AREA FORECAST DISCUSSION

 
 FXUS66 KSEW 161028
 AFDSEW
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE SEATTLE WA
 330 AM PDT MON MAR 16 2009
 
 .SYNOPSIS...A LARGE UPPER LEVEL TROUGH OVER THE NORTHEAST PACIFIC 
 WILL STEER MORE COOL AND SHOWERY WEATHER ACROSS WESTERN WASHINGTON 
 TODAY AND TUESDAY. ANOTHER MOIST PACIFIC WEATHER SYSTEM WILL MOVE 
 OVER THE AREA TUESDAY NIGHT...WITH RAIN OVER THE LOWLAND AND 
 MOUNTAIN SNOW CONTINUING THROUGH WEDNESDAY NIGHT...FOLLOWED BY 
 SHOWERS THURSDAY. COOL SHOWERY WEATHER WILL CONTINUE THROUGH THE 
 WEEKEND AS THE OFFSHORE TROUGH APPROACHES THE AREA FRIDAY...THEN 
 MOVES INLAND SATURDAY.
 
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 .SHORT TERM...SPRING HAS SPRUNG. OK...EARLY SPRING...SINCE TREE 
 POLLENS ARE HIGH AND WE ARE STILL GETTING SNOW MIXED IN WITH 
 SHOWERS. THE CURRENT WEATHER PATTERN IS ALSO TYPICAL OF EARLY 
 SPRING...A LARGE UPPER LEVEL TROUGH IS OFFSHORE OVER THE NE 
 PACIFIC...AIMING STRONG SW FLOW ALOFT OVER W WA. IR SATELLITE IMAGE 
 TELLS THE STORY FOR THE NEXT TWO DAYS...THE UNSTABLE SHOWERY AIR 
 MASS AFFECTING WESTERN WA STRETCHES FOR AROUND 1500 MILES WESTWARD 
 ACROSS THE NE PACIFIC. 
 
 WILL KEEP LIKELY TO CATEGORICAL POPS ACROSS THE AREA THROUGH TUESDAY 
 FOR SHOWERS. SW FLOW ALOFT AT 850 MB IS IN THE 20-35KT RANGE TODAY 
 ON THE NAM AND WRF-GFS...A LITTLE WEAKER IN THE GFS. THIS SUPPORTS 
 THE NAM IDEA OF SNOW ADVISORY AMOUNTS OF SNOW OVER THE MOUNTAINS 
 TODAY. WILL EXTEND THE WINTER WEATHER ADVISORY THROUGH 6 PM FOR 
 ANOTHER 4-10 INCHES OF NEW SNOW. 
 
 MODELS SHOW 850 MB TEMPERATURES COOLING LATE TONIGHT AND EARLY 
 TUESDAY...ENOUGH TO DROP FREEZING LEVELS TO 1500 TO 2500 FEET. THIS 
 IS LOW ENOUGH SO THAT HEAVIER SHOWERS MAY BRING SOME SNOW DOWN TO 
 THE SURFACE OVER THE LOWLANDS. SO WILL MENTION POSSIBLE SNOW MIXED 
 IN THE SHOWERS TONIGHT AND TUESDAY. 
 
 GFS/ECMWF BOTH CARRY ABOUT THE SAME IDEA FOR TUESDAY NIGHT THROUGH 
 WEDNESDAY NIGHT. A FRONTAL SYSTEM DEVELOPING OUT NEAR 165W SHOULD 
 BEGIN SPREADING RAIN OVER W WA TUESDAY NIGHT. THE COLD FRONT BECOMES 
 ROUGHLY PARALLEL TO THE SW FLOW IN THE JET STREAM...KEEPING FRONTAL 
 MOISTURE PUMPING UP OVER THE AREA THROUGH WEDNESDAY NIGHT. THIS WILL 
 BE A WET PERIOD WITH THE GFS QPF SHOWING ABOUT 3 INCHES IN 36 HOURS 
 FOR THE OLYMPICS. THIS ISN`T SUPER WET...BUT WILL HAVE TO KEEP AN 
 EYE ON THIS WITH REGARD TO POTENTIAL FLOODING. KAM
 
 .LONG TERM...BOTH GFS/ECMWF BEGIN SHIFTING THE BIG OFFSHORE UPPER 
 LEVEL TROUGH EASTWARD BY ABOUT FRIDAY...EVENTUALLY MOVING IT ACROSS 
 W WA EITHER SATURDAY OR SUNDAY. BOTTOM LINE...THE WET PATTERN WILL 
 CONTINUE. KAM
 
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 .AVIATION...COLD UNSTABLE AIR MASS REMAINING OVER WESTERN WASHINGTON 
 THROUGH TODAY. PLENTY OF SHOWERS OFF THE COAST ON THE SATELLITE 
 IMAGERY AND WITH THE WEST SOUTHWESTERLY FLOW ALOFT THESE SHOWERS 
 WILL MOVE ACROSS THE AREA TODAY. WEAK SURFACE TROUGH MOVING THROUGH 
 ALONG WITH SOME DAYTIME HEATING WILL HELP TO ENHANCE THE SHOWER 
 ACTIVITY IN THE AFTERNOON AND EARLY EVENING HOURS. CEILINGS WILL BE 
 QUITE VARIABLE WITH CIGS IN THE VICINITY OF SHOWERS IN THE 2000 TO 
 3000 FOOT RANGE WHILE OUTSIDE OF THE SHOWER ACTIVITY CONDITIONS 5000 
 SCATTERED TO BROKEN. VISIBILITY COULD DROP DOWN TO NEAR 3SM IN THE 
 HEAVIER SHOWERS. 
 
 KSEA...LOTS OF TEMPO GROUPS IN THE TAF TODAY WITH SHOWERS MOVING 
 OVER THE AIRPORT THROUGH THE EVENING HOURS. CEILINGS NEAR 2000 FEET 
 WITH THE SHOWERS AND NEAR 5000 FEET DURING THE LONGER BREAKS IN THE 
 SHOWER ACTIVITY. SOUTHWESTERLY WINDS 5 TO 10 KTS WITH GUSTS NEAR 20 
 KNOTS BEGINNING LATER THIS MORNING WITH SHOWER ACTIVITY. FELTON
 
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 .MARINE...PRESSURE GRADIENTS TRENDING DOWN THIS MORNING IN THE WAKE 
 OF THE STRONG FRONT THAT MOVE THROUGH THE AREA SUNDAY. SMALL CRAFT 
 ADVISORIES FOR THE INLAND WATERS WILL COME TO AN END BY EARLY THIS 
 AFTERNOON. SMALL CRAFT ADVISORY FOR WINDS AT THE WEST ENTRANCE WILL 
 ALSO COME TO AN END THIS AFTERNOON BUT A SMALL CRAFT WILL REMAIN IN 
 EFFECT DUE TO THE HAZARDOUS SEAS. SWELL ALONG THE COAST AND AT THE 
 WEST ENTRANCE IN THE 14 TO 18 FOOT RANGE WILL VERY SLOWLY SUBSIDE 
 TODAY THROUGH TUESDAY BUT STILL BE IN DOUBLE DIGITS TUESDAY 
 AFTERNOON. ANOTHER FRONTAL SYSTEM WILL STALL OVER THE AREA ON 
 WEDNESDAY. THIS SYSTEM AT THIS POINT LOOKS TO BE WEAKER THAN THE 
 SYSTEM THAT MOVED THROUGH ON SUNDAY. FELTON  
 
 &&
 
 .SEW WATCHES/WARNINGS/ADVISORIES... 
 WA...WINTER WEATHER ADVISORY FOR THE OLYMPICS AND CASCADES TODAY.
 
 PZ...SMALL CRAFT ADVISORY COAST...WEST ENTRANCE...NORTHERN INLAND    
      AND PUGET SOUND AND HOOD CANAL.
      SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS GRAYS HARBOR BAR.
           
 $$
 
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