FXUS66 KSEW 091046
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
245 AM PST WED DEC 9 2009

.SYNOPSIS...AN UPPER LEVEL RIDGE JUST WEST OF THE BRITISH COLUMBIA 
COAST WILL MAINTAIN A COLD DRY AIR MASS OVER WESTERN WASHINGTON 
THROUGH FRIDAY. WEAK DISTURBANCES EMBEDDED IN THE NORTH FLOW ALOFT 
MAY BRING SOME SNOW FLURRIES TO THE NORTH PART THURSDAY AND FRIDAY. 
A FRONTAL SYSTEM ARRIVING FROM THE WEST WILL BRING WARMER AND WETTER 
WEATHER EARLY NEXT WEEK. 

&&

.SHORT TERM...DEWPOINTS ARE RUNNING A LITTLE HIGHER THAN THIS TIME 
YESTERDAY...BUT LIGHTER GRADIENTS AND SPARSE HIGH CLOUDS ARE 
ALLOWING TEMPERATURES TO FALL MORE QUICKLY OVERNIGHT. THIS SHOULD BE 
THE COLDEST MORNING OF THE WEEK...WITH TEENS AND 20S URBAN AND WATER 
AREAS...AND SINGLE DIGITS IN THE COLDEST LOWLAND VALLEYS. THERE IS 
NOT MUCH CHANGE IN THE FORECAST THINKING THROUGH FRIDAY. ANOTHER 
MOSTLY SUNNY DAY IS EXPECTED TODAY...WITH HIGH TEMPERATURES RUNNING 
11-14 DEGREES BELOW NORMAL. HEIGHTS RECOVER SLIGHTLY THURSDAY INTO 
FRIDAY...WITH 850MB TEMPS OVER THE CENTRAL SOUND RISING FROM NEAR 
-9C OR SO THIS AFTERNOON TO -5C FRIDAY AFTERNOON. THIS TRANSLATES TO 
HIGH TEMPERATURES MODERATING FROM NEAR FREEZING TODAY TO UPPER 30S 
TO NEAR 40 BY FRIDAY. A SERIES OF WEAK DISTURBANCES IN NORTH TO 
NORTHWEST FLOW ALOFT MAY BRING A FEW FLURRIES TO NORTHERN ZONES 
THURSDAY INTO FRIDAY...OTHERWISE LITTLE MORE THAN A GRADUAL INCREASE 
IN CLOUD COVER IS EXPECTED WHICH SHOULD HELP INSULATE US SOMEWHAT 
FROM THE OVERNIGHT TEMPERATURES OF THE PAST 2 OR 3 MORNINGS.

CURRENT LOW CHANCE POPS FOR FRIDAY NIGHT STILL LOOK GOOD AS THE 
MODELS GENERALLY KEEP PRECIP ASSOCIATED WITH THE SOUTHERN BRANCH OF 
THE JET WELL SOUTH OF THE AREA...AND PRECIP WITH THE NORTHERN BRANCH 
MOSTLY OVER BRITISH COLUMBIA. PLENTY OF QUESTIONS REMAIN IN WHAT 
MATERIALIZES BEYOND THEN...AND THAT IS ADDRESSED BELOW. 

.LONG TERM...A LOT OF UNCERTAINTY REMAINS IN THE DETAILS OF WHAT 
WILL OCCUR OVER THE WEEKEND AS WE SWITCH FROM THE COLD DRY PATTERN 
OF LATE TO A MILDER UNSETTLED PATTERN NEXT WEEK. THE EURO MODEL IS 
NOT AS AGGRESSIVE WITH A MODIFIED ARCTIC BOUNDARY PUSHING TOWARD THE 
CANADIAN BORDER ON SUNDAY AS IT WAS THIS TIME YESTERDAY. IN 
FACT...WHEN COMPARED TO THE OPERATIONAL GFS...IT WOULD BE THE MORE 
FAVORABLE FOR A WARMER SCENARIO THIS WEEKEND. 

THE 00Z GFS ACTUALLY DEVELOPS A SURFACE LOW OFF THE COAST EARLY 
SATURDAY...WHICH HELPS DRAW IN SOME COLD AIR FROM THE NORTH. 
THIS IN TURN BRINGS LOW LEVEL THICKNESSES DOWN. THE GFS 
ENSEMBLE MEAN DOES NOT AGREE WITH THIS. THE 00Z SUNDAY GFS 500MB 
STANDARD DEVIATION PLOTS SHOW A LOT OF UNCERTAINTY BOTH TO OUR 
SOUTHWEST AND OUR NORTH. THE LATEST RUN OF THE CANADIAN ADDS SOME 
CREDENCE TO WHAT THE EURO MODEL SHOWS IN SPINNING UP A SURFACE LOW 
OVER VANCOUVER ISLAND ON SUNDAY...WHICH WOULD KEEP MOST OF THE 
LOWLANDS OUT OF THE SNOW SCENARIO EXCEPT PERHAPS IN WHATCOM COUNTY. 
BEYOND THE TRANSITIONAL WEEKEND PERIOD...THE MODELS ARE IN PRETTY 
GOOD AGREEMENT OF A RETURN TO THE FAMILIAR LOWLAND RAINS AND 
MOUNTAIN SNOW WE EXPECT THIS TIME OF YEAR. NO SIGNIFICANT CHANGES 
WERE MADE TO THE EXTENDED FORECAST OVERNIGHT. 27

&&

.HYDROLOGY...WITH THE COLD DRY WEATHER PATTERN IN PLACE THIS WEEK... 
NO RIVER FLOODING WILL OCCUR. ANY PRECIPITATION OVER THE WEEKEND IS 
LIKELY TO BE LIGHT...AND CONTINUED COLD WEATHER WILL MAKE FLOODING 
UNLIKELY THEN AS WELL. EARLY NEXT WEEK A WET AND WARMER WEATHER 
SYSTEM IS FORECAST TO REACH THE PACIFIC NORTHWEST. AT THIS TIME...IT 
APPEARS THAT PRECIP AMOUNTS AND SNOW LEVELS NEXT WEEK WILL REMAIN 
LOW ENOUGH TO KEEP THE FLOODING THREAT LOW. NO FLOODING IS FORECAST 
ON THE GREEN RIVER. 

&&

.AVIATION...LITTLE CHANGE IN THE WEATHER PATTERN THROUGH THU. LIGHT 
NLY FLOW WILL PERSIST OVER WA AND DRY AND STABLE CONDITIONS. HIGH 
PRES E OF THE CASCADES WILL RESULT IN LIGHT OFFSHORE FLOW THROUGH 
THE PERIOD. 

KSEA...SE WIND 4-8 KT..BECOMING NE AFTER 18Z. NO FOG EXPECTED.

&&

.MARINE...SEAS ARE HOVERING AROUND 10 FT THIS MORNING AT BUOY 41. 
WILL LEAVE UP THE SMALL CRAFT FOR HAZARDOUS SEAS TODAY. SEAS SHOULD 
START TO SUBSIDE BELOW 10 FT TONIGHT. 

SURFACE HIGH PRES AND WEAK GRADIENTS ARE RESULTING IN GENERALLY 
LIGHT WINDS ACROSS THE WATERS TODAY. A WEAK SURFACE LOW FORMING OFF 
VANCOUVER ISLAND ON THU AND FRI MAY INDUCE SOME INCREASED SE WIND UP 
TO 20 KT OVER THE NRN COASTAL WATERS AND W ENTRANCE. MODELS KEEP 
WINDS BELOW SMALL CRAFT CRITERIA THROUGH THU. 

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE. 
PZ...SMALL CRAFT ADVISORY COASTAL WATERS TODAY.
    .SMALL CRAFT ADVISORY GRAYS HARBOR THROUGH TONIGHT.
        
$$

WWW.WEATHER.GOV/SEATTLE

FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE 
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.