Small Craft Advisory 817, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
501 AM AKDT SUN OCT 18 2015

.ANALYSIS AND UPPER LEVELS...
LOOKING AT THE UPPER LEVELS...A BROAD TROUGH CONTINUES TO DOMINATE
THE BERING AND MAINLAND ALASKA. LOCATED WITHIN THE LARGE TROUGH
ARE TWO CLOSED LOWS...ONE LOCATED SOUTH OF THE ALEUTIAN CHAIN NEAR
50N 180 AND A SECOND LOW CENTERED OVER BRISTOL BAY. MULTIPLE
SHORTWAVES ARE EVIDENT THIS MORNING IN SATELLITE IMAGERY ROTATING
AROUND THE PERIPHERY OF THE TROUGH. MOST OF THE POLAR JET REMAINS
WELL SOUTH OF THE FORECAST AREA AROUND 35N-45N WITH A NORTHWARD
ASCENT INTO SOUTHEASTERN ALASKA AND BRITISH COLUMBIA.

AT THE SURFACE...THREE SURFACE LOWS ARE IMPACTING THE FORECAST
AREA THIS MORNING. A WEAKENING 993 MB LOW SOUTHWEST OF ADAK IS
MOVING TO THE SOUTHEAST WHERE IT WILL BE ABSORBED INTO A NEW 993
MB LOW IN THE NORTH PACIFIC THAT IS EXPECTED TO DEVELOP THIS
AFTERNOON. A 990 MB LOW NEAR NUNIVAK ISLAND IS BRINGING LOW
CLOUDS...RAIN...AND PATCHY FOG TO MUCH OF THE ALASKA PENINSULA AND
SOUTHWESTERN ALASKA. FINALLY...A 992MB LOW LOCATED IN THE WESTERN
GULF OF ALASKA IS PRODUCING WIDESPREAD RAINFALL TO MUCH OF SOUTH
CENTRAL ALASKA.

&&

.MODEL DISCUSSION...
THE MODELS ARE IN OVERALL AGREEMENT WITH THE SYNOPTIC DETAILS THROUGH
THE SHORT TERM AS THE REGION STAYS IN A PROGRESSIVE PATTERN. THE
ONLY DIFFERENCE TO NOTE IN THE MODELS IS WITH THE DEVELOPMENT OF
THE LOW SOUTH OF DUTCH HARBOR THIS AFTERNOON. MODELS ARE SIMILAR
IN LOCATION AND MOVEMENT OF THE LOW...HOWEVER THE CANADIAN MODEL
IS DEVELOPING THE LOW ABOUT 5 TO 10 MB DEEPER THAN THE NAM/GFS AND
OTHER NON-NORTH AMERICAN MODELS. THIS FORECAST PACKAGE TRENDED TO
THE GFS THROUGH THE SHORT TERM /DAY 3/.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE UPPER LOW IN THE GULF WILL REACH THE GULF COAST LATE THIS
AFTERNOON AND MOVE INLAND TONIGHT. PRECIPITATION WITH THIS SYSTEM
IS ALREADY AT THE COAST AND WILL WORK ITS WAY INLAND TODAY WITH
THE BEST CHANCES OF RAIN DURING THE EVENING/NIGHT HOURS WITH THE
PASSAGE OF THE UPPER LOW. A RIDGE OF HIGH PRESSURE WILL BRIEFLY
BUILD OVER SOUTHERN ALASKA MONDAY AFTERNOON FOR A BREAK IN THE
WET WEATHER...WHICH WILL LAST THROUGH MONDAY NIGHT FOR INLAND
AREAS. A STRONG LOW WILL REACH FULL MATURITY (VERTICALLY STACKED
AND WITH THE SURFACE LOW REACHING A MINIMUM OF ABOUT 968 MB) AROUND
NOON MONDAY ABOUT 300 MILES SOUTH OF CHIGNIK. THE LEADING FRONT
WITH STRONG WINDS AND PRECIPITATION WILL REACH KODIAK ISLAND BY
EARLY MONDAY AFTERNOON AND TO THE NORTH GULF COAST MONDAY NIGHT.

&&

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

AN UPPER LOW SITUATED JUST OFF THE WESTERN CAPES IS RESPONSIBLE
FOR CONTINUED SHOWERS ACROSS SOUTHWEST THIS MORNING...ONSHORE FLOW
ACROSS BRISTOL BAY AND THE KUSKOKWIM DELTA SUPPORTING FOG
DEVELOPMENT. THIS LOW WILL SLOWLY LIFT NORTH THROUGH MONDAY
NIGHT...WITH SHOWER THREATS DIMINISHING IN COVERAGE. A STRONG GALE
FORCE FRONT WILL MOVE SOUTH OF THE ALASKA PENINSULA ON MONDAY AND
MONDAY NIGHT...BRINGING AN INCREASE OF EASTERLY WINDS ACROSS THE BRISTOL
BAY REGION...ALONG WITH RAIN THREATS TO COASTAL LOCATIONS INTO
INTERIOR BRISTOL BAY.

&&

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

A BROAD UPPER TROUGH OVER THE BERING WILL CONTINUE TO SUPPORT
SPOTTY SHOWERS...ALONG WITH INCREASING NORTHERLY WINDS AS THE
OVERALL TROUGH AXIS SHIFTS TO THE EAST. A GALE FORCE FRONT WILL
MOVE INTO THE FAR EASTERN ALEUTIANS ON MONDAY AND MONDAY
NIGHT...WITH ANOTHER WEAKENING OCCLUDED FRONT MOVING INTO THE
WESTERN BERING THAT SAME TIME.

&&

.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...DIFLUENCE/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 119 132 150 155.

&&

SYNOPSIS AND MODEL DISCUSSION...RF
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...MTL