Gale Warning 483, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
540 PM AKDT Wed Aug 30 2023

.ANALYSIS AND UPPER LEVELS...

Some lingering showers early this afternoon across the Alaska
Peninsula and Southcentral as a shortwave trough lifts northward.
This wave is one of several rotating around a larger upper low
sitting just off the southwest coast near the Yukon Delta.

The well-advertised late week low is finally making its appearance
as it becomes more organized and rapidly deepens just south of the
Central Aleutians. Water vapor satellite imagery shows a well-
defined dry slot this afternoon with dry air wrapping around the
low's center. The low is expected to bottom out late this
afternoon/evening before is crosses the Aleutian Chain and
occluding. Surface winds have ramped up considerably since 18Z and
21Z obs located at Adak and Atka showed sustained winds of 25 to 30
kts...gusting 35 to 40 kts. Winds will continue to intensify this
afternoon with gusts on the Pacific side of the Aleutians
approaching 65 kts.

&&

.MODEL DISCUSSION...

Overall model agreement remains good with models generally in line
with the progression of the strong low crossing the Central
Aleutians then tracking the upper low towards the Kuskokwim Bay by
mid-morning Thursday. Still some slight differences on placement and
timing of the surface low, but model consensus has the surface low
starting to move inland by Thursday afternoon southeast of Kuskokwim
Bay. Given the system will have lost quite a bit of steam by then,
these minor difference won't factor in too much in the bigger
picture. Models are in alignment with strong offshore winds
developing south of the Aleutian Chain and the western Gulf Thursday
afternoon. Models continue to be in good agreement with a brief
transition to more zonal flow across the Aleutian Chain and Gulf
with another trough moving into the Bering Sea with longwave trough
setting up by late in the weekend.

&&

.AVIATION...
PANC...Light rain showers will lift north through the evening with
periods of sub-5000 ft ceilings possible. There could be aperiod
of wind shear between 09z and 21Z Thursday depending on how
quickly the Turnagain Arm wind bends into the terminal which is
high confidence for 22Z Thur-00z Friday. Gusts will likely reach
35-45 kts during this time period. An Airport Weather Warning has
been issued.

&&

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

Showers from a front that has provided ample rainfall to coastal
Southcentral, particularly Prince William Sound, will continue to
lift northward out of Southcentral tonight. This exit will bring
a brief reprieve in rainfall for the area, however the remnants
of a tropical system currently approaching the Aleutians and
Southwest Alaska will shift eastward toward Southcentral Alaska
towards the latter part of the work week. By late tonight/early
Thursday morning, rain will move back into our region, firstly
over Kodiak. Rain will soon extend over much of Southcentral
Alaska, with the core reaching the coastline by early Thursday
afternoon. The majority of Southcentral will see at least some
light precipitation associated with this system, with the Copper
River Basin seeing the least of which. Areas of strongest rainfall
are expected to keep along the coastline with prolonged periods
of moderate to heavy rainfall to last until potentially Friday
afternoon. Areas around Prince William Sound such as Whittier,
Valdez, and Cordova may see another 2-3 inches of rain before the
weekend.

Winds from this system will be largely southwesterly, an
orientation that will result in winds being largely blocked when
approaching Kodiak City. Kodiak may see a period of gusty winds as
the flow shifts more westerly by Friday. Gusty winds into the
Anchorage area will begin late tomorrow morning bringing winds up
to 30 knots to portions of the bowl. These winds are currently
anticipated to shift southerly and ease by late Thursday
evening/early Friday morning. Given the early flow pattern being
Southwesterly, periods of gusty winds will become increasingly
likely in Kachemak Bay. In the bay, especially the Outer Bay, wind
gusts 30-40 knots and waves around 10 ft are possible later this
week. There is the potential for some clearing skies on the
backside of this system, freeing up Kodiak Island and portions of
the Southern Kenai Peninsula by Friday afternoon.

&&

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

A strong, deep, North Pacific low is well-defined on Satellite
Imagery this afternoon as it makes it's way towards the Chain. The
aforementioned low is expected to track across the central
Aleutians tonight and bring high-end gale force winds across the
southern Bering and storm force winds across the Pacific side of
the central Aleutians. Although the low is expected to weaken as
it tracks towards the Kuskokwim delta coastline, it will bring
elevated winds region wide and widespread rainfall. Peak forecast
winds for the duration of the low's passage should remain around
40 to 50 knots with peak gusts of 65 knots possible, with the
highest winds expected through favored bays and passes and along
the Pacific side of the Chain. As the low tracks across the Chain
tonight, it will usher widespread rain into the Chain and
Southwest tonight and last until at least Friday morning for
Southwest. Strong winds will reach Southwest late tonight with
high-end southeasterly winds pushing inland through Thursday and
turning more southerly by Thursday afternoon as the low moves
closer to the Kuskokwim Delta coast. By Thursday night into
Friday, winds will continue to shift and turn more westerly as the
low continues inland and weakens. Expect gusty conditions through
terrain gaps, especially the Kamishak Gap through at least
Thursday. By late Thursday into Friday the flow begins to change
as the low tracks into interior Alaska. West to southwest winds
will remain elevated across Bristol Bay and on the Pacific side of
the Alaska Peninsula through Friday morning. Rainfall amounts
associated with this system for Southwest Alaska will not be quite
as high as parts of the Aleutian Islands, but a broad region of
0.40 to 1.00 inch rainfall amounts are forecast across all of
Southwest Alaska through Friday morning. As we head into the
weekend, overall flow will continue to be westerly with small
craft winds encompassing much of the Bering.

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

The thinking has not changed much from yesterday's long-term
discussion. Both deterministic and ensemble guidance continue to
hone in on the pattern being progressive with no one synoptic
feature lingering in place for too long.

There is reasonable agreement amongst the various guidance suites
that a trough will, with embedded shortwaves, will swing eastward
from Kamchatka and to the coast of western and southwestern Alaska
by Sunday morning. The associated frontal system will spread
light to moderate rain through southwest Alaska, the Alaska
Peninsula (AKPEN), and eventually, Southcentral Alaska later in
the day Sunday. Conditions quiet down for Monday along southwest
Alaska and the AKPEN, with leftover showers for Southcentral as
the trough moves away to the east and ridging builds in from the
Bering Sea.

Meanwhile, another system moves eastward from northern Kamchatka
into the northern Bering Monday evening. The progression of this
system is still uncertain with regards to the southern
extent/reach of the front, as well as the track of the low itself.
Currently, the operational GFS is the outlier with this system as
it moves the low quicker, further south, and into western Alaska
Tuesday afternoon. The ensemble mean of the GFS is more in-line
with with the Canadian, ECMWF, and their respective ensembles
means with the system staying further to the north, off the coast,
and simply the front making it to the coast. Because of this
uncertainty, the precipitation forecast for Southcentral remains
in flux. For Wednesday, there is ensemble agreement with the idea
of zonal west-to-east flow resuming over the entire domain with
weak shortwaves embedded within the flow. The resultant weather
would be scattered showers across the Bering, Southwest, as well
as Southcentral.

&&

SYNOPSIS AND MODEL DISCUSSION...PP
SOUTHCENTRAL ALASKA...CL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...NS
LONG TERM...DN