Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 262217
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT TUE AUG 26 2014

.SYNOPSIS...WEDNESDAY WILL BE WARM AND SUNNY AS A HIGH PRESSURE
RIDGE MOVES EAST OUT OF THE AREA. A SHORTWAVE UPPER LEVEL TROUGH
WILL MOVE THROUGH THE AREA THURSDAY...BRINGING COOLER WEATHER AND
AREAS OF DRIZZLE. A LONGWAVE UPPER LEVEL TROUGH WILL MOVE THROUGH
THE AREA THIS WEEKEND...BRINGING A WIDESPREAD CHANCE OF SHOWERS.

&&

.SHORT TERM...AN UPPER LEVEL RIDGE AXIS IS OVER THE REGION
NOW...BRINGING WARM TEMPERATURES AND CLEAR SKIES. WE ARE IN AN
ONSHORE FLOW PATTERN...AND LATE TONIGHT/WEDNESDAY MORNING LOW CLOUDS
AND FOG WILL MOVE INLAND. THE NAM12 BOUNDARY LAYER RH SHOWS THE
CLOUDS REACHING ALL THE WAY TO THE PUGET SOUND REGION BUT NOT
COVERING ALL OF IT. WEDNESDAY...THE UPPER LEVEL RIDGE AXIS WILL MOVE
TO THE EAST...AND TEMPERATURES WILL FALL A FEW DEGREES. HOWEVER...IT
WILL STILL BE WARM AND DRY.

THURSDAY MORNING WE WILL SEE ANOTHER PUSH OF LOW
CLOUDS INLAND. THE MARINE LAYER WILL LIKELY BE A BIT DEEPER AND TAKE
LONGER TO BURN OFF. I HAVE INCLUDED AREAS OF MORNING DRIZZLE IN THE
FORECAST FOR SOME LOWER ELEVATIONS BASED ON THE THICKNESS OF THE
MARINE LAYER. DURING THE DAY ON THURSDAY A SMALL UPPER LEVEL
SHORTWAVE TROUGH WILL MOVE OVER THE AREA...AND THERE IS A SLIGHT
CHANCE OF SHOWERS OVER NORTHWESTERN WASHINGTON...ESPECIALLY IN THE
NORTH CASCADES.

AN UPPER LEVEL LONGWAVE TROUGH WILL BEGIN TO MOVE INTO THE AREA ON
FRIDAY. ANY SHOWERS EARLIER IN THE DAY ON FRIDAY SHOULD BE IN THE
MOUNTAINS...HOWEVER THE GFS SHOWS A CHANCE OF SHOWERS IN THE
LOWLANDS BEGINNING IN THE LATE AFTERNOON. JSMITH

JSMITH

.LONG TERM...AS THE UPPER LEVEL TROUGH MOVES THROUGH THE
REGION...THE CHANCE OF SHOWERS WILL CONTINUE FROM FRIDAY NIGHT
THROUGH SUNDAY...WITH THE SHOWERS MOST WIDESPREAD ON SATURDAY. THE
GFS AND ECMWF DISAGREE BEGINNING MONDAY ON WHAT WILL HAPPEN
NEXT...THE GFS HAS SHOWERS CONTINUING OFF AND ON LABOR DAY AND
TUESDAY...BUT THE ECMWF SHOWS A TREND OF DRYING OUT WITH AN UPPER
LEVEL RIDGE BUILDING IN. BECAUSE OF THIS...THERE IS QUITE A BIT OF
UNCERTAINTY IN THE FORECAST FROM LABOR DAY ONWARDS. JSMITH

&&

.AVIATION...AN UPPER LEVEL RIDGE WILL REMAIN OVER E WA THROUGH
06Z/11 PM TONIGHT WITH W FLOW ALOFT. THE UPPER LEVEL RIDGE WILL
WEAKEN AND MOVE E OVER ID AFTER 06Z. WEAK LOW LEVEL ONSHORE FLOW
WILL CONTINUE. THE AIR MASS WILL REMAIN DRY AND STABLE OVER THE
INTERIOR...BUT MVFR/IFR STRATUS WITH PATCHY FOG WILL SPREAD INLAND
FROM THE COAST OVERNIGHT...PUSHING E THROUGH THE STRAIT TO WHIDBEY
ISLAND IN THE N INTERIOR...AND INLAND THROUGH THE CHEHALIS VALLEY TO
SW PUGET SOUND.

KSEA...CLEAR SKIES THROUGH TONIGHT. STRATUS IS NOT EXPECTED TO REACH
KSEA WEDNESDAY MORNING BUT THE NAM MODEL SHOWS A TINY BIT OF LOW
LEVEL MOISTURE SO I WILL INCLUDE A SCT005 LAYER TO INDICATE
SOMETHING COULD BE AROUND. SURFACE WINDS BE NW 4-8 KT THROUGH 02Z
THEN SHIFT N 3-6 KT. KAM

&&

.MARINE...A SURFACE HIGH OVER THE NE PAC WILL MAINTAIN AN ONSHORE
FLOW PATTERN ACROSS WESTERN WA THROUGH FRIDAY NIGHT. ONSHORE FLOW
WILL BE STRONGEST DURING THE EVENING HOURS AND WEAKEST DURING THE
MORNING. A SCA REMAINS IN EFFECT FOR THE CENTRAL/E STRAIT OF JUAN DE
FUCA THIS EVENING. ONSHORE PRESSURE GRADIENTS ARE A LITTLE AHEAD OF
YESTERDAY SO IT IS MORE LIKELY THAT WINDS WILL REACH SCA LEVELS. THE
INCREASING ONSHORE FLOW WILL LEAD TO MORE STRATUS/FOG FORMING IN THE
STRAIT WEDNESDAY MORNING.

A WEAK DISSIPATING COLD FRONT WILL MOVE OVER THE COASTAL WATERS LATE
WEDNESDAY NIGHT OR THURSDAY MORNING. ONSHORE FLOW WILL INCREASE
AHEAD OF THE FRONT WEDNESDAY EVENING. THE CANADIAN 2.5KM MODEL
INDICATED 30-35 KT WINDS IN THE STRAIT AND THE WRF-GFS TOPPED OUT AT
25 KT. FORECAST WINDS HAVE BEEN BOOSTED TO 20-30 KT. THERE IS A
DECENT CHANCE FOR RACE ROCKS TO REACH GALE FORCE...BUT I WILL HOLD
OFF FOR ANOTHER RUN TO SEE IF THE MODELS REMAIN CONSISTENT. WINDS
HAVE BEEN RAISED TO SCA LEVELS OVER ADMIRALTY INLET AND OVER THE N
INLAND WATERS ADJACENT TO THE E ENTRANCE.

ANOTHER WEAK FRONTAL SYSTEM WILL REACH THE AREA FRIDAY NIGHT OR
SATURDAY MORNING. KAM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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








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