Small Craft Advisory 855, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 PM AKDT Thu Oct 27 2022

.ANALYSIS AND UPPER LEVELS...

Two shortwave troughs embedded in the long wave pattern are
continuing to influence the sensible weather over the Southern
Mainland. The first shortwave is associated with a compact surface
low southeast of Middleton Island. This feature is continuing to
produce bands of showers rotating back toward Cordova and
Middleton Island this afternoon. A weak surface trough extends
west from the surface low, with precipitation developing along
this boundary, over parts of Prince William Sounds and the eastern
Kenai Peninsula. Elsewhere across Southcentral, conditions are
generally dry with partly cloudy skies. Gusty northerly gap winds
persist along the immediate coast and across Kodiak Island;
however, these winds are not quite as strong as yesterday.

The second shortwave trough is digging southward across the AKPen
this afternoon. An associated surface low is also moving south
into Bristol Bay. Snow showers are falling out ahead of the low,
mainly in an area from Igiugig to Port Heiden. Northwesterly
winds persist from the Southwest Alaska coast to the Pribilofs and
eastern Aleutians due to cold air advection in the wake of the
trough and out ahead of a building upper-level ridge over the
western Aleutians.

&&

.MODEL DISCUSSION...

Latest model guidance remains in good agreement with the overall
synoptic pattern through the next several days. One struggle,
though, is with the extent and timing of snow showers across the
northern Gulf, extending over Cordova and the southern Copper
River Basin. Here, guidance is struggling with the interaction of
a trough extending westward from the surface low with a vorticity
lobe moving east from Cook Inlet. The 12z GFS and 18z hi-res runs
are now bringing snow into Cordova for a time this afternoon and
evening. Given the extent of precipitation seen on the Middleton
Island radar, the most recent forecast package weights to this
solution.

Out west, models continue to lock onto what will become a
hurricane-force low traversing the Southern Bering for Friday.
The progression of the low does look slightly faster than in
previous models; however, the position and strength of the core
of strongest winds looks on track, with the strongest winds
expected in an area from Atka to Dutch Harbor. Models are also in
better agreement with placement of the low as it crosses the
AKPen and heads into the western Gulf, although the EC continues
to be the fastest solution. Overall, there is above average
confidence with regard to the forecast and expected impacts moving
into the weekend.

&&

.AVIATION...

PANC...VFR conditions are expected to persist along with
northerly winds throughout the TAF period. An upper-level
shortwave will move from the AKPen into the Gulf of Alaska later
today, helping to reinforce the cold air advection and northerly
winds. Northerly wind gusts to around 20 kt are possible at times
late this afternoon through early morning Friday.

&&.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: This
afternoon through Sunday)...

The surface low southeast of Kayak Island will continue to
produce light precipitation across western Prince William Sound
through tonight this is expected to bring 1 to 3 inches of
snowfall around the Cordova area. Otherwise a quick moving upper
level low is moving into the western Gulf this will bring a few
showers across Kodiak Island this evening. As this low moves
across the southern Gulf tonight expect an occasional thunderstorm
due to cold air aloft over the warmer ocean. These will occur
mainly over the offshore zones. Cold air advection will be
reinforced this evening as the upper low moves across the Gulf.
This will produce gale force winds with storm force gusts across
the western Gulf and near the Barren Islands. Friday the winds
will decrease as an upper level ridge moves across the area so
expect mainly dry conditions across the area. Another strong low
pressure system will quickly move towards Kodiak Island and the
western Gulf Friday evening this could bring a quick shot of snow
around Kodiak city with an inch or less of snowfall as warm air
advects into the area changing the precipitation to rain Friday
night. Winds will be strong around Kodiak but blowing snow will be
limited as temperatures warm above freezing. This is system will
also bring Gales Friday across the Gulf and Barren Islands with
areas of storm force winds Saturday across the northern Gulf and
on the back side of the low across the southern Gulf. Sunday cold
air advection and a strong pressure gradient will produce
northerly gap winds in the usual spots. The area will also be dry
on Sunday as colder drier air advects across the region.

&&

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

Winds and showers will diminish tonight in Southwest Alaska
including the Alaska Peninsula (AKPen). A relatively cold, stable
air mass over Southwest will linger into the beginning of the
weekend. Quiet conditions Friday will be revisited after a
potentially unsettled beginning of the weekend.

A low pressure system approaching the Eastern Bering on Friday
will draw offshore flow from Coastal Southcentral, creating some
potential for gusty conditions in places like the Greater Bristol
Bay area up to Cape Newenham. Precipitation has been a focus for
this forecast, and while some rain is expected along the AKPen,
the northern extent of heavier precipitation does not seem to
reach the mainland of Southwest AK. Chances for precipitation has
been seen in for the Greater Bristol Bay area, such as Dillingham
and King Salmon, for Saturday morning before diminishing as the
day progresses. The temperatures for this precipitation to come
down as snow will be present, however again this requires the
precipitation. Winds and precipitation will both diminish into the
weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A rapidly deepening low in the western Aleutians will move
eastward tonight and Friday. The low will continue pushing
eastward Friday night, however will take a slightly more
southerly track than previously expected. The low will begin to
cross the AKPen Friday night, exiting the region by Saturday. As
this low exits, a ridge will build over the western Aleutians,
spreading eastward. This will provide a calmer pattern for the
weekend in the Aleutians.

Light winds in the Central Aleutians tonight will increase
rapidly Friday as Gales currently in the Western Aleutians will
intensify to a large swath of storm- force winds with areas of
hurricane- force wind between Nikolski and Atka Friday night
possible by Friday afternoon. Hurricane force gusts are likely
along the Central/Eastern Aleutians Friday through Friday night as
well. Confidence on hurricane force winds pushing inland from
Atka to Unalaska is increasing, therefore a High Wind Warning has
been issued for associated zones. Additionally, seas are expected
to reach around 35 feet during the strongest winds.

&&

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

The long term forecast, starting on Monday, comes with many
uncertainties. Deterministic models are all over the place with
the evolution and interaction of two upper lows, one over the
Bering, and the second to the south of the Western Aleutians over
the North Pacific. How and when they eventually phase will pose
significantly different forecast scenarios. So, instead, let's
begin with where we see areas of greater confidence. All models,
deterministic and ensemble, are in good agreement with the
placement of a persistent area of low pressure over the eastern
Gulf and immediately offshore of the Alaska Panhandle on Monday.
Similar agreement exists with the position of a ridge of high
pressure located over the AKPEN. Thus, we can ascertain a greater
degree of confidence in conditions over the Gulf of Alaska and
into Southcentral at the beginning of the long term period.

Dry conditions can be anticipated over the entirety of
Southcentral on Monday. General northwesterly flow aloft and at
the surface will continue to reinforce a colder air mass into the
region and the western Gulf. Enhanced outflow winds spilling out
of the bays and passes will likely be prevalent from the Western
Alaska Range into Prince William Sound. While confidence in
potential winds as well as precipitation, including snow, will
remain low Wednesday into Thursday, a changeover from dry to wet
appears likely across Southcentral by Wednesday afternoon. Warm
air advection into the northern Gulf Coast, as southeasterly winds
pick up in strength ahead of the front and low, may also cause
precipitation issues as the potential for overrunning increases.

That brings us back to the Bering where questions abound. A
colder Bering low to the north of the Western Aleutians, is
forecast to interact with a much warmer North Pacific low tracking
into the Western Aleutians from the south. Combining the
ensembles gives a vague but somewhat trustworthy picture Monday
into Tuesday. Both lows begin to phase Monday night with the
development of one broadscale low forming over the southern Bering
by Tuesday morning. A threat for widespread gales and potential
storm force winds will exist across much of the Bering before the
newly developed low finally moves inland of the Alaska Coast
sometime through Wednesday morning. A strong front will likely
extend southeastward from the low and sweep across the Aleutians
in concert with its parent low over the Bering. Snow, rain and
enhanced winds will all be possible with the low's and front's
progress eastward. The best chance for impacts from snow across
Southwest will exist Tuesday night into Wednesday, with strong
north to northwesterly winds likely to accompany the backside of
the low, through the eastern Bering and over Bristol Bay and the
AKPEN. Finally, high pressure builds into the Bering over the
latter half of the day on Thursday. Regardless of timing, a highly
unsettled weather pattern looks to unfold early to the middle of
next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 185 187.
MARINE...Hurricane 170 172 173 174 175
Storm 138 155 171 176 177 179 412 413 414
Gale 120 130 131 132 136 137 139 150 160 165 178 180 351 352 411.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CL
LONG TERM...BL