FXUS66 KSEW 011122
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST TUE DEC 1 2009

.SYNOPSIS...CLOUD COVER AND PATCHY FOG OVER WESTERN WASHINGTON WILL 
CLEAR THIS AFTERNOON AS AN UPPER RIDGE OFFSHORE APPROACHES THE AREA. 
THE RIDGE WILL PROVIDE DRY WEATHER THROUGH THURSDAY WITH MOSTLY 
SUNNY DAYS. NIGHTS AND MORNINGS WILL BE COOL WITH AREAS OF FOG 
FORMING. THE RIDGE WILL BREAK DOWN FRIDAY AS A FRONT ARRIVES...AND 
COLDER AIR WILL ARRIVE FROM THE NORTH BY THE END OF THE WEEKEND.

&&

.SHORT TERM...WE HAVE NORTHWEST FLOW ALOFT OVER THE PACIFIC 
NORTHWEST EARLY THIS MORNING. THE FRONT THAT BRUSHED WESTERN 
WASHINGTON MONDAY HAS MOVED SOUTHEAST -- IT NOW EXTENDS FROM SOUTH 
CENTRAL CANADA ACROSS MONTANA AND INTO EASTERN OREGON. AN UPPER 
RIDGE IS BUILDING OFFSHORE AND PROGRESSING TOWARD 130W. 
PRECIPITATION IN THE FORECAST AREA HAS ENDED EXCEPT LIGHT AMOUNTS 
FALLING IN THE CASCADES FROM ABOUT SNOQUALMIE PASS SOUTHWARD...AND 
THIS SHOULD END WITHIN THE NEXT FEW HOURS. THERE HAS BEEN SOME 
CLEARING OVER THE NORTHWEST PART OF THE FORECAST AREA ..BASICALLY 
ALONG THE COAST AND IN THE NORTHWEST INTERIOR...BUT MUCH OF THE AREA 
REMAINS CLOUDY AND WE HAVE PATCHY FOG IN JUST A FEW SPOTS.

THE FLOW ALOFT WILL VEER GRADUALLY TODAY AS THE UPPER RIDGE BUILDS 
AND ADVANCES TO 130W. WEAK OFFSHORE FLOW...WHICH DEVELOPED LATE 
MONDAY...WILL CONTINUE AS A SURFACE HIGH SITS OVER THE CANADIAN 
ROCKIES. PATCHY FOG COULD YET FORM IN MORE PLACES AS DAYBREAK 
ARRIVES...BUT IN GENERAL THE AIR MASS WILL CONTINUE TO DRY FROM THE 
NORTHWEST. MOST OF WESTERN WASHINGTON SHOULD HAVE MOSTLY SUNNY SKIES 
THIS AFTERNOON IF NOT SOONER.

THE UPPER RIDGE WILL CONTINUE TO BUILD SLIGHTLY AND SHIFT SLOWLY 
EASTWARD OVER THE NEXT 48 HOURS. BY EARLY THURSDAY MORNING THE UPPER 
RIDGE AXIS WILL BE RIGHT ALONG THE PACIFIC NORTHWEST COAST...FLOW 
ALOFT OVER WESTERN WASHINGTON WILL BE ALMOST DUE NORTH...AND 500 MB 
HEIGHTS WILL BE CLOSE TO 5700 METERS. THE RIDGE WILL KEEP WESTERN 
WASHINGTON DRY AND THE FREEZING LEVEL WILL CLIMB TO AROUND 9000 FT. 
THERE WILL BE AREAS OF NIGHT AND MORNING FOG IN THE VALLEYS...AND 
OVERNIGHT TEMPERATURES CLOSE TO OR BELOW FREEZING WILL BE FAIRLY 
COMMON. 

ON THURSDAY THE UPPER RIDGE WILL SHIFT INLAND AND BEGIN TO COLLAPSE 
...AS A FRONTAL SYSTEM MOVES OVER ITS NORTH SIDE INTO BRITISH 
COLUMBIA. CONCEIVABLY THIS FEATURE COULD BRING SOME PRECIPITATION TO 
THE COAST OR NORTH PART OF THE FORECAST AREA BY LATE THURSDAY NIGHT. 
WHILE THE GFS KEEPS THE SYSTEM FURTHER NORTH...IT IS BEGINNING TO 
LOOK LIKE THE ODD MAN OUT. THE NAM...EUROPEAN...AND CANADIAN MODELS 
ALL DIG THE SYSTEM MORE AGGRESSIVELY SOUTHWARD. I WILL INTRODUCE A 
CHANCE OF PRECIPITATION TO THE COASTAL WATERS AND COAST LATE 
THURSDAY NIGHT. MCDONNAL 

.LONG TERM...DESPITE THE DIVERGING MODEL SOLUTIONS MENTIONED ABOVE 
FOR THURSDAY NIGHT AND FRIDAY...THE LONGER RANGE MODELS GENERALLY 
AGREE ON A TREND TOWARD COLDER WEATHER IN THE EXTENDED PERIOD.

THE EUROPEAN MODEL PUTS A 5350 METER UPPER LOW OVER WASHINGTON LATE 
FRIDAY AND SHOWS COLD AIR FLOWING INTO WESTERN WASHINGTON FROM A 
1046 MB SURFACE HIGH OVER WESTERN CANADA BY THE END OF THE WEEKEND. 

THE GFS SOLUTION -- WHILE IT KEEPS THE FIRST SYSTEM INITIALLY 
FURTHER NORTH ON FRIDAY -- DOES DROP A SIMILARLY COLD UPPER LOW INTO 
WESTERN MONTANA LATE FRIDAY...WITH A SHORTWAVE ROTATING DOWN THE 
WEST SIDE OF THE UPPER LOW ACROSS THE NORTHWEST FRIDAY NIGHT. 
PROBABLY MORE IMPORTANT...IT ALSO SHOWS COLD NORTHERLY SURFACE FLOW 
DEVELOPING BY THE END OF THE WEEKEND WITH A 1050 MB SURFACE HIGH 
OVER WESTERN CANADA.

OUR CURRENT FORECAST...WHICH ACCOUNTS FOR THE LONG RANGE MODEL TREND 
BUT LEANS TOWARD CLIMATOLOGY...UNDERPLAYS THE INFILTRATION OF COLD 
AIR SHOWN BY THE MODELS FOR SUNDAY AND MONDAY. HOWEVER GIVEN THE 
UNCERTAINTY IN THE MODELS THAT FAR IN ADVANCE...AND THEIR TENDENCY 
TO OVERDO THE INTRUSION OF COLD AIR WEST OF THE CASCADES...THE 
EXTENDED FORECAST IS FINE FOR THE TIME BEING. MCDONNAL

&&

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

NONE OF THE MODELS INDICATES SIGNIFICANT...LET ALONE FLOOD-PRODUCING 
...PRECIPITATION DURING THE PERIOD. ON THE CONTRARY...UNREGULATED 
RIVERS IN WESTERN WASHINGTON -- WHICH HAVE GRADUALLY BEEN FALLING 
TOWARD NORMAL STREAMFLOWS OVER THE PAST WEEK -- WILL FALL FURTHER 
WITH DRIER THAN NORMAL AND COOLER THAN NORMAL WEATHER. MCDONNAL

&&

.AVIATION...CEILINGS ARE ALL OVER THE PLACE EARLY THIS MORNING. A 
PATCH OF STRATUS WITH CIGS AROUND 030 SITS FROM THE CENTRAL PUGET 
SOUND SOUTH TO AROUND KOLM WHILE SOME LOCALLY LOWER CIGS SIT ALONG 
HOOD CANAL. THEN SOME PATCHES OF STRATUS WITH CIGS AROUND 020-025 
SIT FROM THE STRAITS OF GEORGIA AND SE VANCOUVER ISLAND THROUGH THE 
SAN JUANS THEN TO WHIDBEY ISLAND AND ALONG THE NORTHERN SLOPES OF 
THE OLYMPICS. A FEW PLACES IN BETWEEN HAVE CLEARED AND WILL LIKELY 
SEE SOME SHALLOW FOG FORM DURING THE EARLY MORNING HOURS. SHORT TERM 
FORECAST MODELS AND STATISTICAL MODELS FROM OVERNIGHT BADLY OVER 
FORECASTED THE EXTENT OF VERY LOW STRATUS AND FOG...PARTICULARLY 
OVER THE CENTRAL AND SOUTHERN INTERIOR. IT APPEARS THAT THE 
IMMEDIATE BOUNDARY LAYER DRIED SLIGHTLY MORE THAN FORECAST. WITH ALL 
THE VARIABILITY IN CONDITIONS AROUND EARLY THIS MORNING...CONFIDENCE 
IN THE DETAILS THROUGH MIDDAY ARE LOW. 

THE TREND WILL BE A DRYING ONE AS HIGH PRES OVER CENTRAL B.C. GIVES 
DRYING NELY OFFSHORE FLOW FOR THE NEXT 36 HOURS. FOG LATER TONIGHT 
SHOULD BE SHALLOW AND CONFINED TO FAVORED SHELTERED VALLEYS AND THE 
VALLEYS OF THE SOUTHWEST INTERIOR FROM ABOUT KOLM SOUTHWARD. 

KSEA...EXPECT LIGHT N/NE WIND. STRATUS IS RATHER HIGH AT THIS TIME. 
WILL NEED TO MONITOR THE VALLEYS TO THE NE OF THE TERMINAL FOR THE 
POSSIBLE FORMATION OF LAKE WASHINGTON STRATUS OR FOG THAT COULD 
DRIFT SW INTO THE TERMINAL THRU MIDDAY. BUT WITH DEWPOINTS RATHER 
LOW WILL KEEP IT OUT OF THE FORECAST FOR NOW. CONFIDENCE IS LOW. 
EXPECT CLEARING BY MIDDAY WITH MAINLY CLEAR CONDITIONS THROUGH WED. 
ALBRECHT

&&

.MARINE...HIGH PRES OVER CENTRAL B.C. TODAY WILL MOVE SLOWLY SE INTO 
FAR EASTERN WA BY LATE WED. THE HIGH WILL GIVE LIGHT OFFSHORE FLOW 
TO THE WATERS. SWELL AROUND 10 FT ALONG THE WASHINGTON COAST WILL 
SUBSIDE LATER THIS MORNING AND THE GRAYS HARBOR BAR WILL DROP INTO 
THE MODERATE CONDITION CATEGORY AFTER THE VERY STRONG EBB LATER THIS 
AFTERNOON. 

ALL WILL BE RATHER QUITE UNTIL THU NIGHT OR FRI WHEN A COLD FRONT 
DROPS SE FROM THE GULF OF ALASKA...POSSIBLY GIVING SOME SMALL CRAFT 
ADVISORY NWLY WINDS TO THE COAST AND N INTERIOR WATERS. THEN FRI 
NIGHT OR SATURDAY WE WILL NEED TO CLOSELY MONITOR THE PROGRESS OF A 
DEVELOPING ARCTIC AIR MASS OVER THE YUKON AS IT HEADS INTO BRITISH 
COLUMBIA. THE LIKELIHOOD OF GALE FORCE ARCTIC OUTFLOW OVER THE 
UPCOMING WEEKEND APPEARS TO BE INCREASING. ALBRECHT 

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE
PZ...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS COASTAL WATERS.
    .SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.
   
$$

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