Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 301618
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
915 AM PDT SAT AUG 30 2014

.SYNOPSIS...AN UPPER LEVEL TROUGH MOVING THROUGH THE PACIFIC
NORTHWEST TODAY WILL KEEP SHOWERS OVER THE AREA TODAY. WEAK UPPER
LEVEL RIDGING WILL BUILD OVER WESTERN WASHINGTON TONIGHT AS THE
UPPER LEVEL TROUGH SHIFTS EAST BRINGING DRIER CONDITIONS TO MUCH OF
THE AREA. ANOTHER RELATIVELY WEAK FRONTAL SYSTEM WILL BEGIN TO
AFFECT THE REGION SUNDAY AFTERNOON AND EVENING. HIGH PRESSURE ALOFT
WILL BUILD OVER THE AREA LABOR DAY FOLLOWED BY ANOTHER WEAK SYSTEM
LATER TUESDAY.

&&

.SHORT TERM...THE UPPER LEVEL TROUGH MOVING OVER WESTERN WASHINGTON
AND OREGON THIS MORNING AND 70 KT JET NOSING INTO CENTRAL OREGON
SUPPORTING WET CONDITIONS FROM CENTRAL OREGON NORTH INTO
SOUTHWESTERN WASHINGTON THIS MORNING. A MORE ORGANIZED BAND OF
PRECIPITATION EXTENDING FROM GRAYS HARBOR INTO THE SOUTHERN SOUND
HAS PRODUCED MODERATE RAIN AT SEVERAL LOCATIONS. EXPECT THIS BAND TO
FILL SHIFT NORTH AND EAST OVER THE NEXT FEW HOURS BRINGING RAIN AND
OR SHOWERS INTO THE METRO AREA THROUGH THE EARLY AFTERNOON. EXPECT
THE HEAVIEST PRECIPITATION TO FALL FROM SEATTLE SOUTHWARD.

PRECIPITATION WILL TAPER OFF LATE IN THE AFTERNOON AND OVERNIGHT AS
HIGH PRESSURE ALOFT BUILDS OVER THE REGION TONIGHT THROUGH SUNDAY
MORNING. BY LATE TONIGHT THE THREAT OF SHOWERS WILL BE LIMITED
MAINLY TO THE CASCADES...WITH LINGERING LIGHT SHOWERS OVER THE COAST
AND MOUNTAINS BY SUNDAY MORNING.

THE NEXT SYSTEM WILL BEGIN TO AFFECT THE AREA LATE SUNDAY AFTERNOON
AND EVENING. OVERNIGHT RUNS OF THE MODELS STRUGGLED SOMEWHAT WITH
THE TRACK...TIMING...AND STRENGTH OF THIS SYSTEM. MOST GUIDANCE
AGREES ON THE BULK OF THE PRECIPITATION FALLING SUNDAY NIGHT WHILE
SOME MODELS REMAIN SLOWER AND WEAKER...DELAYING ANY PRECIPITATION
UNTIL MONDAY MORNING. THE CURRENT FORECAST CONTINUES TO HIGHLIGHT
THE BEST CHANCE OF PRECIPITATION OVER THE NORTHWEST HALF OF THE CWA
SUNDAY NIGHT.  AN UPPER RIDGE WILL BUILD INTO THE PACIFIC NORTHWEST
ON LABOR DAY WITH DRIER CONDITIONS. SOLID ONSHORE FLOW THROUGH THE
PERIOD WILL KEEP TEMPERATURES NEAR OR SLIGHTLY BELOW NORMAL.

.LONG TERM...CONFIDENCE THROUGH THE EXTENDED PERIOD REMAINS LOW.
MEDIUM RANGE MODELS CONTINUE TO SHOW AN UPPER LEVEL TROUGH MOVING
INTO THE REGION TUESDAY NIGHT AND OR WEDNESDAY MORNING. THERE REMAIN
DIFFERENCES ON THE SPECIFIC TIMING...PLACEMENT...AND STRENGTH OF
RAINFALL. MODELS ALL SHOW THE ZONAL FLOW OVER THE NORTHEAST PACIFIC
BREAKING DOWN AT THIS POINT INTO A HIGHER AMPLITUDE PATTERN AS A
RIDGE BUILDS INTO THE GULF OF ALASKA. MODELS STILL DISAGREE ON
DOWNSTREAM CONDITIONS OVER THE PACIFIC NORTHWEST THURSDAY...SOME
SHOWING LINGERING UNSETTLED WEATHER...WHILE OTHERS SHOWING A QUICKER
TRANSITION TO DRIER CONDITIONS. MOVING INTO THE WEEKEND...THERE
CURRENTLY IS SOME CONSENSUS ON THE LARGE SCALE RIDGE SHIFTING EAST
FRIDAY THROUGH THE WEEKEND WITH DRIER AND WARMER WEATHER.

&&

.AVIATION...CLOUDY WITH AREAS OF PCPN TODAY AND THE MTNS WILL BE
MOSTLY OBSCURED. AN UPPER TROUGH OVER THE PACNW WILL SHIFT EAST TO
THE ROCKIES SUN. THE AIR IS MOIST WITH WSW FLOW ALOFT BECOMING WNW
SUN. A WEATHER SYSTEM AND 1012 MB LOW JUST OFF THE SOUTH WA COAST
THIS MORNING WILL MOVE EAST THROUGH MAINLY THE SRN PTN OF WRN WA
TODAY. AT 830AM THERE IS A BAND OF RAIN IN AN ARC FROM AROUND HQM TO
OLM AND EAST INTO THE CASCADES AND ALSO AREAS OF PCPN SOUTH INTO NW
ORE.

KSEA...A BAND OF RAIN WILL MOVE THROUGH THE SEATTLE AREA THROUGH
MIDDAY AND THEN BECOME SCATTERED SHOWERS IN THE AFTERNOON. THIS IS
ASSOCIATED WITH A WEAK LOW PRESSURE SYSTEM MOVING INTO SW WA
TODAY.

&&

.MARINE...A 1012 MB LOW WILL MOVE ASHORE NEAR THE SOUTH WA COAST
THIS MORNING. LIGHT ONSHORE FLOW WILL CONTINUE TONIGHT AND SUNDAY.
ANOTHER WEATHER SYSTEM WILL EACH THE AREA SUN NIGHT...THE 12Z NAM
SHOWS THIS FRONT WEAKENING AFTER IT CROSSES 130W SUNDAY AFTERNOON
AND BREAKING UP OVER WRN WA MONDAY MORNING.


&&

.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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