Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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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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