Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 152139
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
240 PM PDT WED APR 15 2015

.SYNOPSIS...AN UPPER LEVEL RIDGE AND SURFACE HIGH WILL REMAIN OVER
WESTERN WASHINGTON THROUGH THURSDAY FOR DRY WEATHER. A WARM FRONT
OVER BRITISH COLUMBIA WILL BRING CLOUDS MAINLY TO THE NORTH PART
TONIGHT. THE RIDGE WILL BUILD NORTHWARD THURSDAY...BRINGING WARMER
WEATHER AND MORE SUNSHINE. A WEAK UPPER LEVEL TROUGH WILL BRING
CLOUDS OVER THE AREA FRIDAY...AND A CHANCE OF SHOWERS TO THE NORTH
COAST AND NORTH CASCADES FRIDAY AFTERNOON AND EVENING. A STRONGER
UPPER LEVEL RIDGE WILL BRING SUNNY WARMER WEATHER SATURDAY AFTERNOON
THROUGH TUESDAY.

&&

.SHORT TERM...AN UPPER LEVEL RIDGE WILL REMAIN OVER W WA TONIGHT AND
THURSDAY FOR DRY WEATHER. WSW FLOW ALOFT OVER THE NE PACIFIC IS
SENDING LOTS OF WARM ADVECTION MOISTURE THROUGH THE RIDGE AND JUST
TO THE N...ACROSS VANCOUVER ISLAND. PREVIOUS FORECASTS HAD A CHANCE
OF LIGHT RAIN OVER THE N COAST AND N CASCADES IN THE EVENT THE WARM
FRONT BRUSHED THE N PART OF W WA...BUT SO FAR TODAY THE MOISTURE HAS
STAYED UP OVER B.C.  IT LOOKS LIKE THE MOISTURE WILL MISS W WA
TONIGHT AS WELL. ON THURSDAY THE UPPER LEVEL RIDGE WILL BUILD A
LITTLE...PUSHING THE WARM FRONT MOISTURE EVEN FARTHER N. THURSDAY
SHOULD BE MOSTLY SUNNY AND MILD EXCEPT FOR SOME MORNING CLOUDS OVER
THE FAR NORTH PART.

MODELS HAVE CONTINUED THE WEAKENING TREND WITH THE UPPER LEVEL
TROUGH EXPECTED TO CROSS THE AREA LATE FRIDAY. AT THIS POINT THERE
IS VERY LITTLE PRECIP EXPECTED FROM THIS SYSTEM. CLOUD COVER LOOKS
ON THE LOW SIDE WITH JUST A SKINNY FRONT-LIKE BAND CROSSING W WA IN
THE AFTERNOON. THERE ARE STILL MINOR DIFFERENCES IN TIMING...WITH
THE FASTEST GFS TAKING THE TROUGH OVER THE CASCADES BY 00Z WHILE THE
SLOWER ECMWF STILL HAS IT OVER THE OLYMPICS. ALL MODELS SHOW DRYING
NW FLOW ALOFT BEHIND THE TROUGH FRIDAY EVENING...WITH MODERATE
WESTERLY LOW LEVEL ONSHORE FLOW. NAM BOUNDARY LAYER RH PROGS SUGGEST
THAT STRATUS WILL ADVECT OVER THE COASTAL WATERS BY FRIDAY...SO IT
WILL BE IN POSITION TO MOVE INLAND LATE FRIDAY WHEN THE ONSHORE FLOW
INCREASES. MODELS SHOW THE ONSHORE FLOW WEAKENING PRETTY QUICK LATE
FRIDAY NIGHT...SO IT IS UNCERTAIN HOW FAR INLAND AND HOW MUCH
STRATUS WILL FORM BY SATURDAY MORNING. THE FORECAST HEDGES ON
STRATUS OVER THE INTERIOR WITH JUST A PARTLY SUNNY FORECAST SATURDAY
MORNING. NAM SHOWS THE LOW LEVEL MOISTURE MIXING OUT RAPIDLY
SATURDAY AFTERNOON FOR SUNNY SKIES. KAM

.LONG TERM...ANOTHER UPPER LEVEL RIDGE WILL BUILD OFFSHORE ON
FRIDAY...BEHIND THE UPPER LEVEL TROUGH. THE RIDGE WILL CONTINUE
BUILDING THROUGH SATURDAY...BEFORE MOVING OVER THE WA COAST SUNDAY
MORNING. WEAK NW ONSHORE FLOW WILL CONTINUE SATURDAY AFTERNOON
BEFORE SHIFTING TO A NORTHERLY OFFSHORE FLOW ON SUNDAY. THE
RESULTING WARMING SHOULD PUSH MAX TEMPS UP TO THE 60S TO LOWER 70S
SUNDAY AND MONDAY. SKIES WILL BE MOSTLY CLEAR FROM SATURDAY
AFTERNOON ONWARD.

LONG RANGE MODELS START BREAKING THE RIDGE DOWN STARTING
SUNDAY...WHILE IT REMAINS OVER THE REGION. BY TUESDAY
AFTERNOON...WITH THE RIDGE AXIS OVER THE ID PANHANDLE...A LARGE
UPPER LEVEL TROUGH WILL HAVE FORMED JUST OFFSHORE. S FLOW ALOFT WILL
DEVELOP OVER W WA WITH MOISTURE INCREASING AT SOME POINT. MODELS ARE
STRUGGLING WITH CONSISTENCY REGARDING THE STRUCTURE OF THE TROUGH
AND WHEN AND HOW MUCH MOISTURE WILL MOVE N OVER W WA. FOR NOW THE
FORECASTS FOLLOW A ROUGH BLEND OF THE GFS AND ECMWF WITH A CHANCE OF
SHOWERS OVER W WA TUESDAY NIGHT AND WEDNESDAY. KAM

&&

.AVIATION...WESTERLY FLOW ALOFT WILL CONTINUE TONIGHT AS A FLAT
UPPER LEVEL RIDGE REMAINS OVER THE AREA. AT THE SURFACE HIGH PRES
WILL GIVE GENERALLY LIGHT WINDS. THE AIR MASS IS STABLE AND DRYING.
A CUMULUS FIELD OVER LAND AREAS WILL EVAPORATE AFTER SUNSET. SOME
HIGH CLOUDS WILL DRIFT OVER THE AREA FROM TIME TO TIME. CONDITIONS
WILL BE VFR.

KSEA...DISCUSSION ABOVE APPLIES. NORTHERLY WIND 4-6 KT WILL CONTINUE
TONIGHT AND THURSDAY. CHB

&&

.MARINE...HIGH PRES WILL REMAIN OVER THE AREA THROUGH THURSDAY
GIVING GENERALLY LIGHT WINDS. HOWEVER THERE IS STILL SOME ONSHORE
COMPONENT AND PRES GRADIENTS MAY BECOME STRONG ENOUGH FOR SMALL
CRAFT ADVISORY WESTERLY WINDS IN THE STRAIT THIS EVENING. PRES
GRADIENTS WILL BE LIGHT LATER TONIGHT THROUGH THURSDAY NIGHT.

A WEAK FRONT WILL MOVE THROUGH THE AREA ON FRIDAY. THIS FRONT WILL
INCREASE ONSHORE FLOW FRIDAY AFTERNOON THROUGH SATURDAY ALONG THE
COAST AND WESTERLY FLOW IN THE STRAIT OF JUAN DE FUCA DURING THE
AFTERNOON AND EVENING HOURS. SMALL CRAFT ADVISORY STRENGTH WESTERLY
WINDS ARE POSSIBLE EACH AFTERNOON AND EVENING IN THE STRAIT AND ON
THE COAST STARTING FRIDAY AFTERNOON. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT.

&&

$$

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











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