FXUS66 KSEW 040551
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
950 PM PST THU DEC 3 2009

.SYNOPSIS...HIGH CLOUDS FROM AN APPROACHING UPPER LEVEL TROUGH WILL 
CROSS THE AREA TONIGHT. AN UPPER LEVEL TROUGH MOVING SOUTH FROM 
BRITISH COLUMBIA WILL PUSH A WEAK COLD FRONT ACROSS THE AREA FRIDAY 
AFTERNOON AND EVENING FOR A CHANCE OF SHOWERS OVER THE NORTH 
CASCADES. ANOTHER UPPER LEVEL TROUGH WILL PUSH A MODIFIED ARCTIC 
FRONT ACROSS THE AREA SATURDAY NIGHT FOR A CHANCE OF SNOW SHOWERS. 
AN UPPER LEVEL RIDGE JUST OFFSHORE WILL MAINTAIN COLD NORTH TO 
NORTHEAST FLOW ALOFT OVER THE AREA EARLY NEXT WEEK. A CHANGE TO 
WESTERLY FLOW ALOFT AROUND MID WEEK WILL BRING A CHANCE OF RAIN AND 
ALLOW TEMPERATURES TO WARM.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS THE UPPER LEVEL RIDGE THAT WAS 
OVER THE AREA FOR THE LAST 36 HOURS HAS MOVED EAST. MID/HIGH LEVEL 
CLOUDS OUT AHEAD OF AN APPROACHING UPPER LEVEL TROUGH MOVING THROUGH 
THE AREA THIS EVENING. EVEN WITH THE CLOUD COVER TEMPERATURES ARE 
ALREADY IN THE 30S WITH THE COLDER LOCATIONS LIKE ARLINGTON AND 
OLYMPIA ALREADY IN THE UPPER 20S. EXPECT MOST LOCATIONS IN THE CWA 
TO BE BELOW FREEZING FRIDAY MORNING. COOL UPPER LEVEL TROUGH MOVING 
SOUTH WILL REACH WESTERN WASHINGTON FRIDAY MORNING. VERY WEAK COLD 
FRONT WILL STALL OVER SOUTHERN BRITISH COLUMBIA LATE FRIDAY MORNING 
INTO FRIDAY AFTERNOON...CLOSE ENOUGH TO BRING A CHANCE OF SHOWERS TO 
THE NORTH CASCADES. FOR THE REMAINDER OF THE AREA JUST SOME MID/HIGH 
LEVEL CLOUDS ON FRIDAY. HIGH TEMPERATURES WILL REMAIN COOL AND RUN 
AROUND 5 DEGREES COOLER THAN THURSDAY. 850 MB TEMPERATURES AROUND 
PLUS 5C THIS EVENING WILL DROP DOWN TO NEAR -5C BY 00Z SATURDAY. 
UPPER LEVEL TROUGH MOVING OFF TO EAST FRIDAY AFTERNOON INTO FRIDAY 
EVENING LEAVING WESTERN WASHINGTON IN COOL NORTHERLY FLOW ALOFT 
FRIDAY NIGHT INTO SATURDAY, WEAK UPPER LEVEL LOW DROPPING DOWN OVER 
THE AREA SATURDAY NIGHT. MODELS DIFFER SLIGHTLY ON THE TRACK OF THE 
LOW WITH THE NAM MOVING THE LOW RIGHT OVER WESTERN WASHINGTON WHILE 
THE GFS SOLUTION TAKES THE LOW FURTHER TO THE EAST. EVEN WITH THE 
NAM SOLUTION THE PRECIPITATION IS LIGHT AND MAINLY OVER THE 
MOUNTAINS. THE MAIN STORY SATURDAY NIGHT INTO SUNDAY MORNING WILL 
BE THE INCREASING OUTFLOW NORTHEAST WINDS OVER THE NORTHERN 
INTERIOR. CURRENT MODEL SOLUTIONS PUT THE KBLI-CYWL GRADIENT AROUND 
MINUS 14 MB. THIS IS STRONG ENOUGH TO PUSH THE OUTFLOW WINDS INTO 
THE NORTH INTERIOR. DEPENDING ON THE STRENGTH OF THE OUTFLOW WINDS 
SOME UPSLOPE FLURRIES COULD DEVELOP ALONG THE NORTH SLOPES OF THE 
OLYMPICS LATE SATURDAY NIGHT INTO SUNDAY MORNING. THE LOW WILL 
CONTINUE TO SAG SOUTH ON SUNDAY. THIS WILL DRAW THE COLDEST AIR MASS 
OF THE SEASON SO FAR INTO WESTERN WASHINGTON...850 MB TEMPERATURES 
BELOW -10C. DAYTIME HIGHS IN THE 30S WILL BE COMMON ON SUNDAY WITH 
PLACES IN THE NORTH INTERIOR NOT EVEN GETTING UP TO FREEZING. LOWS 
SUNDAY NIGHT/MONDAY MORNING COULD EASILY GET INTO THE TEENS IN THE 
COLDER LOCATIONS. FORECASTS ON TRACK. NO ZONE UPDATE THIS EVENING. 
FELTON

.LONG TERM...PREVIOUS DISCUSSION FOLLOWS. THE HIGH AMPLITUDE RIDGE W 
OF THE B.C. COAST WILL REMAIN IN PLACE THROUGH WEDNESDAY. CONTINUING 
N FLOW ALOFT AND OFFSHORE FLOW KEEP THE AIR MASS COLD AND DRY. 
SHOULD SEE A LITTLE MODERATION IN TEMPS BY WEDNESDAY...BUT IT WILL 
STILL BE COLDER THAN NORMAL. BOTH GFS/ECMWF CONTINUE TO SHOW W FLOW 
ALOFT UNDERCUTTING THE UPPER LEVEL RIDGE STARTING TUESDAY...BUT WITH 
THE WESTERLIES AIMED AT CA THIS TIME. ECMWF HAS KEPT ALL PRECIP S OF 
W WA...BUT THE 12Z GFS RUN HAD RAIN REACHING W WA LATER WEDNESDAY. 
NEWEST 18Z GFS IS DRY...SO GIVEN THE MODEL INCONSISTENCY CONFIDENCE 
IN PRECIP IS LOW. THERE IS A CHANCE OF RAIN IN THE FORECAST BASED 
MAINLY ON THE 12Z GFS AND CLIMO. MORE CERTAIN CHANGE WILL BE A 
MODERATION IN TEMPERATURE. KAM 

&&

.HYDROLOGY...THERE IS NO THREAT OF RIVER FLOODING IN THE FORECAST 
AREA FOR THE NEXT WEEK. THIS INCLUDES THE GREEN RIVER. 

TWO DRY UPPER LEVEL TROUGHS/WEAK SURFACE FRONTS WILL CROSS THE 
AREA...FRIDAY AFTERNOON AND AGAIN SATURDAY EVENING. LITTLE IF ANY 
PRECIP IS EXPECTED FROM EITHER OF THESE SYSTEMS. IN THE LONG 
TERM...WARMER AND POTENTIALLY WETTER WEST FLOW ALOFT RETURNS AROUND 
MID NEXT WEEK. SHOULD SEEM SOME RAIN OVER THE AREA BY LATE NEXT 
WEEK...BUT THE MODELS DEPICT FASTER WESTERLY FLOW WITH FRONTAL 
SYSTEMS PERIODICALLY CROSSING THE AREA...WHICH IS TYPICALLY NOT A 
FLOODING PATTERN. KAM

&&

.AVIATION...AN UPPER LEVEL TROUGH IS PASSING THROUGH SOUTHERN B.C. 
THIS EVENING...WITH A WEAK COLD FRONT MOVING SOUTH THROUGH WESTERN 
WA. THE AIR MASS REMAINS DRY AND STABLE WITH JUST FEW-SCT MID AND 
HIGH LEVEL CLOUDS. PATCHY /FREEZING/ FOG POSSIBLE TONIGHT WITH LIGHT 
WINDS. FAVORED AREA WILL BE SW INTERIOR. THE UPPER LOW WILL DIVE SE 
TOWARD MONTANA ON FRI WITH NORTHERLY FLOW INCREASING ACROSS WESTERN 
WA. 33

KSEA...VFR CONDITIONS WITH LIGHT WINDS. 33

&&

.MARINE...A WEAK COLD FRONT IS PASSING S THROUGH WESTERN WA THIS 
EVENING...BUT SFC WINDS WILL REMAIN LIGHT. SWELLS ARE STILL NEAR 10 
FT SO WILL CONTINUE THE HAZARDOUS SEAS HEADLINES. SEAS WILL SUBSIDE 
FRI MORNING. NORTHERLY PRES GRADIENTS WILL INCREASE ON FRI AS A 
STRONG AREA OF HIGH PRES DEVELOPS OVER INTERIOR B.C. OFFSHORE FLOW 
WILL FURTHER INCREASE OVER THE WEEKEND AS THE SFC HIGH SLIDES 
FARTHER SOUTH. MAY SEE NE GALES IN THE NORTHERN INLAND WATERS SAT 
NIGHT WITH FRASER RIVER OUTFLOW. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY HAZARDOUS SEAS COASTAL WATERS AND WEST     
      ENTRANCE.
     SMALL CRAFT ADVISORY ROUGH BAR CONDITIONS GRAYS HARBOR BAR. 

$$

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