FXUS66 KSEW 222316 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PST TUE DEC 22 2009

.SYNOPSIS...HIGH PRESSURE WILL BUILD OVER THE REGION PROVIDING DRY 
WEATHER AND BELOW AVERAGE TEMPERATURES WEDNESDAY THROUGH AT LEAST 
CHRISTMAS DAY. 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...ISOLATED SHOWERS OVER PUGET SOUND STILL SHOWING UP ON 
RADAR THIS AFTERNOON. AS NLY FLOW WEAKENS THIS EVENING...SHOWER 
ACTIVITY SHOULD COME TO AN END. LEFT SOME LOW CHANCE POPS IN THE 
FORECAST JUST FOR THE PUGET SOUND REGION TO COVER THIS. 
OTHERWISE...A STRETCH OF DRY WEATHER ACROSS WESTERN WASHINGTON WILL 
BEGIN TONIGHT. 

A 1040 MB SURFACE HIGH WILL BUILD OVER SRN B.C. TONIGHT AND 
WEDNESDAY. MODELS HAVE TRENDED MUCH WEAKER WITH THE OFFSHORE 
GRADIENTS AND FRASER OUTFLOW. MESO MODELS STILL INDICATE A SHORT 
PERIOD OF OUTFLOW WINDS ACROSS WRN WHATCOM COUNTY ON WEDNESDAY. THIS 
SHOULD HOLD HIGH TEMPS DOWN LOCALLY AROUND KBLI AS LOWER DEW POINT 
AIR FILTERS IN. TEMPS WILL RUN SLIGHTLY COOLER ACROSS THE REST OF 
THE AREA AS WELL WITH DRIER AND COOLER AIR ALOFT SETTLING IN. THIS 
WILL SET THE STAGE FOR POSSIBLE INVERSIONS OVER THE INTERIOR 
LOWLANDS WHICH COULD CREATE STAGNATE CONDITIONS BY THURSDAY AND 
FRIDAY. MODELS DO INDICATE ELY GRADIENTS TIGHTEN UP TOWARD THE END 
OF THE WEAK WHICH COULD EITHER HELP MIX THINGS OUT OR SUPPRESS THE 
INVERSIONS MAKING IT MORE DIFFICULT TO BREAK OUT OF LOW CLOUDS AND 
FOG WITH COLDER DAYTIME HIGHS. DECIDED TO MAKE SOME CHANGES TO THE 
THURSDAY/FRIDAY PERIOD AND LEAN TOWARD INVERSIONS. HAVE LOWERED 
TEMPS AND INCREASED CLOUDS AND FOG WORDING AROUND THE PUGET SOUND 
REGION TO THE SW INTERIOR. A LITTLE MORE MIXING USUALLY DEVELOPS UP 
NORTH SO WILL KEEP ONLY PATCHY FOG WORDING AND WARM THE HIGH TEMPS 
SLIGHTLY. 

.LONG TERM...THE UPPER LEVEL FLOW REMAINS STRONGLY SPLIT OVER THE 
WEEKEND INTO NEXT WEEK. THIS MEANS THAT WEATHER SYSTEM WILL EITHER 
BYPASS WA TO THE S OR WEAKEN CONSIDERABLY AS THEY PUSH ACROSS THE 
AREA. MODELS HAVE FLIP FLOPPED ON BRINGING A WEAK SYSTEM INTO WA BY 
SATURDAY BUT ARE BACK TO A DRIER SOLUTION WHICH IS PREFERRED. LEFT 
THE WEEKEND FORECAST AS IS WITH LOW POPS ON THE COAST AND FAR S. BY 
MONDAY...MODELS INDICATE WEAK NRN BRANCH SYSTEMS MAY TRY TO PUSH 
ACROSS THE AREA. KEPT THE LOW CHANCE POPS IN FOR EARLY NEXT WEEK.

MERCER

&&

.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 CONTINUES TO EXTEND FROM THE 
NW COAST OF VANCOUVER ISLAND...THROUGH THE S PUGET SOUND AREA THEN 
INTO NW OREGON. THIS TROUGH IS DISSIPATING. A 1035 MB HIGH OVER 
CENTRAL B.C. WILL SLOWLY SLIDE SE.

LOW LEVEL PRESSURE GRADIENTS ARE QUITE LIGHT AND WILL REMAIN RATHER 
LIGHT THROUGH THU. AS A RESULT OF THE LIGHT FLOW AND LACK OF 
INSOLATION THIS TIME OF YEAR EXPECT LOW LEVEL MOISTURE TO ONLY 
SLOWLY DECREASE THIS EVENING. MID AND HIGH LEVEL MOISTURE SLIDING SE 
ALONG THE COAST COAST OF VANCOUVER ISLAND WILL DO LITTLE TO INHIBIT 
RADIATIONAL COOLING LATER TONIGHT...SO SOME LOW STRATUS AND PATCHY 
FOG COULD FORM TONIGHT AS HINTED AT BY MODEL OUTPUT STATISTICS FROM 
BOTH THE GFS AND NAM. 

KSEA...P6SM BKN025 BKN040 LATE THIS AFTERNOON WITH LOWER LEVEL 
SCATTERING OUT AROUND 00Z TAF ISSUANCE TIME AND THE 040 DECK 
SCATTERING OUT DURING THE EVENING. SOME FOG OR STRATUS COULD FORM 
OVER THE VALLEYS TO THE E OF THE TERMINAL TONIGHT AND SPREAD INTO 
THE TERMINAL EARLY WED...BUT CONFIDENCE IN THIS SCENARIO IS LOW. 
LIGHT WIND WILL TREND NE 5 KT OVERNIGHT THRU WED.

&&

.MARINE...A WEAK TROUGH FROM OFF THE WEST COAST OF NORTHERN 
VANCOUVER ISLAND SE INTO THE N OREGON CASCADES WILL DISSIPATE THIS 
EVENING. A 1035 MB HIGH OVER CENTRAL B.C. WILL GRADUALLY SLIDE 
SE INTO EASTERN WA THU AND FRI.

WITH THE OFFSHORE FLOW...THERE IS A RISK OF SMALL CRAFT ADVISORY 
CONDITIONS AT TIMES AT THE WEST ENTRANCE TO THE STRAIT AND ALONG THE 
COAST BEGINNING WED AND CONTINUING THROUGH FRI NIGHT OR SAT MORNING.

A SPLITTING FRONT WILL APPROACH THE COAST ON SATURDAY BUT MAY NEVER 
MOVE THROUGH BEFORE DISSIPATING.

SWELL AROUND 10 FT EARLY THIS EVENING ALONG THE COAST WILL SUBSIDE 
TO BELOW 10 FT BY MIDNIGHT TONIGHT. ALBRECHT

 
$$

WWW.WEATHER.GOV/SEATTLE

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