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516
FXUS66 KSEW 191031
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PDT WED SEP 19 2014

.SYNOPSIS...A WEAK UPPER LEVEL TROUGH OVER THE AREA TODAY WILL KEEP
SKIES MOSTLY CLOUDY WITH A CHANCE OF MORNING SHOWERS. CLOUDS WILL
GRADUALLY DISSIPATE TONIGHT AS A STRONG UPPER LEVEL RIDGE BUILDS
OVER THE PACIFIC NORTHWEST. THE STRONG RIDGE AND LOW LEVEL OFFSHORE
FLOW WILL BRING SUNNY SKIES AND A WARMING TREND THIS WEEKEND WITH
HIGHS REACHING THE MID 70S TO MID 80S ON SUNDAY. A WEAKENING FRONT
WILL BRING INCREASING CLOUDS AND COOLER TEMPERATURES ON MONDAY WITH
A FEW COASTAL SHOWERS. A LARGE UPPER LEVEL TROUGH WILL MAINTAIN A
WETTER AND COOLER PATTERN ALL NEXT WEEK.

&&

.SHORT TERM...SATELLITE IMAGERY AT 09Z/2 AM SHOWED CLOUDS FROM A
WEAK UPPER LEVEL SHORTWAVE TROUGH OVER THE N PART OF W WA AND
OFFSHORE VANCOUVER ISLAND. RADAR WAS PICKING UP SOME SHOWERS OVER
THE SAME AREA. THE AIR MASS OVER THE REST OF W WA REMAINS MOIST WITH
LOTS OF MVFR STRATUS MIXED IN WITH MID LEVEL CLOUDS. LATEST MODELS
INDICATE THE WEAK SHORTWAVE TROUGH WILL WEAKEN RAPIDLY THIS MORNING
AS AN UPPER LEVEL RIDGE BEGINS BUILDING OVER THE REGION. THE CHANCE
OF SHOWERS STILL LOOKS GOOD THIS MORNING ESPECIALLY FOR THE NORTH
PART OF W WA. HOWEVER WITH MODEL GUIDANCE AND MOS GUIDANCE KEEPING
IT DRY OVER THE LOWLANDS THIS AFTERNOON...PRECIP HAS BEEN DROPPED.

BOTH THE NAM AND GFS TIME-HEIGHT SECTIONS SHOW GRADUALLY DECREASING
CLOUDS OVERNIGHT...SO IT SHOULD BE MOSTLY CLEAR BY SATURDAY MORNING.
THE LACK OF CLOUD COVER LATE TONIGHT WILL ALLOW GOOD RADIATIONAL
COOLING CONDITIONS...RESULTING IN AREAS OF FOG OVER MAINLY THE
INTERIOR SATURDAY MORNING. BASED ON THE VERY SHALLOW LOW LEVEL
MOISTURE ON THE NAM TIME-HEIGHT SECTIONS I OPTED TO ELIMINATE THE
LOW CLOUDS AND JUST GO WITH FOG.

THE MODELS REMAIN QUITE CONSISTENT WITH THE STRONG UPPER LEVEL RIDGE
BUILDING OVER THE REGION THIS WEEKEND. THE RIDGE AXIS WILL BE OVER W
WA ON SATURDAY...THEN SHIFT OVER W MT ON SUNDAY. LOW LEVEL N-NE
OFFSHORE FLOW WILL DEVELOP SATURDAY FOR MAX TEMPS MAINLY IN THE 70S
TO LOWER 80S. N FLOW 5-15 KT WILL PROBABLY KEEP THE N INTERIOR AND N
PUGET SOUND REGION A LITTLE COOLER WITH HIGHS NEAR 70. LOW LEVEL
FLOW WILL SHIFT TO E-NE SATURDAY NIGHT AND CONTINUE ON SUNDAY
ALLOWING MAX TEMPS TO PEAK IN THE MID 70S TO MID 80S. MOS GUIDANCE
IS STILL A LITTLE ON THE COOL SIDE...BUT SEEMS TO BE GRADUALLY
CATCHING UP WITH WHAT WE HAVE HAD IN THE FORECAST THE PAST FEW DAYS.

THE EASTERLY OFFSHORE FLOW WILL BRING A THERMALLY INDUCED SURFACE
TROUGH N OVER THE WA COAST ON SUNDAY AND THE TYPICAL COASTAL STRATUS
SURGE WILL FOLLOW IT. LOW STRATUS SHOULD MAKE IT ALL THE WAY UP THE
COAST BY SUNDAY AFTERNOON. THE CURRENT FORECAST FOLLOWS THE NAM
BOUNDARY LAYER RH GUIDANCE BUT THE ACTUAL STRATUS SURGE IS
FREQUENTLY FASTER. THE IMMEDIATE COAST WILL GET SOME COOLING FROM
THE RESULTING ONSHORE FLOW SUNDAY MORNING...BUT A FASTER SURGE COULD
ALLOW THE ONSHORE FLOW TO PENETRATE A BIT FARTHER INLAND IN THE
AFTERNOON. KAM

.LONG TERM...ALL FACTORS ARE IN PLACE FOR A SW MARINE PUSH SUNDAY
NIGHT. STRATUS SURGE UP THE COAST ON SUNDAY...UPPER LEVEL RIDGE AXIS
SHIFTING OVER W MT...AND WEAKENING COLD FRONT APPROACHING THE COAST.
THE SW MARINE PUSH SUNDAY NIGHT SHOULD PUSH STRATUS UP THROUGH AT
LEAST MOST OF PUGET SOUND BY MONDAY MORNING. EARLIER NAM RUNS WERE
LESS EXTENSIVE WITH THE BOUNDARY LAYER RH MONDAY MORNING...BUT THE
LATEST 06Z NAM COVERS ALL OF W WA. THE NAM TIME-HEIGHT SECTION SHOWS
A MARINE LAYER AROUND 4000 FEET DEEP MONDAY MORNING. IF THIS WORKS
OUT THEN CLOUD COVER WILL HAVE TO BE INCREASED FOR MONDAY.

THE MODELS HAVE BEEN FAIRLY CONSISTENT WITH MONDAYS APPROACHING
FRONTAL SYSTEM. MODELS SHOW THE FRONT REMAINING INTACT UNTIL IT
MOVES E OF 130W MONDAY MORNING. AT THAT POINT MODEL CROSS-SECTIONS
SHOW THE UPPER LEVEL TROUGH SHEARING AWAY FROM THE LOW LEVEL
FRONT...LEAVING IT TO STALL ALONG THE COAST. THE MODELS HAVE BEEN
DRY FOR W WA ON MONDAY...BUT WITH A DEEP MARINE LAYER IT IS POSSIBLE
THAT SOME DRIZZLE COULD DEVELOP MONDAY MORNING...CAUSED BY WEAK LIFT
FROM THE UPPER TROUGH AS IT CONTINUES MOVING INLAND.

BOTH GFS AND ECMWF HAVE A LONGWAVE UPPER LEVEL TROUGH DEVELOPING
OFFSHORE OVER THE NE PACIFIC BY TUESDAY WITH SW FLOW ALOFT OVER W
WA. THE TROUGH WILL DOMINATE THE PATTERN ALL NEXT WEEK FOR PERIODS
OF RAIN OR SHOWERS OVER W WA. BOTH MODELS SHOW A COLD FRONT REACHING
W WA TUESDAY NIGHT OR EARLY WEDNESDAY. THIS SYSTEM WILL PROBABLY
SCOOP UP THE REMNANTS OF MONDAYS STALLED FRONT AS IT MOVES INLAND.
KAM

&&

.AVIATION...WESTERLY FLOW ALOFT TODAY AND TONIGHT WITH AN UPPER
RIDGE BUILDING OVER THE PACIFIC NORTHWEST. A WEAKENING FRONT WILL
REACH THE COAST THIS MORNING AND DISSIPATE OVER THE AREA THIS
AFTERNOON. WEAK ONSHORE FLOW WILL CONTINUE TODAY WITH HIGH PRESSURE
ALONG THE COAST AND LOWER PRESSURE INLAND. THE LOW LEVEL FLOW WILL
QUICKLY TURN NORTHERLY TONIGHT AS A TROUGH BUILDS NORTH ALONG THE
COAST.

AIR MASS MOIST AND SLIGHTLY UNSTABLE TODAY...DRYING AND BECOMING
STABLE THIS AFTERNOON AND TONIGHT. THERE WILL BE A MIX OF LOW AND
MID CLOUDS ACROSS THE AREA THIS MORNING. CLOUDS WILL LIFT AND BREAK
UP THIS AFTERNOON WITH MOSTLY CLEAR SKIES TONIGHT. SOME FOG...
ESPECIALLY IN SHELTERED VALLEYS...IS LIKELY LATER TONIGHT.

KSEA...1000-2000 FOOT CEILINGS WILL LIFT LATE THIS MORNING...THEN
BREAK UP DURING THE AFTERNOON. A FEW HOURS OF FOG IS POSSIBLE
SATURDAY MORNING AT THE TERMINAL. SOUTHERLY WIND 4-8 KNOTS...
SWITCHING TO NORTHERLY LATE THIS AFTERNOON OR THIS EVENING. SCHNEIDER

&&

.MARINE...A WEAK AND DISSIPATING FRONT WILL MOVE THROUGH THE WATERS
TODAY. LOW LEVEL NORTHERLY OFFSHORE FLOW WILL QUICKLY DEVELOP
TONIGHT AND CONTINUE THROUGH SUNDAY MORNING AS A TROUGH BUILDS NORTH
ALONG THE WEST COAST. SMALL CRAFT ADVISORY EAST WINDS ARE LIKELY FOR
THE WESTERN STRAIT SUNDAY MORNING.

A WEAKENING TROUGH WILL REACH THE COASTAL WATERS MONDAY. A MUCH
STRONGER FRONTAL SYSTEM WELL OFFSHORE WILL BE APPROACHING FROM THE
WEST ON TUESDAY. SCHNEIDER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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








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