382  
FXUS66 KSEW 181714  
AFDSEW  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SEATTLE WA  
915 AM PST SAT NOV 18 2017  
   
SYNOPSIS
 
WEAK HIGH PRESSURE WILL GIVE WAY TO A VIGOROUS FRONT ON  
SUNDAY. WEAK HIGH PRESSURE WILL BE OVER THE AREA MONDAY. A STRONG  
WARM FRONT WILL ARRIVE TUESDAY. THE MIDDLE OF THE WEEK WILL BE MILD,  
RAINY, AND PERHAPS WINDY AT TIMES.  
 

 
   
SHORT TERM
 
HIGH PRESSURE IS CENTERED OVER IDAHO AND A FRONTAL  
SYSTEM IS HUNG UP AROUND QUEEN CHARLOTTE SOUND. THE FRONT WILL SHIFT  
SOUTHEAST INTO WESTERN WASHINGTON SUNDAY AFTERNOON AND EVENING.  
WINDS ARE LIKELY TO BE STRONG ENOUGH FOR WIND ADVISORIES IN THE  
TYPICALLY WINDY SPOTS WHICH ARE THE COAST AND FOR THE ADMIRALTY  
INLET/WHIDBEY ISLAND AREA NORTH THROUGH THE SAN JUANS AND OVER  
THROUGH BELLINGHAM BAY. THE 12Z GFS SHOWS A PEAK PDX-BLI GRADIENT OF  
+11MB EARLY SUNDAY AFTERNOON WITH THE STRONGEST GRADIENT AROUND  
ADMIRALTY INLET AND WHIDBEY ISLAND.  
 
THERE WILL BE HEAVY RAIN IN THE OLYMPICS AND OF COURSE A DECENT RAIN  
SHADOW IN THE LEE OF THE OLYMPICS. THE NORTH CASCADES OUGHT TO GET  
SOME HEAVY WET SNOW--AND PERHAPS PARADISE ON MOUNT RAINIER AND THE  
HIGHER PEAKS WILL GET HEAVY WET SNOW--BUT IT COULD EASILY CHANGE  
OVER TO RAIN FOR AWHILE. THE FRONT WILL STILL BE HUNG UP OVER  
SOUTHWEST WASHINGTON AND THE CASCADES THROUGH SUNDAY NIGHT--THAT IS  
SLOWER THAN PREVIOUS RUNS. A 1001MB LOW RIDES UP THE FRONT SUNDAY  
EVENING--BUT THE UW WRFGFS SHOWS THE FROPA WIND SHIFT ON A LINE FROM  
ASTORIA TO TACOMA BEFORE MIDNIGHT AND THE PRECIP SHOULD FINISH UP BY  
DAYBREAK MONDAY. MOST OF THE DAY ON MONDAY WILL BE DRY AND THEN RAIN  
STARTS UP AGAIN MONDAY NIGHT--FASTER THAN YESTERDAYS 12Z GFS.  
 
   
LONG TERM
 
A STRONG WARM FRONT MOVES THROUGH WESTERN WASHINGTON  
TUESDAY AND TUESDAY NIGHT. THE HEAVIER RAIN SHIFTS TO THE B.C. COAST  
AND VANCOUVER ISLAND WEDNESDAY AND WEDNESDAY EVENING AND THEN SHIFTS  
BACK DOWN THROUGH THE AREA EARLY THURSDAY--BUT THEN BREAKS UP AND  
DECREASES. IT WILL BE A LITTLE WINDY AT TIMES--MAINLY IN THE  
ADMIRALTY INLET AREA NORTH THROUGH THE SAN JUANS AND ON THE COAST--  
AND PERHAPS IN THE GAPS THROUGH THE CASCADES. THERE IS A MORE  
EASTERLY GRADIENT THAN SOUTHERLY AT TIMES. THE LATEST GFS HAS  
CONTINUED THE TREND OF LOOKING LESS DRAMATIC: PERIODS OF RATHER MILD  
WET AND BREEZY WEATHER ARE CERTAINLY ON TAP TUE-THU, JUST NOT MUCH  
IN THE WAY GENUINE STORMINESS THAT I CAN FIND. THE HIGHEST PDX-BLI  
GRADIENT I CAN FIND IS ONLY +8MB 12Z THU AS A 983MB LOW REACHES  
HAIDA GWAII.  
 

 
   
AVIATION
 
A FLAT UPPER RIDGE AXIS WILL CROSS WESTERN WASHINGTON  
THIS AFTERNOON, THEN EXIT EAST AND FLATTEN A BIT TONIGHT. A COLD  
FRONT WILL SPREAD DOWN THE B.C. COAST TONIGHT AND MOVE SOUTHWARD  
THROUGH WESTERN WASHINGTON LATER SUNDAY, PRECEDED BY HEAVY RAIN AND  
STRONG SOUTHERLY SURFACE WIND. MODERATE WESTERLY FLOW ALOFT TODAY  
WILL BECOME STRONG ON SUNDAY. THE AIR MASS IS STABLE. CLOUDS BELOW  
050 WILL DIMINISH TODAY. THEN MOISTURE AND CLOUDS WILL THICKEN  
RAPIDLY STARTING LATE THIS EVENING.  
 
KSEA...GRADUAL DRYING OF THE AIR MASS BELOW 100 WILL CONTINUE UNTIL  
EARLY THIS EVENING, CAUSING CLOUDS BELOW 100 TO DISSIPATE BETWEEN  
NOW AND 21Z. CLEAR BELOW 100 WILL CONTINUE THROUGH AT LEAST 08Z  
(MIDNIGHT) TONIGHT. CLOUD CIGS AROUND 040 WILL RAPIDLY DEVELOP  
OVERNIGHT, WITH RAIN DEVELOPING LATER SUNDAY MORNING. CIGS AND VSBYS  
LOWERING SUNDAY AFTERNOON IN OCCASIONALLY MODERATE RAIN. SOUTHERLY  
SURFACE WIND FOR THE NEXT 30 HOURS. HANER  
 

 
   
MARINE
 
A STRONG COLD FRONT EXTENDING WSW FROM THE QUEEN  
CHARLOTTE SOUND INTO THE PACIFIC WILL SLOWLY DRAG SOUTHEAST DOWN THE  
BRITISH COLUMBIA COASTLINE TODAY THEN WILL PUSH SOUTHEASTWARD  
THROUGH THE WASHINGTON COASTAL AND INLAND WATERS ON SUNDAY. WINDS  
WILL INCREASE IN ADVANCE OF THE FRONT. THE WIND INCREASE WILL  
PROGRESS FROM NORTH TO SOUTH THROUGH TONIGHT WITH GALES EXPECTED  
OVER ALL OF THE WATERS EXCEPT FOR PUGET SOUND AND HOOD CANAL AND THE  
CENTRAL STRAIT OF JUAN DE FUCA LATE TONIGHT INTO SUNDAY AHEAD OF THE  
FRONT. WINDS WILL RAPIDLY EASE BEHIND THE FRONT LATER SUNDAY INTO  
SUNDAY NIGHT.  
 
HIGH PRESSURE WILL BRING LIGHT WINDS TO THE WATERS ON MONDAY. A  
STRONG WARM FRONT WILL PUSH NORTHWARD THROUGH THE WATERS ON TUESDAY.  
A STRONG LOW PRESSURE CENTER DEVELOPING WELL OFFSHORE WILL PUSH  
NORTHEAST INTO HAIDA GWAII ON WEDNESDAY. HANER  
 

 
   
HYDROLOGY
 
THE SKOKOMISH RIVER IN MASON COUNTY IS A SHOO-IN TO GET  
OVER FLOOD STAGE AND STAY UP AROUND FLOOD STAGE FOR SEVERAL DAYS.  
THE 12Z GFS DOES NOT SHOW ESPECIALLY HEAVY PRECIP OVER THE CASCADES  
AND FOR WEDNESDAY AND WEDNESDAY EVENING SHIFTS THE HEAVY RAIN UP IN  
BRITISH COLUMBIA.  
 

 
   
SEW WATCHES/WARNINGS/ADVISORIES
 
 
WA...FLOOD WATCH FROM SUNDAY MORNING THROUGH MONDAY MORNING FOR HOOD  
CANAL AREA-LOWER CHEHALIS VALLEY AREA-OLYMPICS.  
 
WINTER STORM WATCH FROM SUNDAY AFTERNOON THROUGH LATE SUNDAY  
NIGHT FOR CASCADES OF PIERCE AND LEWIS COUNTIES.  
 
WINTER STORM WATCH FROM LATE TONIGHT THROUGH SUNDAY AFTERNOON  
FOR CASCADES OF WHATCOM AND SKAGIT COUNTIES.  
 
PZ...SMALL CRAFT ADVISORY UNTIL 10 PM PST THIS EVENING FOR COASTAL  
WATERS FROM CAPE FLATTERY TO JAMES ISLAND 10 TO 60 NM-  
COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND OUT 10 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-WEST ENTRANCE U.S. WATERS STRAIT OF  
JUAN DE FUCA.  
 
GALE WARNING FROM 10 PM THIS EVENING TO 3 PM PST SUNDAY FOR  
COASTAL WATERS FROM POINT GRENVILLE TO CAPE SHOALWATER 10  
TO 60 NM-NORTHERN INLAND WATERS INCLUDING THE SAN JUAN  
ISLANDS.  
 
GALE WARNING FROM 10 PM THIS EVENING TO NOON PST SUNDAY FOR  
COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND 10 TO 60  
NM-COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND OUT 10  
NM-COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE 10  
TO 60 NM-WEST ENTRANCE U.S. WATERS STRAIT OF JUAN DE FUCA.  
 
SMALL CRAFT ADVISORY UNTIL 3 AM PST SUNDAY FOR COASTAL WATERS  
FROM JAMES ISLAND TO POINT GRENVILLE OUT 10 NM.  
 
GALE WARNING FROM 3 AM TO NOON PST SUNDAY FOR COASTAL WATERS  
FROM JAMES ISLAND TO POINT GRENVILLE OUT 10 NM.  
 
SMALL CRAFT ADVISORY UNTIL 3 AM PST SUNDAY FOR COASTAL WATERS  
FROM POINT GRENVILLE TO CAPE SHOALWATER OUT 10 NM.  
 
GALE WARNING FROM 3 AM TO 3 PM PST SUNDAY FOR COASTAL WATERS  
FROM POINT GRENVILLE TO CAPE SHOALWATER OUT 10 NM.  
 
SMALL CRAFT ADVISORY UNTIL 3 PM PST THIS AFTERNOON FOR ADMIRALTY  
INLET-EAST ENTRANCE U.S. WATERS STRAIT OF JUAN DE FUCA-  
NORTHERN INLAND WATERS INCLUDING THE SAN JUAN ISLANDS.  
 
GALE WARNING FROM MIDNIGHT TONIGHT TO 3 PM PST SUNDAY FOR  
ADMIRALTY INLET-EAST ENTRANCE U.S. WATERS STRAIT OF JUAN DE  
FUCA.  
 
SMALL CRAFT ADVISORY FROM 6 PM THIS EVENING TO 9 PM PST SUNDAY  
FOR PUGET SOUND AND HOOD CANAL.  
 

 
 

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