FXUS66 KSEW 030554
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
950 PM PST WED DEC 2 2009

.SYNOPSIS...UPPER LEVEL RIDGE OVER WESTERN WASHINGTON THROUGH 
THURSDAY WITH LOW LEVEL OFFSHORE FLOW GIVING THE AREA DRY BUT COOL 
WEATHER. THE RIDGE WILL WEAKEN THURSDAY NIGHT INTO FRIDAY. UPPER 
LEVEL LOW DEVELOPING OVER EASTERN BRITISH COLUMBIA FRIDAY MOVING 
INTO WESTERN MONTANA LATE IN THE DAY. A WEAK WEATHER SYSTEM ON THE 
BACKSIDE OF THE LOW WILL NICK WESTERN WASHINGTON LATE FRIDAY INTO 
FRIDAY NIGHT FOR A CHANCE OF PRECIPITATION MAINLY OVER THE EASTERN 
PORTION OF THE AREA. A COLDER AIR MASS WILL SETTLE OVER THE 
REGION THIS WEEKEND. AN UPPER LOW IS EXPECTED TO SETTLE OVER 
NORTHWEST WASHINGTON SATURDAY NIGHT AND THEN SLIDE SOUTH INTO 
SOUTHWEST OREGON BY SUNDAY AFTERNOON. IT LOOKS LIKE COLD NORTH FLOW 
ALOFT WILL RE-DEVELOP EARLY NEXT WEEK AND PERSIST THROUGH AT LEAST 
THE MIDDLE OF THE WEEK.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS UPPER LEVEL RIDGE OVER WESTERN 
WASHINGTON. OFFSHORE GRADIENTS IN PLACE WITH HIGH PRESSURE CENTERED 
OVER SOUTHEAST BRITISH COLUMBIA. THE SURFACE AND UPPER LEVEL PATTERN 
WILL REMAIN IN PLACE THROUGH THURSDAY GIVING WESTERN WASHINGTON A 
COOL NIGHT TONIGHT FOLLOWED BY A SUNNY BUT COOL DAY ON THURSDAY. 
PATTERN BREAKS DOWN THURSDAY NIGHT INTO FRIDAY WITH AN UPPER LEVEL 
LOW DEVELOPING OVER EASTERN BRITISH COLUMBIA, THE LOW WILL MOVE 
SOUTH AND BE OVER WESTERN MONTANA BY LATE IN THE DAY FRIDAY. A WEAK 
SHORTWAVE ALONG THE BACKSIDE OF THE LOW WILL CLIP WESTERN WASHINGTON 
FRIDAY AFTERNOON INTO FRIDAY EVENING. THE MODELS DIFFER ON IF THIS 
FEATURE WILL PRODUCE AND PRECIPITATION. THE GFS SOLUTIONS ARE DRY 
WHILE THE NAM SOLUTIONS HAVE SOME LIGHT PRECIPITATION MAINLY OVER 
THE EASTERN PORTION OF THE CWA. THE AIR MASS WILL BE FAIRLY DRY OVER 
WESTERN WASHINGTON FRIDAY AFTERNOON AND EVENING WITH NORTHERLY 
SURFACE GRADIENTS. WILL STAY WITH THE CHANCE POPS FOR NOW BUT IF THE 
ECMWF COMES IN DRY POPS COULD GET PULLED FROM THE FORECAST IN THE 
THURSDAY MORNING PACKAGE. NORTHERLY GRADIENTS INCREASING OVER THE 
WEEKEND WITH A COLD AIR MASS DROPPING DOWN OVER WESTERN WASHINGTON 
WITH HIGHS IN THE UPPER 30S AND LOWER 40S COMMON EVEN WITH PLENTY OF 
SUNSHINE. NO UPDATE TO THE FORECAST TONIGHT. FELTON

.LONG TERM...PREVIOUS DISCUSSION FOLLOWS. STILL A LOT OF UNCERTAINTY 
FROM SUNDAY THROUGH THE REST OF THE EXTENDED PERIOD. THE ECMWF AND 
GFS RETAIN AN UPPER LOW OFF THE CENTRAL OREGON COAST SUN WITH 
ANOTHER ONE SOMEWHERE NEAR THE IDAHO AND OREGON BORDER. ANY 
PRECIPITATION THAT FALLS SUN WOULD LIKELY BE AS SNOW...EVEN DOWN TO 
SEA LEVEL. HAVE NOT MADE ANY CHANGES TO THE SUN GRIDS DUE TO THE 
DEGREE OF UNCERTAINTY. THE ECMWF AND GFS THEN CONSOLIDATE BOTH LOWS 
AND RE-CENTER IT OFF THE SOUTH OREGON COAST 12Z MON. PRECIP CHANCES 
REMAIN LIMITED DUE TO THE NORTHEAST TO EAST FLOW COMPONENT. THINGS 
GET MORE COMPLICATED TUE AND WED OF NEXT WEEK. IT LOOKS LIKE THE 
NORTHERN JET STREAM WILL DOMINATE AT THAT TIME WITH SOME SORT OF 
ARCTIC BOUNDARY EXTENDING FROM THE SOUTHERN B.C. COAST THROUGH 
NORTHERN WASHINGTON. THE GFS HAS IT A BIT MORE TO THE NE WED 
COMPARED TO THE ECMWF. MODELS THEN DEVELOP AN OVER-RUNNING SITUATION 
WED...BUT THIS APPEARS TO BE MORE OF AN OREGON PROBLEM. IN ANY 
EVENT...SOMETHING TO KEEP AN EYE ON. HOPEFULLY THE MODELS WILL COME 
INTO BETTER AGREEMENT. WEISHAAR

&&

.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. MCDONNAL

&&

.AVIATION...STRONG AREA OF HIGH PRES REMAINS OVER THE PAC NW 
TONIGHT...LEADING TO CLEAR AND DRY CONDITIONS. PATCHY FOG POSSIBLE 
EARLY THU MORNING...ALTHOUGH WITH DEW POINTS IN THE 20S MAY FROST 
OUT BY SUNRISE. OTHERWISE...VFR CONDITIONS WILL PREVAIL ON THU WITH 
JUST FEW-SCT CIRRUS OVER THE REGION. 

KSEA...CLEAR SKIES WITH LIGHT NLY WINDS. 

&&

.MARINE...NORTH/NORTHEASTERLY OFFSHORE FLOW CONTINUES ACROSS WESTERN 
WA TONIGHT WITH HIGH PRES OVER SE B.C. AND E WA. MEANWHILE SEAS 
EXPECTED TO RISE OVERNIGHT AND MAY NEED SMALL CRAFT ADVISORY FOR 
HAZARDOUS SEAS AS SWELLS RISE TO AROUND 10 FT. HIGH PRES WILL SHIFT 
FARTHER E ON THU WITH GENERALLY LIGHT WINDS IN WESTERN WA. A WEAK 
COLD FRONT WILL PASS THROUGH THE COASTAL WATERS THU NIGHT INTO EARLY 
FRI. OFFSHORE FLOW WILL INCREASE OVER THE WEEKEND AS A STRONG AREA 
OF HIGH PRES DEVELOPS OVER INTERIOR B.C. 33 

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE. 

$$

WWW.WEATHER.GOV/SEATTLE

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