Weather Service Report


650 
FXUS66 KSEW 272150
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT MON APR 27 2015

.SYNOPSIS...A COLD FRONT WILL MOVE INTO THE WASHINGTON OFFSHORE 
WATERS TONIGHT AND ACROSS WESTERN WASHINGTON TUESDAY. RAIN WILL TURN 
TO SCATTERED SHOWERS TUESDAY NIGHT AND WEDNESDAY AS AN UPPER TROUGH 
FOLLOWS THE FRONT. ANOTHER WEAK SYSTEM COULD BRING A FEW LIGHT 
SHOWERS ALONG WITH PARTLY SUNNY SKIES THURSDAY. DRY AND SLIGHTLY 
WARMER WEATHER IS LIKELY FRIDAY THROUGH MONDAY.

&&

.SHORT TERM...THE UPPER RIDGE OVER THE PACIFIC NORTHWEST 
STRENGTHENED AS IT SHIFTED SLOWLY EASTWARD TODAY...AND THE WARM 
FRONT THAT WAS OVER WESTERN WASHINGTON EARLY THIS MORNING MOVED 
NORTH OF THE AREA. MOST OF THE FORECAST AREA HAS HAD SUNNY SKIES 
THIS AFTERNOON...AND 2 PM TEMPERATURES RANGED FROM THE LOWER 60S TO 
THE MID 70S.

THE PACIFIC FRONTAL SYSTEM OFFSHORE WILL DEVELOP FURTHER TONIGHT... 
WITH ITS SURFACE DEEPENING TO AROUND 985 MB AND TRACKING 
NORTH-NORTHEAST TO NEAR HAIDA GWAII LATE TONIGHT. ITS COLD FRONT 
WILL MOVE INTO THE WASHINGTON OFFSHORE WATERS TONIGHT AND ACROSS 
WESTERN WASHINGTON TUESDAY. ITS POST-FRONTAL UPPER TROUGH WILL BE 
OVER THE AREA TUESDAY NIGHT AND WEDNESDAY...BRINGING SOME SHOWERS 
AND A PUGET SOUND CONVERGENCE ZONE. SHOWERS WILL BE LIMITED OVER THE 
LOWLANDS OUTSIDE THE CONVERGENCE ZONE...AND MUCH OF THE AREA WILL 
HAVE PARTLY SUNNY SKIES. 

THE FRONTAL SYSTEM SHOULD BRING MEASURABLE PRECIPITATION TO JUST 
ABOUT THE ENTIRE FORECAST AREA...BUT AMOUNTS WILL NOT BE SUBSTANTIAL 
-- MAYBE UP TO 0.25 INCH OR A LITTLE MORE IN THE MOUNTAINS AND THE 
CONVERGENCE ZONE. THE SNOW LEVEL WILL FALL TO AROUND 3500 FT AS 
PRECIPITATION IN THE MOUNTAINS DECREASES. DAYTIME HIGHS WILL RETURN 
TO NEAR NORMAL IN THE MID 50S TO LOWER 60S.

THE UPPER TROUGH WILL MOVE EAST OF THE CASCADES WEDNESDAY NIGHT... 
LEAVING WESTERN WASHINGTON UNDER THE ZONAL FLOW AHEAD OF A FLAT 
UPPER RIDGE OFFSHORE ON THURSDAY. ANOTHER SYSTEM IN THE ZONAL FLOW 
WILL BE HEADING INTO BRITISH COLUMBIA. THIS FEATURE COULD BRING SOME 
SHOWERS TO WESTERN WASHINGTON...BUT THEY WILL MOSTLY LIKELY BE 
SPOTTY AND LIGHT WITH PARTLY SUNNY SKIES.   MCDONNAL

.LONG TERM...ZONAL FLOW WILL CONTINUE FRIDAY AND SATURDAY...WITH 
UPPER HEIGHTS SLOWLY RISING AHEAD OF AN OFFSHORE UPPER RIDGE. 
PRECIPITATION SHOULD STAY NORTH OF THE FORECAST AREA...WITH PARTLY 
TO MOSTLY SUNNY SKIES OVER WESTERN WASHINGTON. A SMALL UPPER RIDGE 
WILL MOVE OVER THE AREA SUNDAY...FOR DRY AND SLIGHTLY WARMER 
WEATHER. THE UPPER RIDGE SHOULD MOVE EAST ON MONDAY...AND A WEAK 
FRONT COULD APPROACH BUT NOT REACH THE AREA. MONDAY SHOULD BE A DRY 
DAY BUT CLOUDS AND ONSHORE FLOW COULD BE INCREASING.   MCDONNAL

&&

.AVIATION...AN UPPER RIDGE OVER THE PACIFIC NORTHWEST WILL SHIFT 
EAST THIS EVENING. SOUTHWEST FLOW ALOFT. A COLD FRONT ASSOCIATED 
WITH A DEEPENING LOW MOVING INTO WESTERN BRITISH COLUMBIA WILL SWEEP 
ACROSS THE AREA MIDDAY TUESDAY. MODERATE POST FRONTAL ONSHORE FLOW 
WILL GIVE A PUGET SOUND CONVERGENCE ZONE TUESDAY AFTERNOON AND 
EVENING. THE AIR MASS IS STABLE AND WILL BECOME SOMEWHAT UNSTABLE 
TUESDAY AFTERNOON. 

LOW LEVEL MOISTURE THAT WAS OVER WESTERN WASHINGTON THIS MORNING HAS 
ERODED TO NORTH OF THE CANADIAN BORDER THIS AFTERNOON. GOOD VFR 
CONDITIONS WILL PREVAIL THROUGH THIS EVENING WITH LOW LEVEL OFFSHORE 
FLOW ACROSS THE REGION.

STRATUS WITH BASES FL 010-015 WILL DEVELOP OVER THE OREGON COAST 
THIS EVENING AND SURGE NORTHWARD ALONG THE COAST OVERNIGHT AS A COLD 
FRONT WELL OFFSHORE APPROACHES THE AREA FROM THE WEST. THE STRATUS 
WILL QUICKLY MOVE INLAND DURING THE PRE DAWN HOURS ON TUESDAY. 

THE OFFSHORE FRONT WILL MAKE LANDFALL ALONG THE COAST AROUND 15Z TUE 
AND THROUGH THE INTERIOR OF WESTERN WASHINGTON ABOUT 18Z. THE FRONT 
WILL BE PRECEDED BY A COUPLE OF HOURS OF LIGHT RAIN WITH GENERALLY 
MVFR CONDITIONS. CONDITIONS WILL IMPROVE TO MAINLY VFR BEHIND THE 
FRONT MIDDAY TUE...BUT A PUGET SOUND CONVERGENCE ZONE IS LIKELY TO 
FORM BETWEEN KPAE AND KSEA BEHIND THE FRONT MIDDAY TUESDAY AND 
PERSIST INTO THE EVENING HOURS GIVING LOCAL MVFR CONDITIONS THERE. 
ALBRECHT

KSEA...EXPECT GOOD VFR CONDITIONS TO PREVAIL THROUGH ABOUT 10Z THIS 
EVENING. CIGS AROUND FL 010 WILL SPREAD INTO THE TERMINAL AREA 
AROUND 10Z...THEN SOME -RA CAN BE EXPECTED 15Z-18Z TUE AHEAD OF A 
COLD FRONT. VFR CONDITIONS WILL PREVAIL TUE AFTERNOON BUT A PUGET 
SOUND CONVERGENCE ZONE IS LIKELY TO SET UP NORTH OF THE TERMINAL.  
SOUTH WINDS 45KT INCREASING AT FL050 AFTER 06Z TUE WILL CONTINUE 
UNTIL FROPA 18Z TUE. AT THE SURFACE...NW WIND TO 6 KT WILL VEER TO 
NE TO E 03Z-07Z THEN SHIFT TO S-SW 8-12 KT AFTER 10Z. ALBRECHT

&&

.MARINE...A COLD FRONT ASSOCIATED WITH A RAPIDLY DEVELOPING LOW WEST 
OF THE WASHINGTON OFFSHORE WATERS MOVE THROUGH THE COASTAL WATERS 
TUESDAY MORNING THEN THROUGH THE INLAND WATERS MIDDAY TUESDAY. 
EXPECT SMALL CRAFT ADVISORY SOUTH TO SOUTHWEST WINDS TO DEVELOP 
AHEAD OF THE FRONT TONIGHT OVER THE COASTAL WATERS THEN SHIFT INTO 
THE INLAND WATERS LATE TONIGHT INTO TUE MORNING. SMALL CRAFT 
ADVISORY WEST WINDS ARE NOW EXPECTED TO DEVELOP LATER THIS EVENING 
THROUGH THE CENTRAL AND EASTERN PORTIONS OF THE STRAIT OF JUAN DE 
FUCA IN A PRE FRONTAL ONSHORE SURGE. SMALL CRAFT ADVISORY WESTERLY 
WINDS WILL CONTINUE IN THE STRAIT OF JUAN DE FUCA BEHIND THE FRONT 
TUESDAY AFTERNOON AND EVENING...BUT WILL DIMINISH OVER MOST OF THE 
OTHER WATERS. 

ANOTHER COLD FRONT WILL MOVE THROUGH THE WATERS ON WEDNESDAY 
AFTERNOON OR EVENING. THIS FRONT IS EXPECTED TO BE WEAKER THAN THE 
TUESDAY COLD FRONT. AT THIS TIME...WIND SPEEDS ARE EXPECTED TO 
REMAIN BELOW SMALL CRAFT ADVISORY LEVEL AROUND THIS FRONT. 

HIGH PRES WILL BUILD INTO THE COASTAL WATERS ON THU AND WILL 
GRADUALLY SHIFT OFFSHORE FRIDAY INTO SATURDAY. SMALL CRAFT ADVISORY 
NORTHWEST WINDS WILL GRADUALLY DEVELOP OVER THE COASTAL WATERS ON 
FRIDAY AND CONTINUE THROUGH SATURDAY. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE. 
PZ...SMALL CRAFT ADVISORY COASTAL WATERS...CENTRAL STRAIT OF JUAN 
      DE FUCA...AND EAST ENTRANCE TO THE STRAIT STARTING MIDNIGHT
      TONIGHT.
     SMALL CRAFT ADVISORY NORTHERN INLAND WATERS...ADMIRALTY INLET...
      PUGET SOUND/HOOD CANAL STARTING 3 AM PDT TUE MORNING.

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT 
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML



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