Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 291018
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
318 AM PDT FRI APR 29 2016

.SYNOPSIS...A FRONT AND ITS UPPER TROUGH WILL MOVE THROUGH WESTERN
WASHINGTON TODAY FOR OCCASIONAL SHOWERS OVER MOST OF THE AREA. A
STRONG UPPER RIDGE WILL BRING DRY WARMER WEATHER SATURDAY THROUGH
MONDAY. A CHANCE OF SHOWERS WILL RETURN TUESDAY THROUGH THURSDAY
OF NEXT WEEK...WITH MOIST SOUTHERLY FLOW ALOFT AND LOW LEVEL
ONSHORE FLOW.

&&

.SHORT TERM...A WEAK COLD FRONT AND UPPER TROUGH ARE APPROACHING
THE PACIFIC NORTHWEST COAST EARLY THIS MORNING. AT 3 AM THE RADAR
ON THE WASHINGTON COAST WAS JUST STARTING TO SHOW PRECIPITATION
OVER THE COASTAL WATERS. THERE ARE ALSO SOME LIGHT SPOTTY SHOWERS
ALONG THE WEST SLOPES OF THE CASCADES AND IN THE WEAK CONVERGENCE
ZONE OVER SNOHOMISH COUNTY...BUT OTHERWISE WESTERN WASHINGTON WAS
DRY OVERNIGHT. TEMPERATURES WERE IN THE MID 40S TO LOWER 50S.

THE SYSTEM WILL MOVE ACROSS WESTERN WASHINGTON THIS MORNING...
AND MOIST ONSHORE LOW LEVEL FLOW WILL CONTINUE BEHIND THE FRONT
THIS AFTERNOON. MEASURABLE PRECIPITATION SHOULD BE WIDESPREAD...
BUT IT WON`T AMOUNT TO MUCH -- ROUGHLY 0.10 TO 0.20 INCH IN THE
CONVERGENCE ZONE AND MOSTLY LESS THAN 0.10 INCH ELSEWHERE IN THE
LOWLANDS. A BIT MORE WILL FALL IN THE CASCADES...WITH THE SNOW
LEVEL AROUND 4500 FT AND 1 TO 3 INCHES OF SNOW ABOVE THAT.

A STRONG UPPER RIDGE WILL BEGIN BUILDING OFFSHORE TONIGHT...JUST
OUTSIDE OF 130W. DRY NORTHERLY FLOW ALOFT WILL DEVELOP QUICKLY
OVER WESTERN WASHINGTON...SO SHOWERS WILL BECOME MAINLY LIMITED TO
THE CASCADES AND SOUTH INTERIOR IN THE EVENING AND THEN END
OVERNIGHT.

THE UPPER RIDGE WILL SHIFT SLOWLY EAST OVER THE AREA THIS WEEKEND.
THE AIR MASS WILL WARM...WITH 850 MB TEMPERATURES OVER WESTERN
WASHINGTON REACHING AROUND +6C ON SATURDAY AND +12C ON SUNDAY. IN
ADDITION OFFSHORE FLOW WILL DEVELOP ON SUNDAY AS A THERMAL TROUGH
EXTENDS NORTHWARD ALONG THE PACIFIC NORTHWEST COAST FROM NORTHERN
CALIFORNIA. HIGHS SHOULD BE IN THE 60S TO LOWER 70S SATURDAY AND
MOSTLY IN THE 70S TO LOWER 80S SUNDAY. MCDONNAL

.LONG TERM...THE UPPER RIDGE WILL MOVE FURTHER EAST ON MONDAY...
AND THE AIR MASS WILL WARM A BIT MORE AS SOUTHERLY FLOW ALOFT
DEVELOPS. IT IS CONCEIVABLE THAT SOME MID LEVEL MOISTURE WILL
ACCOMPANY WEAK INSTABILITY OVER THE MOUNTAINS TO PRODUCE ISOLATED
SHOWERS BY LATE MONDAY. THE MODELS HAVE NOT BEEN CONSISTENT WITH
THAT IDEA THOUGH...AND IN COORDINATION WITH SURROUNDING OFFICES WE
WILL STAY WITH A DRY FORECAST FOR NOW. THERE IS A BETTER CHANCE OF
SHOWERS DEVELOPING TUESDAY ALONG WITH INCREASING ONSHORE FLOW.
MOIST SOUTHERLY FLOW ALOFT...WEAK INSTABILITY...AND LOW LEVEL
ONSHORE FLOW WILL KEEP A CHANCE OF SHOWERS IN THE FORECAST
WEDNESDAY AND THURSDAY AS WELL. MCDONNAL

&&

.AVIATION...AN UPPER TROUGH AXIS WILL PASS WRN WA THIS MORNING.
AIR MASS HAS ALREADY STARTED TO MOISTEN...WITH INSTABILITY
INCREASING AS TROUGH AXIS PASSES THROUGH. SHOWERS AND FAIRLY
WIDESPREAD IFR CIGS EXPECTED BY MID TO LATE MORNING. KPAE IS ALREADY
SEEING SUCH CONDITIONS THANKS TO A PSCZ THAT DEVELOPED DURING THE
OVERNIGHT HOURS. THE RISK FOR SHOWERS CONTINUES TO LOOK LIKE A
GENERALLY NARROW WINDOW WITH THE THREAT ENDING AND GRADUAL CLEARING
EXPECTED THIS AFTERNOON AND EVENING FROM WEST TO EAST. LIGHT NW
UPPER LEVEL FLOW TODAY...SHIFTING TO THE NORTH AND STRENGTHENING
TONIGHT. SMR

KSEA...INCREASING CLOUDS WEST OF PUGET SOUND AT THE TIME OF THIS
WRITING WILL LIKELY SPREAD OVER KSEA BY BETWEEN 10Z AND
11Z...BRINGING CIGS DOWN TO IFR IN ABOUT THE 1-2K FT RANGE. MODEL
TIMING OF SHOWERS REMAINS CONSISTENT WITH ACTIVITY KICKING OFF BY
MID MORNING AND WRAPPING UP BY MID AFTERNOON. CIGS ARE EXPECTED TO
LIFT TO VFR THIS AFTERNOON AND EVENING...BUT LOOKS LIKE SOME
CLOUDINESS WILL LINGER INTO THE OVERNIGHT HOURS. SMR

&&

.MARINE...A FRONT WILL PASS EASTWARD ACROSS THE WATERS THIS EARLY
MORNING...ENHANCING ONSHORE FLOW INTO THIS EVENING. SMALL CRAFT
ADVISORIES REMAIN IN PLACE FOR CENTRAL AND EASTERN STRAIT AND HAVE
BEEN EXTENDED INTO LATE TONIGHT. WHILE THERE MAY BE A BRIEF PERIOD
WHERE WINDS WILL FALL BELOW ADVISORY CRITERIA...DID NOT WANT TO
SPLIT HAIRS AS WINDS WILL PICK UP AGAIN AS FRONT MOVES THROUGH. WITH
THAT INCREASE IN WINDS...WATERS IMPACTED BY SMALL CRAFT ADVISORY
WIND SPEEDS WILL INCLUDED COASTAL WATERS...NORTHERN INLAND
WATERS...AND THE WEST ENTRANCE. THUS ALL HAVE THEIR OWN ADVISORIES
IN PLACE WITH START AND END TIMES STAGGERED ACCORDINGLY. ADVISORIES
FOR COASTAL WATERS MAY NEED TO BE EXTENDED INTO SUNDAY...BUT THAT IS
BEST LEFT TO FUTURE FORECASTS.

A TURN TO NORTHERLY ONSHORE FLOW IS EXPECTED ON SATURDAY.
NORTHERLY OFFSHORE FLOW IS EXPECTED ON SUNDAY. NORTHERLIES COULD
REACH SMALL CRAFT STRENGTH IN PUGET SOUND ON SUNDAY PM. HANER/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY FROM 11 AM THIS MORNING TO 5 PM PDT
     SATURDAY FOR COASTAL WATERS FROM CAPE FLATTERY TO JAMES
     ISLAND 10 TO 60 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.

     SMALL CRAFT ADVISORY FROM 11 AM THIS MORNING TO 5 AM PDT
     SATURDAY FOR COASTAL WATERS FROM CAPE FLATTERY TO JAMES
     ISLAND OUT 10 NM-COASTAL WATERS FROM JAMES ISLAND TO POINT
     GRENVILLE OUT 10 NM-COASTAL WATERS FROM POINT GRENVILLE TO
     CAPE SHOALWATER OUT 10 NM-WEST ENTRANCE U.S. WATERS STRAIT
     OF JUAN DE FUCA.

     SMALL CRAFT ADVISORY UNTIL 5 AM PDT SATURDAY FOR CENTRAL U.S.
     WATERS STRAIT OF JUAN DE FUCA-EAST ENTRANCE U.S. WATERS
     STRAIT OF JUAN DE FUCA.

     SMALL CRAFT ADVISORY FROM 8 PM THIS EVENING TO NOON PDT SATURDAY
     FOR NORTHERN INLAND WATERS INCLUDING THE SAN JUAN ISLANDS.

&&

$$

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