FXUS66 KSEW 241720 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
915 AM PST THU DEC 24 2009

.SYNOPSIS...HIGH PRESSURE ALOFT OFF THE COAST IS EXPECTED TO MOVE 
ONSHORE TODAY THROUGH CHRISTMAS PROVIDING DRY WEATHER TO WESTERN 
WASHINGTON INTO THE COMING WEEKEND. 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 ON CHRISTMAS DAY. A SERIES OF WEAK SYSTEMS COULD AFFECT 
THE AREA EARLY NEXT WEEK.

&&

.SHORT TERM...THE FOG IS HIT AND MISS THIS MORNING AND MOSTLY 
CONFINED TO THE PUGET SOUND REGION AND SW INTERIOR. THE 12Z KUIL 
SOUNDING SHOWS A RATHER STRONG INVERSION WITH A CAP AROUND 800 FT 
AND ANOTHER MUCH HIGHER UP AROUND 4000 FEET. DAYTIME HEATING IS VERY 
WEAK THIS TIME OF YEAR SO IT'S DIFFICULT TO TELL IF WE CAN BREAK THE 
LOWER CAP AND GET SOME MIXING ALLOWING FOG TO DISSIPATE. I SUSPECT 
THE SW INTERIOR WILL HAVE A HARDER TIME BREAKING OUT...POSSIBLY 
STAYING FOGGY THROUGH THE DAY. MODELS STILL SHOW CONSIDERABLE 
WARMING ALOFT THE NEXT COUPLE DAYS AS HIGH PRESSURE MOVES OVER THE 
PAC NW WITH SUBSIDENCE ALOFT. THIS SHOULD HAVE THE AFFECT OF 
LOWERING INVERSION HEIGHTS THE NEXT COUPLE DAYS. MODELS HOWEVER HAVE 
TRENDED STRONGER WITH THE ELY MOUNTAIN WAVE OR GAP WINDS LATER 
FRIDAY INTO SATURDAY WHICH COULD CAUSE SOME MIXING WITHIN THE 
INVERSION LAYER...LIMITING THE AMOUNT OF STAGNATION. THE 12Z MM5 
VENTILATION INDEX SHOWS MUCH MORE IMPROVEMENT OVER YESTERDAYS RUN 
DUE TO THE ELY WINDS PICKING UP FRIDAY FROM AROUND KING COUNTY NWD. 
CROSS CASCADES GRADIENTS ARE STRONGER ON THE 12Z RUN AND THE 850 MB 
WINDS BECOME DUE ELY 20-25 KT WITH A CRITICAL LEVELS ABOVE THAT 
WHICH BODES WELL FOR EAST WINDS. IF THE MM5 IS CORRECT...A WIND 
ADVISORY WILL PROBABLY BE NEEDED FOR THE FOOTHILLS AND POSSIBLY THE 
E PUGET SOUND LOWLANDS E OF THE 405 LATER FRIDAY INTO SATURDAY. THE 
AIR STAGNATION WILL STAY IN EFFECT THROUGH SATURDAY MORNING GIVEN 
THE UNCERTAINTY IN THE AMOUNT OF MIXING EXPECTED. HOWEVER...WILL 
REFRESH THE STATEMENT THIS AFTERNOON AND INDICATE THE POSSIBILITY OF 
IMPROVING CONDITIONS FROM AROUND KING COUNTY NWD. IT DOES STILL 
APPEAR THAT THE SW INTERIOR AND S SOUND WILL PROBABLY NOT GET ENOUGH 
WIND TO MIX THINGS OUT...REMAINING STAGNANT THE NEXT COUPLE DAYS. 
MAY NEED TO TWEAK DAYTIME HIGHS DOWN A BIT THERE. THE LATEST GFS RUN 
DOES LOWER THE UPPER HEIGHTS BY LATER SATURDAY INTO SUNDAY WITH A 
VERY WEAK SYSTEM TRYING TO PUSH INTO THE AREA. THIS SYSTEM MAY LIFT 
OR BREAK REMAINING INVERSIONS...POSSIBLY ALLOWING THE STAGNATION 
ADVISORY TO BE DROPPED. A DECISION ON THAT WILL BE MADE SATURDAY 
MORNING AFTER COORDINATING WITH STATE AND LOCAL AGENCIES. 

MERCER

.LONG TERM...PREVIOUS DISCUSSION FOLLOWS...UPPER LEVEL RIDGING 
ANCHORED FROM ALBERTA SOUTHWARD INTO THE ROCKIES EARLY IN THE 
EXTENDED PERIOD...SPLIT FLOW ALOFT WILL WEAKEN SYSTEMS AS THEY 
APPROACH WESTERN WASHINGTON EARLY NEXT WEEK. THIS MAKES THEIR 
STRENGTH...TRACK...AND AVAILABLE MOISTURE A TOUGH CALL. NO CHANGES 
WERE MADE TO THE EXTENDED FORECAST OVERNIGHT. HOWEVER...RECENT RUNS 
OF THE BOTH GFS/NAM SUGGEST THAT AT LEAST A CHANCE OF RAIN MAY NEED 
TO BE ADDED TO THE COAST/OLYMPICS/SW INTERIOR FOR THE DAYTIME PERIOD 
ON SUNDAY. THE 00Z CANADIAN AND ECMWF WERE MORE AGGRESSIVE WITH 
BRINGING A CONSOLIDATED SYSTEM INTO THE REGION ON TUESDAY WHILE THE 
GFS PLACES WEAK RIDGING JUST WEST OF 130W. WITH BOTH THE GFS AND 
CANADIAN ENSEMBLES SHOWING A LOT OF UNCERTAINTY UPSTREAM OF THE 
RIDGE OVER WESTERN CANADA EARLY NEXT WEEK...THE CURRENT GENERIC 
CHANCE OF RAIN FORECAST MONDAY AND BEYOND LOOKS GOOD UNTIL MORE 
DETAIL CAN BE SORTED OUT. 27

&&

.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 LATE TONIGHT AND 
FRI. IN THE MEANTIME A 1040 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. 

THE FORECAST CHALLENGE IS FOG EXTENT. AT THIS TIME IT APPEARS THAT 
THE AREA FROM ABOUT TACOMA SOUTHWARD THROUGH THE SW INTERIOR VALLEYS 
WILL REMAIN GENERALLY SOCKED IN WITH FOG AND STRATUS AND TOPS TO 
AROUND 010. N OF SEATTLE THE TOPS ARE SHALLOWER...AROUND 006...SO 
THAT FEEBLE INSOLATION SHOULD BE SUFFICIENT TO KEEP FOG PATCHY. 
EXPECT FOG TO REDEVELOP TONIGHT...BUT TO BE MORE PATCHY FROM KPAE 
NORTHWARD THAN KSEA SOUTHWARD WHERE FOG AND LOW STRATUS WILL BE 
WIDESPREAD. 
12Z MESOSCALE MODELS GENERALLY DEVELOP WEAK MOUNTAIN WAVE CONDITIONS 
EARLY FRI. THIS BODES WELL FOR SCOURING FOG FROM RIGHT AROUND KSEA 
NORTHWARD. SOUTH OF KSEA EXPECT STAGNANT LIFR CONDITIONS TO 
CONTINUE.
 
KSEA...NORTH OR NORTHEAST WIND 6 KT OR LESS TODAY AND TONIGHT. KSEA 
WILL BE ON THE CUTTING EDGE OF PERSISTENT FOG TO THE S AND 
IMPROVEMENT TO THE N. WILL MAINTAIN THE IDEA OF IMPROVEMENT 20Z 
TODAY BEFORE CONDITIONS DETERIORATE AROUND 06Z-08Z TONIGHT. 
HOWEVER...THERE IS A NEARLY 50 PERCENT PROBABILITY THAT FOG COULD 
PERSIST WITH ONLY OCCASIONAL IMPROVEMENT 20Z-02Z TODAY. CONFIDENCE 
IS LOW. ALBRECHT
 
&&

.MARINE...SURFACE HIGH PRESSURE EXTENDING FROM SOUTHEAST BRITISH 
COLUMBIA ALONG THE EASTERN BORDER OF WASHINGTON WILL PRODUCE 
OFFSHORE FLOW ACROSS WESTERN WASHINGTON AND ITS COASTAL WATERS 
THROUGH THE WEEKEND. 

SMALL CRAFT ADVISORY STRENGTH EASTERLY WINDS ARE EXPECTED AT THE 
WEST ENTRANCE TO THE STRAIT AND IN THE COASTAL WATERS...ESPECIALLY 
NEAR GAPS IN THE TERRAIN. EXPECT OFFSHORE FLOW TO INCREASE FRI-SAT 
AS PRESSURE GRADIENTS TIGHTEN OVER W WA. SMALL CRAFT ADVISORY 
CONDITIONS WILL LIKELY SPREAD E THROUGH THE STRAIT TO THE E ENTRANCE 
AND PORTIONS OF THE N INLAND WATERS FRI AND PERSIST INTO SAT. 

THE AREA OF HIGH PRESSURE WILL BEGIN TO WEAKEN LATER SATURDAY AS A 
SPLITTING FRONTAL SYSTEM MOVES INTO THE OFFSHORE WATERS FROM THE 
WEST. CURRENT MODEL FORECASTS SHOW THE FRONTAL SYSTEM MOVING VERY 
SLOWLY EASTWARD SUNDAY AND MONDAY...PROBABLY MORE OR LESS 
DISSIPATING BY THE TIME IT REACHES THE COAST MONDAY. 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

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