Small Craft Advisory 815, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
433 PM AKDT SAT OCT 17 2015

.ANALYSIS AND UPPER LEVELS...
A BROAD UPPER LEVEL TROUGH ENCOMPASSES THE BERING AND WESTERN
ALASKA MAINLAND...WITH AN UPPER LEVEL JET IN THE NORTH
PACIFIC...WELL TO THE SOUTH OF THE BERING SEA. TWO MAIN FEATURES
DOMINATE THE TROUGH...WITH ONE LOW CENTERED NEAR SHEMYA...AND A
SEPARATE LOW CENTERED NEAR THE ALASKA PENINSULA. AT THE
SURFACE...A DISSIPATING LOW PRESSURE SYSTEM IS BRINGING WIDESPREAD
RAINFALL ACROSS SOUTHCENTRAL. SOME MODERATE DOWNSLOPING IS
HAS LIMITED PRECIPITATION ACROSS THE COPPER RIVER BASIN AND AS
BROUGHT WARMING TEMPERATURES. A SECONDARY LOW PRESSURE SYSTEM IS
CENTERED JUST OFFSHORE OF THE KUSKOKWIM DELTA AND SUPPORTING
WIDESPREAD RAIN AND STRATUS ACROSS THE SOUTHWEST MAINLAND.
MEANWHILE...A SEPARATE LOW IS CENTERED NEAR SHEMYA AND SUPPORTING
GUSTY WINDS ACROSS THE WESTERN BERING.

&&

.MODEL DISCUSSION...

IN THE SHORT TERM MODELS ARE GENERALLY IN GOOD AGREEMENT ON THE
DEVELOPMENT OF A SURFACE LOW NEAR KODIAK THIS AFTERNOON AND ITS
PROGRESSION TOWARDS PRINCE WILLIAM SOUND SUNDAY. MODELS DIVERGE
MORE SIGNIFICANTLY ON THE INTENSITY AND TRACK OF A LOW PRESSURE
SYSTEM AS IT APPROACHES THE ALASKA PENINSULA MONDAY AFTERNOON.
CURRENTLY A BLEND OF MODELS WAS USED.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
WHILE THE WIDESPREAD RAIN THAT FELL OVER MUCH OF THE SOUTHCENTRAL
HAS BEGAN TO LIFT NORTHWARD...THE NEXT UPPER LEVEL TROUGH TO BRING
RAIN TO THE REGION CAN ALREADY BE SEEN ENTERING THE SOUTHWEST
GULF. AHEAD OF THIS NEXT SYSTEM...RAIN OVER SOUTHCENTRAL WILL BE
MUCH MORE SHOWERY AND SPOTTY IN NATURE TONIGHT AND SUNDAY MORNING
AS A SERIES OF WEAK SHORTWAVES PASS OVERHEAD. OVER THE SUSITNA
VALLEY AND WESTERN COPPER RIVER BASIN...WIDESPREAD RAIN WILL BEGIN
TO BECOME SHOWERS AS WELL THIS EVENING AS THE DEEP LAYER MOISTURE
MOVES TO THE NORTH.

THE FOCUS THEN SHIFTS TO THE SYSTEM CURRENTLY DEVELOPING JUST EAST
OF KODIAK ISLAND. THIS LOW WILL INTENSIFY SLIGHTLY AS IT
PROGRESSES TOWARDS THE NORTH GULF COAST SUNDAY MORNING...BRIEFLY
GIVING GALE FORCE WINDS TO AREAS EAST OF HINCHINBROOK ENTRANCE.
WEAK DOWNSLOPING WILL INITIALLY LIMIT RAINFALL TO THE GULF COAST
FROM THIS LOW AROUND SOUTHCENTRAL ON SUNDAY MORNING. AS THIS LOW
MAKES LANDFALL IN THE AFTERNOON...STEADY RAIN WILL DEVELOP WITH
THE APPROACH OF THE UPPER LEVEL TROUGH. CONDITIONS WILL THEN BEGIN
TO IMPROVE ON MONDAY MORNING BEHIND THE UPPER LEVEL TROUGH AS WEAK
RIDGING BEGINS TO DEVELOP. THIS RIDGING WILL OCCUR AHEAD OF A
DEVELOPING GALE FORCE LOW THAT WILL BEGIN TO IMPACT THE GULF AND
KODIAK ISLAND ON MONDAY.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...

THE UPPER LOW OVER THE EASTERN BERING SEAS CONTINUES TO HAVE THE
MOST INFLUENCE OVER THE SOUTHWEST MAINLAND REGION. THIS LOW WILL
SLOWLY MOVE NORTH NORTHWEST THROUGH MONDAY AND CONTINUE TO ROTATE
PRECIPITATION ACROSS THE REGION THUS CONTINUING THE PATTERN OF
MORE FREQUENT PERIODS OF RAIN AND SHOWERS DURING THE DAY AND A
MIXTURE OF RAIN AND SNOW OVERNIGHT.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...

THE AFOREMENTIONED LOW IN THE EASTERN BERING SEA IS KEEPING THAT
AREA IN THE SHOWERY REGIME FOR THE NEXT COUPLE OF DAYS. A WEAK
LOW BY BERING SEA STANDARDS FOR THIS TIME OF YEAR IS OVER THE
WESTERN ALEUTIANS. THIS LOW IS PRODUCING SHOWERS AND AS IT EXITS
TO THE SOUTHEAST IT WILL BE BRIEFLY REPLACED BY A WEAK RIDGE OF
HIGH PRESSURE ENDING THE SHOWERS. THE NEXT SYSTEM TO IMPACT THE
WESTERN BERING SEA AND WESTERN ALEUTIANS WILL PUSH A FRONT TO THE
WESTERN ALEUTIANS EARLY MONDAY BRINGING RAIN AND STRONGER WINDS TO
THE REGION.
&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
THE UPPER AIR PATTERN BEGINS MONDAY AFTERNOON WITH THE LONGWAVE
TROUGH SHIFTING TO THE EAST TOWARD THE GULF WITH A SHORTWAVE
TROUGH EMBEDDED WITHIN...BRINGING ABOUT A FAMILIAR CONUNDRUM. WILL
THERE BE A SECONDARY SURFACE CIRCULATION ALONG THE FRONT/TRIPLE
POINT OR WILL IT STAY AN OPEN TROUGH? ANY EXTRA DEVELOPMENT WILL
ACT TO REALLY INCREASE THE PRESSURE GRADIENT AND WINDS ALONG AREAS
OF THE NORTH GULF COAST. SYNOPTICALLY...SIGNS POINT TO SOME
SEMBLANCE OF SECONDARY CYCLOGENESIS. AN APPROACHING JET
STREAK...TRIPLE POINT...DIFFLUENCE/DIVERGENCE ALOFT...AND STRONG
SHORTWAVE TROUGH ROTATING AROUND THE LONGWAVE ARE ALL FACTORS
PUSHING THE FORECAST TOWARD ANOTHER SURFACE LOW. THE JET AND
TRIPLE POINT QUICKLY MOVE OFF TO THE EAST SO IT MAYBE
WEAKER/SHORTER LIVED...WHICH LED THIS SHIFT TO FAVOR THE 12Z ECMWF
SOLUTION OVER THE REST OF GUIDANCE...WHEREAS THE NAM WAS TOO
ROBUST IN DEVELOPMENT AND THE GFS NOT ENOUGH. THIS SYSTEM STAYS
MOSTLY IN THE GULF OF ALASKA THROUGH TUESDAY BEFORE MOVING OFF TO
THE EAST. THE EXTENDED PERIOD BECOMES A LITTLE MORE UNCLEAR AS
ANOTHER BROAD UPPER TROUGH SETS UP IN THE BERING SEA THE REST OF
THE WEEK CONTINUING THE ACTIVE PATTERN.

FROM A SENSIBLE WEATHER PERSPECTIVE...A GALE FORCE FRONT WILL MOVE
INTO THE GULF MON/TUE. THIS FRONT WILL BRING STEADY RAIN TO THE
KODIAK ISLAND AND THE NORTH GULF COAST. MOST OF THIS MOISTURE WILL
STAY CONFINED TO THE COASTAL AREAS WITH A LESSER CHANCE FOR
SHOWERS INLAND. THE FRONT WILL WRAP AROUND AND AFFECT PARTS OF THE
ALASKA PENINSULA. THERE WILL BE A BREAK MIDWEEK...POSSIBLY SOME
SUNSHINE FOR SOUTHCENTRAL BEFORE THE ACTIVE PATTERN FROM THE WEST
ENCROACHES BY LATER IN THE WEEK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 119.

&&

SYNOPSIS AND MODEL DISCUSSION...EN
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SA
LONG TERM...ML