Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 231612
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
915 AM PDT THU APR 23 2015

.SYNOPSIS...A FRONT WILL BRING SHOWERS TODAY. SHOWERS AND COOL
CONDITIONS REMAIN IN THE FORECAST THROUGH SATURDAY AS AN UPPER LEVEL
LOW PASSES. A WEAK UPPER LEVEL RIDGE WILL BRING DRIER WEATHER
SATURDAY NIGHT. A WARM FRONT MAY CLIP THE AREA ON SUNDAY. HIGH
PRESSURE WILL BRING WARMER AND DRIER WEATHER MONDAY AND TUESDAY.

&&

.SHORT TERM...FRONT MOVING ONTO THE COAST LOOKS IMPRESSIVE ON RADAR
BUT HAS ONLY BROUGHT A FEW HUNDREDTHS TO THE NORTH COAST SO FAR.
MODELS AGREE THE FRONT WILL SPREAD LIGHT RAIN INLAND BY THIS
AFTERNOON. UNDER CLOUDY SKIES THE HIGHS TODAY WILL BE IN THE 50S.

A BROAD UPPER LEVEL LOW WILL SLOWLY APPROACH FROM THE NORTHWEST
FRIDAY. BY SATURDAY AFTERNOON IT WILL BE OVER THE CASCADES AND BY
SATURDAY NIGHT IT WILL HAVE DEPARTED. SHOWERS ARE IN THE FORECAST
FOR THE ENTIRE PERIOD WITH FRIDAY AFTERNOON THE MOST LIKELY TIME.
THERE IS SOME INSTABILITY ASSOCIATED WITH THE UPPER LOW WITH 18Z
FRIDAY THROUGH 06Z SATURDAY THE MOST UNSTABLE PERIOD IN THE GFS. MAY
INCLUDE ISOLATED THUNDERSTORMS FOR THAT TIME IN THE NEXT FORECAST
PACKAGE. HIGHS WILL BE IN THE 50S FRIDAY AND SATURDAY.

SATURDAY NIGHT AND SUNDAY ARE PRETTY DRY IN THE GFS...WITH A SYSTEM
ON SUNDAY REMAINING OFFSHORE AND ONLY BRINGING SOME LIGHT RAIN TO
THE COAST. THE EURO HAS A SIMILAR PATTERN BUT IS SLIGHTLY MORE
PROGRESSIVE WITH THE RAIN. IF CURRENT TRENDS CONTINUE MAY BE ABLE TO
REMOVE PRECIPITATION FROM THE INTERIOR ALTOGETHER FOR SUNDAY. BURKE

.LONG TERM...PREVIOUS DISCUSSION...MODELS ARE SHOWING AN UPPER LEVEL
RIDGE BUILDING OVER WESTERN WA EARLY NEXT WEEK FOR A BREAK IN THE
WEATHER. MONDAY IS NOW LOOKING DRIER AND POPS WERE SCALED BACK AS A
FRONT STALLS/WEAKENS OVER B.C. TEMPS WILL REBOUND WITH HIGHS NEAR 70
DEGREES IN THE INTERIOR. THE RIDGE AXIS WILL SHIFT TOWARD IDAHO ON
TUESDAY BUT THE ECMWF SHOWS ANOTHER FRONT MOVING IN. THE GFS HOWEVER
MAINTAINS DRY WEATHER ACROSS WESTERN WA AS THE SAME FRONT STALLS
OFFSHORE. GIVEN THE DISPARITY NO MAJOR CHANGES WERE MADE AT THIS
POINT. THE MODELS ARE LEANING TOWARD LONG WAVE TROUGHING THROUGH THE
REST OF THE EXTENDED PERIOD. WILL STICK WITH THE CURRENT FORECAST
THAT CALLS FOR NEAR NORMAL TEMPS AND A CHANCE OF SHOWERS. 33

&&

.AVIATION...A COLD FRONT OVER THE PACIFIC NORTHWEST COASTAL WATERS
THIS MORNING WILL MOVE ACROSS WESTERN WASHINGTON THIS AFTERNOON. AN
UPPER LOW WILL MOVE SLOWLY SOUTHEAST ALONG THE COAST OF BRITISH
COLUMBIA TONIGHT. SOUTHWEST FLOW ALOFT WILL CONTINUE. THE AIR MASS
WILL REMAIN MOIST...AND IT WILL BECOME WEAKLY UNSTABLE TONIGHT.

FRONTAL PRECIPITATION WAS FALLING OVER THE COAST...OLYMPIC
PENINSULA...AND THE NORTH INTERIOR OF WESTERN WASHINGTON AT 8 AM. IT
WILL SPREAD INLAND ACROSS THE REST OF THE FORECAST AREA DURING THE
NEXT FEW HOURS. CONDITIONS WILL DETERIORATE TO WIDESPREAD MVFR AND
LOCAL IFR...CONTINUING THIS EVENING AS ONSHORE FLOW FOLLOWS THE
FRONT. THERE COULD BE SOME MARGINAL IMPROVEMENT LATE TONIGHT.

KSEA...SOUTHERLY WINDS 6-10 KT WILL STRENGTHEN TO 8-14 KT GUSTING 24
KT THIS AFTERNOON. CEILINGS WILL LOWER TO MVFR LEVELS DURING THE
AFTERNOON WITH RAIN AS THE FRONT MOVES THROUGH THE INTERIOR.
MCDONNAL

&&

.MARINE...A COLD FRONT OVER THE COASTAL WATERS THIS MORNING WILL
MOVE THROUGH WESTERN WASHINGTON THIS AFTERNOON...FOLLOWED BRIEFLY BY
ONSHORE FLOW THIS EVENING. THE FRONT WILL PRODUCE SMALL CRAFT
ADVISORY SOUTHERLY WINDS FOR ALL MARINE ZONES EXCEPT THE CENTRAL
STRAIT OF JUAN DE FUCA. SMALL CRAFT ADVISORY POST-FRONTAL WINDS WILL
CONTINUE THIS EVENING. THE CENTRAL AND EASTERN STRAIT SHOULD HAVE
WESTERLIES OF 20 TO 30 KT...AND A BRIEF MARGINAL GALE IS NOT OUT OF
THE QUESTION.

A WEAK TROUGH WILL MOVE THROUGH THE FORECAST AREA FROM THE OFFSHORE
WATERS FRIDAY AND FRIDAY NIGHT. A WEAK FRONTAL SYSTEM WILL REACH THE
AREA SUNDAY...AND HIGH PRESSURE WILL BUILD OVER THE REGION MONDAY.
MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY ALL MARINE ZONES.

&&

$$

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




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