Small Craft Advisory 118, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
550 AM AKST THU FEB 7 2013

.ANALYSIS AND UPPER LEVELS...
A BROAD UPPER-TROUGH DOMINATES THE BERING SEA/ALEUTIAN CHAIN...
ANCHORED BY A LOW JUST SOUTH OF KISKA WHICH IS FILLING AND DRIFTING
EAST. AN ASSOCIATED FRONT MOVES ASHORE SW AK AND IS DISSIPATING WHILE
SUPPORTING SNOW AND BLOWING SNOW. EMBEDDED WEAK UPPER DISTURBANCES
BEHIND THE FRONT SUPPORT HODGEPODGE SNOW AND RAIN SHOWERS FROM THE
AKPEN/PRIBILOFS BACK THROUGH THE ALEUTIAN CHAIN. FURTHER EAST...
TRANSITORY UPPER RIDGING OVER THE NORTHERN GULF AND SOUTHCENTRAL AK
IS QUICKLY RETREATING EAST TOWARDS YUKON TERRITORY...THOUGH IT HAS
SUPPORTED SOME EARLY MORNING FOG/STRATUS ACROSS COOK INLET AND PRINCE
WILLIAM SOUND. THE SOUTHEASTERN EDGE OF THE AFOREMENTIONED FRONT IS
SWINGING FROM THE WESTERN TO THE NORTHERN GULF. THE FRONT SPREADS
MAINLY SNOW...THOUGH IT MIXES WITH OR CHANGES TO RAIN ALONG COASTAL
AREAS. EARLY HINTS OF A TRIPLE-POINT LOW DEVELOPING ALONG IT'S AXIS
APPEARS NEAR KODIAK ISLAND THIS MORNING TRACKING NORTHEAST TOWARDS
THE KENAI PENINSULA. ALSO OF NOTE IS A DEEPENING SURFACE LOW IN THE
CENTRAL PACIFIC AROUND 1000 MILES SOUTH OF ATKA...WHICH IS BEGINNING
TO PIVOT NORTHEAST.

.MODEL DISCUSSION...
MODELS SATISFACTORILY CAPTURE THE MAIN FEATURES. THE LOW JUST SOUTH
OF KISKA CONTINUES FILLING AS IT MOVES TO AROUND 250 MILES SOUTHEAST
OF ATKA FRI MORNING...THEN BECOMES ABSORBED BY THE SYSTEM MOVING UP
FROM THE CENTRAL PACIFIC. THIS SYSTEM MOVING UP IS PROGGED TO DEEPEN
INTO THE MID-960 MB RANGE ON FRI...TRACK TO AROUND 300 MILES SOUTH OF
SAND POINT EARLY SAT MORNING...THEN MOVE ASHORE MAINLAND BRISTOL BAY
NEAR THE SOUTHERN MOUTH OF SHELIKOF STRAIT ON SAT. THE ECMWF HAS HAD
DECENT RUN TO RUN CONSISTENCY WITH THIS DEVELOPMENT...WITH THE
NAM/UKMET/GEM GLOBAL ALL REASONABLY CLOSE. THE GFS IS CONSIDERABLY
SLOWER WITH MOVEMENT TODAY AND MUCH FURTHER SOUTHEAST...SWINGING THE
SAID LOW ALONG THE SOUTH SIDE OF KODIAK ISLAND SAT NIGHT. EVEN
SO...IT IS NOTABLE THAT ALL MODELS DEPICT A SIGNIFICANT THERMAL
GRADIENT ALONG THE KUSKOKWIM DELTA WHICH INCREASES IN STRENGTH SAT.
THIS OCCURS AS A POLAR VORTEX SWINGS SOUTH OUT OF THE CHUKCHI SEA AND
BEGINS TO INTERACT WITH THE PACIFIC SYSTEM.

FURTHER EAST...THE NAM AND ECMWF HAD SIMILAR DEPICTIONS OF THE TRIPLE
POINT DEVELOPING NEAR KODIAK ISLAND...TRACKING IT TO NEAR SEWARD
TONIGHT. ANOTHER TRANSITORY RIDGE MOVES THROUGH THE GULF/SOUTHCENTRAL
ON FRI...BUT BEGINS TO AMPLIFY SUBSTANTIALLY AS IT PUSHES EAST OF THE
COPPER BASIN ON SAT IN RESPONSE TO THE ELONGATED TROUGHING THROUGH
THE EASTERN BERING AND SW MAINLAND. OVERALL THERE IS DECENT MODEL
AGREEMENT WITH THIS AS WELL.

.SHORT TERM FORECAST...
SOUTHCENTRAL/GULF...
PRECIPITATION OVER KODIAK ISLAND SHOULD TAPER TO SHOWERS WITH
FROPA/TRIPLE POINT PASSAGE THIS AFTERNOON. LOOK FOR A BRIEF RESPITE
AS THE UPPER-RIDGE SWINGS THROUGH ON FRI. HOWEVER A FRONT ASSOCIATED
WITH THE CENTRAL PACIFIC SYSTEM BEGINS MOVING UP FRI NIGHT AND SAT.
THAT NEXT SYSTEM SHOULD FEATURE WARMER TEMPERATURES LEADING MOST
PRECIP TO FALL AS RAIN. SOLID GALES ARE ADVERTISED CURRENTLY...BUT
WILL NEED TO EVALUATE POTENTIAL FOR STORM-FORCE PENDING HOW DEEP THE
CENTRAL PACIFIC LOW WILL STILL BE AS IT ADVANCES TOWARDS THE SW
MAINLAND.

ELSEWHERE...SNOW MOVES BACK THROUGH SOUTHCENTRAL AS RETURN FLOW ALOFT
BEHIND THE RIDGE TRENDS SOUTHERLY. THERE MAY BE SOME LOCAL
DOWNSLOPING ACROSS SOME OF THE MORE TYPICAL AREAS AS 850 MB FLOW
INITIALLY IS A BIT MORE SOUTHEASTERLY ORIENTED AHEAD OF THE FRONT.
BUT ANY DOWNSLOPE COMPONENT DURING THE DAY ERODES BY THIS
EVENING/TONIGHT AS THE FRONT SHEARS OUT WITH SUPPORTING SHORTWAVE
ENERGY ALOFT SPLITTING...WITH ONE LOBE HEADING TOWARDS THE AK
PANHANDLE AND THE OTHER UP THROUGH COOK INLET TO CENTRAL AK.
SHORTWAVE LIFT SHOULD ALLOW SOME LIGHT ACCUMULATIONS ACROSS AREAS
WHICH DO NOT SEE MUCH DURING THE DAY TODAY. HEAVIEST ACCUMULATING
SNOW THE NEXT 24 HOURS EXPECTED ACROSS THE PASSES OF PRINCE WILLIAM
SOUND AND THE SOUTHERN TO EASTERN KENAI PENINSULA. THE BEST SUSTAINED
BLOWING SNOW THREAT IS THROUGH PORTAGE VALLEY AND TURNAGAIN ARM DUE
TO HEAVY PRECIP AND STRONG WINDS. LOCAL AREAS OF REDUCED VISIBILITY
STILL POSSIBLE HOWEVER IN AND AROUND PRINCE WILLIAM SOUND/THOMPSON
PASS. ALL PRECIPITATION TAPERS TO PASSING SHOWERS OR FLURRIES WITH
SHORTWAVE PASSAGE. ATTENTION THEN TURNS TO THE PACIFIC SYSTEM'S FRONT
THAT MOVES TO THE SOUTHERN MAINLAND THIS WEEKEND. AT THIS TIME THERE
APPEARS TO BE AN ELEVATED THREAT OF STRONG WINDS AND HEAVY
PRECIPITATION...THOUGH THE DETAILS STILL NEED TO BE IRONED OUT.

SOUTHWEST/AKPEN...
WHAT'S LEFT OF THE FRONT WILL DISSIPATE OVER BRISTOL BAY AND THE
KUSKOKWIM DELTA TODAY. THE BLOWING SNOW THREAT OVER BRISTOL BAY IS
BECOMING SUBSTANTIALLY REDUCED AS THE PRESSURE GRADIENT RELAXES AND
TEMPERATURES WARM A BIT IN THE WAKE OF FROPA. THE LOSS OF STEADY
PRECIPITATION OVER THE KUSKOKWIM DELTA SHOULD HELP IMPROVE VISIBILITY
HERE TOO...BUT WITH TEMPERATURES REMAINING IN THE SINGLE DIGITS TO
TEENS...ANY FALLEN SNOW REMAINS HIGHLY TRANSPORTABLE AT LEAST INTO
THE EVENING. SOUTHWEST AK WILL BE IN THE CROSSHAIRS OF STRENGTHENING
DYNAMICS THIS WEEKEND...WITH THE POTENTIAL FOR WIDESPREAD BLOWING AND
OR HEAVY SNOW. ALONG THE AKPEN...STRONG WINDS AND RAIN ARE EXPECTED
INITIALLY IN THE WARM AIR...BUT AS STRONG COLD ADVECTION BLEEDS DOWN
FROM THE EASTERN BERING...STRONG WINDS AND SNOW WILL BECOME A CONCERN
HERE AS WELL.

BERING SEA/ALEUTIANS...
THE EASTERN BERING/CHAIN WILL SEE PERIPHERAL WEATHER IMPACTS FROM THE
PENDING PACIFIC SYSTEM BECAUSE IT'S TRACK IS PRETTY FAR EAST.
HOWEVER...TEMPERATURES SHOULD COOL DOWN MARKEDLY. BRISK NORTH GALES
OVER THE WESTERN BERING WILL SEE A RELAXING OF THE PRESSURE GRADIENT
BRIEFLY EARLY FRI AS THE LOW NEAR KISKA DIES OUT...BUT BE REPLACED BY
EASTERLY GALES AS A FRONT ADVANCING FROM KAMCHATKA WILL ADVANCE FRI
NIGHT INTO SAT.

.LONG TERM FORECAST...
MUCH OF THE AOR WILL BE DOMINATED BY A NEAR STATIONARY UPPER-LOW
CENTERED OVER WESTERN AK EARLY NEXT WEEK. A BRIEF PERIOD OF
PROGRESSIVE FLOW MAY DEVELOP FOR A TIME IN IT'S WAKE AS IT SHIFTS TO
THE NORTHEAST. THE PATTERN SHIFTS TO ONE OF A RIDGE OVER THE SOUTHERN
MAINLAND/GULF MID TO LATE NEXT WEEK AS AN UPPER-LOW SLIPS SOUTHEAST
OF KAMCHATKA DOWN TO THE NORTH CENTRAL PACIFIC HELPING TO RE-AMPLIFY
THE FLOW.

.AER/ALU...WATCH/WARNING SUMMARY
PUBLIC...BLIZZARD WARNING...125
         WINTER WEATHER ADVISORY FOR BLOWING SNOW...155 161
MARINE...GALE...120 125 130 170 175 176 179 180 185 352 411 412 413
                414
         HEAVY FREEZING SPRAY...179 411 412 413 414
FIRE WEATHER...NONE

MALOY