FXUS66 KSEW 242311 CCA
AFDSEW

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

.SYNOPSIS...A STRONG RIDGE OF HIGH PRESSURE WILL REMAIN OVER THE 
PACIFIC NORTHWEST THROUGH CHRISTMAS DAY. STRONG WINTER TEMPERATURE 
INVERSIONS WILL LEAD TO WIDESPREAD NIGHT AND MORNING FOG OVER THE 
INTERIOR LOWLANDS ALONG WITH COOL DAYTIME TEMPERATURES. INCREASING 
OFFSHORE FLOW WILL BRING BREEZY EASTERLY WINDS TO THE CASCADES AND 
FOOTHILLS LATE CHRISTMAS DAY THROUGH SATURDAY. HIGH PRESSURE WEAKENS 
ON SUNDAY ALLOWING A SERIES OF WEAK SYSTEMS TO POSSIBLY AFFECT THE 
AREA NEXT WEEK.

&&

.SHORT TERM...FOG CLEARED IN MOST SPOTS TODAY EXCEPT IN A FEW 
SHELTERED POCKETS OF THE SW INTERIOR. THE INVERSIONS REMAIN IN PLACE 
HOWEVER WITH THE TOP AROUND 4-5K FT. THIS WILL LOWER TONIGHT WITH 
RADIATIONAL COOLING AND SUBSIDENCE. FOG WILL REDEVELOP TONIGHT 
ACROSS THE INTERIOR LOWLANDS...MOST EXTENSIVELY FROM AROUND SEATTLE 
SOUTHWARD. ADDITIONAL WARMING ABOVE THE INVERSION LAYER WILL RESULT 
IN A STRONGER CAP TONIGHT AND CHRISTMAS DAY WITH STAGNATION LIKELY 
WORSENING. THE AIR STAGNATION ADVISORY REMAINS IN EFFECT. 

MODELS ARE WAFFLING ON THE DEVELOPMENT OF A MOUNTAIN WAVE/GAP WIND 
EVENT CHRISTMAS NIGHT AND SATURDAY. THE 12Z MODEL RUNS WERE 
DEFINITELY MORE AGGRESSIVE WITH THE MM5 INDICATING ADVISORY LEVEL 
WINDS IN THE FOOTHILLS...POSSIBLY EXTENDING INTO THE EAST PUGET 
SOUND LOWLANDS. THE MM5 ALSO SHOWED SOME BREEZY WINDS ACROSS OTHER 
AREAS FROM KING COUNTY TO THE N INTERIOR RESULTING IN SOME LOW LEVEL 
MIXING AND BETTER VENTILATION. KEPT TEMPS ALONG THE FOOTHILLS AND 
COAST THE WARMEST AND LOWERED TEMPS A BIT OVER THE S SOUND/SW 
INTERIOR WHERE INVERSIONS WILL PROBABLY PERSIST THROUGH SATURDAY 
MORNING. FROM KING COUNTY NWD...SOME MIXING SEEMS PROBABLE SO I 
STUCK CLOSE TO MOS IN THE LOW TO MID 40S. 

THE UPPER RIDGE WEAKENS ON SATURDAY AND SHIFTS NE OVER CENTRAL 
CANADA SUNDAY WITH SPLIT FLOW AND LOWER HEIGHTS ALONG THE W COAST. 
THIS SHOULD ALLOW INVERSIONS TO BREAK AND IN THE CASE OF THE SW 
INTERIOR RISE ENOUGH TO IMPROVE STAGNANT CONDITIONS. A SYSTEM 
APPROACHES WA ON SUNDAY BUT SPLIT FLOW WILL LIKELY SEND THE LOW 
S...BYPASSES WA. THE ECMWF IS DRY AND THE PREFERRED SOLUTION AS THE 
GFS APPEARS TO BE A LITTLE TO AGGRESSIVE BRINGING SOME MOISTURE INTO 
THE AREA.

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

&&

.AVIATION...A RIDGE OF HIGH PRES ALOFT IN THE COASTAL WATERS WILL 
SHIFT ACROSS W WA TONIGHT THEN E OF THE CASCADES FRI. IN THE 
MEANTIME A 1034 MB HIGH OVER SE B.C. WILL SAG SLOWLY SE INTO MT. 
EXPECT LIGHT TO MODERATE OFFSHORE FLOW TO CONTINUE.

THE AIR MASS IS QUITE STABLE AND A TEMPERATURE INVERSION FROM THE 
SURFACE TO AROUND 050 IS IN PLACE. EXPECT THE INVERSION TO 
CONTINUE...AND MAYBE EVEN STRENGTHEN SOMEWHAT THRU FRI. 

WHILE CONDITIONS HAVE MOSTLY CLEARED THIS AFTERNOON EXPECT FOG TO 
REFORM OVER LOWER TERRAIN THIS EVENING AND CONTINUE THROUGH THE 
MORNING HOURS FRI. TOPS WILL BE QUITE SHALLOW...ABOUT 600 FT NORTH 
AND LESS THAN 1000 FT S. EASTERLY FLOW ACROSS THE CASCADES WILL 
INCREASE EARLY FRI...AND SHOULD ERODE THE FOG FROM THE N AND E. BUT 
MANY AREAS S OF KTCM IN THE INTERIOR MAY HAVE DIFFICULTY CLEARING. 
FORECAST MODELS ARE HAVING DIFFICULTY MAINTAINING CONSISTENCY WITH 
POSSIBLE MOUNTAIN WAVE DEVELOPMENT...WHICH WOULD MORE EFFECTIVELY 
ERODE FOG...AND STRAIT GAP FLOW THAT WOULD KEEP THE EROSION TO 
NEARER THE CASCADES. 

KSEA...NORTH OR NORTHEAST WIND 6 KT OR LESS TODAY AND TONIGHT. FOG 
WILL FORM TO THE S OF THE TERMINAL EARLY AND POSSIBLY SPREAD INTO OR 
VERY NEAR THE TERMINAL 06Z-10Z TIME FRAME. WITH THE TERMINAL AT 
THE EDGE OF THE DEVELOPING FOG BANK...CONFIDENCE IN THE FORECAST AT 
KSEA IS LOW. EXPECT THE FOG TO CLEAR RIGHT AT THE TERMINAL 18Z-20Z 
FRI. ALBRECHT
 
&&

.MARINE...A 1034 MB HIGH OVER SE BRITISH COLUMBIA WILL MOVE SE INTO 
MONTANA AND EASTERN WA. EXPECT MODERATE OFFSHORE FLOW THROUGH SAT. 
THE OFFSHORE FLOW WILL DIMINISH SUN AS A SPLITTING FRONT DISSIPATES 
OVER THE WATERS. 

EXPECT SMALL CRAFT ADVISORY CONDITIONS ALONG THE COAST AND AT THE 
WEST ENTRANCE WITH STRONGEST WINDS NEAR GAPS IN THE TERRAIN. SMALL 
CRAFT ADVISORY CONDITIONS WILL GRADUALLY SPREAD E THROUGH THE STRAIT 
AND TO THE EAST ENTRANCE AND POSSIBLY THE N INLAND WATERS FRI NIGHT 
INTO SAT.  ALBRECHT

&&

.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.