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AREA FORECAST DISCUSSION

 
 FXUS66 KSEW 241155
 AFDSEW
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE SEATTLE WA
 300 AM PST TUE NOV 24 2009
 
 .SYNOPSIS...A WEAK COLD FRONT OVER WESTERN WASHINGTON WILL EXIT TO 
 THE EAST TODAY WHILE A WEAK WARM FRONT ARRIVES FROM THE WEST...AND 
 MOST AREAS WILL RECEIVE A LITTLE RAIN. A WEAK COLD FRONT WILL SIT 
 OFF THE COAST TONIGHT AND WEDNESDAY...THEN BRING WETTER WEATHER 
 WEDNESDAY NIGHT THROUGH THURSDAY NIGHT AS IT STRENGTHENS AND MOVES 
 INLAND. COOL SHOWERY WEATHER WILL FOLLOW ON FRIDAY.
 
 &&
 
 .SHORT TERM...A WEAK COLD FRONT MOVED INTO WESTERN WASHINGTON LAST 
 EVENING...WHICH HAS BEEN WEAKENING FURTHER AS IT MOVES SLOWLY 
 EASTWARD ACROSS THE REGION. LIGHT RAIN CONTINUES TO FALL ACROSS MUCH 
 OF THE FORECAST AREA...GRADUALLY DECREASING FROM THE WEST. 
 TEMPERATURES ARE MILD -- MOSTLY IN THE MID 40S -- AND THE SNOW LEVEL 
 HAS CLIMBED WELL ABOVE 5000 FT. 
 
 AN UPPER LEVEL RIDGE CURRENTLY OVER THE OFFSHORE WATERS WILL SHIFT 
 INTO THE PACIFIC NORTHWEST THIS MORNING AND EAST OF THE CASCADES 
 LATE TODAY...ALLOWING THE SOUTHEAST END OF ANOTHER WARM FRONT 
 IMMEDIATELY UPSTREAM OF THE RIDGE TO REACH WESTERN WASHINGTON. 
 BETWEEN THE DEPARTURE OF THE COLD FRONT THIS MORNING AND THE ARRIVAL 
 OF THE WARM FRONT...MOST OF WESTERN WASHINGTON WILL GET A LITTLE BIT 
 OF RAIN TODAY. IN THE INTERIOR LOWLANDS IT WILL AMOUNT TO ONLY A FEW 
 HUNDREDTHS OF AN INCH. THE HEAVIEST AMOUNTS -- UP TO A HALF INCH -- 
 WILL FALL ALONG THE COAST AND OVER THE OLYMPIC PENINSULA.
 
 THE WARM FRONT WILL MOVE NORTHEAST INTO BRITISH COLUMBIA TONIGHT... 
 LEAVING A WEAK COLD FRONT OR BAROCLINIC BAND EXTENDING FROM 
 VANCOUVER ISLAND SOUTHWEST ACROSS THE OFFSHORE WATERS. THE FRONT 
 WILL MOVE SLOWLY EASTWARD INTO THE FORECAST AREA ON WEDNESDAY. IT 
 WILL BRING SOME RAIN TO THE AREA DURING THIS PERIOD...ESPECIALLY 
 ALONG THE COAST...OVER THE OLYMPIC PENINSULA...AND IN THE NORTHWEST 
 INTERIOR. THE FORECAST WILL CALL FOR A CHANCE OF RAIN FOR THE 
 WESTERN WASHINGTON INTERIOR LOWLANDS TONIGHT AND WEDNESDAY...MEANING 
 IT SHOULD REMAIN MOSTLY DRY FROM THE EVERETT AREA SOUTHWARD. AGAIN 
 THE ONLY AREA THAT WILL RECEIVE SIGNIFICANT PRECIPITATION WILL BE 
 THE COAST AND OLYMPICS.
 
 THE FORECAST FOR WEDNESDAY NIGHT THROUGH THURSDAY NIGHT...INCLUDING 
 THANKSGIVING DAY OF COURSE...IS LOOKING MORE INTERESTING -- AND MORE 
 UNCERTAIN. THE MODELS HAVE SHOWN A VARIETY OF SOLUTIONS FOR THIS 
 PERIOD OVER THE PAST FEW DAYS...BUT THEY ARE BEGINNING TO AGREE THAT 
 A FRONTAL WAVE WILL DEVELOP ALONG THE EXISTING BAROCLINIC BAND LATE 
 WEDNESDAY WHERE IT EXTENDS INTO THE WATERS WELL OFF THE COAST OF 
 NORTHERN CALIFORNIA. 
 
 HOW THIS WILL PLAY OUT IS STILL A BIG QUESTION -- IN FACT THERE IS A 
 FAIRLY BIG DIFFERENCE EVEN BETWEEN THE 00Z TUE SUITE OF MODELS AND 
 THE 06Z TUE RUN...WHICH JUST CAME IN OVER THE PAST COUPLE HOURS. 
 MOST MODEL RUNS HAVE SHOWN VARIOUS TAKES WITHIN THE GENERAL IDEA 
 THAT IT WILL BE RATHER WET AND MILD...THOUGH THE LATEST EUROPEAN 
 MODEL SHOWS A DEEP (IE COLD) UPPER LOW FOLLOWING THE FRONTAL WAVE 
 INTO WESTERN WASHINGTON LATE THURSDAY NIGHT. 
 
 AT THIS POINT I THINK WE CAN BE FAIRLY CONFIDENT THAT WEDNESDAY 
 NIGHT THROUGH THURSDAY NIGHT WILL BE WET...WITH SOMETHING LIKE 3 TO 
 4 INCHES OF PRECIPITATION FALLING OVER THE OLYMPIC MOUNTAINS AND 
 MAYBE THE NORTH CASCADES AS THE SNOW LEVEL FALLS FROM 7000 FT TO 
 4500 FT.
 
 A BIGGER QUESTION IS THE POTENTIAL FOR WIND. SOME MODEL SOLUTIONS 
 ...FOR EXAMPLE THE 00Z TUE GFS...HAVE SHOWN A DEVELOPING SURFACE LOW 
 WITH THE FRONTAL WAVE TRACKING NORTHEAST ACROSS WESTERN WASHINGTON 
 THURSDAY AFTERNOON AND EVENING. HOWEVER THE 06Z TUE GFS RUN SHOWS A 
 MARKEDLY DIFFERENT SCENARIO WITH THE LOW MOVING ACROSS WESTERN 
 OREGON. AT ANY RATE THERE IS AT LEAST A POSSIBILITY OF A STORMY 
 THANKSGIVING IN WESTERN WASHINGTON. I KNOW...HARD TO BELIEVE. 
 MCDONNAL 
 
 .LONG TERM...THE BEST BET IS THAT WE WILL HAVE COOL SHOWERY 
 CONDITIONS ON FRIDAY IN THE POST-FRONTAL AIR MASS. THERE IS NOT A 
 GREAT DEAL OF CONFIDENCE IN ANY DETAILS BEYOND FRIDAY THOUGH...AS 
 THE LONGER RANGE MODEL RUNS HAVE BEEN SOMEWHAT INCONSISTENT. BEST 
 BET IS THAT A DIRTY RIDGE WILL BE OVER THE REGION ALLOWING WEAK 
 SYSTEMS TO BRING A BIT OF RAIN TO THE AREA AT TIMES. MCDONNAL
 
 &&
 
 .HYDROLOGY...AS THE FORECAST STANDS...THE ONLY SIGNIFICANT THREAT OF 
 RIVER FLOODING IS FOR THE SKOKOMISH RIVER AS A RESULT OF RAIN THIS 
 WEDNESDAY NIGHT THROUGH THURSDAY NIGHT. DETAILS FOLLOW.
 
 THE WEAK SYSTEMS AFFECTING THE AREA TODAY AND TONIGHT WILL HAVE 
 LITTLE OR NO AFFECT ON WESTERN WASHINGTON RIVERS.
  
 PRECIPITATION WILL INCREASE OVER THE OLYMPIC MOUNTAINS WEDNESDAY 
 AFTERNOON AND...AS MENTIONED ABOVE...THE WEDNESDAY NIGHT THROUGH 
 THURSDAY NIGHT PERIOD WILL BE RATHER WET. IT LOOKS REASONABLE THAT 3 
 TO 4 INCHES OF PRECIPITATION WILL FALL OVER THE OLYMPIC MOUNTAINS 
 AND PERHAPS THE NORTH CASCADES DURING THAT 36 HOUR PERIOD WITH THE 
 SNOW LEVELS FAIRLY HIGH -- PROBABLY FALLING FROM 7000 FT TO AROUND 
 4500 FT DURING THE PERIOD. MOST LIKELY THAT AMOUNT OF RAIN WOULD 
 PRESENT A FLOOD THREAT ONLY FOR THE SKOKOMISH RIVER.
 
 A BIT LESS PRECIPITATION IS EXPECTED OVER THE CENTRAL CASCADES -- 
 LIKELY IN THE 1.5 TO 3 INCH RANGE. AT THE OUTSIDE THAT WOULD PRESENT 
 A FLOOD THREAT FOR ONLY THE MOST FLOOD-PRONE RIVER REACHES LIKE THE 
 PUYALLUP RIVER NEAR ORTING. 
 
 COOLER SHOWERY WEATHER WILL ARRIVE FRIDAY...AND THE WEATHER LOOKS 
 LESS WET IN THE EXTENDED PERIOD FOR SATURDAY AND BEYOND. 
 
 NO FLOODING IS EXPECTED ON THE GREEN RIVER DURING THE PERIOD. 
 
 &&
 
 .AVIATION...A WEAK UPPER LEVEL RIDGE WILL MOVE TO THE WA COAST 
 AROUND 18Z...AND ACROSS THE CASCADES BY 02Z. LOW AND MID LEVEL 
 MOISTURE WILL HANG AROUND THE W WA INTERIOR TODAY. EXPECT THE STABLE 
 AND MOIST AIR MASS TO KEEP MAINLY MVFR CIGS TODAY WITH SOME IFR THIS 
 MORNING ESP ALONG THE COAST AND CENTRAL SOUND. A WEAK WARM FRONT 
 WILL BRUSH THE AREA THIS EVENING KEEPING SOME MVFR CIGS AROUND. 
 
 KSEA...CIGS WILL BE MAINLY MVFR TODAY...BUT THE IFR CIGS WILL STAY 
 FOR SOME TIME THIS MORNING. EXPECT SURFACE WINDS TO REMAIN S THROUGH 
 THIS EVENING. 13
 
 &&
 
 .MARINE...SEAS ARE HIGH ENOUGH FOR SCA ALONG THE COAST AND W 
 ENTRANCE THIS MORNING. S-SE GRADIENTS RISE LATE THIS AFTERNOON AS 
 THE NE PACIFIC LOW TAKES OVER AND ANOTHER COLD FRONT APPROACHES 
 SLOWLY FROM THE NW. SCA WINDS EXPECTED COAST AND ENTRANCES THIS 
 EVENING THROUGH WED AFTERNOON. WILL STICK WITH THE GFS SOLUTION 
 BRINGING A LOW ACROSS THE AREA ON THURSDAY...THOUGH THERE IS STILL A 
 BIT OF DOUBT ABOUT JUST WHAT TRACK THE LOW WILL REALLY TAKE. STRONG 
 ONSHORE FLOW WILL FOLLOW THE LOW FOR GALES ON THE COAST AND THROUGH 
 THE STRAIT THU NIGHT. 
 
 &&
 
 .SEW WATCHES/WARNINGS/ADVISORIES...
 WA...NONE.
 
 PZ...SMALL CRAFT ADVISORY COAST...AND ENTRANCES TO THE STRAIT. 
      SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS AT GRAYS
       HARBOR BAR. 
    
 

 
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