FXUS66 KSEW 011718
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
918 AM PST TUE DEC 1 2009

.SYNOPSIS...AN UPPER LEVEL RIDGE BUILDING OVER THE REGION AND A 
SURFACE HIGH OVER BRITISH COLUMBIA WILL PRODUCE DRY NORTH FLOW ALOFT 
ACROSS WESTERN WASHINGTON TODAY WITH SKIES CLEARING. THE RIDGE AND 
SURFACE HIGH WILL MAINTAIN MOSTLY CLEAR COOL AND DRY WEATHER THROUGH 
THURSDAY...EXCEPT FOR AREAS OF MORNING FOG. THE RIDGE WILL BREAK 
DOWN AFTER MIDWEEK...ALLOWING AN UPPER LEVEL LOW TO MOVE OVER THE 
AREA FRIDAY THROUGH MONDAY. THIS WILL BRING BACK CLOUDIER WEATHER 
WITH A CHANCE OF RAIN.   

&&

.SHORT TERM...EARLY MORNING VISIBLE IMAGERY SHOWS AREAS OF CLOUDS 
ACROSS W WA. NORTH PART AND COAST IS GENERALLY PARTLY SUNNY...WITH 
MOSTLY CLOUDY CONDITIONS FROM THE SOUTH PUGET SOUND SOUTHWARD. HAVE 
ALREADY SENT AN UPDATE TWEAKING THE CLOUD COVER FOR TODAY. WITH DRY 
N FLOW ALOFT FROM THE UPPER LEVEL RIDGE ALONG 130W AND N LOW LEVEL 
FLOW FROM THE SURFACE HIGH OVER B.C. SHOULD SEE MOSTLY SUNNY SKIES 
THIS AFTERNOON. 

THIS DRY PATTERN WILL CONTINUE THROUGH THURSDAY AS THE RIDGE MOVES 
SLOWLY EAST AND THE SURFACE HIGH SLIDES DOWN OVER THE NORTHERN 
ROCKIES. WITH MOSTLY CLEAR SKIES TONIGHT...GOOD RADIATIONAL COOLING 
CONDITIONS SHOULD DROP LOWS INTO THE UPPER 20S AND 30S...AND ALLOW 
AREAS OF FOG TO FORM. MOS GUIDANCE INDICATES THAT WEDNESDAY NIGHT 
WILL BE A FEW DEGREES COOLER...PROBABLY THE MID 20S TO MID 30S. A 
SHIFT TO WEAK EASTERLY FLOW AND THE DESCENDING SUBSIDENCE INVERSION 
SHOULD REDUCE THE FOG COVERAGE THURSDAY MORNING. 

THE UPPER LEVEL RIDGE BREAKS DOWN ON THURSDAY AS A STRONG UPPER 
LEVEL SHORTWAVE TROUGH MOVES INLAND ACROSS CENTRAL B.C.  MODELS HAVE 
BEEN AT ODDS WITH THE EVOLUTION OF THIS WAVE BUT SEEM TO BE EDGING 
CLOSER TO CONSENSUS. THURSDAY SHOULD STILL BE MOSTLY SUNNY...BUT 
WITH THE FLOW ALOFT SHIFTING TO W...THERE WILL PROBABLY BE AN 
INCREASE IN CIRRUS IN THE AFTERNOON. 
  
.LONG TERM...THE NEW 12Z GFS IS PRETTY MUCH THE SAME AS THE 06Z 
RUN...WHICH IN TURN WAS A LITTLE FASTER AND FURTHER EAST WITH THE 
THURSDAY UPPER LEVEL SHORTWAVE TROUGH COMPARED TO THE 00Z ECMWF AND 
00Z CANADIAN. ENOUGH CONSENSUS EXISTS TO START SETTLING ON A 
FORECAST OTHER THAN JUST CLIMO. THE ECMWF/CANADIAN SOLUTION LOOKS A 
LITTLE BETTER...WITH THE UPPER LEVEL TROUGH DEEPENING INTO A CLOSED 
LOW THAT MOVES S RIGHT OVER W WA. GFS PATH FOR THE LOW KEPT W WA 
DRY...BUT REGARDLESS...THE LOW IS CLOSE ENOUGH TO MAINTAIN CHANCE 
POPS FOR RAIN FRIDAY AND SATURDAY. THE LOW CONTINUES S OVER OREGON 
SUNDAY/MONDAY WITH ANOTHER ROUND OF COOL NORTH FLOW DEVELOPING OVER 
W WA. THIS LOOKS LIKE AND EVEN COOLER PERIOD THAN ON 
WEDNESDAY/THURSDAY. KAM 

&&

.HYDROLOGY...THERE IS NO THREAT OF RIVER FLOODING EXPECTED IN THE 
FORECAST AREA THROUGH DECEMBER 11TH. THIS INCLUDES THE GREEN RIVER.

ONLY LIGHT PRECIPITATION IS EXPECTED AS THE UPPER LEVEL LOW MOVES 
OVER THE AREA FRIDAY AND SATURDAY. DRY NORTH FLOW RETURNS SUNDAY 
THROUGH EARLY NEXT WEEK. THIS HIGH AMPLITUDE PATTERN SHOULD 
EVENTUALLY BREAK DOWN LATE NEXT WEEK WITH W FLOW UNDERCUTTING THE 
RIDGE...ALLOWING A SOMEWHAT WETTER PATTERN TO RETURN. KAM 

&&

.AVIATION...DRY NWLY FLOW ALOFT WITH HIGH PRESSURE AT THE SURFACE IS 
GIVING LARGELY CLEAR SKIES AROUND PUGET SOUND THIS MORNING. HOWEVER 
A BAND OF CLOUDS WITH ABOUT 1500 FOOT CEILINGS LIES JUST EAST OF 
PUGET SOUND AND IT WILL AFFECT THE TERMINALS AT TIMES. TREND IS 
TOWARD CLEARING...AND WITH DEW POINTS FALLING TO AROUND 30...FOG 
WILL LIKELY BE MINIMAL.

KSEA...HAS GOT A CEILING FROM THE BAND OF STRATUS MENTIONED ABOVE. 
THIS COULD LAST A FEW HOURS BUT THE TREND IS TOWARD DRYING AND 
CLEARING. WINDS NORTH OR NORTHEAST 4-8 KT. CHB

&&

.MARINE...HIGH PRES OVER CENTRAL BRITISH COLUMBIA WILL GIVE OFFSHORE 
FLOW TODAY THROUGH THE REST OF THE WEEK. HAVE HOISTED SMALL CRAFT 
ADVISORIES FOR THE WEST ENTRANCE AND NORTHERN INLAND WATERS...WHERE 
WINDS ARE ALREADY PUSHING ADVISORY LEVELS. THE PATTERN WITH THIS 
KIND OF FLOW IS STRONGER FLOW IN THE MORNINGS...WEAKER AT NIGHT. 
HAVE INDICATED THIS FOR TODAY AND TONIGHT.

THERE IS SOME CHANCE OF ARCTIC OUTFLOW IN THE NORTH THIS WEEKEND. A 
SOMEWHAT MORE LIKELY SOLUTION IS THE COLDEST AIR WILL SLIP INTO 
EASTERN WASHINGTON AND MONTANA...WITH ONLY MINOR OUTFLOW IN WESTERN 
WASHINGTON. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE
PZ...SMALL CRAFT ADVISORY WEST ENTRANCE AND NORTHERN INLAND WATERS.
    .SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.
   
$$

WWW.WEATHER.GOV/SEATTLE

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