AREA FORECAST DISCUSSION

 
 FXUS66 KSEW 232257
 AFDSEW
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE SEATTLE WA
 300 PM PST MON NOV 23 2009
 
 .SYNOPSIS...A COLD FRONT WILL MOVE INLAND TONIGHT BRINGING RAIN TO 
 MOST OF WESTERN WASHINGTON. WEAK HIGH PRESSURE WILL MOVE ACROSS THE 
 AREA TUESDAY BUT RESIDUAL MOISTURE MAY STILL PRODUCE SOME LIGHT RAIN 
 AT TIMES. A WEAK WARM FRONT WILL MOVE THROUGH THE AREA TUESDAY 
 NIGHT...FOLLOWED BY ANOTHER FRONTAL SYSTEM TO STALL ACROSS THE AREA 
 WEDNESDAY NIGHT THROUGH THANKSGIVING DAY. WEAK SYSTEMS WILL MOVE 
 ACROSS FRIDAY AND OVER NEXT WEEKEND FOR CONTINUED UNSETTLED WEATHER.
 
 &&
 
 .SHORT TERM...LATEST RADAR AND OBS INDICATE LIGHT TO MODERATE RAIN 
 IS SPREADING FROM THE COAST INTO THE INTERIOR AS EXPECTED. THE 
 LATEST IR IMAGE INDICATES PERHAPS A LITTLE BIT OF WEAKENING 
 BEGINNING TO OCCUR BUT THERE IS STILL ABUNDANT MOISTURE OFFSHORE YET 
 TO MOVE IN. MODELS STALL AND WEAKEN THE FRONTAL BAND OVER WRN 
 WA TONIGHT WITH THE SURFACE STRUCTURE WASHING OUT. THE LIGHTER 
 GRADIENTS AND RESIDUAL MOISTURE UNDER BUILDING UPPER HEIGHTS ARE A 
 RECIPE FOR LOW CLOUDS AND LIGHT RAIN/DRIZZLE. THIS MAY HANG IN MUCH 
 OF THE DAY TUESDAY AS MODELS MEANDER THE WARM AIR ADVECTION BOUNDARY 
 AROUND BEFORE LIFTING NWD TUE EVENING. THERE COULD BE A BREAK IN 
 RAIN TUESDAY NIGHT INTO WEDESDAY MORNING AS THE FLOW LIFTS NWD IN 
 RESPONSE TO THE NEXT TROUGH DIGGING OFFSHORE. WILL KEEP SOME LOW 
 POPS AROUND IN CASE THE ECMWF SOLUTION IS CORRECT WHICH DOES NOT 
 LIFT THE FLOW AS FAR N AND KEEPS SOME LIGHT QPF AROUND PARTS OF THE 
 AREA.
 
 THINGS BECOME MUCH MORE INTERESTING FROM LATER WEDNESDAY THROUGH 
 THANKSGIVING NIGHT. MODELS HAVE ALL MOSTLY TRENDED TOWARD A STRONGER 
 ELONGATED FRONTAL SYSTEM TO STALL ACROSS WRN WA DURING THE PERIOD. 
 THE NAM AND GFS BOTH SHOW A PERSISTENT BAND OF RAINFALL OVER THE 
 AREA...WITH HEAVIEST AMOUNTS OVER THE OLYMPIC PENINSULA AND N 
 CASCADES. THIS IS DIFFERENT THAN PREVIOUS MODEL SOLUTIONS WHICH 
 SHOWED A FRONTAL BAND SHEARING APART AND WEAKENING AS IT MOVED 
 ACROSS. PART OF THE CHANGE IS DUE TO THE NEW SECONDARY LOW 
 DEVELOPMENT BEING DEPICTED BY MODELS. AS THE TROUGH DIGS OFFSHORE ON 
 WEDNESDAY...MODELS SHOW A SURFACE LOW DEVELOPING AT THE BASE AND 
 DEEPENING AS IT TRACKS ALONG 40 N. THIS HOLDS THE FRONTAL BAND OVER 
 WA...PREVENTING IT FROM PROGRESSING THROUGH THE AREA. THE PROLONGED 
 RAINFALL COULD LEAD TO FLOOD CONCERNS BUT CONFIDENCE IS VERY LOW AT 
 THIS POINT. SEE THE HYDRO SECTION FOR DETAILS.  THE DEVELOPING 
 SURFACE LOW COULD ITSELF POSE SOME REAL FORECAST CHALLENGES. THE 
 NAM/GFS/UKMET ALL DEEPEN THE LOW AND TRACK IT SOMEWHERE INTO OREGON 
 OR EVEN NRN CA. THIS WOULD NOT POSE A RISK FOR HIGH WINDS IN WA. 
 HOWEVER...THE GFS TRACK COMES AWFULLY CLOSE AND A DEEPER/FURTHER 
 OFFSHORE SOLUTION COULD POSE PROBLEMS. THE ECMWF...ONE OF THE MORE 
 TRUSTED MODELS ACTUALLY SHOWS A HIGH WIND EVENT FOR WRN WA 
 THANKSGIVING NIGHT WITH A LOW TAKING A CLASSIC WIND STORM TRACK INTO 
 WA. AGAIN...THIS IS THE ONLY MODEL AND FIRST SOLUTION TO SHOW THIS 
 SO ITS SIMPLY SOMETHING TO KEEP AN EYE ON FOR NOW. 
 
 .LONG TERM...THE UPPER TROUGH MOVES ACROSS WRN WA ON FRIDAY WITH 
 STRONG POST FRONTAL ONSHORE FLOW AND SHOWERS ACROSS THE AREA. SNOW 
 LEVELS WILL FALL TO AROUND 3000-3500 FEET WITH THE STRONG FLOW 
 LIKELY DROPPING SOME SNOW IN THE PASSES. PRECIPITATION SHOULD 
 LIGHTEN UP FRIDAY NIGHT AS THE TROUGH SHIFTS EAST. MODELS BRING 
 ANOTHER SHORT WAVE ACROSS THE AREA ON SATURDAY WITH A CHANCE OF 
 LIGHT RAIN OR SHOWERS. SNOW LEVELS WILL LOW...DOWN TO MOST PASSES SO 
 A LITTLE MORE SNOW COULD BE POSSIBLE IN THE PASSES. RIDGING MAY 
 DEVELOP ON SUNDAY POSSIBLY LEADING TO A PERIOD OF DRY WEATHER BUT 
 THAT IS LONG WAY OFF. WILL KEEP SOME POPS IN THE SUNDAY/MONDAY 
 PERIOD FOR NOW.
 
 &&
 
 .HYDROLOGY...THE SKOKOMISH RIVER REMAINS JUST ABOVE FLOOD STAGE THIS 
 AFTERNOON. WATER HAS STOPPED RELEASING FROM CUSHMAN DAM #2 ON THE 
 NORTH FORK OF THE RIVER. THIS SHOULD ALLOW THE RIVER TO FALL BELOW 
 FLOOD STAGE THIS EVENING. THE WARNING REMAINS IN EFFECT THROUGH LATE 
 TONIGHT TO COVER RESIDUAL FLOODING AS THE RIVER FALLS. SEE THE 
 LATEST FLOOD STATEMENT FOR DETAILS.
 
 THE FRONT TONIGHT MAY BRING UP TO AN INCH OF RAIN TO THE OLYMPICS 
 WITH LESSER AMOUNTS ELSEWHERE. THIS SHOULD NOT BE ENOUGH TO BRING 
 RIVERS TO FLOOD...INCLUDING THE SKOKOMISH. HOWEVER...THE SKOKOMISH 
 WILL CONTINUE TO RUN HIGH...POSSIBLY JUST BELOW FLOOD STAGE THROUGH 
 TUESDAY GIVEN THE RENEWED RAINFALL TONIGHT.
 
 THE 12Z/18Z MODEL RUNS HAVE MOSTLY ALL TRENDED TOWARD A WETTER 
 SOLUTION FOR THE LATE WEDNESDAY THROUGH THANKSGIVING DAY SYSTEM. 
 PREVIOUS GFS SOLUTIONS HAD SHOWN A MORE PROGRESSIVE AND SPLIT TROUGH 
 TO MOVE THROUGH WHICH WOULD NOT HAVE LED TO FLOOD CONCERNS. THE NEW 
 NAM AND GFS NOW SHOW THIS ELONGATED TROUGH TO REMAIN INTACT WITH A 
 RATHER VIGOROUS FRONTAL BAND TO STALL ACROSS THE OLYMPIC PENINSULA 
 AND N CASCADES FROM WEDNESDAY AFTERNOON THROUGH THANKSGIVING DAY. 
 THE 24 HOUR QPF AMOUNTS VARY FROM 3-4 INCHES FROM THE NAM TO 2 OR 
 MAYBE 3 INCHES ON THE GFS. THESE AMOUNTS COULD CONCEIVABLY PUSH SOME 
 RIVERS CLOSE TO FLOOD STAGE...ESPECIALLY THE SKOKOMISH RIVER. 
 HOWEVER...THERE ARE A LOT OF THINGS GOING AGAINST FLOODING. FIRST 
 OFF...THE ECMWF KEEPS THE FRONTAL BAND HUNG UP ALONG THE FAR N WA 
 COAST AND S VANCOUVER ISLAND...BYPASSING THE N CASCADES AND POSSIBLY 
 THE OLYMPICS WITH THE HEAVIEST RAIN. ALSO...THE FLOW ALOFT IS WEAK 
 WITH ALMOST NO OROGRAPHIC CONTRIBUTION TO RAINFALL. SIMPLE DYNAMICS 
 WOULD HAVE TO DO THE TRICK. PW VALUES ARE NOT TOO IMPRESSIVE 
 EITHER...1.5 INCHES MAX IN THE PLUME. WITH ALL THIS SAID...I WILL 
 HOLD OFF ON ISSUING AN ESF BASES ON JUST THE MORNING RUNS. MAY NEED 
 TO ISSUE SOMETHING ON TUE IF MODELS CONTINUE TO TREND WET...STAY 
 TUNED TO THE LATEST FORECAST. IF FLOODING DOES DEVELOP IT WOULD NOT 
 OCCUR UNTIL LATER THANKSGIVING DAY INTO FRIDAY.
 
 NO FLOODING IS EXPECTED ON THE GREEN RIVER AT ANY TIME DURING THE 
 FORECAST PERIOD.
 
 &&
 
 .AVIATION...AN UPPER LEVEL TROUGH CROSSING SOUTHERN BRITISH COLUMBIA 
 WILL PUSH A WEAKENING COLD FRONT NEAR THE WA COAST AT 00Z ACROSS W 
 WA TONIGHT. THE FRONT WILL DISSIPATE OVER SW WA LATE TONIGHT. 
 COASTAL OBS HAVE DROPPED TO LOW END MVFR JUST AHEAD OF THE FRONT AND 
 EXPECT THIS LOW MOISTURE TO FILTER SOUTH ACROSS THE W WA INTERIOR 
 TONIGHT AS THE FRONT SAGS S...BEFORE DISSIPATING OVER S WA LATE 
 TONIGHT. EXPECT THE MOIST AIR MASS WITH MVFR AND MAYBE IFR CIGS TO 
 CONTINUE TUESDAY AS A WEAK WARM FRONT BRUSHES THE AREA. 
 
 KSEA...WILL STICK WITH THE TREND OF SLOWLY LOWERING CIGS 
 TONIGHT...THOUGH TIMING IS STILL IFFY. SHOULD BE MAINLY MVFR...BUT 
 WITH CIGS DROPPING BELOW BKN020 AROUND OR AFTER MIDNIGHT. MIGHT BE 
 SOME IFR CIGS AROUND TUESDAY MORNING. EXPECT SURFACE WINDS TO REMAIN 
 S THROUGH THIS EVENING. KAM
 
 &&
 
 .MARINE...LA PEROUSE BUOY 206 ON S VANCOUVER ISLAND SHIFTED NW 20Z 
 SO THE PER THE OB AND IR IMAGERY THE FRONT SHOULD BE ALMOST TO THE N 
 COAST. GRADIENTS ARE STILL UP ENOUGH TO PRODUCE HIGH END SCA WINDS 
 ALONG THE IMMEDIATE COAST AND CENTRAL PART OF THE NEW SALISH SEA /E 
 ENTRANCE AND N INLAND WATERS/. EXPECT GRADIENTS AND WINDS TO EASE 
 THIS EVENING...SO WILL END THE GALE WARNINGS BUT MAINTAIN SMALL 
 CRAFT ADVISORIES THROUGH MUCH OF THE EVENING. SEAS ARE UP HIGH 
 ENOUGH FOR SCA ALONG THE COAST AND W ENTRANCE. S-SE GRADIENTS RISE 
 AGAIN LATER TUESDAY AS THE NE PACIFIC LOW TAKES OVER AND ANOTHER 
 COLD FRONT APPROACHES SLOWLY FROM THE NW. WILL GO 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. KAM
 
 
 &&
 
 .SEW WATCHES/WARNINGS/ADVISORIES...
 WA...A FLOOD WARNING REMAINS IN EFFECT FOR THE SKOKOMISH RIVER       
      IN MASON COUNTY.    
 PZ...SMALL CRAFT ADVISORY COAST...ENTRANCES TO THE STRAIT...         
      ADMIRALTY INLET AND N INLAND WATERS. 
      SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS AT GRAYS          
      HARBOR BAR. 
 
 $$
 
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