Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 201722
AFDSEW

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

.SYNOPSIS...A WARM FRONT WILL MOVE ACROSS WESTERN WASHINGTON TODAY.
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...A STRONG WARM FRONT WILL MOVE INLAND TODAY FOR MORE
RAIN AND HEAVY MOUNTAIN PRECIP. HIGH WIND IS MARGINAL OVER THE NORTH
INTERIOR AND WILL LET THE WARNING CONTINUE THROUGH NOON TODAY. WA
DOT SHOWS SNOW IN THE PASSES THIS MORNING AND A WINTER WEATHER
ADVISORY REMAINS IN EFFECT. THE SNOW LEVELS WILL RISE TO 5000-6000
FT THIS AFTERNOON AS WARMER AIR MOVES INLAND...WITH SNOW CHANGING
OVER TO RAIN IN THE PASSES. STRONG PERSISTENT WESTERLY FLOW WILL
PREVAIL TONIGHT WITH HEAVY RAIN CONTINUING IN THE MOUNTAINS. THIS
WILL LEAD TO POSSIBLE RIVER FLOODING AND A FLOOD WATCH REMAINS IN
EFFECT. MODELS SHOW SEATTLE IN THE RAIN SHADOW TONIGHT DUE TO
WESTERLY DOWNSLOPE FLOW OFF THE OLYMPICS...RAINFALL TOTALS WILL BE
LIGHT IF ANYTHING.

STRONG WESTERLY UPSLOPE FLOW WILL CONTINUE ON SUN WITH MORE HEAVY
RAIN ACROSS THE WEST SLOPES OF THE MOUNTAINS. AN UPPER LEVEL RIDGE
WILL THEN BUILD INLAND SUN NIGHT WITH HEAVY PRECIP TAPERING OFF. THE
RIDGE WILL REMAIN IN PLACE ON MON FOR DRIER WEATHER. 33

.LONG TERM...PREVIOUS DISCUSSION...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...THREE TO FIVE INCHES OF RAIN IS EXPECTED TO FALL OVER
THE OLYMPICS AND OVER THE CASCADES FROM SNOHOMISH THROUGH LEWIS
COUNTIES. A BULLSEYE OF SEVEN 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. LESS PRECIP IS FORECAST OVER THE CASCADES OF WHATCOM AND
SKAGIT COUNTIES WITH TWO TO FOUR INCHES...THE FLOOD THREAT IS LESS
OVERALL.

PRECIPITATION IS STARTING AS SNOW OVER THE CASCADES THIS MORNING BUT
SNOW LEVELS WILL RISE RAPIDLY...TO 5000 TO 7000 FEET THIS AFTERNOON
AND EVENING. THE HEAVIEST RAIN WILL BE OVER THE OLYMPICS TODAY AND
TONIGHT...AND OVER THE CASCADES FROM LATE THIS AFTERNOON THROUGH
SUNDAY MORNING.

ANY FLOODING WOULD LIKELY BEGIN AS EARLY AS LATE THIS AFTERNOON OR
EVENING ON THE SKOKOMISH RIVER. THE CASCADE RIVERS COULD FLOOD
TONIGHT OR SUNDAY. MODELS SHOW HEAVY RAIN ENDING SUNDAY NIGHT.

&&

.AVIATION...A STRONG FRONTAL SYSTEM WILL MOVE THROUGH WESTERN
WASHINGTON TODAY AND TONIGHT WITH WESTERLY FLOW ALOFT. AT THE
SURFACE STRONG SOUTHEAST GRADIENTS TODAY WILL TURN ONSHORE TONIGHT.
THE AIR MASS WILL BE MOIST AND STABLE WITH WIDESPREAD RAIN.

CEILINGS ARE GENERALLY AROUND 3K FT THIS MORNING WITH VISIBILITY
RESTRICTIONS TO 2SM IN RAIN AT TIMES. THERE MIGHT BE A FEW HOURS OF
IMPROVEMENT EARLY THIS AFTERNOON...WITH SOME 4-5K FT CEILINGS...BUT
CONDITIONS WILL DETERIORATE LATE THIS AFTERNOON AND EVENING. THE
MOUNTAINS AND FOOTHILLS WILL BE OBSCURED.

KSEA...RAIN AND AN INCREASING SOUTHERLY WIND OF 12G22KT LATER TODAY
WITH THE PASSAGE OF A SECOND FRONTAL WAVE. CHB

&&

.MARINE...A STRONG SYSTEM WILL MOVE THROUGH WESTERN WASHINGTON TODAY
AND TONIGHT. GALE SOUTHEAST WINDS WILL SWITCH TO SOUTHWEST OVER THE
COAST THIS AFTERNOON. OVER THE INLAND WATERS GALE SOUTHEAST WINDS
WILL BECOME SMALL CRAFT ADVISORY STRENGTH SOUTHWEST WINDS. SEAS OVER
THE COASTAL WATERS WILL BE AROUND 20 FEET TODAY. THERE IS A HIGH
SURF ADVISORY FOR THE COASTAL BEACHES THROUGH TODAY AND A COASTAL
FLOOD ADVISORY FOR MINOR FLOODING OF LOW AREAS NEAR THE BEACHES
DURING THE TIME OF HIGH TIDE THIS MORNING.

SUNDAY SHOULD BE A POSTFRONTAL DAY WITH STRONG ONSHORE FLOW EASING
SUNDAY NIGHT AND MONDAY. A RIDGE OF HIGH PRES WILL BUILD OVER THE
AREA ON MONDAY. A WEAKER SYSTEM WILL ARRIVE AROUND TUESDAY. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...FLOOD WATCHES IN EFFECT FOR PORTIONS OF WESTERN WA.
     HIGH WIND WARNING WESTERN WHATCOM...SAN JUAN
     ISLANDS..ADMIRALTY INLET.
     WIND ADVISORY WESTERN SKAGIT COUNTY.
     WINTER WEATHER ADVISORY CASCADES.
     HIGH SURF ADVISORY NORTH AND CENTRAL COAST.
     COASTAL FLOOD ADVISORY NORTH AND CENTRAL COAST SATURDAY MORNING.

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.

&&

$$

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




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