Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 150347
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT TUE APR 14 2015

.SYNOPSIS...SCATTERED SHOWERS AND A PUGET SOUND CONVERGENCE ZONE
WILL WEAKEN TONIGHT AS AN UPPER LEVEL TROUGH OVER THE AREA MOVES
EAST. AN UPPER LEVEL RIDGE WILL MOVE OVER THE AREA TONIGHT THEN
REMAIN THROUGH THURSDAY FOR DRY WEATHER...EXCEPT FOR A CHANCE OF
SPOTTY LIGHT RAIN OVER THE FAR NORTH PART. AN UPPER LEVEL TROUGH
WILL CROSS THE AREA LATE FRIDAY WITH A LITTLE LIGHT RAIN. A STRONGER
UPPER LEVEL RIDGE WILL BRING MOSTLY SUNNY AND WARMER DAYS BY SUNDAY.

&&

.SHORT TERM...RADAR INDICATES SHOWERS ARE DECREASING ACROSS WRN WA
WITH LOSS OF DIURNAL INSTABILITY. MOST THE RESIDUAL SHOWERS SHOULD
DISSIPATE OVER THE NEXT COUPLE HOURS. THE PUGET SOUND CONVERGENCE
ZONE IS PRODUCING THE ONE AREA OF CONCENTRATED SHOWERS OVER KING
COUNTY AND ALONG THE PIERCE COUNTY LINE EXTENDING INTO THE CASCADES.
THIS ACTIVITY SHOULD ALSO DISSIPATE AS SURFACE GRADIENTS RELAX. WITH
BUILDING UPPER HEIGHTS AND WEAKER FLOW...PRECIPITATION SHOULD BE
FINISHED BY LATER TONIGHT.

MODELS HAVE TRENDED STRONGER WITH THE RIDGE OVER THE PAC NW
WEDNESDAY THROUGH FRIDAY. THE 18Z GFS IS DRY THROUGH THE PERIOD WITH
WAVES MOVING TO THE N OF THE AREA ACROSS BRITISH COLUMBIA. LIGHT
RAIN OVER THE N COAST IS IN THE FORECAST THE NEXT COUPLE DAYS WHICH
MAY NEED TO BE REMOVED. THERE WILL BE A WARMING TREND WITH HIGHS
GOING FROM THE MID/UPPER 50S WEDNESDAY TO THE MID OR POSSIBLY UPPER
60S BY FRIDAY. A FRONT WILL BRUSH THE AREA FRIDAY NIGHT BUT CURRENT
MODEL RUNS SHOW JUST SOME INCREASED CLOUD COVER AND A SWITCH TO
ONSHORE FLOW FRIDAY NIGHT. MERCER

.LONG TERM...FROM PREVIOUS DISCUSSION...MODERATE WESTERLY ONSHORE
FLOW WILL DEVELOP BEHIND THE UPPER LEVEL TROUGH FRIDAY NIGHT. THIS
SHOULD MOISTEN THE LOWER AIR MASS FOR SATURDAY WITH MOSTLY CLOUDY
CONDITIONS OVER THE INTERIOR LOWLANDS. A SOMEWHAT BROAD AND HIGHER
AMPLITUDE UPPER LEVEL RIDGE WILL BUILD OFFSHORE FRIDAY NIGHT BEHIND
THE SHORTWAVE TROUGH THEN SHIFT SLOWLY E OVER THE COAST BY MONDAY.
THIS WILL PUT W WA IN A DRYING AND WARMING TREND...SO ONCE SATURDAYS
LOW LEVEL MOISTURE IS MIXED OUT...SUNDAY AND MONDAY SHOULD BE SUNNY
AND WARM. MAX TEMPS SHOULD GET UP INTO THE 60S TO LOWER 70S. GFS AND
ECMWF WEAKEN THE RIDGE BY TUESDAY AND ALLOW A SHORTWAVE TROUGH TO
MOVE UP FROM THE S WITH SOME SHOWERS. THE CANADIAN IS SLOWER...SO I
WILL JUST INCREASE THE CLOUD COVER OVER THE AREA AND HOLD OFF ON
SHOWER FOR NOW. KAM

&&

.AVIATION...NORTHWEST FLOW ALOFT WILL CONTINUE THROUGH WEDNESDAY AS
AN UPPER LEVEL RIDGE BUILDS OVER THE AREA. AT THE SURFACE...ONSHORE
FLOW WILL DIMINISH TONIGHT AS HIGH PRES BUILDS INTO WESTERN
WASHINGTON. THE AIR MASS IS SOMEWHAT MOIST AND UNSTABLE THIS
EVENING. THE AIR MASS WILL GRADUALLY STABILIZE TONIGHT THROUGH
WEDNESDAY.

ISOLATED SHOWERS OVER THE COASTAL WATERS ARE DIMINISHING. IN THE
INTERIOR...A CONVERGENCE ZONE EXTENDS FROM THE NORTHEASTERN OLYMPIC
PENINSULA SOUTHEAST TO SEATAC AIRPORT AND INTO THE CASCADES AROUND
SNOQUALMIE PASS. ANOTHER CONVERGENCE ZONE CAN BE SEEN FROM VICTORIA
BC AIRPORT SE ACROSS THE SAN JUAN ISLANDS AND INTO STEVENS PASS. THE
NORTHERN ZONE WILL DISSIPATE BY 07Z WHILE THE SOUTHERN CONVERGENCE
ZONE WILL LIKELY LINGER UNTIL ABOUT 12Z. CONDITIONS ARE VFR...EVEN
IN SHOWERS. LOCAL MVFR IN SHOWERS IS POSSIBLE OVERNIGHT.

DESPITE THE INCREASED RIDGING ACROSS THE AREA ON WEDNESDAY...A WEDGE
OF COLD AIR BELOW AN INVERSION AT ABOUT 5000 FEET WILL ALLOW
BKN-OVC040 TO DEVELOP MIDDAY WED AND CONTINUE THROUGH THE EVENING
HOURS. ALBRECHT

KSEA...A CONVERGENCE ZONE RIGHT OVER THE TERMINAL WILL GIVE VARIABLE
WINDS 4-6 KT THIS EVENING. WIND WILL TEND TO BECOME LIGHT SLY ON
WED. DESPITE SHOWER ACTIVITY...EXPECT CIGS TO REMAIN AROUND
BKN035-040 THROUGH WEDNESDAY. ALBRECHT

&&

.MARINE...MODERATE ONSHORE FLOW WILL EASE TONIGHT AS HIGH PRES
BUILDS INTO THE WATERS. THE ONLY REMAINING SMALL CRAFT ADVISORY IS
FOR THE CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA UNTIL 3 AM PDT
WED MORNING AND FOR A ROUGH GRAYS HARBOR BAR OVERNIGHT.

HIGH PRES OVER THE WATERS WED WILL CONTINUE UNTIL THU AS A WEAK
FRONT MOVES BY THE WATERS TO THE NORTH.

A WEAK FRONT WILL MOVE THROUGH THE WATERS ON FRIDAY. THIS FRONT WILL
INCREASE NORTHWEST FLOW FRIDAY AFTERNOON AND FRIDAY NIGHT. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA....
PZ...SMALL CRAFT ADVISORY CENTRAL STRAIT AND EAST ENTRANCE.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

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

















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