FXUS66 KSEW 092234
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
230 PM 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...AND 
POSSIBLY SUNDAY AS WELL. A FRONTAL SYSTEM ARRIVING FROM THE WEST 
WILL BRING WARMER AND WETTER WEATHER EARLY NEXT WEEK. 

&&

.SHORT TERM...FEW CHANGES MADE TO THE FORECAST. TEMPERATURES REMAIN 
WELL BELOW NORMAL...BOTH THE HIGHS AND THE LOWS...WITH BELOW 
FREEZING TEMPERATURES AT NIGHT AND HIGHS IN THE 30S. CLOUDS AND SNOW 
FLURRY CHANCES WILL SLOWLY WORK THEIR WAY SOUTH TONIGHT THROUGH 
FRIDAY. BY FRIDAY NIGHT THE NORTHERN STREAM MOISTURE COULD BE WELL 
INTO NORTHERN WASHINGTON...AND THE GFS STILL LIKES THE IDEA OF SOME 
MOISTURE FROM THE SOUTH CLIPPING THE SOUTHERN PART OF THE STATE. 
THERE COULD ALSO BE NO PRECIPITATION. SO THE FORECAST STILL CALLS 
FOR A CHANCE OF SNOW NEARLY EVERYWHERE FRIDAY NIGHT AND SATURDAY. 
THE EXCEPTIONS ARE THE SOUTHWEST INTERIOR WHERE THE WARMER SYSTEM TO 
THE SOUTH MAY CAUSE A MIX OF RAIN AND SNOW...AND THE COAST WHERE IT 
SHOULD BE WARM ENOUGH DURING THE DAY TO JUST RAIN. BURKE

.LONG TERM...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...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 
TONIGHT AND THURSDAY. SURFACE HIGH PRESSURE OVER THE ROCKIES AND 
LOWER PRESSURE OVER CALIFORNIA WILL RESULT IN WEAK OFFSHORE FLOW. 
LOCAL SHALLOW SURFACE-BASED MOISTURE COULD PRODUCE PATCHY FOG IN THE 
MOST VULNERABLE SPOTS LATE TONIGHT AND THURSDAY MORNING. A WEAK 
SYSTEM EMBEDDED IN THE NORTHERLY FLOW COULD PRODUCE LIGHT FLURRIES 
OVER THE NORTH PART OF THE FORECAST AREA LATE TONIGHT AND THURSDAY. 
THE AIR MASS WILL REMAIN STABLE.

KSEA...NORTH WIND 4-8 KT BECOMING NORTHEAST LATER TONIGHT AND 
THURSDAY. MCDONNAL

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE. 
PZ...SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS GRAYS HARBOR THIS 
     EVENING.
   
$$

WWW.WEATHER.GOV/SEATTLE

FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE 
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