Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 210418
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT WED AUG 20 2014

.SYNOPSIS...LOW LEVEL ONSHORE FLOW IS EXPECTED TO CONTINUE INTO
FRIDAY MAINTAINING AREAS OF MARINE CLOUDS AND COOLER THAN AVERAGE
TEMPERATURES. A WEAK LOW PRESSURE SYSTEM ALOFT IS EXPECTED TO MOVE
ONSHORE FROM THE GULF OF ALASKA LATER THURSDAY AND FRIDAY BRINGING A
THREAT OF SHOWERS MAINLY TO THE MOUNTAINS. HIGHER PRESSURE ALOFT IS
FORECAST TO BUILD ONSHORE OVER THE WEEKEND AND INTO NEXT WEEK FOR
WARMER WEATHER AND MORE SUNSHINE.

&&

.SHORT TERM...WEAK ONSHORE FLOW AND AN EXISTING MOIST MARINE AIR
MASS WILL RESULT IN AREAS OF LOW CLOUDS DEVELOPING AGAIN LATER
TONIGHT AND THURSDAY MORNING. A SHORT WAVE EMBEDDED IN NW FLOW ALOFT
WILL APPROACH THE AREA ON THURSDAY. THE MAIN EFFECT OF THIS SYSTEM
WILL BE TO DEEPEN THE MARINE LAYER AND ALLOW MARINE STRATUS TO MIX
OUT FASTER THAN THIS MORNING. THE SYSTEM WILL ONLY BRING A LIMITED
AMOUNT OF MID LEVEL MOISTURE BUT THE ASSOCIATED WEAK LIFT COULD
PRODUCE A FEW SHOWERS OVER THE NORTH CASCADES AND EVEN THE FAR N
INTERIOR. THE GFS MOS GUIDANCE SHOWS LOW CHANCE POPS ACROSS MAINLY
NRN PORTIONS OF THE FORECAST AREA WHICH IS ALREADY INDICATED IN THE
FORECAST SO NO UPDATES PLANNED. HIGH TEMPERATURES WILL REMAIN BELOW
AVERAGE IN THE UPPER 60S TO AROUND 70.

MODELS INDICATE THE UPPER LOW WILL CLOSE OFF OVER WA THURSDAY NIGHT
AND SHIFT SE ON FRIDAY. COOLER AIR ALOFT AND A SLIGHT INCREASE IN
LOW/MID LEVEL MOISTURE SHOULD RESULT IN WEAKLY UNSTABLE CONDITIONS
ACROSS THE AREA...ESPECIALLY THE MOUNTAINS. GFS LI VALUES ARE BELOW
ZERO E OF PUGET SOUND AND LOWEST OVER THE CASCADES AND OLYMPICS. THE
BEST CHANCE OF SHOWERS WILL BE IN THE FOOTHILLS AND MOUNTAINS BUT A
FEW SHOWERS OVER THE LOWLANDS CANNOT BE RULED OUT FRIDAY. GRADIENTS
ARE LIGHTER AND MORE NWLY ON FRIDAY WITH LESS CLOUD COVER SO HIGH
TEMPERATURES SHOULD WARM A FEW DEGREES INTO THE LOW/MID 70S AROUND
PUGET SOUND.

HIGH PRESSURE ALOFT OVER THE ERN PAC NUDGES INTO THE PAC NW
SATURDAY. RISING UPPER HEIGHTS AND WEAK NW ONSHORE FLOW SHOULD
RESULT IN DRY WEATHER AND A WARMING TREND. EXPECT SUNSHINE WITH
HIGHS HEADING TOWARD 80 ON SATURDAY. MERCER

.LONG TERM...FROM PREVIOUS DISCUSSION...ALL THE PROGS SEEM TO BE
GENERALLY ON THE SAME PAGE THROUGH WEDNESDAY. THE RIDGING ALOFT AND
WARMING TREND CONTINUES INTO SUNDAY. SO THE WEEKEND LOOKS TO HAVE
MORE SUNSHINE AND HIGH TEMPS RISING TO AROUND OR A BIT ABOVE
AVERAGE. A WEAK SHORTWAVE DIGS SE MONDAY IN THAT NW FLOW ALOFT
SUNDAY NIGHT AND MONDAY. THE PRIMARY IMPACT ON WESTERN WA WILL BE TO
INCREASE LOW LEVEL ONSHORE FLOW FOR A BRIEF COOL DOWN. THEN THE
RIDGE ALOFT REBOUNDS TUESDAY AND WEDNESDAY WITH LOW LEVEL ONSHORE
FLOW AGAIN EASING FOR MORE SUNSHINE AND TEMPS RISING A BIT ABOVE
SEASONAL NORMS. BUEHNER

&&

.AVIATION...NORTHWESTERLY FLOW ALOFT BECOMING WESTERLY ON THURSDAY
AS AN UPPER LEVEL TROUGH MOVES INTO THE AREA. LOW LEVEL FLOW WILL
REMAIN ONSHORE WITH PERSISTENT HIGH PRESSURE OVER THE OFFSHORE
WATERS AND LOWER PRESSURE INLAND. AIR MASS STABLE BECOMING SLIGHTLY
UNSTABLE THURSDAY AFTERNOON.

LAST VISIBLE IMAGERY OF THE DAY SHOWED JUST A BROKEN THIN LAYER OF
STRATUS ALONG THE COAST AND SOME SC OVER THE MOUNTAINS. SURFACE
GRADIENTS ARE WEAK ONSHORE WITH KOLM-KHQM AND KOLM-KBLI NEAR 1 MB.
K0TH-KSEA IS LESS THAN A MILLIBAR WITH THE CROSS CASCADE GRADIENT
AROUND PLUS 8 MB. BOTH THE KUIL SOUNDING AND THE VAD WIND PROFILER
AT KATX SHOW LIGHT SOUTHERLY WINDS IN THE LOWER LEVELS. THE KLGX VAD
WIND PROFILE SHOWS WEST NORTHWESTERLY WINDS AT 2000 FEET. WITH THE
ONSHORE GRADIENTS NOT EXPECTED TO INCREASE MUCH MORE OVERNIGHT THE
STRATUS OVER THE INTERIOR THURSDAY MORNING WILL HAVE TO FORM IN
PLACE. THIS DOES NOT BODE WELL FOR A WIDESPREAD DEEP MARINE LAYER
OVER THE INTERIOR IN THE MORNING. WITH THE CEILINGS ALONG THE COAST
IN THE 1400-1800 FOOT RANGE WILL GO FOR CEILINGS OVER THE INTERIOR
THURSDAY MORNING NEAR THIS HEIGHT BUT WILL SLOW DOWN THE ONSET OF
THE STRATUS OVER THE INTERIOR IN THE 06Z TAFS TO CLOSER TO 12Z. WITH
THE LAYER NOT BEING VERY DEEP EXPECT A LATE MORNING OR MIDDAY
BREAKOUT FOR THE INTERIOR WITH A LATER BREAKOUT FOR THE COAST.

KSEA...MOSTLY CLEAR SKIES INTO THE EARLY MORNING HOURS WITH CEILINGS
AROUND 1500 FEET DEVELOPING AROUND 12Z. STRATUS DECK SCATTERING OUT
MIDDAY. SOUTHWEST WINDS 4 TO 8 KNOTS THROUGH THURSDAY AFTERNOON.
FELTON

&&

.MARINE...THE VERY STATIC ONSHORE FLOW PATTERN WILL CONTINUE FOR THE
NEXT SEVERAL DAYS. HIGH PRESSURE OVER THE OFFSHORE WATERS AND LOWER
PRESSURE INLAND WILL RESULT IN SMALL CRAFT ADVISORY WINDS FOR THE
CENTRAL AND EASTERN STRAIT IN THE EVENING AND EARLY MORNING HOURS.
FELTON

&&

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

&&

$$

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