Gale Warning 595, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 PM AKDT Mon Oct 9 2023

.ANALYSIS AND UPPER LEVELS...

A persistent upper-level longwave trough remains stationary over
the western half of the state, extending down into the Gulf of
Alaska. This trough is anchored by two distinct shortwaves, the
first moving northwest parallel to the Alaska Panhandle. The
second of these waves is well south of Kodiak Island and
continues to move east toward the west coast of British Columbia.
The first wave is associated with a compact surface low southeast
of Yakutat. An initial band of precipitation out ahead of the low
is beginning to advance toward Middleton Island and Cordova.

Gusty gap winds along the coast continue this afternoon,
especially for Seward and Whittier, as an already northerly flow
is further enhanced by a tightening pressure gradient due to the
advancing area of low pressure to the east.

Elsewhere, fog and low stratus persist from the Matanuska Valley
south over Cook Inlet. This area of cloud, however, is eroding as
the increasing northwesterly flow advects drier air into the
region. Gusty northwesterly winds are also increasing across
Bristol Bay and Kodiak Island as a weaker wave moves south across
Southwest Alaska helping to further draw down the colder airmass
over the interior of the state.

Farther west, high pressure dominates much of the Bering and
Aleutians with plenty of low stratocumulus across this region.
Northerly wind gusts continue for the eastern Aleutians and
Alaska Peninsula.

&&

.MODEL DISCUSSION...

Models are in good agreement with the overall synoptic features
through the next several days. The biggest challenge in the short
term for Southcentral Alaska will be how the shortwave digging
south across the Barren Islands for Tuesday morning interacts with
the wave and surface low moving northwest from the Alaska
Panhandle. All models have a new, albeit weaker, surface low
developing southeast of Kenai in response to the first wave. The
discrepancy is with positioning and eventual movement by midweek.
The extent of precipitation west of Prince William Sound will be
dependent on the movement of this feature. Whatever the solution,
moisture looks to linger across portions of the Copper River
Basin, Prince William Sound, and the Chugach and Talkeetnas
Mountains through midweek,

Farther west, a likely hurricane-force low remains forecast to
move over the Western and Central Aleutians beginning Wednesday
morning. Small differences in the position of the center of the
low are evident, but mostly insignificant to the overall picture.
The bigger difference amongst models is the actual strength of the
low with guidance bottoming out the low between 945 mb and 952 mb.

&&

.AVIATION...

PANC...Low MVFR ceilings will likely prevail through mid
afternoon with the potential for MVFR ceilings to climb to near
3,000 ft AGL this evening. A redevelopment of a FEW or SCT low
stratus deck (~ 500 ft AGL) is possible by late evening and
continuing into early Tuesday morning; however, LIFR ceilings
and/or fog is not expected. There is also low chance (10% to 30%)
of flurries Tuesday morning.

&&

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

An upper-level longwave trough remains located over much of the
state and extends down over the Gulf of Alaska. Embedded within
the trough are various upper shortwaves, notably over Kodiak
Island and near the Panhandle. At the surface, a low pressure
system is centered near Yakutat. An initial band of precipitation
out ahead of the low is beginning to advance toward Middleton
Island and Cordova. Precipitation will continue to spread across
the Copper River Basin tonight into Tuesday morning. Areas of
light snow are forecast for interior regions of the Copper River
Basin, with about 1 to 3" anticipated for the western parts. As
the low tracks closer to Southcentral, moisture wraps up around
the low and gets pulled west across the Chugach Front Range. Light
snow is expected to reach west into the Palmer area Tuesday
morning, and there is also low chance (10% to 30%) of flurries
Tuesday morning for Anchorage, as well. Nonetheless, most of the
snow should stay up on the mountains.

The aforementioned upper shortwave dropping southeastward across
Kodiak Island is helping enhance cold air advection for that area.
The resulting strong, gusty northwesterly gap winds across
Shelikof Strait and Kamishak Gap will continue increasing into the
evening. Going into Wednesday, widespread precipitation focused
over the Copper River Basin becomes more showery as the low
continues to weaken and linger over the northern Gulf near Prince
William Sound. The best chances for precipitation late Wednesday
into Thursday look to be generally over/near Prince William Sound.
Later on Thursday, models are advertising a surface low tracking
into the eastern Gulf. At this time, however, the models are
struggling with its exact track and evolution heading into the end
of the work week.

&&

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

On satellite imagery, there is an area of high pressure from the
Western Bering to the Central Bering and there are two lows over
the NE Pacific. However, the models have pinged into deep
cyclogenesis over the NW Pacific which will impact the Aleutians
beginning Wednesday. The run to run continuity with the models,
bodes well for high confidence. The expectation is that this
strong North Pacific low that will lift northward to the Western
Bering Sea by Wednesday night and send its front eastward. This
low will reach 945 mb to 950 mb and is being steered by a jet
with a 170 knot core. A tight pressure gradient forces will result
in widespread storm force winds and the potential for hurricane
force gusts Wednesday into Thursday. The High Wind Watch remains
in effect for the Western and Central Aleutian Chain. A Special
Weather Statement has been issued for the Eastern Aleutians to
address that strong winds will be possible late Wednesday night
into Thursday morning.

&&

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

A weakening upper level low across northern half of the state
will give way to a ridge nosing in across eastern Alaska into the
weekend. This ridge will push much of the colder air entrenched
out across the Bering as it starts to dive south. The powerful
Aleutian system that will have brought the forecast high winds and
hurricane force wind conditions will be unable to progress towards
mainland Alaska and will stall and weaken Saturday as it loses all
upper level support. Overall rain and unsettled weather will be a
mainstay through the weekend in the Aleutians and the AKPEN.

Southcentral will see moderation. Early into the weekend a storm
across the panhandle will lift north, keeping the Gulf coast and
the Copper River Basin in the cross hairs. Temperatures should
swing back into the normal range, after an early week cool down.
For the most part, no appreciable snow is expected in low areas
through the weekend. Overnight Sat/Sun, the Anchorage/MatSu could
light precipitation, mainly focused higher in the mountains. Much
of the extended forecast plays into the evolution of TS Bolaven,
which is currently in the tropics, east of Japan. Beyond the
weekend timeframe, confidence is increasing that the remnants of
this storm will move into the Gulf of Alaska. Model trends
continue to tilt this scenario towards likely.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...AF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...MK