Gale Warning 571, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 AM AKDT Wed Sep 4 2024

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

The transient upper-level ridge in place over the Southern
Mainland yesterday has lifted to the north and east overnight in
response to a North Pacific low moving toward the western Gulf
and a second upper-level trough digging south over the west coast
of Alaska. The result is an increase in cloud cover across
interior Southcentral early this morning that is helping to keep
morning low temperatures a bit warmer than the previous morning.
The cloud cover is also resulting in less radiational cooling,
keeping any fog development at bay.

An occluded front attached to the North Pacific low has moved
north well in advance of the parent low, positioned in a east-
west orientation across the eastern Gulf to Kodiak Island.
Widespread rain and strong easterly gales continue along and out
ahead of the front, pushing into the northern Gulf and along the
immediate Southcentral coast this morning. A band of rain as also
moved inland across northern Cook Inlet north into the Susitna
Valley.

Strong east to southeast flow will pick up as the front
approaches the coast, supporting a strong upslope enhancement and
downslope drying signature that will last for much of this long
duration rainfall event. However, there may be some light rain or
sprinkles that make it to the ground across the western Kenai
Peninsula up into the Mat-Su and Anchorage Wednesday Morning on
the leading edge before downslope flow and drying mostly shuts
rainfall off for the rest of the day. On the flip side, much of
the eastern Kenai Peninsula up to the Prince William Sound will
stay quite wet and windy as the front and parent low lift north
through Thursday.

The forecast for winds over interior valleys of Southcentral
remains marred by a bit of uncertainty, particularly towards the
Mat Valley and Anchorage areas. Southeasterly gap wind through
Turnagain Arm will persist this morning, but should bend south and
away from the Anchorage Bowl as a strong northeast to southwest
pressure gradient sets up between the low entering the Gulf and
higher pressure over the Mainland and Yukon. This gradient will
set up a battle between flow out of the Mat Valley from the
northeast and the Knik Valley form the southeast, and wind
direction/intensity will fluctuate over the next day or two for
much of the Mat Valley as subtle changes in the gradient occur.
Farther south over Anchorage, winds along the Turnagain Arm could
briefly return from around noon today through mid to late
afternoon as the low level profile becomes more unstable with
daytime heating and as pressures lower over the Susitna Valley,
perhaps briefly overcoming the down-inlet gradient before bending
back south as the low moves near Kodiak Island on Thursday
morning.

From Thursday into Friday, the pattern will become a bit less
coherent with time as forecast uncertainty grows with the
interaction of multiple small waves embedded within a very large
region of cyclonic flow resulting from the merge of the Gulf low
with an upper trough moving over the Bering/Southwest. The
original Gulf low will also weaken as it approaches the Kenai
Peninsula and effectively merge with the system out west as well
by early Thursday. This entire larger system will advance east
into the Gulf on Friday. This will allow the pattern with east to
southeast flow and rain focused along the coast to give way to a
much more showery and less windy regime going towards the end of
the work week.

-AS/TM

&&

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

A showery and gusty day is in store for much of Southwest Alaska
on Wednesday as a shortwave trough pushing into the west coast of
Alaska begins to interact with a broad low lifting into the
western Gulf. Unlike yesterday, areas of fog have remained
offshore of the coast with no inland areas experiencing any
significant reductions in visibility. Morning radar shows numerous
showers spreading across Cook Inlet into the Western Alaska
Range. Areas of rain should expand in coverage across Southwest
Alaska throughout the day as low-level convergence increases
between the shortwave trough and incoming Gulf low. A tightening
pressure gradient will also allow for gusty easterly winds winds
to span from the Western Alaska Range into the Kuskokwim Delta.

Thursday's rain chances for Southwest Alaska will be even higher
and more widespread than today, as a broad trough, previously over
Northeast Russia, slides across the eastern Bering. Multiple
smaller waves of low pressure are forecast to dive southeastward
across the Aleutians, helping to dig the broader trough eastward
and into Southcentral through Friday. Primary impacts from the
trough's arrival will be for small craft winds gusting to gale
force across the Central and Eastern Aleutians.

The departure of the trough into interior Alaska will help to
clear out precipitation late Friday into Saturday across Southwest
Alaska. Further west, models are coming into better agreement
with the next trough and front sweeping across the Aleutians and
Bering Friday and Saturday with another round of wind and rain.

-BL

.LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)...

The period begins with relief from heavy rainfall along the Gulf
Coast as the northern Gulf surface low dissipates and the upper
level trough drifts to the east through the weekend. Rainfall
chances linger through Saturday morning for Cordova and eastward
in the forecast area but decrease throughout Saturday afternoon
into evening. A weak ridge building over the Alaska Peninsula
throughout the weekend progresses into the Gulf and South Central
Alaska early next week, keeping conditions calm throughout the
period.

The focus shifts to the Bering Sea for the weekend as a front
pushes across the Bering Sea and approaches Western Alaska Sunday
morning. This system may bring periods of modest rainfall to the
Aleutians Saturday morning through afternoon and Western Alaska
Sunday morning through Sunday night. Sub-gale force winds are
anticipated for the Bering Sea and the coastal areas of Western
Alaska. Greater model differences appear heading into Tuesday
regarding the next system for the Bering Sea and Aleutians.

-JH

&&

.AVIATION...

PANC...VFR conditions are expected through the TAF period with
periods of light rain possible early this morning. LLWS is likely
in the morning with northerly surface winds and southeasterlies
around 1500 to 2000ft. These conditions may dissipate through the
afternoon as the Turnagain Arm winds bend back into the terminal
and the winds become unidirectional. Light rain will become
increasingly likely Thursday morning.

&&