FXUS66 KSEW 021623
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
823 AM PST WED DEC 2 2009

.SYNOPSIS...AN UPPER LEVEL RIDGE WILL PROVIDE TWO MORE DAYS OF COOL 
SUNNY WEATHER WITH AREAS OF NIGHT AND MORNING FOG. A COLD FRONT AND 
UPPER LEVEL TROUGH WILL REACH THE AREA LATE IN THE WEEK FOR A CHANCE 
OF SHOWERS FRIDAY AND SATURDAY...POSSIBLY FALLING AS SNOW LATE 
FRIDAY NIGHT AND EARLY SATURDAY. THE PATTERN WILL KEEP COOL 
TEMPERATURES AND A THREAT OF LIGHT PRECIPITATION IN THE FORECAST 
SUNDAY AND EARLY NEXT WEEK.

&&

.SHORT TERM...THE AIR MASS HAS DRIED OUT OVERNIGHT AND DEWPOINTS ARE 
IN THE 20S. SKIES ARE CLEAR...WITH ONLY ISOLATED FOG PATCHES. 
NORTHERLY PRESSURE GRADIENTS ARE UP ENOUGH TO KEEP NORTH WINDS IN 
THE 5-15 MPH RANGE TODAY. AS A RESULT...THE MAX TEMP FORECAST FOR 
TODAY MAY BE 1-2 DEGREES HIGH...BUT NOT QUITE ENOUGH TO MERIT A 
MORNING UPDATE. 

AS THE UPPER LEVEL RIDGE OVER B.C. TILTS FURTHER INLAND THROUGH 
THURSDAY...AND THE SURFACE HIGH OVER SE B.C. SLIDES DOWN OVER THE 
NORTHERN ROCKIES...THE NORTHERLY PRESSURE GRADIENTS WILL EASE. THERE 
MAY BE A MINOR INCREASE IN FOG THURSDAY MORNING.

MODELS BRING AN UPPER LEVEL SHORTWAVE TROUGH CURRENTLY OUT NEAR 
50N/150W TO THE CENTRAL B.C. COAST THURSDAY AFTERNOON...THEN DIG IT 
SOUTH OR SE OVER WA AS A CLOSED LOW BY LATE FRIDAY. THE FORECAST 
PROBLEM THE PAST COUPLE OF DAYS HAS BEEN WHERE THE LOW WILL 
GO...EITHER THE ECMWF SOLUTION FAVORING SLOWER AND OVER W WA...OR 
THE FASTER GFS SOLUTION WHICH TAKES THE LOW OVER ID. MODEL 
DIFFERENCES CONTINUE...BUT MODEL TRENDS FROM THE ECMWF HAVE BEEN 
GRADUALLY FAVORING DRIER. CONSENSUS SEEMS TO FAVOR A SLOWER SOLUTION 
LIKE THE ECMWF AND NAM OVER THE GFS. FORECASTS ARE PRETTY MUCH ON 
TRACK. MAY HAVE TO COOL TEMPERATURES DOWN A BIT MORE ON FRIDAY...BUT 
THIS WILL DEPEND UPON WHAT THE 12Z ECMWF SHOWS. KAM

.LONG TERM...IN THE LONGER RANGE THE MODELS AGREE THAT THE UPPER LOW 
WILL MOVE SOMEWHERE TO OUR SOUTH OVER THE WEEKEND...FOLLOWED BY 
NORTHERLY FLOW ALOFT. MORE IMPORTANTLY THEY AGREE THAT A STRONG 
SURFACE HIGH OF AROUND 1050 MB OVER WESTERN CANADA -- THE REFLECTION 
OF AN ARCTIC MASS MOVING SOUTHWARD -- WILL EVENTUALLY PUSH COLDER 
AIR INTO WESTERN WASHINGTON. THEY ALSO AGREE THAT THE THREAT OF 
PRECIPITATION WILL BE LIMITED...HENCE ONLY CHANCE POPS IN THE 
FORECAST. 

EARLY NEXT WEEK THE UPPER LOW WILL PROBABLY WEAKEN AND SHIFT 
EASTWARD...WITH A WEAK UPPER RIDGE APPROACHING THE WEST COAST OF THE 
UNITED STATES. THE MODEL SOLUTIONS DIFFER IMPORTANTLY AGAIN -- 
ESPECIALLY IF YOU LIVE IN CALIFORNIA WHICH COULD BE IN FOR A BIG 
RAIN EVENT -- BUT FOR WESTERN WASHINGTON THEY ALL SUGGEST A COOL 
NORTHWEST FLOW ALOFT THROUGH TUESDAY WITH LIMITED THREAT OF 
PRECIPITATION. 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. MCDONNAL

&&

.AVIATION...THE COMBINATION OF AN UPPER LEVEL RIDGE AND MODERATE 
OFFSHORE FLOW AFT THE SURFACE IS PROVIDING CLEAR SKIES ACROSS THE 
REGION. SOME PATCHES OF FOG WILL DISSIPATE BY 10 AM THIS MORNING.  
THE OFFSHORE FLOW WILL CONTINUE INTO THURSDAY WITH SKIES REMAINING 
CLEAR. PATCHES OF FOG ARE EXPECTED TO FORM AGAIN LATE TONIGHT IN THE 
MOST FOG PRONE AREAS. 

KSEA...CLEAR SAILING THROUGH THURSDAY WITH A GENERAL NORTHERLY WIND 
CRANKING AROUND TO NORTHEAST AFT 06Z-08Z TONIGHT.  CERNIGLIA

&&

.MARINE...HIGH PRES OVER SE BRITISH COLUMBIA WILL SAG SLOWLY SE INTO 
MONTANA AND EASTERN WA BY THU. THIS WILL MAINTAIN THE OFFSHORE FLOW 
THROUGH THE PERIOD.  THE ONLY AREA WHERE THIS WILL POSE A PROBLEM 
WIND WISE IS THE WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA WHERE 
THERE WILL BE SMALL CRAFT EASTERLIES AT TIMES.  

A FRONT WILL SLIDE S THROUGH THE AREA FRI AS IT DISSIPATES.  THIS 
WILL SERVE TO ENHANCE THE OFFSHORE FLOW THROUGH THE WEEKEND WITH 
POSSIBLE SCA TO GALE NE WINDS ACROSS PORTIONS OF THE NORTHER PUGET 
SOUND WATERS AND PARTS OF THE STRAIT.  CERNIGLIA

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY WEST ENTRANCE TO THE STRAIT.
   
$$

WWW.WEATHER.GOV/SEATTLE

FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE 
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.