Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 210540
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
940 PM PST SAT DEC 20 2014

.SYNOPSIS...STRONG AND MOIST WESTERLY FLOW ALOFT WILL PERSIST
THROUGH SUNDAY. HIGH PRESSURE ALOFT WILL BUILD OVER THE REGION
SUNDAY NIGHT AND WILL REMAIN OVER THE AREA ON MONDAY FOR A DRYING
TREND. THE HIGH WILL WEAKEN ON ON TUESDAY...ALLOWING A FRONT TO
SAG INTO THE AREA FROM THE NORTHWEST.

&&

.SHORT TERM...
THE STEADY PRECIP HAS BECOME SHOWERY ACROSS THE CWA. HOWEVER
STRONG OROGRAPHIC ENHANCEMENT WILL RESULT IN LOCALLY HEAVY SHOWERS
AT TIMES ON THE WEST FACING SLOPES OF THE MOUNTAINS. STRONG
SHADOWING TO THE LEE OF THE OLYMPIC RANGE WILL RESULT IN PARTS OF
THE PUGET SOUND REGION BEING MAINLY DRY OVERNIGHT.

STRONG AND MOIST ZONAL FLOW ALOFT WILL CONTINUE ON SUNDAY. FAST-
MOVING DISTURBANCES EMBEDDED IN THIS FLOW WILL RESULT IN LOCALLY
HEAVY SHOWERS ON THE WEST FACING SLOPES OF THE CASCADES. THE SNOW
LEVEL SHOULD BE MAINLY IN THE 4000 TO 5000 FOOT RANGE DURING THE
DAY...SO EXPECT THE PRECIP TO FALL MAINLY AS SNOW ABOVE THE 4000
FOOT LEVEL. IN FACT...IT LOOKS LIKE A WINTER WEATHER ADVISORY WILL
BE NEEDED FOR THE CASCADES FOR SUNDAY AS AMOUNTS COULD END UP IN
THE 6 TO 11 INCH RANGE...ESPECIALLY IN THE CENTRAL CASCADES.

EXPECT A GRADUAL DRYING TREND SUNDAY NIGHT DUE TO AN UPPER RIDGE
BUILDING OVER THE REGION. THIS RIDGE WILL PREVAIL OVER THE AREA ON
MON FOR DRY CONDITIONS ACROSS MUCH OF THE CWA.

MODELS SHOWED THE RIDGE FLATTENING ON TUE WITH A BAROCLINIC ZONE
SAGGING SOUTH OVER THE CWA DURING THE DAY. 33/05

.LONG TERM FROM THE PREVIOUS DISCUSSION...
A COOL UPPER LEVEL TROUGH WILL CUT THROUGH WESTERN WA ON WEDNESDAY
FOR COOLER WEATHER AND MORE SHOWERS. SNOW LEVELS WILL DROP TO
AROUND 2000 FT. DRY NORTHERLY FLOW WILL PREVAIL ON CHRISTMAS DAY
FOR MAINLY DRY WEATHER...WITH JUST ISOLATED SHOWERS IN THE CASCADES.
THIS PATTERN WILL CONTINUE INTO FRIDAY. MODELS THEN SHOW ANOTHER
COOL UPPER TROUGH CROSSING THE PAC NW ON SATURDAY...WITH SHOWERS
LINGERING INTO SUN. 33

&&

.HYDROLOGY...
A FLOOD WARNING WAS ISSUED FOR THE SKOKOMISH RIVER LATE IN THE
AFTERNOON. THE RIVER WAS STILL RISING AND WAS AT 16.9 FEET AROUND
915 PM PST. OTHER RIVERS SUCH AS THE ELWHA WERE SHOWING SHARP
RISES. A FLOOD WATCH REMAINS IN EFFECT FOR MUCH OF THE RIVERS
DRAINING OFF THE OLYMPICS AND CENTRAL CASCADES THROUGH LATE SUNDAY
NIGHT.

SNOW LEVELS ALSO ROSE THIS EVENING...WITH THE SNOW CHANGING OVER
TO RAIN ABOVE THE 3000 FOOT LEVEL. A MILDER AIR MASS ALONG WITH
STRONG UPSLOPE FLOW WILL RESULT IN LOCALLY HEAVY SHOWERS AT
TIMES...MAINLY ON THE CASCADES...THROUGH SUNDAY. THE MODELS
INDICATED AN ADDITIONAL 1-2 INCHES OF PRECIP WILL FALL ON THE
CASCADES OVERNIGHT INTO SUNDAY MORNING. HOWEVER THE SNOW LEVELS
WILL LOWER INTO THE 4500 TO 5500 FOOT RANGE BY SUNDAY MORNING...SO
SOME OF THIS PRECIP WILL BE IN THE FORM OF SNOW.

THE RIVERS THAT DO EXPERIENCE FLOODING SHOULD CREST DURING THE
DAY SUNDAY...SUNDAY NIGHT AT THE LATEST.

FLOODING IS NOT EXPECTED MON THROUGH SATURDAY.

&&

.AVIATION...STRONG WESTERLY FLOW ALOFT WILL PREVAIL TONIGHT AND
SUNDAY. STRONG ONSHORE FLOW AT THE SURFACE WILL START TO EASE LATER
SUNDAY. THE AIR MASS WILL BE MOIST AND STABLE. LOW CLOUDS TONIGHT.
CEILINGS SHOULD LIFT A LITTLE DURING THE DAY ON SUNDAY.

KSEA...LOW CLOUDS. CEILINGS LIFTING A BIT SUNDAY AFTERNOON.
SOUTHERLY WIND 15G30KT...EASING A BIT SUNDAY AFTERNOON. SCHNEIDER

&&

.MARINE...A STRONG FRONTAL SYSTEM WILL MOVE ONSHORE TONIGHT...
BRINGING GALES TO MOST WATERS. SUNDAY WILL BE A POST-FRONTAL DAY WITH
ONSHORE FLOW EASING SUNDAY NIGHT AND MONDAY. HIGH PRESSURE WILL
BUILD OVER THE AREA ON MONDAY. A WEAKER FRONTAL SYSTEM WILL ARRIVE
AROUND TUESDAY. SCHNEIDER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNINGS OR WATCHES ARE IN EFFECT FOR MUCH OF THE CWA.

PZ...GALE WARNINGS ARE IN EFFECT FOR THE COASTAL WATERS...STRAIT
     OF JUAN DE FUCA...AND NORTHERN INLAND WATERS.

     SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE REMAINING
     WATERS.

     SMALL CRAFT ADVISORY FOR ROUGH BAR IN EFFECT TIL 6 PM SUNDAY
     FOR THE GRAYS HARBOR BAR.

&&

$$

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