FXUS66 KSEW 012244
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
245 PM PST TUE DEC 1 2009

.SYNOPSIS...AN UPPER LEVEL RIDGE BUILDING OVER THE REGION AND A 
SURFACE HIGH OVER BRITISH COLUMBIA WILL PRODUCE DRY NORTH FLOW ALOFT 
ACROSS WESTERN WASHINGTON THROUGH THURSDAY. THE RIDGE AND 
SURFACE HIGH WILL MAINTAIN MOSTLY CLEAR COOL AND DRY WEATHER THROUGH 
THURSDAY...EXCEPT FOR AREAS OF MORNING FOG. THE RIDGE WILL BREAK 
DOWN AFTER MIDWEEK...ALLOWING AN UPPER LEVEL LOW TO MOVE OVER THE 
REGION FRIDAY THROUGH MONDAY. THIS WILL BRING BACK CLOUDIER WEATHER 
WITH A CHANCE OF RAIN OR SHOWERS.   

&&

.SHORT TERM...VISIBLE SATELLITE IMAGERY SHOWS SKIES ARE PRETTY MUCH 
CLEAR OVER W WA THIS AFTERNOON. STILL SOME SMALL AREAS OF CLOUDS 
OVER THE SW INTERIOR...BUT THIS SHOULD DISSIPATE EARLY THIS EVENING. 
WITH CLEAR SKIES OVERNIGHT...RADIATIONAL COOLING CONDITIONS WILL BE 
EXCELLENT. NORTH WINDS WILL SLOW DOWN THE FOG FORMATION...BUT WITH 
DEW POINTS CURRENTLY IN THE MID TO UPPER 30S FROM PUGET SOUND 
SOUTHWARD...AND FORECAST LOWS EXPECTED TO BE COLDER...THERE WILL BE 
AREAS OF FOG AROUND WEDNESDAY MORNING. DEWPOINTS UP AROUND 
BELLINGHAM AND THE SAN JUANS HAVE ALREADY DIPPED INTO THE MID 
20S...SO NOT MUCH FOG IS EXPECTED THERE. 

MODEL TRENDS LOOK GOOD THROUGH THURSDAY...SLOWLY MOVING THE UPPER 
LEVEL RIDGE ALONG 130W INLAND OVER B.C. ON THURSDAY...AND SHIFTING 
THE B.C. SURFACE HIGH SE OVER THE NORTHERN ROCKIES. THE SHIFT TO 
EASTERLY LOW LEVEL FLOW BY THURSDAY...COMBINED WITH SUBSIDENCE OVER 
THE AREA...SHOULD GRADUALLY REDUCE THE FOG COVERAGE THURSDAY AND 
FRIDAY MORNINGS. MORNINGS SHOULD BE CHILLY WITH THE COLDEST AREAS 
GETTING DOWN TO THE MID 20S BY THURSDAY MORNING. 

MODELS ARE IN BETTER CONSENSUS FOR FRIDAY BUT STILL HAVE IMPORTANT 
DIFFERENCES. HAVE GONE WITH THE ECMWF SOLUTION...SUPPORTED BY THE 
CANADIAN. THIS SOLUTION TAKES THE UPPER LEVEL SHORTWAVE TROUGH 
APPROACHING THE B.C. COAST THURSDAY AFTERNOON...SOUTHEASTWARD OVER 
W WA AS A CLOSED LOW BY FRIDAY AFTERNOON. THE GFS HAS CONSISTENTLY 
TAKEN THE LOW FURTHER EAST WHICH WOULD RESULT IN A DRY FORECAST. 
WILL KEEP THE CHANCE OF RAIN FOR FRIDAY...BUT HAVE NUDGED POPS UP A 
BIT MORE IN LINE WITH THE ECMWF. KAM 

.LONG TERM...BOTH 12Z GFS AND 12Z ECMWF MOVE THE LOW SLOWLY SOUTH 
OVER OREGON BY SUNDAY. HAVE KEPT A CHANCE OF RAIN OVER W WA FRIDAY 
NIGHT AND SATURDAY PER THE ECMWF. WITH THE UPPER LOW OVER OREGON 
...MODELS HAD SUNDAY/MONDAY DRY...BUT THE BEST COURSE WITH THE LOW 
IN THE VICINITY FOR DAYS 5 ONWARD IS TO HANG ON TO A CHANCE OF 
SHOWERS. LATEST 18Z GFS RUN HAS COME IN WITH A FEW CHANGES IN 
DETAILS DURING THIS PERIOD WHICH SUPPORTS TAKING THE CONSERVATIVE 
APPROACH. BOTH MODELS SET UP COOL N-NE LOW LEVEL FLOW OVER THE AREA 
BY SUNDAY AS THE UPPER LOW MOVES S OF WA AND A STRONG SURFACE HIGH 
FORMS OVER B.C.   THIS WILL ALLOW A COLDER AIR MASS TO SLIDE DOWN 
OVER THE AREA DURING THE SUNDAY-TUESDAY PERIOD. GFS MAY BE OVERDOING 
THE COLD AIR ADVECTION AS USUAL...BUT HAVE STILL COOLED OFF MAX/MIN 
TEMPS FROM SATURDAY ON TO BE MORE IN LINE WITH THE PATTERN. KAM 

&&

.HYDROLOGY...THERE IS NO THREAT OF RIVER FLOODING EXPECTED IN THE 
FORECAST AREA THROUGH DECEMBER 11TH. THIS INCLUDES THE GREEN RIVER.

ONLY LIGHT PRECIPITATION IS EXPECTED AS AN UPPER LEVEL LOW MOVES 
OVER THE AREA FRIDAY AND SATURDAY. THE UPPER LEVEL LOW MAY STILL BE 
IN THE VICINITY SUNDAY AND MONDAY SO WILL MAINTAIN A LOW CHANCE OF 
SHOWERS. GFS/ECMWF ARE SHOWING A SHIFT BACK TO A WETTER PATTERN 
SOMETIME NEXT WEEK AS WESTERLY FLOW ACROSS THE NE PACIFIC UNDERCUTS 
THE LARGE UPPER LEVEL RIDGE ALONG 140W. EXACTLY WHEN THIS RETURN TO 
A WETTER PATTERN HAPPENS IS STILL UNCERTAIN...BUT DOES NOT 
NECESSARILY MEAN RETURNING TO A FLOOD PATTERN. KAM

&&

.AVIATION...DRY NWLY FLOW ALOFT AND OFFSHORE SURFACE FLOW IS GIVING 
LARGELY CLEAR SKIES AROUND PUGET SOUND. SKIES WILL BE MOSTLY CLEAR 
TONIGHT AND WEDNESDAY. PATCHY FOG IS POSSIBLE LATE TONIGHT AND EARLY 
WEDNESDAY MORNING AT SUSCEPTIBLE LOCATIONS...KOLM AND POSSIBLY KPAE.

KSEA...SHOULD BE CLEAR TONIGHT...WITH A SMALL CHANCE OF FOG IN THE 
MORNING BUT WILL NOT MENTION IT IN THE TAF. WINDS NORTH 8-12 KT 
THIS EVENING...BECOMING NE 4-8 KT OVERNIGHT. CHB

&&

.MARINE...HIGH PRES OVER CENTRAL BRITISH COLUMBIA WILL GIVE OFFSHORE 
FLOW TODAY THROUGH THE REST OF THE WEEK. HAVE KEPT SMALL CRAFT 
ADVISORIES FOR THE WEST ENTRANCE AND NORTHERN INLAND WATERS GOING 
INTO THE EVENING...WHERE WINDS ARE STILL PUSHING ADVISORY LEVELS. 
ALSO ADDED PUGET SOUND INTO THE ADVISORY CATEGORY AS OBSERVATIONS IN 
THE SOUTH END ARE NEAR ADVISORY LEVELS. WINDS OVER ALL AREAS WILL 
DIMINISH SOMEWHAT...AT THE WEST ENTRANCE THEY ARE LIKELY TO RISE 
AGAIN IN THE MORNING. 

A VERY WEAK FRONT IS FORECAST TO DISSIPATE OVER THE AREA ON FRIDAY. 
IT WILL HAVE LITTLE IMPACT BUT COULD TURN THE NORTHERLIES AROUND TO 
SOUTHEAST WINDS FOR AWHILE.

THERE IS SOME CHANCE OF ARCTIC OUTFLOW IN THE NORTH THIS WEEKEND. A 
SOMEWHAT MORE LIKELY SOLUTION IS THE COLDEST AIR WILL SLIP INTO 
EASTERN WASHINGTON AND MONTANA...WITH ONLY MODERATE OUTFLOW IN 
WESTERN WASHINGTON. HAVE KEPT WINDS BELOW 30 KT THROUGH THE WEEKEND. 
A LITTLE STRENGTHENING IN THE GRADIENTS COULD BOOST NORTHEAST WINDS 
INTO THE GALE CATEGORY AT THE WEST ENTRANCE AND OVER THE NORTHERN 
INLAND WATERS THIS WEEKEND. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE
PZ...SMALL CRAFT ADVISORY WEST ENTRANCE...NORTHERN INLAND WATERS...
     AND PUGET SOUND AND HOOD CANAL.
    .SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.
   
$$

WWW.WEATHER.GOV/SEATTLE

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