Gale Warning 570, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKDT Mon Sep 2 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Today through Wednesday evening)...

An upper-level trough continues to sit elongated over
Southcentral, with its axis centered from Kodiak Island to the
Copper River Basin. Aloft, a weak area of deformation is occurring
from Cook Inlet northeast to across the Talkeetnas. Closer to the
surface, there is weak convergence as the flow over the Susitna
Valley is from the northwest and the flow over Prince William
Sound is from the east. Low-level moisture is also lingering
across this same area. The result is persistent, slow-moving
showers from Kalgin Island to Hatch Pass. The weak flow is also
keeping these showers from moving much as they develop, resulting
in periods of heavy downpours. Between 1AM and 4 AM, Wasilla
received 0.49" of rain, with higher amounts to the north.
Elsewhere, rainfall amounts have been lighter, but are still
ranging between 0.10" and 0.30" this morning for Anchorage to
Palmer.

This area of shower activity will be slow to move out today as yet
another weak upper-level wave (vorticity lobe) drops south over
the area. By later this afternoon, the trough slides southeast and
the flow aloft turns more westerly, helping to taper off shower
activity as it moves over the higher terrain. While temperatures
aloft today will not be quite as cool, and thus the atmosphere not
quite as unstable, an isolated thunderstorm cannot be ruled out
along the foothills of the Talkeetna, western Chugach, and Kenai
Mountains. An isolated thunderstorm is also possible along the
higher terrain of Kodiak Island today.

Elsewhere, scattered showers remain for Prince William Sound and
the Copper River Basin today. One forecast challenge tonight is
that there is indication of a return up inlet flow underneath a
building ridge for early Tuesday morning. If this occurs, it could
draw low clouds and moisture back over places like Kenai and
Anchorage for Tuesday morning. There also is the potential for
some fog Tuesday morning in low lying areas that clear out Monday
night.

Our next system will be in the form of a North Pacific low that
moves northward bringing a front into the southern Gulf late
Tuesday into Wednesday with stronger easterly winds and rain for
Kodiak Island and the eastern Kenai Peninsula. Inland areas are
likely to mainly downslope out the rain in the cross-barrier
flow. As the front moves toward Prince William Sound late
Wednesday, a developing southerly flow behind it coupled with
another potential shortwave lifting north over the western Gulf
will allow for a second round of heavy rain from Kodiak Island to
the southern half of the Kenai Peninsula. Total rainfall rates
with this system could approach 1.5" to 2.5" for Kodiak City and
Seward, 2.5" to 3.5" for Cordova, and 3" to 5" for Whittier and
Portage.

-TM

&&

.SHORT TERM FORECAST FOR SOUTHWEST ALASKA, THE BERING SEA/ALEUTIANS
(Today through Thursday morning)...

A weak upper-level low near the Alaska Peninsula (AKPEN) will
continue to dissipate today while a ridge continues moving
eastward in the Bering Sea towards mainland Southwest Alaska. The
anticipation is for nicer weather both for today and Tuesday.
However, like yesterday in the vicinity of the Aleutian Range,
there could be enough instability to fire off a couple of isolated
thunderstorms this afternoon as a weak upper-level disturbance in
the area could provide enough lift against the Aleutian Range in
the Bristol Bay land zone. Weather across Southwest then turns
more active for Wednesday and Thursday.

The unsettled weather slated for Southwest is already getting its
act together out towards the Western Aleutians and western Bering
this morning. A weak Kamchatka front will move eastward through
the Bering through Tuesday and deliver a round of light rain to
both the Western and Central Aleutians for today, and perhaps the
Pribilof Islands by Tuesday before it weakens.

Behind this system, another stronger shortwave sliding eastward
from Kamchatka will give another round rain across the Aleutians
Tuesday through Wednesday. At the same time, there will be a North
Pacific low that lifts northward with its front lashing the
Pacific side of the AKPEN Tuesday night. There looks to be some
interaction between these two on Wednesday and could enhance
rainfall across the southwestern mainland by then. Regardless, the
heaviest precipitation with the stronger shortwave, now in the
Bering Tuesday, looks to be across the Pribilof Islands and
Eastern Aleutians Tuesday night into Wednesday morning. Most of
the Bering will remain showery for Thursday as additional weak
shortwaves move across the area from the northwest.

Meanwhile, as the North Pacific low continues to lift northward
near southern Kodiak Island, it weakens. The majority of gale-
force winds look to occur on Tuesday along the Pacific side marine
zones of the AKPEN as the front advances to the AKPEN. Winds
weaken back to small-craft for Wednesday and Thursday as the storm
weakens. Expect southeastern Kamishak Gap winds to pick up
Tuesday through Thursday across Bristol Bay as pressure gradients
tighten between the northward advancing North Pacific low and the
low and front in the eastern Bering. In general, expect most of
mainland Southwest to be showery and unsettled both Wednesday and
Thursday with the rainiest conditions across the Kuskokwim Delta
and northwest Bristol Bay.

&&

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

An impressive looking closed low in the Eastern North Pacific
approaches Kodiak, reaching the island by early Thursday.
Widespread, moderate to heavy rainfall is expected along the
northern coast for Kodiak, the Eastern Kenai Peninsula, Prince
William Sound, and Cordova. Elsewhere, lighter rainfall will
spread across much of Southcentral, Southwest during this period.
Gusty winds up to gale force are anticipated for marine zones in
the Northern Gulf. This low briefly lingers, then weakens into a
trough by Thursday afternoon. A second low inbound from Eastern
Russia crosses the Bering on Thursday. After merging with broader
Gulf of Alaska trough, this system forms a closed low in the
Northern Gulf for Saturday. Weak high pressure ridging across the
Eastern Bering persists as the next front pushes into the Western
Bering on Saturday.

&&

.AVIATION...VFR conditions and light winds are expected through
the TAF period with scattered showers in the morning. There is a
chance for brief MVFR conditions early in the period should a
moderate to heavy shower develop over the area.

&&