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[Printable]
723
FXUS66 KSEW 211701
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PST SUN DEC 21 2014

.SYNOPSIS...STRONG MOIST WEST FLOW ALOFT WILL CONTINUE TODAY THEN
WEAKEN TONIGHT...BRINGING LIGHT RAIN TO THE LOWLANDS AND HEAVIER
RAIN TO THE CASCADES. AN UPPER LEVEL RIDGE WILL BUILD OVER THE
REGION MONDAY FOR DRIER WEATHER. A FRONTAL SYSTEM WILL MOVE OVER
WESTERN WASHINGTON LATE MONDAY NIGHT AND TUESDAY FOLLOWED BY AN
UPPER LEVEL TROUGH ON WEDNESDAY FOR PERIODS OF RAIN. DRIER AND
COOLER NORTHWEST FLOW ALOFT WILL DEVELOP AFTER MIDWEEK.

&&

.SHORT TERM...STRONG WESTERLY WINDS ALOFT WILL CONTINUE TO BRING
MOISTURE TO WESTERN WA TODAY. SHOWERS WILL MAINLY AFFECT THE COAST
AND MOUNTAINS. MODELS PLACE SEATTLE IN THE RAIN SHADOW WITH
DOWNSLOPE WINDS OFF THE OLYMPICS...EXPECT LITTLE RAIN IF ANYTHING.
THE CASCADES WILL SEE ANOTHER 1-2 INCHES OF PRECIP WITH RAIN
EXPECTED AT THE PASSES. TEMPS WILL REMAIN MILD WITH HIGHS IN THE
50S. THE WINTER SOLSTICE IS TODAY...303 PM PST.

AN UPPER LEVEL RIDGE WILL START TO BUILD OVER THE PAC NW TONIGHT
WITH SHOWERS TAPERING OFF ACROSS WESTERN WA. THIS RIDGE WILL REMAIN
IN PLACE ON MON FOR MAINLY DRY WEATHER. THE RIDGE WILL THEN FLATTEN
ON TUE WHILE A COLD FRONT SLIDES S/SE FROM B.C. INTO WESTERN WA.
THIS SYSTEM WILL BRING RAIN TO MAINLY THE OLYMPIC PENINSULA AND
NORTH CASCADES. 33

.LONG TERM...PREVIOUS DISCUSSION...MODELS STILL AGREE THAT AN UPPER
LEVEL TROUGH WILL MOVE ACROSS W WA ON WEDNESDAY. THE REMNANTS OF THE
TUESDAY FRONTAL SYSTEM WILL STILL BE OVER W WA ON WEDNESDAY AND THE
APPROACHING TROUGH WILL MAKE GOOD USE OF THE MOISTURE. POPS HAVE
BEEN INCREASED WITH RAIN EXPECTED OVER ALL OF W WA. DRIER NW FLOW
ALOFT SETS UP OVER THE REGION BEHIND THE UPPER LEVEL TROUGH THURSDAY
AND FRIDAY. THE CURRENT FORECAST IS DRY...BUT MAY BE A LITTLE
OPTIMISTIC. THERE COULD STILL BE SOME LEFTOVER RAIN AS THE TROUGH
DEPARTS WEDNESDAY EVENING...AND THERE IS A CHANCE THAT ANY VERY WEAK
SHORTWAVES EMBEDDED IN THE NW FLOW COULD BRING A CHANCE OF RAIN
THURSDAY AND FRIDAY. THE GFS/ECMWF SEEM A LITTLE MORE CONSISTENT FOR
THE LATE WEEK PERIOD...BUT CONFIDENCE IS STILL ON THE LOW SIDE. KAM

&&

.HYDROLOGY...THOUGH SHOWERY PRECIPITATION CONTINUES THIS MORNING IN
THE MOUNTAINS...HYDROLOGICALLY SIGNIFICANT PRECIPITATION HAS ENDED.
A FLOOD WATCH REMAINS IN EFFECT ONLY FOR PIERCE...THURSTON...AND
LEWIS COUNTIES. FLOODING IN PIERCE AND LEWIS COUNTIES IS LOOKING
VERY UNLIKELY AND THERE IS ONLY A SMALL CHANCE OF MINOR FLOODING ON
THE LOWER COWLITZ RIVER IN LEWIS COUNTY. IT SHOULD BECOME CLEAR IN
THE NEXT FEW HOURS WHETHER THE COWLITZ AT RANDLE WILL CREST JUST
ABOVE OR BELOW FLOOD STAGE. WILL WAIT TO CANCEL THE FLOOD WATCH OR
KEEP IT GOING UNTIL THEN.

THE ONLY RIVER WITH A FLOOD WARNING IN EFFECT EARLY THIS MORNING IS
THE SKOKOMISH. THE SKOKOMISH WILL SLOWLY RECEDE TODAY AND FALL BELOW
FLOOD STAGE TONIGHT.

FLOODING IS UNLIKELY MONDAY THROUGH SATURDAY. BURKE

&&

.AVIATION...MODERATELY STRONG WLY FLOW ALOFT TONIGHT...BECOMING
LIGHTER NW FLOW ALOFT LATER TONIGHT AND MONDAY. HIGHER PRESSURE
ALOFT WILL BUILD MON AND MON NIGHT. AIR MASS MOIST AND SLIGHTLY
UNSTABLE. THERE WILL BE AREAS OF LOW CLOUDS TODAY BUT ALSO SOME
BREAKS...MAINLY THIS AFTERNOON. A WEAK PUGET SOUND CONVERGENCE ZONE
COULD DEVELOP AT TIMES TODAY...AFFECTING MAINLY THE WRN SNOHOMISH
AND KING/SNOHOMISH BORDER AREA. KSEA/KBFI ARE NOT EXPECTED TO BE
AFFECTED. KPAE WILL POSSIBLY BE AFFECTING WITH PERIODS OF LOWER MVFR
CIGS IN ANY HEAVIER SHOWERS.

KSEA...SSW WIND 8-12 KT W/ GUSTS TO 25 KT...EASING THIS AFTERNOON TO
SW WIND 8-10 KT. DTM

&&

.MARINE...MODERATE TO STRONG ONSHORE FLOW WILL PREVAIL TODAY WITH
SOLID WESTERLY SMALL CRAFT IN THE STRAIT AND MARGINAL SMALL CRAFT
ADVISORIES ALL OTHER WATERS. ONSHORE FLOW WILL EASE TONIGHT WITH
HIGH PRESSURE BUILDING OVER THE AREA ON MONDAY. WINDS OVER SOME OF
THE INTERIOR WATERS WILL GRADUALLY EASE TONIGHT. HIGHER PRESSURE
DEVELOPS MONDAY WITH WINDS GENERALLY UNDER 20 KT MOST AREAS. A
FRONTAL SYSTEM WILL ARRIVE AROUND TUESDAY WITH SOME SMALL CRAFT
WINDS AGAIN POSSIBLE. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNINGS OR WATCHES ARE IN EFFECT FOR PARTS OF THE
     CWA.
PZ...SMALL CRAFT ADVISORY ALL WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH TO 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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