Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 312159
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT TUE MAR 31 2015

.SYNOPSIS...AN UPPER TROUGH WILL BRING SHOWERS AND COOLER WEATHER
TONIGHT AND WEDNESDAY. SHOWERS WILL DIMINISH THURSDAY AS WEAK HIGH
PRESSURE BUILDS OVER THE AREA AHEAD OF A FRONTAL SYSTEM THAT WILL
AFFECT THE AREA ON FRIDAY. AN UPPER LEVEL LOW WILL MAINTAIN A CHANCE
OF SHOWERS THROUGH THE WEEKEND.

&&

.SHORT TERM...AN UPPER LEVEL TROUGH WILL MAINTAIN COOL AND SHOWERY
WEATHER ACROSS WESTERN WA TONIGHT. TEMPS WERE IN THE LOW TO MID 50S
THIS AFTERNOON...AROUND 10 DEGREES COOLER THAN YESTERDAY. A PUGET
SOUND CONVERGENCE ZONE WILL CONTINUE TO WAVER THROUGH SNOHOMISH AND
KING COUNTIES TONIGHT. SHOWERS FROM THE CONVERGENCE ZONE MAY PRODUCE
HEAVY SNOW IN THE CASCADES...ESPECIALLY AT STEVENS PASS...AND
A WINTER WEATHER ADVISORY REMAINS IN EFFECT. THE SNOW LEVEL IS
AROUND 4000 FT BUT WILL LOWER TO AROUND 3000 FT TONIGHT. LESS SNOW
IS EXPECTED AT SNOQUALMIE PASS WITH 1-3 INCHES.

THERE IS STILL A RISK OF THUNDERSTORMS ACROSS WESTERN WA THIS
AFTERNOON AND EVENING AS THE UPPER LEVEL TROUGH AXIS SWINGS THROUGH.
SO FAR THERE HAVE BEEN A FEW STRIKES IN EASTERN JEFFERSON COUNTY AND
ADMIRALTY INLET...AND A CALL FROM A SPOTTER IN NORTH BEND REPORTING
THUNDER. SMALL HAIL HAS ALSO BEEN REPORTED WITH THE HEAVIER CELLS.
T-STORM ACTIVITY SHOULD DIMINISH AFTER SUNSET.

THE AIR MASS WILL REMAIN COOL AND UNSTABLE ON WEDNESDAY AS ANOTHER
SHORT WAVE TROUGH CLIPS THROUGH WESTERN WA. EXPECT MORE SHOWERS AND
A SLIGHT CHANCE OF THUNDERSTORMS ONCE AGAIN. TEMPS WILL REMAIN A FEW
DEGREES BELOW NORMAL...SIMILAR TO TODAY.

OVERALL THE FORECAST LOOKS DRIER ON THURSDAY AS A WEAK UPPER LEVEL
RIDGE MOVES IN. TEMPS WILL ALSO BUMP UP A FEW DEGREES...CLOSER TO
NORMAL. THE RIDGE WILL FLATTEN THU NIGHT AS OUR NEXT COLD FRONT
APPROACHES. THIS FRONT WILL REACH THE COAST EARLY FRI MORNING...WITH
RAIN SPREADING INTO THE INTERIOR BY THE AFTERNOON. 33

.LONG TERM...THE AIR MASS WILL REMAIN COOL AND UNSTABLE THIS WEEKEND
THROUGH EARLY NEXT WEEK AS AN UPPER LEVEL LOW SPINS OFFSHORE. THIS
SYSTEM WILL CONTINUE TO PUMP MOISTURE INTO WESTERN WA FOR A CHANCE
OF SHOWERS EACH DAY. THE UPPER LOW WILL WEAKEN AND MOVE INLAND ON
MONDAY. A WARM FRONT WILL THEN ARRIVE MONDAY NIGHT OR TUE FOR MORE
RAIN. THERE IS GOOD AGREEMENT SO FAR BETWEEN THE GFS AND ECMWF. 33

&&

.HYDROLOGY...FLOODING IS UNLIKELY OVER THE NEXT 7 DAYS.

&&

.AVIATION...WESTERLY FLOW ALOFT WILL PREVAIL THROUGH THIS EVENING AS
A SOMEWHAT BROAD UPPER LEVEL TROUGH MOVES ACROSS THE PACIFIC NW. THE
AIR MASS IS UNSTABLE AND GENERALLY MOIST WITH SCATTERED SHOWERS.
THERE IS STILL A CHANCE OF AN ISOLATED THUNDERSTORM THROUGH ABOUT
03Z. AN AREA OF MORE DEVELOPED SHOWERS ASSOCIATED WITH AN EMBEDDED
UPPER LEVEL SHORTWAVE TROUGH WAS OVER W WA AT 21Z...WITH THE BACK
EDGE OF THE AREA STILL ABOUT 60 NM OFFSHORE. THE SHOWER BAND SHOULD
BE COMPLETELY E OF THE AREA BETWEEN 03Z AND 06Z. A CONVERGENCE ZONE
OVER N PUGET SOUND MAY CONTINUE OFF AND ON THROUGH TONIGHT.

CIGS ARE PREDOMINANTLY VFR SCT-BKN035-060. CIGS MAY LOWER BRIEFLY TO
MVFR IN SHOWERS.

FLOW ALOFT WILL SHIFT TO NW ON WEDNESDAY...AND ANOTHER UPPER LEVEL
SHORTWAVE TROUGH WILL SEND ANOTHER LINE OF SHOWERS ACROSS W WA. THE
GFS SHOWS LIFTED INDICES AROUND -2 WEDNESDAY AFTERNOON AND NAM HIGH
RES SOUNDINGS ARE UNSTABLE...SO THERE IS A CHANCE OF THUNDERSTORMS.

KSEA...A LINE OF SHOWERS WILL BE MOVING ACROSS PUGET SOUND THROUGH
ABOUT 03Z. THIS WILL KEEP CIGS VFR BKN030-050...BUT CIGS COULD STILL
LOWER BRIEFLY TO BKN020-030 IF A SHOWER GOES RIGHT OVER THE
TERMINAL. CIGS SHOULD LIFT TO SCT-BKN050-060 LATER THIS EVENING.
SURFACE WINDS WILL REMAIN SW 5-15 KT BUT GUSTS TO 22 KT ARE POSSIBLE
AND THE WINDS MAY SHIFT AROUND AS SHOWERS CROSS THE AREA. KAM

&&

.MARINE...ONSHORE FLOW HAS WEAKENED SIGNIFICANTLY THIS
AFTERNOON...BUT THE UNSTABLE AIR MASS HAS ALLOWED HIGHER GUSTS TO
MIX DOWN TO THE SURFACE. WINDS IN THE STRAIT HAVE REMAINED BELOW
GALE FORCE TODAY...BUT A LINE OF SHOWERS ASSOCIATED WITH AN UPPER
LEVEL SHORTWAVE TROUGH WILL MOVE ACROSS THE AREA LATE THIS AFTERNOON
AND EVENING. THE PASSING SHORTWAVE TROUGH COMBINED WITH THE UNSTABLE
AIR MASS MAY BE ENOUGH TO GENERATE BRIEF PERIODS GALE FORCE WINDS
OR GUSTS LATE THIS AFTERNOON AND EVENING. THE GALE WARNING HAS BEEN
EXTENDED UNTIL 9 PM THIS EVENING BUT CONFIDENCE IS ONLY MODERATE AND
ANY GALE FORCE WINDS THAT OCCUR WILL BE BRIEF.

SMALL CRAFT ADVISORIES REMAIN UP FOR THE REST OF THE INLAND WATERS
DUE PRIMARILY TO WIND GUSTS IN THE SCA RANGE. WINDS WILL EASE THIS
EVENING.

THE SCA FOR HAZARDOUS SEAS WAS EXTENDED OVER ALL THE COASTAL WATERS
UNTIL 8 PM. MODEL FORECASTS HAVE THE SWELL SUBSIDING TO 9 FT
TONIGHT...BUT WITH CURRENT BUOY OBS ARE STILL SHOWING 10 FEET AT
TIMES...THE EXTENSION ALLOWS A LITTLE MORE TIME FOR THE SEAS TO
SUBSIDE.

A SURFACE RIDGE FORMING OFFSHORE WILL MAINTAIN WEAKER ONSHORE FLOW
THROUGH THURSDAY. THE FLOW MAY STILL BE STRONG ENOUGH WEDNESDAY
EVENING FOR BRIEF SCA W WINDS IN THE STRAIT.

MODELS REMAIN CONSISTENT IN BRINGING A COLD FRONT ACROSS W WA ON
FRIDAY...WITH SCA WINDS EXPECTED OVER THE COASTAL WATERS. KAM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER WEATHER ADVISORY CASCADES OF SNOHOMISH AND KING
     COUNTIES.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS FOR HAZARDOUS SEAS.
     SMALL CRAFT ADVISORY ADMIRALTY INLET...NORTHERN INLAND
      WATERS...PUGET SOUND AND HOOD CANAL.
     GALE WARNING CENTRAL AND EAST STRAIT OF JUAN DE FUCA.
     SMALL CRAFT ADVISORY GRAYS HARBOR FOR ROUGH BAR.

&&

$$

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












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