FXUS66 KSEW 232347
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 PM 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 WITH GENERALLY LIGHT WINDS. THE RESULTING
INVERSIONS WILL RESULT STAGNANT CONDITIONS ACROSS THE INTERIOR
LOWLANDS WITH AREAS OF LOW CLOUDS AND FOG. A WEAK FRONT COULD REACH
THE AREA MONDAY.
&&
.SHORT TERM...VISIBLE IMAGERY SHOWS LOW CLOUDS RAPIDLY CLEARING OUT
LATE TODAY WITH CIRRUS CONTINUING TO STREAM ACROSS WESTERN
WASHINGTON. THE UPPER RIDGE CURRENTLY NEAR 135W IS FORECAST TO MOVE
OVER THE PAC NW THURSDAY AND FRIDAY. MODEL TIME HEIGHTS AND FORECAST
SOUNDINGS STILL INDICATE SUBSIDENCE AND CONSIDERABLE WARMING ALOFT
THROUGH THE END OF THE WEEK. THIS WILL HAVE THE EFFECT OF STRONGLY
CAPPING INVERSIONS ACROSS THE LOWLANDS OF WRN WA. THE MM5 IS SHOWING
POOR TO VERY POOR VENTILATION AS THE INVERSIONS BECOME SHALLOWER
THROUGH THE END OF THE WEEK WHICH IMPLIES LITTLE MIXING IN THE LOWER
LEVELS. SOME FLOW IS LIKELY ALONG THE COAST...THE STRAIT...AND
ACROSS THE SAN JUANS WHICH ARE EXPOSED TO FLOW AND TEND NOT FORM
PERSISTENT INVERSIONS. IN COORDINATION WITH OTHER OFFICES AND OTHER
STATE AND LOCAL AGENCIES...DECIDED TO GO WITH AN AIR STAGNATION
ADVISORY FOR MUCH OF THE INTERIOR LOWLANDS OF WESTERN WASHINGTON
THROUGH SATURDAY MORNING.
THE EXPECTED INVERSIONS WILL CREATE A VERY TRICKY TEMPERATURE AND
CLOUD COVER FORECAST THIS WEEK. AREAS THAT MIX LIKE THE
COAST...STRAIT...FOOTHILLS...PORTIONS OF THE N INTERIOR WILL LIKELY
SEE THE WARMEST TEMPS...MAINLY IN THE 40S. HOWEVER...LOCATIONS THAT
REMAIN STAGNANT LIKE THE PUGET SOUND REGION AND SW INTERIOR COULD
REMAIN STUCK IN THE 30S. NAM/GFS MOS GUIDANCE IS SHOWING RATHER
UNIFORM HIGH TEMPS THE NEXT FEW DAYS BUT IS PROBABLY NOT PICKING UP
ON THE INVERSIONS ALL THAT WELL. HAVE UNDERCUT MOS BY SEVERAL
DEGREES IN THE AREAS MOST LIKELY TO HOLD ONTO STRONG INVERSIONS. NOT
SURE WHAT TO DO ABOUT THE LOW CLOUD AND FOG POTENTIAL. ALL THE
FORECAST SOUNDING ARE FAIRLY DRY THROUGH THE PERIOD AND WOULD IMPLY
SOME PATCHY NIGHT/MORNING FOG. HOWEVER...MODELS OFTEN MISS THE
EXTENT OF LOW LEVEL MOISTURE TRAPPED WITHIN SHALLOW BUT STRONGLY
CAPPED INVERSIONS. HAVE ADDED SOME LOW CLOUDS AND AREAS OF FOG TO
MUCH OF THE INTERIOR WITH THIS IN MIND. WILL NEED TO KEEP AN EYE ON
THE POTENTIAL FOR GAP WINDS ON FRIDAY. THE LATEST MM5 RUNS DO
DEVELOP SOME BREEZY CONDITIONS EXTENDING INTO THE FOOTHILLS AND
PERHAPS INTO THE PUGET SOUND LOWLANDS. THIS COULD PARTIALLY OR FULLY
CLEAR OUT STAGNATION IN THOSE AREAS. AS FOR REMAINING AREAS...ELY
FLOW DOES NOT SEEM STRONG ENOUGH TO MIX THINGS OUT AND THE WIND MAY
SIMPLY OVERSHOOT THE INVERSION LAYER.
A FRONT APPROACHES THE COAST ON SATURDAY AND SEVERELY SPLITS WITH
MOST OR ALL OF THE ENERGY HEADING SOUTH AND REMAINING OFFSHORE.
HOWEVER...ANY UPPER LEVEL ENERGY MOVING IN COULD ALLOW THE
INVERSIONS TO LIFT OR BREAK. ON THE OTHER HAND...THE INVERSIONS AND
STAGNATION COULD PERSIST INTO THE WEEKEND.
.LONG TERM...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.
MERCER
&&
.AVIATION...UPPER RIDGE AXIS HAS SHIFTED INSIDE OF 130W WITH
NORTHWEST FLOW ALOFT. THE REMNANTS OF A SYSTEM PUSHING INTO B.C. IS
STILL ALLOWING SOME HIGH CLOUD TO STREAM DOWN OVER THE RIDGE.
STRATUS THAT FORMED THIS MORNING HAS LIFTED AND MIXED OUT SOMEWHAT
TO THE BASE OF THE INVERSION AROUND 010-020. THE TOP OF THE
SUBSIDENCE INVERSION HAS LOWERED TO 044 ACCORDING TO ACARS SOUNDINGS
AS WARM AIR ALOFT CONTINUES TO MOVE INTO THE REGION. MODEL SOUNDINGS
ARE FAIRLY DRY TONIGHT...BUT WITH LEFTOVER MOISTURE FROM TODAY AND
LITTLE MIXING THERE SHOULD BE AREAS OF SHALLOW FOG FORMING TONIGHT.
SURFACE HIGH PRES WILL SLIDE SE INTO EASTERN WA ON THURSDAY
STRENGTHENING OFFSHORE GRADIENTS. THIS SHOULD HELP MIX OUT TERMINALS
DURING THE AFTERNOON...ALTHOUGH FOG/LOW CLOUD WILL LIKELY LINGER
LONGEST IN THE SOUTHWEST INTERIOR AND CENTRAL SOUND.
KSEA...LIGHT NORTHERLY WINDS BECOMING LIGHT NE OVERNIGHT. PREFER NAM
MOS GUIDANCE THAT HAS FOG/STRATUS FORMING AROUND 09Z AND STICKING
AROUND THROUGH AT LEAST LATE MORNING. DAMICO
&&
.MARINE...OFFSHORE FLOW WILL CONTINUE THROUGH THE WEEKEND AS SURFACE
HIGH PRES OVER SOUTHERN BRITISH COLUMBIA SLIDES SOUTHWARD INTO
EASTERN WA. OFFSHORE GRADIENTS WILL TIGHTEN ON THURSDAY...LIKELY
PRODUCING SMALL CRAFT WINDS FOR THE WESTERN STRAIT AND COASTAL GAP
AREAS BEGINNING AROUND 00Z FRIDAY. HIGH PRES WILL WEAKEN ON SATURDAY
AS A SPLITTING FRONTAL SYSTEM ENTERS THE COASTAL WATERS. CURRENT
MODEL FORECASTS SHOW THE FRONTAL SYSTEM STALLING OFFSHORE THROUGH
SUNDAY. DAMICO
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...AIR STAGNATION ADVISORY FOR PORTIONS OF WESTERN WASHINGTON.
PZ...NONE.
$$
WWW.WEATHER.GOV/SEATTLE
FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.
