FXUS66 KSEW 231715
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PST WED DEC 23 2009

.SYNOPSIS...UPPER RIDGE MOVING SLOWLY EASTWARD ACROSS THE REGION 
WILL PROVIDE DRY WEATHER TO WESTERN WASHINGTON INTO THE COMING 
WEEKEND. TEMPERATURES WILL REMAIN SOMEWHAT BELOW AVERAGE THROUGH AT 
LEAST CHRISTMAS DAY...AND GENERALLY LIGHT WINDS WILL RESULT IN AREAS 
OF FOG. A WEAK FRONT COULD REACH THE AREA SUNDAY NIGHT OR MONDAY.

&&

.SHORT TERM...UPPER LEVEL HIGH PRES CENTERED ALONG 130W WILL SLOWLY 
SHIFT EWD OVER WA BY FRIDAY. THIS WILL PROVIDE DRY WEATHER THROUGH 
CHRISTMAS DAY AND LIKELY INTO MUCH OF THE WEEKEND. A 1042 MB SURFACE 
HIGH CENTERED OVER SRN B.C. E OF THE ROCKIES TODAY WILL PROVIDE 
LIGHT OFFSHORE FLOW. LIGHT OUTFLOW WINDS FROM THE FRASER SHOULD HELP 
SCATTER OR CLEAR CLOUD COVER ACROSS THE FAR N INTERIOR. NOT SURE HOW 
MUCH CLEARING THE REST OF THE INTERIOR WILL SEE TODAY. WITH 
SUBSIDENCE ALOFT DEVELOPING...CLOUDS MAY ACTUALLY LOWER AND FILL IN 
FURTHER. THE SUBSIDENCE INVERSION SHOULD STRENGTHEN TONIGHT THROUGH 
SATURDAY MORNING WITH MUCH WARMER AIR ALOFT KEEPING THE CAP IN 
PLACE. THIS WILL LEAD TO POOR VENTILATION AND STAGNATION PROBLEMS.  
BY FRIDAY...THE MM5 DOES DEVELOP SOME GAP WINDS BUT THE NAM DOES 
NOT. THE MOST LIKELY SCENARIO IS THAT SOME MIXING DEVELOPS ALONG THE 
FOOTHILLS AND PERHAPS THE EAST PUGET SOUND LOWLANDS BUT THE ELY 
WINDS WILL PROBABLY OVERSHOOT THE INVERSION ELSEWHERE. IN 
COORDINATION WITH OTHER OFFICES...STATE AND LOCAL AGENCIES...WILL BE 
ISSUING AN AIR STAGNATION ADVISORY THIS AFTERNOON FOR THE THURSDAY 
THROUGH SATURDAY MORNING PERIOD.  WILL EXCLUDE THE MOUNTAINS ABOVE 
THE INVERSION AND ALSO THE STRAIT AND COAST WHERE MORE FLOW 
DEVELOPS.  WILL ALSO INDICATE SOME BETTER CONDITIONS CLOSER TO THE 
FOOTHILLS WHERE SOME ELY GAP WINDS PROVIDE SOME MIXING. 

THE OTHER FORECAST CHALLENGE OVER THE NEXT FEW DAYS WILL BE 
TEMPERATURES. IF INVERSIONS DO STAY IN PLACE...OUR CURRENT HIGH 
TEMPS ARE PROBABLY ON THE WARM SIDE. MAY NEED TO KNOCK HIGHS BACK 
INTO THE 30S ACROSS MUCH OF THE INTERIOR.  MERCER

.LONG TERM...PREVIOUS DISCUSSION FOLLOWS...MODEL AGREEMENT IN THE 
LONGER RANGE REMAINS SOMEWHAT SKETCHY. THERE IS GENERAL AGREEMENT 
HOWEVER THAT THE STRETCH OF DRY WEATHER WILL CONTINUE SATURDAY 
THROUGH SUNDAY...AS THE UPPER RIDGE SHIFTS EAST OF THE AREA AND A 
WEAK FRONTAL SYSTEM SLOWLY APPROACHES FROM THE WEST. BY EARLY NEXT 
WEEK...WEAK SYSTEMS COULD BEGIN TO AFFECT WESTERN WASHINGTON BUT 
NOTHING VERY SIGNIFICANT APPEARS TO BE ON THE WAY. MCDONNAL 

&&

.HYDROLOGY...THERE IS NO THREAT OF FLOOD-PRODUCING RAINFALL IN THE 
FORECAST AREA FOR THE NEXT TEN DAYS. THIS INCLUDES THE GREEN RIVER. 

MERCER

&&

.AVIATION...UPPER RIDGE AXIS HAS SHIFTED INSIDE OF 130W AND 
CURRENTLY WE HAVE NORTHWEST FLOW ALOFT OVER THE AREA. A WARM FRONT 
PUSHING INTO SOUTH/CENTRAL B.C. WILL CONTINUE TO SPREAD SOME MID AND 
HIGH CLOUD OVER THE NORTHERN HALF OF WESTERN WASHINGTON TODAY. 
TRICKY FORECAST FOR TODAY WITH PERSISTENT SCT-BKN 040-050 STRATUS 
DECK STILL IN PLACE FROM OVERNIGHT. LAYER IS FAIRLY THIN AND MODELS 
STILL SHOW A LATE BREAK OUT AROUND 00Z. HOWEVER THINGS ARE LOOKING A 
BIT MURKY LOOKING NORTH HERE FROM THE FORECAST OFFICE.  LIGHT WINDS 
AND POOR MIXING ARE ALLOWING SHALLOW FOG/STRATUS LAYER TO AROUND THE 
AREA...ESPECIALLY NORTH OF SEATTLE. THINKING IS THAT THIS WILL ONLY 
BE A PROBLEM THROUGH MIDDAY...WITH A SLIGHT UPTICK IN NORTHERLY 
GRADIENTS. THE LOW LEVEL INVERSION WILL STRENGTHEN TONIGHT AND 
SHALLOW FOG IS LIKELY FOR TO FORM...ESPECIALLY FOR LOW LYING 
TERMINALS. 

KSEA...LIGHT WINDS SHOULD BECOME LIGHT NORTHERLY...22-00Z.  

&&

.MARINE...CURRENTLY LIGHT NORTHERLY GRADIENTS EXIST AS A 1040 MB 
HIGH OVER SOUTHERN BRITISH COLUMBIA SLIDES SE INTO ERN WA BY THU. 
THE BLI-YWL GRADIENT HAS PEAKED AROUND -7MB AND AS A RESULT ONLY 
LIGHT NORTHEASTERLY FRASER OUTFLOW IS EXPECTED.  THE HIGH WILL 
REMAIN OVER ERN WA THROUGH SAT BUT SLOWLY WEAKEN. WITH THE OFFSHORE 
FLOW...SMALL CRAFT ADVISORY CONDITIONS AT TIMES AT THE WEST ENTRANCE 
TO THE STRAIT AND ALONG THE COAST BEGINNING THURSDAY NIGHT. A 
SPLITTING FRONT WILL APPROACH THE COAST ON SAT. FELTON/DAMICO

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...NONE.
PZ...NONE.

 
$$

WWW.WEATHER.GOV/SEATTLE

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