Gale Warning 494, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 AM AKDT Fri Sep 15 2023

.ANALYSIS AND UPPER LEVELS...

The overall pattern continues to be mostly dictated by a large,
complex upper low that just does not want to budge from its
current location over the far northern Bering Sea. A number of
shortwaves embedded in cyclonic flow in place across much of the
outlook area are continuing to rotate mostly east on the south
side of the broad upper low. Most notably, a pair of stronger
troughs are now passing near and to the south of the AKPen and
Kodiak Island. An attendant surface low is now approaching the
west side of Kodiak Island as it deepens, producing areas of light
rain across the AKPen and much of Kodiak Island, as well as winds
up to small craft range in the western Gulf. Southerly and onshore
flow continues to stream into Southcentral out ahead of the
approaching Gulf low. Southcentral is now seeing a break in
between systems and a resulting lack in upper level forcing over
the region. Thus, rainfall is now limited to areas where upslope
enhancement is favored in this regime, including most of the north
Gulf Coast and Prince William Sound.

Out in the west, strong zonal flow continues to push over the
southern Bering Sea as a 160 kt upper jet streak moves near the
crest of a flat, transient ridge centered over the Aleutian Chain.
Farther upstream and even farther west, the next more potent low
is just starting to come into the picture, with the center now
moving quickly eastwards a ways off to the south of the western
Aleutians.

&&

.MODEL DISCUSSION...

Model agreement is beginning to improve for the pair of strong
lows expected to eventually merge into one distinct center later
this weekend over the Gulf. However, there are still some slight
timing discrepancies for both systems at this juncture. For the
first low expected to move over the Gulf through Saturday, the
ECMWF and Canadian models have generally depicted the low rapidly
weakening and arcing northeast towards the Kenai Peninsula and
Prince William Sound faster than the NAM and especially the GFS
through Saturday afternoon. There is a similar split for the low
skirting right along the Aleutians and AKPen today and tonight,
with the GFS and NAM taking the low east slightly faster and
farther to the south than the Canadian and ECMWF models. This
spread persists as the low enters the Gulf and merges with the
remnants of the first system through Saturday night. Overall,
confidence is improving for the active low track persisting into
the weekend, but there are still some details to iron out.

&&

.AVIATION...

PANC...Periods of rain will continue as a series of short-waves
transit the region. Persistent low level southeasterly flow
should be enough to maintain VFR conditions through the TAF
period. The low level flow will begin to weaken and back toward
the northeast Saturday morning as the weakening low pushes onshore
along the north Gulf coast. This may allow ceilings to lower to
MVFR sometime Saturday morning, with steady rain developing.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Sunday night)...

Key messages:

*Wet weather will continue across the region through Saturday,
 with some areas drying out on Sunday.

*A storm will strengthen over the Gulf today through tonight, then
 be replaced by a larger and stronger storm Saturday through Sunday.
 This will bring gale to storm force winds to much of the Gulf
 waters along with building seas.

A couple of out of phase short-wave troughs digging southeastward
from the Alaska Peninsula into the southern Gulf of Alaska and
northeast Pacific will lead to development and strengthening of a
surface low in the southern Gulf today. Ridging out ahead of this
system will aid in creating a strong frontal boundary across the
northern to eastern Gulf waters, with gale to storm force winds
expected this afternoon through tonight. This system will lift
northward toward Southcentral Saturday morning as a deepening low
tracking eastward along the Aleutian chain moves into the Gulf.
This second system will be more potent and impacts more widespread.
While the low center will head quickly for the eastern Gulf, a
high amplitude ridge upstream in the Bering Sea combined with cold
air advection and unstable air behind the Gulf low will lead to
strong northwesterly winds across the western to central Gulf
(and Kodiak Island) Saturday night through Sunday.

Rain will be continuous along the coast and Kodiak Island through
Saturday. Interior areas of Southcentral will see some occasional
breaks in the rain. While multiple inches of rain will fall along
the coast (and certainly in the mountains), rain rates do not
look heavy enough to pose a big threat for flooding. As the low
heads for Southeast Alaska on Sunday, rain will diminish across
Southcentral. Persistent cloud cover and rain will maintain cool
temperatures along with a narrow range between daytime highs and
nighttime lows.

&&

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

The status-quo of the active and progressive pattern looks to
continue through the short-term period. A broad trough stretching
from Siberia to southwest Alaska will maintain its position. This
configuration will keep most of mainland southwest in a showery
regime through at least Saturday before a shortwave ridge works in
for Sunday. This could bring a little reprieve from precipitation
before showers return Monday for coastal locations and the Alaska
Peninsula (AKPEN).

Further out west, the parade of low pressure systems will continue
for the Bering Sea/Aleutian Chain. This first system arrives this
morning for the western Chain and tracks in close proximity to the
Chain to the southern AKPEN while deepening/strengthening. This
system is expected to bring widespread sustained gale-force winds
along the North Pacific side of the Aleutians, with pockets of
storm-force winds along its track. The other story with this
system is the moderate to heavy rain. Albeit a relatively quick
moving system, 1.5 to 2 inches of rain are expected from the
western to central Aleutians this morning through Saturday
morning. Lesser amounts closer to an inch of rain are expected
along the southern AKPEN. As the system exits to the Gulf Saturday
afternoon, a ridge builds in behind for much of the Chain on
Saturday, yielding a brief break from the rain and strong winds.

The next systems enter both the western Bering Sea with a separate
one tracking in close proximity to the Aleutian Chain on the North
Pacific side on Sunday. Both of these systems do not look quite
as strong as the system slated for today. Nonetheless, rain looks
likely, along with at least small-craft winds, across the Bering
and Aleutians for Sunday into Monday.

&&

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

The latest long term forecast has a low moving through the Gulf
of Alaska and a second low moving into the Bering on Monday.
Surprise! Unsettled conditions with continued rain chances look to
continue for the foreseeable future. Yesterday there was mention
of Invest 93W, currently located in the West Pacific. Models have
struggled with whether this low will be picked up by an earlier
trough or get missed initially and lifted into the Aleutians over
the weekend. We will lean with the latter scenario and Monday's
Gulf low should possess a fair amount of moisture associated with
it. Thus, the Gulf will remain quite showery with potential for
heavy rain lifting into eastern portions of Prince William Sound
and Southeast Alaska. Additionally, a threat for high winds may
develop within barrier jet flow from Yakutat to Icy Bay as the low
finally moves into the eastern Bering. To the west, the low
moving through the Bering on Monday is expected to be much broader
in size, but also less intense, as it moves across the Aleutian
Chain through the middle of next week. Showery conditions will
plague both the Aleutians and the Bering through Wednesday. The
same low expands into Southcentral Wednesday into Thursday. Thus,
precipitation chances, initially low for Southwest, will increase
significantly by late Tuesday through Wednesday. Southcentral may
see a brief break from precipitation on Tuesday before precipitation
becomes more substantial and widespread across Southcentral Tuesday
night into Wednesday.

&&

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DN
LONG TERM...BL