FXUS66 KSEW 110414
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
815 PM PST THU DEC 10 2009

.SYNOPSIS...NORTHWEST FLOW ALOFT WILL HELP KEEP THE AIR MASS OVER 
WESTERN WASHINGTON COLDER THAN NORMAL THROUGH THE WEEKEND. AN AREA 
OF LOW CLOUDS OVER THE NORTH PART MAY PRODUCE A FEW FLURRIES TONIGHT 
AND FRIDAY. A WEAK UPPER LEVEL DISTURBANCE EMBEDDED IN THE NORTHWEST 
FLOW WILL BRING A CHANCE OF SNOW FLURRIES TO THE AREA FRIDAY NIGHT 
THROUGH SATURDAY. A STRONGER UPPER LEVEL TROUGH WILL BRING A CHANCE 
OF SNOW AS IT MOVES SOUTH ON SUNDAY. A FRONTAL SYSTEM ARRIVING FROM 
THE WEST WILL BRING WARMER AND WETTER WEATHER EARLY NEXT WEEK...BUT 
PRECIPITATION COULD BEGIN AS SNOW.

&&

.SHORT TERM...THE UPPER LEVEL RIDGE JUST W OF THE B.C. COAST AND A 
DEEP LOW OVER NORTHERN CANADA WILL KEEP THE COOL NW FLOW ALOFT GOING 
THROUGH THE WEEKEND. SOUTHERN BRANCH WESTERLIES HAVE BROKEN THROUGH 
OVER CA AND WILL SPREAD NORTHWARD ACROSS OREGON THROUGH 
SATURDAY...LEAVING W WA IN-BETWEEN THE TWO BRANCHES. THIS 
EFFECTIVELY DELAYS THE ONSET OF SIGNIFICANT PRECIP OVER THE AREA 
UNTIL PROBABLY SUNDAY. 

NW FLOW ALONG THE B.C. COAST HAS PUSHED A LARGE AREA OF LOW CLOUDS 
ACROSS THE N PART OF W WA...EXTENDING FROM ABOUT KALALOCH ON THE 
COAST...ACROSS PORT ANGELES...AND N WHIDBEY ISLAND. MODELS SEEM 
REASONABLE IN PUSHING THE CLOUD DECK A LITTLE FURTHER SOUTH 
TONIGHT...THEN SLOWLY ENGULFING ALL OF W WA FRIDAY NIGHT. THE AIR 
MASS IS COLD ENOUGH FOR SNOW...AND THERE COULD BE A FEW FLURRIES 
COMING OUT OF THE CLOUD DECK...BUT NO ACCUMULATION IS EXPECTED.

SATURDAY WILL BE INTERESTING AS THE MODELS SHOW A VERY WEAK UPPER 
LEVEL SHORTWAVE TROUGH EMBEDDED IN THE N BRANCH MOVING S FROM B.C. 
WHILE AT THE SAME TIME A WEAK FRONTAL BAND IN THE S BRANCH MOVES N 
FROM OREGON. NAM/GFS BOTH SHOW SPOTTY PRECIP MAINLY N AND S OF 
SEATTLE...THOUGH IT WOULD NOT BE HARD FOR ONE OF THE AREAS TO REACH 
SEATTLE. UPWARD MOTION IS WEAK BUT THE AIR MASS WILL REMAIN COLD 
ENOUGH FOR SNOW. QPF FROM THE DRIER NORTHERN TROUGH MAY END UP BEING 
A FEW HUNDREDTHS...FOR LESS THAN AN INCH OF SNOW. THE SOUTHERN 
FRONTAL BAND LOOKS WETTER AND COULD POSSIBLY BRING A COUPLE OF 
INCHES OF SNOW TO LEWIS COUNTY. 

ALL MODELS SHOW A STRONGER NORTHERN BRANCH UPPER LEVEL TROUGH MOVING 
DOWN OVER THE AREA SUNDAY. THIS LOOKS LIKE IT WILL HAVE STRONGER AND 
MORE WIDESPREAD UPWARD MOTION. THE AIR MASS WILL PROBABLY BE 
BORDERLINE FOR EITHER RAIN OR SNOW...BUT THE BEST CHANCES WILL BE IN 
THE MORNING AND NORTH OF SEATTLE. THE COMPLICATION FOR THE GREATER 
PUGET SOUND REGION WILL BE 30 KT WESTERLY FLOW AT 850 MB...WHICH 
WOULD PRODUCE A RAIN SHADOW OVER AREA. KAM 

.LONG TERM...PREVIOUS VERSION...THE MODELS DIFFER ABOUT MONDAY AS 
WELL. THE EURO SEEMS SOLIDLY IN THE RAIN CAMP DURING THE DAY BUT THE 
GFS IS STILL COLD. SO THE FORECAST REMAINS RAIN OR SNOW ON MONDAY. 
IF THE GFS IS RIGHT THERE COULD BE BANDS OF SNOW IN A ZONE OF 
CONVERGENCE BETWEEN THE ARCTIC AIR AND THE WARMER AIR TO THE SOUTH. 

BOTH MODELS BRING WARM AND MOIST PACIFIC AIR INLAND MONDAY NIGHT 
WHICH SHOULD MEAN THE COLD SNAP ENDS. THE NORTHWEST INTERIOR COULD 
HAVE SNOW FOR A FEW MORE HOURS BEFORE THE COLD AIR RETREATS. THERE 
APPEARS TO BE NOTHING MORE THAN RAIN FROM TUESDAY ON. HAVE BROAD 
BRUSHED THE POP GRIDS WITH LIKELY COAST AND MOUNTAINS AND CHANCE 
ELSEWHERE. BURKE

&&

.HYDROLOGY...PREVIOUS VERSION...A COOL DRY AIR MASS REMAINS IN PLACE 
ACROSS WESTERN WA WITH NO FLOOD CONCERNS ACROSS THE REGION. THE SNOW 
LEVEL WILL BE ESSENTIALLY ZERO THROUGH THE WEEKEND INTO EARLY NEXT 
WEEK...SO ANY PRECIPITATION WILL BE SNOW AND WILL NOT RUN OFF INTO 
THE RIVERS. A TRANSITION TO A WETTER AND WARMER PATTERN IS FORECAST 
NEXT WEEK.  PRECIPITATION AMOUNTS ARE NOT PARTICULARLY HIGH...AND 
SNOW LEVELS WILL STAY ON THE LOW SIDE. RIVERS WILL RISE BUT ANY 
FLOODING IS UNLIKELY. NO FLOODING IS EXPECTED ON THE GREEN RIVER. 

THE GFS SHOWS A POTENTIAL FOR HEAVY RAIN OVER THE OLYMPICS ABOUT 
MIDWEEK BUT THE EURO SENDS MOST OF THE RAIN NORTH INTO VANCOUVER 
ISLAND. NOT GOING TO HIT THIS ONE VERY HARD YET. BURKE

&&

.AVIATION...LIGHT NW FLOW ALOFT OVER THE REGION THROUGH FRI. SURFACE 
HIGH PRESSURE OVER THE ROCKIES AND LOWER PRESSURE OFFSHORE WILL 
RESULT IN CONTINUING WEAK LOW LEVEL OFFSHORE FLOW. THE AIR 
MASS WILL REMAIN STABLE. HIGHER STRATUS AROUND THE 3-4K FT LEVEL 
OVER THE N COAST...STRAIT...AND N INTERIOR WILL INFILTRATE SLIGHTLY 
FURTHER S OVERNIGHT PARTIALLY INVADING THE N SOUND/HOOD CANAL. 
STRATUS WILL PERSIST THROUGH FRI MORNING...WITH THE SW INTERIOR 
STAYING MOSTLY CLEAR. THIS AFFECTS THE IFR FORECAST FOR THE 
KBFI/KSEA TERMINALS. WILL REMOVE THIS AND ADD IN A VFR STRATUS DECK 
FOR LATE TONIGHT/EARLY FRI. THE SURFACE TEMP/DEW POINT SPREADS N OF 
KBFI SUPPORT SOME POSSIBLE IFR FOG/STRATUS IN ADDITION TO THE 3-4K 
FT STRATUS...SO WILL MAKE FEW CHANGES TO THE KPAE/KBFI TERMINAL 
FORECASTS.

KSEA...NE WIND 3-6 KT. SCT-BKN CLOUDS AROUND 3-4K FT TO THE NORTH 
MAY REACH THE TERMINAL AFTER 09Z. IF THIS OCCURS...RADIATIONAL 
COOLING WILL BECOME LIMITED AND IFR CONDITIONS UNLIKELY TO DEVELOP. 
DECIDED TO GO WITH THE LATEST SATELLITE TRENDS WHICH SUGGEST THE SCT 
OR BKN STRATUS IS A BETTER BET THAN THE IFR DECK. THIS CHANGE WILL 
BE REFLECTED IN THE 06Z TAF ISSUANCE. DTM

&&

.MARINE...SURFACE HIGH PRESSURE ALONG THE ROCKY MOUNTAINS WILL 
PRODUCE WEAK OFFSHORE OR NLY FLOW THROUGH FRIDAY. A LOW PRES SYSTEM 
WILL BYPASS WA TO THE S ON SAT. SURFACE LOW PRES WILL FORM OVER 
VANCOUVER ISLAND ON SUN INDUCING STRONGER SLY FLOW OVER WRN 
WA...POSSIBLY CLOSE TO SMALL CRAFT CRITERIA FOR THE COAST/N 
INTERIOR. AS THIS LOW MOVES E...AN APPROACHING FRONT WILL MOVE 
ACROSS THE FORECAST AREA FROM THE NORTHWEST MONDAY. DTM

&&

.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.