FXUS66 KSEW 022250 CCA
AFDSEW
AREA FORECAST DISCUSSION...CORRECTION
NATIONAL WEATHER SERVICE SEATTLE WA
ISSUED BY NATIONAL WEATHER SERVICE PORTLAND OR
245 PM PST WED DEC 2 2009
CORRECTION FOR WARNING/ADVISORY SECTION
.SYNOPSIS...NORTH TO NORTHEAST LOW-LEVEL FLOW WILL CONTINUE THROUGH
THURSDAY. A WEAKENING WEATHER SYSTEM WILL APPROACH THE FORECAST FROM
THE NORTHWEST LATE THURSDAY AND THURSDAY NIGHT...WHICH WILL WEAKEN
THE OFFSHORE GRADIENT. A REINFORCING LOW PRESSURE SYSTEM WILL DROP
SOUTH OUT OF CANADA FRIDAY. A COLDER AIR MASS WILL SETTLE OVER THE
REGION THIS WEEKEND. AN UPPER LOW IS EXPECTED TO SETTLE OVER
NORTHWEST WASHINGTON SATURDAY NIGHT AND THEN SLIDE SOUTH INTO
SOUTHWEST OREGON BY SUNDAY AFTERNOON. IT LOOKS LIKE COLD NORTH FLOW
ALOFT WILL RE-DEVELOP EARLY NEXT WEEK AND PERSIST THROUGH AT LEAST
THE MIDDLE OF THE WEEK.
&&
.SHORT TERM...VISIBLE SATELLITE IMAGERY SHOWS CLEAR SKIES OVER THE
FORECAST AREA THIS AFTERNOON. THE AIR MASS HAS DRIED CONSIDERABLY
OVER THE PAST 24 TO 36 HOURS WITH DEW POINTS IN THE TEENS AND 20S
DUE TO NORTHEAST LOW-LEVEL FLOW. THE KSEA-KYKM GRADIENT WAS RUNNING
CLOSE TO -5.0 MB AS OF 21Z...WHICH IS NOT TOO OVERLY STRONG. LATEST
MODELS MAINTAIN THIS NORTHEAST TO EAST LOW-LEVEL FLOW TONIGHT WHICH
SHOULD KEEP VALLEY FOG TO A MINIMUM. MADE SOME MINOR MIN TEMP
ADJUSTMENTS FOR TONIGHT...ESPECIALLY IN SOUTHWEST INTERIOR VALLEYS
THAT ARE MORE WIND-PROTECTED.
THE OFFSHORE OR NORTH FLOW STARTS TO WEAKEN LATE THU AS THE NEXT
WEAK WEATHER SYSTEM APPROACHES FROM THE NORTHWEST. SHOULD SEE A
LITTLE MORE FOG COVERAGE THU NIGHT WITH WEAKER SURFACE GRADIENTS.
THIS NEXT SYSTEM WILL BE AN INSIDE-SLIDER. THE LATEST GFS HAS THE
UPPER LOW OVER THE EXTREME NORTHERN IDAHO PANHANDLE 18Z FRI WHILE
THE NAM PUTS IT OVER SE BRITISH COLUMBIA. MOISTURE LOOKS TO BE
RATHER LIMITED WITH THIS SYSTEM. THUS...HAVE TRIMMED BACK POPS FOR
FRI AFTERNOON AND FRI NIGHT. COOLED FRI MAX TEMPS A COUPLE OF
DEGREES.
THE UPPER RIDGE RE-AMPLIFIES NEAR 140W FRI NIGHT RESULTING IN
STRONGER NORTH FLOW ALOFT TO SPREAD OVER THE AREA. 12Z GFS INDICATES
1000-500 MB THICKNESSES OF 525-530 DM 18Z SAT. A REINFORCING SHOT OF
COLDER AIR IS EXPECTED TO ENTER NORTHWEST WA LATE SAT OR SAT
EVENING. BY 06Z SUN THICKNESSES DROP TO BELOW 520 DM WITH 850 MB
TEMPS CLOSE TO -10C. WEISHAAR
.LONG TERM...STILL A LOT OF UNCERTAINTY FROM SUNDAY THROUGH THE REST
OF THE EXTENDED PERIOD. THE ECMWF AND GFS RETAIN AN UPPER LOW OFF
THE CENTRAL OREGON COAST SUN WITH ANOTHER ONE SOMEWHERE NEAR THE
IDAHO AND OREGON BORDER. ANY PRECIPITATION THAT FALLS SUN WOULD
LIKELY BE AS SNOW...EVEN DOWN TO SEA LEVEL. HAVE NOT MADE ANY
CHANGES TO THE SUN GRIDS DUE TO THE DEGREE OF UNCERTAINTY. THE ECMWF
AND GFS THEN CONSOLIDATE BOTH LOWS AND RE-CENTER IT OFF THE SOUTH
OREGON COAST 12Z MON. PRECIP CHANCES REMAIN LIMITED DUE TO THE
NORTHEAST TO EAST FLOW COMPONENT. THINGS GET MORE COMPLICATED TUE
AND WED OF NEXT WEEK. IT LOOKS LIKE THE NORTHERN JET STREAM WILL
DOMINATE AT THAT TIME WITH SOME SORT OF ARCTIC BOUNDARY EXTENDING
FROM THE SOUTHERN B.C. COAST THROUGH NORTHERN WASHINGTON. THE GFS
HAS IT A BIT MORE TO THE NE WED COMPARED TO THE ECMWF. MODELS THEN
DEVELOP AN OVER-RUNNING SITUATION WED...BUT THIS APPEARS TO BE MORE
OF AN OREGON PROBLEM. IN ANY EVENT...SOMETHING TO KEEP AN EYE ON.
HOPEFULLY THE MODELS WILL COME INTO BETTER AGREEMENT. WEISHAAR
&&
.HYDROLOGY...THERE IS NO THREAT OF RIVER FLOODING IN THE FORECAST
AREA FOR THE NEXT WEEK. THIS INCLUDES THE GREEN RIVER.
NONE OF THE MODELS INDICATES SIGNIFICANT...LET ALONE FLOOD-PRODUCING
PRECIPITATION DURING THE PERIOD. MCDONNAL
&&
.AVIATION...THE COMBINATION OF AN UPPER LEVEL RIDGE AND MODERATE
OFFSHORE FLOW AFT THE SURFACE IS PROVIDING CLEAR SKIES ACROSS THE
REGION TODAY. THE OFFSHORE FLOW WILL CONTINUE THROUGH THURSDAY WITH
SKIES REMAINING CLEAR. PATCHES OF FOG ARE EXPECTED TO FORM AGAIN
LATE TONIGHT IN THE MOST FOG PRONE AREAS.
KSEA...CLEAR SAILING THROUGH THURSDAY WITH A GENERAL NORTHERLY WIND
CRANKING AROUND TO NORTHEAST AFT 06Z-08Z TONIGHT.
&&
.MARINE...HIGH PRES OVER SE BRITISH COLUMBIA WILL SAG SLOWLY SE INTO
MONTANA AND EASTERN WA ON THU. THIS WILL MAINTAIN THE OFFSHORE FLOW
THROUGH THE PERIOD. A FRONT WILL SLIDE S THROUGH THE AREA FRI AS AND
DISSIPATE. THIS WILL SERVE TO ENHANCE THE OFFSHORE FLOW THROUGH
THE WEEKEND WITH POSSIBLE SCA TO GALE NE WINDS ACROSS PORTIONS OF
THE NORTHER PUGET SOUND WATERS AND PARTS OF THE STRAIT. TW
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY FOR SOUTH PUGET SOUND UNTIL 9PM.
$$
WWW.WEATHER.GOV/SEATTLE
FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.
