348
FXUS66 KSEW 242309
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
310 PM PST TUE NOV 24 2009
SYNOPSIS
A WEAK WARM FRONT OVER THE AREA WILL MOVE NORTH TONIGHT.
AN ASSOCIATED COLD FRONT WILL PUSH INLAND WEDNESDAY EVENING THEN
STALL OVER THE REGION. A LOW PRESSURE CENTER WILL MOVE UP ALONG THE
FRONT LATER THURSDAY INTO FRIDAY BRINGING MORE RAIN TO THE AREA.
ADDITIONAL FRONTAL SYSTEMS WILL BRUSH THE NORTHWEST THROUGH THE
WEEKEND WITH DRIER WEATHER ON TAP FOR NEXT WEEK.
SHORT TERM
A WEAK WARM FRONT OVER THE AREA WILL LIFT SLOWLY
NORTHWARD THIS EVENING. AS IT LIFTS NORTH...THE CLOUDS SHOULD BREAK
UP A BIT WITH A LIGHT SOUTHERLY BREEZE AND CONTINUED MILD
TEMPERATURES. EXPECTING THE LITTLE BIT OF RAIN THAT IS FALLING HERE
AND THERE TO BASICALLY COMING TO AN END LATER THIS EVENING EXCEPT
FOR FAR NORTHERN AREAS.
STILL EXPECTING DRY CONDITIONS INTERIOR AREAS WEDNESDAY MORNING WITH
JUST A CHANCE OF LIGHT RAIN ELSEWHERE. BEING SOLIDLY IN THE WARM
SECTOR...MAY HAVE A FEW BREAKS OF SUNSHINE IN THE MORNING INTERIOR
AREAS BEFORE THE CLOUDS FILL IN AGAIN. WILL BE A MILD DAY WITH THE
WARM AIR MASS IN PLACE. THEN AS THE COLD FRONT APPROACHES...THE
RAIN WILL REACH THE NORTH COAST BY NOON THEN SPREAD SE ACROSS THE
REGION THROUGH THE AFTERNOON.
THIS FRONT WILL MOVE INLAND OVERNIGHT THEN STALL OVER THE AREA WITH
THE RAIN BECOMING A BIT LIGHTER AFTER MIDNIGHT AS THE MAIN AREA OF
UPWARD FORCING MOVES EAST OF THE AREA. THEN A FRONTAL WAVE IS
EXPECTED TO DEVELOP ALONG THE FRONT OVER THE PACIFIC WELL SW OF HERE
AND THEN MOVE NE. THE EVOLUTION OF THIS FRONTAL WAVE IS NOT PINNED
DOWN AS TIGHT AS WOULD BE NICE AND A 50-100 MILE N-S DIFFERENCE WILL
MAKE A BIG DIFFERENCE.
CURRENT THINKING IS THAT IS WILL HAVE A PATH SIMILAR TO THE 12Z
GFS...ALTHOUGH SOMEWHAT STRONGER WITH THE CENTER OF CIRCULATION.
THAT PATH IS MOVING ONSHORE NEAR ASTORIA THEN MOVE NE FROM THERE
ACROSS THE CENTRAL CASCADES. THIS DIMINISHES THE THREAT OF STRONGER
WINDS...BUT PLACES MORE RAIN OVER THE AREA. IT WILL STILL BE A BIT
BREEZY ON THE BACK SIDE OF THIS SYSTEM...BUT THAT IS LOOKING MORE
LIKE IT WILL BE FRIDAY MORNING. SO FOR NOW WILL NOT GO WITH BIG
WIND PROBLEMS...BUT THIS NEEDS TO BE WATCHED AS 100 MILES FURTHER
NORTH MEANS WINDY AGAIN.
THEN FRIDAY THE THREAT OR RAIN DIMINISHES AS THE SYSTEM PULLS OFF TO
THE NORTHEAST. SNOW LEVELS DROP THURSDAY NIGHT INTO
FRIDAY...DROPPING BELOW ALL THE PASSES THURSDAY NIGHT WITH SNOW
THERE INTO FRIDAY BEFORE TAPERING OFF LATER IN THE DAY. PLAN
ACCORDINGLY IF YOU WILL BE TRAVERSING THE PASSES FRIDAY. CERNIGLIA
LONG TERM
THROUGH THE WEEKEND WE ARE EXPECTING TO HAVE A BROAD
LONGWAVE RIDGE BUILDING OVER THE REGION...HOWEVER THERE LOOKS AS
THOUGH THERE WILL BE ABUNDANT MOISTURE MOVING THROUGH THE RIDGE AND
ACROSS WASHINGTON. SO ALTHOUGH IT WILL BE MILD WITH LIFTING
FREEZING LEVELS...I CAN NOT REMOVE THE THREAT OF RAIN FROM THE
FORECAST FOR THE WEEKEND AS A RESULT OF ALL THE MOISTURE.
BEYOND THAT...THE LONGER RANGE MODELS ARE BEGINNING TO HINT AT A
PATTERN CHANGE BEGINNING WITH A SYSTEM DIVING SE ACROSS THE
NORTHWEST MONDAY NIGHT. IF THIS ACTUALLY OCCURS IT WOULD USHER IN A
PERIOD OF DRY WEATHER BY THE MIDDLE NEXT WEEK WITH SOME SUNSHINE.
BEING THAT MODELS AT THAT RANGE ARE SUSPICIOUS AT BEST WITH
TIMING...I LEFT THE SLIGHT CHANCE OF RAIN IN THE FORECAST FOR MONDAY
NIGHT AND TUESDAY. IF THIS TREND CONTINUES TOMORROW...I WILL LIKELY
MAKE SIGNIFICANT CHANGES TO THIS PERIOD OF THE FORECAST. CERNIGLIA
HYDROLOGY
THE EVOLUTION OF THE NEXT SYSTEM HAS CHANGED A
BIT...ENOUGH TO IMPACT THE RAIN FORECAST. THE CHANGE HAS BEEN TO BE
A BIT MORE PROGRESSIVE WITH THE APPROACHING COLD FRONT...BUT THEN
DRIVE THE SURFACE LOW UP THE FRONT A BIT FURTHER SOUTH THAN EARLIER
PROGGED. WHILE THIS LESSONS THE THREAT OF STRONG WINDS IT ALSO
AFFECTS THE PATTERN OF PRECIPITATION.
THERE IS NOW SOMEWHAT LESS OF AN IMPACT ON THE OLYMPICS AND MORE
ACROSS SOUTHWEST WASHINGTON THE THE CENTRAL AND SOUTHERN CASCADES.
FOR THE OLYMPICS...WHAT THIS WOULD MEAN IS STORM TOTALS MORE LIKE
2-3 INCHES RATHER THAN 3.5-4 INCHES. FOR THE CASCADES IT MEANS
INSTEAD OF 1-2 INCHES IT IS MORE LIKE 2-3 INCHES OVERALL. THIS IS
OVER A 24 HOUR PERIOD. NORMALLY THIS WOULD NOT POSE TOO MANY
PROBLEMS...BUT WITH RIVERS STILL DRAINING WATER FROM THE RECENT
EVENTS...THIS AMOUNT OF RAIN COULD CAUSE PROBLEMS FOR SOME OF THE
RIVERS DRAINING THE CENTRAL AND SOUTHERN CASCADES AND POSSIBLY THE
SKOKOMISH RIVER. THIS SYSTEM WILL HAVE A FAIR AMOUNT OF MOISTURE TO
WORK WITH SO PERIODS OF LOCALLY HEAVY RAIN IS POSSIBLY. THE ONE
BENEFIT IS THAT THE RAIN WILL COME IN TWO SEPARATE WAVES...ONE WITH
THE FRONT AND THE NEXT WITH THE SURFACE LOW TRAVELING UP THE FRONT.
THE BREAK BETWEEN THE TWO WILL HELP SMOOTH OUT THE FLOW PEAKS.
THIS PATTERN IS NOT GOOD FOR GENERATING SIGNIFICANT RAINS IN THE
GREEN RIVER BASIN HOWEVER SO NO FLOODING IS EXPECTED ON THE GREEN
RIVER DURING THE PERIOD.
I WILL BE ISSUING A FLOOD POTENTIAL OUTLOOK HIGHLIGHTING THE
POTENTIAL FOR FLOODING PROBLEMS ON SOME OF THE RIVERS IN WESTERN
WASHINGTON LATER THURSDAY INTO FRIDAY. CERNIGLIA
AVIATION
A RATHER WEAK UPPER LEVEL RIDGE WILL BE OVER W WA
THROUGH 12Z WEDNESDAY. THE LOWER AIR MASS REMAINS VERY MOIST AND
STABLE WITH WIDESPREAD IFR/MVFR. EXPECT WEAK WARM ADVECTION TO
CONTINUE THIS EVENING WHICH WILL MAKE IT HARD FOR CIGS TO LIFT. THE
FRONTAL SYSTEM OFFSHORE WILL GRADUALLY APPROACH THE AREA...WITH S
WINDS EXPECTED TO SLOWLY INCREASE. EXPECT CIGS TO RISE AS THE
INCREASING WINDS ALOFT PROMOTE MORE MIXING IN THE LOWER AIR
MASS...WHICH SHOULD ALLOW SOMEWHAT HIGHER MVFR CIGS TO FORM. SOME
GUIDANCE SUGGESTS HIGHER VFR CIGS OVERNIGHT...BUT THIS IS A TOUGHER
BET. COULD STILL SEE AREAS OF IFR REFORMING EARLY WEDNESDAY
MORNING...ESPECIALLY IF MORE OVERNIGHT LIFTING/CLEARING DEVELOPS
THAN ANTICIPATED.
KSEA...WILL KEEP CIGS LOW THROUGH MOST OF THE EVENING...WITH A
GRADUAL LIFT TO 010-020 BY LATE EVENING. VIS SHOULD GO UP FASTER.
COULD POSSIBLY SEE CIGS HIGHER THAN 030 LATER TONIGHT...BUT
REGARDLESS WILL DROP CIGS BACK TO IFR EARLY WEDNESDAY MORNING.
EXPECT SURFACE WINDS TO REMAIN LIGHT S. KAM
MARINE
A WAVE MOVING UP THE OFFSHORE FRONT SEEMS TO HAVE WEAKENED
THE SE PRESSURE GRADIENTS ACROSS W WA THIS AFTERNOON. REGARDLESS
GRADIENTS WILL BE STRENGTHENING AGAIN AS THE FRONT CONTINUES ITS
SLIDE DOWN THE B.C. COAST. HAVE SCAS UP IN ALL SE EXPOSED PLACES
INLAND IN ANTICIPATION OF THAT. HAVE OPTED TO GO WITH A GALE ON THE
COAST BASED ON THE WRF-GFS SOLUTION...SHOWING AN AREA OF 35 KT WINDS
OFF THE N COAST. THE SOUTH PORTION OF THE COASTAL WATERS WILL
PROBABLY TOP OUT AT 30 KT AT BEST. MODELS ALSO INDICATE THE
STRONGEST WINDS WILL BE LATE TONIGHT AND WEDNESDAY MORNING...BEFORE
GRADIENTS BEGIN WEAKENING IN THE AFTERNOON.
THOUGHT THAT THE MODELS HAD BEGUN TO CONVERGE ON A TRACK AND
STRENGTH FOR THE LOW APPROACHING THE AREA THURSDAY. WRONG. 18Z GFS
JUST IN...HAS FLIP-FLOPPED BACK CLOSER TO THE EARLIER SOLUTIONS
BRINGING THE LOW FURTHER NORTH ACROSS W WA...RATHER THAN NW OREGON.
FORECAST IS BASED ON THE MORE SOUTHERN TRACK FROM THE 12Z RUNS.
SYSTEM TIMING AND WINDS AHEAD OF THE LOW ARE IN QUESTION. ALL RUNS
SHOW STRONG ONSHORE FLOW BEHIND THE SYSTEM. THESE WILL BE THE
STRONGEST WINDS IN ANY SCENARIO. KAM
SEW WATCHES/WARNINGS/ADVISORIES
WA...NONE.
PZ...GALE WARNING COAST.
SMALL CRAFT ADVISORY...ENTRANCES TO THE STRAIT...
ADMIRALTY INLET...AND NORTH INLAND WATERS.
SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS AT 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.
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