Small Craft Advisory 221, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
339 PM AKST THU MAR 3 2016

.SHORT TERM...A WEAKENING OCCLUDED FRONT OVER THE N-CENTRAL
PANHANDLE AND NE GULF WILL DISSIPATE TONIGHT. A WEAK TROF WILL
MOVE N THRU THE FAR ERN GULF AND SE AK TONIGHT. A GALE FORCE LOW
WILL MOVE N INTO THE FAR SE GULF LATE TONIGHT...THEN TURN NW
TOWARD THE N-CENTRAL GULF FRI. MODEL DIFFERENCES WITH THESE
FEATURES WERE MINOR...WITH THE MOST DIFFERENCE WITH THE LOW. USED
12Z GFS AS IT LOOKED LIKE A REASONABLE COMPROMISE ON TRACK OF THE
LOW.

MAIN FORECAST CONCERNS WILL BE PRECIP POTENTIAL/PTYPE...AND WINDS.
A BAND OF LIGHT PRECIP WAS FALLING ALONG THE FRONT OVER THE
N-CENTRAL AREA WITH SOME DEFORMATION ENHANCEMENT ALOFT. AS THE
FRONT DISSIPATES...EXPECT THE PRECIP THREAT WITH IT TO DIMINISH
TONIGHT. THE TROF HAS A BAND OF SHOWERS OVER THE SRN AREA ATTM.
THESE SHOWERS WILL MOVE N BUT WILL ENCOUNTER OFFSHORE FLOW...WHICH
WILL DECREASE AREAL COVERAGE AND INTENSITY OF THE SHOWERS AS THEY
MOVE INTO THE CENTRAL AND NRN AREAS. THEN...PRECIP WITH THE GALE
FORCE LOW SHOULD REACH THE FAR SRN AREA OVERNIGHT TONIGHT...THEN
MOVE NWD THRU THE INNER CHANNELS FRI BUT WILL BECOME MORE SPOTTY
DUE TO THE OFFSHORE FLOW. THE PRECIP WITH THE LOW WILL HOLD
TOGETHER MORE ALONG THE OUTER COAST AND ERN GULF AREA.

AS FOR PTYPE...ENOUGH COOL/DRY AIR IN LOWER LEVELS TO CAUSE
EVAPORATIVE COOLING AND ALLOW PRECIP TO REACH SFC AS SNOW WITH THE
DYING OCCLUDED FRONT. THIS WILL MOSTLY BE IN THE PAJN-PAOH AREAS
NWD EARLY THIS EVENING. THERE MAY BE ENOUGH OF THE COOLER AIR FOR
SOME OF THE SHOWERS TO FALL AS SNOW OVER THE NRN AREA LATER
TONIGHT. BY TIME PRECIP WITH GALE FORCE LOW MOVES IN...THINK ONLY
THREAT FOR SNOW WILL BE OVER THE NE AREA...MAINLY N PAJN.

STRONGEST WINDS OVER THE NEXT 24 HRS WILL BE MOSTLY ASSOCIATED
WITH THE LOW MOVING IN LATE TONIGHT AND FRI. EXPECT 40-45 KT WINDS
ON N AND E SIDE OF THE SYSTEM. LOOKS LIKE LOW WILL BE CLOSE ENOUGH
FOR STRONG WINDS TO AFFECT THE SRN OUTER COAST LATE TONIGHT AS
WELL. SOME OF THESE WINDS MAY TRY TO REACH THE CENTRAL COAST FRI
MORNING BUT BY THEN THE LOW WILL START TO MOVE FURTHER FROM THE
COAST. OVER THE INNER CHANNELS...NLY/ELY WINDS WILL INCREASE AHEAD
OF THE LOW WITH MOST PLACES GETTING AT LEAST SCA LEVEL WINDS. SOME
GALES ARE LIKELY OVER THE SRN CHANNELS AND OUT OF CROSS SOUND
LATER TONIGHT INTO FRI MORNING.

OTHERWISE...DECIDED TO TAKE OUT TSTMS FROM THE FORECAST WITH THE
TROF MOVING IN THIS EVENING. NO LIGHTNING HAS BEEN DETECTED WITH
THE CONVECTIVE CELLS ALONG THE TROF SINCE LATE MORNING...AND
EFFECT OF THE LOW TO THE S WILL BE TO WARM TEMPS ALOFT OVER THE
SHOWERS. THERE HAS BEEN SOME TSTM ACTIVITY WITH THE LOW MOVING IN
BUT IT HAS DIMINISHED GREATLY SINCE THIS MORNING...AND MODEL
FORECAST CAPES DECREASE AS SYSTEM MOVES N...SO DID NOT INCLUDE
ANY TSTMS WITH THAT SYSTEM IN OUR AREA LATE TONIGHT AND FRI.

.LONG TERM...EXTENDED RANGE FORECAST PERIOD BEGINS WITH MULTIPLE
UPPER LEVEL LOWS ROTATING CYCLONICALLY ABOUT EACH OTHER. UPPER
LEVEL CIRCULATION DOES NOT COALESCE INTO A SINGLE, LARGE
CIRCULATION UNTIL WEDNESDAY. THAT SINGULAR FEATURE WILL WEAKEN AS
IT DRIFTS NORTH AND EVENTUALLY BECOME ABSORBED BY ANOTHER UPPER
LOW THAT MOVES INTO THE GULF FROM THE WEST LATE FRIDAY NIGHT.
CHAOTIC UPPER LEVEL STEERING ASSOCIATED WITH THIS PATTERN WILL
COMPLICATE FORECASTING TRACK AND SPEED OF THE VARIOUS SURFACE
FEATURES AND THIS IS REFLECTED IN MODE DISAGREEMENT TODAY.
AFTERNOON MODELS ARE ACTUALLY IN PRETTY GOOD AGREEMENT THROUGH
TUESDAY NIGHT CONSIDERING THE STATE OF THE UPPER ATMOSPHERE.

UPPER LEVEL DYNAMICS ARE AS DISORGANIZED AS THE STEERING FLOW.
CONSEQUENTLY, PRECIP BEING DEPICTED BY THE VARIOUS MODELS IS, AS
WITH THE PLACEMENT OF THE LOWS THEMSELVES, QUITE DISPARATE.
BIGGEST CHANGE IN THIS REGARD FROM YESTERDAY WAS TO EXTEND THE
POTENTIAL FOR DRY WEATHER OVER THE CENTRAL INNER CHANNELS AWAY
FROM THE OUTER COAST FROM SATURDAY MORNING THROUGH SUNDAY
AFTERNOON. APART FROM THAT TIME FRAME, HIGH CONFIDENCE IN AN
OVERALL WET PATTERN BUT LOW CONFIDENCE IN THE PRECISION OF JUST
WHERE AND WHEN IT WILL RAIN.

MODELS CONTINUE TO MAINTAIN A TIGHT NORTHEAST-SOUTHWEST PRESSURE
GRADIENT ACROSS THE PANHANDLE AND THIS IS AMPLIFIED BY THE PASSAGE
OF EACH LOW/WAVE THAT TRACKS NORTHWEST OVER THE EASTERN GULF. PEAK
CONDITIONS LOOK LIKE SUNDAY THROUGH SUNDAY AFTERNOON, WITH GALES
OVER SEVERAL OF THE INNER CHANNELS.

BEGAN THE EXTENDED RANGE WITH GFS22 FOR PRESSURE AND WIND TO BLEND
BETTER WITH THE END OF THE SHORT RANGE FORECAST PERIOD. SHIFTED TO
ECMWF FOR SATURDAY THROUGH SUNDAY NIGHT. FOLLOWED UP WITH THE
LATEST WPC. EMPLOYED AN ENSEMBLE APPROACH TO POP AND QPF BASED ON
DIFFERING MODEL SOLUTIONS. NO SIGNIFICANT CHANGES TO TEMPS. SNOW
LEVELS ADJUSTED UP BASED ON FORECAST OVERNIGHT LOW TEMPS. OVERALL
FORECAST CONFIDENCE IS AVERAGE.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...STRONG WIND FROM MIDNIGHT AKST TONIGHT THROUGH LATE TONIGHT FOR
     AKZ027.
MARINE...GALE WARNING FOR PKZ022-033>036-041>043-051.
     SMALL CRAFT ADVISORY FOR PKZ011>013-021-031-032-052-053.
&&

RWT/FRITSCH

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