Small Craft Advisory 733, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
715 PM AKDT Sun Sep 27 2020

.ANALYSIS AND UPPER LEVELS...

A high amplitude trough encompasses all of the Alaska region from
the Bering Sea and mainland Alaska southward across the North
Pacific. High amplitude ridges bookend either side of the trough,
over the northwest Pacific and along the west coast of the
Continental U.S. An occluded, storm-force low over the middle of
the Gulf is weakening as it tracks northward toward Prince William
Sound. This system is producing widespread strong, gusty winds
and moderate to heavy rainfall across the Gulf, Kodiak, and Prince
William Sound regions. Winds are rapidly diminishing after
peaking this morning. Some of the more impressive wind
observations from this morning (at the time the low reached its
strongest point) include peak wind gusts of 70 kt (81 mph) at
Middleton Island, 61 kt (70 mph) at Amatuli Island, and 51 kt (59
mph) at Kodiak Airport. Meanwhile, waveheights have risen to 20
feet or more across much of the northern Gulf. As the low heads
northward, light rain is beginning to spread from the Gulf over
the coastal mountains and into the Copper River Basin. Otherwise,
most of the rest of interior Southcentral is dry.

Southwest Alaska remains relatively calm under northerly flow.
A little bit of light rain from the Gulf low is crossing the
Alaska Peninsula into Bristol Bay. A pool of cold air aloft that
brought a period of light snow to Bethel Saturday night continues
making its way south across the Bering Sea under broad northerly
flow. This is leading to gusty winds, low clouds and rain showers
from the Pribilof Islands to the Aleutians and southern Alaska
Peninsula.

&&

.MODEL DISCUSSION...

Models show good overall synoptic agreement. There is excellent
agreement with the occluded system over the Gulf, and good
indication of a second low forming from the trough over the North
Pacific. Models disagree when the low center forms and the
strength of it, with some models showing peak strength ~8-10mb
higher than others. Despite these differences, confidence is high
that the associated warm front will cross Kodiak and the Alaska
Peninsula. The track of this system is fairly consistent across
the models.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

A broad, weakening low over the North Gulf of Alaska will bring
more showers across Prince William Sound, Seward and Kenai
Peninsula tonight, with isolated thunderstorms passing over
Middleton Island and mainly south of Prince William Sound.
Wind observations from the Gulf and along Prince William Sound
indicated a downward trend in response to the existing low, except
winds over the Gulf will remain at gale force but will diminish
late tonight.

A secondary low is expected to emerge from the North Pacific
before trekking toward the western Gulf on Wednesday. It will
bring a lot of rain over Kodiak and Kenai Peninsula, then
spreading across Prince William Sound in the days ahead. This
very same system will elevate winds and wave heights beginning
Tuesday, which will keep the Gulf of Alaska, Kenai Peninsula and
Prince William Sound under a wet weather pattern through the
middle of the week.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 3: Sunday
afternoon through Wednesday)...

An occluded front with low level warm air advection across
Southwest Alaska combined with an upper level trough will continue
to bring rain tonight across the AKPEN. Most of the rain will
come to an end by Monday morning with a few showers possible over
the higher terrain through Monday evening. Widespread fog is not
expected Monday morning despite clearing skies in the YK delta.
This is due to drier northerly flow and winds around 10 knots
which will keep the atmosphere mixed. A compact upper level low
will move towards the YK delta. This will bring colder air with
850 temperatures in the -5 to -8 degree range, so colder
temperatures expected in the YK delta for Tuesday. This would
support some snow especially in the morning, however moisture is
limited so not much precipitation is expected with this system.

Another strong low pressure system will move up towards the AKPEN
from the northern Pacific Tuesday night into Wednesday. This will
bring storm-force winds with a few gusts to hurricane force along
with areas of moderate to heavy rain. Strong winds are expected
from Cold Bay up to Port Heiden and other coastal locations with
wind gusts near 50 knots.

&&

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

Northerly flow will dominate the area with a couple of shortwave
troughs bringing areas of rain from the Pribilof Islands south
towards the Aleutians through Monday morning. Monday afternoon,
the pressure gradient will begin to loosen up so winds will
decrease.

A frontal system will graze the western Aleutians and
Pacific side marine zones with small craft winds and chance of
rain Monday into Tuesday.

Tuesday into Wednesday, a compact low pressure system will move
down from the north enhancing precipitation over the Pribilof
Islands.

Our attention then turns to the next storm system that will be
approaching the eastern Aleutians Tuesday night through Wednesday
with storm force winds, increased wave heights and periods of
rain.

&&

.MARINE (Days 3 through 5/Wednesday through Friday)...

A strong North Pacific low tracks over the Alaska Peninsula late
Wed. The Gulf will have widespread gale force, with storm force
winds across the Kodiak Island and western Gulf waters. These
winds will diminish a bit Wednesday, but remain elevated. A new
North Pacific low approaches the Alaska Peninsula on Thursday.
There is uncertainty in the exact track and strength of this
second storm, but it looks like it will affect the same areas.
The current forecast will go with widespread gales for now, but
storm force winds are possible Thursday afternoon through Thursday
night from the western Gulf to Bristol Bay, likely spreading into
the Bering Sea on Friday.

For the Bering and Aleutians, A North Pacific low crosses the
Alaska Peninsula and dissipates in the Eastern Bering Thu. This
will produce widespread small craft winds, with smaller areas of
gale force winds likely. The second storm mentioned above has
potential to bring much more widespread gales to the region by
Thursday night/Friday, but confidence is low at this point in
time.

&&

.LONG TERM FORECAST (Days 4 through 7/Thursday through Sunday)...

The pattern will become even more amplified as the upper trough
over the eastern Bering/Aleutians deepens and the upper ridge over
the west coast of North America builds. The tilt of the upper
trough will change from positive to negative which will help
support cyclogenesis along the eastern Aleutians/AKPEN on
Wednesday as a 972 mb low is expected to strengthen south of the
AKPEN on Wednesday and move northwestward into the eastern Bering.
As this low moves northwestward into the Bering it will gradually
weaken due to moving farther away from the core of the jet. The
low then will remain stalled in the eastern Bering through Sunday.
One major impact of the orientation of this jet is a good
potential for strong Turnagain Arm winds during this period as the
core of the jet is over Southcentral Alaska on Wednesday
(favorable for strong Turnagain Arm winds). Moreover, the
northwestward motion of this low into the eastern Bering will
enhance the pressure gradients across Turnagain Arm and cause
these strong winds to be prolonged. This system will also produce
heavy rain across Kodiak and along the windward (south-east
facing) side of the Kenai Mountains. The highest rainfall amounts
will be over Kodiak and along the southern Kenai Peninsula.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale warning 119 130 150 351 352.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KM/SEB
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RC
MARINE/LONG TERM...MK/BG/SEB