Gale Warning 551, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
526 AM AKDT Fri Oct 7 2022

.ANALYSIS AND UPPER LEVELS...

Southern mainland Alaska sits at the boundary of two large-scale
air masses this morning. To the west, a broad upper-level
longwave trough is situated over Far Eastern Russia, extending
across the northern Bering Sea and western Alaska. To the east, a
ridge stretches up from the western United States into the
Canadian Yukon and Northwest Territories. Between these two
air masses, an occluded front with a well-defined presentation on
infrared satellite imagery extends across Southwest Alaska. Radar
indicates, however, relatively light rain showers associated with
this feature across the Bristol Bay region and Western Alaska
Range. Behind the front, a cooler air mass with relatively few
clouds is moving in over the Kuskokwim Delta, while the rest of
the Southwest remains covered in stratus.

A cut-off low continues to spin over the North Pacific, rotating
yet another shortwave toward Southcentral Alaska. This, in
combination with a secondary filament of moisture streaming
northward across the western Gulf, is producing a band of
moderate-to-heavy rain for Prince William Sound. Southwesterly
flow tapping into this stream of moisture and the dynamic lift
ahead of the approaching front has also contributed to the
development of scattered rain showers across the Cook Inlet
region. This feed of moisture is particularly evident on
satellite as a deck of stratus spreading northward from the Gulf
of Alaska. Lastly, a developing low over the western Aleutians
will be the next feature to produce significant impacts across
southern Alaska.

&&

.MODEL DISCUSSION...

Models continue to be in good synoptic agreement through the short
term. The confluence of features over southern mainland Alaska,
however, makes for a complex forecast on the local scale.
Differences in the timing and placement of the approaching front
from the west and moisture stream out of the Gulf of Alaska
contribute to uncertain timing with rainfall in the Cook Inlet
region through Friday night. Other uncertainties remain beyond
that point, including with regard to how much of the cold air
mass behind the front gets through the Alaska Range and into
Southcentral on Sunday. This will ultimately drive snow levels in
the higher elevations. We also continue to closely monitor a low
expected to approach the Southwest coast Sunday night into
Monday. Differences in track and timing with this low decreases
confidence in pinpointing the locations of highest impact along
the coast.

&&

.AVIATION...
PANC...VFR conditions will prevail through Friday afternoon.
Gusty southeasterly Turnagain winds are expected to gradually
diminish through the day. Ceilings will then lower to MVFR by
this afternoon in advance of steadier rain approaching early
Saturday morning.

&&

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

The main weather story for today is the front approaching from
the west. This is turning flow southwesterly aloft with a shortwave
propagating over Cook Inlet and into the Prince William Sound
areas. Over the Sound it is tapping into a very moist plume
associated with a low well south of the Gulf, bringing in a band
of heavy precipitation to the Cordova area early this morning.
This area of rain will not move much today as it will have its
westward motion stopped by the incoming front from the west. Rain
over the Susitna Valley and Cook Inlet areas is associated with
that front, but there is not really a lot of reinforcing moisture
past the initial push until the upper level trough really digs in
later this weekend. Therefore, the rain over Cook Inlet,
including the Anchorage area, will mainly occur early this
morning, with probabilities decreasing afterward.

The weather gets a little more interesting as that upper low digs
southeastward and the big push of moisture and lift moves over
Southcentral. That should come in Saturday morning from west-to-
east and bring in widespread rain. The big question will be
whether there will be enough cold air moving in before the rain
stops to change that rain into snow Saturday night into Sunday. It
still looks unlikely for that to occur at sea level, especially
with 850 mb temperatures remaining at or above 0C until Sunday
morning. This is due to the way the upper low will track over
Kodiak Island on Saturday night which will keep the warmer air in
place over Southcentral for longer as the warm sector remains over
the mainland areas. By late Saturday night into Sunday morning,
the colder air will probably bring snow levels down to 1500 ft to
2500 ft across the area. This will mean that places such as
Turnagain Pass, Chickaloon Pass, Broad Pass and the Anchorage
Hillside are likely to see some snow. Places lower in elevation
than that will likely remain rain. Seward is also looking at
somewhere around 3.5 inches of rain from late Friday night
through Sunday morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

Generally quiet conditions are seen across the region this
morning with the presence of zonal flow at 500 mb. Scattered
showers occurring over the Middle Kuskokwim Valley will taper off
by this afternoon. A deep upper trough will continue digging to
the west through Saturday and southwesterly flow aloft, combined
with its associated surface low, will bring rain across the
region. Highest totals of around an inch look to be most likely in
the Bristol Bay area. Temperatures at 850 mb will fall to around
-10C on Sunday and low temperatures overnight Saturday into Sunday
will drop below freezing over much of the area. Although the
majority of the precipitation will have shifted eastward by then,
some light snowfall accumulations are likely east of Sleetmute.
Weak ridging will lead to more benign conditions on Sunday. A
strong low tracking across the Gulf Monday will push a long
southwesterly fetch into the Kuskokwim and Bristol Bays.
Confidence is increasing for the likelihood of storm surge and
potential coastal flooding/erosion in these areas. We will
continue to monitor.

&&

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

A trough will deepen over the Bering through Saturday resulting
in a strengthening surface low pressure system tracking west to
east across the Aleutians. Strong cold air advection on the
backside of the low will lead to gusty winds late Saturday into
Sunday for the south side of the Alaska Peninsula. A trailing
ridge will bring a brief reprieve before a much stronger low
enters the western Bering and strengthens through Monday. While
the strongest core of winds will remain offshore, its associated
front will bring gusty southerly winds and rain across the
Aleutians on Sunday, followed by stronger westerly winds on the
south side of the low. Conditions are expected to be worse for the
Pribilofs closer to the center of circulation.

&&

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

Expect multiple lows this forecast period. The latest run of the
GFS/ECMWF/Canadian are well clustered on Monday and it looks like
Monday morning is going to be pretty cold across Southcentral.
Recent runs of the GFS have come in line with more amplified
mainland to Gulf of Alaska upper trough development. By Wednesday,
divergence in solutions has emerged over the Bering and the NW
Pacific but the Gulf Of Alaska is still in good agreement with the
low there. By midweek next week there is a high probability of
additional pattern amplification as an upper low dropping
southeast out of the Kamchatka Peninsula merges with a system.
This will place Southcentral in due southerly flow with a deep
moisture fetch on Wednesday. In a nutshell, another atmospheric
river is gearing up and headed towards Alaska with potential for
heavy rain along the coast and possibly inland as well.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 132 150 155 172 174.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CQ
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
LONG TERM...PJS

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
152 PM AKDT Fri Oct 7 2022

AKZ155-161-072300-
Kuskokwim Delta-Bristol Bay-
Including the cities of Bethel, Nunivak Island, King Salmon,
Dillingham, Naknek, and Pilot Point
152 PM AKDT Fri Oct 7 2022

Strong southwesterly winds could potentially result in coastal
flooding and erosion across the coastal Kuskokwim Delta and far
western Bristol Bay with the potential track of this system.
Therefore, a Coastal Flood Watch has been issued for late Sunday
night through Tuesday morning.