Gale Warning 738, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
545 AM AKST Tue Nov 28 2023

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Thursday night)...

Conditions are fairly quiet across Southcentral this morning, with
just a few areas of showers due to low level instability and some
weak short-waves aloft. However, the upper level flow remains
progressive, so this is just a very brief break in the impactful
weather. A moderate strength low will track northward across
Southwest Alaska today with a trailing cold front tracking east
into the western Gulf and Cook Inlet. A ridge will build
downstream of this, leading to rapidly tightening pressure
gradients. This will lead to strengthening southeasterly winds
region-wide, with the typical strongest winds through gaps in the
Kenai and Chugach Mountains, including Turnagain Arm and the Knik
River Valley. The cold air aloft that moved into Southcentral
yesterday has resulted in fairly steep low level lapse rates (in
excess of 7 degrees C/km on the 12Z Anchorage sounding). This
instability will lead to vertical mixing in the low levels, with
stronger winds aloft mixing down to the surface. As a result,
winds will be more widespread than they would be in a stable
environment. Have issues a special weather statement to highlight
strong and gusty winds this afternoon and evening from the Kenai
Peninsula to Anchorage, the Matanuska Valley, and the southern to
western Susitna Valley.

Meanwhile, the arrival of the cold front combined with upslope
flow will lead to increasing precipitation for Kodiak Island, the
Gulf coast and portions of the Kenai Peninsula. Despite the cold
air advection yesterday, there is still an above freezing layer
near the surface and the wind today will lead to further warming.
Therefore, precipitation will be primarily in the form of rain at
sea level, with snow in the mountains.

An amplifying trough over the North Pacific will lead to
cyclogenesis this afternoon, with a deepening low then tracking
northward into the Gulf of Alaska tonight through Wednesday. Low
level flow over Southcentral will gradually back from southeast
to northeast as this low moves into the Gulf, allowing winds to
diminish overnight tonight through Wednesday morning. There
remains some uncertainty in the ultimate track of this low, with
some model guidance keeping the low down over the Gulf offshore
and other solutions tracking the low up toward the Kenai
Peninsula. This will make a big difference in the wind and
precipitation forecast for Wednesday/Wednesday night. The farther
north track would push the leading front onshore bringing stronger
winds and heavier precipitation, especially for the interior
Kenai Peninsula from Girdwood down the Seward Highway corridor. It
could also lead to some precipitation spreading over the coastal
mountains to inland areas of Southcentral. For now, have taken a
middle ground solution.

The upper trough will ultimately lift northward across Southcentral
Thursday, driving the remnants of the low into Southcentral. It
looks like it will be in a much weakened form, but will bring a
chance of snow to interior areas. The upper levels keep moving,
with another trough moving into the Gulf Thursday/Thursday night.
It does look like the upper flow will become more zonal, so any
surface lows that spin up look like they will remain well south in
the Gulf as we head through the end of the week. This would mean
mostly dry weather for Southcentral with seasonable temperatures.

&&

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

An upper-level trough spreads from the Kamchatka Peninsula across
the Aleutians and into Southwest Alaska with an embedded upper
low located to the west of St. Lawrence Island. Convective snow
showers continue to blanket much of the Bering and Aleutians as
cold air streams east over the comparatively warm sea surface.

A notably potent shortwave and attendant surface low over the
Alaska Peninsula this morning are tracking northeastward into the
Bristol Bay area. The low will continue moving very quickly
northward through the Lower Kuskokwim Valley today before exiting
to the north by this evening. An intense but fairly brief round of
snow will follow this low north today, while enough warm air
works in to the east of the low center for a period of mixing or
changeover to rain today across the southeastern portions of
Bristol Bay. The heaviest snowfall will mostly be focused over the
Kilbucks and Kuskokwim Mountains, but as much as 3 to 5 inches of
accumulation will be possible for surrounding spots including
Togiak, Aniak and Sleetmute before the low moves off to the north
late on Tuesday. Strong and gusty winds across Bristol Bay and
Lower Kuskokwim Valley, in addition to gale force winds over
Bristol Bay and the coastal waters south of the Alaska Peninsula,
will diminish by tonight with the exit of the low. Conditions
across Southwest will become a bit calmer and cooler later in the
week.

Further to the west, a strong front reaches the western Aleutians
on Wednesday morning and will continue lifting east across the
Bering and rest of the Aleutians through Thursday. Steady
precipitation and gale force winds will accompany the front. The
low itself will track near the western Aleutians by early Thursday
morning. Model agreement with this system begins to diverge later
Thursday morning with respect to low center placement, the
progression of the front, and a subsequent secondary low formation
along the front. Nonetheless, the Bering and Aleutians can expect
to see generally active weather through at least the end of the
week with this system.

AF

&&

.LONG TERM FORECAST (Days 4 through 7)...

The long-term forecast period begins Friday morning with fair
model agreement depicting a broad upper-level trough stretched
across the Bering Sea into the Gulf of Alaska, creating several
surface low pressure centers in the southeastern Bering and
northwestern Gulf. These lows will draw westerly gales across the
Aleutian Chain and easterly gales across the northern Gulf coast
into Prince William Sound. Generally a showery, wet pattern will
be in place across all of Southern Alaska, with areas such as the
lower Kuskokwim Valley, and Copper River Basin seeing relatively
less precipitation than surrounding regions. Looking into Saturday
model uncertainty increases, with some solutions keeping a low
pressure center spinning near Bristol Bay, while other solutions
retreat the low to the Western Bering. These discrepancies lead to
a difficult forecast as to the behavior of the Bering early next
weekend. By Sunday and Monday, model disagreements become severe
with some solutions keeping a low in the Southern Bering and
granting Southcentral a reprieve in rain and winds. While other
solutions spin up a new low pressure center in the northern Gulf
of Alaska coast that would bring another strong shot of
precipitation and strong winds to the Southcentral coastline,
while keeping the Bering relatively benign. Overall, a wet pattern
for the Aleutians and Southcentral regions, and looking colder
and drier from mid to late week for Southwest Alaska.

CL

&&

.AVIATION...

PANC...Gusty southeasterly winds will continue through tonight,
with the highest winds expected mid-afternoon through this
evening, ranging from 15-22 knots sustained and gusts reaching
30-35 knots. Winds diminish and transition to northerly early
Wednesday morning. Flow aloft may hold on to southeast winds
longer after surface winds have shifted, which may lead to some
wind shear beginning around 12Z Wednesday.

KM

&&