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000
FXUS66 KSEW 160451
AFDSEW

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

.SYNOPSIS...A WEAK UPPER LEVEL SHORTWAVE TROUGH CROSSING WESTERN
WASHINGTON TONIGHT WILL KEEP SCATTERED SHOWERS OVER THE AREA. A
BROAD WEAK UPPER LEVEL TROUGH WILL REMAIN OVER WESTERN WASHINGTON
THURSDAY AND FRIDAY FOR A CHANCE OF SHOWERS AND PARTLY CLOUDY SKIES.
WESTERLY FLOW ALOFT WILL BRING ANOTHER UPPER LEVEL TROUGH INTO THE
AREA OVER THE WEEKEND FOR AN INCREASING CHANCE OF SHOWERS. AN UPPER
LEVEL LOW WILL MOVE SOUTH OVER THE REGION TUESDAY THROUGH THURSDAY
FOR SCATTERED SHOWERS AND SLIGHTLY COOLER TEMPERATURES.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS WEAK UPPER LEVEL SHORTWAVE
MOVING SLOWLY THROUGH WESTERN WASHINGTON THIS EVENING. DOPPLER
RADARS HAVE PLENTY OF PRECIPITATION ECHOES BUT AMOUNTS HAVE BEEN
LIGHT...FROM A TRACE TO JUST A COUPLE OF HUNDREDTHS. TEMPERATURES
HAVE BEEN IN THE LOWER TO MID 50S ALL EVENING.

SHORTWAVE MOVING OUT OF THE AREA OVERNIGHT WITH A DECREASE IN THE
SHOWER ACTIVITY AFTER MIDNIGHT. WEAK UPPER LEVEL TROUGH REMAINING
OVER WESTERN WASHINGTON ON THURSDAY WITH THE JET WELL TO THE SOUTH
OF THE AREA...OVER NORTHERN CALIFORNIA. ANOTHER WEAK SYSTEM WILL
ARRIVE THURSDAY AFTERNOON BUT IT WILL BE SPLITTING AS IT REACHES
WESTERN WASHINGTON SO WILL STAY WITH THE CHANCE POPS SCENARIO.

WEAK TROUGHINESS REMAINING OVER WESTERN WASHINGTON THURSDAY NIGHT
INTO FRIDAY KEEP A CHANCE OF SHOWERS IN THE FORECAST. THE AIR MASS
BECOMES A LITTLE MORE UNSTABLE FRIDAY AFTERNOON WITH THE BEST
CHANCES FOR SHOWERS AT THAT TIME.

AFTER A BRIEF BREAK FRIDAY NIGHT A NEGATIVELY TILTED TROUGH ARRIVES
ON SATURDAY. MODELS HAVE BEEN FAIRLY CONSISTENT IN KEEPING THIS
FEATURE INTACT FOR THE MOST PART UNTIL IN MOVES INLAND SATURDAY
AFTERNOON INTO SATURDAY EVENING. CURRENT FORECAST HAS LIKELY POPS
ALONG THE COAST ON SATURDAY WITH CHANCE POPS INLAND. THIS LOOKS GOOD
AT THIS POINT.

TEMPERATURES THROUGHOUT THE SHORT TERM WILL REMAIN NEAR NORMAL FOR
THIS TIME OF YEAR WITH HIGHS IN THE MID 50S TO MID 60S AND LOWS IN
THE 40S. CURRENT FORECAST HAS THE TRENDS COVERED. NO UPDATE THIS
EVENING. FELTON

.LONG TERM...PREVIOUS DISCUSSION FOLLOWS. 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...WEAK UPPER LEVEL TROUGHING OVER THE PAC NW WILL MAINTAIN
SHOWERS ACROSS WESTERN WA TONIGHT. THE FLOW ALOFT IS WESTERLY. MVFR
CONDITIONS ARE ANTICIPATED TONIGHT WITH PLENTY OF LOW LEVEL MOISTURE
AND WEAK PRES GRADIENTS. MOIST WESTERLY FLOW WILL CONTINUE ON THU
FOR ADDITIONAL SHOWERS...ISOLATED TO SCATTERED IN COVERAGE. VFR
CONDITIONS ARE EXPECTED BY THE AFTERNOON. 33

KSEA...SHOWERS WILL REMAIN IN THE VICINITY TONIGHT WITH MVFR
CEILINGS. SURFACE WINDS ARE LIGHT SOUTHERLY. EXPECT ISOLATED TO
SCATTERED SHOWERS ON THU WITH CIGS LIFTING IN THE AFTERNOON...VFR
CONDITIONS WILL PREVAIL. 33

&&

.MARINE...A WEAK 1013 LOW WILL REMAIN NEARLY STATIONARY OFFSHORE
TONIGHT THEN DISSIPATE ON THU. PRES GRADIENTS AND WINDS ACROSS
WESTERN WA WILL REMAIN LIGHT. ONSHORE FLOW WILL PREVAIL THU NIGHT
AND FRI WITH HIGHEST WINDS THROUGH THE STRAIT OF JUAN DE FUCA. A
COLD FRONT WILL THEN CROSS THE AREA ON SAT FOR INCREASING WINDS.
HIGH PRES OVER THE NE PAC WILL MAINTAIN ONSHORE FLOW THROUGH THE
WEEKEND. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

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