Gale Warning 529, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
508 AM AKDT Tue Aug 13 2024

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

The surface low and upper-level trough responsible for the
widespread showers yesterday are slowly moving out of the region
this morning. Scattered clouds and a few isolated showers linger
over the southern and eastern Copper River Basin this morning,
still under the influence of the trough.

Elsewhere, conditions are mainly clear and cool to start the day
as a ridge of high pressure builds from the west. The exception to
this is areas of fog that have developed overnight and continue
through this morning due to the cooling temperatures and residual
moisture both at and near the surface. This most widespread area
of dense fog at 4am is over the Matanuska Valley, south and west
of Wasilla. A Dense Fog Advisory is in effect until 9 am for this
area due to locally dense fog, especially along the Parks Highway.
Areas of fog are also beginning to develop near Willow,
Talkeetna, and across parts of the Kenai Peninsula.

Gap winds have also redeveloped overnight for Seward and Whittier,
with gusts around 25 to 35 mph, respectively. These winds will
persist into early afternoon before diminishing.

Fog will be a concern yet again tonight as the flow aloft weakens,
low level moisture lingers, and overnight temperatures again fall
under clear skies. A weak southeasterly flow developing across the
northern and western Gulf by early Wednesday morning could lead to
the advection of low stratus along the immediate Southcentral
coast and up Cook Inlet. How expansive this cloud deck will be,
however, is uncertain.

A Bering Low moving through Southwest could begin to impact
Southcentral as early as Wednesday evening with southerly winds
changing to more easterly/southeasterly late Wednesday evening
moving from Kodiak Island through Kamishak Bay and the eastern
Kenai Thursday morning. Though southerly winds into the wind gusts
into Homer and Shelikof Strait ranging from 20 to 30 mph are
likely. Wednesday evening, winds shifting to a more
easterly/southeasterly direction Thursday into Seward, Kenai
Peninsula and Whittier Thursday.

-TM/DJ

&&

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

Surface high pressure with ridging aloft extends across much of
the Bering Sea, Aleutians, and Southwest Alaska this morning,
promoting widespread dry and relatively calm conditions. The
region is blanketed in stratus, with some fog present in the
eastern Bering Sea. A weak surface trough also in the eastern
Bering will push onshore of Southwest Alaska later this morning,
bringing potential for some light rain showers to the Kuskokwim
Delta which will diminish in the evening hours and overnight.
That will be the only significant chance for precipitation over
Southwest Alaska until Wednesday night. Stratus over the Lower
Kuskokwim Valley and much of Bristol Bay looks to mix out later
today allowing temperatures to reach into the lower 60s this
afternoon and again on Wednesday before the arrival of the next
storm system.

A low centered approximately 200 nm west of Attu this morning will
be the main feature of concern across the region this week as it
moves eastwards along the Aleutians through Wednesday before
lifting across the Bering Sea late Wednesday into Thursday. The
associated frontal boundary will sweep across the Aleutians
bringing moderate to heavy rainfall and gusty southerly winds
(mostly below gale force) before lifting over the Alaska Peninsula
and towards the Southwest Coast by Wednesday night.

The low intensifies as it tracks across the Bering Sea, with gale
force south to southeasterly winds developing in the eastern
Bering Wednesday night. While there is some concern for high
surf along the coast, particularly for the northern Kuskokwim
Bay, the wind direction and intensity is similar to another event
earlier this month which appeared to produce only a minor onshore
push of water. As a result, thinking impacts to water levels will
be fairly minimal. Models have come into better agreement with
the evolution of the low, lending greater confidence to the timing
and rainfall amounts. The low draws up an abundance of moisture
from the tropical West Pacific, bringing precipitable water values
of 1.0 to 1.5 inches into Southwest Alaska for Thursday.
Therefore, from when the front first pushes onshore along the
Southwest Coast Wednesday night and lifts across Southwest Alaska
through Thursday, the area can expect half an inch to upwards of
one inch of rainfall. The low lifts into Western Alaska Thursday
night as rain showers quickly taper off from southwest to
northeast. Looking ahead to Friday and into the weekend, there is
much greater uncertainty regarding the track and strength of a
second low tracking across the Bering Sea.

Quesada

&&

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

The long term forecast begins with a complex and transitional
weather map. An extended Central Bering upper level trough moves
into and bisects Mainland Alaska North to South through the
Weekend. The closed Bering Strait low migrates into the Arctic
late Saturday. Embedded systems add into the dynamic weather
picture. The forecast weather is handled with a GFS- ECMWF-
Canadian blend through Saturday before switching to a GEFS- ECMWF-
Canadian ensemble mix the rest of the way. The surface map starts
with a well developed Northern Bering low and a front that
extends into Western Alaska. The low dissipates over Northwest
Alaska late Friday, while the front continues to spread areas of
heavy rain and gusty winds into the Interior through late
Saturday. A North Pacific low developing South of the Aleutians
brings a bit of tropical influence across the Central and Eastern
Aleutians by Friday night. The low crosses the Central Aleutians
around midday Saturday and its front pushes a second round of
heavy rain into Southwest Alaska. This area of heavy rains also
spreads over the Alaska Peninsula, Kodiak Island through Saturday
and the Southcentral coasts from the Kenai Peninsula, Prince
William Sound, and the Lower Copper River Basin by Monday. A
second axis of rainfall drives inside the Alaska Range into the
Southern Interior for Sunday and Monday.

Towards the end of this forecast period, a new North Pacific low
center approaches the Western Aleutians for Sunday and Monday.
This developing system looks to have support from a second upper
level trough coming out of Russia over Kamchatka, plus some
additional influence from the North Pacific. It is expected to
bring heavy rains and potentially high-end gale force winds into
the Western Aleutians and Bering for the start of next week.

- Kutz

&&

.AVIATION...

PANC...
VFR conditions and light winds are generally expected to persist.
There will be some potential for fog or very low stratus to form
near the terminal this morning where skies have remained
relatively clear through the early morning. However, any fog or
low stratus is likely to break up with daytime heating by late
this morning or early this afternoon.

&&