FXUS66 KSEW 052332
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 PM PST SAT DEC 5 2009

.SYNOPSIS...WINTER WEATHER WILL CONTINUE TONIGHT WITH 
WINDY CONDITIONS AND BELOW NORMAL TEMPERATURES. BRIEF SNOW FLURRIES 
ARE POSSIBLE TONIGHT AS AN ARCTIC OUTFLOW SLIDES SOUTH THROUGH THE 
AREA. DRY WEATHER WILL RETURN SUNDAY WITH AND CONTINUED COLD 
CONDITIONS AND PERSIST THROUGH AT LEAST THURSDAY...WITH A CHANCE OF 
PRECIPITATION RETURNING LATE IN THE WEEK.

&&

.SHORT TERM...A PRONOUNCED UPPER LEVEL RIDGE JUST EAST OF 140W 
CONTINUES DOMINATE THE FLOW PATTERN OVER THE EAST PACIFIC INTO 
WESTERN NORTH AMERICA. NORTHERLY FLOW ALOFT EXTENDS FROM YUKON 
TERRITORY ALL THE WAY DOWN INTO NORTHERN CALIFORNIA WHILE A SURFACE 
HIGH CONTINUES TO BUILD OVER EASTERN BC...STRENGTHENING NORTHERLY 
SURFACE PRESSURE GRADIENTS. ENHANCED NORTHERLY SURFACE GRADIENTS 
WILL SUPPORT A DECENT ARCTIC OUTFLOW DEVELOPING THIS EVENING THROUGH 
THE FRASER VALLEY BRINGING WINDY AND CONTINUED COLD CONDITIONS TO 
PORTIONS OF THE INTERIOR. AFTERNOON MODEL RUNS CONTINUE TO FORECAST 
CYWL-KBLI PRESSURE GRADIENTS IN THE NEIGHBORHOOD OF -15 TO -18 MB 
OVERNIGHT TONIGHT...REMAINING STRONG THROUGH SUNDAY. STRONGEST WINDS 
CONTINUE TO BE EXPECTED IN AREAS EXPOSED TO FRASER OUTFLOW...HAVE 
ACCORDINGLY ISSUES A WIND ADVISORY FOR THE NORTHWEST INTERIOR AND 
EAST STRAIT THROUGH EARLY MONDAY MORNING. 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.

THE OTHER CONCERN IN THE SHORT TERM WILL BE THE POTENTIAL FOR SNOW 
OVERNIGHT WITH THE STRONG SHORT WAVE TROUGH DROPPING OVERHEAD 
TONIGHT AND UPSLOPE FLOW ALONG THE NORTH OLYMPICS.  MESOSCALE MODELS 
CONTINUE TO SUPPORT THE IDEA OF SOME SNOW SHOWER ACTIVITY ASSOCIATED 
WITH INSTABILITY ALOFT AND LIFT ALONG THE OUTFLOW LATE TONIGHT.  
MOISTURE REMAINS THE LIMITING FACTOR...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 AN INCH OR SO OF 
SNOW LATER TONIGHT THRU EARLY SUN. 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...MEDIUM RANGE MODELS IN DECENT AGREEMENT THROUGH MUCH OF 
THE WEEK...BREAKING 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. TEMPERATURES WILL REMAIN BELOW NORMAL WITH COLDEST CONDITIONS 
EARLY IN THE WEEK. MEDIUM RANGE MODELS NOW HOLDING OFF ON THE THREAT 
OF PRECIPITATION UNTIL SATURDAY INTO SUNDAY FRIDAY... WITH THE 18Z 
GFS COMING IN MUCH CLOSER TO THE 12Z ECMWF THAN PREVIOUS RUNS. 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 EXTENDS INTO THE YUKON AND 
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 SUN. NLY FLOW ALF BECMG NE. 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...BUT CANNOT RULE OUT A 
FLURRY OR TWO BY MIDNIGHT TONIGHT...MAINLY OVER THE SAN JUANS AND 
STRAIT OF JUAN DE FUCA AND THE NRN AND ERN SIDE OF THE OLYMPICS. 19

KSEA...INCREASING NNE BREEZE OVERNIGHT. WATCHING THE SATELLITE 
IMAGERY TODAY SUGGESTS THAT THE RISK OF A FLURRY IS NOT ALL THAT 
HIGH TONIGHT...PERHAPS A FEW HOURS OF FLURRIES WITH BKN050. THE 
CLOUD LAYER DVLPG WITH THIS MOSTLY DRY UPPER TROF HAS GIVEN OMAK 
BKN048 BKN070 AT 2PM...AND IT IS THAT LAYER OF CLOUDS THAT WILL MOVE 
SW OVER THE CASCADES THIS EVENING AND IS THE REASON I WILL KEEP THE 
TAF AS IT IS WITH A LITTLE -SHSN ACTIVITY IN THE MIDDLE OF THE 
NIGHT...WE ARE NOT EXPECTING ANY ACCUMULATION AND THE CHANCE FOR 
MEASURABLE PRECIP TNGT IS JUST 20 PCT...SO JUST A TRACE/FLURRY. 19

&&

.MARINE...GRADIENTS DIDN'T DO MUCH TODAY OVER WRN WA AND FCST DIDN'T 
WORK OUT TOO WELL WITH THE EXCEPTION OF SMITH ISLAND UP THRU KBLI 
WHERE NE WIND 20-30KT WORKED OUT OKAY. BUT FCSTS SHOULD WORK OUT 
BETTER TONIGHT AS NE PRES GRADIENTS INCREASE WITH COLD CANADIAN 
SURFACE HIGH PRES SAGGING SOUTH A NOTCH AND FRASER OUTFLOW CRANKING 
UP OVERNIGHT...NO NEED TO CHANGE THE FCSTS AND I AM LEAVING UP ALL 
THE SMALL CRAFT ADVISORIES AND THE GALE WARNINGS FOR THE NORMALLY 
WINDY SPOTS FOR FRASER OUTFLOW. ADMIRALTY INLET AND PUGET SOUND MAY 
SEE THE NLY WIND COME UP TONIGHT ONLY TO BECOME MORE ERRATIC AS THE 
GRADIENT BECOMES MORE EASTERLY LATE SUN...BUT WITH THE MODELS 
SHOWING AT LEAST -5MB OLM-BLI SUNDAY THE SMALL CRAFT ADVISORY FOR 
PUGET SOUND SHOULD FINALLY VERIFY...IF A DAY LATE. 19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WIND ADVISORY FOR THE NORTHWEST INTERIOR AND EAST STRAIT.
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.