FXUS66 KSEW 071054
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
315 AM PST MON DEC 7 2009

.SYNOPSIS...STRONG SURFACE HIGH PRESSURE OVER THE INTERIOR OF 
WESTERN CANADA IS PUSHING MODIFIED ARCTIC AIR INTO WESTERN 
WASHINGTON. THE HIGH IS EXPECTED TO SLIDE SOUTHEAST INTO THE MIDDLE 
OF THE U. S. THE NEXT FEW DAYS. THE COLD DRY CONDITIONS ARE EXPECTED 
TO CONTINUE THROUGH MUCH OF THIS WEEK WITH THE JETSTREAM WELL SOUTH 
OF THE PACIFIC NORTHWEST INTO CALIFORNIA.

&&

.SHORT TERM...THE WEATHER OVER WESTERN WASHINGTON WILL BE DOMINATED 
BY A STRONG AND PERSISTENT REX BLOCK BETWEEN 140W AND 150W THROUGH 
THE END OF THE WEEK.

WHILE THE CENTER OF THE ARCTIC HIGH PRES IS OVER SOUTHERN ALBERTA 
AND IS SLIDING SE...A RIDGE EXTENDING W INTO CENTRAL B.C. WILL 
CONTINUE TO KEEP FLOW OFFSHORE...BUT WINDS WILL DIMINISH TODAY AS 
THE NORTHEASTERLY PRESSURE GRADIENTS RELAX. WILL END THE WIND 
ADVISORY FOR THE NORTH INTERIOR AND EASTERN STRAIT OF JUAN DE FUCA 
ZONES AS WINDS ARE SLOWLY DIMINISHING.

LIGHTER WINDS MEAN LESS MIXING. SO EXPECT RADIATIONAL COOLING AT 
NIGHT TO BE QUITE STRONG MON NIGHT AND TUE NIGHT. COMBINED WITH LOW 
SUN ANGLES AND LIGHT OFFSHORE PRESSURE GRADIENTS EXPECT DAYTIME 
TEMPERATURES TO STRUGGLE TO GET MUCH ABOVE FREEZING THROUGH WED 
AFTER NIGHTTIME LOWS FALL IN TO THE TEENS TO LOWER 20S. 

A WEAK SHORTWAVE TROUGH DROPPING SOUTHWARD TO THE EAST OF THE RIDGE 
WILL ALLOW HIGH LEVEL CLOUDINESS TO INCREASE SOMEWHAT TONIGHT AND 
EARLY TUE. THIS MAY KEEP LOWS TONIGHT FROM GETTING AS COLD AS THEY 
COULD. BUT THERE IS INSUFFICIENT LIFT OR MOISTURE TO PRODUCE ANY 
PRECIPITATION AT ALL AND CLOUDINESS WILL BE LESS THAN SOLID AND WILL 
BE THIN. ALBRECHT

.LONG TERM...GLOBAL MODELS ALL FORECAST THE EVENTUAL BREAKDOWN OF 
THE REX BLOCK AND BRING SOME SOUTHERN STREAM MOISTURE INTO WESTERN 
WASHINGTON BY NEXT WEEK. THROUGH THE END OF THE WEEK THE MODELS 
AGREE THAT LIGHT OFFSHORE FLOW AT THE LOWER LEVELS WILL CONTINUE 
WHILE ALOFT THE FLOW IS DOMINATED BY THE NORTHERN STREAM. THE UKMO 
AND CANADIAN MODELS HINT AT ANOTHER SURGE OF ARCTIC AIR JUST TO THE 
NORTH OF THE AREA FRI INTO SAT. THE ECMWF HAS THE ARCTIC AIR JUST TO 
THE NORTH AS WELL BUT SHOWS IT REMAINING BOTTLED UP JUST TO THE 
NORTH OF THE BORDER. THE GFS APPEARS MOST AGGRESSIVE IN ALLOWING 
SOUTHERN STREAM FLOW AND MOISTURE TO COME NORTH INTO WASHINGTON BY 
SAT INTO SUN. THE TREND HAS BEEN IN THE DIRECTION OF THE ECMWF THAT 
KEEPS WASHINGTON IN NO-MANS LAND BETWEEN THE NORTHERN AND SOUTHERN 
STREAMS...BUT WITH SOME WEAK NORTHERN STREAM SYSTEMS PASSING OVER OR 
JUST TO THE NORTH OF THE AREA FRI INTO SAT FOR A CHANCE OF LIGHT 
PRECIPITATION IN MODERATING TEMPERATURES. 

EXPECT MODEL SKILL TO SUFFER SOMEWHAT OVER THE NEXT SEVERAL DAYS AS 
THE BLOCK BREAKS DOWN BUT WE TRANSITION TO A TRADITIONAL EL-NINO 
SPLIT FLOW REGIME. THE MAIN MESSAGE WILL BE THAT SYSTEMS THAT DO 
COME OVER WA SHOULD BE QUITE WEAK AND COLD AIR IN THE LOWER LEVELS 
WILL SLOWLY MODERATE. ALBRECHT

&&

.HYDROLOGY...COLD AND DRY WEATHER WILL PERSIST THIS WEEK. WHILE 
TEMPERATURES WILL MODERATE STARTING LATE IN THE WEEK..PRECIPITATION 
THAT FALLS IS EXPECTED TO BE LIGHT. THE RISK OF RIVER FLOODING IN 
THE FORECAST AREA FOR THE NEXT WEEK OR MORE FOR ALL 
RIVERS...INCLUDING THE GREEN RIVER...WILL REMAIN QUITE LOW.
ALBRECHT 

&&

.AVIATION...VFR CONDITIONS WILL PREVAIL TODAY/TONIGHT. SKIES REMAIN 
CLEAR THIS MORNING WITH A DRY AIR MASS IN PLACE. NE FLOW EASING 
ACROSS THE NORTHERN PORTION OF THE CWA THIS MORNING. A WEAK 
DISTURBANCE WILL NUDGE INTO WESTERN WA TONIGHT WITH INCREASING 
HIGH/MID LEVEL CLOUDS EXPECTED. 33

KSEA...CLEAR SKIES TODAY WITH INCREASING HIGH/MID LEVEL CLOUDS 
TONIGHT. VFR CONDITIONS WILL PREVAIL. LIGHT E FLOW THIS MORNING 
BECOMING NE. 33

&&

.MARINE...NORTHEASTERLY FLOW IS BEGINNING TO RELAX ACROSS WESTERN 
WASHINGTON AS HIGH PRES EAST OF THE CASCADES WEAKENS. WINDS STILL 
NEAR GALE FORCE STRENGTH IN THE NORTHERN INLAND WATERS AND 
CENTRAL/EAST STRAIT OF JUAN DE FUCA...WITH WINDS EASING TO 10 TO 20 
KT LATE THIS MORNING. OTHERWISE...LIGHT OFFSHORE FLOW EXPECTED 
ACROSS THE REGION DURING THE NEXT SEVERAL DAYS. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING CENTRAL AND EAST STRAIT OF JUAN DE FUCA...NORTHERN 
      INLAND WATERS THIS MORNING. 
     SMALL CRAFT ADVISORY COASTAL WATERS...WEST STRAIT OF JUAN DE 
      FUCA. 

$$

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