Small Craft Advisory 743, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
512 AM AKDT SUN SEP 18 2016

.ANALYSIS AND UPPER LEVELS...
Ridging has built in over the central Bering and is tracking to
the east. It has helped to bring stratus and fog for areas around
the Bering and southwest Alaska. The upper level disturbance that
was over southwest Alaska has moved south and is now moving into
the North Pacific. West of the ridging the next system is moving
into the western Bering. This system is still deepening and is
bringing up a strong push of moisture with it.

&&

.MODEL DISCUSSION...
The models are in good agreement with the main features until
early on Tuesday when a triple point low develops on the frontal
boundary of the system moving into the Bering. The 00Z runs are
starting to come into better agreement on the placement of the
triple point low, but still vary on its intensity. However, the
NAM is still struggling to place a true triple point low and is
much faster than the others.

&&

.AVIATION...

Main forecast issue this morning is with possibility of IFR or
MVFR ceilings later this morning through early afternoon. Guidance
indicates low stratus banked mainly along the mountains but will
watch carefully to see if it backs west to PANC. Possible that
low stratus hangs around a while well into the late afternoon or
evening, but went the more optimistic route for now with 12z
package.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level low is moving through the Copper River Basin this
morning causing showers to come to an end and winds to turn to north
to west. The biggest question for today is how long the patchy
fog and stratus will linger in areas west of mountain ranges such
as the Anchorage to Palmer region. It is possible that the
northwest flow may pack up the very moist air that is in place
along the mountains causing the stratus not to burn off until well
into the afternoon.

Gusty north to west outflow winds have already begun along the
Gulf coast and will persist through tonight and rapidly diminish
Monday as the ridge is displaced by the strong storm moving into
the area for the middle of the week. Clearing skies tonight may
bring below freezing temperatures to colder areas of the Anchorage
Bowl and Mat-Su valleys as well as the Kenai Peninsula and Copper
River Basin.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sun and Mon)...
Today represents the relative break in the pattern before a major
Fall storm moves in Monday. Weak ridging is building over the
area this morning. This is resulting in some drying to the mid and
upper levels, helping to bring an end to much of the
precipitation. However, it is also serving to trap some of the
low-level moisture creating pockets of fog and low stratus. Expect
this fog and low stratus to build in a little bit more this
morning as subsidence increases. It is fighting against some
northwest flow, so even as it develops, it should be short-lived
and burn off by noon. The initial warm occlusion from the Bering
will start to spread high clouds in across the area today and
could clip the Y-K Delta coast with some light precipitation.

The bigger show is definitely on Monday as a gale-force front
arrives from the Bering. South winds will overspread the coastline
by Monday morning and then push inland through Monday afternoon.
These winds, even though they will be short-lived, should be
enough to bring some high surf into areas stretching from
Kuskokwim Bay northward. A High Surf Advisory has been issued for
this. With a slightly NE to SW orientation, rain will hit the Y-K
Delta first in the late morning and then move into the Bristol Bay
Coast by evening. Some of this rain could be moderate to heavy at
times as the front plows inland overnight Monday and Tuesday. The
silver lining is that the front moves quickly off to the east,
which will allow winds to diminish and the heaviest precipitation
to end.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Sun and
Mon)...
Ridging is holding on over the Eastern Bering this morning, but is
quickly being supplanted by the strong front developing over the
Western Aleutians. This will be the story for the next 24 hours.
The parent low is located just south of the Gulf of Anadyr. It
will begin a slow and methodical track across the Bering starting
today. The low will maintain its strength courtesy of some
lower-level thermodynamics and a finger of the subtropical jet in
the upper levels. As it tracks across, it will push a vigorous
near-storm-force front across the Bering. The front will bring
moderate to heavy rain to the Central Aleutians by this afternoon
and to the Eastern Aleutians by Monday morning. After the front
sweeps through, the low itself will have plenty of cold air aloft
to support widespread showers, gusty winds, and even some isolated
thunderstorms over the Central Bering.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Models are converging toward a consensus solution with the low
pressure that is forecasted to develop south of Kodiak and the
Alaska Peninsula along the strong north-south oriented weather
front Tuesday into Wednesday. It appears as though the primary
low moves north to the west of the Alaska Range, with secondary
and tertiary lows forming along the front over upper Cook Inlet
and the North Gulf Coast, respectively. This has enabled us to
trend stronger with winds with greater confidence for this storm
beginning late Tuesday and extending through Wednesday, especially
along the North Gulf Coast, Prince William Sound, Portage Valley,
Turnagain Arm and the higher elevations of Anchorage. If
confidence continues to improve with respect to higher winds,
watches or warnings for high winds over land and/or marine areas
may be needed in the next day or two with this potentially major
storm.

From previous long-term discussion...

Forecast confidence is improving on the timing and intensity of
the strong gale force front that will move from the Alaska
Peninsula into the Gulf for the middle of next week. This will
bring gusty winds and heavy rain to Kodiak Island starting Tuesday
evening, before spreading into the Gulf Coast where it will bring
widespread gale force winds and heavy rain to much of the Gulf
Coast through Wednesday. For interior Southcentral, downsloping
will prevent the heaviest rain from falling over the population
centers, however a significant amount of available moisture from
this system will likely keep light rain over the area for the
middle of next week. There is also decent agreement that a triple
point low developing over the western gulf on Wednesday will bring
gusty northerly winds from Cook Inlet into the Matanuska Valley,
however there isn't enough confidence at this point to determine
how strong these might be. As a result, winds were conservatively
increased throughout the Cook Inlet region today, but these may
need to be increased further if confidence increases. The other
concern from this system rests with how much rain may fall along
the Gulf Coast. The good news is that there is good confidence
that ridging over the panhandle will be much weaker than expected
a few days ago, and as a result the chances of a prolonged period
of heavy rain during the middle of next week is lower than it was
a few days ago.

By late next week the models really begin to struggle as another much
deeper system quickly develops in the North Pacific and moves
into the Gulf. The exact impacts from this system are tough to
tell with large model spread; however, it is very likely that
active weather will continue throughout the Gulf and south
Mainland through the work week. This could include a potential for
continued heavy rain and significant winds throughout the Gulf, as
well as the chance of a significant wind storm into Southcentral.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...150 155 165 170 172-181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...CC/DEK