FXUS66 KSEW 061644
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
845 AM PST SUN DEC 6 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. YET 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...A STRONG REX BLOCK HAS PARKED ALONG 140W WITH AN UPPER 
LOW MOVING SOUTHWARD THROUGH OREGON AND ANOTHER NEAR 47/170W. 
NORTHERLY FLOW ALOFT OVER WA. 

AT THE SURFACE..A STRONG HIGH IS OVER NORTHERN B.C. WITH QUITE COLD 
AIR IN THE INTERIOR OF WESTERN CANADA. OUTFLOW OF MODIFIED ARCTIC 
AIR UNDERWAY ACROSS WESTERN WA. THE AIR MASS IS QUITE COOL AND DRY. 
THE WILLIAMS LAKE TO BELLINGHAM PRES GRADIENT IS NOW NEAR 15 
MILLIBARS WITH THE STRONGEST ASSOCIATED WINDS IN THE NORTH INTERIOR. 
WINDS ELSEWHERE SHOULD MIX DOWN TO THE SURFACE LATER TODAY.

COLDER AIR BOTH AT THE SURFACE AND ALOFT WILL CONTINUE TO SPILL INTO 
WESTERN WA TODAY AND TONIGHT. THE HIGHEST TEMPERATURES OF THE DAY 
ARE LIKELY THIS MORNING WITH READINGS EITHER HOVERING AND DIPPING A 
BIT AS THE DAY WEARS ON. 

THE CLOUD COVER INVOLVED IN THE OREGON UPPER LOW SHOULD CLEAR FROM 
THE NORTH LATER TODAY AS THE LOW CONTINUES SOUTH. RADAR SHOWS A BIT 
OF LIGHT PRECIP IN THE MOUNTAINS AND SHOULD TAPER OFF LATER TODAY. 

THE BIG STORY IS THE PACIFIC NW SHOULD REMAIN IN THE COLD NORTHERLY 
FLOW ALOFT THIS WEEK. THE STRONG HIGH IN B.C. IS EXPECTED TO SLIDE 
SE INTO THE MIDDLE OF THE U.S. MON AND TUE. SO THE OUTFLOW WINDS 
SHOULD EASE STARTING LATER TONIGHT. BUT WITH THE COLD DRY AIR MASS 
NOW HERE...SHOULD GET QUITE A BIT OF RADIATIONAL COOLING DURING OUR 
LONG NIGHTS. LOWS FALLING INTO THE TEENS AND 20S AT NIGHT LOOKS ON 
TRACK AND STRUGGLING TO GET ABOVE FREEZING DURING THE DAY. BUEHNER

.LONG TERM...FROM PREVIOUS DISCUSSION..THE REX BLOCK IS ESTABLISHED 
AND WILL TAKE ITS TIME TO BREAK DOWN. NEW LONG TERM MODELS ARE 
GENERALLY HOLDING WESTERN WASHINGTON IN THE NORTHERN STREAM THROUGH 
ABOUT FRI AND KEEP HIGHER SURFACE PRES TO THE NE OF THE AREA 
PREVENTING THE INFLUX OF MARINE AIR. SOUTHERN STREAM SYSTEMS COMING 
UNDER THE BLOCK APPEAR TARGETED TO HIT INTO CENTRAL AND SOUTHERN CA 
THROUGH THE WEEK. AS A RESULT LOWERED POPS THROUGH THU NIGHT OR FRI 
AND BROUGHT TEMPERATURES DOWN A FEW DEGREES WITH LESS CLOUD COVER. 
EVENTUALLY A VERY DEEP 500 MB LOW OVER THE NORTHERN CANADIAN ARCTIC 
MOVES W INTO THE YUKON AND THE BLOCK PINCHES OFF AND RETROGRADES 
INTO SIBERIA. WHILE FLOW IN THE FAR EXTENDED REMAINS SPLIT IN WHAT 
LOOKS LIKE AN INCREASINGLY TYPICAL EL NINO FASHION...ONSHORE FLOW 
SHOULD INCREASE SUFFICIENTLY TO ALLOW MILD MARINE AIR TO PUSH INLAND 
AND GIVE A CHANCE OF LIGHT RAIN NEXT WEEKEND. AT THIS TIME A WARM 
ADVECTION SNOW PATTERN TO END THE ARCTIC INTRUSION DOES NOT APPEAR 
TO BE LIKELY. ALBRECHT

&&

.HYDROLOGY...COLD AND DRY WEATHER WILL PERSIST THROUGH THE UPCOMING 
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 IS QUITE LOW 
AND INCLUDES THE GREEN RIVER. BUEHNER

&&

.AVIATION...STRONG NE FLOW ALF WITH A FEW BANDS OF LIGHT SNOW NEAR 
THE CASCADE CREST SOUTH OF STEVENS PASS THIS MORNING WITH CLOUDS AND 
FLURRIES OVER THE OLYMPICS. OTHERWISE...CLEARING FROM THE NORTH. NE 
PRESSURE GRADIENTS ARE STRONG WITH FRASER OUTFLOW LIKELY TO BE 
STEADY AND STRONG THRU THE DAY AND THRU THIS EVENING...THAT WILL 
EASE ON MONDAY. CLEAR AND COLD WITH A DRY AIR MASS TNGT AND MON.

KSEA...DRYING AIR MASS AND NE BREEZE TODAY...SKIES SHOULD BE CLEAR 
THIS AFTERNOON AND TONIGHT BUT FOR THE MORNING HOURS IT LOOKS LIKE 
MORE OF THIS BKN080 WILL CONTINUE BLOWING OFF THE CASCADES IN THE NE 
FLOW ALF.  

&&

.MARINE...STRONG NE FLOW WITH FRASER OUTFLOW CONTINUES AND NO CHANGE 
TO THE FCST EXPECTED. AT DAYBREAK OLM-BLI IS ARND -5MB AND THE 
BLI-WILLIAMS LIKE GRADIENT IS ARND -14MB. THE CROSS CASCADE GRADIENT 
IS ONLY A COUPLE MB SO FAR...BUT THAT WILL CLIMB AND THRU MONDAY THE 
WINDS WILL TREND TO MORE EASTERLY WHILE THEY DECREASE IN MOST SPOTS. 
19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WIND ADVISORY NORTHWEST INTERIOR...AND EAST STRAIT THROUGH 4 AM 
     MONDAY.
PZ...GALE WARNING STRAIT OF JUAN DE FUCA...NORTHERN INLAND           
     WATERS.    
     SMALL CRAFT ADVISORY COASTAL WATERS...ADMIRALTY INLET...        
     PUGET SOUND AND HOOD CANAL.  

$$

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