000 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. $$ WWW.WEATHER.GOV/SEATTLE FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.