FXUS66 KSEW 251727
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
928 AM PST FRI DEC 25 2009

.SYNOPSIS...HIGH PRESSURE ALOFT OVER THE PACIFIC NORTHWEST WILL 
CONTINUE TODAY. STRONG WINTER TEMPERATURE INVERSIONS WILL MAINTAIN 
AREAS OF NIGHT AND MORNING FOG IN THE INTERIOR LOWLANDS ALONG WITH 
COOL DAYTIME TEMPERATURES. STRONG OFFSHORE FLOW WILL BRING BREEZY 
EASTERLY WINDS TO THE CASCADES AND FOOTHILLS TODAY AND SATURDAY. A 
SERIES OF WEAK SYSTEMS WILL AFFECT THE AREA NEXT WEEK.

&&

.SHORT TERM...A STRONG UPPER LEVEL RIDGE AXIS WAS SITUATED DIRECTLY 
OVER WRN WA THIS MORNING. IN ASSOCIATION WITH THE RIDGE...WARM AIR 
ALOFT AND OUR 15-16 HOUR CLEAR NIGHTS THIS TIME OF YEAR ARE LEADING 
TO STRONG SURFACE-BASED TEMPERATURE INVERSIONS. THE 12Z KUIL 
SOUNDING SHOWED A TEMPERATURE OF 56F AT ONLY 1300 FT MSL...SO THERE 
IS PLENTY OF WARM AIR ABOVE THE FROSTY LOWLANDS. THE 12Z NAM SHOWS 
THE 1000-850 MB LAPSE RATE REMAINING WELL BELOW ZERO THROUGHOUT THE 
LOWLANDS TODAY...MEANING INVERSIONS WILL NEVER BREAK. FROM AN AIR 
QUALITY STANDPOINT...THIS WILL PROVIDE LITTLE IF ANY VERTICAL MIXING 
OF SMOKE AND POLLUTANTS TODAY. 

HOWEVER...STRONG EASTERLY GRADIENTS WILL PROVIDE SOME HORIZONTAL 
TRANSPORT AND CLEAR THE AIR IN QUITE A FEW PLACES. THIS WILL 
PARTICULARLY BE THE CASE WHERE EASTERLY GAP WINDS AFFECT THE CASCADE 
FOOTHILLS...PARTS OF KING COUNTY...AND OVER THE NORTH INTERIOR FROM 
BELLINGHAM OUT THROUGH THE STRAIT OF JUAN DE FUCA. THE MAIN AREAS OF 
CONCERN FOR AIR QUALITY WOULD BE WHERE WINDS STAY LIGHT OR CALM ALL 
DAY. THIS WOULD BE MOST LIKELY IN PIERCE...THURSTON...LEWIS AND 
MAYBE MASON COUNTIES. 

UPPER HEIGHTS OVER WRN WA WILL LOWER SOME ON SATURDAY AS THE UPPER 
RIDGE MOVES INTO THE INTERIOR OF B.C. AND EVENTUALLY ALBERTA. THIS 
WILL WEAKEN INVERSIONS A BIT ON SATURDAY WITH LOW-LEVEL INVERSIONS 
EXPECTED TO DISSIPATE ON SUNDAY. 

FROM A MORE SENSIBLE STANDPOINT...THE WEATHER PATTERN WILL CONTINUE 
TO PROVIDE CLEAR AND COOL WEATHER WITH FROSTY NIGHTS THROUGH THE 
WEEKEND. A WAVE LIFTING NORTH WELL PAST 130W ON SUNDAY WILL TRY TO 
BRING SOME LIGHT RAIN ONSHORE FROM THE SOUTHWEST...BUT THE RAIN WILL 
LIKELY DISSIPATE BEFORE REACHING THE FORECAST AREA AS IT ENCOUNTERS 
THE DRY AIR IN PLACE OVER WRN WA AND UNFAVORABLE OFFSHORE LOW-LEVEL 
FLOW. BASICALLY...CLOUDS SHOULD INCREASE ON SUNDAY...BUT THE VAST 
MAJORITY OF WRN WA WILL REMAIN DRY.     HANER

.LONG TERM...FROM PREVIOUS DISCUSSION...THE EXTENDED MODELS CONTINUE 
TO HAVE TROUBLE WITH DETAILS AS WE TRANSITION FROM A SPLIT TO A MORE 
CONSOLIDATED FLOW ALOFT NEXT WEEK. THE 00Z ECMWF BRINGS A SYSTEM 
ONSHORE LATE TUESDAY...BUT TAKES MOST OF THE ENERGY ASSOCIATED WITH 
IT TO OUR SOUTH. THE GFS/CANADIAN ARE BRINGING A MORE ORGANIZED 
SYSTEM ONSHORE INTO WESTERN WASHINGTON AROUND THE SAME TIME. THE 
BROADBRUSH CHANCE OF RAIN IN THE EXTENDED PERIOD LOOKS OK FOR 
NOW...BUT IT APPEARS WE WILL HAVE A GOOD SHOT AT SOME RAIN FOR THE 
MIDDLE AND END OF NEXT WEEK. 27

&&

.HYDROLOGY...THERE IS NO THREAT OF FLOOD-PRODUCING RAINFALL FOR THE 
NEXT TEN DAYS. THIS INCLUDES THE GREEN RIVER.        HANER

&&

.AVIATION...LIGHT FLOW ALOFT UNDER A STRONG UPPER RIDGE...GRADUALLY 
SWITCHING TO LIGHT SOUTHEAST FLOW TONIGHT AS THE RIDGE MOVES SLOWLY 
EAST. AT THE SURFACE STRONG HIGH PRESSURE EXTENDING FROM SOUTHEAST 
BRITISH COLUMBIA INTO NORTHERN IDAHO WILL GIVE CONTINUING OFFSHORE 
FLOW.

THE COMBINATION OF WARM AIR ALOFT WITH THE RIDGE AND A RELATIVELY 
COOL AIR MASS AT THE SURFACE IS PRODUCING A VERY STABLE AIR MASS. A 
STRONG INVERSION FROM THE SURFACE TO AROUND 5000 FT WILL CONTINUE 
THROUGH TONIGHT.

LOW LEVEL MOISTURE TRAPPED IN THE INVERSION AND LIGHT WINDS HAS 
CAUSED FOG ACROSS THE SOUTH PUGET SOUND AREA AND SOUTH INTERIOR. 
HOWEVER THERE HAS BEEN ENOUGH WIND FROM ROUGHLY SEATTLE NORTHWARD TO 
LIMIT OR PREVENT FOG ALTOGETHER. THESE CONDITIONS WILL PREVAIL TODAY 
AND TONIGHT...WITH ONLY LIMITED FOG BURNOFF IN THE SOUTH THIS 
AFTERNOON. 

TONIGHT SHOULD BRING A NEAR REPEAT...WITH THE FOGGIEST CONDITIONS IN 
THE SOUTH PUGET SOUND AND SOUTHWEST INTERIOR AND PATCHIER FOG TO THE 
NORTH.

KSEA...NORTHEAST WIND 8-12 KT THIS MORNING...EASING TO 6-8 KT THIS 
AFTERNOON THROUGH TONIGHT. WIND SHOULD BE STRONG ENOUGH INTO THIS 
EVENING AT LEAST TO PREVENT A LONG PERIOD OF LIFR CONDITIONS. CHB
 
&&

.MARINE...A 1034 MB HIGH EXTENDING FROM SOUTHEAST BRITISH COLUMBIA 
INTO NORTHERN IDAHO WILL REMAIN IN PLACE THROUGH SATURDAY NIGHT. 
EASTERLY GALES DEVELOPED OVERNIGHT AT THE WEST ENTRANCE TO THE 
STRAIT OF JUAN DE FUCA...AND WILL CONTINUE INTO THE AFTERNOON. 
GRADUALLY INCREASING WINDS ARE LIKELY OVER THE CENTRAL STRAIT AND 
EAST ENTRANCE TONIGHT AND THEN OVER ADMIRALTY INLET AND THE NORTHERN 
INLAND WATERS SATURDAY AND SATURDAY NIGHT. THESE WATERS SHOULD ALL
EVENTUALLY HAVE SMALL CRAFT ADVISORY WINDS.

THE STRENGTH OF OFFSHORE FLOW WILL EASE SOMEWHAT SUNDAY AND MONDAY 
AS A SPLITTING FRONT DISSIPATES OVER THE OFFSHORE WATERS. ANOTHER 
WEAK FRONT WILL PROBABLY MOVE INTO THE OFFSHORE WATERS FROM THE WEST 
ON TUESDAY. 

IN ADDITION WESTERLY SWELLS OF 9 TO 10 FT OVER THE COASTAL WATERS 
TODAY WILL GRADUALLY BUILD TO AROUND 15 FT ON SATURDAY NIGHT...THEN 
GRADUALLY SUBSIDE SUNDAY THROUGH TUESDAY. EXPECT SMALL CRAFT 
ADVISORY LEVEL HAZARDOUS SEAS AND ROUGH BAR CONDITIONS AT GRAYS 
HARBOR FOR THE NEXT SEVERAL DAYS. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...AIR STAGNATION ADVISORY FOR MOST OF THE WESTERN WASHINGTON   
      LOWLANDS THROUGH SATURDAY MORNING.
PZ...GALE WARNING UNTIL 3 PM FOR WEST ENTRANCE STRAIT OF JUAN DE 
     FUCA AND COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND OUT 
     10 NM.
    .SMALL CRAFT ADVISORY FOR THE REMAINDER OF THE COASTAL WATERS.
    .SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

$$

WEATHER.GOV/SEATTLE

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