Small Craft Advisory 15, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
320 PM AKST MON JAN 4 2016

.SHORT TERM...WITH HIGH PRESSURE FIRMLY IN PLACE OVER SOUTHEAST
ALASKA...THE WEATHER, OR FOR THAT MATTER, THE FORECAST, WILL NOT
CHANGE APPRECIABLY OVER THE NEXT 24 HOURS. THE RIDGE WILL
POSITION ITSELF FARTHER TO THE NORTH OVER THE CANADIAN YUKON BY
TUESDAY IN RESPONSE TO A CUT-OFF LOW ON ITS WAY NORTHWEST ACROSS
SOUTHERN AND CENTRAL BC.

UNDER HIGH PRESSURE, THE WEATHER ELEMENT OF MOST CONCERN IS FOG,
WHICH IS NOTORIOUSLY FICKLE. WE OFTEN RELY ON PATTERN RECOGNITION
AND BASIC PREMISES...SUCH AS INVERSION STRENGTH, PERIODS OF LIGHT
WINDS, AVAILABILITY OF MOISTURE SOURCE, AND TIME OF DAY.
PERSISTENCE ALSO PLAYS A ROLE. FOG FORMED EARLY YESTERDAY MORNING
IN JUNEAU AFTER THE WEEKEND'S RAINS RELENTED AND PERSISTED THROUGH
THE DAY INTO THE EVENING BEFORE THINNING. AN EBBING TIDE OFFERS
ONE GOOD MOISTURE SOURCE FOR FOG FORMATION, EVEN DURING PERIODS OF
SUB-FREEZING TEMPERATURES, AS THE WARMER WATERS CAN QUICKLY BE
COLLECTED BY THE IMMEDIATE ATMOSPHERE. BUT IT NOW APPEARS...DRYING
OCCURRED OVERNIGHT AS MELTED SNOW FROZE AND EFFECTIVELY LIMITED
THE MOISTURE SOURCE...OVER-COMPENSATING FOR THE GAIN FROM AN
EBBING TIDE. FOG PERSISTED ALONG THE SOUTHERN SIDE OF DOUGLAS
ISLAND OVER THE MUCH WIDER STEPHENS PASSAGE. KETCHIKAN, ANNETTE,
AND PETERSBURG ALSO EXPERIENCED FOG DEVELOPMENT AROUND SUNSET
YESTERDAY. HOWEVER, TEMPERATURES WERE SLIGHTLY WARMER AND MOISTURE
MORE ABLY ABSORBED. IN THE CASE OF PETERSBURG, THE SHELTERED
LOCATION OFFERED SUCH AN IDEAL ENVIRONMENT OF STABILITY THAT THE
FOG APPEARS TO HAVE STRATIFIED INTO THE AFTERNOON. WE DO EXPECT
THE INVERSION TO SHARPEN OVERNIGHT AND CEILINGS AND VISIBILITIES
TO DROP AGAIN THIS EVENING. FOG MAY ALSO BE PREVALENT ACROSS THE
SOUTHERN INNER CHANNELS INCLUDING KETCHIKAN WITH WARMER
TEMPERATURES, LIGHT WINDS, AND WIDER CHANNELS. POTENTIALLY A SMALL
INCREASE IN WINDS AS A SURFACE SYSTEM MOVES TOWARD DIXON ENTRANCE
TUESDAY AFTERNOON MAY MIX THE ATMOSPHERE SUFFICIENTLY TO END THE
FOG. MORE LIKELY...COOLER MORNING TEMPERATURES MAY LIMIT ITS
MOISTURE SOURCE. FINALLY A MID-AFTERNOON CALL TO ONE OF OUR
SPOTTERS IN TENAKEE CONFIRMED WEBCAMS THAT FOG HAD FORMED AND WAS
BECOMING MORE DENSE. WITH LIGHT WINDS AND A STEEPENING INVERSION,
WE WILL ADD TENAKEE TO THE DENSE FOG ADVISORY RAISED THIS MORNING
FOR PETERSBURG. THE DENSE FOG ADVISORY FOR JUNEAU WAS CANCELLED
THIS MORNING WHEN POSITIVE TRENDS FAILED TO MATERIALIZE.

THE STRONG GRADIENTS INDICATED BY THE NAM HAS NOT APPEARED ACROSS
LYNN CANAL...AND WE HAVE NOW LIMITED THE WINDS TO 25 KT THROUGH
THE TIME PERIOD. BUT STRONG OUTFLOW WINDS WILL CONTINUE AT TIMES
THROUGH SKAGWAY THROUGH TUESDAY AND BEYOND. WITH WINDS DOWNTOWN
JUNEAU BEING A VIRTUAL NO-SHOW DURING THE LAST FEW DAYS, AND
MODELS INDICATING THE STRONGEST GRADIENT ALONG THE COASTAL
MOUNTAINS THEMSELVES, WE REDUCED WINDS TO LIGHT. SMALL CRAFT WINDS
ACROSS THE CENTRAL GULF AND CROSS SOUND WILL SLOWLY FALL THROUGH
TONIGHT AND TUESDAY.

WE NUDGED TO HIGH RES MODELS FOR GULF WIND GUIDANCE. WE MADE SOME
MODIFICATIONS TO TEMPERATURES INCLUDING COOLING DOWNTOWN JUNEAU AS
WELL AS WHITE PASS TO ADJUST TRENDS FOR CURRENT CONDITIONS...BUT
OTHERWISE TEMPERATURES LOOKED GOOD. OUR CONFIDENCE IN THE GENERAL
FORECAST IS GOOD EVEN AS OUR CONFIDENCE IN FOG IS ALWAYS SHAKY.
FOG IS A VERY FICKLE THING.

.LONG TERM...OVERALL, A DRY WEEK IS IN STORE FOR SOUTHEAST
ALASKA. OUTFLOW WINDS WILL BE THE MAIN STORY THROUGH THE FIRST
HALF OF THE WEEK WITH A WEAK UPPER LOW ROTATING NORTHWEST INTO THE
PANHANDLE WEDNESDAY AND THURSDAY THEN A WEAK FRONT CROSSING THE
GULF LATER FRIDAY AND SATURDAY. MODELS THEN INDICATE ANOTHER DRY
PERIOD OF WEATHER IS POSSIBLE THROUGH EARLY NEXT WEEK.

OUTFLOW WINDS WILL BE FAVORED IN AREAS IN THE VICINITY OF THE
COAST MOUNTAINS AS THE PRESSURE GRADIENT WILL BE PACKED TIGHTLY
ALONG THE COAST RANGE. DO NOT FORSEE OUTFLOW WINDS BEING AS MUCH
OF AN ISSUE TOWARDS THE OUTER COAST. ANOTHER HINDERING FACTOR
TOWARDS A STRONGER AND MORE WIDESPREAD OUTFLOW EVENT IS THE LACK
OF COLD AIR IN CANADA. THEREFORE HAVE DECREASED WIND SPEEDS
ACROSS A LARGE PORTION OF THE AREA FROM THE PREVIOUS FORECAST.
HOWEVER...SMALL CRAFT TO GALE FORCE WINDS ARE STILL ANTICIPATED IN
THE INNER CHANNELS ALONG THE COAST MOUNTAINS INCLUDING LYNN
CANAL...STEPHENS PASSAGE AND FREDERICK SOUND. OUTFLOW AREAS ON
LAND CAN ALSO EXPECT GUSTY WINDS THROUGH MID WEEK. THE GRADIENT
AND WINDS WILL BEGIN TO WEAKEN AS A WEAK FRONT MOVES INTO THE
EASTERN GULF BY THE WEEKEND.

PRECIPITATION WILL NOT BECOME AN ISSUE UNTIL LATE WEDNESDAY NIGHT
AND THURSDAY AS AN UPPER LOW NEAR VANCOUVER ISLAND ROTATES
NORTHWEST INTO THE EASTERN GULF. MODELS HAVE TRENDED WETTER WITH
THIS FEATURE SO HAVE INCLUDED A CHANCE OF RAIN AND SNOW OVER THE
SOUTHERN PANHANDLE AND PORTIONS OF THE OUTER COAST. HOWEVER WITH A
DRY AIRMASS IN PLACE AND THE FACT MUCH OF THE ENERGY WITH THIS
SYSTEM REMAINS ALOFT EXPECT PRECIPITATION AMOUNTS TO BE VERY LIGHT
AND NOT AMOUNT TO MUCH. A LOW MOVING INTO THE WEST CENTRAL GULF
WILL SEND A WEAKENING WEATHER FRONT INTO THE EASTERN GULF FRIDAY.
THIS WILL BRING A BAND OF PRECIPITATION TO THE PANHANDLE EARLY IN
THE WEEKEND. WARM AIR ALOFT RESIDING OVER THE AREA COMBINED WITH
WEAKENING OUTFLOW WINDS WOULD FAVOR MAINLY RAIN WITH SOME
POTENTIAL SNOW AT THE ONSET. HOWEVER...WOULD NOT BE SURPRISED TO
SEE MODELS CONTINUE TO TREND WEAKER WITH THIS SYSTEM AS IT LOSES
ITS UPPER LEVEL SUPPORTS AND ENCOUNTERS THE RIDGE OVER WESTERN
NORTH AMERICA. DO NOT EXPECT ANY SIGNIFICANT SNOWFALL AMOUNTS AT
SEA LEVEL WITH THIS SYSTEM REGARDLESS.

MODELS WERE ALL IN FAIRLY GOOD AGREEMENT THROUGH THE EXTENDED
RANGE. USED THE 12Z NAM/ECMWF FOR UPDATES TO THE INHERITED
FORECAST THROUGH DAY 4 THEN THE LATEST WPC THEREAFTER. MODEL
AGREEMENT LEADS TO INCREASED FORECASTER CONFIDENCE IN THE LONG
RANGE.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...DENSE FOG ADVISORY UNTIL NOON AKST TUESDAY FOR AKZ026.
         DENSE FOG ADVISORY UNTIL NOON AKST TUESDAY FOR AKZ021.
         STRONG WIND THROUGH LATE TUESDAY NIGHT FOR AKZ018.
MARINE...GALE WARNING FOR PKZ052.
         SMALL CRAFT ADVISORY FOR PKZ012-022-043-051.
         SMALL CRAFT ADVISORY DUE TO SEAS FOR PKZ041-042-053.

&&

JWA/TPS

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