Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 180359
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT THU JUL 17 2014

.SYNOPSIS...MODERATE ONSHORE FLOW TONIGHT WILL BRING WIDESPREAD LOW
CLOUDS FRIDAY MORNING. FRIDAY AFTERNOON WILL BE PARTLY SUNNY AS HIGH
CLOUDS FROM AN APPROACHING UPPER LEVEL LOW SPREAD INLAND. THE UPPER
LEVEL LOW WILL BRING SCATTERED SHOWERS TO THE AREA FRIDAY
NIGHT...WITH SHOWERS INCREASING SATURDAY AFTERNOON AND EVENING AS
THE LOW NEARS THE WASHINGTON COAST. A BROAD UPPER LEVEL TROUGH WILL
REMAIN OVER THE REGION THROUGH MID NEXT WEEK FOR MOSTLY CLOUDY SKIES
AND SCATTERED SHOWERS.

&&

.SHORT TERM...MODERATE NW FLOW ALOFT WILL CONTINUE OVER THE PACNW
INTO EARLY NEXT WEEK PRODUCING A MORE ACTIVE WEATHER PATTERN.

FOR TONIGHT...THE MAIN PROBLEM IS HOW MUCH STRATUS WILL TONIGHTS
ONSHORE FLOW PRODUCE FRIDAY MORNING. THE SW ONSHORE FLOW IS ALMOST
AS STRONG AS LAST NIGHT SO THE MAIN DIFFERENCE IS THE LACK OF
STRATUS ALONG THE COAST THIS EVENING. WITH SOMEWHAT LESS MOIST AIR
THAN NORMAL BEING ADVECTED INLAND...FRIDAY MORNINGS STRATUS COVERAGE
MAY FORM LATER AND HAVE A LITTLE LESS COVERAGE. THE CURRENT FORECAST
LOOKS PRETTY CLOSE...BUT MAY BE OVERDOING THE STRATUS IN THE STRAIT
OF JUAN DE FUCA AND OVER THE FAR N INTERIOR. BASED ON NAM
TIME-HEIGHT SECTIONS THE STRATUS WILL BE SHALLOW ONCE AGAIN AND
LIKELY BURN OFF BY MIDDAY.

THE NW FLOW ALOFT WILL BRING HIGH LEVEL MOISTURE AHEAD OF AN
APPROACHING UPPER LEVEL LOW OVER W WA FRIDAY AFTERNOON. MID LEVEL
AND AND LOWER LEVEL MOISTURE WILL INCREASE FRIDAY EVENING. GIVEN THE
MID DAY STRATUS BURN OFF AND PERHAPS MAINLY HIGH CLOUDS IN THE
AFTERNOON...THERE MIGHT BE ENOUGH SUNSHINE AROUND TO UPGRADE TO
PARTLY SUNNY FRIDAY AFTERNOON. I WILL LET THE MIDSHIFT MAKE THAT
CALL.

MODELS AGREE THAT THE SMALL UPPER LEVEL LOW NEAR 55N/160W AT 00Z/5
PM THIS AFTERNOON WILL DRIVE A WETTER AIR MASS ACROSS W WA FRIDAY
NIGHT THROUGH SATURDAY NIGHT. THE FIRST CHANCE OF SHOWERS WILL BE
FRIDAY NIGHT...WITH CHANCE POPS EVERYWHERE...HIGHER END OVER THE
NORTH AND TAPERING DOWN ACROSS PUGET SOUND AND THE SW INTERIOR. THE
WETTEST PERIOD APPEARS TO BE SATURDAY AFTERNOON AND NIGHT WHEN WE
HAVE LIKELY POPS OVER THE COAST AND NORTH PART TAPERING TO CHANCE
POPS OVER THE CENTRAL/SOUTHERN INTERIOR. CONFIDENCE IS LOW ON HOW
MUCH PRECIP WILL FALL...MAINLY BECAUSE SOME OF THE MODELS APPEAR TO
BE OVERLY HEAVY ON THE PRECIP FOR THIS DRY TIME OF YEAR.

BY SUNDAY A BROAD UPPER LEVEL TROUGH WILL HAVE SETTLED IN OVER THE
PACNW AND W CANADA. W FLOW ALOFT THROUGH THE TROUGH AND ACROSS W WA
MAY BRING A WEAKER DISTURBANCE OR TWO ACROSS THE AREA...BUT
ALL-IN-ALL SUNDAY MAY BE MORE DRY THAN WET WITH JUST SOME SCATTERED
SHOWERS AROUND.

WITH NW FLOW ALOFT THEN THE TROUGH OVER THE AREA...THE NEXT SEVERAL
DAYS WILL BE SLIGHTLY COOLER THAN NORMAL WITH HIGHS FROM THE MID 60S
ALONG THE COAST TO THE LOWER TO MID 70S INLAND. KAM

.LONG TERM...A LONGWAVE TROUGH REMAINS OVER BRITISH COLUMBIA THROUGH
ABOUT TUESDAY NIGHT. THIS WILL KEEP A CHANCE OF SHOWERS IN THE
FORECAST THROUGH WEDNESDAY...THOUGH ANY SHOWER ACTIVITY IS LIKELY TO
BE CONFINED TO THE MOUNTAINS AND PERHAPS THE EXTREME NORTH. THE GFS
IS LARGELY DRY MONDAY THROUGH WEDNESDAY AND EVEN THE LATELY WETTER
EURO IS PRETTY DRY.

ON THURSDAY THE UPPER TROUGH IS WELL TO THE EAST AND HEIGHTS REBOUND
IN BOTH MODELS TO 580 DM. THAT SHOULD BE ENOUGH TO MAKE THURSDAY
SUNNY AND WARM. BURKE


&&

.AVIATION...LIGHT NW FLOW TONIGHT AS AN UPPER TROUGH MOVES EWD.
MODERATE W-NW FLOW WILL DEVELOP LATER ON FRIDAY AS ANOTHER STRONGER
TROUGH APPROACHES. THE AIR MASS WILL REMAIN STABLE AND DRY EXCEPT
FOR MOISTURE IN THE SURFACE-BASED MARINE LAYER.

A MIX OF WIDESPREAD LOW-END MVFR AND IFR STRATUS IS EXPECTED TO TO
DEVELOP ACROSS THE WESTERN WASHINGTON INTERIOR LOWLANDS ROUGHLY 10Z
THROUGH 18Z. TOPS WILL PROBABLY BE AROUND 2500 FT. THE LOW CLOUDS
SHOULD SCATTER OUT AGAIN MIDDAY FRIDAY.

KSEA...SOUTH TO SOUTHWEST WIND 5-10 KT. STRATUS WILL PROBABLY MOVE
INTO KSEA AGAIN LATE TONIGHT WITH IFR OR LOW-END MVFR CONDITIONS
ROUGHLY 10Z-18Z. DTM

&&

.MARINE...ONSHORE FLOW WILL CONTINUE TO INCREASE THIS EVENING. A
GALE WARNING REMAINS IN EFFECT FOR THE CENTRAL STRAIT OF JUAN DE
FUCA AND EAST ENTRANCE STRAIT...WITH MODERATE CONFIDENCE. SMALL
CRAFT ADVISORIES ARE ALSO IN EFFECT FOR THE COASTAL
WATERS...ADMIRALTY INLET...AND THE NORTHERN INLAND WATERS.

MODERATELY STRONG ONSHORE FLOW FRIDAY AFTERNOON AND NIGHT COULD
PRODUCE ANOTHER ROUND OF WESTERLY GALES IN THE CENTRAL AND EASTERN
STRAIT. A GALE WATCH IS IN EFFECT FOR THAT.

A SERIES OF WEAK UPPER TROUGHS WILL MOVE THROUGH THE REGION FRIDAY
NIGHT THROUGH TUESDAY. ONSHORE FLOW WILL CONTINUE AS HIGH SURFACE
PRESSURE REMAINS OVER THE NORTHEAST PACIFIC OCEAN...WITH LOWER
PRESSURE EAST OF THE CASCADES.  DTM/MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING CENTRAL STRAIT OF JUAN DE FUCA AND EAST STRAIT
       TONIGHT.
     SMALL CRAFT ADVISORY COASTAL WATERS THROUGH NOON FRIDAY.
     SMALL CRAFT ADVISORY ADMIRALTY INLET...NORTHERN INLAND WATERS
       TONIGHT.
     GALE WATCH CENTRAL STRAIT OF JUAN DE FUCA AND EAST STRAIT
       TONIGHT.

&&

$$

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








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