FXUS66 KSEW 231130
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST WED DEC 23 2009
.SYNOPSIS...WITH THE EXCEPTION OF LOCAL DRIZZLE THIS MORNING...AN
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...NORTHWEST FLOW CONTINUES ALOFT OVER WESTERN WASHINGTON
EARLY THIS MORNING AHEAD OF AN UPPER LEVEL RIDGE OFFSHORE THAT IS
PROGRESSING SLOWLY EASTWARD ALONG 130W. AT THE SURFACE A 1041 MB
HIGH MOVING SOUTHWARD THROUGH BRITISH COLUMBIA. THIS IS PRODUCING
WEAK NORTHERLY OFFSHORE FLOW ACROSS WESTERN WASHINGTON...BUT THE
PUSH OF COLDER AIR HAS BEEN QUITE WEAK. BELLINGHAM HAS NOT REPORTED
A NORTH WIND GREATER THAN 3 KT DURING THE PAST 24 HOURS. SKIES
REMAIN GENERALLY CLOUDY AND THE TEMPERATURES ARE MILDER THAN
EXPECTED -- MOSTLY IN THE LOWER TO MID 30S.
IN THE VERY SHORT TERM...THE MAIN CONCERN IS THE POSSIBILITY OF MORE
SPOTTY LIGHT PRECIPITATION OVER LAND AREAS ADJACENT TO THE INLAND
WATERS OF WESTERN WASHINGTON. WE HAVE HAD SOME PRECIPITATION ALONG
THESE LINES OVERNIGHT...MAINLY TUESDAY EVENING...BUT THEY COULD
FLARE UP AGAIN THIS MORNING AS A RESULT OF THE COOL MOIST AIR...
RELATIVELY WARM WATER...AND LIGHT LOW LEVEL WINDS. THE GFS AND NAM
MODELS INDICATE THIS WILL OCCUR...SO THE FORECAST WILL INCLUDE AREAS
OF DRIZZLE THIS MORNING. THE TEMPERATURE OF THE LOWER LEVEL AIR MASS
IS TOO WARM TO WARRANT A MENTION OF FLURRIES.
THE UPPER RIDGE WILL CONTINUE ITS SLOW PROGRESSION TOWARD THE COAST
TODAY THROUGH THURSDAY...WITH THE AIR MASS ALOFT WARMING QUICKLY.
THE FREEZING LEVEL ALONG THE COAST WILL CLIMB TO AROUND 6500 FT LATE
TONIGHT. GENERALLY LIGHT WINDS WILL CONTINUE WITH THE SURFACE HIGH
SHIFTING INTO THE NORTHERN ROCKIES...CAUSING THE PRESSURE GRADIENT
TO BECOME MORE EASTERLY. CLOUD COVER WILL GRADUALLY CLEAR THIS
AFTERNOON AND TONIGHT...AND RADIATIONAL COOLING WILL CAUSE THE AIR
MASS AT THE SURFACE TO FALL INTO THE MID 20S TO LOWER 30S TONIGHT.
A VERY STRONG AND RELATIVELY SHALLOW INVERSION WILL DEVELOP AS A
RESULT. WITH WINDS REMAINING LIGHT...VENTILATION WILL REMAIN POOR TO
VERY POOR THROUGH THURSDAY AT LEAST. FOGGY AREAS WILL BECOME MORE
WIDESPREAD...AND THE LACK OF A MECHANISM TO DISPERSE POLLUTANTS
COULD NECESSITATE THE ISSUANCE OF AN AIR STAGNATION ADVISORY LATER
THIS MORNING. THIS MATTER WILL BE COORDINATED BY WASHINGTON NWS
OFFICES...STATE OFFICES...AND LOCAL CLEAN AIR AGENCIES LATER THIS
MORNING.
THE UPPER RIDGE WILL SHIFT INLAND THURSDAY NIGHT AND FRIDAY...AND
THE STRENGTH OF THE EASTERLY SURFACE PRESSURE GRADIENT WILL INCREASE
SOMEWHAT. MORE IMPORTANTLY...AS PREVIOUS DISCUSSIONS HAVE MENTIONED
...SEVERAL INGREDIENTS THAT PRODUCE MOUNTAIN WAVE CONDITIONS ARE
LIKELY TO COME TOGETHER. NAMELY THE INVERSION HEIGHT WILL BE NEAR
THE CREST OF THE CASCADES...AND EASTERLY FLOW AT THE CREST WITH A
REVERSAL TO NORTHWEST FLOW HIGHER ALOFT. THIS FEATURE WILL NOT BE
STRONG ENOUGH TO PRODUCE HIGH WINDS...BUT IT CERTAINLY COULD PRODUCE
LOCALLY BREEZY CONDITIONS -- 15 TO 25 MPH WINDS -- ALONG THE WEST
SLOPES OF THE CASCADES AND NEAR LOW LEVEL GAPS IN THE TERRAIN. MANY
OF THE DETAILS ARE YET TO COME TOGETHER...BUT THE UW WRF-GFS MODEL
NICELY DEPICTS HOW THIS MIGHT PLAY OUT ON FRIDAY MORNING.
THE BREEZY CONDITIONS WILL IMPROVE AIR STAGNATION CONDITIONS WHERE
THE WINDS OCCUR...BUT WILL NOT HELP MUCH IN THE AREAS WHERE WINDS
REMAIN CALM. BREEZY CONDITIONS WILL ALSO CAUSE TO TEMPERATURES TO
JUMP AS THE WINDS MIX DOWN SOME OF THE WARMER AIR ALOFT. MCDONNAL
.LONG TERM...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.
&&
.AVIATION...SATELLITE IMAGERY SHOWS STRATUS ACROSS MOST OF WESTERN
WASHINGTON AT 11Z. CEILINGS RANGING FROM NEAR 2500 FEET ON THE COAST
TO AROUND 4000 FEET OVER THE INTERIOR. LIGHT SURFACE GRADIENTS
COMBINING WITH LIGHT FLOW IN THE LOWER LAYERS OF THE AIR MASS WILL
MAKE IT TOUGH FOR THE STRATUS TO BURN OFF THIS MORNING. THE LAYER IS
NOT VERY THICK WITH THE TOPS ON THE ARO PROFILER AT WESTPORT AROUND
2000 FEET ABOVE THE BASES WITH THE CLOUD DECK THINNING THE CLOSER
ONE GETS TO THE CASCADES. TOUGH CALL ON WHETHER THE STRATUS DECK
DISSIPATES LATER TODAY INTO TONIGHT. MODEL CROSS SECTIONS SHOW AN
INCREASE IN THE LOW LEVEL OFFSHORE FLOW BEGINNING THIS AFTERNOON
INTO THIS EVENING WITH THE 850 MB WINDS COMING AROUND EAST
NORTHEASTERLY. WITH THIS IN MIND WILL GO AHEAD AND GO FOR SCATTERED
CLOUDS AROUND 00Z IN THE TAFS. WITH THE CLEARING SKIES THE NEXT
FORECAST ISSUE WILL BE FOG THURSDAY MORNING. SURFACE GRADIENTS
REMAIN FAIRLY LIGHT AND WITH THE WINDS RIGHT OFF THE GROUND ARE 10
KNOTS OR LESS THURSDAY MORNING. EXPECT WIDESPREAD FOG TO FORM
THURSDAY MORNING LOWERING THE VISIBILITIES DOWN TO BELOW A MILE IN
MANY PLACES. FELTON
KSEA...STRATUS DECK REMAINING INTACT WITH CEILINGS NEAR 4000 FEET
THROUGH MID AFTERNOON. SKIES CLEARING LATE AFTERNOON INTO THE
EVENING HOURS. FOG POSSIBLE THURSDAY MORNING. POSSIBLE THE FOG WILL
FORM IN THE GREEN RIVER VALLEY AND LIFT UP INTO THE AIRPORT NEAR OR
JUST AFTER SUNRISE. LIGHT AND VARIABLE WINDS TODAY. FELTON
.
&&
.MARINE...A 1040 MB HIGH OVER SOUTHERN BRITISH COLUMBIA TODAY AND
TONIGHT WILL SLIDE SE INTO ERN WA THU. 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
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.
$$
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