Storm Warning 114, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
435 PM AKDT Tue Oct 10 2023

.ANALYSIS AND UPPER LEVELS...

The overarching pattern across southern Alaska has changed little
in the past day or so. Two main shortwave troughs are continuing
to rotate slowly east across parts of the outlook area, anchored
by a stalled upper level low sitting over the Seward Peninsula.
The first of these main troughs is now becoming more negatively-
tilted as it lifts into the Kenai Peninsula on it's way towards
the Copper River Basin. A small surface low now sits about
underneath the trough axis, wandering aimlessly just off to the
west of Middleton Island. Areas of light rain showers (and snow
showers for higher elevations) are drifting around and to the
north of this feature, affecting mainly the Prince William Sound
and the Chugach Mountains. A shortwave ridge is shifting into the
Alaska Peninsula in the wake of the trough passage, and northwest
flow remains set up between the northern Gulf low and higher
pressure over the Bering. However, winds are continuing towards
as slow downward trend as the gradient along the Alaska Peninsula
weakens.

Out towards the west, the second, broader shortwave trough is
progressing slowly into the eastern Bering and having little
impact to sensible conditions near the surface. Most of Southwest
is experiencing calm and cool conditions under a pervasive deck of
stratus moving overhead near the transient ridge moving past.
Farther west, the outer reaches of a front stemming off a
Kamchatka low are beginning to reach the far western
Bering/Aleutians, with increased southerly winds and areas of
rain showers advancing slowly northeast.

&&

.MODEL DISCUSSION...

Models are staying in good agreement overall at the synoptic
level, but discrepancies remain for smaller scale details.
Agreement is decent overall for the Hurricane force low weakening
to storm force as it enters the southern Bering from Wednesday to
Thursday. However, there has been a distinct southeast jog in the
consensus for the low's path prior to it stalling out late on
Thursday. This cast some uncertainty on the wind forecast across
the central and western parts of the Aleutian Chain, with the
overall potential for higher wind gusts appearing to shift east
with the new model consensus for the low track. Models have also
struggled with the exact path of a low expected to curve
northwest towards the Prince William Sound between Thursday and
Friday. Most notably, there is still a fairly large east-west
spread for an axis of precipitation following this low into
Southcentral late Thursday into Friday, ranging from as far west
as the Kenai Peninsula to as far east as the Copper River Basin.

&&

.AVIATION...

PANC...Ceilings are expected to remain mostly in VFR range through
at least tonight, but will likely stay at or below the 5000 ft
threshold. Periods of very light rain, perhaps mixing with
flurries again overnight, will be possible as waves of showers
attempt to move west into Anchorage through Wednesday morning.
However, significant reductions to ceilings/visibility are not
anticipated with this activity at this time.

&&

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

An upper-level longwave trough lingers over the state and has an
embedded upper low over the Seward Peninsula. An upper shortwave
pinwheeling around the southern periphery tracks across the Kenai
Peninsula towards the Copper River Basin this afternoon. A
weakening surface low just west of Middleton Island is promoting
areas of light rain showers (snow for higher elevations) for
portions of Southcentral, mainly Prince William Sound and the
Chugach Front Range.

Gusty northwesterly gap winds across Shelikof Strait and Kamishak
Gap will diminish tonight into Wednesday morning. Also for
Wednesday, light scattered showers continue across the region as
the weak low lingers near Middleton Island. The best chances for
precipitation for Wednesday into Thursday look to be generally
near Prince William Sound, the Chugach Front Range, the Kenai
Mountains, and northern Copper River Basin. Precipitation chances
along the Gulf coast increase going into Friday as a strong upper
shortwave and attendant surface low lift north from the Gulf.
Models continue to struggle with the track and evolution of the
low. However, generally expect an increase in precipitation and
winds for Southcentral coastal and marine regions late Thursday
through Friday morning before the system weakens later in the day.

&&

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

A low in the north Pacific will rapidly intensify, deepening at a
rate of more than 30 mb in a 24 hour period. This low that is
undergoing "bomb cyclogenesis" will trek into the Bering. A High
Wind Warning for the Western and Central Aleutians remains in
effect and the Special Weather Statements have been issued for the
Pribilof Islands and the Eastern Aleutians.

As of 3 PM AKDT or 23Z, on Himawari satellite imagery, the low of
interest is approximately 600 miles southwest of Attu Island with
an estimated surface pressure of 985 mb. The model consensus is
that this feature will move northeastward. Wednesday morning the
center of the surface circulation will be approximately 275 miles
south of Attu and by Wednesday afternoon it will be over the Rat
Islands of the Aleutians (west of Adak) at 949 mb Wed afternoon.
Wednesday morning the center of circulation for the surface low
will be 50 miles north of Adak at 952 mb, continuing into the
Central Bering at 956 mb by Thursday afternoon. This low is being
steered by a jet with a 165 knot core. The pressure gradient field
is enormous. Expect widespread storm force winds along with areas
of hurricane force winds.

While the Aleutians and the Bering are getting hammered, Southwest
Alaska can expect mild autumn weather conditions this forecast
period.

&&

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

An unsettled and soggy extended forecast on tap for much of the AOR
as multiple low pressure systems work along the Aleutians and into
the Gulf of Alaska through the weekend/early next week.

Out West, the powerful Bering Sea/Aleutian system that is expected
to bring Storm to Hurricane force winds to the western and central
Aleutians should weaken/open by Friday night, with localized gale
force winds lingering over the central Aleutians as the system slows
and stalls through Saturday morning. As the system stalls and
loses steam fighting an upper level ridge, expecting a transfer
of energy to a separate developing low pressure system within the
N Pacific. This developing low brings about sporadic near-Gale
force winds along the southern periphery of the east/central
Aleutians by Sunday through Tuesday.

For Southcentral, rain chances increase moving into the late weekend
as multiple low pressure systems push into the Gulf of Alaska. At
this time, not anticipating impactful rain/snowfall amounts through
the weekend, however, can't rule out light precip across the
Anchorage/MatSu saturday night into Sunday as a low pressure system
pushes into Prince William Sound. Otherwise, much of the extended
forecast plays into the evolution of TS Bolaven, currently located N
of Guam. Beyond the weekend timeframe, confidence is increasing that
the remnants of this storm will move into the Gulf of Alaska,
bringing with it potential for heavy rain and increased winds.

&&

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