Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 190437
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PST THU DEC 18 2014

.SYNOPSIS...A FRONT WILL BRING RAINY AND LOCALLY WINDY WEATHER TO
WESTERN WASHINGTON TONIGHT...FOLLOWED BY AN UPPER TROUGH AND
SCATTERED SHOWERS FRIDAY. A FRONTAL SYSTEM WILL MOVE THROUGH
SATURDAY...FOLLOWED BY MOIST WESTERLY FLOW ON SUNDAY. PRECIPITATION
WILL BE ESPECIALLY HEAVY IN THE MOUNTAINS. THE WEATHER WILL BE
SOMEWHAT DRIER EARLY NEXT WEEK WITH AN UPPER RIDGE OVER THE AREA.

&&

.SHORT TERM...A VIGOROUS FRONTAL SYSTEM IS MOVING INTO THE PACIFIC
NORTHWEST THIS EVENING...WITH RAIN FALLING ACROSS MOST OF WESTERN
WASHINGTON. SOUTHEAST WINDS ARE GUSTING INTO THE 25 TO 35 MPH RANGE
ALONG THE COAST AND IN THE NORTH INTERIOR.

THE SURFACE FRONT MADE LANDFALL ON THE NORTH WASHINGTON COAST JUST
AFTER 7 PM...AND IT SHOULD MOVE THROUGH THE WESTERN WASHINGTON
INTERIOR LATE THIS EVENING OR AROUND MIDNIGHT. LIGHT TO MODERATE
PRECIPITATION AND LOCALLY WINDY CONDITIONS WILL CONTINUE THROUGH THE
FRONTAL PASSAGE. WINDS WILL EASE AND PRECIPITATION WILL DECREASE
OVERNIGHT. THE SNOW LEVEL WILL BE IN THE 4000 TO 5000 FT RANGE AS
THIS SYSTEM MOVES THROUGH...ALTHOUGH EASTERLY WINDS IN THE PASSES
COULD KEEP THE SNOW LEVEL LOCALLY LOWER. SEVERAL INCHES MORE
SNOWFALL CAN BE EXPECTED OVERNIGHT.

THE POST-FRONTAL UPPER TROUGH WILL MOVE THROUGH THE AREA FRIDAY...
AND IT SHOULD BE A SHOWERY DAY IN THE COOLER AND WEAKLY UNSTABLE AIR
MASS. THE SNOW LEVEL WILL BE 3000 TO 4000 FT WITH UP TO 4 INCHES OF
ADDITIONAL SNOW IN THE MOUNTAINS.

ANOTHER MORE VIGOROUS FRONTAL SYSTEM WILL SPREAD PRECIPITATION INTO
WESTERN WASHINGTON FRIDAY NIGHT...WITH THE CORE OF THE SYSTEM MOVING
THROUGH SATURDAY AND SATURDAY NIGHT. THIS SYSTEM WILL BRING SOMEWHAT
WINDIER CONDITIONS TO MAINLY THE COAST AND NORTH INTERIOR...WHERE A
HIGH-END WIND ADVISORY EVENT IS LIKELY -- MEANING SUSTAINED WINDS UP
TO 35 MPH AND GUSTS UP TO 55 MPH. THE SNOW LEVEL WILL BE 2500 TO
3500 FT AS PRECIPITATION BEGINS...BUT IT WILL RISE QUICKLY TO 5500
TO 7000 FT SATURDAY NIGHT. PRECIPITATION AMOUNTS WITH THIS SYSTEM
WILL BE SIGNIFICANT...AS DISCUSSED IN MORE DETAIL IN THE HYDROLOGY
SECTION BELOW.

MOIST WESTERLY FLOW WILL FOLLOW THE SYSTEM ON SUNDAY WITH AREAS OF
MODERATE TO HEAVY PRECIPITATION CONTINUING...ESPECIALLY IN THE
MOUNTAINS.   MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE AFTERNOON
FORECAST DISCUSSION -- THE EXTENDED FORECAST REMAINS CHALLENGING
WITH A WIDE RANGE OF SOLUTIONS AMONG THE MODELS. THERE IS SOME
AGREEMENT THAT MONDAY WILL GENERALLY FEATURE SHOWERS OR A LULL IN
RAINFALL AS HIGHER PRESSURE BUILDS ALOFT. MODELS DO NOT AGREE ON THE
STRENGTH OR TRACK OF THE NEXT SYSTEM APPROACHING FROM THE W ON
TUESDAY. THE LATEST 12/18Z GFS BRINGS THE LOW IN FURTHER N WHICH
WOULD STILL PRODUCE WARM ADVECTION RAINFALL ACROSS THE AREA...THOUGH
NOT AS MUCH AS WAS SHOWN BY PREVIOUS SOLUTIONS.

MOST MODELS DIG A COLD UPPER TROUGH SWD OUT OF B.C. THROUGH WRN WA
ON WEDNESDAY. THIS COULD USHER IN COOLER AIR BUT MODELS DISAGREE ON
THE EXTENT OF RIDGING OFFSHORE AND WHERE IT SETS UP. THE ECMWF
CONTINUES TO BE ONE OF THE COLDER SOLUTIONS AND HAS THE KONA LOW CUT
OFF FURTHER W...THUS A FURTHER W DEVELOPMENT OF THE UPPER RIDGE. THE
KONA LOW IS FURTHER E ON THE GFS40 WITH A RIDGE NEAR 135-140 WHICH
WILL STILL USHER IN COOLER NLY FLOW BUT PERHAPS LESS CHANCE OF
PRECIPITATION. IT WILL BE INTERESTING TO SEE HOW THIS PANS OUT BUT A
SWITCH TO COOLER OR COLD TEMPERATURES IS A POSSIBILITY BY THURSDAY.
POPS WERE TRIMMED WAY BACK AND MAY NEED TO BE REMOVED ALTOGETHER IF
MODEL TRENDS CONTINUE. SNOW LEVELS WILL ALSO BE MUCH
LOWER...POSSIBLY TO SEA LEVEL...BUT IT IS VERY UNCLEAR AS TO HOW
MUCH MOISTURE WILL REMAIN BY THAT TIME.

&&

.HYDROLOGY...THOUGH RAIN WILL FALL AT TIMES...FLOODING IS UNLIKELY
ON ANY RIVER THROUGH SATURDAY.

HEAVY RAIN COULD CAUSE FLOODING BY LATE THIS WEEKEND. A WEATHER
SYSTEM WITH STRONG WESTERLY FLOW ALOFT WILL BRING HEAVY RAIN AT
TIMES TO THE COAST AND OLYMPICS ON SATURDAY. HEAVY RAIN WILL SPREAD
INTO THE CASCADES SATURDAY AFTERNOON.

MODELS CURRENTLY TAPER RAIN OVER THE OLYMPICS ON SUNDAY BUT KEEP
HEAVY RAIN GOING IN THE CASCADES...ESPECIALLY OVER THE CASCADES FROM
ABOUT KING COUNTY SOUTH. THE TOTAL MODEL RAINFALL OVER ABOUT 48
HOURS FROM SATURDAY MORNING THROUGH SUNDAY NIGHT AMOUNTS TO AROUND
FOUR TO SIX INCHES OVER THE OLYMPICS AND CENTRAL CASCADES WITH A
TEN-INCH BULLSEYE OVER MOUNT RAINIER. THE SNOW LEVEL WILL BE ABOVE
5000 FEET ON SATURDAY AND SUNDAY BEFORE FALLING TO 3500 FEET OR SO
SUNDAY NIGHT.

THESE RAINFALL AMOUNTS ARE 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.

A COOLER AND DRIER AIR MASS IS FORECAST TO BE OVER THE AREA NEXT
WEEK. AFTER MONDAY THE RIVERS WILL RECEDE AND ANY FURTHER FLOODING
IS UNLIKELY. BURKE

&&

.AVIATION...A FRONT OVER WESTERN WA THIS EVENING WILL MOVE INLAND
OVERNIGHT. A STRONGER FRONT WILL REACH THE AREA FRIDAY NIGHT AND
SATURDAY MORNING. RAIN TONIGHT WILL CHANGE TO SHOWERS...AND THEN
BACK TO RAIN FRIDAY NIGHT. THE AIR WILL BE MOIST...THEN SOME
SUNBREAKS FRIDAY...THEN INCREASING MOISTURE AGAIN FRIDAY NIGHT. SW
FLOW ALOFT.

KSEA...RAIN TURNING TO SHOWERS...SOME SUNBREAKS POSSIBLE
FRIDAY...THEN BACK TO RAIN FRIDAY NIGHT. SOUTHERLY BREEZE.

&&

.MARINE...A FRONT OVER WESTERN WA THIS EVENING WILL MOVE INLAND
OVERNIGHT. A STRONGER FRONT WILL REACH THE AREA FRIDAY NIGHT AND
SATURDAY MORNING. THE WIND HAS FALLEN OFF OVER THE COASTAL WATERS
THIS EVENING BEHIND THE FRONT...BUT THERE IS STILL A GALE WARNING UP
FOR THE EAST ENTRANCE WHERE SMITH ISLAND STILL HAD 30-40KT SE WINDS
AT 8PM. THE SWELL WILL RISE THROUGH FRIDAY NIGHT...PEAKING AROUND 22
FT.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...WIND ADVISORY THROUGH TONIGHT FOR THE SAN JUAN ISLANDS...
     WESTERN WHATCOM COUNTY...AND THE ADMIRALTY INLET AREA.
     WINTER WEATHER ADVISORY THROUGH TONIGHT FOR THE OLYMPICS.
     WINTER WEATHER ADVISORY FOR THE WEST SLOPES OF THE CASCADES
     4 PM PST THIS AFTERNOON THROUGH TONIGHT.
     HIGH SURF ADVISORY FOR THE NORTH AND CENTRAL COAST FROM LATE
     FRIDAY MORNING THROUGH EARLY SATURDAY MORNING.
     COASTAL FLOOD WATCH FOR THE NORTH AND CENTRAL COAST FROM
     SATURDAY MORNING THROUGH SATURDAY AFTERNOON.

PZ...GALE WARNING THROUGH TONIGHT COASTAL WATERS AND EAST
     ENTRANCE.
     SMALL CRAFT ADVISORY ALL REMAINING WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR TONIGHT THROUGH
     FRIDAY.
     GALE WATCH COAST...ENTRANCES TO THE STRAIT OF JUAN DE FUCA...
     ADMIRALTY INLET...AND NORTHERN INLAND WATERS FRIDAY NIGHT
     THROUGH SATURDAY.

&&

$$

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




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