FXUS66 KSEW 100443 CCA
AFDSEW

AREA FORECAST DISCUSSION...CORRECTED WARNINGS SECTION
NATIONAL WEATHER SERVICE SEATTLE WA
830 PM PST WED DEC 9 2009

.SYNOPSIS...NORTH TO NORTHWEST FLOW ALOFT FROM STRONG UPPER LEVEL 
RIDGE JUST WEST OF THE BRITISH COLUMBIA COAST WILL HELP MAINTAIN THE 
COLD DRY AIR MASS OVER WESTERN WASHINGTON THROUGH FRIDAY. WEAK 
DISTURBANCES EMBEDDED IN THE NORTHWEST FLOW MAY BRING A CHANCE OF 
SNOW FLURRIES TO THE NORTH FRIDAY AND AND TO THE REST OF THE AREA 
FRIDAY NIGHT THROUGH THE WEEKEND. A FRONTAL SYSTEM ARRIVING FROM THE 
WEST WILL BRING WARMER AND WETTER WEATHER MONDAY OR TUESDAY. 

&&

.SHORT TERM...THE STRONG UPPER LEVEL RIDGE CENTERED JUST W OF THE 
B.C. COAST STILL HAS ENOUGH AMPLITUDE TO MAINTAIN COLD DRY N-NW FLOW 
FROM THE YUKON ACROSS B.C. AND DOWN OVER W WA INTO THIS WEEKEND. 
SOUTHERN BRANCH W FLOW HAS ALREADY UNDERCUT THE WEAKENING RIDGE AND 
WILL BE STEERING WEATHER SYSTEMS OVER CA THE NEXT FEW DAYS. 

THE UPPER LEVEL RIDGE WILL CONTINUE WEAKENING THROUGH THIS 
WEEKEND...AND THE SURFACE HIGH PREVIOUSLY OVER B.C. HAS ALREADY 
SHIFTED OVER THE NORTHERN ROCKIES...ALLOWING THE LOW LEVEL FEED OF 
MODIFIED ARCTIC AIR TO END. THESE COMBINED WILL ALLOW THE CURRENT 
COLD DRY AIR MASS OVER W WA TO MODERATE SLOWLY THROUGH THE WEEKEND. 

LATEST 18Z/00Z NAM/GFS HAVE GONE PRETTY MUCH DRY THROUGH FRIDAY. 
LOOKING UPSTREAM OVER B.C. ON LATE AFTERNOON VISIBLE AND EARLY 
EVENING SATELLITE IMAGERY...NOTHING JUMPS OUT THAT WOULD INDICATE 
ANY SORT OF ORGANIZED SYSTEM MOVING S TOWARD THE AREA. THEREFORE THE 
FORECASTS WERE UPDATED EARLIER THIS EVENING TO DROP THE MENTION OF 
SNOW FLURRIES OVER THE NORTH PART THROUGH THURSDAY NIGHT. THAT 
SAID...SOMETIMES THE SUREST WAY TO MAKE IT SNOW SEEMS LIKE TO TAKE 
IT OUT OF THE FORECAST. THAT IS 
MOTHER-NATURE-INDUCED-METEOROLOGIST-PARANOIA...MNIMP...WHICH COULD 
ALSO BE THE SOUND ONE MIGHT REPEATEDLY MAKE WITH WILDLY-WIDE EYES 
WHEN THE FORECAST BUSTS AFTER A WRONG-WAY UPDATE. A MINOR INCREASE 
IN MOISTURE OVER THE N PART FRIDAY AND FRIDAY MIGHT SPIT OUT A SNOW 
FLURRY...SO HAVE LEFT THAT IN THE FORECAST. 

WITH THE UPPER LEVEL RIDGE WEAKENING...BY FRIDAY W WA IS IN-BETWEEN 
THE N BRANCH AND S BRANCH STREAMS. LATEST MODELS STILL HAVE RESIDUAL 
COOL NW FLOW ALOFT OVER W WA...BUT BOTH GFS/ECMWF SHOW A S BRANCH 
SYSTEM CROSSING OREGON. THE MODELS HAVE HAD A HARD TIME LATELY 
SETTLING ON WHETHER W WA WILL REMAIN UNDER THE INFLUENCE OF THE DRY 
N BRANCH OR WHETHER S STREAM MOISTURE WILL CREEP UP OVER THE AREA. 
AT THIS POINT BOTH MODELS SHOW S BRANCH MOISTURE ARRIVING ON 
SATURDAY. THE AIR MASS MAY STILL BE COLD AND DRY ENOUGH FOR THIS 
PRECIP TO FALL AS SNOW...OR OVER THE S PART AS A RAIN/SNOW MIX. 
ENOUGH UNCERTAINTY BASED ON RECENT MODEL PERFORMANCE TO KEEP LOW 
CHANCE POPS OVER ALL OF W WA...THOUGH THE BEST CHANCE OF PRECIP WILL 
BE SOUTH. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 230 PM AFD...ON SUNDAY THE 
EURO AND GFS ARE IN BROAD AGREEMENT. BOTH DEVELOP A WEAK SURFACE LOW 
ON VANCOUVER ISLAND AND SLIDE IT SE ON SUNDAY. WITH COLD AIR IN 
PLACE THE GENERAL EXPECTATION IS FOR SNOW ON THE NORTH SIDE OF THE 
LOW AND RAIN ON THE SOUTH SIDE. 850-1000 MB THICKNESSES AGREE WITH 
THIS IDEA...WITH THE 1300M CONTOUR AROUND THE MIDDLE OF THE STATE. 
THE TRACK OF THE LOW VARIES BETWEEN MODELS SO CONCEIVABLY THERE 
COULD BE SNOW EVERYWHERE...OR RAIN EVERYWHERE. THIS IS A LONG WAY OF 
SAYING THE FORECAST FOR SUNDAY AND SUNDAY NIGHT NOW CALLS FOR A 
CHANCE OF RAIN OR SNOW SHOWERS.

AFTER SUNDAY THE EURO AND GFS DIVERGE. THE EURO BRINGS MUCH WARMER 
AIR INLAND WITH A STRONG PACIFIC SYSTEM...AND RAINS MONDAY AND 
MONDAY NIGHT WITH RAIN SHOWERS TUESDAY. THE 12Z GFS BROUGHT MUCH 
COLDER ARCTIC AIR INTO WESTERN WASHINGTON WHILE STILL BRINGING WARM 
OVERRUNNING MOISTURE INTO THE STATE FROM THE PACIFIC...IN OTHER 
WORDS A WIDESPREAD HEAVY SNOW EVENT. THE 18Z GFS HAS BACKED OFF 
CONSIDERABLY FROM THAT SOLUTION...WITH NEITHER VERY COLD AIR NOR 
MUCH IN THE WAY OF PRECIPITATION. IN GENERAL THE EURO IS SUPERIOR TO 
THE GFS IN THE MEDIUM RANGE. BUT CONSIDERING THE UNCERTAINTY...HAVE 
CHANGED THE FORECAST FOR MONDAY TO CHANCE OF RAIN OR SNOW FOR MOST 
ZONES. TUESDAY LOOKS WARMER WITH RAIN...LIKE A NORMAL WINTER STORM 
IN WESTERN WASHINGTON. SHOWERS WEDNESDAY. AT SOME POINT EARLY NEXT 
WEEK THE COLD SNAP SHOULD BE OFFICIALLY OVER. BURKE

&&

.HYDROLOGY...PREVIOUS DISCUSSION FROM THE 230 PM AFD...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 RAIN 
AMOUNTS AND SNOW LEVELS NEXT WEEK WILL REMAIN LOW ENOUGH TO KEEP THE 
FLOODING THREAT LOW. NO FLOODING IS FORECAST ON THE GREEN RIVER. 
BURKE

&&

.AVIATION...LIGHT NORTHERLY FLOW ALOFT WILL CONTINUE OVER THE PAC NW 
THROUGH THU. THE ATMOSPHERE WILL REMAIN DRY AND STABLE...EXCEPT 
PATCHY SURFACE MOISTURE IN SHELTERED VALLEYS. LOW LEVEL OFFSHORE 
FLOW WILL PREVENT FOG FROM FORMING IN MOST AREAS EXCEPT POSSIBLY IN 
PRONE SHELTERED VALLEYS OF THE SW INTERIOR. LITTLE CHANGE IN THE 
WEATHER PATTERN IS EXPECTED THE NEXT COUPLE DAYS.

KSEA...NE WIND 4-8 KT. FOG NOT EXPECTED. DTM

&&

.MARINE...SURFACE HIGH PRES WILL PERSIST OVER THE WATERS THROUGH 
FRI. WINDS GENERALLY 15 KT OR LESS...HIGHEST OVER THE COASTAL WATERS 
AND W ENTRANCE TO THE STRAIT. A COLD FRONT WILL MOVE THROUGH THE 
FORECAST AREA FROM THE NORTHWEST SUNDAY AND MONDAY.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE. 
PZ...NONE.
   
$$

WWW.WEATHER.GOV/SEATTLE

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