Small Craft Advisory 757, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
516 PM AKDT Mon Sep 26 2022

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low is positioned in the western Bering Sea
with a surface front extending across the central Aleutians,
bringing rain and gusty winds to these areas. A second, weaker low
is positioned over Bristol Bay. Southerly flow out ahead of it is
driving scattered showers over much of Southwest Alaska and along
the Gulf coast. Weak ridging is seen between these two features
and relatively calm conditions are observed across the eastern
Bering Sea.

&&

.MODEL DISCUSSION...

Confidence remains moderate to high in the short term as models
show good agreement with large scale features. Exact timing of
precipitation over Southcentral tonight into Tuesday remains
somewhat uncertain. GFS is slightly faster with the associated
upper wave, although all synoptic models indicate it moving
through in the same general time period. Models have slowed the
progression of a front moving into the western Gulf late Tuesday
into Wednesday. As such, stronger local gap winds may be likewise
slightly delayed.

&&

.AVIATION...

PANC...Generally expect lowering ceilings as showers move into
the terminal late this afternoon and evening. Low level SSE flow
across the Kenai Mountains should initially keep ceilings low
end VFR, but as we head through the night flow will weaken and
ceilings will likely drop to MVFR. The bigger question is tomorrow
morning. With rain ending around 12Z and decreasing moisture
aloft, expect there will be some IFR clouds and maybe vis in the
vicinity. Best case scenario, there will just be some ground fog
at the terminal. Worst case, IFR/LIFR could settle in between
between 12 and 18Z. Ultimately, expect clearing by midday Tuesday
behind the exiting short-wave.

&&

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

Expect showery conditions to continue over Southcentral Alaska
as a robust shortwave pivots over the region from tonight
through Tuesday morning. The forecast remains on track with
this shortwave. The heaviest precipitation is still expected
over Eastern Prince William Sound, with snow showers at higher
elevations due to cold air aloft. By Tuesday afternoon and
evening, the entrance of a transient shortwave ridge should
allow for clearer skies, with the potential for temperatures
to drop and some areas of fog due to radiational cooling.

Active weather returns by Wednesday morning as a mature low
tracking along the Eastern Aleutians pushes its front across
Southcentral. Newer model runs have the front moving more slowly
into Southcentral, so the timing of precipitation and the onset
of elevated winds have been delayed slightly. Regardless, we are
still expecting gales across much of the northern Gulf of Alaska,
with gusty gap winds through the Turnagain and Knik Arms.
Precipitation will likely be heavy along Prince William Sound
and the Western Kenai Peninsula, with interior Kenai Peninsula
and the Anchorage Bowl maintaining drier conditions due to
downsloping, at least until Thursday morning. Then, as the front
weakens and winds diminish, precipitation should fill in over
these areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Thu evening)...

Generally quiet weather is expected across much of Southwest
tonight as an upper level low continues to weaken. Weak upper
level shortwaves continue to rotate around this low causing hit
and miss rain showers. Activity will be the most widespread this
afternoon through around midnight before diminishing overnight
with any remaining activity mostly confined to higher terrain.
Any areas that can clear out have the opportunity to dip to
freezing or slightly below freezing.

Then, a storm system currently impacting the Western Aleutians/western
Bering Sea will continue moving eastward along the Aleutian Chain
overnight. The front associated with this system is expected to
begin impacting part of the southern Alaska Peninsula Tuesday
morning with rain and wind. This front continues moving northward
reaching the Bristol Bay and Kuskokwim Delta coastlines Tuesday
afternoon. As the front moves onshore and then inland, expect
gusty easterly to southeasterly winds to combine with periods of
rain leading to a wind-driven rain, especially in and around the
Dillingham area. Winds are expected to increase throughout the day
Tuesday for Bristol Bay and the Kuskokwim Delta with wind gusts
peaking Tuesday evening/night. Peak wind gusts are expected to be
around 35 kts (40 mph) around Dillingham and up to 45 kts or 50
mph around Iliamna Wednesday am. Peak gusts on the Kuskokwim Delta
are expected to be upwards of 30 kts or 35 mph.

Lastly, after the initial front moves through, expect unsettled
conditions to continue Wednesday through Thu afternoon as the
energy with the main upper level low/trough meanders near Cape
Newenham. This will maintain periods of rain throughout this time
frame. Rainfall amounts of 0.75-1.25" are expected for Bristol
Bay with locally 2"+ amounts across the higher terrain northwest
of Dillingham. For the Middle Kuskokwim Valley and Kuskokwim
Delta, expecting amounts of 0.5-1" with locally 1"+.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Thu evening)...

A gale force storm system currently located in between Shemya and
Amchitka will continue moving eastward along the Aleutian Chain
tonight. Therefore, expect periods of rain showers along with
gusty winds to continue through tonight. Colder air wrapping in on
the backside of the low over a relatively warm water surface will
create for an unstable atmospheric profile leading to convective
showers tonight through Tuesday and Wednesday. As this cold air
wraps in on the backside of the storm, the pressure gradient at
the surface will tighten in combination with upper level support
leading to an increase to gale force winds across the eastern
Bering Sea for Wednesday/Wednesday night. This will also lead to
an increase in the wave heights with seas of 14-18 ft along the
northern Alaska Peninsula and on the northern side of the eastern
Aleutian Chain Wednesday/Wednesday night.

A weak storm system is expected to move through the Western
Aleutians Tuesday evening through Wednesday afternoon with some
more rain and gusty winds for the Western Aleutians, though, not
nearly as strong as this initial system.

Then attention turns late in the period to a much stronger low
pressure system moving towards the Aleutians/Bering Sea Thursday.
One thing of note right off the bat is this is not expected to be
anything like the previous ex-typhoon Merbok in terms of track and
intensity. The current track takes this system northward from the
North Pacific to around Shemya where it will reach its peak
intensity. The storm is then expected to begin moving off to the
east and slowly weaken as it traverses the Bering Sea. Right now,
it appears storm force winds with hurricane force wind gusts are
possible for the Western and Central Aleutians as this system
moves through along with periods of heavy rain. Seas up to 40 ft
are also possible mainly on the Pacific side of the Western
Aleutians. The weather scenario playing out is a current tropical
storm named "Kulap" will move northeastward and get absorbed
(phase) into an upper level trough coming off the Asia continent.
This will help the storm to become extratropical as colder air is
absorbed into the system with the exact strength still uncertain,
though not as strong as ex-Merbok. The potential exists for this
to be a hurricane force low as it moves into the Western Aleutians,
weakening as it moves across the Bering Sea with the center
staying near or west of the Pribilofs.

&&

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

The long-wave trough will remain over the Bering Sea along with
negative height anomalies at 500 mb through Day 7. The pattern
remains unsettled for the Bering as Tropical Storm Kulap becomes
extra-tropical and moves into region. At the onset of this
forecast period, Friday, model divergence remains regarding the
intensity and the track of the surface low. The divergence amongst
the global models becomes more amplified on Saturday and Sunday.
But the general consensus is that the remnants of Kulap will push
eastward across the Central Bering, then take more of a
southeastward turn back toward the Pacific. The evolution of this
system is being rigorously scrutinized. The divergence in model
solutions is not atypical at these longer time scales. As newer
model data become available the confidence level in the track of
this low will increase.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 130 131 132 136 137 150 160 170 171 172 173
                      174 175 176 177 178 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...KC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
LONG TERM...PS