Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 022300
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
400 PM PDT WED SEP 2 2015

.SYNOPSIS...A LARGE UPPER LEVEL LOW WILL MOVE SLOWLY SOUTH OVER
WESTERN WASHINGTON TONIGHT THROUGH THURSDAY...BRINGING COOL SHOWERY
WEATHER. THERE IS ALSO A CHANCE OF THUNDERSTORMS DURING THE
AFTERNOON AND EVENING HOURS. THE LOW WILL MOVE DOWN OVER OREGON ON
FRIDAY...FOR A GRADUAL DRYING AND WARMING TREND FRIDAY AFTERNOON AND
SATURDAY. A WEAK UPPER LEVEL TROUGH MAY BRING SOME SHOWERS ON
SUNDAY...AND NORTHWEST FLOW ALOFT MAY BRING ADDITIONAL WEAK TROUGHS
AND SHOWERS AT TIMES EARLY NEXT WEEK.

&&

.SHORT TERM...A COLD DEEP UPPER LEVEL LOW CENTERED OVER CENTRAL B.C.
THIS AFTERNOON WILL MOVE SLOWLY S OVER SOUTHERN VANCOUVER ISLAND BY
THURSDAY AFTERNOON. THE AIR MASS IS UNSTABLE AND SCATTERED SHOWERS
AND ISOLATED THUNDERSTORMS HAVE BEEN MOVING ACROSS W WA THIS
AFTERNOON. MODELS AND WATER VAPOR IMAGERY SHOW A SMALL UPPER LEVEL
SHORTWAVE TROUGH JUST OFF THE WA COAST THIS AFTERNOON WHICH SHOULD
KEEP ISOLATED THUNDERSTORMS GOING THIS EVENING AS IT MOVES ACROSS W
WA. THE AIR MASS SHOULD STABILIZE JUST ENOUGH LATER THIS EVENING TO
END THE THREAT OF THUNDERSTORMS.

MODELS BRING THE UPPER LEVEL LOW DOWN OVER S VANCOUVER ISLAND AND
THE NW INTERIOR THURSDAY AFTERNOON. GFS HAS LIFTED INDICES 0 TO -3
THURSDAY AFTERNOON WHILE THE NAM SHOWS 0 TO -2. NAM BUFR SOUNDING
FOR KSEA SHOWS A SKINNIER CAPE PROFILE AND LOWER CAPE VALUE ON
THURSDAY COMPARED TO TODAY...BUT THIS IS STILL ENOUGH TO INCLUDE
AFTERNOON AND EVENING THUNDERSTORMS FOR THURSDAY.

MODELS REMAIN IN FAIR AGREEMENT THAT THE UPPER LEVEL LOW WILL DROP S
OVER OREGON FRIDAY AFTERNOON WITH DRIER N FLOW DEVELOPING OVER W WA.
THE 12Z GFS WAS A LITTLE FASTER IN DEVELOPING THE N FLOW THAN THE
ECMWF BUT THE 18Z GFS HAS MIGRATED TO THE SLOWER ECMWF SOLUTION.
STILL...BOTH MODELS SHOW DRIER N FLOW DEVELOPING OVER W WA IN THE
AFTERNOON AND RESTRICT SHOWERS TO MAINLY OVER THE MOUNTAINS. THE
CHANCE OF SHOWERS OVER THE LOWLANDS WAS DROPPED FOR FRIDAY AFTERNOON
WITH CLEARING W OF PUGET SOUND.

THE UPPER LEVEL LOW WILL BE OVER SE OREGON BY SATURDAY MORNING. THE
DRIER N FLOW ALOFT BEHIND THE LOW WILL HAVE ALL FRIDAY NIGHT TO
CLEAR OUT THE CLOUDS...SO SATURDAY SHOULD BE MOSTLY SUNNY AND
WARMER.

THE UPPER LEVEL LOW WILL BRING A COOLER AIR MASS OVER W WA. HIGHS
TODAY AND THURSDAY WILL BE MAINLY IN THE LOWER TO MID 60S...OR ABOUT
10 DEGREES BELOW NORMAL. LOWS IN THE COLDEST SPOTS WILL DROP INTO
THE MID TO UPPER 40S TONIGHT...THEN REACH THE LOWER 40S THURSDAY AND
FRIDAY NIGHTS. SNOW LEVELS OVER THE MOUNTAINS WILL LOWER TO AROUND
5500 FEET TODAY AND THURSDAY...WITH LOCAL SNOW ACCUMULATIONS OF
A FEW INCHES POSSIBLE OVER THE HIGHER ELEVATIONS. KAM

.LONG TERM...NW FLOW ALOFT DEVELOPS OVER THE REGION ON SUNDAY AND
CONTINUES THROUGH TUESDAY. THE GFS AND ECMWF HAVE BEEN PRETTY
INCONSISTENT WITH THE TIMING AND STRENGTH OF THE UPPER LEVEL
SHORTWAVES EMBEDDED IN THE NW FLOW. THE MODELS AGREE THAT A WEAK
SHORTWAVE TROUGH WILL CROSS W WA ON SUNDAY...WHICH IS JUST ENOUGH TO
ADD A CHANCE OF SHOWERS TO THE FORECAST. HOWEVER...CONFIDENCE IS
STILL ONLY MODERATE BECAUSE THE MODEL AGREEMENT WAS JUST REACHED
OVER THE PAST 2-3 RUNS. FOR MONDAY AND TUESDAY CONFIDENCE IS LOW.
THE MODELS HAVE NOT REACHED ANY SORT OF CONSENSUS AND EACH HAS BEEN
INCONSISTENT FROM RUN-TO-RUN. I WILL LEAVE LABOR DAY DRY...BUT KEEP
THE CHANCE OF SHOWERS MONDAY NIGHT THROUGH TUESDAY NIGHT. IT LOOKS
LIKE AN UPPER LEVEL RIDGE MAY BE BUILDING AROUND MIDWEEK SO I WENT
DRY FOR WEDNESDAY. KAM

&&

.AVIATION...AN UPPER LEVEL TROUGH CONTINUES TO WORK SOUTH ALONG THE
BRITISH COLUMBIA COAST THIS AFTERNOON. THERE IS ENOUGH INSTABILITY
AND MOISTURE THAT IN ADDITION TO SCATTERED SHOWERS...THUNDERSTORMS
HAVE BEEN SPREADING MAINLY INTO THE STRAIT AND NORTH INTERIOR INTO
THE AREA FROM THE NORTHWEST. VFR CONDITIONS WITH BROKEN TO SCATTERED
SKIES OVER MUCH OF THE AREA THIS AFTERNOON. SHOWERS AND
THUNDERSTORMS WILL CONTINUE TO SPREAD SOUTH INTO THE CENTRAL AND
SOUTH SOUND THIS AFTERNOON AND EVENING. AN EMBEDDED SHORTWAVE TROUGH
ROTATING UNDER THE UPPER LEVEL TROUGH WILL MOVE INTO WESTERN
WASHINGTON THIS EVENING...HELPING ENHANCE SHOWERS AND THUNDERSTORMS
THIS EVENING. EXPECT VFR CONDITIONS TO PREVAIL THIS EVENING WITH
LOWER CEILINGS AND GUSTY WINDS OCCURRING WITH SHOWERS. CEILINGS WILL
LOWER OVERALL INTO THURSDAY MORNING TO MVFR LEVELS.

KSEA...VFR CEILINGS WILL LINGER THROUGH THE AFTERNOON...EXCEPT WITH
SCATTERED SHOWERS AS THEY WORK SOUTH INTO KING COUNTY FROM THE NORTH
THIS AFTERNOON AND EVENING. ABUNDANT LOW LEVEL MOISTURE WILL HELP
CEILINGS LOWER TO MVFR LEVELS EARLY WEDNESDAY MORNING. WINDS WILL
REMAIN SOUTHERLY AT 10-15 KNOTS.

&&

.MARINE...LOW LEVEL ONSHORE FLOW CONTINUING TODAY WITH NORTHWESTERLY
WESTERLY FLOW OVER THE COASTAL WATERS. 7 TO 9 FOOT SWELL THAT HAS
BEEN AFFECTING THE COASTAL WATERS WILL SUBSIDE OVERNIGHT TO 6 TO 7
FEET AT AROUND 10 SECONDS. SHOWERS AND THUNDERSTORMS OVER AREA
WATERS WILL KEEP WINDS GUSTY BUT BELOW SMALL CRAFT ADVISORY
CRITERIA. LOW LEVEL ONSHORE FLOW WILL WEAKEN SOMEWHAT LATE TONIGHT
INTO THURSDAY...AND AGAIN THURSDAY NIGHT INTO FRIDAY AS A WEAK
SURFACE LOW MOVES THROUGH THE OFFSHORE WATERS. HIGH PRESSURE WILL
BUILD OVER THE WATERS FRIDAY INTO THE WEEKEND.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY GRAYS HARBOR BAR.

&&

$$

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