Small Craft Advisory 641, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
530 AM AKDT Fri Aug 28 2020

.ANALYSIS AND UPPER LEVELS...

An upper level jet extends from the Kamchatka Peninsula through
the North Pacific. A 500 mb low is currently intensifying in its
left exit region. A surface low is vertically stacked with this
upper low and will slowly move to the east through today. Its
associated front will bring moderate rain and gusty southeasterly
winds to the western and central Aleutians. A ridge over the
eastern Bering and Southwest Alaska is leading to quiet conditions
across the region, though the usual low stratus is observed on
satellite along the Kuskokwim Delta in an onshore northwesterly
flow. This upper ridge will move eastward today as a low departs
over the eastern Gulf, bringing settled weather to much of
Southcentral.

&&

.MODEL DISCUSSION...

Models remain in good synoptic agreement through Saturday. A front
will move into the northern Gulf Saturday night and all models
show a Turnagain wind developing overnight into Sunday morning.
There are still differences in the track, though all major models
likewise indicate a strong low moving over the central Aleutians
by Sunday morning. The more southerly GFS track remains preferred
at this time. See the marine/long term discussions for further
details, including potential impacts.

&&

.AVIATION...

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

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

North-to-northwest surface flow is increasing in Southcentral as a
trough strengthens along the north Gulf coast with higher pressure
over the mainland of the state. While some higher level clouds are
around, the drying trend is intact today with the only potential
precipitation in the form of a few afternoon and evening showers
over the mountains.

A front will move over Kodiak Island from the southwest late
tonight into Saturday morning and then move along the north Gulf
coast Saturday night. This will increase winds to gale force over
many of the marine areas at that time. Winds will also increase in
Turnagain Arm on Saturday with the gusty Southeast winds moving
into Anchorage Saturday afternoon and evening. Due to the strength
of the front, the pressure gradient down Cook Inlet will
greatly increase which will turn the winds to the north in town
around midnight Saturday night though they will increase even
more through Turnagain Arm, Prince William Sound and along the
north Gulf coast. Winds turning left out of Turnagain Arm due to a
strong low is more common during winter months, and this is the
first time we have seen this in a while. Combine that with
numerous temperatures dropping into the 30s early Friday morning
and the change in seasons looks to be starting.

By Sunday, an unseasonably strong low will move into Bristol Bay
bringing strong winds and areas of heavy rainfall across
Southcentral. This storm is the first strong Fall storm and there
is the potential for hazardous conditions over parts of
Southcentral so check for forecast updates to stay up on the
latest information.

&&

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

Weather will be fairly quiet today, with high pressure in place
over Southwest AK. Just some afternoon/evening showers are
expected. This will change tomorrow with the arrival of a frontal
system from the Bering Sea. This front is expected to bring
steady rain and will also cool temperatures due to the more
maritime (cooler, moist air) influence it brings with it.

Sunday will see a larger and more powerful weather system move
into Southwest Alaska. Our first significant low pressure of the
fall season (albeit a bit early) is expected to arrive. This low
will bring windy conditions mainly along the coast and rain for
all of Southwest AK. The big impact of this system may be coastal
flooding for Bristol Bay due to sustained storm force winds out of
the southwest over the open water. While the NAM and GFS both
showed storm force winds, there is has been a lot of model
uncertainty with this system. Therefore, we are still watching
what will develop at this point, so a determination on the
possibility of coastal flooding has not yet been made. So, stay
tuned to the forecast in the coming days.

&&

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

A weak frontal system continues to push east through the Bering
Sea/Aleutians Islands. Satellite shows this system currently in
the Central Aleutians. It is expected to push east and into
Southwest AK tomorrow, bringing weak gale force winds as it goes.

Saturday night into Sunday a stronger storm will develop over the
Central Aleutians, with storm force winds forecasted with this
low. This low looks to be the and early arrival of the first big
low of the fall storm season, A couple changes made to the
forecast tonight were to extend low end storm force winds into
Bristol bay and increase wave heights to accompany the stronger
winds. The NAM and the GFS have at least decent agreement on this.
However, models are still moving the low track around and still
getting honed in on it. So, while there is growing confidence in
the forecast, right now confidence is moderate. More updates will
certainly be made to the forecast, so stay tuned in coming days.

&&

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

Model confidence has increased with a low approaching Bristol Bay
Sunday afternoon. The strength of this storm will produce storm
force winds, along the southern periphery of the low, south of the
eastern Aleutians. There is some uncertainty beyond this, with
the potential track up towards the northeast and gales across the
western Gulf Monday. After that, a fairly benign wind pattern is
expected across the Gulf and the Bering Sea for the remainder of
the forecast period. All in all, there will be plenty of
refinement associated with the latter part of this system as we
head towards the weekend, but mariners in the area should pay
close attention.

&&

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

A fairly active pattern is expected into the middle of next week.
A strong upper low moves across the north Pacific Sunday moves
over the southern mainland Sunday evening then weakens. In its
wake, numerous short waves will track eastward across the Alaska
Peninsula and Southern Mainland. The general mean position of the
nearly zonal upper jet remains over the northeast Pacific. After
the remnant of Typhoon Bavi makes landfall Sunday the pattern will
become more showery with support of upper short waves.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 120 130-132 160 170 173 175 351 411-413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AP
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...CK/RMC

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1016 AM AKDT Fri Aug 28 2020

AKZ101-111-121-125-131-135-141-145-291900-
Anchorage-Matanuska Valley-Western Kenai Peninsula-
Western Prince William Sound-Northeast Prince William Sound-
Southeast Prince William Sound-Copper River Basin-Susitna Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, Chickaloon, Kenai, Soldotna, Homer,
Cooper Landing, Whittier, Seward, Girdwood, Moose Pass, Valdez,
Thompson Pass, Cordova, Glennallen, Eureka, McCarthy, Paxson,
Slana, Talkeetna, Willow, and Cantwell
1016 AM AKDT Fri Aug 28 2020

...WET AND WINDY ACROSS SOUTHCENTRAL THIS WEEKEND...

A couple of weather systems will sweep across Southcentral this
weekend bring generally widespread wind and wet conditions. The
first culprit will be a weather front which moves north across
Southcentral Saturday evening/night. This system will bring gusty
easterly gales over the Gulf coast waters with gusty locally
driven winds through Turnagain Arm, Knik River Valley, and along
the Copper River Saturday afternoon/evening. The brunt of the rain
will be along the Gulf Coast.

The next system which will be stronger is associated with a
rapidly developing storm force low which moves into Bristol Bay
late Sunday afternoon. It will bring widespread winds and rain to
Southcentral. Winds in the aforementioned areas will strengthen
with gusts expected in the 60 mph range along Turnagain Arm and
higher elevations Sunday afternoon through evening. Rainfall will
be moderate to heavy at times along the Gulf Coast from Kenai
Peninsula near Seward to Cordova with up to a couple of inches of
rain expected Sunday evening through early Monday.

Please stay tuned for updates

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1056 AM AKDT Fri Aug 28 2020

AKZ152-155-161-291900-
Lower Kuskokwim Valley-Kuskokwim Delta-Bristol Bay-
Including the cities of Aniak, Sleetmute, Bethel, Hooper Bay,
Nunivak Island, King Salmon, Dillingham, Naknek, and Pilot Point
1056 AM AKDT Fri Aug 28 2020

...WET AND WINDY CONDITIONS EXPECTED AS THE FIRST BIG STORM OF
THE FALL SEASON MOVES INTO SOUTHWEST ALASKA SUNDAY INTO MONDAY...

A strong low moving in from the Bering on Sunday into Monday will
produce strong winds, periods of heavy rain, and potentially high
water levels along the Bristol Bay Coast.

Rainfall will move into Southwest Alaska Sunday afternoon
associated with this system, continuing through Monday. Areas
across interior Bristol Bay and the Kuskokwim Valley can expect to
see over an inch of rain through this time period. Southwesterly
storm force winds of 55 mph will move into Bristol Bay by Sunday
afternoon, with strong winds impacting coastal locations from
Togiak to Port Heiden, with the possibility for high water levels.

Please stay tuned for the latest forecast throughout the weekend.

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1155 AM AKDT Fri Aug 28 2020

AKZ152-155-161-291900-
Lower Kuskokwim Valley-Kuskokwim Delta-Bristol Bay-
Including the cities of Aniak, Sleetmute, Bethel, Hooper Bay,
Nunivak Island, King Salmon, Dillingham, Naknek, and Pilot Point
1155 AM AKDT Fri Aug 28 2020

...WET AND WINDY CONDITIONS EXPECTED AS THE FIRST BIG STORM OF
THE FALL SEASON MOVES INTO SOUTHWEST ALASKA SUNDAY INTO MONDAY...

A strong low moving in from the Bering on Sunday into Monday will
produce strong winds, periods of heavy rain, and potentially high
water levels along the Bristol Bay Coast.

Rainfall will move into Southwest Alaska Sunday afternoon
associated with this system, continuing through Monday. Areas
across interior Bristol Bay and the Kuskokwim Valley can expect to
see over an inch of rain through this time period. Southwesterly
storm force winds of 55 mph will move into Bristol Bay by Sunday
afternoon, with strong winds impacting coastal locations from
Togiak to Port Heiden, with the possibility for high water levels.
Elevated water levels are also expected along the Iliamna River
late Sunday into Monday.

Please stay tuned for the latest forecast throughout the weekend.

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1256 PM AKDT Fri Aug 28 2020

AKZ101-111-121-125-131-135-141-145-300030-
Anchorage-Matanuska Valley-Western Kenai Peninsula-
Western Prince William Sound-Northeast Prince William Sound-
Southeast Prince William Sound-Copper River Basin-Susitna Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, Chickaloon, Kenai, Soldotna, Homer,
Cooper Landing, Whittier, Seward, Girdwood, Moose Pass, Valdez,
Thompson Pass, Cordova, Glennallen, Eureka, McCarthy, Paxson,
Slana, Talkeetna, Willow, and Cantwell
1256 PM AKDT Fri Aug 28 2020

...WET AND WINDY ACROSS SOUTHCENTRAL THIS WEEKEND...

A couple of weather systems will sweep across Southcentral this
weekend bring generally widespread wind and wet conditions. The
first culprit will be a weather front which moves north across
Southcentral Saturday evening/night. This system will bring gusty
easterly gales over the Gulf coast waters with gusty locally
driven winds through Turnagain Arm, Knik River Valley, and along
the Copper River Saturday afternoon/evening. The brunt of the rain
will be along the Gulf Coast.

The next system which will be stronger is associated with a
rapidly developing storm force low which moves into Bristol Bay
late Sunday afternoon. It will bring widespread winds and rain to
Southcentral. Winds in the aforementioned areas will strengthen
with gusts expected in the 60 mph range along Turnagain Arm and
higher elevations Sunday afternoon through evening. Rainfall will
be moderate to heavy at times along the Gulf Coast from Kenai
Peninsula near Seward to Cordova with up to a couple of inches of
rain expected Sunday evening through early Monday.

Rivers and streams in the Seward area and Western Susitna Valley
(Yentna and Skwentna) will experience elevated water levels
beginning Sunday night and continuing into Tuesday.

Please stay tuned for forecast updates