FXUS66 KSEW 302300
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PST MON NOV 30 2009
.SYNOPSIS...A COLD FRONT OVER WESTERN WASHINGTON WILL MOVE SOUTH
THIS EVENING...WITH CLEARING BEHIND IT. AN UPPER LEVEL RIDGE AND
SURFACE HIGH BUILDING OVER THE REGION TONIGHT WILL KEEP THE AREA DRY
THROUGH THURSDAY. AN UPPER LEVEL LOW WILL BRING A CHANCE OF RAIN TO
THE AREA FRIDAY THOUGH MONDAY.
&&
.SHORT TERM...THE RAPIDLY WEAKENING COLD FRONT OVER WESTERN WA THIS
AFTERNOON SEEMS ABOUT FINISHED. SEEMS WHAT IS LEFT IS WEAK
CONVERGENCE BETWEEN THE NORTHERLY WINDS BEHIND THE COLD FRONT AND
SOUTH WINDS RUNNING UP PUGET SOUND. WILL KEEP A CHANCE OF RAIN GOING
THIS EVENING TO COVER THIS...THEN DRY AND SOME CLEARING AFTER
THAT...AS THE STRONG UPPER LEVEL RIDGE BUILDING OFFSHORE TAKES OVER.
WITH CLEARING LATER TONIGHT THIS IS A GOOD SETUP FOR AREAS OF FOG
AND LOW CLOUDS TO FORM BY TUESDAY MORNING. WITH FAIRLY DRY AIR OVER
THE N INTERIOR WOULD EXPECT LESS FOG/LOW CLOUD FORMATION THERE.
HOWEVER THE NAM BOUNDARY LAYER RH SHOWS A VERY MOIST AIR MASS OVER
PUGET SOUND FROM EVERETT SOUTHWARD THAT HANGS AROUND INTO THE
AFTERNOON. WILL PLAN ON SOME PARTIAL CLEARING FROM SEA-TAC NORTHWARD
TUESDAY AFTERNOON...BUT KEEP TACOMA SOUTHWARD CLOUDED UP. WITH NE
FLOW ALSO EXPECT UPSLOPE CLOUDS/FOG TO PLAGUE THE STRAIT OF JUAN DE
FUCA INTO EARLY TUESDAY AFTERNOON.
THE SURFACE HIGH BUILDING OVER B.C. TONIGHT EVENTUALLY MOVES SE OVER
THE NORTHERN ROCKIES BY THURSDAY. THIS WILL SHIFT THE SURFACE FLOW
AROUND TO AN EASTERLY DIRECTION ALLOWING SOME DOWNSLOPE DRYING FROM
THE CASCADES TO DEVELOP. DRY AIR FROM THE SUBSIDENCE INVERSION
SHOULD ALSO WORK DOWN TOWARD THE SURFACE. NET EFFECT WILL BE TO CUT
BACK ON THE FOG/LOW CLOUD COVERAGE WEDNESDAY AND THURSDAY. MAX/MIN
TEMPS WILL BE CLOSE TO CLIMO. OVERNIGHT CLEARING WILL ALLOW MINS TO
DROP INTO THE UPPER 20S IN THE COLDER AREAS WITH 30S ELSEWHERE.
HIGHS SHOULD TOP OUT IN THE 40S. COLDEST MORNINGS WILL BE WEDNESDAY
THURSDAY...AND IT WOULD NOT BE SURPRISING TO SEE A COUPLE SPOTS
REACH THE MID 20S. KAM
.LONG TERM...GFS AND ECMWF ARE STILL BUTTING HEADS WITH OPPOSITE
SOLUTIONS WITH AN APPROACHING UPPER LEVEL SHORTWAVE TROUGH ON
FRIDAY. OPERATIONAL GFS HAS TRENDED DRIER AND FASTER AND TAKES THE
SHORTWAVE ACROSS SE B.C. FRIDAY WITH NO PRECIP OVER W WA. ECMWF IS
SLOWER AND FURTHER SOUTH WITH THE SHORTWAVE...FORMING A CLOSED LOW
JUST OFFSHORE NEAR 48N/130W 12Z FRIDAY...THEN KEEPING IT AROUND THE
AREA THROUGH THE WEEKEND. GFS EVENTUALLY HAS A CLOSED LOW OVER ID ON
SATURDAY BUT STAYS DRY FOR W WA. ECMWF IS A LITTLE WETTER. THE 12Z
CANADIAN SHIFTED AROUND TOWARD THE ECMWF SOLUTION AND 7 OF THE 12
ENSEMBLE MEMBERS FROM THE 12Z GFS FAVOR SOME FORM OF CLOSED LOW OVER
THE REGION SATURDAY. NONE OF THE SOLUTIONS SEEMS COMPLETELY
REASONABLE BUT THE VARIOUS SOLUTIONS TILT TOWARD SOMETHING CLOSER TO
THE ECMWF SCENARIO. THE BEST COURSE FOR NOW IS TO CONTINUE WITH LOW
CHANCE POPS FOR RAIN FRIDAY THROUGH SUNDAY. THE ECMWF IS NOT
PARTICULARLY WET SO POPS BELOW CLIMO WORK OK. KAM
&&
.HYDROLOGY...NO HYDROLOGICALLY SIGNIFICANT RAINFALL IS EXPECTED FOR
THE NEXT 10 DAYS...OR THROUGH WEDNESDAY DEC 9TH. A WEAK COLD FRONT
AND RESIDUAL CONVERGENCE WILL BRING SOME LIGHT RAIN FROM PUGET
SOUND SOUTHWARD THIS EVENING. AN UPPER LEVEL LOW MAY BRING A LITTLE
MORE LIGHT RAIN TO THE AREA FRIDAY THROUGH MONDAY...ALTHOUGH THERE
IS STILL A LOT OF UNCERTAINTY ABOUT HOW THE UPPER LOW PATTERN WILL
EVOLVE. REGARDLESS...THE CURRENT TENDENCY FOR HIGH AMPLITUDE RIDGING
OFFSHORE ALONG 135W OR SO IS NOT A HEAVY RAIN OR FLOODING PATTERN.
THE HIGH-AMPLITUDE RIDGING WILL PROBABLY BREAK DOWN DURING THE
MIDDLE OF DECEMBER...WITH A SHIFT TO A WETTER PATTERN. KAM
&&
.AVIATION...WEAK COLD FRONT MOVING INTO THE CASCADES THIS AFTERNOON.
RADAR SHOWS A PROBABLE PUGET SOUND CONVERGENCE ZONE FORMING IN
SNOHOMISH COUNTY...SURFACE WINDS SUPPORT THIS IDEA. HIGH PRES WILL
BUILD OVER CENTRAL BRITISH COLUMBIA BEHIND THE FRONT. WESTERLY FLOW
ALOFT WILL BECOME MORE NORTHWESTERLY THIS EVENING.
THE PSCZ WILL LIKELY MOVE SOUTH THIS EVENING AND DISSIPATE. WINDS...
MOST OF WHICH ARE CURRENTLY SOUTHERLY THROUGH THE PUGET SOUND
TERMINALS...WILL TURN NORTHERLY THIS EVENING. VISIBLE SATELLITE
IMAGERY SHOWS SOME CLEARING BEHIND THE FRONT...BUT ANY CLEARING IS
LIKELY TO BE SHORT LIVED. CEILINGS WILL DEFINITELY LIFT A BIT AFTER
THE FRONT PASSES...FROM 2K FT TO MORE LIKE 4K FT. HOWEVER CEILINGS
ARE LIKELY TO FALL AGAIN LATE TONIGHT...POSSIBLY INTO THE IFR OR
EVEN LOW IFR CATEGORY.
KSEA...SW WIND 10-20 KT AS THE FRONT PASSES THIS AFTERNOON...THEN
SHIFTING TO NLY 5-8 KT BEHIND THE FRONT BY 03Z OR SO. A LITTLE LIGHT
RAIN IS POSSIBLE UNTIL 03Z. PUGET SOUND CONVERGENCE ZONE LIKELY TO
KEEP CEILINGS AROUND 2K FT THIS EVENING...THERE COULD BE BRIEF
IMPROVEMENT FOR AWHILE...THEN IFR OR LOW IFR CONDITIONS ARE POSSIBLE
AFTER 10Z...CONTINUING INTO TUESDAY MORNING. CHB
&&
.MARINE...WINDS HAVE TURNED WESTERLY IN THE STRAIT AND NORTHWESTERLY
IN THE NORTHERNMOST INLAND WATERS. WIND ALL AREAS THROUGH PUGET
SOUND WILL TURN NORTHERLY THIS EVENING. AS TO STRENGTH OF WINDS...
THERE ARE PROBABLY SOME SMALL CRAFT ADVISORY WINDS OFF THE COAST AND
WILL MAINTAIN THAT ADVISORY INTO THE EVENING. EXPECTING THE REST OF
THE ZONES TO FALL BELOW 20 KT...IF NOT IMMEDIATELY THEN SOON.
HIGH PRES WILL BUILD BEHIND THE FRONT OVER CENTRAL BRITISH COLUMBIA
TONIGHT THEN SHIFT SOUTHEAST INTO EASTERN WASHINGTON BY WEDNESDAY.
WINDS WILL TURN MORE NORTHEASTERLY OVERNIGHT AND DIMINISH IN
STRENGTH.
SWELL ON THE COAST HAS SUBSIDED INTO THE 12-14 FT RANGE...AND IS
FORECAST TO FALL BELOW 10 FT ON TUESDAY.
MODELS DISAGREE ABOUT WHETHER ANOTHER COLD FRONT COULD MOVE
SOUTHEAST THROUGH THE WATERS LATER THURSDAY OR FRIDAY...SO WILL NOT
HIT THIS EVENT HARD FOR NOW. CHB
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE
PZ...SMALL CRAFT ADVISORY COAST.
SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.
$$
WWW.WEATHER.GOV/SEATTLE
FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.
