FXUS66 KSEW 041005
 AFDSEW
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE SEATTLE WA
 300 AM PDT SAT JUL 4 2009
 
 .SYNOPSIS...SUNNY AND WARM WEATHER WILL CONTINUE THROUGH THE HOLIDAY 
 WEEKEND...WITH AREAS OF MORNING LOW CLOUDS AND FOG CONFINED MAINLY 
 TO THE COAST. AN UPPER TROUGH WILL MOVE INTO THE REGION SUNDAY NIGHT 
 AND MONDAY FOR A TRANSITION TO COOLER WEATHER WITH A CHANCE OF 
 SHOWERS FOR NEXT NEXT WEEK.
 
 &&
 
 .SHORT TERM...AN UPPER LEVEL RIDGE REMAINS OVER THE PACIFIC 
 NORTHWEST WITH 500 MB HEIGHTS OVER WESTERN WASHINGTON AROUND 5800 
 METERS. THERE HAVE BEEN SOME CHANGES AT LOW LEVELS THAT WILL MAKE 
 THE FORECAST FOR THE NEXT COUPLE DAYS SLIGHTLY TRICKY. THE SURFACE 
 GRADIENTS TURNED WEAKLY ONSHORE FRIDAY EVENING AND A SHALLOW LAYER 
 OF COASTAL STRATUS HAS BEEN MOVING INLAND SLOWLY OVERNIGHT...MAINLY 
 THROUGH THE CHEHALIS GAP AND TO A LESSER EXTENT DOWN THE STRAIT OF 
 JUAN DE FUCA. 
 
 IN THE INTERIOR THE STRATUS WILL PROBABLY REMAIN LIMITED TO THE 
 SOUTHWEST PART OF THE PUGET SOUND REGION -- IE SHELTON AND PERHAPS 
 BREMERTON -- WITH ONLY PATCHY COVERAGE IN THE STRAIT AND NORTH 
 INTERIOR. IT SHOULD BURN OFF RELATIVELY QUICKLY. BUT MORE 
 IMPORTANTLY THE INFLUX OF SHALLOW MARINE AIR SHOULD HAVE AT LEAST A 
 MINOR EFFECT ON HIGH TEMPERATURES ACROSS THE INTERIOR LOWLANDS. IN 
 GENERAL THE MODELS HAVE UNDERESTIMATED THE STRENGTH OF ONSHORE 
 GRADIENTS. THE BOTTOM LINE IS THAT WE ARE LIKELY TO BE NOT QUITE AS 
 WARM AS FRIDAY ACROSS THE INTERIOR LOWLANDS.
 
 THE UPPER RIDGE AXIS WILL MOVE EAST TOWARD THE NORTHERN ROCKIES 
 TONIGHT...AND THE UPPER TROUGH CURRENTLY JUST OUTSIDE 140W WILL 
 APPROACH THE PACIFIC NORTHWEST COAST SUNDAY. LOW LEVEL ONSHORE FLOW 
 WILL PROBABLY REMAIN ONLY WEAK...BUT THE 500 MB HEIGHTS WILL FALL TO 
 AROUND 5750 METERS AS COOLER AIR MOVES IN ALOFT. TEMPERATURES OVER 
 THE LOWLANDS WILL BE SIMILAR OR ONLY SLIGHTLY COOLER ON SUNDAY...BUT 
 THE COOLER AIR ALOFT WITH THE TROUGH WILL DESTABILIZE THE AIR MASS 
 FOR A LOW CHANCE OF SHOWERS IN THE CASCADES.
 
 IT SHOULD BE MENTIONED THAT THE GUIDANCE IS NOT UNANIMOUS WITH THE 
 IDEA OF SLIGHT COOLING EACH DAY THIS WEEKEND. IN PARTICULAR THE NAM 
 MOS IS FORECASTING TEMPERATURES OVER THE INTERIOR TO BE ABOUT THE 
 SAME AS FRIDAY FOR BOTH DAYS THIS WEEKEND.
  
 THE MODELS REMAIN IN GENERAL AGREEMENT THAT THE UPPER TROUGH WILL 
 REACH THE AREA SUNDAY NIGHT. UPPER LEVEL HEIGHTS WILL CRASH 
 FURTHER...RESULTING IN A SUFFICIENT INCREASE IN ONSHORE FLOW FOR A 
 MODERATE TO STRONG ONSHORE PUSH. BY MONDAY THE PATTERN CHANGE WILL 
 BE MORE OR LESS COMPLETE -- AN UPPER TROUGH WILL BE SETTLING OVER 
 THE REGION AND LOW LEVEL FLOW WILL BE ONSHORE FOR COOLER WEATHER AND 
 A CHANCE OF SHOWERS.      
 
 MCDONNAL
 
 .LONG TERM...THE LONGER RANGE MODELS AGREE THAT THE UPPER TROUGH 
 WILL BE PARKED OVER THE REGION FOR THE EXTENDED FORECAST PERIOD 
 TUESDAY THOUGH FRIDAY. THE CURRENT BROAD BRUSH FORECAST LOOKS GOOD. 
 THERE WILL BE MORE CLOUDS THAN SUN ALONG WITH A CHANCE OF SHOWERS. 
 ONSHORE FLOW WILL CONTINUE WITH A FAIRLY STRONG MARINE INFLUENCE... 
 AND TEMPERATURES WILL BE A LITTLE BELOW NORMAL.
 
 MCDONNAL
 
 &&
 
 .AVIATION...AN UPPER RIDGE WILL REMAIN OVER THE REGION THRU TONIGHT. 
 INCREASING SW FLOW ALOFT. LIGHT ONSHORE FLOW TODAY WILL BECOME FLAT 
 OR WEAK OFFSHORE OVERNIGHT.
 
 STRATUS/FOG NOW BLANKETED THE COAST...AND HAS MADE IT AS FAR E AS 
 THE FAR S SOUND. THE DEPTH OF THE MARINE LAYER WAS FAIRLY SHALLOW OR 
 BETWEEN 1K AND 1500 FT DEEP. EXPECT THE LOW CLOUDS/FOG TO DISSIPATE 
 OR RETREAT OFFSHORE BETWEEN 18Z AND 20Z...SOONER OVER THE INTERIOR.
 
 .KSEA...CONTINUED VFR CONDITIONS. LOW CLOUDS/FOG SHOULD REMAIN WELL 
 S AND W OF THE TERMINAL. EXPECT LIGHT AND VARIABLE WINDS...BECMG NW 
 IN THE AFTERNOON.
 
 &&
 
 .MARINE...
 LIGHT ONSHORE FLOW WILL PREVAIL OVER THE AREA TODAY. THE FLOW WILL 
 BECOME WEAK OFFSHORE OVERNIGHT BUT THIS WILL BE BRIEF. EXPECT LIGHT 
 ONSHORE FLOW TO RETURN SUNDAY AFTERNOON...WITH A STRONG MARINE PUSH 
 OCCURING SUNDAY NIGHT. IT IS POSSIBLE THAT THERE WILL BE A BRIEF 
 PERIOD OF WIND GUSTS NEAR 35 KNOTS WITH THE PUSH AS IT SURGES NORTH
 ACROSS THE PUGET SOUND...ADMIRALTY INLET...AND THE NORTHERN INLAND 
 WATERS. ANTICIPATE MODERATE ONSHORE FLOW ACROSS THE AREA ON MON.  
 
 &&
 
 .SEW WATCHES/WARNINGS/ADVISORIES...
 WA...RED FLAG WARNING IN EFFECT FOR THE ENTIRE CWA UNTIL MIDNIGHT
      SUNDAY NIGHT.
 PZ...SMALL CRAFT ADVISORY FOR WINDS IN EFFECT FOR THE CENTRAL AND
      EASTERN STRAIT OF JUAN DE FUCA FROM 3 PM THIS AFTERNOON UNTIL
      MIDNIGHT TONIGHT.
 
 
 
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