AREA FORECAST DISCUSSION

 
 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. 
     
 

 
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