FXUS66 KSEW 241120 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 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 AXIS OF A DIRTY UPPER LEVEL RIDGE OVER THE 
OFFSHORE WATERS WILL SLIDE OVER WESTERN WASHINGTON LATE THIS 
AFTERNOON. A SURFACE RIDGE OVER INTERIOR BRITISH COLUMBIA WILL SHIFT 
SOUTHEAST INTO EASTERN WASHINGTON TODAY...WITH OFFSHORE FLOW
INCREASING THROUGH CHRISTMAS DAY AS A TROUGH MOVES INTO THE 
OFFSHORE WATERS TO TIGHTEN THE GRADIENT. SURFACE BASED INVERSIONS 
DEVELOPING OVERNIGHT HAVE LIKELY BEEN SLOWED SOMEWHAT BY AREAS OF 
HIGH AND MID LEVEL MOISTURE STREAMING OVERHEAD. THE BULK OF THE 
STRATUS AND FOG THAT HAS DEVELOPED THROUGH 10Z IS EXACTLY WHERE IT 
FEELS MOST AT HOME IN THE STRETCH FROM OLYMPIA SOUTHWARD INTO KELSO. 
I DECIDED TO TRIM BACK SKY COVER A LITTLE TODAY...AND REDUCE FOG 
COVERAGE OVER THE SW INTERIOR TO PATCHY FOR THIS AFTERNOON. HIGH 
TEMPERATURES WERE BOOSTED A LITTLE...TRENDING CLOSER TO THE GFS MOS 
NUMBERS BUT COOLER THAN THE NAM NUMBERS THAT SEEM TOO WARM FOR 
INTERIOR AREAS GIVEN THE PRESENT WEATHER PATTERN. MUCH LIKE THIS 
MORNING...LOW CLOUD COVER TONIGHT WILL BE A FUNCTION OF HOW MUCH 
HIGHER LEVEL MOISTURE GETS THROUGH THE UPPER LEVEL RIDGE. 
ADDITIONALLY...INCREASING EASTERLY GRADIENTS MAY LIMIT FOG COVERAGE 
OUTSIDE OF THE FOG-FRIENDLY SW INTERIOR AND SOUTH PUGET SOUND. FOR 
CHRISTMAS DAY...A NEAR PERSISTENCE FORECAST WAS USED...EXCEPT FOR A 
LITTLE LESS FOG AND A LITTLE WARMER TEMPERATURES IN THE SPOTS THAT 
WILL BENEFIT MOST FROM THE DOWNSLOPE COMPONENT OF OFFSHORE 
FLOW...ESPECIALLY THE COAST.

BY SATURDAY...THE UPPER LEVEL RIDGE WILL HAVE SHIFTED EAST OF THE 
CASCADES. BOTH THE NAM/GFS SUGGEST THAT LATE SATURDAY THE RIDGE WILL 
GET PINCHED OFF OVER ALBERTA AS AN OFFSHORE UPPER LEVEL TROUGH 
UNDERCUTS IT AND ATTEMPTS TO SPREAD SOME MOISTURE INTO OREGON. 
OFFSHORE FLOW CONTINUES FOR WESTERN WASHINGTON WITH INCREASING HIGH 
AND MID LEVEL CLOUDS CREEPING IN FROM THE SOUTHWEST DURING THE DAY 
AS THE OFFSHORE TROUGH EXERTS A LITTLE MORE INFLUENCE ON THE REGION.

.LONG TERM...WITH 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...NORTHWEST FLOW ALOFT WILL CONTINUE OVER WESTERN 
WASHINGTON TODAY AND TONIGHT...AS AN UPPER LEVEL RIDGE OVER THE 
OFFSHORE WATERS PROGRESSES SLOWLY TOWARD THE COAST. AT THE SURFACE 
AN AREA OF HIGH PRESSURE EXTENDING FROM SOUTHEAST BRITISH COLUMBIA 
INTO NORTHEAST WASHINGTON WILL PRODUCE CONTINUE OFFSHORE FLOW...WITH 
GENERALLY LIGHT NORTHEASTERLY WINDS ACROSS THE WESTERN WASHINGTON 
INTERIOR LOWLANDS. SOMEWHAT STRONGER EASTERLY WINDS WILL DEVELOP 
ALONG THE COAST...ESPECIALLY NEAR GRAYS HARBOR AND THE WEST ENTRANCE 
TO THE STRAIT OF JUAN DE FUCA.

THE COMBINATION OF WARMING ALOFT WITH THE UPPER RIDGE...A RELATIVELY 
COLD AIR MASS AT THE SURFACE...AND LIGHT WINDS WILL KEEP THE AREA 
UNDER A STRONG INVERSION THROUGH THE PERIOD. SURFACE-BASED MOISTURE 
TRAPPED UNDER THE INVERSION CAUSED AREAS OF FOG AND STRATUS TO FORM 
OVERNIGHT...AND THIS WILL BECOME MORE WIDESPREAD OVERNIGHT AS THE 
MID LEVEL CLOUDS GRADUALLY CLEAR AND TEMPERATURES FALL. MUCH OF THE 
WESTERN WASHINGTON INTERIOR WILL HAVE IFR CONDITIONS BETWEEN 12Z AND 
20Z. AREAS WITH DENSER FOG THIS MORNING AND ESPECIALLY LIGHT WINDS 
TODAY...MEANING MAINLY THE SOUTH PUGET SOUND AND SOUTHWEST INTERIOR 
...WILL BE SLOW TO CLEAR AND PROBABLY HANG ON TO SOME FOG EVEN IN 
THE AFTERNOON. FOG AND STRATUS WILL REDEVELOP TONIGHT AND LIKELY BE 
MORE WIDESPREAD.

KSEA...NORTH OR NORTHEAST WIND 6 KT OR LESS TODAY AND TONIGHT. KSEA 
WILL LIKELY HAVE PERIODS OF IFR CONDITIONS DUE TO FOG FROM ROUGHLY 
13Z THROUGH 20Z TODAY AND AGAIN AFTER 08Z FRIDAY. MCDONNAL
 
&&

.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 WILL DEVELOP OVER THE 
COASTAL WATERS AND AT THE WEST ENTRANCE TO THE STRAIT OF JUAN DE 
FUCA TODAY...THEN SPREAD TO THE CENTRAL STRAIT AND EAST ENTRANCE 
FRIDAY AS PRESSURE GRADIENTS TIGHTEN OVER WESTERN WASHINGTON. 

THE AREA OF HIGH PRESSURE WILL WEAKEN 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. 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 THROUGH FRIDAY.
    .SMALL CRAFT ADVISORY FOR GRAYS HARBOR DUE TO ROUGH BAR 
     CONDITIONS THROUGH FRIDAY.

$$

WEATHER.GOV/SEATTLE

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