Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 141012
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PDT TUE APR 14 2015

.SYNOPSIS...AN UPPER TROUGH WILL MOVE ACROSS WASHINGTON TODAY FOR
SCATTERED SHOWERS...A CHANCE OF THUNDERSTORMS...AND SOME SNOW IN THE
MOUNTAINS. AN UPPER RIDGE WILL MOVE OVER THE AREA WEDNESDAY AND
THURSDAY FOR DRY WEATHER...EXCEPT PERHAPS IN THE FAR NORTH PART. A
WEAK FRONT WILL CROSS THE AREA FRIDAY OR FRIDAY NIGHT. A STRONGER
UPPER RIDGE WILL BRING MOSTLY SUNNY AND WARMER DAYS THIS WEEKEND.

&&

.SHORT TERM...A RATHER SHARP UPPER LEVEL TROUGH IS MOVING INTO THE
PACIFIC NORTHWEST EARLY THIS MORNING...FOLLOWING THE COLD FRONT THAT
MOVED THROUGH THE AREA MONDAY EVENING. THERE ARE SCATTERED SHOWERS
ACROSS WESTERN WASHINGTON...WITH WHAT APPEARS TO BE A LOOSELY
ORGANIZED PUGET SOUND CONVERGENCE ZONE FORMING ALONG THE BORDER OF
SKAGIT AND SNOHOMISH COUNTIES. TEMPERATURES AT 2 AM RANGED FROM THE
UPPER 30S TO THE 40S.

THE UPPER TROUGH WILL MOVE QUICKLY ACROSS THE CASCADES TODAY...AND
THE FLOW ALOFT WILL VEER TO NORTHWESTERLY AHEAD OF AN APPROACHING
UPPER RIDGE OFFSHORE. THE AIR MASS IS UNSTABLE EARLY THIS MORNING
WITH THE 500 MB TEMPERATURE AROUND -32C...AND THERE HAVE BEEN
ISOLATED THUNDERSTORMS OFFSHORE NEAR THE TROUGH AXIS. THE UPPER
HEIGHTS WILL RISE THIS AFTERNOON AS THE TROUGH MOVES EAST...BUT THE
AIR MASS WILL REMAIN FAIRLY UNSTABLE TO AROUND 600 MB. THE
CONVECTIVE TEMPERATURE WILL STILL BE ONLY AROUND 50F LATE TODAY. IN
ADDITION THE CONVERGENCE ZONE SHOULD BECOME ORGANIZED...PROBABLY
MOVING SOUTH INTO SEATTLE LATE TODAY AS THE FLOW ALOFT VEERS.

SCATTERED SHOWERS SHOULD CONTINUE...WITH A CHANCE OF THUNDERSTORMS
AT THE COAST AND IN THE CONVERGENCE ZONE. THE SNOW LEVEL WILL BE
AROUND 2000 FT TODAY. UP TO 4 INCHES OF NEW SNOW SHOULD FALL IN THE
MOUNTAINS...ESPECIALLY WHERE THE CONVERGENCE ZONE EXTENDS INTO THE
CASCADES AROUND STEVENS PASS AND SNOQUALMIE PASS.

THE OFFSHORE UPPER RIDGE WILL PROGRESS TO THE WASHINGTON COAST LATE
TONIGHT. SHOWERS SHOULD BECOME MAINLY LIMITED TO THE MOUNTAINS AND
THE WEAKENING CONVERGENCE ZONE DURING THE EVENING.

THE UPPER RIDGE WILL MOVE OVER WASHINGTON ON WEDNESDAY. A WEAK WARM
FRONT WILL BRUSH THE NORTHWEST CORNER OF THE FORECAST AREA AS IT
MOVES OVER THE NORTH SIDE OF THE RIDGE INTO BRITISH COLUMBIA. THE
WARM FRONT COULD BRING SOME LIGHT PRECIPITATION TO THE COAST...
OLYMPICS...NORTH INTERIOR...AND MOUNT BAKER AREA WEDNESDAY AND
WEDNESDAY NIGHT. THE PUGET SOUND REGION SOUTHWARD SHOULD HAVE PARTLY
SUNNY SKIES WEDNESDAY.

THE UPPER RIDGE WILL BULGE NORTHWARD OVER THE REGION ON THURSDAY...
WHICH SHOULD PUSH THAT WARM FRONTAL PRECIPITATION ALMOST ENTIRELY
NORTH OF THE FORECAST AREA.   MCDONNAL

.LONG TERM...A WEAK UPPER TROUGH WILL MOVE ACROSS THE REGION FRIDAY
OR FRIDAY NIGHT. THE MODELS ARE NOT HANDLING THE FEATURE VERY
WELL...BUT IT SHOULD AT MOST POSE A THREAT OF LIGHT PRECIPITATION
MAINLY IN THE NORTH. THEN AN UPPER RIDGE WILL MOVE SLOWLY FROM THE
OFFSHORE WATERS INTO THE PACIFIC NORTHWEST...BRINGING MOSTLY SUNNY
AND WARMER WEATHER THIS WEEKEND AND MONDAY.   MCDONNAL

&&

.AVIATION...NORTHWEST FLOW ALOFT. THE AIR MASS WILL BE MOIST AND
UNSTABLE TODAY. THE AIR MASS WILL GRADUALLY DRY AND STABILIZE
TONIGHT. AT THE SURFACE...MODERATE TO STRONG ONSHORE FLOW WILL
GRADUALLY DIMINISH THROUGH THIS EVENING.

NUMEROUS SHOWERS CONTINUE OVER THE COASTAL WATERS AND FROM THE
NORTHEASTERN OLYMPIC PENINSULA EASTWARD INTO THE CASCADES OF SKAGIT
AND SNOHOMISH COUNTIES IN A PUGET SOUND CONVERGENCE ZONE. THE
CONVERGENCE ZONE IS EXPECTED TO SHIFT BACK TO THE SOUTH TO A
POSITION BETWEEN SEATAC AIRPORT AND EVERETT LATER THIS MORNING
THROUGH THIS EVENING AS THE UPPER LEVEL FLOW TURNS MORE
NORTHWESTERLY. THE CONVERGENCE ZONE WILL THEN GRADUALLY DISSIPATE
TONIGHT. THERE WILL BE A CHANCE OF THUNDERSTORMS OVER THE COASTAL
WATERS AND COASTAL ZONES...AND WITH THE PUGET SOUND CONVERGENCE
ZONE...THROUGH THIS AFTERNOON. CONDITIONS WILL MAINLY BE VFR ACROSS
THE AREA...BUT WILL OCCASIONALLY FALL TO MVFR IN SHOWERS AND
ISOLATED TSTMS. MVFR CEILINGS AND VISIBILITIES WILL BE MORE
PERSISTENT WITH THE PUGET SOUND CONVERGENCE ZONE. ALBRECHT

KSEA...AT 930Z A WEAK PUGET SOUND CONVERGENCE ZONE WAS WELL NORTH OF
THE AREA. THE CONVERGENCE ZONE IS EXPECTED TO SAG SOUTH TOWARD THE
TERMINAL DURING THE AFTERNOON HOURS THEN DISSIPATE AROUND THE
TERMINAL LATE THIS AFTERNOON AND THIS EVENING. EXPECT P6SM BKN030
THIS MORNING TO IMPROVE TO BKN040 MIDDAY. CEILINGS MAY DROP TO
BKN-OVC015-025 DURING THE LATE AFTERNOON AND EVENING HOURS AS THE
CONVERGENCE ZONE MOVES INTO THE TERMINAL AREA. THERE IS THE RISK OF
AN ISOLATED TSTM 22Z-02Z. WINDS WILL GENERALLY BE SOUTH 8-12 KT
TODAY WITH GUSTS TO 25 KT DURING THE AFTERNOON HOURS. OPPOSING FLOW
BETWEEN KBFI AND KSEA IS LIKELY DURING THE LATE AFTERNOON AND
EVENING HOURS. ALBRECHT

&&

.MARINE...MODERATE TO STRONG ONSHORE FLOW EARLY THIS MORNING WILL
GRADUALLY RELAX THROUGH TONIGHT AS HIGH PRES GRADUALLY BUILDS INTO
THE WATERS.

A GALE WARNING REMAINS IN EFFECT FOR THE CENTRAL AND EASTERN STRAIT
OF JUAN DE FUCA UNTIL 8 AM PDT. A SMALL CRAFT ADVISORY WILL REMAIN
IN EFFECT FOR LATER THIS MORNING THROUGH MUCH OF TONIGHT AS ONSHORE
FLOW CONTINUES. SMALL CRAFT ADVISORIES ARE IN EFFECT FOR ADMIRALTY
INLET AND THE NORTHERN INLAND WATERS TIL 8 AM THIS MORNING...THEN
AGAIN AFTER ABOUT 3 PM AS GRADIENTS TEMPORARILY INCREASE DURING THE
LATE AFTERNOON AND EVENING HOURS.

COASTAL BUOYS MEASURED A WESTERLY SWELL OF 16 TO 18 FEET OVERNIGHT.
THE SWELL WILL GRADUALLY DIMINISH TODAY AND WILL FALL TO BELOW 10
FEET TONIGHT. SMALL CRAFT ADVISORIES FOR HAZARDOUS SEAS CONTINUE FOR
THE WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA AND THE COASTAL
ZONES. A SMALL CRAFT ADVISORY IS IN EFFECT FOR A ROUGH GRAYS HARBOR
BAR.

HIGH PRES WILL BUILD INTO THE WATERS TONIGHT AND CONTINUE OVER THE
WATERS THROUGH THU. THIS WILL GIVE LIGHT WINDS TO THE AREA. A WEAK
FRONT WILL MOVE THROUGH THE WATERS ON FRI. THIS FRONT WILL INCREASE
ONSHORE FLOW FRI AFTERNOON AND EVENING AND WILL INCREASE THE NWLY
FLOW ALONG THE COAST. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER WEATHER ADVISORY CASCADES TILL 11 AM.
PZ...GALE WARNING CENTRAL AND EAST STRAIT UNTIL 8 AM PDT.
     SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT AFTER 8 AM PDT.
     SMALL CRAFT ADVISORY NORTHERN INLAND WATERS AND ADMIRALTY
      INLET.
     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS COASTAL WATERS AND WEST
      ENTRANCE TO THE STRAIT OF JUAN DE FUCA.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

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




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