351  
FXUS66 KSEW 291111  
AFDSEW  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SEATTLE WA  
411 AM PDT MON MAY 29 2017  
   
SYNOPSIS
 
ONSHORE FLOW WILL INCREASE THROUGH TUESDAY AS UPPER  
RIDGING EXITS TO THE EAST. MARINE MOISTURE, IN THE FORM OF LOW  
CLOUDS WILL PUSH FURTHER INLAND AND PERSIST LONGER EACH DAY. AN  
UPPER TROUGH AND SOUTHERLY FLOW ALOFT WILL BRING AN INCREASING  
CHANCE OF SHOWERS AND A FEW THUNDERSTORMS, MAINLY TUESDAY AND  
TUESDAY NIGHT. WEAK WEATHER DISTURBANCES WILL KEEP CLOUDS AND A  
CHANCE OF SHOWERS IN THE FORECAST THROUGH THE END OF THE WEEK  
AHEAD.  
 

 
   
SHORT TERM
 
THIS WEEKEND'S WARM AND DRY UPPER RIDGE WILL EXIT  
EAST TODAY TOWARD THE NORTHERN ROCKIES, WITH INCREASING SOUTHWEST  
FLOW ALOFT. THIS HAS ALREADY OPENED THE DOOR TO COOLER MARINE AIR  
SPREADING INLAND OVERNIGHT, ALONG WITH LOW MARINE CLOUDS. ASOS OBS  
AND DERIVED FOG IMAGERY SHOW LOW CLOUDS HAVE ALREADY PUSHED INLAND  
THROUGH THE STRAIT TO NEAR PAE, AND UP FROM THE SOUTHWEST TO PWT  
AND SEA. BY SUNRISE, MARINE CLOUDS SHOULD FILL IN FURTHER OVER THE  
LOWLANDS. AS A RESULT OF THE WELL-DEFINED ALBEIT SHALLOW MARINE  
AIR, TODAY'S AFTERNOON TEMPERATURES WILL BE DOWN 10 OR MORE  
DEGREES OVER THE PUGET SOUND LOWLANDS FROM SUNDAY. OTHERWISE, A  
FEW SHOWERS MAY POP UP THIS AFTERNOON OVER THE CASCADE CREST AND  
THE INTERIOR OF THE OLYMPIC MTNS, BUT WEAK WIND PROFILES AND  
PLENTY OF CONVECTIVE INHIBITION SUGGESTS THAT ANY SHOWERS WILL BE  
SHORT-LIVED, JUST A QUICK PULSE.  
 
ON TUESDAY, WE WILL BE EYEING A DISTURBANCE THAT WILL SWING UP  
FROM THE SOUTHWEST. MID-LEVEL LAPSE RATES WILL STEEPEN WITH THE  
APPROACH OF THIS FEATURE, WITH LARGE-SCALE LIFT HELPING SHOWERS TO  
BLOSSOM OVER WESTERN WASHINGTON ON TUESDAY. THE MARINE LAYER WILL  
DEEPEN, AND MARINE CLOUDS ARE LIKELY TO BE STUBBORN, LEADING TO A  
COOL DAY WITH BELOW NORMAL TEMPERATURES. OWING TO A COOL AND  
STABLE BOUNDARY LAYER BUT UNSTABLE MID-LEVELS, CONVECTION ON  
TUESDAY WILL BE ELEVATED, INSTEAD OF SURFACE-BASED. THE MID-LEVEL  
WIND PROFILE WILL HAVE QUITE A BIT OF DIRECTIONAL AND SPEED SHEAR,  
SO SOME ORGANIZATION INTO A FEW CLUSTERS OF THUNDERSTORMS SEEMS  
PLAUSIBLE. THE FOCUS OF ANY THUNDERSTORMS WILL SHIFT FROM THE  
OLYMPIC PENINSULA LATE IN THE MORNING TO THE CASCADES BY EARLY TUE  
EVENING, PASSING ACROSS THE SEATTLE METRO AREA DURING THE  
AFTERNOON. WITH LOW-HANGING MARINE CLOUDS, SKY WATCHERS WILL NOT  
BE ABLE TO SEE THE CONVECTIVE CLOUDS ABOVE.  
 
AN UPPER LOW OFF THE B.C. COAST AND W-SW FLOW ALOFT WILL CONTINUE  
TO DEEPEN THE MARINE AIR ON WEDNESDAY. PRECIP CHANCES GO DOWN ON  
WED DUE TO LESS LARGE-SCALE LIFT, BUT OROGRAPHIC LIFT WITH THE  
ONSHORE FLOW PATTERN AND MOIST BOUNDARY LAYER COULD LEAD TO A FEW  
LIGHT SHOWERS. HANER  
   
LONG TERM
 
THURSDAY AND FRIDAY WILL FEATURE ONSHORE FLOW AND  
BROAD UPPER TROUGHING OVER THE PAC NW. THIS WILL FAVOR NEAR OR  
SLIGHTLY BELOW NORMAL TEMPERATURES ALONG WITH A FEW LIGHT SHOWERS,  
MAINLY ON WINDWARD MOUNTAIN SLOPES AND IN A PUGET SOUND  
CONVERGENCE ZONE. THE ECMWF BASICALLY MAINTAINS THIS PATTERN  
SATURDAY AND POSSIBLY INTO NEXT SUNDAY, WHILE THE GFS BECOMES A  
BIT MORE ZONAL WITH 500 MB HEIGHTS RISING ABOVE 570 DECAMETERS.  
THIS WILL MAINLY AFFECT THE INLAND EXTENT OF MARINE CLOUDS AND  
WOULD MAKE A FEW DEGREES DIFFERENCE IN HIGH TEMPERATURES, BUT  
NOTHING MORE IMPACTFUL THAN THAT. HANER  
 

 
   
AVIATION
 
WRN WA WILL REMAIN BTWN AN UPPER LEVEL TROF OFFSHORE AND  
A RIDGE OF HIGH PRES OVER THE INTERIOR W. SW FLOW ALOFT WILL BECOME  
MORE SLY WITH TIME.  
 
MEANWHILE, EXPECT WDSPRD IFR CIGS OVER THE COAST, PUGET SOUND REGION  
SWD, AND AREAS ADJACENT TO THE STRAIT OF JUAN DE FUCA THIS MORNING,  
WITH AREAS OF IFR CIGS FROM WHIDBEY ISLAND NWD. THERE WILL BE AREAS  
OF LIFR CIGS/VSBYS OVER THE INTERIOR LOWLANDS. CIGS WILL LIFT INTO  
THE MVFR CATEGORY RANGE AND THE FOG WILL DISSIPATE BY 1900 UTC.  
EXPECT THE CLOUDS TO SCATTER OUT OVER THE INTERIOR BTWN 1800 AND  
2000 UTC; HOWEVER, STRATUS WILL LINGER OVER THE COAST ALL DAY.  
 
KSEA...THE STRATUS WAS MOVING IN FROM THE SOUTH AT THIS TIME. EXPECT  
IFR CIGS TO LIFT INTO THE MVFR CATEGORY RANGE (1-2K FT) BY LATE THIS  
MORNING. THERE WILL LIKELY BE FOG REDUCING THE SURFACE VISIBILITY TO  
5 SM, POSSIBLY AS LOW AS 2SM, THIS MORNING. EXPECT THE CLOUDS TO  
SCATTER OUT BETWEEN 11 AM AND NOON. WINDS WILL BE SOUTHERLY 5-10  
KNOTS, BECOMING WESTERLY THIS AFTERNOON.  
 

 
   
MARINE
 
 
HIGH PRESSURE OFFSHORE COUPLED WITH LOWERING PRESSURE EAST OF THE  
CASCADES WILL RESULT IN STRENGTHENING ONSHORE FLOW THROUGH TONIGHT.  
THE FLOW WILL WEAKEN TUESDAY AS THE OFFSHORE HIGH SHIFTS EAST,  
EVENTUALLY REACHING THE WA COAST TUE EVENING. A 1014 MB HIGH ON THE  
WA COAST WITH LOWER PRESSURE EAST OF THE CASCADES WILL RESULT IN  
LIGHT ONSHORE FLOW ON WEDNESDAY.  
 

 
   
SEW WATCHES/WARNINGS/ADVISORIES
 
 
WA...NONE.  
PZ...GALE WARNING UNTIL 5 AM PDT TUESDAY FOR CENTRAL U.S. WATERS  
STRAIT OF JUAN DE FUCA-EAST ENTRANCE U.S. WATERS STRAIT OF  
JUAN DE FUCA.  
 
SMALL CRAFT ADVISORY UNTIL 5 AM PDT TUESDAY FOR NORTHERN INLAND  
WATERS INCLUDING THE SAN JUAN ISLANDS-WEST ENTRANCE U.S.  
WATERS STRAIT OF JUAN DE FUCA.  
 
SMALL CRAFT ADVISORY UNTIL 3 AM PDT TUESDAY FOR ADMIRALTY INLET.  
 

 
 

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