000 FXUS66 KSEW 081732 AFDSEW AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SEATTLE WA 930 AM PST SUN NOV 8 2009 .SYNOPSIS...A MOIST AND UNSTABLE AIR MASS WILL CONTINUE TO SUPPORT SHOWERY WEATHER TODAY. A COLD FRONT WILL SPREAD RAIN AND WIND TO THE COAST LATE TONIGHT AND INLAND ON MONDAY. A LARGE UPPER LEVEL TROUGH WILL BRING SHOWERS TO THE AREA TUESDAY THROUGH THURSDAY AS IT CROSSES THE REGION. ANOTHER FRONTAL SYSTEM WILL REACH THE AREA FRIDAY OR SATURDAY. && .SHORT TERM...A MOIST AND UNSTABLE AIR MASS IS STILL IN PLACE OVER WRN WA THIS MORNING...THOUGH THE INSTABILITY ON THIS MORNINGS 12Z KUIL SOUNDING WAS WEAKER THAN WHAT WAS SHOWN ON THE PREVIOUS TWO MORNINGS. IN ADDITION...PRESSURE GRADIENTS HAVE BEEN TRENDING LESS SOUTHERLY AND LESS ONSHORE DURING THE PAST 12-18 HOURS. IN FACT...THE 16Z UIL-BLI GRADIENT WAS -2.3 MB...SO LOW-LEVEL FLOW IS ACTUALLY NOW TRENDING OFFSHORE. THESE FACTORS WILL MEAN A MUCH LESS VIGOROUS FIELD OF SHOWERS TODAY WITH NO MORE THUNDER EXPECTED. LOOKS LIKE THE SHORTWAVE ABOUT TO CROSS 150W WILL AMPLIFY THE LONGWAVE TROUGH OVER THE ERN PAC...TURNING OUR FLOW ALOFT MORE SOUTHERLY TODAY AND THIS EVENING. AS THE SHORTWAVE EXITS THE BASE OF THE LONGWAVE TROUGH THIS AFTERNOON...IT WILL ENERGIZE THE BAROCLINIC BOUNDARY DRAPED OUT ALONG 40N WITH CYCLOGENESIS TAKING PLACE. BOTH THE 12Z NAM AND GFS AGREE THAT BY 12Z MON...A DEEPENING 996 MB LOW WILL REACH THE CENTRAL VANCOUVER ISLAND COAST AS IT TRACKS N-NE. AS THE LOW PASSES BY...WINDY CONDITIONS WILL DEVELOP...MOST NOTABLY FOR THE COAST AND THE NORTH INTERIOR. AT ITS PEAK AROUND 12Z MON...THE GFS SHOWS 850 MB WINDS REACHING 70 KT ALONG THE NORTH COAST. THE NAM SHOWS 925 MB WINDS REACHING 55-65 KT ALONG THE NORTH COAST AND NORTH INTERIOR AROUND 12-15Z. THE NAM-BASED MARINE MOS GUIDANCE BRINGS SUSTAINED WIND SPEEDS OF 41-45 KT AT TATOOSH ISLAND DURING THIS TIME. BEST IDEA AT THIS TIME IS THAT WIND SPEEDS WILL FALL JUST SHY OF WARNING CRITERIA...BUT SUFFICE IT TO SAY THAT IT WILL BE A WINDY LATE NIGHT AND MONDAY MORNING FOR MANY. WIND ADVISORIES WILL LIKELY BE ISSUED FOR THE NORTH INTERIOR AND WESTERN STRAIT WITH THE AFTERNOON FORECAST PACKAGE. TRAILING THE DEEPENING LOW CENTER WILL BE A COLD FRONT...WHICH WILL CROSS WRN WA DURING THE DAY ON MON. IT WILL BE ACCOMPANIED BY RAIN AND FOLLOWED BY A DECREASE IN WIND. A SHORTWAVE RIDING UP IN SW FLOW ALOFT WILL KEEP THE RAIN GOING ON MON NIGHT. SNOW LEVELS SHOULD GENERALLY RUN NEAR 3500 FEET ON MON NIGHT FOLLOWING THE COLD FRONT. THE UPPER PATTERN WILL AGAIN AMPLIFY ON TUE. THIS TIME...THE WRN U.S. UPPER RIDGE WILL TRY NUDGING INTO WRN WA...SUPPORTING ON TUE NIGHT THE BEST CHANCE FOR SOME DRY WEATHER THROUGH MID-WEEK. THE ERN PAC TROUGH WILL THEN START TO MARCH EAST...PUSHING YET ANOTHER COLD FRONT THROUGH WRN WA ON WED WITH ANOTHER ROUND OF RAIN. AN UPPER TROUGH AXIS SHOULD BE NEARBY LATER ON WED AND WED NIGHT. THE ACCOMPANYING COLD AIR ALOFT AND INSTABILITY SHOULD SUPPORT SHOWERY WEATHER THROUGH AT LEAST WED NIGHT...POSSIBLY INTO THU. HANER .LONG TERM...FROM PREVIOUS DISCUSSION...GFS AND ECMWF AGREE ON BRINGING AN UPPER LEVEL TROUGH AXIS ACROSS THE AREA WEDNESDAY NIGHT...WITH A MORE ORGANIZED BAND OF SHOWERS OR RAIN MOVING OVER THE AREA JUST AHEAD OF THE TROUGH AXIS ON WEDNESDAY. SHOWERS LIKELY FORECAST WORKS FINE FOR NOW. SNOW LEVELS SHOULD DROP DOWN BELOW PASS LEVELS WEDNESDAY NIGHT...BUT SHOWER ACTIVITY SHOULD BE DECREASING AT THAT POINT. GFS/ECMWF SOLUTIONS DIVERGE SIGNIFICANTLY AFTER THURSDAY...SO THE FRIDAY/SATURDAY SYSTEM REMAINS IN DOUBT. WILL LEAVE THE FORECASTS AS IS FOR NOW SINCE CONFIDENCE IN THE MODELS IS LOW. KAM && .HYDROLOGY...NO SIGNIFICANT FLOODING IS EXPECTED IN WESTERN WASHINGTON DURING THE NEXT WEEK. THIS INCLUDES THE GREEN RIVER. NO SIGNIFICANT FRONTAL SYSTEMS ARE EXPECTED THIS WEEK. MONDAYS FRONT WILL BRING SOME BRIEF MODERATE RAIN...WITH SNOW LEVELS AROUND 4500 OR 5000 FEET. THE PRECIPITATION BAND THAT IS FORECAST TO FORM AHEAD OF THE UPPER LEVEL TROUGH ON WEDNESDAY MAY BRING SOME BRIEF MODERATE SHOWERS TO THE AREA...BUT SNOW LEVELS WILL BE DOWN TO 3000 TO 3500 FEET. MODELS ARE HAVING TROUBLE IN THE LONG TERM WITH THE FRIDAY OR SATURDAY FRONTAL SYSTEM...BUT SO FAR ALL THE SOLUTIONS INDICATE A PROGRESSIVE SYSTEM THAT ONLY BRINGS A RELATIVELY SHORT PERIOD OF MODERATE TO HEAVY PRECIPITATION. IN THE EXTENDED WAY OUT PERIOD...THE GFS HEMISPHERIC 5-WAVE CHARTS SHOW A MEAN LONG WAVE TROUGH OUT NEAR 150W OR SO WHICH WOULD MAINTAIN THE CURRENT PROGRESSIVE PATTERN OF WEATHER SYSTEMS CROSSING THE AREA...BUT NOT LINGERING TO PRODUCE LENGTHY PERIODS OF RAIN. KAM && .AVIATION...BASIC PATTERN HAS CHANGED LITTLE...WEST-SOUTHWEST FLOW ALOFT WITH A DEVELOPING LOW PRESSURE CENTER OFFSHORE. AIR MASS IS MOIST AND SLIGHTLY UNSTABLE. SCATTERED SHOWERS WILL CONTINUE. THIS MORNING THE SHOWERS SEEM TO BE CONCENTRATED IN SOUTHERN AREAS. CEILINGS AGAIN RANGE FROM NO CEILING TO MVFR IN OR NEAR SHOWERS. AFTER 00Z HIGH AND MID LEVEL CLOUDS WILL INCREASE AS NEXT SYSTEM BEGINS TO AFFECT THE AREA. KSEA...PARAGRAPH ABOVE APPLIES. SOUTH WIND 5-10 KT WITH HIGHER GUSTS IN SHOWERS. WIND WILL TURN SOUTHEAST AND INCREASE TO 10-15 KT AFTER 03Z. CHB && .MARINE...NO REASON TO DOUBT MODELS WHICH SPIN UP A DEEP LOW SOUTHWEST OF WASHINGTON AND BRING IT NORTHEAST THEN NORTH ALONG THE COAST LATE TONIGHT. THIS WILL BRING GALES TO MOST WATERS AND HAVE CONVERTED THE REMAINING GALE WATCHES TO WARNINGS. WIND IS SURPRISINGLY STRONG THIS MORNING SO HAVE SIMPLIFIED MATTERS BY HAVING THE GALE WARNED ZONES HAVE SMALL CRAFT ADVISORIES UNTIL THE GALE WARNING GOES INTO EFFECT TONIGHT. MODELS HAVE BACKED OFF A BIT ON THE PEAK STRENGTH OF THE SYSTEM...NO 50 KT SURFACE WINDS AROUND TATOOSH. WILL INSPECT THE 12Z MM5-GFS CLOSELY. ANOTHER IMPULSE MOVES THROUGH THE FLOW MONDAY NIGHT...WITH ANOTHER FRONT WEDNESDAY. PATTERN IS QUITE PROGRESSIVE AND TIMING DETAILS LIKELY TO CHANGE. AFTER WEDNESDAY SPLIT FLOW APPEARS TO DEVELOP WHICH KEEPS WASHINGTON FAIRLY DRY. CHB && .SEW WATCHES/WARNINGS/ADVISORIES... WA...NONE. PZ...GALE WARNING COAST...ENTRANCES...ADMIRALTY INLET...CAMANO ISLAND TO POINT ROBERTS TONIGHT AND MONDAY. SMALL CRAFT ADVISORY SAME ZONES TODAY. $$ WWW.WEATHER.GOV/SEATTLE FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.