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000
FXUS66 KSEW 151553
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
853 AM PDT WED MAY 15 2013

.SYNOPSIS...A WEAK UPPER LEVEL SHORTWAVE TROUGH JUST OFFSHORE WILL
MOVE NORTHEAST ACROSS WESTERN WASHINGTON TODAY BRINGING CLOUDS AND
SCATTERED SHOWERS WITH IT. ANOTHER WEAK SHORTWAVE TROUGH WILL KEEP
SCATTERED SHOWERS GOING THIS EVENING. A BROAD WEAK UPPER LEVEL
TROUGH WILL REMAIN OVER THE AREA THURSDAY AND FRIDAY FOR A CHANCE OF
SHOWERS AND PARTLY CLOUD SKIES. WESTERLY FLOW ALOFT WILL BRING
ANOTHER UPPER LEVEL TROUGH TO THE REGION SATURDAY AND AGAIN EARLY
NEXT WEEK. TYPICAL MAY WEATHER WILL PREVAIL.

&&

.SHORT TERM...WATER VAPOR IMAGERY SHOWS THE UPPER LEVEL SHORTWAVE
TROUGH JUST OFFSHORE FROM THE S WA COAST NEAR 45N/128W. IR IMAGERY
SHOWS CLOUD TOPS COOLING OFFSHORE AHEAD OF THE SHORTWAVE...AND THE
COASTAL RADAR IS PICKING UP A LARGE AREA OF LIGHT RAIN UNDER THE
COOLING CLOUD TOPS. PER THE GFS THIS AREA IS CLOSE TO THE FAVORABLE
SPOT FOR UPWARD MOTION BETWEEN THE RIGHT ENTRANCE REGION OF A WEAK
JET STREAK OVER B.C. AND THE LEFT EXIT REGION OF ANOTHER WEAK JET
STREAK APPROACHING THE OREGON COAST. THE GFS SEEMS TO HOLD THIS AREA
TOGETHER TODAY AS IT MOVES NE OVER W WA...SO I WILL STICK WITH THE
CURRENT FORECAST OF SHOWERS LIKELY TODAY. SHOWERS WILL SPREAD TO THE
COAST AND WESTERN PART OF THE SW INTERIOR BY LATE THIS
MORNING...WITH SHOWERS CONTINUING TO SPREAD ENE ACROSS PUGET SOUND
THIS AFTERNOON. THE FAR N INTERIOR AROUND BELLINGHAM MAY END UP ON
THE NORTHERN EDGE OF THE SHOWERS.

SCATTERED SHOWERS SHOULD CONTINUE INTO AT LEAST EARLY THIS EVENING
AS THE LAST EFFECTS FROM THIS AFTERNOONS SHORTWAVE TROUGH MOVE NE
AND THE TRAILING SHORTWAVE TROUGH...CURRENTLY NEAR 45N/133W AT
15Z...MOVES E ALONG THE WA OREGON BORDER. SHOWER ACTIVITY SHOULD HIT
A LULL OVERNIGHT.

ON THURSDAY THE MODELS AGREE THAT THERE WILL BE A BROAD WEAK UPPER
LEVEL TROUGH CARVED OUT OVER W WA. THERE ARE NOT ANY FEATURES THAT
STAND OUT THAT WOULD BE A SOURCE OF UPWARD MOTION...BUT THE NAM BUFR
SOUNDINGS INDICATE THE LOWER AIR MASS BELOW ABOUT 12000 FEET WILL
BECOME UNSTABLE BY AFTERNOON. AND THE AIR MASS SHOULD BE SOMEWHAT
MOIST. THE GFS MID-LEVEL LAPSE RATES SUPPORT THIS INSTABILITY. THIS
SHOULD BE ENOUGH FOR A CHANCE OF SHOWERS...WITH MOST ACTIVITY IN THE
AFTERNOON. THE RESULTING CLOUD COVER WILL PROBABLY END UP PARTLY
SUNNY...BUT WITH PERIODS OF CLEARING MIXED IN WITH MOSTLY CLOUDY
SKIES AT TIMES. TEMPERATURES WILL REMAIN VERY CLOSE TO NORMAL FOR
MID-MAY.

THE BROAD WEAK UPPER LEVEL TROUGH REMAINS OVER THE AREA FRIDAY AND
THE AIR MASS REMAINS WEAKLY UNSTABLE...SO FRIDAY SHOULD BE SIMILAR
TO THURSDAY...WITH PERHAPS SLIGHTLY LESS SHOWER ACTIVITY. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 AM AFD...THE NEXT UPPER
TROUGH...AN ELONGATED FEATURE EXTENDING FROM SOUTHEAST ALASKA AND
THE BRITISH COLUMBIA COAST INTO THE PACIFIC NORTHWEST...WILL MOVE
INLAND SATURDAY. ONSHORE FLOW WILL INCREASE AS THIS FEATURE MOVES
INLAND...AND IT WILL BRING A BETTER CHANCE OF SHOWERS TO THE
FORECAST AREA.

A MINOR UPPER RIDGE WILL BUILD OVER THE REGION SUNDAY...BUT WEAK
ONSHORE FLOW AND A CHANCE OF SHOWERS WILL CONTINUE. THE LONGER RANGE
MODELS ARE IN GENERAL AGREEMENT THAT ANOTHER UPPER TROUGH WILL MOVE
TOWARD THE PACIFIC NORTHWEST ON MONDAY...WITH A FAIRLY DEEP UPPER
LOW MOVING OVER THE AREA FROM THE NORTHWEST ON TUESDAY.   MCDONNAL

&&

.AVIATION...AN UPPER LEVEL TROUGH OFF THE PAC NW COAST THIS MORNING
WILL MOVE OVER WRN WA LATER TODAY AND TONIGHT. LIGHT SW FLOW
ALOFT...BECOMING LIGHT NW FLOW ALOFT THURSDAY AS THE TROUGH DIGS
SWD. MOIST IN THE MID AND HIGHER LEVELS THIS MORNING...BECOMING
MOIST AT ALL LEVELS THIS AFTERNOON AS LIGHT RAIN INCREASES ACROSS
THE AREA. GENERALLY VFR CONDITIONS ALTHOUGH PERIODS OF MVFR CLOUDS
COULD DEVELOP ALONG THE COAST AND POSSIBLY SW INTERIOR LATER THIS
AFTERNOON AND EVENING. STABLE ATMOSPHERE...BECOMING WEAKLY UNSTABLE
THURSDAY BUT THUNDERSTORMS ARE NOT EXPECTED.

KSEA....LIGHT RAIN EXPECTED TO DEVELOP AFTER MIDDAY BUT CIGS SHOULD
REMAIN ABOVE 3000-3500 FEET. LIGHT SE WIND LESS THAN 6 KT WILL TURN
SW THIS AFTERNOON...THEN BACK TO SLY TONIGHT. DTM

&&

.MARINE...A WEAK SURFACE TROUGH MOVING INTO THE COASTAL WATERS THIS
MORNING WILL DISSIPATE AS IT PUSHING INTO THE INLAND WATERS LATER
TODAY. SE WINDS MAY REACH 15 TO 25 KT OVER THE OUTER COASTAL WATERS
TODAY AND THE SMALL CRAFT ADVISORY REMAINS IN EFFECT FOR MARGINAL
CONDITIONS. WINDS WILL BE MUCH LIGHTER OVER THE INNER COASTAL WATERS
AND INLAND WATERS AS THE SURFACE TROUGH DISSIPATES LATER TODAY
BEFORE MOVING INLAND.

THE OFFSHORE LOW WILL STALL AND WEAKEN INTO A BROAD TROUGH OVER THE
COASTAL WATERS TONIGHT AND THURSDAY AND WILL RESULT IN LIGHT WINDS
ACROSS THE AREA.

A DEVELOPING RIDGE OF HIGH PRESSURE OFFSHORE IS FORECAST TO MOVE
EAST TOWARD THE COASTAL WATERS ON FRIDAY. STRONG ONSHORE PRES
GRADIENTS WILL DEVELOP ON FRIDAY AND CONTINUE THROUGH FRIDAY NIGHT.
ONSHORE GRADIENTS WILL CONTINUE INTO THE UPCOMING WEEKEND.
DTM/ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY OUTER COASTAL WATERS THROUGH THIS
      AFTERNOON.

&&

$$

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