FXUS66 KSEW 091658
AFDSEW

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

.SYNOPSIS...A COLD DRY AIR MASS WILL REMAIN OVER WESTERN WASHINGTON 
THROUGH FRIDAY. WEAK DISTURBANCES EMBEDDED IN THE NORTHWESTERLY FLOW 
ALOFT MAY BRING SNOW FLURRIES TO THE NORTH PART THURSDAY AND FRIDAY. 
THERE IS A CHANCE OF SNOW EVERYWHERE FRIDAY NIGHT AND SATURDAY. A 
FRONTAL SYSTEM ARRIVING FROM THE WEST WILL BRING WARMER AND WETTER 
WEATHER EARLY NEXT WEEK. 

&&

.SHORT TERM...FEW CHANGES THROUGH FRIDAY...WITH COLD DRY AIR OVER 
THE AREA...A SLOW MODERATION IN TEMPERATURES...AND CLOUDS GRADUALLY 
INVADING THE NORTH WITH FLURRIES POSSIBLE THERE AT TIMES. THE 
MODERATING TEMPERATURES ARE RELATIVE...HIGHS WILL STILL STRUGGLE TO 
REACH THE MID 30S...BUT LOWS SHOULD RISE INTO THE UPPER TEENS AND 
20S. BY FRIDAY CLOUDS WILL PROBABLY COVER THE WHOLE AREA.

FRIDAY NIGHT STILL LOOKS LIKE WHEN SNOW MIGHT BEGIN. SHORT TERM MOS 
POPS NOW COVER THIS PERIOD...AND ARE UNIFORMLY IN THE CHANCE 
CATEGORY. TEMPERATURES FRIDAY NIGHT WILL BE BELOW FREEZING SO ANY 
PRECIPITATION WILL BE SNOW. MODELS ARE UNIMPRESSIVE WITH AMOUNTS. 
THE NAM AND EURO HAVE NO SOUTHERN SYSTEM IMPACT THROUGH SATURDAY. 
THE GFS CLIPS THE SOUTH WITH SOME PRECIPITATION ON SATURDAY. 

ALL MODELS HAVE DRIBS AND DRABS OF SNOW IN THE NORTH FRIDAY NIGHT 
THROUGH SATURDAY...WITH THE COAST WARMER AND LIKELY TO TURN OVER TO 
RAIN ON SATURDAY. THE EURO IS THE MOST INTERESTING...WITH A BROAD 
SURFACE LOW FORMING ALONG THE ARCTIC BOUNDARY NORTH OF WASHINGTON. 
THIS LOW SAGS SOUTH OVER THE AREA ON SUNDAY AND COULD BE A SNOW 
PRODUCER. WILL DEFER ANY FURTHER DISCUSSION OF THE EXTENDED UNTIL 
THIS AFTERNOON. BURKE

.LONG TERM...PREVIOUS DISCUSSION...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 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...NORTHERLY FLOW ALOFT ACROSS THE REGION WILL CONTINUE 
THROUGH TONIGHT. SURFACE HIGH PRESSURE OVER THE ROCKIES AND LOWER 
PRESSURE OVER CALIFORNIA WILL RESULT IN WEAK OFFSHORE FLOW. THE AIR 
MASS WILL REMAIN STABLE AND DRY...EXCEPT LOCAL SHALLOW SURFACE-BASED 
MOISTURE COULD PRODUCE PATCHY FOG IN THE MOST VULNERABLE SPOTS LATE 
TONIGHT AND THURSDAY MORNING.

KSEA...SOUTHEAST WIND 4-8 KT BECOMING NORTHEAST THIS AFTERNOON AND 
TONIGHT. MCDONNAL

&&

.MARINE...SURFACE HIGH PRESSURE ALONG THE ROCKY MOUNTAINS WILL 
PRODUCE WEAK OFFSHORE FLOW THROUGH THURSDAY NIGHT. AN AREA OF LOW 
PRESSURE MOVING INTO CALIFORNIA WILL STRENGTHEN THE OFFSHORE FLOW 
OVER WESTERN WASHINGTON ON SATURDAY. A COLD FRONT WILL MOVE INTO THE 
FORECAST AREA FROM THE NORTHWEST ON SUNDAY.

&&

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

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