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000
FXUS66 KSEW 202202
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT MON MAY 20 2013

.SYNOPSIS...BIG CHANGES IN THE WEATHER WILL BEGIN LATE TONIGHT AND
PERSIST INTO NEXT WEEKEND AS AN UPPER LEVEL LOW MOVES SOUTHEAST FROM
THE GULF OF ALASKA THEN SPINS AROUND OVER THE PACIFIC NORTHWEST. A
FRONT WILL MOVE ACROSS THE AREA TUESDAY MORNING...THEN SHOWERS ARE
LIKELY THROUGH WEDNESDAY. CONDITIONS WILL BE MOIST AND COOLER THAN
NORMAL ALL WEEK.

&&

.SHORT TERM...RADAR SHOWS A BAND OF RAIN APPROACHING THE COAST. RAIN
WILL DEVELOP ON THE COAST THIS EVENING THEN SPREAD INLAND AFTER
MIDNIGHT. MODELS KEEP A FEATURE THAT LOOKS LIKE A FRONT INTACT ALL
THE WAY THROUGH WESTERN WASHINGTON BEFORE THE PRECIPITATION PATTERN
TURNS MORE SHOWERY TUESDAY AFTERNOON. THERE COULD BE SOME AFTERNOON
SUN BREAKS AFTER THE FRONT PASSES. THIS WILL ALLOW TEMPERATURES TO
RISE INTO THE MID 50S BUT NO HIGHER.

ONCE THE UPPER LEVEL LOW MOVES OVER THE REGION...MODELS DO NOT MOVE
IT AWAY FOR THE REST OF THE WEEK. IT SPINS AROUND BOTH WEDNESDAY AND
THURSDAY IN THE VICINITY OF WESTERN WASHINGTON. HEIGHTS SLOWLY BUILD
FROM THE MID 540S OF TUESDAY TO THE MID 550S BY THURSDAY. THIS IS
A SHOWERY PATTERN AND POPS REMAIN HIGH...LIKELY OR BETTER...FOR EACH
PERIOD THROUGH THURSDAY. THERE WILL BE SUN BREAKS AT TIMES AND ALSO
A SLOW WARMING TREND...WITH HIGHS IN THE LOW 60S OVER INLAND AREAS
THURSDAY. WITH ALL THE COLD AIR ALOFT THE SHOWERS ARE LIKELY TO
PRODUCE SMALL HAIL AT TIMES. BURKE

.LONG TERM...ONLY MINOR DIFFERENCES BETWEEN THE EURO AND GFS IN THAT
BOTH KEEP AN UPPER LOW IN THE AREA. THE EURO KEEPS IT MORE OR LESS
OVER WESTERN WASHINGTON THROUGH MONDAY WHILE THE GFS SPINS THE
ORIGINAL LOW AWAY AND DEVELOPS A NEW ONE OVER THE AREA. IN BOTH
MODELS HEIGHTS SLOWLY RISE INTO THE 560S. HAVE A BROAD BRUSH
FORECAST FOR THE ENTIRE PERIOD FRIDAY THROUGH MONDAY WITH CHANCE
POPS AND HIGHS IN THE UPPER 50S TO MID 60S. BURKE

&&

.AVIATION...SOUTHWEST FLOW ALOFT OVER WESTERN WASHINGTON THIS
AFTERNOON WILL STRENGTHEN AND BACK TOWARD SOUTHERLY TONIGHT. A DEEP
UPPER LOW WILL MOVE INTO THE OFFSHORE WATERS LATE TONIGHT AND REMAIN
THERE TUESDAY. LOW LEVEL FLOW ONSHORE FLOW WILL INCREASE TONIGHT AND
CONTINUE TUESDAY. THE AIR MASS WILL BE MOIST AND SLIGHTLY UNSTABLE.

VFR CONDITIONS PREVAIL OVER THE MOST OF THE FORECAST AREA WITH
CEILINGS MAINLY ABOVE 120. THE EXCEPTION IS AREAS OF MVFR CEILINGS
ALONG THE COAST AND IN THE SOUTHWEST INTERIOR. PRECIPITATION WILL
REACH THE COAST THIS EVENING AND SPREAD INLAND TONIGHT WITH
CONDITIONS DETERIORATING ACROSS THE FORECAST AREA TO MVFR AND
IFR.MARGINAL IMPROVEMENT IS POSSIBLE LATE TUESDAY.

KSEA...SOUTHWEST WIND 3-8 KT BECOMING NORTHWEST 6-10 KT LATE
AFTERNOON...THEN BACKING TO SOUTHWEST AGAIN AROUND MIDNIGHT. RAIN
WILL LIKELY BEGIN AT KSEA AROUND 4 AM TUESDAY...AND CONDITIONS WILL
LIKELY DETERIORATE TO MVFR OCCASIONALLY IFR.   MCDONNAL

&&

.MARINE...ONSHORE FLOW WILL INCREASE ACROSS WESTERN WASHINGTON AND
ITS COASTAL WATERS TONIGHT. SMALL CRAFT ADVISORY WESTERLIES ARE
LIKELY IN THE CENTRAL STRAIT OF JUAN DE FUCA AND EAST ENTRANCE
STRAIT TONIGHT...WITH MARGINAL SMALL CRAFT ADVISORY WINDS IN
ADMIRALTY INLET AND THE NORTHERN INLAND WATERS.

NORTHWEST FLOW OVER THE COASTAL WATERS WILL INCREASE TONIGHT AND
TUESDAY AS A WEAK SURFACE LOW OF AROUND 1013 MB TRACKS
EAST-SOUTHEAST ACROSS THE OREGON OFFSHORE WATERS AND INTO SOUTHWEST
OREGON. SMALL CRAFT ADVISORY NORTHWEST WINDS ARE LIKELY OVER THE
COASTAL WATERS TUESDAY.

ANOTHER LOW WILL PROBABLY MOVE SOUTHEAST ACROSS THE WASHINGTON
OFFSHORE WATERS WEDNESDAY THROUGH THURSDAY. THERE IS LOTS OF
UNCERTAINTY IN THE SPECIFICS OF THE WIND FORECAST THOUGH...AS MODELS
HAVE BEEN HIGHLY INCONSISTENT.   MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL STRAIT OF JUAN DE FUCA...EAST
        ENTRANCE STRAIT...ADMIRALTY INLET...NORTHERN INLAND WATERS.

&&

$$

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