONG W FLOW ALOFT CONTINUES SUNDAY AFTERNOON AND NIGHT...BUT THE
MOISTURE EMBEDDED IN THE FLOW DIMINISHES...SO THE HEAVY RAIN WILL
END.

MODELS REMAIN ON TRACK BUILDING AN UPPER LEVEL RIDGE JUST OFFSHORE
ON MONDAY...WHILE A SURFACE HIGH SETTLES OVER W WA. WINDS WILL
WEAKEN AND THE AIR MASS WILL DRY...BUT THERE WILL STILL BE ENOUGH
CLOUD COVER AROUND FOR MOSTLY CLOUD SKIES. POPS OVER THE LOWLANDS
HAVE BEEN LOWERED INTO THE 20-30 RANGE...BUT IF THE MODELS KEEP IT
DRY THE CHANCE OF RAIN COULD PROBABLY BE DROPPED IN A LATER
FORECAST. KAM

.LONG TERM...MODEL SOLUTIONS REMAIN IN FLUX FROM TUESDAY ONWARD.
ONCE AGAIN THE LATEST GFS AND ECMWF SOLUTIONS ARE IN FAIR
AGREEMENT...BUT THE SOLUTIONS KEEP CHANGING WITH EACH NEW RUN. THE
WEATHER PATTERN WILL REMAIN ACTIVE NEXT WEEK...BUT NOT AS WET AND
WINDY AS THIS WEEKEND. THE LATEST SOLUTIONS FLATTEN THE MONDAY
RIDGE...ALLOWING WARM ADVECTION RAIN TO CREEP THROUGH IT AND OVER W
WA ON TUESDAY. A NORMAL STRENGTH FRONTAL SYSTEM CROSSES THE AREA ON
WEDNESDAY. NO SIGNIFICANT SYSTEMS SHOW UP THURSDAY AND FRIDAY...BUT
THE NW FLOW ALOFT MAY STILL BE MOIST ENOUGH TO KEEP A CHANCE OF
RAIN. CONFIDENCE REMAINS LOW. KAM

&&

.HYDROLOGY...STRONG MOIST W FLOW ALOFT WITH A COUPLE OF WARM FRONTS
WILL BRING HEAVY RAIN TO THE MOUNTAINS TODAY THROUGH SUNDAY. THREE
TO FIVE INCHES OF RAIN IS EXPECTED TO FALL OVER THE OLYMPICS...AND
OVER THE CASCADES FROM ROUGHLY STEVENS PASS SOUTHWARD. A BULLSEYE OF
7 INCHES IS POSSIBLE OVER MT RAINIER. THIS AMOUNT OF RAIN IS GREAT
ENOUGH TO CAUSE FLOODING ON SOME OF THE RIVERS FLOWING OFF THE
OLYMPICS AND CASCADES. A FLOOD WATCH REMAINS IN EFFECT.

PRECIPITATION WILL START AS SNOW OVER THE CASCADES THIS MORNING BUT
SNOW LEVELS WILL RISE RAPIDLY...PEAKING OUT NEAR 5500 FEET NORTH TO
7500 FEET SOUTH THIS EVENING. THE HEAVIEST RAIN WILL BE OVER THE
OLYMPICS TODAY AND TONIGHT...AND OVER THE CASCADES FROM LATE THIS
AFTERNOON THROUGH SUNDAY MORNING...WHEN LOW LEVEL W FLOW AROUND 850
MB RISES TO 40-50 KT.

ANY FLOODING WOULD LIKELY BEGIN AS EARLY AS SATURDAY NIGHT ON THE
SKOKOMISH RIVER. THE CASCADE RIVERS COULD FLOOD SATURDAY NIGHT OR
SUNDAY. MODELS SHOW HEAVY RAIN ENDING SUNDAY NIGHT. KAM

&&

.AVIATION...A STRONG FRONTAL SYSTEM WILL MOVE THROUGH WESTERN WA
TODAY AND TONIGHT...THERE ARE ACTUALLY TWO FRONTAL WAVES...ONE THIS
MORNING AND ANOTHER LATER THIS AFTERNOON AND EVENING. STRONG WSW
FLOW ALOFT. THE AIR WILL BE MOIST AND STABLE WITH RAIN AND OVERALL
LOWERING CIGS. THERE MIGHT BE A FEW HOURS OF IMPROVEMENT EARLY THIS
AFTERNOON...BUT THERE WONT BE MUCH OF A BREAK BETWEEN THE STRONG
FRONTAL WAVE THIS MORNING...AND THE NEXT ONE LATER THIS AFTERNOON
AND EVENING. THE MOUNTAINS AND FOOTHILLS WILL BE OBSCURED.

KSEA...RAIN AND AN INCREASING SOUTHERLY BLUSTERY SSW WIND LATER
TODAY WITH THE PASSAGE OF THE SECOND FRONTAL WAVE.

&&

.MARINE...A STRONG SYSTEM WILL MOVE THROUGH WESTERN WA TODAY AND
TONIGHT. SUNDAY SHOULD BE A POSTFRONTAL DAY WITH STRONG ONSHORE FLOW
EASING BY SUNDAY NIGHT AND MON. SEAS OVER THE COASTAL WATERS WILL BE
AROUND 20FT TODAY AND THERE IS A HIGH SURF ADVISORY FOR THE COASTAL
BEACHES THROUGH SATURDAY AND A COASTAL FLOOD ADVISORY FOR MINOR
FLOODING OF LOW AREAS NEAR THE BEACHES DURING THE TIME OF HIGH TIDE
THIS MORNING.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...HIGH WIND WARNINGS ARE IN EFFECT FOR THE COAST AND PARTS OF THE
     NORTH INTERIOR.

     HIGH SURF ADVISORY IN EFFECT TIL 4 PM SATURDAY FOR THE COAST.

     WIND ADVISORY IN EFFECT UNTIL NOON FOR WESTERN SKAGIT COUNTY.

     WINTER WEATHER ADVISORY IN EFFECT FROM UNTIL 4 PM TODAY FOR THE
      CASCADES.

     COASTAL FLOOD ADVISORY IN EFFECT FROM UNTIL 1 PM TODAY FOR THE
      COAST.

     FLOOD WATCHES ARE IN EFFECT FOR MUCH OF THE CWA FROM SATURDAY
     AFTERNOON THROUGH LATE SUNDAY NIGHT.

PZ...GALE WARNING COAST...STRAIT...NORTHERN INLAND WATERS.
     SMALL CRAFT ADVISORY ADMIRALTY INLET AND PUGET SOUND AND HOOD
      CANAL.
     SMALL CRAFT ADVISORY FOR SEVERE CONDITIONS GRAYS HARBOR BAR.

&&

$$

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