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000
FXUS66 KSEW 141622
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
922 AM PDT FRI JUN 14 2013

.SYNOPSIS...THE LINGERING EFFECTS OF ONSHORE FLOW WILL KEEP SKIES
MOSTLY CLOUDY THROUGH MID THIS AFTERNOON. SKIES WILL CLEAR LATE THIS
AFTERNOON AND TONIGHT AS A SMALL UPPER LEVEL RIDGE MOVES ACROSS THE
AREA. A LARGE UPPER LEVEL LOW WELL OFFSHORE WILL BRING WARM
SOUTHERLY FLOW ALOFT THIS WEEKEND LEADING TO MOSTLY SUNNY AND WARM
CONDITIONS. MOSTLY CLOUDY SKIES WITH A CHANCE OF SHOWERS WILL RETURN
NEXT WEEK AS THE UPPER LEVEL LOW GRADUALLY NEARS THE AREA THEN MOVES
INLAND ON WEDNESDAY.

&&

.SHORT TERM...ONSHORE FLOW OVERNIGHT HAS BROUGHT A SUBSTANTIAL
MARINE LAYER TO W WA THIS MORNING. VISIBLE SATELLITE
IMAGERY...OBS...AND THE NEW EXPERIMENTAL LOW CLOUD THICKNESS DATA
SHOW CLOUD BASES AROUND PUGET SOUND AROUND 3000 FEET WITH TOPS
ANYWHERE FROM 4600 TO 5500 FEET. CLOUD BASES ARE LOWER OVER THE N
INTERIOR IN SPOTS WITH MVFR CIGS. CLOUD THICKNESSES SEEM TO BE
RUNNING 1000-1500 FEET IN AREAS WHERE THERE ARE NO BREAKS IN THE
OVERCAST. WITH THE HIGH CLOUD BASES AND FAIRLY THICK CLOUD LAYERS IT
WILL TAKE QUITE A WHILE FOR CLEARING TO DEVELOP. THERE WILL BE
OCCASIONAL BREAKS IN THE OVERCAST TODAY...BUT CLEARING WILL HOLD OFF
UNTIL LATE THIS AFTERNOON AND THIS EVENING. WITH 8 AM TEMPERATURES
IN THE LOWER TO MID 50S...HIGHS TODAY IN THE 60S STILL LOOK GOOD.

A SMALL UPPER LEVEL RIDGE WILL MOVE OVER W WA THIS AFTERNOON AND
TONIGHT...WHICH WILL ALTER THE SURFACE RIDGE ALONG THE COAST JUST
ENOUGH FOR DRIER N LOW LEVEL FLOW TO DEVELOP LATE THIS AFTERNOON AND
EVENING. COMBINED WITH THE NORMAL DIURNAL CLEARING...THIS WILL HELP
SET UP A MOSTLY SUNNY WEEKEND.

THE DOMINANT FEATURE THIS WEEKEND THROUGH NEXT WEEK WILL BE THE
LARGE UPPER LEVEL LOW CURRENTLY OFFSHORE NEAR 49N/140W. MODELS AGREE
THAT THE LOW WILL CONTINUE MOVING SLOWLY S THROUGH THIS
WEEKEND...THEN HEAD SLOWLY E TOWARD THE COAST SUNDAY EVENING. THE
POSITION OF THE LOW WILL ALLOW WARM S FLOW ALOFT TO DEVELOP OVER W
WA SATURDAY...LEADING TO ABOUT 10 DEGREES OF WARMING AND HIGHS IN
THE 70S. WITH CONTINUING WARM S FLOW ALOFT SUNDAY LOOKS LIKE A
REPEAT OF SATURDAY. ONSHORE FLOW WILL BE WEAK THIS WEEKEND AND
STRATUS WILL CONTINUE TO BLANKET THE COASTAL WATERS. STRATUS SHOULD
COVER THE COAST EACH MORNING THIS WEEKEND AND THERE WILL PROBABLY BE
SOME PATCHY STRATUS OVER THE INTERIOR...EXCEPT THAT THE 12Z NAM
SHOWED A BIT MORE OVER THE SW INTERIOR SUNDAY MORNING.

THE S FLOW ALOFT OVER THE AREA THIS WEEKEND MAY ALSO LEAD TO
AFTERNOON CONVECTION ALONG THE CASCADE CREST. THIS LOOKS LIKE A
COMBINATION OF DIURNAL INSTABILITY AIDED AT TIMES BY SMALL WEAK
SHORTWAVE TROUGHS CROSSING THE AREA. THE CURRENT FORECAST FOR A
CHANCE OF AFTERNOON AND EVENING THUNDERSTORMS OVER THE CASCADES
LOOKS FINE. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 340 AM AFD...EXTENDED
MODELS THROWING A BIT OF A WRINKLE INTO THE EXTENDED FORECAST THIS
MORNING. FOR THE LAST FEW MODEL RUNS BEFORE THE 00Z RUN THE MODELS
HAD BEEN REALLY CONSISTENT WITH THE TIMING OF THE EASTWARD MOVEMENT
OF THE UPPER LEVEL LOW OFFSHORE MOVING THE LOW OVER WESTERN
WASHINGTON ON TUESDAY. 00Z RUNS ARE SLOWER WITH THE EASTWARD
PROGRESSION OF THE LOW BY ABOUT 12 HOURS WITH THE LOW STILL WEST OF
130W ON MONDAY. WITH THE SLOWER TRACK OF THE LOW MODELS HAVE REDUCED
THE CHANCES FOR SHOWERS OVER WESTERN WASHINGTON ON MONDAY AND BUMPED
UP THE HIGH TEMPERATURES BY UP TO 5 DEGREES. SINCE THIS IS A NEW
SOLUTION WILL STAY WITH THE CHANCE OF SHOWERS BEGINNING SUNDAY NIGHT
BUT WILL UP THE MAX TEMPERATURES ON MONDAY CLOSER TO THE GUIDANCE.
BOTH THE GFS AND ECMWF ARE VERY SIMILAR WITH THE UPPER LEVEL LOW
MOVING INTO THE OFFSHORE WATERS ON TUESDAY...OVER WESTERN WASHINGTON
ON WEDNESDAY...THEN MOVING OFF TO THE EAST ON THURSDAY. WILL STAY
WITH THE BROADBRUSH CHANCE OF SHOWERS FORECAST THROUGH THURSDAY WITH
HIGH TEMPERATURES A FEW DEGREES BELOW NORMAL. FELTON

&&

.AVIATION...ONSHORE FLOW WILL CONTINUE TODAY WITH ISOLATED SHOWERS
OVER THE NORTH INTERIOR ENDING THIS MORNING. UPPER LEVEL RIDGING
WILL MAINTAIN A STABLE AIR MASS. THE STRATUS LAYER OVER THE LOWLANDS
IS AROUND 1000FT THICK WITH CIGS 2000-3000FT. EXPECT A SLOW BURN-OFF
TODAY WITH CIGS BKN-OVC THROUGH 21-24Z. VFR CONDITIONS WILL PREVAIL
TONIGHT WITH INCREASING CIRRUS CLOUDS ALOFT. 33

KSEA...EXPECT A SLOW BURN-OFF TODAY WITH CIGS HOVERING AROUND 3000
FT. SURFACE WINDS WILL REMAIN S/SW AROUND 10KT. WINDS WILL TURN
NORTHERLY TONIGHT BY 06Z. 33

&&

.MARINE...HIGH PRES OVER THE OFFSHORE WATERS AND LOWER PRES EAST OF
THE CASCADES WILL PRODUCE LIGHT TO MODERATE ONSHORE FLOW ACROSS
WESTERN WA THROUGH MON. THE CENTRAL/EAST STRAIT OF JUAN DE FUCA MAY
SEE SMALL CRAFT ADVISORY WINDS SAT NIGHT AND SUN NIGHT. A BROAD
SURFACE LOW /1012 MB/ WILL SPIN OVER THE OFFSHORE WATERS ON
TUE...THEN WEAKEN AND SHIFT INLAND TUE NIGHT. 33

&&

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

&&

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