FXUS66 KSEW 101709
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PST THU DEC 10 2009
.SYNOPSIS...A COLD DRY AIR MASS WILL REMAIN OVER WESTERN WASHINGTON
THROUGH FRIDAY. WEAK DISTURBANCES EMBEDDED IN THE NORTHWEST FLOW
WILL BRING A CHANCE OF SNOW FLURRIES TO THE NORTH TODAY THROUGH
FRIDAY AND A CHANCE OF SNOW TO THE EVERYWHERE FRIDAY NIGHT THROUGH
THE WEEKEND. A FRONTAL SYSTEM ARRIVING FROM THE WEST MAY BRING
WARMER AND WETTER WEATHER MONDAY OR TUESDAY.
&&
.SHORT TERM...STRATUS HAS MOVED INTO FAR NORTHERN AREAS THIS
MORNING. BOTH CLOUDS AND FLURRIES WERE DOWNPLAYED FOR SOME REASON IN
THE PREVIOUS FORECAST. WILL INCREASE CLOUD COVER NORTH...PERHAPS ADD
FLURRIES...AND MAY UPDATE FOR THESE CHANGES. REST OF THE AREA SHOULD
REMAIN COLD AND CLEAR WITH HIGHS AROUND FREEZING TODAY...EXCEPT THE
COAST SHOULD GET INTO THE MID OR EVEN UPPER 30S.
MODELS CONTINUE TO SHOW CLOUDS INCREASING OVER ALL AREAS FROM THE
NORTH TONIGHT AND FRIDAY. THIS SHOULD HELP KEEP OVERNIGHT LOWS FROM
PLUNGING INTO THE SINGLE DIGITS AGAIN...THOUGH IN THE FAR SOUTH THIS
COULD STILL HAPPEN. THE COLD AIR MASS WILL BE STILL BE IN PLACE
FRIDAY SO HIGHS WILL BE IN THE LOW TO MID 30S. THERE WILL BE A
CHANCE OF FLURRIES IN THE NORTH.
MODELS ARE IN NOW FAIR AGREEMENT THAT A SOUTHERN STREAM SYSTEM WILL
MOVE INTO OREGON AND SPREAD CLOUDS INTO SOUTHERN WASHINGTON FRIDAY
NIGHT. SO FRIDAY NIGHT WILL BE CLOUDY EVERYWHERE. MOS POPS FOR THAT
TIME ARE ALL IN THE CHANCE CATEGORY EVERYWHERE. MODELS DO NOT LOOK
PARTICULARLY WET...ONLY THE EURO HAS ANY PRECIPITATION...BUT ANY
PRECIPITATION WILL CERTAINLY BE SNOW AND THE FORECAST OF CHANCE OF
SNOW EVERYWHERE STILL LOOKS GOOD.
ON SATURDAY THE EURO IS WETTEST WITH PRECIPITATION IN THE NORTH IN
THE COLD AIR AND ALSO PRECIPITATION IN THE SOUTH FROM THE SOUTHERN
SYSTEM. THE NAM AND GFS ONLY HAVE THE SOUTHERN PRECIPITATION. ALL
MODELS ARE COLD WITH THE 1300M 850-1000MB THICKNESS LINE REMAINING
NEAR THE COAST. EXCEPT FOR THE COAST...PRECIPITATION ON SATURDAY
WILL BE SNOW. HOWEVER POPS ARE STILL IN THE CHANCE CATEGORY AND
AMOUNTS WILL BE LIGHT...PROBABLY AMOUNTING TO LESS THAN AN INCH.
FINALLY...THE SUNDAY FORECAST REMAINS PROBLEMATIC. THE EURO WARMS
THINGS UP CONSIDERABLY...THE GFS WARMS UP THE SOUTHERN PART...AND
THE NAM KEEPS COLD AIR OVER THE WHOLE AREA EXCEPT FOR THE COAST. ALL
THREE MODELS SPIN UP A WEAK SURFACE LOW OVER VANCOUVER WASHINGTON
AND SHIFT IT SOUTHEAST IN THE NORTHWESTERLY FLOW...SPAWNING SHOWERS
AS IT ARRIVES. IT IS ON SUNDAY THAT THE EXTENDED MOS FIRST HAS
LIKELY POPS...IN THE NORTH AND ON THE COAST. SO DEPENDING ON HOW THE
TEMPERATURES PLAY OUT...SUNDAY COULD HAVE SIGNIFICANT SNOWFALL IN
SOME ZONES. THE FORECAST SHOULD DEFINITELY SAY RAIN OR SNOW SHOWERS
ON SUNDAY. BURKE
.LONG TERM...PREVIOUS DISCUSSION...AS THE PACIFIC FRONTAL SYSTEM
MOVES FARTHER INLAND ON SUN...LOOKS LIKE WE WILL GET A SLY PUSH AND
FINALLY DRAW IN WARMER MARINE AIR INTO THE REGION. THIS WILL FINALLY
BREAK OUR COLD SNAP...BUT WITH POSSIBLE CONSEQUENCES. MODELS SHOW
ANOTHER STRONG AREA OF HIGH PRES BUILDING OVER INTERIOR B.C. WITH
COLD AIR POSSIBLY INVADING W WA THROUGH THE FRASER SUN NIGHT. THIS
IS A SETUP FOR POSSIBLE HEAVY SNOW...RIGHT ALONG THE
BOUNDARY...WHICH IS QUITE DIFFICULT TO NAIL DOWN AT THE MOMENT.
MODEL SO FAR DO NOT MATCH UP ...THE EURO IS MUCH SLOWER WITH THE
FRASER OUTFLOW AND WE COULD STAY AS RAIN. WILL NEED TO WATCH THIS
CLOSELY OVER THE NEXT FEW DAYS.
OTHERWISE...NO OTHER CHANGES IN THE LONG TERM PACKAGE. IT STILL
LOOKS LIKE WE WILL MODERATE FURTHER AS WE HEAD INTO THE MIDDLE OF
THE WEEK WITH TEMPS BACK NEAR NORMAL. THE PATTERN IS WETTER BUT NOT
EXCEPTIONAL. 33
&&
.HYDROLOGY...A COOL DRY AIR MASS REMAINS IN PLACE ACROSS WESTERN WA
WITH NO FLOOD CONCERNS ACROSS THE REGION. A TRANSITION TO A WETTER
AND WARMER PATTERN IS FORECAST NEXT WEEK BUT PRECIPITATION AMOUNTS
ARE NOT SIGNIFICANT FOR HYDROLOGIC CONCERNS. NO FLOOD ISSUES FOR THE
GREEN RIVER THROUGH THE EXTENDED FORECAST.
&&
.AVIATION...NORTHWEST FLOW ALOFT OVER THE REGION WILL CONTINUE TODAY
AND TONIGHT...GRADUALLY BACKING DURING THE PERIOD. SURFACE HIGH
PRESSURE OVER THE ROCKIES AND LOWER PRESSURE OFFSHORE WILL RESULT IN
CONTINUING WEAK OFFSHORE LOW LEVEL FLOW. THE AIR MASS WILL REMAIN
STABLE.
WE HAVE ISOLATED FOG AND STRATUS THIS MORNING -- MAINLY IN THE
SOUTHWEST INTERIOR -- AND THE SOUTH END OF A BANK OF CLOUDS OVER THE
STRAIT OF GEORGIA IS BRINGING CEILINGS IN THE 2500-3500 FT RANGE TO
THE NORTHWEST INTERIOR. MID AND HIGH CLOUD COVER WILL SLOWLY
INCREASE FROM THE NORTH DURING THE NEXT 24 HOURS...AND IT IS
POSSIBLE THAT AREAS OF STRATUS WILL DEVELOP FRIDAY MORNING.
KSEA...NORTHEAST WIND 4-8 KT. 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 NORTHERLY OFFSHORE FLOW
OVER WESTERN WASHINGTON ON SATURDAY. A FRONT WILL MOVE ACROSS THE
FORECAST AREA FROM THE NORTHWEST SUNDAY AND MONDAY. MCDONNAL
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.
$$
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