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000
FXUS66 KSEW 152221
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
322 PM PDT WED MAY 15 2013

.SYNOPSIS...A COUPLE OF WEAK UPPER LEVEL SHORTWAVE TROUGHS CROSSING
WESTERN WASHINGTON THIS EVENING WILL BRING CLOUDY SKIES AND
SCATTERED SHOWERS. A BROAD WEAK UPPER LEVEL TROUGH WILL REMAIN OVER
THE AREA THURSDAY AND FRIDAY FOR A CHANCE OF SHOWERS AND PARTLY
CLOUDY SKIES. WESTERLY FLOW ALOFT WILL BRING ANOTHER UPPER LEVEL
TROUGH AND A CHANCE OF SHOWERS TO THE REGION SATURDAY AND SUNDAY. AN
UPPER LEVEL LOW WILL MOVE SOUTH OVER THE REGION TUESDAY THROUGH
THURSDAY FOR SCATTERED SHOWERS AND SLIGHTLY COOLER TEMPERATURES.

&&

.SHORT TERM...STUPID UPPER LEVEL SHORTWAVE TROUGH HAS BEEN TAKING
ITS SWEET TIME MOVING INLAND TODAY. WATER VAPOR IMAGERY SHOWS THE
TROUGH AXIS RIGHT ALONG THE S WA COAST AT 20Z. RADAR SHOWS THE RAIN
SHIELD STILL HOLDING TOGETHER EXTENDING FROM THE N COAST ACROSS SW
WA AT 20Z. THE SOUTHERN PART OF THE RAIN SHIELD IS STARTING TO
DEVELOP MORE HOLES WHICH MEANS IT IS PROBABLY STARTING TO WEAKEN.
THE UPSTREAM UPPER LEVEL SHORTWAVE TROUGH IS STILL DRIVING STEADILY
EASTWARD OFFSHORE NEAR 45N/130W...AND BOTH GFS/NAM TAKE IT E ACROSS
NW OREGON THIS EVENING. THIS WILL CONTINUE PUSHING THE DOWNSTREAM
SHORTWAVE NNE ACROSS W WA THROUGH THIS EVENING...SO MOST OF THE
INTERIOR SHOULD GET AT LEAST A TRACE OF RAIN THROUGH THIS EVENING.

AFTER THE TWO SHORTWAVE TROUGHS EXIT LATE TONIGHT A BROAD WEAK UPPER
LEVEL TROUGH WILL REMAIN OVER THE REGION THURSDAY AND FRIDAY. NO
SIGNIFICANT FEATURES JUMP OUT THAT WOULD PROVIDE A SOURCE OF UPWARD
MOTION. LOW LEVEL ONSHORE FLOW WILL BE PRESENT BUT WEAK...AND THE
AIR MASS WILL BE SOMEWHAT MOIST. THE MAIN SHOWER GENERATOR WILL
PROBABLY BE THE UNSTABLE LOW LEVEL AIR MASS. NAM BUFR SOUNDINGS SHOW
INSTABILITY DEVELOPING BY MID MORNING THURSDAY WITH AN EQUILIBRIUM
LEVEL ABOVE 12000 FEET. THIS SHOULD BE ENOUGH TO GENERATE SCATTERED
SHOWERS. FORECAST SOUNDINGS SHOW A MORE STABLE AIR MASS ON
FRIDAY...BUT PROBABLY MORE PERSISTENT CLOUD COVER. TEMPERATURES WILL
REMAIN NEAR NORMAL FOR MID MAY.

ON SATURDAY THE BROAD WEAK UPPER LEVEL TROUGH WILL SHIFT EAST AS
STRENGTHENING W FLOW ALOFT OFFSHORE PUSHES A MORE CONSOLIDATED UPPER
LEVEL SHORTWAVE TROUGH TOWARD THE REGION. THIS SYSTEM DOES NOT LOOK
ALL THAT STRONG BUT SHOULD BE ENOUGH FOR MORE CLOUDS AND A CHANCE OF
SHOWERS SATURDAY. KAM

.LONG TERM...THE SATURDAY UPPER LEVEL TROUGH WILL SHIFT OVER E WA
SATURDAY NIGHT BUT THE MODELS STILL SHOW SOME RESIDUAL SHOWER
ACTIVITY OVER THE AREA ON SUNDAY.

MONDAY HAS A CHANCE AT BEING MORE OF A DRY DAY AS THE PATTERN
TRANSITIONS TO A LARGE UPPER LEVEL LOW OVER THE REGION. THIS IS A
RECENT DEVELOPMENT IN THE MODELS SO I HAVE HUNG ONTO A CHANCE OF
SHOWERS A LITTLE LONGER.

FOR TUESDAY AND WEDNESDAY BOTH THE GFS AND ECMWF AGREE ON DROPPING A
CLOSED UPPER LEVEL LOW DOWN THE B.C. COAST AND OVER W WA TUESDAY
AFTERNOON. THE LOW IS COLD WITH ABOUT A 542 DAM CENTER AND 500 MB
TEMPS ANYWHERE FROM -28C TO -35C. THIS IS ENOUGH INSTABILITY TO
CONTINUE THE CHANCE OF SHOWERS AND TO KEEP AND EYE ON THE
THUNDERSTORM POTENTIAL. MAX TEMPS HAVE BEEN LOWERED TO NEAR 60 AND
THEY MAY END UP SLIGHTLY COOLER THAN THAT. SNOW LEVELS MAY BE A BIT
HIGH. KAM

&&

.AVIATION...UPPER LEVEL LOW PRESSURE WILL STALL OVER THE PAC NW
THROUGH TONIGHT...THEN DIG SWD ON THURSDAY. LIGHT SW FLOW ALOFT
TONIGHT...THEN LIGHT NWLY. INCREASING MOISTURE THIS EVENING WITH
LIGHT RAIN AT TIMES. DECREASING MOISTURE THURSDAY MORNING BECOMING
SOMEWHAT DRY BY AFTERNOON. STABLE ATMOSPHERE...BECOMING WEAKLY
UNSTABLE THURSDAY BUT THUNDERSTORMS ARE NOT EXPECTED.

KSEA...RAIN HAS BEEN SLOW TO DEVELOP OVER WRN WA THIS AFTERNOON.
RADAR TRENDS SUGGEST LIGHT RAIN WILL DEVELOP AT THE TERMINAL BEFORE
4 PM PDT...THEN CONTINUE AT TIMES THROUGH THE EVENING BEFORE
BECOMING SHOWERY. VFR CIGS...POSSIBLY LOWERING TO 2-3K FT AT TIMES
AFTER 03-04Z TONIGHT. S-SW WIND 5-9 KT. DTM

&&

.MARINE...A SURFACE TROUGH WILL WEAKEN OVER THE COASTAL WATERS AND
DISSIPATE OVERNIGHT. SE WINDS MAY OCCASIONALLY REACH 25 KT THROUGH
EARLY THIS EVENING OVER THE OUTER COASTAL WATERS BEFORE EASING. A
SMALL CRAFT REMAINS IN EFFECT THROUGH 6 PM. ELSEWHERE...WINDS
GENERALLY BELOW 15 KT OUT OF THE S/SE.

LOWER PRES E OF THE CASCADES ON THURSDAY WILL RESULT IN ONSHORE FLOW
WHICH STRENGTHENS ON FRIDAY. SMALL CRAFT WINDS MAY DEVELOP IN THE
CENTRAL/E STRAIT DURING THE LATE AFTERNOON AND EVENINGS.

A WEAK FRONT MOVES THROUGH THE WATERS SATURDAY...THEN VARYING
DEGREES OF ONSHORE FLOW SUNDAY AND MONDAY. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY OUTER COASTAL WATERS THROUGH 6 PM PDT THIS
     EVENING.

&&

$$

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