Small Craft Advisory 174, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE
JUNEAU AK 404 PM AKST THU FEB 18 2016

.SHORT TERM...DRY NORTHWEST FLOW ALOFT WITH BUILDING HEIGHTS IN
THE UPPER LEVELS CONTINUES TO SUSTAIN MOSTLY CLEAR SKIES ACROSS
THE NORTH AND CENTRAL PANHANDLE. SOME LOWER LEVEL MOISTURE HAS
EFFECTIVELY BEEN TRAPPED OVER THE SOUTHERN PANHANDLE AS STRATOCU
HAS BEEN MORE PERSISTENT OVER SOME AREAS THAN ORIGINALLY THOUGHT.
STILL SOUTHEAST FROM YAKUTAT TO HYDER HAS EXPERIENCED DRY WEATHER
THROUGHOUT. A WEAK SYSTEM TO THE SOUTH BELOW HAIDA GWAII WILL
ADVANCE NORTHWARD TONIGHT AND LIKELY BRING SOME LIGHT RAIN TO THE
FAR SOUTHERN INNER CHANNELS LATE TONIGHT. BUT AS THIS SYSTEM
TRAVELS FURTHER NORTH AND WEST IT WILL WEAKEN. THUS FOR TOMORROW
PRECIPITATION KEPT AT A CHANCE FOR THE SOUTHERN AND CENTRAL
PANHANDLE BUT WILL FAIL TO REACH POINTS NORTH OF ANGOON. ALONG THE
NORTHERN EDGE...PRECIPITATION MAY REACH THE GROUND AS LIGHT SNOW,
BUT IMPACTS SHOULD BE MINIMAL AS THE SYSTEM'S TRAJECTORY SUPPORTS
ONLY LIGHT AMOUNTS OF PRECIPITATION. NO WHERE IS LIGHT SNOW
LIKELY...BUT SIMPLY THERE IS A CHANCE OF IT. ELSEWHERE THE NEXT
24 HOURS WILL BE CONTINUED DRY.

OUR TEMPERATURE FORECAST WAS NOT BAD LAST NIGHT...ALTHOUGH FOR THE
COLD SPOTS IN THE EXTREME NORTH LIKE GUSTAVUS...YAKUTAT...AND
WHITE PASS, WE MAY HAVE BEEN A FEW DEGREES TOO WARM. MODELS ARE
INDICATING CONTINUED COLD AIR ADVECTION OVERNIGHT, SO WITH ANOTHER
CLOUD FREE NIGHT OVER NORTHERN SOUTHEAST WE COOLED TEMPERATURES BY
A FEW MORE DEGREES THAN LOWS THIS MORNING. THIS CAUSED CONCERN
ALONG THE HIGHWAY AS WITH THE CONTINUED STRONG GRADIENTS AND
TEMPERATURES FALLING BELOW ZERO TONIGHT, WE NOW EXPECT WINDCHILLS
TO FALL TO 30 BELOW LATE TONIGHT AND REMAIN SO THROUGH FRIDAY
MORNING. ELSEWHERE WE EXPECT HAINES BORDER TO APPROACH ZERO AND
YAKUTAT AND GUSTAVUS TO NEAR TEN ABOVE. WITH WINDS EXPECTED IN
DOWNTOWN JUNEAU WE EXPECT TEMPERATURES TO BE MUCH WARMER THAN
WINDLESS MENDENHALL VALLEY TONIGHT. THUS THE JUNEAU AREA COULD
EXPERIENCE A 20 DEGREE SPREAD ACROSS A RELATIVELY SMALL GEOGRAPHIC
AREA. APPROACHING CLOUD COVER AND SOME WEAK WARM AIR ADVECTION
WITH THE SYSTEM TO THE SOUTH WILL KEEP TEMPERATURES ABOVE FREEZING
FOR THE SOUTHERN ZONES.

WE KEPT THE STRONG WIND FOR DOWNTOWN JUNEAU AS A CRITICAL LAYER OF
LIGHT WINDS WILL DEVELOP AS THE 500 RIDGE AXIS CROSSES THE SPINE
OF THE COAST MOUNTAINS. THIS WILL BREAK THE INVERSION AND
ACCELERATE WINDS DOWNTOWN FRIDAY MORNING. WINDS MAY BEGIN TO
DIMINISH IN THE AFTERNOON...BUT THESE TAKU EVENTS TEND TO
DIMINISH MORE SLOWLY SO OUR UNCERTAINTY IS GREAT.

AN ENSEMBLE OF NAM, GFS, AND ECMWF USED TO FORGE A COMPROMISE ON
WEAK LOW POSITION TRAVERSING THE EASTERN GULF TONIGHT. THERE WAS
FAIR AGREEMENT AMONG THE THREE. FORECAST CONFIDENCE BETTER THAN
AVERAGE.

.LONG TERM...SIGNIFICANT CHANGES IN THE WEATHER IN STORE FOR
SOUTHEAST ALASKA EARLY ON IN THE EXTENDED RANGE FORECAST. THE
STRONG WINDS OVER THE NORTHERN INNER CHANNELS AND MARINE ZONES
WILL CONTINUE, AS WILL THE INCREASING CLOUDINESS FROM SOUTH TO
NORTH. A WEAK LOW CURRENTLY FORMING NEAR 50N 140W WILL TRACK
NORTHEAST TO A POSITION WEST OF THE DIXON ENTRANCE BY FRIDAY
EVENING. THIS LOW WILL PUSH A FRONT OVER THE EASTERN GULF AND
PANHANDLE THROUGH SATURDAY. COLD SURFACE TEMPERATURES WILL RESULT
IN ACCUMULATING SNOWFALL FOR MUCH OF SOUTHEAST ALASKA FRIDAY NIGHT
THROUGH SATURDAY AFTERNOON WITH SNOW CONTINUING FOR THE FAR
NORTHERN INNER CHANNELS THROUGH SATURDAY NIGHT.

THIS LOW WILL PRESS UP AGAINST AN ALREADY TIGHT NORTHERLY PRESSURE
GRADIENT, RESULTING IN GALES FOR SEVERAL OF THE INNER CHANNELS AND
SMALL CRAFT WINDS ELSEWHERE. ADDITIONALLY, STRONG WIND GUSTS WILL
BE WIDESPREAD FRIDAY NIGHT INTO SATURDAY.

AS THE FIRST LOW WEAKENS OVER THE NORTHEAST GULF, A SECOND GALE
FORCE FRONT WILL BE ENTERING THE EASTERN GULF FROM THE WEST. A
DEVELOPING WAVE ALONG THE FRONTAL BOUNDARY WILL TUG THE FRONT TO
THE WEST, SO MUCH OF THE PRECIP FROM THIS SYSTEM WILL REMAIN
OFFSHORE THROUGH SUNDAY AFTERNOON, BUT EVENTUALLY, THIS FRONT WILL
MOVE EAST. THE MAIN ISSUE WITH THIS SECOND FRONT WILL BE SUSTAINED
STRONG WINDS FOR THE CENTRAL AND SOUTHERN OUTER COAST AND MAXIMUM
GALE FORCE WINDS OVER THE EASTERN GULF SATURDAY AFTERNOON THROUGH
SUNDAY AFTERNOON. AS THIS SECOND FRONT WEAKENS OVER LAND,
SOUTHEASTERLY FLOW WILL SHIFT AND BECOME SOUTHWEST, THUS BREAKING
DOWN THE VERY STRONG OFFSHORE PRESSURE GRADIENT AND ALLOWING
WINDS ACROSS SOUTHEAST ALASKA TO RELAX SUNDAY AFTERNOON INTO THE
EVENING.

THE BREAK WILL BE SHORT LIVED, AS A THIRD FRONT WILL APPROACH
FROM THE WEST MONDAY NIGHT, RESULTING IN MORE SOUTHEASTERLY GALES
OVER THE EASTERN GULF WITH SMALL CRAFT CONDITIONS ON THE INSIDE BY
TUESDAY MORNING.

USED A BLEND OF NAM, GFS22, AND THE CANADIAN NH FOR UPDATES TO
PRESSURE AND WIND THROUGH SATURDAY AFTERNOON. BLENDED GFS22 AND
ECMWF SATURDAY EVENING THROUGH SUNDAY NIGHT. POP AND QPF UPDATED
THROUGH SUNDAY NIGHT WITH ECMWF AND THE CANADIAN WITH SOME HELP
FROM GFS22. TEMPS UNCHANGED EXCEPT FOR LOWERING TEMPS NEAR WHITE
PASS FOR FRIDAY NIGHT. GRADUAL WARMING IS STILL EXPECTED WITH EACH
SUCCESSIVE FRONT. OVERALL FORECAST CONFIDENCE IS AVERAGE.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND FROM FRIDAY MORNING THROUGH FRIDAY AFTERNOON FOR
     AKZ025.
     STRONG WIND THROUGH FRIDAY AFTERNOON FOR AKZ018.
     WIND CHILL ADVISORY FROM MIDNIGHT TONIGHT TO NOON AKST FRIDAY
     FOR AKZ018.
MARINE...SMALL CRAFT ADVISORY FOR PKZ012-013-022-031-034>036-041-043-052.
     SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ042-051.
&&

JWA/FRITSCH

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