FXUS66 KSEW 251115
AFDSEW

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

.SYNOPSIS...HIGH PRESSURE ALOFT OVER THE PACIFIC NORTHWEST WILL 
CONTINUE THROUGH TODAY. STRONG WINTER TEMPERATURE INVERSIONS WILL
MAINTAIN AREAS OF NIGHT AND MORNING FOG IN THE INTERIOR LOWLANDS 
ALONG WITH COOL DAYTIME TEMPERATURES. INCREASING 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...AN UPPER LEVEL RIDGE AXIS IS CENTERED OVER WESTERN 
WASHINGTON ON THIS CHRISTMAS MORNING...WITH SURFACE RIDGING FIRMLY 
ESTABLISHED OVER EASTERN WASHINGTON. OFFSHORE GRADIENTS HAVE BEEN 
STEADILY COMING UP OVERNIGHT. BOTH THE 00Z KUIL SOUNDING AND KSEW 
PROFILER TIME SERIES DATA SHOW CONSIDERABLE WARMING FROM JUST ABOVE 
THE SURFACE THROUGH 5000 FEET SINCE THIS TIME YESTERDAY. WITH THE 
WARMING ALOFT AND CLEAR SKIES...VERY STRONG SURFACE BASED INVERSIONS 
ARE KEEPING SHALLOW DENSE FOG IN PLACE FROM NEAR SEATAC SOUTHWARD 
THROUGH THE SOUTHWEST INTERIOR AND CHEHALIS VALLEY. OFFSHORE 
GRADIENTS ARE PLAYING ENOUGH OF A PART TO KEEP FOG PATCHY FOR MOST 
OF THE CENTRAL AND NORTH SOUND THIS MORNING. THE FOG WILL BE SLOW TO 
CLEAR...IF AT ALL...OVER THE SOUTHWEST INTERIOR. ELSEWHERE...IT 
SHOULD BE A BEAUTIFUL CHRISTMAS DAY. AS THE CROSS CASCADE GRADIENT 
INCREASES LATER TODAY...WINDS MAY APPROACH ADVISORY LEVEL IN THE 
CASCADE FOOTHILLS. THE MM5-WRF HAS A FEW POCKETS OF 30-40 KNOTS 
DEVELOPING IN THE FOOTHILLS AFTER 03Z TONIGHT...WITH THE NAM12 
SHOWING WEAKER WINDS. AT 09Z...A RAWS OB JUST OUTSIDE NORTH BEND HAD 
SE WINDS 20 GUSTING 35 MPH. EASTERLY WINDS WILL KEEP TEMPS AT OR 
BELOW FREEZING IN THE CASCADES PASSES TODAY...BUT CONSIDERABLY 
WARMER JUST ABOVE AS FREE AIR FREEZING LEVELS WILL BE IN THE 9000 TO 
10000 FOOT RANGE.  

THE UPPER LEVEL RIDGE AXIS SHIFTS EAST THE CASCADES TONIGHT. 
MODERATE OFFSHORE GRADIENTS CONTINUE WELL INTO SATURDAY AFTERNOON AS 
A SURFACE RIDGE REMAINS CENTERED OVER NORTHEAST WASHINGTON AND A 
TROUGH MOVES IN THE OFFSHORE WATERS WEST OF 130W. BY SATURDAY 
AFTERNOON...UPPER LEVEL RIDGING IS CENTERED OVER CENTRAL ALBERTA 
WITH AN OFFSHORE SYSTEM ATTEMPTING TO UNDERCUT THE RIDGE AND MOVE 
INTO THE PACIFIC NORTHWEST. THIS SYSTEM WILL ENCOUNTER SPLIT FLOW 
ALOFT AND SHOULD NOT PRODUCE MUCH MORE THAN A FEW MID/HIGH CLOUDS 
FOR THE REGION. ON SUNDAY...THE GFS BRINGS SOME LIGHT PRECIP INTO 
WESTERN WASHINGTON FROM A WEAK FRONT ENCOUNTERING THE SPLIT. THE 
NAM/ECMWF/CANADIAN KEEP THE REGION DRY. MINIMAL 20 PERCENT POPS WERE 
PUT IN THE FORECAST FOR THE CENTRAL COAST SUNDAY AFTERNOON WITH THE 
REMAINDER OF THE AREA NOT EXPECTED TO SEE ANY PRECIP.

.LONG TERM...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...NO THREAT OF FLOOD-PRODUCING RAINFALL IN THE FORECAST 
AREA FOR THE NEXT TEN DAYS. THIS INCLUDES THE GREEN RIVER. 

&&

.AVIATION...WE HAVE NORTHWEST FLOW ALOFT OVER WESTERN WASHINGTON 
EARLY THIS MORNING WITH AN UPPER LEVEL RIDGE ALONG THE COAST. THE 
UPPER FLOW WILL GRADUALLY SWITCH TO SOUTHEAST LATER TODAY AND 
TONIGHT AS THE RIDGE PROGRESSES SLOWLY EASTWARD ACROSS THE PACIFIC 
NORTHWEST. AT THE SURFACE A 1034 MB HIGH EXTENDING FROM SOUTHEAST 
BRITISH COLUMBIA INTO NORTHERN IDAHO WILL PROVIDE CONTINUING 
OFFSHORE FLOW ACROSS OUR FORECAST AREA.

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 LED 
TO FOG FORMATION ACROSS THE SOUTH PUGET SOUND AREA AND SOUTH 
INTERIOR. HOWEVER THERE HAS BEEN ENOUGH WIND FROM ROUGHLY SEATTLE 
NORTHWARD TO LIMIT OR PREVENT FOG FORMATION ALTOGETHER. THE WIND WAS 
EXPECTED TO EASE AROUND MIDNIGHT...SO THERE IS MUCH LESS FOG THAN 
FORECAST IN THE CENTRAL PUGET SOUND AND FURTHER NORTH. FOG WILL 
LIKELY BECOME SOMEWHAT MORE WIDESPREAD BY DAYBREAK...BUT THE 
FORECAST FOR KSEA...KBFI...AND KPAE THIS MORNING WILL BE RATHER LOW 
CONFIDENCE. FOGGY CONDITIONS WILL PROBABLY LINGER THROUGH THE DAY 
OVER THE SOUTHWEST INTERIOR.

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 5-10 KT EARLY THIS MORNING...EASING TO 6 KT OR 
LESS AROUND DAYBREAK AND CONTINUING THROUGH TONIGHT. GIVEN THE FACT 
THAT WINDS WERE EXPECTED TO EASE AND FOG WAS EXPECTED TO FORM 
SEVERAL HOURS AGO...THE CONFIDENCE IN THE FORECAST FOR FOG TO FORM 
LATER THIS MORNING IS NOT GREAT. BEST GUESS NOW IS THAT THE WIND 
WILL HANG ON LONG ENOUGH TO PREVENT A LONG PERIOD OF LIFR 
CONDITIONS. MCDONNAL
 
&&

.MARINE...A 1034 MB HIGH EXTENDING FROM SOUTHEAST BRITISH COLUMBIA 
INTO NORTHERN IDAHO WILL REMAIN IN PLACE THROUGH SATURDAY NIGHT. 
MARGINAL EASTERLY GALES DEVELOPED OVERNIGHT AT THE WEST ENTRANCE TO 
THE STRAIT OF JUAN DE FUCA...AND 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 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. THUS WE CAN EXPECT SMALL 
CRAFT ADVISORY LEVEL HAZARDOUS SEAS AND ROUGH BAR CONDITIONS AT 
GRAYS HARBOR FOR THE NEXT SEVERAL DAYS. MCDONNAL
&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...AIR STAGNATION ADVISORY FOR MOST OF THE WESTERN WASHINGTON   
      LOWLANDS THROUGH SATURDAY MORNING.
PZ...GALE WARNING THIS MORNING 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 GRAYS HARBOR DUE TO ROUGH BAR
      CONDITIONS.

$$

WEATHER.GOV/SEATTLE

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