814
FXAK68 PAFC 070028
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
428 PM AKDT Sat Sep 6 2025

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

Rain, of varying intensities, continues across Southcentral
through Monday. Today`s rainfall across most of Southcentral
through 4:00 pm ranged from a few hundredths to a tenth of an
inch. Through the evening hours, widespread light rainfall will
persist. Gap winds in the Copper River and Knik river will remain
gusty through the overnight hours. Winds through Turnagain Arm
increase overnight through tomorrow afternoon.

Overnight and continuing through tomorrow evening, winds in the
Barren Islands and favored windy locations in the Prince William
Sound increase as the trough moves through the region. Widespread
moderate to heavy rain will fall over Southcentral with the
heaviest amounts along the eastern Kenai Peninsula northwest to
along the Prince William Sound coastline where amounts in these
areas range from 1 to 2 inches. In western Kenai Peninsula,
Anchorage Bowl and areas of the Matanuska Valley and Copper River
Basin, where during easterly flow, down sloping effects dominate,
rainfall amounts range from a trace to a few tenths of an inch.
Farther north in the Susitna Valley, heavy rain is expected. A
flood watch has been issued for the Susitna Valley where runoff
from this rain into the Yentna and Skwentna rivers may result in
water rising out of their banks. This watch is in effect through
Monday afternoon. Across Southcentral, rainfall tapers off Monday.

-Johnston

&&


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

A high amplitude trough is centered over the central to eastern
Bering Sea and anchored by a slowly weakening vertically stacked
low. With a high amplitude ridge just downstream over mainland AK,
the trough and low are very slow to progress eastward. A series
of vorticity-maxima rounding the trough are helping to produce
areas of light rain from the Pribilof Islands to the Southwest
mainland. The more impressive aspect of this system is winds.
With another ridge upstream of the low over the western Bering,
surface pressure gradients are tight. Weak cold air advection
and instability are enhancing winds further, with a band of gale
force winds being observed today from the central Bering Sea
(including the Pribilof Islands) down to the Aleutians (Adak to
Atka to Nikolski). Satellite imagery shows widespread mountain
waves along the Aleutian chain and Alaska Peninsula as west-
southwest winds move right along the volcanic mountains. Gusty
winds extend across this region and up across most of SW AK.

The forecast is on track with this system as it tracks inland
across western AK Sunday, with little or no change made to the
forecast. Attention shifts to a deeper more impactful system
moving up from the northwest Pacific. Typhoon Peipah transitioned
to an extra-tropical low which is now curving northeastward
toward the Aleutians. Models have struggled with the ultimate
track and strength of this low in recent days. However, now that
has undergone the transition from tropical to extra-tropical, the
latest suite of model guidance has locked on to a similar solution
and forecast confidence in the track and weather impacts is
greatly increased. Thus, made large changes to the forecast as
this system tracks along the Aleutians Sunday afternoon through
Tuesday. These changes represent a notably further north storm
track with a deeper low (and stronger winds).

The low will be deepening as it nears the central to western
Aleutians Sunday afternoon. It will interact and ultimately phase
with a short-wave tracking from the Kamchatka Peninsula to the
Aleutians. This will lead to continued deepening of the low
through at least Monday morning, before it occludes and becomes
vertically stacked. With very warm tropical air accompanying the
low, the baroclinicity (strength of the temperature gradient) of
this system looks quite impressive. These types of storms tend to
"overproduce" on strength of winds. As a result, expect widespread
gale force winds, with some chance of storm force winds,
especially with the cold front on the back side of the low.
Forecast soundings for Adak show low level lapse rates are not
all that impressive (roughly 4 to 6 degrees C/KM). Based on this,
have maxed sustained winds in the forecast at around 47 kts.
Still, there will be stronger wind gusts through bays and passes,
including for Adak and Atka and nearby islands. The core of
strongest winds will continue to be on the back side of the low
as it progresses eastward through Tuesday, though the overall wind
field will begin to weaken by late Monday.

Meanwhile, a zonal jet streak will drive the highest moisture air
off the east (remaining south of the Aleutian Chain). Still,
high moisture content combined with strong upward vertical motion
and instability aloft will lead to periods of heavy precipitation,
particularly for Adak and Atka where the forecast calls for nearly
2" of rain in the 24 hour period from late Sunday afternoon
through late Monday afternoon. Thunderstorms with this storm are
also likely, especially south of the Aleutian chain as the upper
jet streak (and dry air intrusion) cross over the low level
warm/moist theta-e ridge. There is a chance of thunderstorms for
both Adak and Atka, but expect if it occurs it would be brief and
would be coincident with the steady heavy rain out ahead of the
warm front. As the low occludes and the atmospheric river gets
shunted eastward across the north Pacific Monday, rainfall
intensity will lessen. Thus, rainfall amounts for Nikolski, Dutch
Harbor/Unalaska on over to Alaska Peninsula communities will not
be nearly as high as what Adak and Atka see.

The primary area of uncertainty is the northward extent of impacts
from this storm system. The leading frontal system will reach
Bristol Bay Monday night, with rain and wind overspreading the
region through Tuesday. However, it is not clear whether the
front will make it far enough north to impact the Kuskokwim Delta
and Kuskokwim Valley.

-SEB

&&


.LONG TERM FORECAST (Days 4 through 7 - Wednesday through
Saturday)...

Models appear to be coming into better agreement with synoptic
features for the long term forecast, beginning on Wednesday of
next week. The remnants of a tropical system pushing through the
Aleutians on Tuesday will likely peak in intensity over the
western Gulf of Alaska on Wednesday morning as a broad ~982 mb low
southeast of Kodiak Island. The implications of this being gusty
northerly winds across the Alaska Peninsula on the backside of the
low, and the potential development of a 25 to 30 knot barrier jet
just outside of Prince William Sound on the north side of the
low. The low quickly fills in and diminishes through Thursday
where guidance has it persisting as a weak stationary low into the
weekend. In addition to winds, rain chances will continue for
coastal areas across the Gulf through the long term forecast, with
drier conditions forecast further inland. The presence of the low
on Wednesday could also lend itself to northerly gap flow into
Kachemak Bay and Resurrection Bay.

To the west and over the Bering and the Aleutian Chain, zonal
flow is forecast to persist, resulting in milder conditions
through the end of the week.

BL

&&

.AVIATION...

PANC...Showers will continue to bring cigs down to MVFR through
early this evening. Cigs should then rise to VFR as some cross
mountain winds and Turnagain winds develop later this evening.
Rain may develop again late tomorrow morning, resulting in a
return of MVFR or at least cigs below 5000ft.

&&
$$