FXUS66 KSEW 031720
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
920 AM PST THU DEC 3 2009

.SYNOPSIS...AN UPPER LEVEL RIDGE WILL BRING COOL DRY WEATHER TODAY 
WITH ONLY HIGH CLOUDS OVER THE AREA. 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. 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. 

&&

.SHORT TERM...THE UPPER LEVEL RIDGE AXIS HAS SHIFTED OVER W WA THIS 
MORNING...BUT HIGH CLOUDS FROM THE APPROACHING UPPER LEVEL TROUGH 
TROUGH JUST OFFSHORE HAVE BEEN CRUISING INLAND THROUGH THE RIDGE 
AXIS. TODAY WILL HAVE A FAIR AMOUNT OF SUNSHINE...JUST NOT AS MUCH 
AS EXPECTED. MORNING LOWS BOTTOMED OUT IN THE MID 20S TO LOWER 30S 
SO MAX TEMPS WILL ONLY REACH THE LOWER 40S THIS AFTERNOON...JUST A 
LITTLE COLDER THAN EXPECTED.

MODELS ARE IN GENERAL AGREEMENT WITH THE EVOLVING PATTERN OVER THE 
SHORT TERM. THE UPPER LEVEL TROUGH APPROACHING THE COAST PUSHES 
INLAND ACROSS SOUTHERN B.C. TONIGHT THEN MOVES SOUTH OVER NE WA...ID 
OR W MT ON FRIDAY. GFS STILL APPEARS TO TAKE THE UPPER LEVEL LOW TOO 
FAR TO THE EAST...SO THE ECMWF/NAM SOLUTION IS PREFERRED. HOWEVER 
THE DIFFERENCE REALLY IS NOT THAT SIGNIFICANT AT THIS POINT. WILL GO 
WITH THE NAM SOLUTION WHICH BRING SOME SPOTTY SHOWERS TO W WA FRIDAY 
AFTERNOON AS A WEAK COLD FRONT PUSHES SOUTH ACROSS THE AREA JUST 
AHEAD OF AN UPPER LEVEL TROUGH. THE FRONT LOOKS QUITE WEAK AND MOS 
POPS ARE LOW. WILL MAKE CHANGES IN THE AFTERNOON PACKAGE TO SPEED UP 
THE TIMING OF THE SHOWERS...AND PROBABLY DECREASE POPS FRIDAY NIGHT 
AND SATURDAY. MAIN EFFECT OF THE FRONT/TROUGH WILL BE TO ADVECT 
COLDER DRIER AIR SOUTH OVER THE AREA.

ALL MODELS SHOW ANOTHER SOMEWHAT STRONGER SHORTWAVE TROUGH MOVING S 
OR SE ACROSS THE AREA SATURDAY NIGHT. THIS LOOKS LIKE IT PUSHES A 
MODIFIED ARCTIC FRONT ACROSS THE AREA WITH IT. UPWARD MOTION LOOKS A 
LITTLE STRONGER AND DEEPER...WITH PERHAPS A BIT MORE MOISTURE. THE 
AIR MASS SHOULD BE COLD ENOUGH FOR SNOW LEVELS AT THE SURFACE...SO 
WILL KEEP THE MENTION OF SNOW SHOWERS...AND POSSIBLY DITCH THE 
MENTION OF RAIN SHOWERS. KAM

.LONG TERM...BOTH GFS/ECMWF AGREE ON KEEPING AN UPPER LEVEL RIDGE 
JUST W OF THE B.C. COAST SUNDAY THROUGH TUESDAY. THIS MAINTAINS COOL 
DRY N FLOW OVER W WA THROUGH THE PERIOD. CURRENT FORECAST STILL HAS 
A CHANCE OF SHOWERS MENTIONED EARLY NEXT WEEK...BUT WILL LOOK AT 
GOING WITH A DRY FORECAST. GFS/ECMWF ALSO AGREE ON PINCHING OFF THE 
UPPER LEVEL RIDGE AROUND MID NEXT WEEK...ALLOWING MILDER AND WETTER 
W FLOW ALOFT TO UNDERCUT THE RIDGE AND REACH THE AREA. WILL PROBABLY 
GET BACK TO A CHANCE OF RAIN BY NEXT THURSDAY...BUT WITH THE 
JETSTREAM AIMED S OF THE AREA...THE PRECIP PROBABLY WILL NOT BE 
SIGNIFICANT. KAM

&&

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

&&

.AVIATION...THE UPPER LEVEL RIDGE AXIS ALONG THE WA COAST WILL SHIFT 
INLAND TODAY AS A TROUGH DIGS DOWN ACROSS B.C. OFFSHORE FLOW WILL 
CONTINUE AT THE SURFACE THROUGH THE PERIOD.  VFR CONDITIONS WILL 
PREVAIL WITH JUST FEW-SCT CIRRUS OVER THE REGION. THE VERY DRY AIR 
MASS WILL CONTINUE TO LIMIT THE AMOUNT OF FOG ACROSS THE REGION 
TONIGHT.  

KSEA...CLEAR AOB 120 WITH LIGHT NORTH TO NORTHEAST WINDS. CERNIGLIA

&&

.MARINE...HIGH PRES EAST OF THE CASCADES WILL MAINTAIN OFFSHORE FLOW 
ACROSS WESTERN WA TODAY.  WINDS ARE GENERALLY LIGHT N/NE EXCEPT AT 
THE WEST ENTRANCE TO THE STRAIT AND NEAR OTHER GAPS IN THE COASTAL 
TERRAIN WHERE THE EASTERLY BREEZE IS A BIT STRONGER.  SEAS ARE 
RUNNING A BIT HIGH AND SITTING AROUND 10 FT AT THE MOMENT AND THAT 
IS EXPECTED TO CONTINUE THROUGH THE DAY SO A SMALL CRAFT ADVISORY IS 
IN EFFECT FOR THAT.  PRIMARY SWELL PERIOD IS FAIRLY LONG AT 15 
SECONDS. 

WINDS WILL PICK UP ON FRI AS A COLD FRONT PASSES SOUTH THROUGH THE 
REGION LATER FRI.  OFFSHORE FLOW WILL INCREASE DURING THE WEEKEND AS 
HIGH PRES BUILDS OVER INTERIOR B.C. AND SAGS SOUTH INTO E WA/ID AND 
MONTANA...PRODUCING MODERATE TO LOCALLY STRONG OFFSHORE FLOW. 
CERNIGLIA

&&

.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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