FXUS66 KSEW 281138
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PST SAT NOV 28 2009

.SYNOPSIS...A WEAK...SYSTEM PASSING TO OUR NORTH WILL BRING LIGHT 
PRECIPITATION TO THE AREA THROUGH TONIGHT. EXPECT DRY WEATHER TO 
RETURN ON SUNDAY UNDER STRENGTHENING HIGH PRESSURE ALOFT. HIGH 
PRESSURE ALOFT WILL THEN PREVAIL OVER THE AREA SUNDAY NIGHT. A WEAK 
LOW PRESSURE SYSTEM CLIPPING THE AREA ON MONDAY WILL RESULT IN A 
CHANCE OF SHOWERS OVER MAINLY THE NORTH CASCADES IN THE AFTERNOON.

&&

.SHORT TERM...
RAIN HAS OVERSPREAD THE NORTH COAST...WITH ONLY SPOTTY LIGHT PRECIP 
OCCURRING ELSEWHERE. LOOK FOR THE PRECIP TO BECOME MORE WIDESPREAD 
TODAY IN RESPONSE TO A SYSTEM PASSING TO OUR N. THIS WILL BE A WAA 
PATTERN...THIS ANTICIPATE SNOW LEVELS TO RISE DURING THE DAY AND 
EVEN MORE SO TONIGHT. THE UPPER RIDGE WILL AMPLIFY FURTHER 
TONIGHT...THEREBY SHUNTING THE PRECIP NWD WITH TIME. BY SUNDAY 
MORNING...THE RAIN THREAT WILL BE CONFINED TO THE FAR NW PORTION OF 
THE CWA. EXPECT DRY AND MILD WEATHER SUNDAY AFTERNOON UNDER AN UPPER 
RIDGE.

HIGH PRES ALOFT WILL PREVAIL OVER THE AREA SUNDAY AFTERNOON AND 
NIGHT FOR DRY WEATHER. THE SFC PRES GRADIENT WILL WEAKEN DURING THE 
DAY SUNDAY AND EVEN MORE SUNDAY NIGHT. THIS COMBINED WITH NOTICEABLY 
LESS CLOUD COVER AND RESIDUAL MOISTURE IN THE LOWER BOUNDARY LAYER 
WILL TRANSLATE TO WIDESPREAD FOG FORMATION ACROSS THE LOWLANDS 
SUNDAY NIGHT. THE FOG WILL PROBABLY BE SLOW TO LIFT ON MON DUE TO 
THE PRESENCE OF A STRONG TEMP INVERSION INITIALLY. THE MODELS DID
INDICATE THAT A SHORT WAVE TROF CLIPPING WRN WA MON AFTERNOON.
THIS SHOULD SERVE TO BREAK UP THE INVERSION. AT ANY RATE...DAYTIME 
TEMPS ACROSS THE LOWLANDS WILL BE COOLER ON MON COMPARED TO SUNDAY 
DUE TO LESS MIXING.

THE TROF CLIPPING THE AREA ON MON MAY BRING A THREAT OF SHOWERS TO 
THE CWA IN THE AFTERNOON. HOWEVER HAVE CONFINED THE RISK OF SHOWERS 
TO THE FAR NRN PORTION OF THE CWA SINCE THE MUCH OF THE ENERGY AND 
MOISTURE WITH THIS SYSTEM WILL REMAIN TO OUR N AND E.    

.LONG TERM FROM THE PREV DISCUSSION...
THE OVERALL PATTERN WILL CONTINUE TO MAKE A MAJOR SHIFT AND AN UPPER 
LEVEL RIDGE WILL BUILD OVER THE EASTERN PACIFIC AND BLEED OVER INTO 
THE WEST COAST.  THIS WILL BRING ABOUT A MUCH DRIER PATTERN FOR THE 
REGION NEXT WEEK.  

THERE IS A BUNDLE OF ENERGY THAT THE VARIOUS LONG RANGE MODELS TRY 
TO GRAB ONTO AND MOVE EAST EITHER THROUGH OR OVER THE RIDGE.  
HOWEVER...EACH RUN AND MODEL IS DIFFERENT WITH THE TRACK...DEPTH... 
AND WHAT DAY THIS WILL MOVE THROUGH THE WESTERLIES.  IN EACH CASE 
THE RIDGE POSITION IS PRETTY MUCH LOCKED IN WITH THIS FEATURE 
PLOWING THROUGH OR OVER THE TOP OF IT.  IF THIS COMES TO PASS IN 
SOME FORM IT COULD BRING A CHANCE OF SHOWERS TO THE AREA FOR ONE 
DAY.  FOR NOW WILL CONTINUE TO GO WITH THE DRY FORECAST AS THIS IS 
WHAT IS THE MOST PROBABLE SOLUTION FOR NOW.  CERNIGLIA 

&&

.HYDROLOGY...THE WARM ADVECTION SYSTEM THAT WILL BRING LIGHT 
PRECIPITATION TO WESTERN WASHINGTON TODAY AND TONIGHT WILL BE
WEAK...AND WILL NOT PRODUCE ENOUGH RAIN TO RESULT IN ANY RIVER 
PROBLEMS.  

BEYOND TONIGHT...THE FORECAST WAS ESSENTIALLY DRY WELL INTO NEXT
WEEK. THUS...RIVER FLOODING WILL NOT BE A CONCERN. EXPECT AREA 
RIVERS TO RETURN TO LOWER FLOW LEVELS.  

NO FLOODING IS EXPECTED ON THE GREEN RIVER THROUGH NEXT WEEK.  
05/CERNIGLIA

&&

.AVIATION...NORTHWEST FLOW WILL CONTINUE ALOFT OVER WESTERN 
WASHINGTON TODAY AND THIS EVENING...GRADUALLY BACKING TO WESTERLY 
TONIGHT. A WEAK UPPER RIDGE CURRENTLY OFFSHORE WILL SHIFT INTO THE 
PACIFIC NORTHWEST TODAY...AND A WEAK WARM FRONT MOVING OVER THE 
NORTH SIDE OF THE RIDGE WILL BRUSH THE REGION. THE FRONT WILL BRING 
SOME LIGHT RAIN TO THE FORECAST AREA TODAY AND TONIGHT...MAINLY 
ALONG THE COAST...OVER THE MOUNTAINS...AND IN THE INTERIOR LOWLANDS 
NORTH OF SEATTLE.

THE AIR MASS WILL REMAIN STABLE WITH SOME MOISTURE TRAPPED AT LOW 
LEVELS AND INCREASING MOISTURE AT MID AND HIGH LEVELS DUE TO THE 
WARM FRONT. CONDITIONS ACROSS MUCH OF WESTERN WASHINGTON HAVE 
ALREADY LOWERED TO MVFR WITH A BKN-SCT LAYER AROUND 2000-3000 FT AND 
A OVC-BKN LAYER 4000 FT. MVFR CEILINGS WILL PREDOMINATE THROUGH 
TONIGHT WITH OCCASIONAL LOCAL IFR CONDITIONS TOO.

KSEA...SOUTHERLY WIND 6-12 KT THROUGH TONIGHT. 

MCDONNAL

&&

.MARINE...A WARM FRONT WILL MOVE ONTO THE BRITISH COLUMBIA COAST 
TODAY AND TONIGHT...WITH ITS SOUTHERN END BRUSHING NORTHWEST 
WASHINGTON. THIS FEATURE WILL PRODUCE SMALL CRAFT ADVISORY WINDS 
OVER THE COASTAL WATERS...AT THE ENTRANCES TO THE STRAIT...IN 
ADMIRALTY INLET...OVER THE NORTHERN INLAND WATERS...AND IN PUGET 
SOUND AND HOOD CANAL THROUGH THIS EVENING. 

WEAK HIGH PRESSURE WILL SIT OVER WESTERN WASHINGTON SUNDAY THROUGH 
MONDAY AS ANOTHER FRONT MOVES BY TO THE NORTH. THEN HIGH PRESSURE 
WILL STRENGTHEN AND SHIFT NORTHWARD INTO BRITISH COLUMBIA MONDAY 
NIGHT...PROVIDING OFFSHORE FLOW ACROSS WESTERN WASHINGTON THROUGH 
WEDNESDAY.

MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.    
PZ...SMALL CRAFT ADVISORY IN EFFECT FOR ALL WATERS EXCEPT THE
     CENTRAL STRAIT OF JUAN DE FUCA TIL 10 PM TONIGHT.

     A SMALL CRAFT ADVISORY IS IN EFFECT FOR ROUGH GRAYS HARBOR
     BAR CONDITIONS TIL 4 PM SUNDAY.

$$

WWW.WEATHER.GOV/SEATTLE

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