FXUS66 KSEW 240440
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
840 PM PST WED DEC 23 2009

.SYNOPSIS...HIGH PRESSURE BUILDING ALOFT OFF THE COAST IS EXPECTED 
TO MOVE ONSHORE THURSDAY AND FRIDAY PROVIDING DRY WEATHER TO WESTERN 
WASHINGTON INTO THE COMING WEEKEND. WITH CLEARING SKIES... 
TEMPERATURE INVERSIONS ARE LIKELY TO DEVELOP IN THE INTERIOR WITH 
RESULTING EVOLVING STAGNANT CONDITIONS AND COOL DAYTIME TEMPERATURES 
WITH AREAS OF LOW CLOUDS AND FOG. EASTERLY WINDS ARE EXPECTED TO 
KEEP CONDITIONS MORE CLEAR IN THE MOUNTAINS AND COASTAL REGIONS. A 
WEAK FRONT COULD REACH THE AREA MONDAY OR TUESDAY OF NEXT WEEK.

&&

.SHORT TERM...DEEP FULLY MATURE STORM NEAR 46N/161W WITH A 960 
MILLIBAR LOW CENTER IS PUMPING UP THE DOWNSTREAM UPPER RIDGE NOW 
NEAR 130W. HENCE WESTERN WA IS NOW BEGINNING TO SEE WARMING ALOFT 
AND SUBSIDENCE. SATELLITE IMAGERY SHOWS THE MID AND HIGH LEVEL 
CLOUDS THAT HAVE BEEN MOVING SE ACROSS THE REGION ARE STARTING TO 
DECREASE AND SHIFT EAST. THERE IS SOME LINGERING STRATOCUMULUS 
HUGGING PARTS OF THE OLYMPIC PENINSULA AND FAR NORTH INTERIOR.

00Z PROGS IN THUS FAR CONTINUE TO SHOW THE UPPER RIDGE MOVING OVER 
WESTERN WA THU AND FRI. IN ADDITION THE SURFACE HIGH IN THE B.C. 
INTERIOR IS SLATED TO SLIDE SE AND FOLLOW THE STRONG STORM IN THE 
MIDDLE OF THE U.S. HENCE N TO S SURFACE GRADIENTS PEAKED EARLIER 
TODAY AND WILL BECOME MORE E TO W THROUGH FRI.

IN THE MEANTIME..THE CLEARING SKY TREND WILL PERMIT MORE RADIATIONAL 
COOLING DURING OUR LONG NIGHTS AND COMBINED WITH THE WARMING ALOFT 
WITH THE INCOMING UPPER RIDGE..A TEMPERATURE INVERSION SHOULD SET 
UP. WITH LINGERING LOW LEVEL MOISTURE..SHOULD GET AREAS OF FOG AND 
LOW CLOUDS TO DEVELOP IN FAVORED LOCATIONS SUCH AS WESTERN SNOHOMISH 
COUNTY..SOUTH SOUND AND SW INTERIOR. 

WINDS HAVE DROPPED OFF THIS EVENING. EASTERLY WINDS SHOULD DEVELOP 
BY FRI THROUGH THE CASCADE GAPS INTO THE FOOTHILLS..OUT THE STRAIT 
AND COASTAL GAPS. BUT NOT STRONG ENOUGH TO BREAK DOWN THE INTERIOR 
TEMPERATURE INVERSION LEAVING QUITE LIMITED VENTILATION THERE. THE 
AIR STAGNATION ADVISORY ISSUED EARLIER TODAY FOR INTERIOR/ I-5 
CORRIDOR AREAS LOOKS ON TRACK.

IF LOOKING FOR RELATIVELY WARM DAYTIME TEMPERATURES..THE COAST LOOKS 
TO BE THE PLACE WITH READINGS CRACKING THE 50 DEGREE MARK WITH THE 
OFFSHORE FLOW. AREAS WITH SOME WIND SHOULD BE WARM IN COMPARISON 
WITH THE INTERIOR INVERSION LOCATIONS THAT WILL STRUGGLE TO REACH 40.

LOOKING AHEAD INTO THE WEEKEND..THE FRONT ASSOCIATED WITH THE DEEP 
STORM IS EXPECTED TO SPLIT APART AS SPLIT FLOW ALOFT DEVELOPS ALONG 
THE WEST COAST OF NORTH AMERICA. IN FACT THE PATTERN LOOKS VERY EL 
NINO-ISH WITH THE SOUTHERN BRANCH OF THE JETSTREAM AIMED INTO CAL. 
SO IT LOOKS DRY THROUGH THE WEEKEND BUT THE INTERIOR STAGNATION MAY 
PERSIST. BUEHNER 

.LONG TERM...FROM PREVIOUS DISCUSSION..MODELS ARE TRENDING TOWARD 
KEEPING UPPER LEVEL SPLIT FLOW ALONG THE W COAST SUNDAY INTO EARLY 
NEXT WEEK. THIS MEANS THAT ANY SYSTEMS THAT APPROACH THE AREA ARE 
LIKELY TO WEAKEN CONSIDERABLY AND PERHAPS DIVE S BEFORE EVEN 
REACHING WA. SOME PREVIOUS MODEL SOLUTIONS WERE BRINGING A WEAK 
SYSTEM AND LIGHT QPF INTO WRN WA EARLY NEXT WEEK. DECIDED TO KEEP 
THE LOW CHANCE CLIMO POPS IN PLACE FOR NOW. MERCER

&&

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

&&

.AVIATION...NORTHWEST FLOW ALOFT WILL CONTINUE OVER WESTERN 
WASHINGTON TONIGHT AND THURSDAY...AS AN UPPER LEVEL RIDGE OVER THE 
OFFSHORE WATERS PROGRESSES SLOWLY TOWARD THE COAST. AT THE SURFACE 
AN AREA OF HIGH PRESSURE EXTENDING FROM SOUTHEAST BRITISH COLUMBIA 
INTO NORTHEAST WASHINGTON WILL PRODUCE CONTINUE OFFSHORE FLOW...WITH 
GENERALLY LIGHT NORTHEASTERLY WINDS ACROSS THE WESTERN WASHINGTON 
INTERIOR LOWLANDS. SOMEWHAT STRONGER EASTERLY WINDS WILL DEVELOP 
ALONG THE COAST.

THE COMBINATION OF WARMING UNDER THE RIDGE ALOFT...A RELATIVELY COLD 
AIR MASS AT THE SURFACE...AND LIGHT WINDS WILL KEEP THE AREA UNDER A 
STRONG INVERSION. SURFACE-BASED MOISTURE TRAPPED UNDER THE INVERSION 
HAS ALREADY RESULTED IN PATCHY FOG AND STRATUS THIS EVENING...AND 
THIS WILL BECOME MORE WIDESPREAD OVERNIGHT AS THE MID LEVEL CLOUDS 
GRADUALLY CLEAR AND TEMPERATURES FALL. MUCH OF THE WESTERN 
WASHINGTON INTERIOR WILL HAVE IFR CONDITIONS BETWEEN 12Z AND 20Z. 
AREAS WITH ESPECIALLY LIGHT WINDS ON THURSDAY...ESPECIALLY THE SOUTH 
PUGET SOUND AND SOUTHWEST INTERIOR...WILL BE SLOW TO CLEAR EVEN IN 
THE AFTERNOON.

KSEA...NORTH OR NORTHEAST WIND 6 KT OR LESS TONIGHT AND THURSDAY. 
KSEA WILL LIKELY HAVE IFR CONDITIONS DUE TO FOG FROM ROUGHLY 13Z 
THROUGH 20Z THURSDAY. MCDONNAL
 
&&

.MARINE...SURFACE HIGH PRESSURE EXTENDING FROM SOUTHEAST BRITISH 
COLUMBIA ALONG THE EASTERN BORDER OF WASHINGTON WILL PRODUCE 
OFFSHORE FLOW ACROSS WESTERN WASHINGTON AND ITS COASTAL WATERS 
THROUGH THE WEEKEND. 

SMALL CRAFT ADVISORY STRENGTH EASTERLY WINDS WILL DEVELOP AT THE 
WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA THURSDAY...THEN SPREAD 
TO THE CENTRAL STRAIT AND EAST ENTRANCE THURSDAY NIGHT AND FRIDAY AS 
PRESSURE GRADIENTS TIGHTEN. 

THE AREA OF HIGH PRES WILL WEAKEN SATURDAY AS A SPLITTING FRONTAL 
SYSTEM MOVES INTO THE OFFSHORE WATERS FROM THE WEST. CURRENT MODEL 
FORECASTS SHOW THE FRONTAL SYSTEM STALLING OFFSHORE SUNDAY AND MORE 
OR LESS DISSIPATING ON MONDAY. MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

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

$$

WEATHER.GOV/SEATTLE

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