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000
FXUS66 KSEW 170500
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
955 PM PDT THU MAY 16 2013

.SYNOPSIS...UPPER LEVEL LOW PRESSURE OVER THE PACIFIC NORTHWEST WILL
RESULT IN PARTLY TO MOSTLY CLOUDY SKIES AND SCATTERED SHOWERS
THROUGH FRIDAY. A WEAK FRONT WILL MOVE ACROSS WESTERN WASHINGTON
SATURDAY BRINGING LIGHT RAIN TO MOST THE AREA. SUNDAY AND MONDAY
WILL SEE A RETURN TO PARTLY SUNNY SKIES AND JUST SPOTTY SHOWERS. AN
UPPER LOW WILL MOVE SOUTH OVER THE REGION TUESDAY THROUGH WEDNESDAY
BRINGING WIDESPREAD SHOWERS AND COOLER TEMPERATURES. THE UPPER LOW
MAY BEGIN TO SHIFT SOUTH BY THURSDAY WITH DECREASING SHOWERS.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS PESKY LITTLE LOW JUST OFF THE
SOUTH WASHINGTON COAST. ORGANIZED BAND OF SHOWERS ROTATING
AROUND THE LOW EXTENDING FROM COWLITZ COUNTY TO THE NEAR PACIFIC
BEACH ALONG THE CENTRAL COAST WILL CONTINUE TO MOVE NORTHWEST
TONIGHT INTO THE EARLY MORNING HOURS. HAVE DONE A QUICK ZONE UPDATE
TO INCREASE THE POPS OVERNIGHT IN THESE AREAS.

FOR THE REMAINDER OF THE CWA...CONVECTIVE ACTIVITY THAT POPPED UP
LATE IN THE AFTERNOON IS STILL GOING ALONG THE CASCADE FOOTHILLS OF
KING AND SNOHOMISH COUNTY. THERE WAS A COUPLE OF LIGHTNING STRIKES
NEAR MOUNT SI AND OVER THE KITSAP PENINSULA EARLIER THIS EVENING BUT
THERE HAS NOT BEEN ANY STRIKES REPORTED SINCE 0245Z. THE REMAINING
SHOWERS ARE BEGINNING TO WEAKEN ON THE DOPPLER RADAR ECHOES. THIS
TREND WILL CONTINUE THIS EVENING WITH THE SHOWERS DISSIPATING IN THE
EARLY MORNING HOURS.

NOT MUCH GOING ON FRIDAY WITH THE LOW LEVEL ONSHORE FLOW INCREASING
A TOUCH AND WEAK TROUGHINESS ALOFT REMAINING OVER THE AREA. THE AIR
MASS WILL BECOME SLIGHTLY UNSTABLE AGAIN FRIDAY AFTERNOON WITH SOME
MORE SHOWERS DEVELOPING BY LATE AFTERNOON. MOST OF THE SHOWERS WILL
BE OVER THE MOUNTAINS.

SHORT BREAK FRIDAY EVENING BEFORE THE NEXT SPLITTING FRONT MOVES
INTO WESTERN WASHINGTON ON SATURDAY. MODEL TIMING FOR THE FRONT HAS
BEEN BOUNCING AROUND BUT THE LAST THREE RUNS HAVE SETTLED ON THE
FRONT GETTING TO THE COAST AROUND 00Z SUNDAY. THE FRONT WILL BE
WEAKENING HAS IT MOVES INLAND WITH THE HIGHEST POPS ALONG THE COAST
SATURDAY. POSSIBLE PUGET SOUND CONVERGENCE ZONE BEHIND THE FRONT
SATURDAY NIGHT.

WEAK UPPER LEVEL BUILDING OVER WESTERN WASHINGTON ON SUNDAY WITH LOW
LEVEL ONSHORE FLOW. SUNDAY LOOKS TO BE THE DRIEST DAY IN THE SHORT
TERM. FELTON


.LONG TERM...LONG RANGE MODELS CONTINUE TO ADVERTISE A LARGE CLOSED
UPPER LEVEL LOW DIGGING SWD MONDAY ALONG THE B.C. COASTLINE TOWARD
THE PAC NW. IT WILL LIKELY BE MOSTLY DRY MONDAY ACROSS WRN WA BUT
ONSHORE FLOW AHEAD OF THE SYSTEM COULD SQUEEZE OUT A FEW SHOWERS
ALONG THE MOUNTAINS. THIS SYSTEM WILL HAVE COLDER AIR ASSOCIATED
WITH IT DUE TO ITS NORTHERN ORIGIN. WHILE THERE ARE SOME SLIGHT
TIMING DIFFERENCES....THE GFS/ECMWF BOTH DIG THIS LOW SWD AND STALL
THE SYSTEM OVER WA TUESDAY THROUGH AT LEAST WEDNESDAY. THE MAIN
IMPACT WILL BE A CHANCE TO COOLER TEMPERATURES AND LOWER SNOW
LEVELS. HIGHS WILL ONLY BE IN THE MID 50S FOR THE
LOWLANDS...POSSIBLY 10 DEGREES OR MORE BELOW AVERAGE. SNOW LEVELS
WILL FALL AS LOW AS 4000 FEET BUT PRECIPITATION WILL BE LIGHT AND
SHOWERY SO THERE WILL BE LITTLE IN THE WAY OF ACCUMULATION. ISOLATED
THUNDERSTORMS AND SMALL HAIL CANNOT BE RULED OUT IN THIS TYPE OF
PATTERN BUT NOT WORTH MENTIONING IN THE FORECAST THIS FAR OUT.
MODELS DIFFER ON HOW LONG THE TROUGH STALLS OVER THE AREA WHICH MAY
START TO SHIFT AWAY FROM THE AREA TOWARD THE END OF THE WEEK.
MERCER

&&

.AVIATION...A WEAK UPPER LEVEL TROUGH OVER THE PAC NW WILL MAINTAIN
ISOLATED SHOWERS ACROSS WESTERN WA TONIGHT. THERE WERE A FEW
LIGHTING STRIKES WITH THE STRONGER CELLS...BUT LOOKS LIKE THIS
ACTIVITY IS FADING AS THE SUN SETS. THIS TROUGH WILL SLOWLY LIFT
E/NE ON FRIDAY FOR ADDITIONAL SHOWERS ACROSS THE REGION. MAY SEE
POCKETS OF MVFR CIGS IN THE MORNING...OTHERWISE EXPECTING VFR
CONDITIONS BY THE AFTERNOON. 33

KSEA...ISOLATED LIGHT SHOWERS WILL REMAIN IN THE VICINITY TONIGHT.
THE THREAT OF T-STORMS IS FADING WITH SUNSET. SURFACE WINDS WILL
REMAIN SOUTHERLY. 33

&&

.MARINE...WEAK ONSHORE FLOW WILL PREVAIL TONIGHT THEN INCREASE ON
FRI. HIGHEST WINDS WILL BE FOUND 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