Small Craft Advisory 847, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
518 AM AKDT Sat Sep 30 2023

.ANALYSIS AND UPPER LEVELS...

A deep vertically stacked low is centered around 150 nm S of
Unimak Pass this morning. Its occluded front extends from Bristol
Bay across the southeastern Bering Sea to the central Aleutians,
wrapping back around the center of circulation. This front is
bringing widespread rain and strong gusty winds to Southwest
Alaska with a warm front extending off the occluded front bringing
the same sensible weather to Kodiak Island. A weak surface trough
over Prince William Sound is producing scattered showers that are
moving west toward the eastern Kenai Peninsula, a result of the
developing southeasterly flow aloft. Low stratus and patchy fog
have persisted over much of the Copper River Basin overnight, but
are beginning to erode as the higher clouds from the
aforementioned front moves over the region. Benign weather also
prevails across the western Aleutians under the influence of high
pressure.

&&

.MODEL DISCUSSION...

While models have a good handle on the track and evolution of the
current storm-force low, there continues to be minor shifts in
timing/placement of both the surface front as it enters the
northern Gulf and what happens to the remnant trough as a second
upper-level arctic trough and developing weak surface low move
west to east across the state. The GFS has trended a bit more to
the NAM, with better interaction between these features and
development of precipitation over Cook Inlet and the Mat-Su
Valleys for Sunday night into Monday. The biggest difference is
the timing in progression of the upper-level wave and extent of
cold air advection. The NAM is still the fastest and coldest
solution, with snow levels dropping below 2,000 ft by Monday
morning and northerly gap winds quickly developing. The GFS is
slower with a more moderated airmass. The morning forecast package
is a blend of both models with higher weight to the GFS.

&&

.AVIATION...

PANC...VFR conditions will persist with light winds early this
morning. A northerly wind will develop mid to late morning,
continuing through Sunday morning. A strong southeasterly wind out
of Turnagain Arm will also develop today as a frontal system moves
toward the Southcentral coast; however, the orientation of the
frontal system will allow the core of strongest winds through
Turnagain Arm to bend quickly down inlet and away from the
terminal. As such, LLWS is not expected over the terminal.
Occasional sprinkles or very light rain is also possible through
the day; however, ceilings/vis should remain safely in the VFR
category.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Monday)...

A strong, vertically stacked low south of Unimak Pass this
morning will continue to track northeast through the day,
reaching Kodiak Island by Sunday morning. Its associated warm
front is draped across the western Gulf this morning, with
widespread gales and rain from the southern Gulf to Kodiak Island.
This front will remain potent as it occludes and lifts to the
northern Gulf this evening. As the front lifts north toward the
Southcentral coast, expect rain and high-end gales, along with a
small core of storm-force winds near Kayak Island, for the latter
half of Saturday. Northeasterly winds will also develop across
Cook Inlet, the Anchorage Bowl, and the Matanuska Valley late
Saturday into early Sunday as the front continues north and the
pressure gradient between weak ridging over the Yukon and the low
over the AKPen tightens. Strong east-southeasterly winds will
also increase for Prince William Sound on Saturday night with the
heaviest rain falling along the immediate coast. Inland areas,
especially those in the lee of the Chugach and Kenai Mountains,
will likely see only occasional sprinkles or very light rain
throughout the day Saturday due to a strong cross-barrier flow
allowing for downsloping.

The front will weaken on Sunday as it pushes on shore, with winds
and precipitation both subsequently diminishing in intensity. The
parent low also weakens as it moves east and stalls east of
Middleton Island. By Monday, another upper-level trough moves
over the region while an associated weak surface low develops over
the northern Gulf/Prince William Sound area and phases with the
previous system. The combination of a stalled mid-level trough
over the western half of Southcentral, moisture advection, and
colder air moving in from the interior, will all allow for the
development of another round of precipitation overspreading
Southcentral from west to east. The colder air will also allow
snow levels to fall below 2,000 ft for Monday morning, with
accumulating snow possible for parts of the Copper River Basin,
mountain passes, and the Anchorage Hillside.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3)...

The strong system currently south of Unalaska has entered its
weakening phase and will spend the next few days tracking towards
the Gulf of Alaska and slowly dissipating. Gale force winds will
continue along the northern and western edges of the low, with
elevated winds along the Kuskokwim Delta and out of Kamishak Gap.
Rain showers began yesterday along the Aleutians and southern
Alaska Peninsula and have begun pushing onshore into Bristol Bay
overnight. Cooler air will begin to filter in on the backside of
the system for Sunday. This cold air will be re-enforced Sunday
night and Monday as an upper-level shortwave trough digs southward
from the Arctic to over the Southwestern Mainland. The main
impact will be increased gap winds through the favored terrain of
the AKPEN. Higher pressure moves into the western Bering on Monday
and will shift eastward to encompass most of the Bering Sea by
Tuesday. This will bring about benign conditions for much of the
Bering Sea, Aleutians, and Southwest Alaska for early to mid next
week.

&&

.LONG TERM FORECAST (Days 4 through 7: Tuesday through Friday)...

The start of the long term period next Tuesday begins with poor
deterministic model agreement. Thus, more weight has been given to
the ensemble means. Ensembles depict a large upper low anchored
over the Beaufort Sea on Tuesday. A second upper low is forecast
over the North Pacific well to the south of the Eastern Aleutians
by roughly 600 nm. Through the end of the week a new developing
low pressure center over Kamchatka will overtake the North Pacific
low, with a longwave trough becoming established from the
southern Bering into the North Pacific by next Friday. As the
Kamchatka low moves into the Bering Tuesday and Wednesday a large
front will spread across the Western and Central Aleutians. This
should bring widespread showers and potentially elevated winds
into the Eastern Aleutians and then northeastward into the
Kuskokwim Delta as the front weakens and begins to dissipate. Much
of Southwest Alaska will be the beneficiary of high pressure,
likely experiencing generally dry conditions through the long term
period. Interior Southcentral may also see initial shower
activity on Tuesday and Wednesday dry out for Thursday and Friday.
Along the coast, however, a wet pattern may be ongoing as
southwesterly flow sees multiple shortwave disturbances digging
into coast through the end of the long term period. It should be
noted that the deterministic GFS has much drier conditions in
place while across the Gulf coast and the Gulf itself, as high
pressure builds back into the region. On the other hand, the EC is
more in line with ensembles. Which bring potential heavy rain
over Prince William Sound and along the coast, with lighter rain
moving further inland into Southcentral. All that being said,
forecast confidence in the long term is low.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
LONG TERM...BL