124  
FXUS66 KSEW 212240  
AFDSEW  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SEATTLE WA  
340 PM PDT THU SEP 21 2017  
   
SYNOPSIS
 
AN OFFSHORE UPPER LEVEL RIDGE APPROACHING WESTERN  
WASHINGTON WILL BRING MORE SUNSHINE AND A LITTLE WARMING FRIDAY AND  
SATURDAY MORNING. THE WEAK RIDGE WILL REMAIN OVER THE REGION SUNDAY  
THROUGH TUESDAY BUT WILL PROBABLY ALLOW TRANSIENT WEAK WEATHER  
SYSTEMS TO BRING MORE CLOUDS AND SOME SPOTTY LIGHT RAIN TO MAINLY  
THE COAST AND NORTH PART OF WESTERN WASHINGTON. THE RIDGE WILL BUILD  
TUESDAY OR WEDNESDAY FOR MOSTLY SUNNY AND WARMER WEATHER.  
 

 
   
SHORT TERM
 
A LARGE UPPER LEVEL TROUGH CENTERED OVER E OREGON  
WILL DRIFT SLOWLY E TONIGHT ALLOWING AN OFFSHORE UPPER LEVEL RIDGE  
TO TILT EASTWARD OVER SOUTHERN B.C. DRIER N FLOW ALOFT AHEAD OF THE  
RIDGE HAS ALREADY ALLOWED CLOUDS TO DIMINISH OVER ESPECIALLY THE  
NORTH PART OF W WA. HOWEVER, RADAR IS STILL PICKING UP SCATTERED  
SHOWERS OVER MAINLY THE MOUNTAINS THIS AFTERNOON.  
 
CLEARING OVERNIGHT SHOULD SET UP A MOSTLY SUNNY DAY ON FRIDAY,  
HOWEVER THE AIR MASS WILL STILL BE A LITTLE COOLER THAN NORMAL WITH  
HIGHS IN THE 60S. THE UPPER LEVEL RIDGE WILL BE OVER SOUTHERN B.C.  
ON SATURDAY, BUT IT WILL BE WEAK ENOUGH TO ALLOW TRANSIENT WEATHER  
SYSTEMS TO PASS THROUGH THE RIDGE AND BRUSH MAINLY THE N PART OF W  
WA WITH SOME LIGHT RAIN. MID LEVEL CLOUDS FROM THE FIRST SYSTEM WILL  
PROBABLY SPREAD OVER THE AREA FROM THE NW SATURDAY MORNING, BUT THIS  
SYSTEM SHOULD BE PRETTY MUCH DRY. THE MODELS HAVE BEEN INCONSISTENT  
WITH THE SYSTEM ON SUNDAY, ESPECIALLY WITH THE COVERAGE OF LIGHT  
RAIN, RANGING FROM ZERO TO SOME SPOTTY LIGHT RAIN OVER THE N PART OF  
THE AREA. WHAT IS MORE CERTAIN IS THAT IT WILL BE CLOUDY WITH HIGHS  
REMAINING A LITTLE BELOW NORMAL. KAM  
   
LONG TERM
 
THE WEAK UPPER LEVEL RIDGE OVER SOUTHERN B.C. WILL  
REMAIN IN PLACE SUNDAY AND MONDAY, STILL WIDE OPEN TO WEAK TRANSIENT  
WEATHER SYSTEMS. THE GFS BRINGS ANOTHER SYSTEM MONDAY THROUGH MONDAY  
NIGHT WITH A BIT MORE COVERAGE OF LIGHT RAIN. THE ECMWF IS IN BETTER  
AGREEMENT BUT STILL A LITTLE SLOWER, BRINGING SOME LIGHT RAIN MONDAY  
AFTERNOON THROUGH TUESDAY MORNING. DESPITE THE BETTER MODEL  
AGREEMENT, CONFIDENCE IS STILL LOW.  
 
MODELS ARE IN BETTER AGREEMENT THAT THE UPPER LEVEL RIDGE WILL BUILD  
ENOUGH BY WEDNESDAY FOR MOSTLY SUNNY WEATHER WEDNESDAY AND  
THURSDAY WITH TEMPERATURES NEAR OR SLIGHTLY ABOVE NORMAL. KAM  
 

 
   
AVIATION
 
A BROAD UPPER LEVEL LOW OVER THE INTERIOR WEST WILL  
MIGRATE FURTHER EASTWARD THROUGH FRIDAY. HIGH PRESSURE CENTERED  
ALONG 140W WILL PERSIST, MAINTAINING LIGHT NORTHERLY FLOW ALOFT. THE  
AIR MASS WILL BECOME STABLE TONIGHT AND SOMEWHAT MOIST IN THE LOWER  
AND MID LEVELS. NORTHERLY FLOW WILL ALLOW THE AIR MASS TO BECOME  
MOSTLY DRY FRIDAY INTO SATURDAY EXCEPT AT THE SURFACE. PATCHY FOG OR  
LOW STRATUS IS POSSIBLE LATE TONIGHT AND FRIDAY MORNING, MAINLY IN  
THE MOST PRONE VALLEYS SUCH AS THE SOUTHWEST INTERIOR, LOWER  
CHEHALIS, COAST, AND RIVER VALLEYS. ANY FOG OR STATUS SHOULD MIX OUT  
BEFORE NOON. OTHERWISE VFR CONDITIONS.  
 
KSEA...LIGHT OR WEST-NORTHWEST WIND TO 5 KT BECOMING NORTHEAST TO 5  
KT THIS EVENING. SCATTERED CLOUDS TONIGHT. PATCHY LOW STRATUS OR FOG  
WILL DEVELOP IN PLACE AROUND PARTS OF THE CENTRAL/SOUTH SOUND. A FEW  
HOURS OF IFR CIGS/VIS MAINLY BETWEEN 12-16Z FRIDAY MORNING CANNOT BE  
RULED OUT (20-25 PERCENT CHANCE). MOST LIKELY, STRATUS OR FOG WILL  
JUST BE IN THE VICINITY OF THE KPAE/KSEA/KBFI TERMINALS AND WILL  
INDICATE THIS IN THE TAFS.  
 

 
   
MARINE
 
HIGH PRESSURE OVER THE NE PACIFIC WILL BUILD OVER THE  
WATERS LATER TONIGHT AND FRIDAY. WINDS SHOULD REMAIN FAIRLY LIGHT,  
LESS THAN 15 KT OR SO THROUGH THE WEEKEND. A WEAK FRONT COULD BRING  
A LITTLE INCREASE IN SOUTHERLY WIND MONDAY.  
 

 
   
SEW WATCHES/WARNINGS/ADVISORIES
 
 
WA...NONE.  
PZ...NONE.  

 
 

 
 
WWW.WEATHER.GOV/SEATTLE  
 
YOU CAN SEE AN ILLUSTRATED VERSION OF THIS DISCUSSION AT  
WWW.WRH.NOAA.GOV/SEW/GAFD/LATEST_WEBAFD.HTML  
 
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