FXUS66 KSEW 250425 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
830 PM PST THU DEC 24 2009

.SYNOPSIS...HIGH PRESSURE ALOFT OVER THE PACIFIC NORTHWEST WILL 
CONTINUE THROUGH CHRISTMAS DAY. THE ASSOCIATED WARM AIR ALOFT HAS 
ESTABLISHED STRONG WINTER TEMPERATURE INVERSIONS THAT MAINTAIN AREAS 
OF NIGHT AND MORNING FOG IN THE INTERIOR LOWLANDS ALONG WITH COOL 
DAYTIME TEMPERATURES. INCREASING OFFSHORE FLOW SHOULD BRING BREEZY 
EASTERLY WINDS TO THE CASCADES AND FOOTHILLS LATE CHRISTMAS DAY AND 
SATURDAY. THE HIGH PRESSURE MOVES INLAND SUNDAY BUT IS EXPECTED TO 
BUMP AN INCOMING PACIFIC WEATHER SYSTEM SOUTH OF THE AREA. A SERIES 
OF WEAK SYSTEMS ARE MORE LIKELY TO AFFECT THE AREA NEXT WEEK.

&&

.SHORT TERM...A SILENT NIGHT ON WESTERN WA WEATHER TONIGHT. HIGH 
PRESSURE ALOFT HAS STRENGTHENED JUST OFF THE COAST DOWNSTREAM FROM 
THE STRONG DEEP MATURE STORM NEAR 160W. SATELLITE IMAGES SHOW THE 
ASSOCIATED COLD FRONT OFF THE COAST HAS BECOME N-S ORIENTED..A SIGN 
IT IS NOT GOING TO MAKE MUCH MORE PROGRESS E. THE VORTICITY MAX 
APPROACHING 40N/150W SHOULD BEGIN THE SPLITTING PROCESS ADVERTISED 
IN THE PROGS.  

WARMER AIR ASSOCIATED WITH THE UPPER RIDGE IS APPARENT WITH FREEZING 
LEVELS VIA THE RAOBS AT QUILLAYUTE - 9400 FT - AND SALEM AT 9500 FT. 
SKIES CLEARED TODAY AND THE REMAINING CIRRUS CLOUDS ARE IN THE 
PROCESS OF DISSIPATING FOR A MIDNIGHT CLEAR. THE EXCEPTION WAS 
PERSISTENT FOG IN THE SOUTH SOUND DOWN THE I-5 CORRIDOR THROUGH 
LEWIS COUNTY. SO ON A COLD WINTERS NIGHT UNDER THE TEMPERATURE 
INVERSION..EXPECT MORE FOG TO DEVELOP OVERNIGHT PRIMARILY IN THE 
SNOHOMISH VALLEY AND FROM ABOUT KSEA SOUTH INTO THE SOUTH SOUND. 
WITH THE INVERSION..THE AIR STAGNATION ADVISORY LOOKS SOLID.

WITH THE BULK OF THE WESTERN CANADIAN SURFACE HIGH SLIDING SE BEHIND 
THE MIDDLE OF U.S. MAJOR WINTER STORM..SURFACE GRADIENTS HERE ARE 
MORE E TO W VS N TO S. PROGS ARE STILL INCONSISTENT REGARDING HOW 
MUCH WIND WILL DEVELOP THRU THE CASCADE GAPS AND INTO THE FOOTHILLS 
LATER CHRISTMAS DAY AND INTO BOXING DAY. AT THIS POINT..EXPECT SOME 
WINDS TO MAKE SOME PROGRESS DOWN THE CASCADE FOOTHILLS INTO LOCALES 
SUCH AS ENUMCLAW. WILL NEED TO KEEP AN EYE ON THE POTENTIAL FOR A 
WIND ADVISORY FOR THE EAST PUGET SOUND LOWLANDS. ALSO SOME OFFSHORE 
WINDS THROUGH THE STRAIT AND COASTAL GAPS LIKE GRAYS HARBOR WHERE 
HIGH TEMPS SHOULD BE WARMER. DO NOT EXPECT MUCH CIRCULATION FOR THE 
REST OF WESTERN INTERIOR. SO FOG MAY AGAIN BE PERSISTENT PRIMARILY 
IN THE SOUTH SOUND AND INTO WESTERN LEWIS COUNTY. 

OVER THE WEEKEND..THE UPPER RIDGE SHIFTS INLAND AS THE SPLIT FLOW 
PATTERN DEVELOPS. THE AFORE MENTIONED VORTICITY MAX DRIVES THAT PART 
OF THE COLD FRONT INTO THE WEST COAST SOUTH OF WA..LEAVING WA DRY. 
MAY NOT BE ENOUGH OFFSHORE FLOW SAT OR SUN TO MIX OUT THE FOG AND 
TEMPERATURE INVERSION IN PARTS OF THE INTERIOR..SOMETHING TO MONITOR.

DECK THE HALLS..THIS JUST IN FROM AIR TRAFFIC CONTROL. A SLEIGH LED 
BY EIGHT REINDEER AND A MAN IN A RED SUIT HAS BEEN SPOTTED MAKING 
ITS WAY SOUTH IN THE AREA...HO HO HO AND TO ALL A GOOD NIGHT. BUEHNER

.LONG TERM...FROM PREVIOUS DISCUSSION..LONG RANGE MODELS MAINTAIN 
THE SPLIT UPPER FLOW NEXT WEEK. HOWEVER...BOTH MODELS INDICATE SOME 
WEAK SYSTEMS EMBEDDED IN THE NRN STREAM TO POSSIBLY BRING LIGHT RAIN 
INTO THE AREAS AT TIMES. THE GFS/ECMWF DISAGREE ON THE STRENGTH AND 
TIMING SO I MAINTAINED THE CHANCE POPS FOR EACH DAY BEGINNING 
MONDAY. MERCER

&&

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

&&

.AVIATION...WE HAVE NORTHWEST FLOW ALOFT OVER WESTERN WASHINGTON 
THIS EVENING AHEAD OF AN UPPER LEVEL RIDGE JUST OFF THE COAST ALONG 
125W. THE RIDGE WILL PROGRESS SLOWLY EASTWARD TO THE CASCADES LATE 
FRIDAY. 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 
TONIGHT AND FRIDAY.

LOW LEVEL MOISTURE TRAPPED IN THE INVERSION AND LIGHT WINDS HAS 
ALREADY ALLOWED FOG TO FROM IN THE SOUTH PUGET SOUND AREA AND 
ESPECIALLY OVER THE SOUTHWEST INTERIOR. THIS WILL BECOME MORE 
WIDESPREAD AS TEMPERATURES COOL TONIGHT...AND IFR CONDITIONS WILL 
PREVAIL ACROSS MUCH OF THE WESTERN WASHINGTON LOWLANDS FROM ROUGHLY 
MIDNIGHT TONIGHT THROUGH NOON FRIDAY. MOST AREAS WILL CLEAR FRIDAY 
AFTERNOON...BUT FOGGY CONDITIONS WILL PROBABLY LINGER THROUGH THE 
DAY OVER THE SOUTHWEST INTERIOR.

KSEA...NORTH OR NORTHEAST WIND 5-10 KT THIS EVENING...EASING TO 6 KT 
OR LESS LATER TONIGHT THROUGH FRIDAY. KSEA WILL LIKELY HAVE IFR 
CONDITIONS -- MAINLY LOW IFR CONDITIONS WITH VISIBILITY LESS THAN OR 
EQUAL TO 1/2SM IN FOG OR FREEZING FOG -- FROM ROUGHLY MIDNIGHT 
TONIGHT THROUGH NOON FRIDAY. MCDONNAL
 
&&

.MARINE...A 1034 MB HIGH EXTENDING FROM SOUTHEAST BRITISH COLUMBIA 
INTO NORTHERN IDAHO WILL REMAIN IN PLACE THROUGH SATURDAY NIGHT. 
GRADUALLY INCREASING OFFSHORE FLOW WILL PREVAIL ACROSS WESTERN 
WASHINGTON AND ITS COASTAL WATERS THROUGH SATURDAY. SMALL CRAFT 
ADVISORY WINDS OCCURRING OVER THE COASTAL WATERS AND WEST ENTRANCE 
TO THE STRAIT OF JUAN DE FUCA TONIGHT WILL SPREAD TO THE CENTRAL 
STRAIT AND EAST ENTRANCE FRIDAY NIGHT AND THEN TO ADMIRALTY INLET 
AND THE NORTHERN INLAND WATERS SATURDAY.

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 10 FT OVER THE COASTAL WATERS TONIGHT 
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...SMALL CRAFT ADVISORY FOR THE COASTAL WATERS AND WEST
      ENTRANCE STRAIT OF JUAN DE FUCA.
    .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.