Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 191143
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
344 AM PST FRI DEC 19 2014

.SYNOPSIS...AN UPPER LEVEL TROUGH WILL BRING SHOWERS TO THE COAST
AND SCATTERED SHOWERS INLAND TODAY. A PAIR OF STRONG WARM FRONTS
WILL BRING RAIN AND WIND TO WESTERN WASHINGTON TONIGHT THROUGH THIS
WEEKEND. AN UPPER LEVEL RIDGE WILL MOVE BRIEFLY OVER THE REGION
MONDAY FOR DRIER WEATHER. ADDITIONAL FRONTAL SYSTEMS WILL REACH THE
REGION NEXT WEEK.

&&

.SHORT TERM...IR SATELLITE IMAGERY AND RADAR SHOWS LAST EVENINGS
COLD FRONT HAS MOVED EAST OF THE AREA. THE UPPER LEVEL TROUGH JUST
OFFSHORE WILL MOVE INLAND OVER W WA LATE THIS MORNING THEN WEAKEN
RAPIDLY AS IT MOVES E OF THE CASCADES THIS AFTERNOON. THE AIR MASS
WILL BE UNSTABLE TODAY WITH NUMEROUS SHOWERS ALONG THE COAST AND
SCATTERED SHOWERS INLAND. GFS SHOWS LIFTED INDICES AS LOW AS -2 OVER
THE COASTAL WATERS TODAY...BUT WITH VERY LITTLE LIGHTNING ACTIVITY
SO FAR I WILL LEAVE OUT THE MENTION OF THUNDERSTORMS.

A STRONG ZONAL JETSTREAM HAS FORMED OVER THE NE PACIFIC AND WILL
DRIVE A COUPLE OF STRONG WET WARM FRONTS ACROSS W WA TONIGHT THROUGH
THIS WEEKEND. MODEL SOLUTIONS ARE IN FAIRLY GOOD AGREEMENT THROUGH
SATURDAY...BUT CONTINUE TO EVOLVE REGARDING SOME DETAILS AND
ESPECIALLY WITH REGARD TO PRECIP AMOUNTS.

THERE IS GOOD CONSISTENCY AND AGREEMENT THAT LIGHT RAIN WILL DEVELOP
THIS EVENING AS THE FIRST STRONG WARM FRONT APPROACHES THE REGION.
GFS/ECMWF BRING THIS FIRST WARM FRONT TO THE COAST EARLY SATURDAY
MORNING. STRONG S-SE WINDS WILL DEVELOP ALONG THE COAST AND OVER THE
N INTERIOR LATE TONIGHT AHEAD OF THE WARM FRONT. A HIGH WIND WATCH
WILL BE ISSUED FOR THESE AREAS.

A NEW WRINKLE IS THAT THE MODELS RESOLVE A SECOND WARM FRONT
TRAILING RIGHT BEHIND THE FIRST. THE SECOND WARM FRONT SHOULD REACH
THE WA COAST LATE SATURDAY AFTERNOON...WITH AN ASSOCIATED WEAK
SURFACE LOW MOVING ACROSS VANCOUVER ISLAND EARLY SATURDAY EVENING.
IT NOW LOOKS LIKE THERE WILL BE A BRIEF LULL IN THE STRONG S-SE
WINDS BETWEEN THE TWO WARM FRONTS...BUT THERE WILL BE A SURGE OF
W-SW WIND BEHIND THE SURFACE LOW SATURDAY EVENING. THE HIGH WIND
WATCH RUNS THROUGH SATURDAY EVENING TO COVER THE SECOND WARM FRONT
AND THE TRAILING SURFACE LOW. MODELS HAVE BEEN PRETTY CONSISTENT
SHOWING UP TO HIGH END WIND ADVISORY WINDS FOR THE COAST/N INTERIOR
THROUGH THIS PERIOD...BUT THERE IS STILL SOME UNCERTAINTY REGARDING
THE STRENGTH OF EACH OF THESE FEATURES. SOME SPOTTY HIGH WINDS ARE
WITHIN REACH...HENCE THE HIGH WIND WATCH.

PERIODS OF HEAVY RAIN ARE STILL EXPECTED OVER THE MOUNTAINS...BUT
RESOLVING THESE FEATURES INTO TWO WARM FRONTS WITH A SURFACE LOW HAS
HAD AN IMPACT ON THE MID LEVEL WIND FIELD WHICH WILL GENERATE
THE UPSLOPE FLOW OVER THE MOUNTAINS. STRONG S FLOW DEVELOPS TONIGHT
AHEAD OF THE FIRST WARM FRONT...BUT RATHER THAN SHIFTING DIRECTLY TO
W BEHIND THE FIRST WARM FRONT...THE FLOW SHIFTS SW WITH A BRIEF LESS
WET PERIOD THROUGH MIDDAY SATURDAY BETWEEN THE WARM FRONTS. STRONG W
FLOW ALOFT DEVELOPS BEHIND THE SECOND WARM FRONT EARLY SATURDAY
EVENING AND CONTINUES THROUGH SATURDAY NIGHT AS THE SURFACE LOW
MOVES INLAND. THE STRONG W FLOW IS WHAT WILL BRING HEAVY RAIN TO THE
CASCADES. OVERALL THE MODELS HAVE BACKED OFF ON RAINFALL AMOUNTS IN
SOME OF THE PERIODS THIS WEEKEND...BUT NOT ALL THE MODELS ARE
CONSISTENT. HEAVY RAIN WILL STILL FALL OVER THE MOUNTAINS WITH 3-5
INCHES OVER THE OLYMPICS AND 3 TO 6+ INCHES OVER MAINLY THE CENTRAL
CASCADES IN THE 36 HOURS FROM 12Z SATURDAY THROUGH 00Z MONDAY...BUT
CONFIDENCE IN THE FORECAST RAINFALL AMOUNTS IS LOWER.

LASTLY THERE COULD BE SOME LOCALLY HEAVY SNOW OVER THE CASCADES
SATURDAY MORNING. COOL AIR FROM TODAYS UPPER LEVEL TROUGH WILL DROP
SNOW LEVELS DOWN TO AROUND 3000 FEET EARLY SATURDAY MORNING. SNOW
LEVELS WILL GRADUALLY RISE TO 5000-7000 FEET BY LATE SATURDAY
AFTERNOON AS THE WARM FRONTS CROSS THE AREA...BUT DURING THE
TRANSITION...UP TO A FOOT OF SNOW COULD FALL OVER MT RAINIER AND MT
BAKER...WITH POSSIBLY UP TO 6-9 INCHES AROUND STEVENS PASS SATURDAY
MORNING. OF COURSE THE SAME UNCERTAINTY REGARDING RAINFALL AMOUNTS
AFFECTS SNOW ACCUMULATIONS. A WINTER WEATHER ADVISORY MAY BE NEEDED
TO ADDRESS THE EARLY SATURDAY SNOW.

W FLOW ALOFT WILL GRADUALLY WEAKEN ON SUNDAY. A CONVERGENCE ZONE MAY
DEVELOP OVER PUGET SOUND ON SUNDAY. KAM

.LONG TERM...BOTH THE GFS AND ECMWF HAVE A MODERATE AMPLITUDE UPPER
LEVEL RIDGE FORMING OFFSHORE MONDAY MORNING THEN SHIFTING E OVER W
WA MONDAY AFTERNOON. BOTH MODELS ALSO SHOW A SURFACE RIDGE MOVING
OVER W WA ON MONDAY...ALTHOUGH THE GFS IS STRONGER THAN THE ECMWF.
THIS WILL END THE WEEKENDS WET WINDY PERIOD. ANOTHER WEAKER WARM
SYSTEM MOVES OVER THE TOP OF THE RIDGE MONDAY NIGHT AND
TUESDAY...BRINGING A QUICK END TO MONDAYS BRIEF DRY PERIOD. THE GFS
AND ECMWF SEEM TO BE IN FAIR AGREEMENT WITH THE LARGER PATTERN PAST
MID NEXT WEEK...BUT WITH THE MODELS CONTINUING TO EVOLVE FROM
RUN-TO-RUN...CONFIDENCE REMAINS LOW. BOTTOM LINE IS THAT ACTIVE
WEATHER WILL PROBABLY BE CONTINUING. KAM

&&

.HYDROLOGY...THOUGH RAIN WILL FALL AT TIMES...FLOODING IS UNLIKELY
ON ANY RIVER THROUGH SATURDAY.

HEAVY RAIN COULD CAUSE FLOODING BY LATE THIS WEEKEND. A PAIR OF
STRONG WARM FRONTS WILL BRING HEAVY RAIN AT TIMES TO THE COAST AND
OLYMPICS ON SATURDAY. HEAVY RAIN WILL SPREAD INTO THE CASCADES
SATURDAY AFTERNOON BUT MAY NOT BE AS HEAVY AS INITIALLY EXPECTED.

MODELS CURRENTLY TAPER RAIN OVER THE OLYMPICS SATURDAY NIGHT AND
SUNDAY...BUT THE STRONGER W FLOW ALOFT SATURDAY NIGHT WILL BRING
HEAVY RAIN TO THE CASCADES...ESPECIALLY OVER THE CASCADES FROM ABOUT
KING COUNTY SOUTH. THE CURRENT RAINFALL FORECAST IN THE 36 HOUR
PERIOD FROM 12Z SATURDAY THROUGH 00Z MONDAY HAS 3-5 INCHES OVER THE
OLYMPICS AND ROUGHLY 3 TO 5 INCHES OVER THE CENTRAL CASCADES WITH
ABOUT 7 INCH BULLSEYE OVER MT RAINIER. THIS IS LESS THAN THE
PREVIOUS FORECAST. AS MENTIONED IN THE SHORT-TERM SECTION
ABOVE...WITH THE EVOLUTION OF THE MID LEVEL WIND FIELD...CONFIDENCE
IN TOTAL RAIN FALL AMOUNTS IS LOWER.

THE SNOW LEVEL WILL RISE TO 5000 TO 7000 FEET BY LATE SATURDAY
AFTERNOON ND NIGHT...THEN LOWER BACK TO AROUND 5000 FEET ON SUNDAY.

THESE RAINFALL AMOUNTS MAY BE ENOUGH TO CAUSE FLOODING. AT THIS TIME
THE RIVERS MOST LIKELY TO FLOOD ARE THE ONES FLOWING OFF THE
CASCADES OF KING...PIERCE...AND LEWIS COUNTIES...AS WELL AS THE
UNUSUALLY FLOOD-PRONE SKOKOMISH RIVER IN MASON COUNTY. ANY FLOODING
WOULD LIKELY BEGIN AS EARLY AS SATURDAY NIGHT ON THE SKOKOMISH
RIVER. THE CASCADE RIVERS COULD FLOOD SUNDAY WITH CRESTS SUNDAY
NIGHT OR MONDAY. KAM

&&

.AVIATION...A FRONT OVER WESTERN WA EARLY THIS MORNING WAS MOVING
INLAND WITH RAIN TURNING TO SHOWERS. A STRONGER FRONTAL SYSTEM WILL
REACH THE AREA LATER TONIGHT AND SATURDAY. THE AIR WILL STAY MOIST
BUT THERE MAY BE SOME SUNBREAKS TODAY...THEN THERE WILL BE
INCREASING MOISTURE AGAIN TONIGHT AND SATURDAY WITH STRONG SW FLOW
ALOFT. A DEEP LOW PRESSURE AREA WILL BE IN THE NE PACIFIC.

KSEA...RAIN TURNING TO SHOWERS EARLY THIS MORNING...SOME SUNBREAKS
AREA POSSIBLE TODAY. THEN RAIN WILL INCREASE AGAIN TONIGHT AND
SATURDAY WILL BE WET AND BLUSTERY.

&&

.MARINE...A FRONT OVER WESTERN WA EARLY THIS MORNING WILL MOVE
INLAND. A STRONGER FRONTAL SYSTEM WILL REACH THE AREA TONIGHT AND
SATURDAY. THERE ARE SMALL CRAFT ADVISORIES UP FOR MOST AREA INTO
THIS EVENING...AND THEN OVERNIGHT TONIGHT AND SATURDAY THERE WILL BE
GALES MOST WATERS. COASTAL SEAS ARE EXPECTED TO PEAK THIS EVENING
AROUND 22 FT.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...WINTER WEATHER ADVISORY THROUGH 6 AM FOR THE OLYMPICS AND
      CASCADES.
     HIGH WIND WATCH FOR THE COAST AND NORTH INTERIOR LATE TONIGHT
      THROUGH SATURDAY EVENING.
     HIGH SURF ADVISORY FOR THE NORTH AND CENTRAL COAST FROM LATE
      THIS MORNING THROUGH EARLY SATURDAY MORNING.
     COASTAL FLOOD WATCH FOR THE NORTH AND CENTRAL COAST FROM
      SATURDAY MORNING THROUGH SATURDAY AFTERNOON.

PZ...GALE WARNING EAST ENTRANCE STRAIT.
     SMALL CRAFT ADVISORY ALL REMAINING WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH/SEVERE GRAYS HARBOR BAR
       CONDITIONS.
     GALE WATCH COAST...ENTRANCES TO THE STRAIT OF JUAN DE FUCA...
     ADMIRALTY INLET...AND NORTHERN INLAND WATERS FRIDAY NIGHT
     THROUGH SATURDAY.

&&

$$

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








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