FXUS66 KSEW 221705 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PST TUE DEC 22 2009

.SYNOPSIS...SPOTTY LIGHT RAIN OR SNOW SHOWERS WILL FALL ACROSS 
WESTERN WASHINGTON TODAY AS COLDER AIR FILTERS INTO THE REGION FROM 
THE NORTH. AN UPPER RIDGE WILL KEEP THE REGION DRY WEDNESDAY THROUGH 
FRIDAY WHILE THE COLD AIR MASS REMAINS OVER THE AREA. THE UPPER 
RIDGE WILL WEAKEN AND MOVE EAST LATE THIS WEEKEND OR EARLY NEXT WEEK 
AND ALLOW A WEAK WEATHER SYSTEM TO REACH THE AREA.

&&

.SHORT TERM...THE LATEST RADAR LOOP SHOWS A BAND OF LIGHT RAIN AND 
SNOW SHOWERS OVER THE N PUGET SOUND WATERS MOVING SWD AND DECREASING 
IN COVERAGE. A RAIN/SNOW MIX WAS REPORTED THIS PAST HOUR AT WHIDBEY 
ISLAND BUT THE PRECIP MIX THAT WAS OCCURING EARLIER ALONG THE STRAIT 
HAS SINCE DISSIPATED. TEMPS AND DEW POINTS IN THE MID 30S...AND A 
FREEZING LEVEL OF 1900 FT ON THE 12Z KUIL SOUNDING DO NOT SUPPORT 
ANYTHING MORE THAN A BRIEF MIX IN HEAVIER SHOWERS THIS MORNING AND 
NO ACCUMULATION. WILL UPDATE THE STRAIT ZONE TO REMOVE THE LIGHT 
ACCUMULATIONS MENTIONED THERE. OVERALL...THE TREND SHOULD BE TOWARD 
DECREASING SHOWERS WITH DRY CONDITIONS BY LATER THIS AFTERNOON AS 
THE MAIN UPPER LOW CONTINUES SWD. GRADIENTS WILL ALSO DECREASE SO 
THERE WILL NO LONGER BE ANY SUPPORT FOR CONVERGENCE ZONE DEVELOPMENT.

A 1040 MB SURFACE HIGH SETTLES IN OVER SRN B.C. E OF THE ROCKIES 
TONIGHT AND WEDNESDAY. MODELS CONTINUE TO BACK OFF ON THE OUTFLOW 
THROUGH THE FRASER AND THE AMOUNT OF COLD AIR ALOFT THAT MANAGES TO 
DRIVE IN FROM THE N. THE NAM STILL SHOWS A BRIEF PERIOD OF WEAK 
OUTFLOW WINDS ACROSS THE N INTERIOR ON WEDNESDAY WHICH WILL LIKELY 
BE THE COLDEST DAY THERE. ELSEWHERE...THE DRIER AND COOLER AIR ALOFT 
WILL SETTLE INTO THE LOWER LEVELS WHICH WILL TAKE TEMPS DOWN TO 
SLIGHTLY BELOW AVERAGE. HIGH PRESSURE ALOFT...LACK OF CLOUDS AND 
STRONG RADIATIONAL COOLING WILL ALLOW MUCH OF THE AREA TO FALL BELOW 
FREEZING DURING THE EARLY MORNING HOURS THROUGH AT LEAST THURSDAY. 
AN EXTENDED PERIOD OF DRY WEATHER IS ANTICIPATED TONIGHT THROUGH AT 
LEAST CHRISTMAS DAY. 

MERCER

.LONG TERM...PREVIOUS DISCUSSION...THE UPPER RIDGE WILL KEEP WESTERN 
WASHINGTON DRY ON FRIDAY. CONFIDENCE IN THAT PART OF THE EXTENDED 
PERIOD IS HIGH. MODEL AGREEMENT FOR SATURDAY THROUGH MONDAY IS AGAIN 
NOT TOO STRONG AS MODEL SOLUTIONS ARE WAFFLING ON HOW LONG THE UPPER 
RIDGE WILL KEEP THE AREA DRY. THE CURRENT FORECAST...WHICH KEEPS THE 
AREA DRY THROUGH SUNDAY...MAY BE A LITTLE OPTIMISTIC. HOWEVER ANY 
SYSTEMS THAT DO REACH THE AREA THIS WEEKEND OR EARLY NEXT WEEK WILL 
BE WEAK. MCDONNAL 

&&

.HYDROLOGY...THERE IS NO THREAT OF RIVER FLOODING IN THE FORECAST 
AREA FOR THE NEXT WEEK. THIS INCLUDES THE GREEN RIVER. MERCER

&&

.AVIATION...A WEAK TROUGH OF LOW PRES EXTENDING FROM OFF THE 
NORTHWEST COAST OF VANCOUVER ISLAND SE INTO THE S INTERIOR OF W WA 
WILL DISSIPATE LATER TODAY AS HIGH PRES OVER N CENTRAL B.C. SAGS SE. 
LOW LEVEL MOISTURE WITH CIGS 025-030 AND SOME WEAK LOW LEVEL 
CONVERGENCE IS PRODUCING SOME VERY LIGHT RAIN/SNOW SHOWERS FROM 
AROUND KNUW-W OF KPAE. THESE LIGHT SHOWERS ARE MOVING S AND MAY 
PRODUCE A BRIEF LIGHT SHOWER OF RAIN OR SNOW THRU ABOUT 20Z. 

LOW LEVEL MOISTURE WILL GRADUALLY ERODE FROM N TO S LATER TODAY AND 
TONIGHT. THE AIR MASS WILL GRADUALLY DRY LATER TODAY AND THIS 
EVENING.

RESIDUAL LOW LEVEL MOISTURE IN THE BOUNDARY LAYER WILL ADD SOME 
UNCERTAINTY TO THE FORECAST EARLY WED AS THE OFFSHORE FLOW IS NOT 
EXPECTED TO BE STRONG ENOUGH TO SCOUR IT OUT. ADDITIONALLY...STATIC 
STABILITY WILL INCREASE BEGINNING EARLY WED MORNING AS HIGH PRES 
BUILDS ALOFT AND SUBSIDENCE PRODUCES SOME ADIABATIC WARMING. SOME 
VERY STRATUS AND FOG AS SHOWN IN THE GFS MODEL OUTPUT STATISTICS IS 
POSSIBLE TOWARD MORNING...BUT BOUNDARY LAYER MOISTURE COULD VERY 
WELL FROST OUT.

KSEA...P6SM BKN025-035 WITH LIGHT WIND BECOMING LIGHT NLY. SOME VERY 
LIGHT SPRINKLES...THAT MAY MIX WITH -SN...ARE POSSIBLE UNTIL ABOUT 
20Z AS THE ACTIVITY OVER CENTRAL PUGET SOUND MOVES S...BUT THESE ARE 
NOT EXPECTED TO BE SIGNIFICANT. SCT025 BKN035-040 AFTER 22Z THROUGH 
THIS EVENING. AT THIS POINT WILL OPT TO KEEP THE LOWER LEVELS MAINLY 
DRY LATER THIS EVENING THRU 00Z THU. ALBRECHT

&&

.MARINE...A WEAK TROUGH FROM OFF THE WEST COAST OF NORTHERN 
VANCOUVER ISLAND SE INTO THE N OREGON CASCADES WILL DISSIPATE. HIGH 
PRES OVER CENTRAL B.C. WILL GRADUALLY SLIDE SE INTO EASTERN WA THU 
AND FRI. A SPLITTING FRONT WILL APPROACH THE COAST ON SATURDAY. 
SWELL AROUND 11-13 FT EARLY THIS MORNING WILL SUBSIDE TO BELOW 10 FT 
EARLY THIS EVENING. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...NONE.
PZ...SMALL CRAFT ADVISORY COAST AND WEST ENTRANCE TO THE  STRAIT     
     FOR HAZARDOUS SEAS.
    .SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS GRAYS HARBOR.

 
$$

WWW.WEATHER.GOV/SEATTLE

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