Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

000
FXUS66 KSEW 211129
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 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 WEST FLOW ALOFT WILL CONTINUE OVER W WA
TODAY...THEN EASE A LITTLE TONIGHT. LOW LEVEL MOISTURE AND LARGE
SCALE LIFT HAS DIMINISHED AS WELL...SO MUCH OF THE LOWLANDS IS
CURRENTLY FREE OF RAIN. GIVEN THE STRONG W FLOW THE MAIN FORCING
TODAY WILL BE OROGRAPHIC IN NATURE...WITH PRECIPITATION MAINLY OVER
THE MOUNTAINS. THE STRONGEST OROGRAPHIC FLOW WILL BE FROM ABOUT
SNOHOMISH COUNTY SOUTHWARD. LOW LEVEL ONSHORE FLOW IS ALSO PRESENT
SO COMBINED WITH THE STRONG W FLOW ALOFT THIS MAY CREATE SOME NARROW
CONVERGENCE LIKE RAIN BANDS OVER PARTS OF THE PUGET SOUND REGION.
OTHER THAN THESE RAIN BANDS THE PUGET SOUND REGION WILL BE IN THE
EAST RAIN SHADOW OF THE OLYMPICS SO OVERALL NOT MUCH RAIN IS
EXPECTED. THE ONSHORE FLOW IS STRONG ENOUGH TO KEEP WINDS BREEZY IN
MANY AREAS.

THE AIR MASS IS RATHER WARM THIS MORNING WITH SNOW LEVELS 5000 FEET
AROUND MT BAKER TO 6500 FEET AROUND MT RAINIER. THE SNOW LEVEL WILL
GRADUALLY LOWER TO 4000 TO 5500 FEET BY LATE THIS AFTERNOON. WITH
SNOW STARTING TO ACCUMULATE AT LOWER ELEVATIONS WHILE PRECIP RATES
DECLINE...SNOW ACCUMULATIONS SHOULD STAY BELOW ADVISORY LEVELS AT MT
BAKER AND STEVENS PASS. UP TO 10 INCHES MAY FALL ON MT RAINIER BUT
THIS SHOULD BE JUST ABOVE THE CRITICAL 5500 FOOT LEVEL OF PARADISE.

A MODERATE AMPLITUDE UPPER LEVEL RIDGE WILL BE BUILDING OFFSHORE
TONIGHT THEN SLIDE E OVER W WA ON MONDAY. GRADUAL DRYING WILL LOWER
THE CHANCE OF RAIN TONIGHT. ACCUMULATIONS OVER THE LOWLANDS WILL
GENERALLY BE A TENTH OF AN INCH OR LESS. ACCUMULATIONS OVER THE
CASCADES WILL BE HIGHER BUT HYDROLOGICALLY INSIGNIFICANT.

WITH THE UPPER LEVEL RIDGE AXIS JUST OFFSHORE...MONDAY WILL BE
PRETTY MUCH DRY. I KEPT LOW CHANCE POPS MONDAY MORNING MAINLY
BECAUSE THE AIR MASS WILL STILL BE ON THE MOIST SIDE. MONDAY
AFTERNOON WILL BE DRY WITH A LITTLE BIT OF SUNSHINE. THE DRY PERIOD
SHOULD EXTEND INTO MONDAY EVENING.

MODELS HAVE BEEN PRETTY CONSISTENT IN SLIDING THE UPPER LEVEL RIDGE
INLAND AND FLATTENING IT MONDAY NIGHT. THIS WILL ALLOW A FRONTAL
SYSTEM PUSHING INTO THE UPSTREAM SIDE OF THE RIDGE TO MOVE INLAND
OVER W WA. WEAK WARM ADVECTION WILL BRING LIGHT RAIN TO THE AREA
LATE MONDAY NIGHT AND TUESDAY. KAM

.LONG TERM...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...RAINFALL AMOUNTS FOR THE PAST 12 HOURS ENDING AT 3 AM
HAVE GENERALLY BEEN BETWEEN 1 AND 3 INCHES IN THE OLYMPICS AND
CASCADES WITH THE HIGHEST AMOUNT AT WHITE PASS WITH A LITTLE MORE
THAN 3 AND A HALF INCHES. RAINFALL RATES HAVE SLOWED CONSIDERABLY IN
THE LAST HOUR OR TWO...DOWN TO AROUND A TENTH TO TWO TENTHS AN HOUR.
RAINFALL RATES WILL CONTINUE TO DECREASE THIS MORNING IN THE
MOUNTAINS. IN ADDITION THE PRECIPITATION WILL TURN OVER TO SNOW
ABOVE ABOUT 5000 FEET AS A SLIGHTLY COOLER AIR MASS MOVES INTO THE
AREA.

WITH THE DECREASING RAINFALL RATES THE FLOOD WATCH FOR CLALLAM AND
JEFFERSON COUNTY WILL BE CANCELLED WITH THE EARLY MORNING FORECAST
PACKAGE. THERE IS A GOOD CHANCE THE WATCH FOR THE REMAINING COUNTIES
GRAYS HARBOR...KING...LEWIS...PIERCE...SNOHOMISH...AND THURSTON WILL
BE CANCELLED AS WELL DURING THE DAY TODAY.

THE ONLY RIVER WITH A FLOOD WARNING IN EFFECT EARLY THIS MORNING IS
THE SKOKOMISH. WITH THE HEAVIEST RAIN OVERNIGHT FROM MOUNT RAINIER
SOUTH THE BEST CHANCE FOR ANY ADDITIONAL RIVER FLOODING IS ON THE
COWLITZ RIVER AND EVEN THAT WILL BE MINOR IF THE RIVER REACHES FLOOD
STAGE.

FLOODING IS NOT EXPECTED MON THROUGH SATURDAY. FELTON

&&

.AVIATION...STRONG WESTERLY FLOW ALOFT WILL PREVAIL TODAY AND
TONIGHT. AT THE SURFACE...MODERATE TO STRONG ONSHORE FLOW WILL
GRADUALLY EASE TODAY AND TONIGHT WITH HIGH PRESSURE BUILDING OVER
THE COAST TONIGHT. AIR MASS MOIST AND SLIGHTLY UNSTABLE. THERE WILL
BE AREAS OF LOW CLOUDS TODAY BUT ALSO SOME BREAKS. THERE WILL LIKELY
BE A PUGET SOUND CONVERGENCE ZONE OVER NORTHERN PUGET SOUND TODAY.

KSEA...THERE SHOULD BE LOW CLOUDS AT TIMES TODAY BUT ALSO SOME
BREAKS. SOUTHERLY WIND 15G30KT...EASING A BIT BY AFTERNOON. SCHNEIDER

&&

.MARINE...MODERATE TO STRONG ONSHORE FLOW WILL PREVAIL TODAY WITH
WESTERLY GALES IN THE STRAIT AND SMALL CRAFT ADVISORIES ALL OTHER
WATERS. ONSHORE FLOW WILL EASE TONIGHT WITH HIGH PRESSURE BUILDING
OVER THE AREA ON MONDAY. A FRONTAL SYSTEM WILL ARRIVE AROUND
TUESDAY. SCHNEIDER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNINGS OR WATCHES ARE IN EFFECT FOR PARTS OF THE
      CWA.
PZ...GALE WARNING CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.
     SMALL CRAFT ADVISORY ALL OTHER WATERS.
     SMALL CRAFT ADVISORY FOR SEVERE CONDITIONS GRAYS HARBOR BAR.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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






USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.