FXUS66 KSEW 090442
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
845 PM PST TUE DEC 8 2009

.SYNOPSIS...AN UPPER LEVEL RIDGE JUST WEST OF THE BRITISH COLUMBIA 
COAST WILL MAINTAIN A COLD DRY MODIFIED ARCTIC AIR MASS OVER WESTERN 
WASHINGTON THROUGH FRIDAY. WEAK DISTURBANCES EMBEDDED IN THE NORTH 
FLOW ALOFT MAY BRING SOME SNOW FLURRIES TO THE NORTH PART THURSDAY 
AND FRIDAY. A SLIGHTLY STRONGER DISTURBANCE MAY BRING A CHANCE OF 
SNOW FRIDAY NIGHT OR SATURDAY. A FRONTAL SYSTEM ARRIVING FROM THE 
WEST WILL BRING WARMER AND WETTER WEATHER EARLY NEXT WEEK. 

&&

.SHORT TERM...THE BIG HIGH AMPLITUDE UPPER LEVEL RIDGE ALONG 135W 
EXTENDS ALL THE WAY N OVER THE INTERIOR OF AK...WITH COLD DRY N FLOW 
ALOFT FROM THE YUKON ACROSS B.C. AND DOWN OVER WA. THIS PATTERN WILL 
CONTINUE THROUGH THE END OF THE WEEK MAINTAINING THE MODIFIED ARCTIC 
AIR MASS OVER THE AREA. 

CURRENT OBS SUPPORT THE FRIGID NIGHT TONIGHT...SO WEDNESDAY SEEMS 
LIKE IT WILL BE THE COLDEST MORNING. 24 HOUR CHANGES SHOW THE LATEST 
TEMPERATURE READINGS A LITTLE COLDER THAN THE SAME TIME 
YESTERDAY...AND WITH CLEAR SKIES...VERY DRY AIR (DEWPOINTS IN THE 
SINGLE DIGITS AWAY FROM WATER SOURCES) AND GENERALLY LIGHT WINDS 
TONIGHT...RADIATIONAL COOLING CONDITIONS ARE PERFECT. EXPECT LOWS IN 
THE COLDER OUTLYING AREAS TO REACH THE SINGLE DIGITS AGAIN WITH 
MAINLY TEENS ELSEWHERE. 

N FLOW ALOFT CONTINUES THROUGH FRIDAY. SMALL WEAK UPPER LEVEL 
TROUGHS  EMBEDDED IN THE N FLOW MIGHT BRING SOME CLOUDS AND A CHANCE 
OF SNOW FLURRIES TO THE NORTH PART THURSDAY THROUGH FRIDAY. MODELS 
SEEM TO BRING A SLIGHTLY STRONGER TROUGH DOWN OVER THE AREA FRIDAY 
NIGHT FOR A CHANCE OF LIGHT SNOW THROUGHOUT W WA. ECMWF HAS WIDER 
PRECIP COVERAGE THAN THE 18Z GFS OR 00Z NAM. CURRENT LOW POPS 25-30 
PERCENT SEEM APPROPRIATE FOR THIS PATTERN...GIVEN THE DRY TRAJECTORY 
THE TROUGH WOULD HAVE TO TAKE AND THE DISPARITY IN THE MODEL 
SOLUTIONS. 

EXPECT A SLOW MODERATION IN TEMPERATURES THE NEXT COUPLE OF DAYS AS 
THE UPPER LEVEL RIDGE WEAKENS...AND THE SURFACE HIGH CURRENTLY OVER 
THE INTERIOR OF B.C. SLIDES SE OVER THE N ROCKIES AND THE LOW LEVEL 
FLOW SHIFTS MORE EASTERLY. MODEL 850 MB TEMPERATURES WARM STEADILY 
AND LOW LEVEL 1000-850 MB THICKNESS VALUES RISE. SHOULD SEE LOWS 
MAINLY IN THE 20S WITH HIGHS IN THE 30S BY FRIDAY. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 230 PM AFD...ON SATURDAY 
THICKNESSES WARM ENOUGH ON THE COAST TO PERHAPS CHANGE THE CHANCE OF 
SNOW TO A CHANCE OF RAIN. WITH A WARMER SYSTEM JUST TO THE SOUTH IN 
OREGON...HAVE GIVEN THE SOUTHWEST INTERIOR AND LOWER CHEHALIS ZONES 
MIXED PRECIPITATION ON SATURDAY. THIS PATTERN CONTINUES SATURDAY 
NIGHT. MODELS HAVE SOMETHING OF A FRONTAL PASSAGE ON SUNDAY...WITH 
WEAK ONSHORE FLOW AND SOME WARMING. THUS HAVE MADE THE DAY SHOWERY 
WITH A LOW SNOW LEVEL...NOT TO SEA LEVEL HOWEVER AND LOTS OF PLACES 
SHOULD GET RAIN SHOWERS.

A WARM AND WET SYSTEM ARRIVES MONDAY AND TUESDAY...WHICH SHOULD END 
THE RECENT COLD SPELL. DAYTIME TEMPERATURES WILL BE SAFELY IN THE 
40S WITH OVERNIGHT LOWS ABOVE FREEZING. THERE IS A CHANCE THAT THE 
MOUNTAINS WILL GET LOTS OF PRECIPITATION BUT FOR NOW MOST OF THE 
RAIN APPEARS HEADED FOR VANCOUVER ISLAND. BURKE

&&

.HYDROLOGY...PREVIOUS DISCUSSION FROM THE 230 PM AFD...WITH THE COLD 
DRY WEATHER PATTERN IN PLACE THIS WEEK... NO RIVER FLOODING WILL 
OCCUR. ANY PRECIPITATION OVER THE WEEKEND IS LIKELY TO BE 
LIGHT...AND CONTINUED COLD WEATHER WILL MAKE FLOODING UNLIKELY THEN 
AS WELL. EARLY NEXT WEEK A WET AND WARMER WEATHER SYSTEM IS FORECAST 
TO REACH THE PACIFIC NORTHWEST. AT THIS TIME MOST OF THE RAIN LOOKS 
LIKE IT WILL TO NORTH INTO CANADA. NO FLOODING IS FORECAST ON THE 
GREEN RIVER. BURKE

&&

.AVIATION...LIGHT NORTHERLY FLOW ALOFT WILL PERSIST THROUGH WED. THE 
ATMOSPHERE WILL BE DRY AND STABLE EXCEPT FOR PATCHY SURFACE MOISTURE 
IN SHELTERED VALLEYS. HIGH PRES AT THE SURFACE WITH LIGHT OFFSHORE 
LOW LEVEL FLOW.  

KSEA...NORTH WIND 3-8 KT BECOMING EAST BY MIDNIGHT. FOG NOT 
EXPECTED. DTM

&&

.MARINE...LITTLE CHANGE IN THE PATTERN THROUGH THU WITH HIGH PRES 
AND LIGHT OFFSHORE FLOW CONTINUING. WINDS LESS THAN 15 KT EXPECTED.

WESTERLY SWELL OVER THE COASTAL WATERS WILL BUILD TO 10 TO 12 FT 
TONIGHT THROUGH WED...THEN GRADUALLY SUBSIDE WED NIGHT. A SMALL 
CRAFT ADVISORY HAS BEEN ISSUED FOR HAZARDOUS SEAS...ALONG WITH A 
SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS AT GRAYS HARBOR. 

A WEAK SURFACE LOW WILL FORM OFF VANCOUVER ISLAND ON FRI WHICH MAY 
INDUCE SOME SLY FLOW OVER THE COASTAL WATERS AND EVENTUALLY OVER THE 
INTERIOR WATERS. HOWEVER...MODELS STILL KEEP WINDS BELOW 20 KT. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE. 
PZ...SMALL CRAFT ADVISORY COASTAL WATERS TONIGHT AND WEDNESDAY FOR 
     HAZARDOUS SEAS.
    .SMALL CRAFT ADVISORY GRAYS HARBOR TONIGHT AND WEDNESDAY FOR 
     ROUGH BAR CONDITIONS.
    
$$

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FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE 
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