605  
FXUS66 KSEW 222200  
GPHAFD  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SEATTLE WA  
300 PM PDT SAT JUL 22 2017  
   
SYNOPSIS
 
A WEAK WEATHER SYSTEM MOVING INTO WESTERN CANADA WILL  
INCREASE LOW LEVEL ONSHORE FLOW INTO WESTERN WASHINGTON TONIGHT FOR  
AN INCREASE IN MORNING CLOUDS SUNDAY. THEN HIGHER PRESSURE ALOFT IS  
FORECAST TO BUILD OVER THE REGION SUNDAY NIGHT THROUGH TUESDAY WITH  
DRY AND WARMING CONDITIONS. LOW LEVEL ONSHORE FLOW IS THEN EXPECTED  
TO INCREASE AGAIN RESULTING IN MODERATING HIGH TEMPERATURES CLOSER  
TO LATE JULY SEASONAL CONDITIONS FOR THE REST OF NEXT WEEK.  
 

 
   
SHORT TERM
 
THE UPPER LEVEL LOW OFF THE SOUTHEAST ALASKAN COAST  
(PIC0) IS EXPECTED TO CONTINUE MOVING INLAND THROUGH WESTERN CANADA  
THROUGH SUNDAY NIGHT. THIS PATTERN SHOULD ENCOURAGE STRONGER LOW  
LEVEL ONSHORE FLOW TONIGHT RESULTING IN MARINE CLOUDINESS BLANKETING  
MUCH OF WESTERN WASHINGTON SUNDAY MORNING. (PIC1) THE CLOUDS WILL  
DISSIPATE IN THE AFTERNOON IN MANY AREAS BUT PRODUCE COOLER HIGH  
TEMPERATURES THAN THOSE OF TODAY, PARTICULARLY IN THE INTERIOR.  
 
PROGS AGREE ON RISING HEIGHTS ALOFT FOR A WARMING AND CLEARING TREND  
MONDAY AND TUESDAY (PIC2) WITH HIGH TEMPERATURES RISING INTO THE 70S  
ALONG THE COAST AND 80S INLAND. BUEHNER  
   
LONG TERM
 
THE OVERALL LONG-WAVE PATTERN WITH THE UPPER RIDGE OVER  
THE INTERMOUNTAIN WEST AND THE TROUGH IN THE VICINITY OF 140-150W  
(PIC3) LOOKS TO CONTINUE THROUGH THE REST OF NEXT WEEK. THIS PATTERN  
LEAVES WESTERN WASHINGTON PRIMARILY IN A SW FLOW ALOFT. THE PROGS  
CONCUR ON A SEQUENCE OF WEAK SHORTWAVES ROTATING OUT OF THE OFFSHORE  
LONG-WAVE TROUGH WITH THE FIRST ARRIVING TUESDAY NIGHT AND  
WEDNESDAY. (PIC4) HEIGHTS ALOFT DECLINE AND LOW LEVEL ONSHORE FLOW  
INCREASES FOR ANOTHER SURGE INLAND OF MARINE CLOUDS WITH MODERATING  
HIGH TEMPERATURES. (PIC5)  
 
GUIDANCE CONTINUES TO HINT AT THE POSSIBILITY OF CONVECTION NEAR THE  
CASCADE CREST WEDNESDAY NIGHT. UPON COLLABORATION WITH NEIGHBORING  
FORECAST OFFICES, WILL INCREASE A THREAT OF THUNDERSTORMS THERE.  
 
PROGS DIFFER ON TIMING, BUT A FEW MORE EJECTING SHORTWAVES SHOULD  
MAINTAIN LOW LEVEL ONSHORE FLOW INTO THURSDAY AND FRIDAY FOR MORE OF  
THE MORNING CLOUD/AFTERNOON SUN REGIME WITH CLOSE TO SEASONAL  
TEMPERATURES. (PIC6) THE COMING WEEKEND MAY OFFER THE START OF A  
WARMING TREND AS HEIGHTS ALOFT RISE. NO RAIN IS IN SIGHT, SO THE DRY  
STREAK IN SEATTLE ON DAY 35 TODAY LOOKS TO EXTEND THROUGH AT LEAST  
THE COMING WEEK. BUEHNER  
 

 
   
AVIATION
 
A BROAD UPPER RIDGE OVER THE AREA WILL SHIFT INLAND WITH  
A WEAK UPPER TROUGH MOVING OVER WESTERN WASHINGTON TONIGHT AND  
SUNDAY (PIC11). THE FLOW ALOFT WILL BE WESTERLY. AT THE SURFACE,  
ONSHORE FLOW WILL CONTINUE WITH HIGH PRESSURE OFFSHORE AND LOWER  
PRESSURE INLAND (PIC12). THE AIR MASS IS STABLE. LOW AND MID LEVEL  
MOISTURE NEAR THE COAST WILL SPREAD INLAND TONIGHT. THE LOW LEVELS  
WILL REMAIN MOIST FOR MOST OF THE DAY ON SUNDAY.  
 
KSEA...MID CLOUDS OR VFR LOW CLOUDS SHOULD DEVELOP THIS EVENING,  
FOLLOWED BY MVFR LOW CLOUDS LATE TONIGHT. THE LOW CLOUDS WILL LIFT  
TO VFR SUNDAY AFTERNOON AND EVENTUALLY SCATTER LATE IN THE DAY.  
SOUTHWEST WIND 4-8 KNOTS SHOULD SWITCH TO NORTHWEST IN THE NEXT FEW  
HOURS AND REMAIN NORTHERLY INTO SUNDAY. SCHNEIDER  
 

 
   
MARINE
 
VARYING AMOUNT OF ONSHORE FLOW WILL PREVAIL FOR THE NEXT  
SEVERAL DAYS WITH HIGH PRESSURE OFFSHORE AND LOWER PRESSURE INLAND  
(PIC12). THE CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA SHOULD HAVE  
SMALL CRAFT ADVISORY WEST WINDS MOST EVENINGS. GALES FORCE WINDS ARE  
POSSIBLE TUESDAY EVENING. THE COASTAL WATERS WILL ALSO HAVE SMALL  
CRAFT ADVISORY NORTHWEST WINDS AT TIMES FOR THE NEXT FEW DAYS,  
ESPECIALLY THE OUTER WATERS. SCHNEIDER  
 

 
   
SEW WATCHES/WARNINGS/ADVISORIES
 
 
WA...NONE.  
PZ...SMALL CRAFT ADVISORY FROM 6 PM THIS EVENING TO 3 AM PDT SUNDAY  
FOR CENTRAL U.S. WATERS STRAIT OF JUAN DE FUCA-EAST  
ENTRANCE U.S. WATERS STRAIT OF JUAN DE FUCA.  
 
SMALL CRAFT ADVISORY FROM 6 AM SUNDAY TO 6 AM PDT MONDAY FOR  
COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND 10 TO 60  
NM-COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE 10  
TO 60 NM-COASTAL WATERS FROM POINT GRENVILLE TO CAPE  
SHOALWATER 10 TO 60 NM.  
 

 
 

 
 
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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