Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 240415
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT SAT APR 23 2016

.SYNOPSIS...SHOWERS WILL INCREASE THIS EVENING AS A FRONTAL SYSTEM
MOVES ACROSS THE AREA AND IS FOLLOWED BY ONSHORE FLOW. AN UPPER
LEVEL TROUGH WILL MOVE OVER WESTERN WASHINGTON ON SUNDAY GIVING COOL
TEMPERATURES...SHOWERS...AND A CHANCE OF THUNDERSTORMS. WEAK HIGH
PRESSURE WILL BUILD INTO THE AREA LATER MONDAY THROUGH TUESDAY
GIVING DRY WEATHER TO THE AREA.

&&

.SHORT TERM...RADARS...SATELLITE IMAGERY...AND SURFACE OBSERVATIONS
SHOW A COLD FRONT EXTENDING SOUTHEASTWARD ALONG THE WEST COAST OF
VANCOUVER ISLAND THEN SOUTHWARD THROUGH THE WASHINGTON AND OREGON
COASTAL WATERS. SHORT TERM MODELS SHOW THE FRONT MOVING EAST INTO
THE CASCADES BY MIDNIGHT...AND IT WILL BE FOLLOWED BY ONSHORE
MODERATE TO STRONG ONSHORE FLOW AND COOLING ALOFT. EXPECT SHOWERS
WITH THE FRONT TO CONTINUE ALONG THE WEST SLOPES OF THE
MOUNTAINS...IN A PUGET SOUND CONVERGENCE ZONE FROM EVERETT TO ABOUT
MT VERNON...AND OVER THE WESTERN PORTION OF THE OLYMPIC PENINSULA
OVERNIGHT. COOL AIR ALOFT CAN BE SEEN OFFSHORE WITH 500 MB
TEMPERATURES LATER TONIGHT OVER THE COASTAL WATERS FALLING TO -31C.

FURTHER COOLING ALOFT VERY LATE TONIGHT OVER THE COASTAL WATERS THEN
SPREADING INLAND DURING THE AFTERNOON HOURS AS A SHORT WAVE UPPER
TROUGH NOW SEEN AROUND 133W MOVES ACROSS THE AREA. THE GOING
FORECAST SHOWS A SLIGHT CHANCE OF TSTMS FOR THE LATE NIGHT AND
MORNING HOURS ALONG THE COAST AND IN THE COASTAL WATERS...AND IN THE
INTERIOR DURING THE AFTERNOON HOURS. THE BEST CHANCE OF TSTMS WILL
LIKELY BE AROUND A PUGET SOUND CONVERGENCE ZONE DURING THE AFTERNOON
AND EVENING HOURS...FIRST AROUND EVERETT THEN SLIDING SOUTHWARD TO
AROUND SEATTLE. THE GFS20 SHOWS LOW LEVEL FORCING WITH THE
CONVERGENCE ZONE LINING UP WITH THE INCOMING TROUGH...LIFTED INDICES
AROUND -2...500 MB TEMPERATURES AROUND -33C...AND CAPE VALUES NEAR
700 J/KG.

THE CONVERGENCE ZONE SLOWLY WEAKENS SUNDAY NIGHT INTO MONDAY. THE
GFS SEEMS TO KEEP SOME CONVERGENCE GOING RIGHT INTO MONDAY EVENING
AS RIDGING ALOFT BUILDS OFFSHORE AND LOW LEVEL ONSHORE FLOW
CONTINUES. IF THE INCOMING 00Z MODELS CONTINUE TO SHOW THE
CONVERGENCE ZONE AND WEST SLOPE SHOWERS GOING THROUGH MONDAY
EVENING...THE NEXT SHIFT MAY NEED TO BOOST POPS FOR MONDAY AND
MONDAY NIGHT.

IT DOES APPEAR THAT THE BUILDING RIDGE OFFSHORE WILL SHIFT INLAND
ONTUESDAY WITH 500 MB HEIGHTS RISING TO OVER 560 DAM. TUESDAY SHOULD
SEE HIGHS RISING TO THE LOWER 60S FROM SEATTLE OR SO SOUTHWARD.

CURRENT FORECASTS APPEAR TO BE IN GOOD SHAPE. NO UPDATES ARE
REQUIRED. ALBRECHT

.LONG TERM...FROM THE PREVIOUS LONG TERM DISCUSSION...AFTER THE WEAK
UPPER RIDGE AND SOME SUNSHINE ON TUESDAY A FRONT WILL APPROACH THE
AREA TUE NIGHT. THE FRONT WILL PROBABLY SPLIT AND THEN THAT UPPER
TROUGH WILL DROP INTO OREGON AND CALIF WED AND THU AS ANOTHER WEAK
RIDGE BUILDS INTO WRN WA. MORE CLOUDS AND A CHANCE OF SHOWERS ARE IN
THE FORECAST AT TIMES...BUT OVERALL PATTERN WILL BE PRETTY QUIET AND
BENIGN.

&&

.AVIATION...AN UPPER LEVEL TROF OFFSHORE WILL CONT MOVG SLOWLY
E. A CDFNT ON THE WA COAST IS ANTICIPATED TO BE E OF THE CASCADES
BY 0700 UTC. STRONG LOW LEVEL ONSHORE FLOW WILL DVLP BEHIND THE
FRONT. CONTD SLY FLOW ALOFT. THE AIR MASS WILL BECOME SLIGHTLY
UNSTABLE OVER THE COASTAL WATERS LATE TONIGHT FOR A RISK OF ISOLD
TSTMS. EXPECT AREAS OF GENERALLY MVFR CIGS /1-3K FT/ AND VSBYS
3-5SM SHRA OVERNIGHT.

KSEA...FORECASTING CIGS OVER THE TERMINAL WILL BE PROBLEMATIC
OVERNIGHT. AT THIS TIME...ANTICIPATING OCNL MVFR CIGS OVERNIGHT.
THERE IS A CHANCE THAT THE CLOUDS COULD BECOME SCATTERED LATE
TONIGHT OR AFTER 0900 UTC. A PUGET SOUND CONVERGENCE ZONE IS
EXPECTED TO DVLP AFTER 0600 UTC BTWN KPAE AND KAWO.

&&

.MARINE...A COLD FRONT ON THE WASHINGTON COAST EARLY THIS EVENING
WILL MOVE ACROSS THE INLAND WATERS BY MIDNIGHT TONIGHT. EXPECT
ONSHORE FLOW TO STRENGTHEN AND BECOME WESTERLY EARLY SUNDAY DUE TO
HIGH PRES BUILDING OVER THE OFFSHORE WATERS. GALE FORCE WESTERLIES
ARE LIKELY OVER THE PARTS OF THE STRAIT OF JUAN DE FUCA LATE TONIGHT
THROUGH SUNDAY DUE TO STRENGTHENING OF THE ONSHORE PRESSURE GRADIENT
IN THE WAKE OF THE COLD FRONT.

ONSHORE FLOW WILL THEN PERSIST THROUGH TUE BUT IT WILL WEAKEN
WITH TIME.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY UNTIL 6 PM PDT SUNDAY FOR ADMIRALTY INLET-
     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 JAMES ISLAND TO POINT
     GRENVILLE OUT 10 NM-COASTAL WATERS FROM POINT GRENVILLE TO
     CAPE SHOALWATER 10 TO 60 NM-COASTAL WATERS FROM POINT
     GRENVILLE TO CAPE SHOALWATER OUT 10 NM-WEST ENTRANCE U.S.
     WATERS STRAIT OF JUAN DE FUCA.

     GALE WARNING UNTIL 6 PM 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 UNTIL 5 PM PDT SUNDAY FOR NORTHERN INLAND
     WATERS INCLUDING THE SAN JUAN ISLANDS.

     SMALL CRAFT ADVISORY UNTIL 6 AM PDT SUNDAY FOR PUGET SOUND AND
     HOOD CANAL.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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



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