000
FXUS66 KSEW 091555
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
854 AM PDT THU SEP 9 2010

.SYNOPSIS...A WEAK UPPER LEVEL RIDGE WILL MOVE OVER WESTERN
WASHINGTON THIS AFTERNOON AND REMAIN THROUGH FRIDAY...BRINGING A
SHORT DRY SPELL TO THE LOWLANDS. MORNING CLOUDS WILL DISSIPATE
SLOWLY IN THE MOIST AIR MASS WITH PARTLY SUNNY SKIES EXPECTED BY
LATE AFTERNOON BOTH DAYS. A LARGE UPPER LEVEL TROUGH WILL FORM OVER
SOUTHERN BRITISH COLUMBIA THIS WEEKEND WITH WEST FLOW ALOFT OVER
WASHINGTON. WEAK WEATHER SYSTEMS MOVING ACROSS THE AREA WILL BRING A
CHANCE OF SHOWERS AT TIMES FROM SATURDAY NIGHT THROUGH THE MIDDLE OF
NEXT WEEK.

&&

.SHORT TERM...WATER VAPOR IMAGERY SHOWS THE WEAK CIRCULATION ALOFT
THAT WAS OVER THE CENTRAL CASCADES LAST NIGHT HAS MOVED OVER THE
DALLES THIS MORNING. THERE WAS SOME SPOTTY VERY LIGHT RAIN OR
DRIZZLE EARLIER THIS MORNING BUT THAT SHOULD BE HISTORY BY 10 AM.
RADAR IS STILL SHOWING SOME ISOLATED SHOWERS OVER THE CASCADES. THE
MAIN PROBLEM FOR TODAY IS THE MOIST STABLE LOWER AIR MASS. IT WILL
TAKE QUITE A WHILE FOR THE SUN TO ERODE THE CLOUD COVER AND AT BEST
IT LOOKS LIKE IT WILL BE PARTLY SUNNY LATE THIS AFTERNOON. N
INTERIOR HAS A BETTER CHANCE OF BREAKING OUT THAN THE PUGET SOUND
REGION OR THE SW INTERIOR. 06Z NAM BUFR SOUNDING FOR KSEA WAS NOT
VERY OPTIMISTIC...SHOWING A SHALLOW LAYER MOIST ENOUGH FOR A BKN
CLOUD LAYER PERSISTING THROUGH 5 PM.

OTHERWISE THE WEAK UPPER LEVEL RIDGE MOVING OVER THE AREA THIS
AFTERNOON WILL KEEP THE LOWLANDS DRY. NW FLOW THROUGH THE RIDGE
JUSTIFIES MAINTAINING A CHANCE OF SHOWERS OVER THE MOUNTAINS THE
NEXT TWO DAYS.

THE AIR MASS REMAINS A LITTLE ON THE COOL SIDE AND FACTORING IN THE
PERSISTENT CLOUD COVER...HIGHS WILL REMAIN 6-10 DEGREES BELOW
NORMAL.

BY SATURDAY...THE LARGE UPPER LEVEL LOW CURRENTLY OVER THE GULF OF
AK WILL HAVE MOVED SE OVER THE NORTHERN B.C. COAST...WITH AN UPPER
LEVEL TROUGH AND WEST FLOW ALOFT DEVELOPING OVER W WA. BOTH GFS/NAM
SHOW AN UPPER LEVEL SHORTWAVE TROUGH CROSSING THE AREA LATE FRIDAY
NIGHT/SATURDAY MORNING WHICH WILL BRING A CHANCE OF SHOWERS TO THE
COAST AND N PART. THE CHANCE OF A LITTLE LIGHT RAIN SHOULD BE
ENHANCED WITH THE LOW LEVEL AIR MASS REMAINING MOIST. WILL LOOK AT
THIS MORE CLOSELY AFTER THE 12Z MODELS ARRIVE. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 340 AM AFD...SOME GENERAL
AGREEMENT IN THE EXTENDED MODELS THIS MORNING. ALL OF THE MODELS
BRING A COOL UPPER LEVEL TROUGH INTO BRITISH COLUMBIA ON SUNDAY BUT
DIFFER JUST HOW FAR SOUTH THE WEAK FRONT ASSOCIATED WITH THE TROUGH
GETS DURING THE DAY ON SUNDAY. THE GFS KEEPS MOST OF THE SHOWER
ACTIVITY CONFINED TO NORTH OF THE BORDER WHILE THE ECMWF AND
CANADIAN MOVE THE WEAKENING SYSTEM OVER WESTERN WASHINGTON. THE GFS
ENSEMBLES SHOW THE OPERATIONAL RUN IS A LITTLE HIGHER WITH THE 500
MB HEIGHTS THAN THE OPERATIONAL MEAN. AT THIS POINT WILL HAVE TO
KEEP THE CHANCE OF SHOWERS IN THE FORECAST. BEYOND SUNDAY THE MODELS
ARE STARTING TO SHOW A LITTLE MORE RIDGING FOR MONDAY THROUGH
WEDNESDAY. THIS IS A NEW TREND WITH THE PREVIOUS EXTENDED MODELS NOT
AS STRONG WITH THE RIDGING. EVEN THE STRONGEST RIDGE SOLUTIONS PUT
THE 500 MB HEIGHTS ONLY IN THE MID 570 DMS SO ITS NOT A VERY STRONG
RIDGE SOLUTION AT THIS POINT. WITH THIS BEING A NEW TREND WILL KEEP
THE CHANCE OF SHOWERS BROADBRUSH GOING FOR NOW MONDAY THROUGH
WEDNESDAY. IF THE CURRENT TREND CONTINUES LOOK A DRIER FORECAST FOR
THE FIRST PART OF NEXT WEEK. FELTON

&&

.AVIATION...AN ELONGATED NORTH-SOUTH ORIENTED UPPER LEVEL TROUGH
REMAINS OVER THE REGION. AN UPPER LOW CENTER THAT MOVED INTO WESTERN
WASHINGTON OVERNIGHT IS WEAKENING AND MAKING ITS WAY SOUTH OF THE
FORECAST AREA THIS MORNING. NORTHERLY FLOW ALOFT WILL BECOME
NORTHWEST LATER TODAY AND TONIGHT.

A MOIST AND STABILIZING AIR MASS OVERNIGHT ALONG WITH LIGHT SURFACE
GRADIENTS HAS LEAD TO WIDESPREAD DEVELOPMENT OF IFR AND LOW-END MVFR
CEILINGS. WE SHOULD SEE GRADUAL IMPROVEMENT TO VFR CONDITIONS DURING
THE AFTERNOON -- ROUGHLY 20Z-22Z -- BUT LOW MVFR CEILINGS ARE
EXPECTED TO DEVELOP ACROSS MUCH OF THE REGION AGAIN TONIGHT AFTER
06Z.

KSEA...MVFR AND OCCASIONAL IFR CEILINGS THIS MORNING. LOW CEILINGS
ARE EXPECTED TO SCATTER OUT AFTER 20Z. SOUTH TO SOUTHWEST WIND 3-8
KT...BECOMING SOUTHWEST 6-12 KT IN THE AFTERNOON. MCDONNAL

&&

.MARINE...LIGHT TO MODERATE ONSHORE FLOW WILL CONTINUE THROUGH
FRIDAY...WITH SMALL CRAFT ADVISORY WESTERLIES EXPECTED IN THE
CENTRAL STRAIT OF JUAN DE FUCA AND EAST ENTRANCE IN THE LATE
AFTERNOON AND EVENING HOURS.

SURFACE HIGH PRESSURE OVER THE COASTAL AND OFFSHORE WATERS WILL
WEAKEN AND SHIFT INLAND ON SUNDAY AS A SURFACE LOW APPROACHES THE
OFFSHORE WATERS FROM THE WEST...WITH THE FLOW BECOMING WEAKLY
OFFSHORE SUNDAY AND MONDAY. MCDONNAL

&&

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

$$

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