FXUS66 KSEW 051707
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
905 AM PST SAT DEC 5 2009

.SYNOPSIS...COLD AND GENERALLY DRY WEATHER WILL REMAIN IN PLACE OVER 
WESTERN WASHINGTON THROUGH THE MIDDLE OF NEXT WEEK. TEMPERATURES 
WILL RUN WELL BELOW NORMAL THROUGH THIS PERIOD. SOMEWHAT WINDY 
CONDITIONS AND BRIEF SNOW FLURRIES ARE POSSIBLE TONIGHT AS AN ARCTIC 
BOUNDARY SLIDES SOUTH THROUGH THE AREA. COOL AND DRY NORTHERLY UPPER 
LEVEL FLOW WILL REMAIN IN PLACE OVER THE REGION THROUGH NEXT WEEK 
WITH A CHANCE OF PRECIPITATION LATE IN THE WEEK.

&&

.SHORT TERM...LIGHT OFFSHORE FLOW AND STRONG RADIATIONAL COOLING 
OVERNIGHT HAVE BUMPED TEMPS DOWN INTO THE HIGH 20S AND LOW 30S THIS 
MORNING WITH PATCHY FOG AND FROST. SKIES WILL REMAIN CLEAR UNTIL 
EARLY THIS AFTERNOON AS SOME MID AND HIGH CLOUDS BEGIN TO MOVE IN 
FROM THE NORTH AHEAD OF AN UPPER LEVEL SHORTWAVE THAT WILL MOVE 
OVERHEAD TONIGHT. BIG STORY REMAINS THE COLD WEATHER THAT LOOKS TO 
PERSIST OVER THE REGION AT LEAST THROUGH MID WEEK. THE PRONOUNCED 
UPPER LEVEL RIDGE JUST EAST OF 140W CONTINUES TO SUPPORT NORTHERLY 
FLOW ALOFT...WHILE STRONG NORTHERLY SURFACE GRADIENTS CONTINUE TO 
BUILD AS A SURFACE HIGH SLIDES INTO BRITISH COLUMBIA FROM THE 
NORTHWEST TERRITORIES. END RESULT WILL BE A DECENT ARCTIC OUTFLOW  
DEVELOPING THIS EVENING THROUGH THE FRASER VALLEY THAT WILL PERSIST 
THROUGH MONDAY. DAYTIME HIGHS TODAY WILL LIKELY BE A COUPLE DEGREES 
COOLER THAN YESTERDAY.

THE STRONG SHORT WAVE TROUGH OVER NORTHERN B.C. THIS MORNING WILL 
DROP THRU THE AREA THIS EVENING. 12Z NAM CONTINUES TO SHOW THE  
SHORT WAVE TROUGH CLOSING OFF AS IT MOVES OVER W WA...CONTINUING TO 
SUPPORT THE IDEA OF SOME SNOW SHOWER ACTIVITY ASSOCIATED WITH 
INSTABILITY ALOFT AND LIFT ASSOCIATED WITH THE SOUTHWARD MOVING 
MODIFIED ARCTIC FRONT LATE TONIGHT. ATMOSPHERE REMAINS DRY SO 
MOISTURE REMAINS THE LIMITING FACTOR...SO NOT EXPECTING MORE THAN  
FLURRIES WITH LITTLE IF ANY ACCUMULATION MOST AREAS. BEST BET FOR 
ACCUMULATION REMAINS THE NORTH SHORE OF THE OLYMPIC PENINSULA WHERE 
LIFT IS EXPECTED TO PERSIST LONGER THAN MOST AREAS WITH UPSLOPE 
FLOW. PLACES LIKE SEQUIM AND PORT ANGELES MAY SEE A COUPLE INCHES OF 
SNOW LATER TONIGHT THRU EARLY SUN. 

THE MOST SIGNIFICANT IMPACT OVERNIGHT WILL BE COLD TEMPERATURES AND 
WINDS WITH AND BEHIND THE ARCTIC OUTFLOW. 12Z NAM CONTINUES TO 
FORECAST CYWL-KBLI PRESSURE GRADIENTS IN THE NEIGHBORHOOD OF -15 TO 
-18 MB OVERNIGHT TONIGHT AND REMAINING STRONG THROUGH SUNDAY. 
AREAS EXPOSED TO FRASER OUTFLOW WILL LIKELY SEE WIND ADVISORY 
CONDITIONS DEVELOP THIS EVENING...ELSEWHERE IN THE PUGET SOUND AREA 
AND IN THE SOUTHWEST INTERIOR EXPECT BRISK NLY FLOW TO DEVELOP 
TONIGHT AND CONTINUE THROUGH SUNDAY WITH THE STRONG NORTHERLY 
GRADIENTS. GRADIENTS AND WINDS COME DOWN SOMEWHAT SUN NIGHT INTO 
MON...WHICH WILL HELP BUMP TEMPS DOWN INTO THE TEENS TO LOWER 20S 
ACROSS WESTERN WASHINGTON AND NOT MUCH ABOVE ABOVE FREEZING MONDAY. 
COOK

.LONG TERM...12Z GFS CONTINUES TO BREAK SOME WESTERLY FLOW ALOFT 
UNDER THE OFFSHORE BLOCK...WITH SYSTEMS SNEAKING INTO CALIFORNIA AND 
SOUTHERN OREGON. PRECIPITATION SHOULD GENERALLY REMAIN SOUTH OF THE 
AREA IN DEVELOPING SPLIT FLOW. FLOW ALOFT OVER THE NORTHWEST REMAINS 
NORTHERLY AND DRY WITH SURFACE HIGH PRESSURE TO OUR NE THROUGH NEXT 
WEEK. EXPECT COLDEST CONDITIONS EARLY IN THE WEEK...WITH TEMPS 
REMAINING BELOW NORMAL THROUGH THE PERIOD. MEDIUM RANGE MODELS HOLD 
OFF ON THE THREAT OF PRECIPITATION UNTIL FRIDAY...BUT SHOWING 
SIGNIFICANT DIFFERENCES IN THE TIMING AND MODE OF PRECIPITATION. COOK

&&

.HYDROLOGY...COLD AND DRY WEATHER WILL PERSIST THROUGH THE FORECAST 
PERIOD... WITH NO THREAT OF RIVER FLOODING IN THE FORECAST AREA 
FOR THE NEXT WEEK. THIS INCLUDES THE GREEN RIVER.

RIVERS WILL CONTINUE TO RECEDE AS A RESULT OF BELOW NORMAL 
TEMPERATURES AND BELOW NORMAL PRECIPITATION THROUGH WEDNESDAY. A 
WEAK AND COLD WEATHER SYSTEM MAY IMPACT THE AREA  FRIDAY...BUT 
SHOULD HAVE LITTLE OR NO IMPACT ON THE RIVERS. COOK
&&

.AVIATION...AN UPPER RIDGE ALONG 140W EXTENDING INTO THE YUKON WILL 
BUILD A BIT MORE THROUGH SUNDAY WHILE A SHORTWAVE TROF DROPS DOWN IN 
THE NLY FLOW ALF AND DEVELOPS INTO AN UPPER LOW OVER NRN CALIF. 
STRONG HIGH PRES AT THE SURFACE OVER WRN CANADA WILL GIVE INCREASING 
NE PRESSURE GRADIENTS OVER THE REGION WITH FRASER OUTFLOW PICKING UP 
TNGT AND SUN MORNING. THE AIR MASS WILL BE MOSTLY CLEAR DRY AND 
COLD...A FEW POCKETS OF LOW LEVEL MOISTURE ARE AROUND THIS MORNING 
BUT THEY WILL NOT LAST LONG. AS THE MODIFIED ARCTIC AIR SPILLS INTO 
THE REGION TONIGHT AND THE UPPER TROF MOVES ACROSS THE AREA A FEW 
FLURRIES COULD BE TOUCHED OFF...MAINLY OVER THE SAN JUANS AND STRAIT 
OF JUAN DE FUCA/NE OLYMPICS. 19

KSEA...A PATCH OF LAKE WASHINGTON STRATUS MIGHT CLIP SEA TAC FOR AN 
HOUR THIS MORNING...OTHERWISE CLEAR SKIES AND INCREASING NLY WIND 
THRU TNGT AND SUN. THE LEADING EDGE OF MODIFIED ARCTIC AIR THAT 
ARRIVES IN WRN WA OVERNIGHT TNGT COULD TOUCH OFF A FLURRY BUT MAINLY 
IT WILL JUST BE CLEAR AND BREEZY BY SUNDAY. 19

&&

.MARINE...INCREASING NE PRES GRADIENTS AS COLD CANADIAN SURFACE HIGH 
PRES SAGS SOUTH AND FRASER OUTFLOW BEGINS. NO CHANGES TO FCSTS 
EXCEPT TO DELAY THE BREEZY WEATHER FOR PUGET SOUND A BIT...WE HAVE 
HAD A SMALL CRAFT ADVISORY UP FOR PUGET SOUND AND IT WILL EVENTUALLY 
GET BREEZY BUT PROBABLY NOT TIL TONIGHT OR SUN MORNING. THE NE GALES 
FOR THE NORTHERN WATERS WHERE THE FRASER OUTFLOW SPILLS OUT AND THE 
GALE FOR THE WEST ENTRANCE SHOULD START UP AT THE LATEST MIDNIGHT 
TONIGHT BUT POSSIBLY AS EARLY AS THIS AFTERNOON. THE BELLINGHAM 
WILLIAMS LAKE GRADIENT IS -9MB AND SHOULD BE -15MB SUNDAY MORNING. 19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING WEST ENTRANCE JUAN DE FUCA...NORTHERN INLAND       
     WATERS. 
     SMALL CRAFT ADVISORY ALL REMAINING WATERS. 
     SMALL CRAFT ADVISORY ROUGH BAR CONDITIONS GRAYS HARBOR BAR. 

$$

WWW.WEATHER.GOV/SEATTLE

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