Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 191726
AFDSEW

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

.SYNOPSIS...AN UPPER LEVEL TROUGH WILL BRING SCATTERED SHOWERS
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...AN UPPER LEVEL TROUGH IS PASSING THROUGH WESTERN WA
THIS MORNING FOR SCATTERED SHOWERS ACROSS THE AREA. THERE WERE A FEW
LIGHTNING STRIKES OVER THE COASTAL WATERS BUT THESE CELLS SHOULD
DRIFT N/NE INTO B.C. THE TROUGH AXIS WILL EXIT THIS AFTERNOON WITH
SHOWERS TAPERING OFF. EXPECT HIGHS IN THE LOWER 50S TODAY.

ANOTHER ROUND OF WEST AND BLUSTERY WEATHER IS ANTICIPATED LATE
TONIGHT THROUGH SATURDAY AS OUR NEXT SYSTEM ARRIVES. THIS SYSTEM MAY
BRING HIGH WIND TO THE COAST AND NORTH INTERIOR AS THE OCCLUDED
FRONT MOVES INLAND SATURDAY MORNING. A HIGH WIND WATCH IS IN EFFECT
FOR THESE ZONES. A PLUME OF MOISTURE IS BEING TRANSPORTED EAST ALONG
A STRONG 175 KT JET AIMED AT WASHINGTON...WHICH WILL LIKELY RESULT
IN HEAVY PRECIP ALONG THE COAST AND IN THE MOUNTAINS. INITIALLY SNOW
LEVELS WILL START AROUND 3000-4000 FT SATURDAY MORNING BUT WILL RISE
TO 5000-7000 FT SATURDAY AFTERNOON AS WARMER AIR MOVES IN FROM
ANOTHER WARM FRONT. PARADISE AND MT BAKER COULD SEE BRIEF HEAVY SNOW
BEFORE CHANGING OVER TO RAIN AND MAY NEED A WINTER WEATHER ADVISORY
TO COVER THIS PERIOD. THE SNOW LEVELS WILL REMAIN HIGH SAT NIGHT
THROUGH SUN WITH MAINLY RAIN IN THE MOUNTAINS. THIS MAY RESULT IN
RIVER FLOODING AND A HYDROLOGIC OUTLOOK HAS BEEN ISSUED. AN UPPER
LEVEL RIDGE WILL BUILD OVER THE AREA SUN NIGHT WHICH SHOULD BRING AN
END TO THE STEADY/HEAVY PRECIP IN THE MOUNTAINS. 33

.LONG TERM...PREVIOUS DISCUSSION...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...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. THE SNOW LEVEL WILL RISE TO 5000 TO 7000 FEET BY
LATE SATURDAY AFTERNOON AND 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.

&&

.AVIATION...A FRONT MOVED INTO THE CASCADES EARLY THIS MORNING.
SCATTERED SHOWERS REMAIN OVER WESTERN WASHINGTON. A STRONGER FRONTAL
SYSTEM WILL REACH THE AREA LATER TONIGHT AND SATURDAY WITH WESTERLY
FLOW ALOFT. THE AIR MASS WILL REMAIN MOIST BUT THERE MAY BE SOME SUN
BREAKS TODAY. INCREASING MOISTURE AGAIN TONIGHT AND SATURDAY WITH
STRONG WESTERLY FLOW ALOFT. AT THE SURFACE STRONG SOUTHEAST WINDS
WILL DEVELOP TONIGHT. THE FLOW WILL TURN ONSHORE SATURDAY NIGHT.

CEILINGS ARE GENERALLY VFR WITH SOME HIGH END MVFR. SHOWERS AND SUN
BREAKS WILL DEVELOP TODAY WITH VFR CEILINGS EXCEPT MVFR IN SHOWERS.
CLOUDS WILL THICKEN AND LOWER TONIGHT WITH SATURDAY BEING MOSTLY
MVFR WITH RAIN.

KSEA...DISCUSSION ABOVE APPLIES. WIND TODAY WILL BE SOUTHERLY 8-12
KT BECOMING SOUTHEAST TONIGHT AND INCREASING SATURDAY. CHB

&&

.MARINE...SMALL CRAFT ADVISORY WINDS ARE LIKELY OVER MOST WATERS
TODAY IN THE WAKE OF A FRONT THAT PASSED LAST NIGHT. A STRONGER
SYSTEM WILL APPROACH TONIGHT WITH GALES LIKELY OVER MOST WATERS.
BOTH THE ADVISORIES AND WARNINGS ARE ALREADY UP FOR THESE WINDS.
AFTER A BRIEF LULL IN WINDS SATURDAY AFTERNOON...MODELS SHOW A
PERIOD OF STRONG ONSHORE FLOW SATURDAY NIGHT THAT WILL ALSO HAVE
GALE SOUTHWEST WINDS OVER MOST WATERS.

A WEAKER SYSTEM ARRIVES MONDAY NIGHT OR TUESDAY.

SWELL ON THE COAST WILL REACH 20 FEET OR SO TONIGHT AND SATURDAY.
COUPLED WITH HIGH TIDES AROUND NOON SATURDAY THERE COULD BE SOME
MINOR TIDAL OVERFLOW AT SPOTS. SO FAR BUOY 5...ABOUT 300 MILES OFF
THE COAST...SHOWS ONLY 20 FOOT SWELLS. THESE WILL DECAY A BIT BEFORE
ARRIVING AT THE COAST. WILL DECIDE WHAT TO DO WITH THE COASTAL FLOOD
WATCH THIS AFTERNOON. CHB

&&

.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...SMALL CRAFT ADVISORY ALL WATERS EXCEPT PUGET SOUND AND
      THE CENTRAL STRAIT OF JUAN DE FUCA THROUGH THIS EVENING.
     GALE WARNING ALL WATERS EXCEPT THE CENTRAL STRAIT AND PUGET
      SOUND LATER TONIGHT THROUGH SAT.
     SMALL CRAFT ADVISORY GRAYS HARBOR BAR.

&&

$$

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





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