Gale Warning 450, NWS Anchorage, AK

629 
FZAK61 PAFC 042322
OFFAER

Offshore Waters Forecast
National Weather Service Anchorage AK
322 PM AKDT Fri Sep 4 2020

Offshore waters forecast for the Gulf of Alaska West of 144W

Wind forecasts reflect the predominant speed and direction
expected. Sea forecasts represent an average of the highest
one-third of the combined wind wave and swell height.

PKZ399-051245-
322 PM AKDT Fri Sep 4 2020
.SYNOPSIS FOR THE WESTERN GULF OF ALASKA...
A trough of low pressure dissipates over the western Gulf 
through Fri evening. A front moves into the southern Gulf 
Sat morning, then slowly moves to the central Gulf by 
Sun afternoon.

$$

PKZ351-051245-
Gulf of Alaska Offshore North of 57N and West of 144W-
322 PM AKDT Fri Sep 4 2020

.TONIGHT...Variable wind 10 kt becoming NE up to 10 kt after
midnight. Seas 3 ft.
.SAT...E wind up to 15 kt. Seas 4 ft.
.SAT NIGHT...E wind 10 to 20 kt. Seas 3 to 6 ft.
.SUN...SE wind 15 to 30 kt. Seas 4 to 9 ft.
.SUN NIGHT...S wind 20 to 30 kt. Seas 8 to 11 ft.
.MON THROUGH TUE...S wind 10 to 20 kt. Seas 8 to 13 ft.
.WED...W wind 15 kt. Seas 6 ft.

$$

PKZ352-051245-
Gulf of Alaska Offshore South of 57N North of 55N and West of
144W-
322 PM AKDT Fri Sep 4 2020

.TONIGHT...E wind up to 15 kt. Seas 3 ft.
.SAT...E wind 15 to 30 kt. Seas 4 to 9 ft.
.SAT NIGHT...E wind 15 to 30 kt. Seas 7 to 12 ft.
.SUN...SE wind 15 to 30 kt. Seas 6 to 11 ft.
.SUN NIGHT...S wind 25 kt. Seas 8 to 12 ft.
.MON THROUGH TUE...S wind 10 to 25 kt. Seas 9 to 14 ft.
.WED...SW wind up to 15 kt. Seas 5 ft.

$$

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 PM AKDT Thu Sep 3 2020

.ANALYSIS AND UPPER LEVELS...

An upper level low along the north Gulf coast continues to spin
up showers and isolated thunderstorm activity over the coastal
waters. A few lightning strikes were observed south of Seward and
east of the Barren Islands this morning, with more potential for
isolated thunderstorms into Prince William Sound and Cordova into
this afternoon. Elsewhere general coverage of the showers has
diminished a bit from this time yesterday afternoon, and this
trend is expected to continue as the upper level low supporting
the activity weakens.

A second upper level low and associated surface low are over the
western Bering this afternoon. These features are slowly moving
south and associated precipitation is not expected to advance
eastward any further than the Adak and Atka area. Weak upper level
ridging between these features is building across the central and
eastern Bering, allowing for lots of sunshine in those areas.

&&

.MODEL DISCUSSION...

Model agreement is good for the first day to day and a half of
the short-term forecast, but rapidly diminishes regarding a
North Pacific low strengthening south of the Eastern Aleutians on
Saturday. A separate low to the east of the main circulation will
approach the Alaska Peninsula from the south on Saturday. It will
rapidly weaken as it does so, eventually becoming a trough of low
pressure by Saturday afternoon. That said, there are quite a few
differences in how long the trough persists north and east of the
main low, which impacts the wind and precipitation forecasts for
the Alaska Peninsula and Eastern Aleutians.

The 18Z NAM has come into much better agreement regarding the
strength and track of the main low as it moves northward into the
Bering on Sunday. The 12Z NAM was much stronger and faster than
the other models, making it a severe outlier. Regardless, the
agreement on that storm has improved markedly today, which in
turn leads to higher confidence on precipitation timing and wind
forecast Sunday night into Monday, which is the next big low
expected to impact southern Alaska. More on that below.

&&

.AVIATION...
PANC...VFR conditions will persist with an occasional light shower
or two moving over the airport. Similar to yesterday, it looks
favorable for Turnagain Arm winds to bend into the terminal this
evening, then die off over night. The overall shower activity will
diminish overnight into Friday. As more breaks in the clouds
develop, could see some patchy stratus (IFR) and fog in the
vicinity of the airport.

&&

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

A vertically stacked (the surface low and upper level low are on
top of each other) low south of the Kenai Peninsula will continue
to bring showers to Southcentral. Isolated thunderstorms are
possible in Prince William Sound and over the western Kenai
Peninsula as this is where the maximum instability is. GOES-17 Day
Cloud Phase Distinction Band shows ample vertical growth occurring
inside of towering cumulus (TCu) clouds across those aforementioned
areas. Due to this low being vertically stacked, it will continue
to weaken through Friday afternoon.

A second low will move northward from the North Pacific into
Bristol Bay. This low will bring another round of rain and wind to
the area. The primary core of the jet will remain east of the low
center and create the potential for an Atmospheric River (a train
of moisture that has a tropical/subtropical connection) in
Southcentral beginning Sunday evening. Latest guidance suggests
the core of this Atmospheric River will remain east of Anchorage
towards the northeast Gulf. Comparing today's runs to yesterday's
runs, the GFS shows the most consistency with keeping the core of
the moisture to the east of Anchorage with today's ECMWF runs
trending towards yesterday's GFS runs. The GEM showed a large
degree of consistency with the GFS and thus a GFS/GEM blend was
the preferred solution.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3:
Friday through Monday morning)...

A surface low in the Gulf of Alaska is bringing some instability
and showers to the Kuskokwim Valley and Bristol Bay today. This
low is expected to slowly shift to the east and an upper level
ridge will move over the region beginning Friday. Expect improving
weather with partly cloudy skies across Southwest Alaska for the
beginning of the Labor Day weekend. By Sunday, a strong storm will
begin to move up from the south across the Aleutian Chain. High
level clouds will begin increasing across the area late Saturday.
By Sunday, a front associated with this low will move onshore in
Bristol Bay and the Kuskokwim Delta, making a wet and windy Sunday
and Monday for Southwest Alaska. Models are still varying on the
track of the low, but the current track could bring another round
of high seas for the Bristol Bay and Kuskokwim Delta coastline, so
it bears watching as the weekend approaches.

&&

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

A low pressure system continues to linger over the western
Aleutians, bringing rain and some gusty winds. This system will
continue to slowly weaken over the next couple of days, keeping
rain in the area through Friday night and giving a brief break
to the central and eastern Aleutian Islands. On Saturday, the
atmospheric energy will move into the base of the upper level
trough, supporting a developing low in the Northern Pacific.
Current model track has this low crossing the Aleutian Chain on
Sunday morning, bringing another round of wind and rain to Atka
and eastward. There is still some uncertainty on the exact track
and strength of this storm, with a lot of difference in the
central pressure values between models and run to run. Regardless,
this storm should be weaker than the one that moved through last
weekend, but how much weaker is still not clear at this time.

&&

.MARINE (Days 3 through 5/Sunday through Tuesday)...

There is increasing confidence that a gale force low will move
northward across the central/eastern Aleutians on Sunday. Storm
force winds and waves are a possibility with this system as there
is still model discrepancy with the strength of the storm. Gales
are likely to continue with this low as it moves into the northern
Bering on Monday but should dissipate Tuesday. The front
associated with the aforementioned low will enter the northwestern
Gulf Sunday. As it propagates northward across the Gulf into
Monday, gale to storm force winds are expected before it
dissipates Monday night. Models are picking up on a small craft
advisory front entering the western Aleutians on Tuesday and
moving to the east into Wednesday.

&&

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

The amplified pattern will continue through Wednesday. A large
area of low pressure over the eastern Bering will bring widespread
rain and winds to much of the Bering Sea, the Aleutian Chain and
across the Gulf and Southcentral on Monday. Rain will continue
into Tuesday for much of the area as that low moves northward.
An upper level ridge continues to strengthen over the North
Pacific/eastern Gulf at this time, which will begin to interact
with the trough digging down from the Bering and into the North
Pacific. This will set up an opportunity for an atmospheric river,
a long and narrow region in the atmosphere - like rivers in the
sky - that transport most of the water vapor outside of the
tropics. This will make its way into the southwestern Gulf on
Tuesday and should start precipitation through Thursday. However,
model discrepancies still haven't resolved if the moisture will be
over Southcentral or if it will get shunted to the southeastern
part of the state. All in all, expect a wet and windy, fall-like
pattern, as we move further into September.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...ED
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LF
MARINE/LONG TERM...CK