Gale Warning 511, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
541 AM AKDT Wed Sep 30 2020

.ANALYSIS AND UPPER LEVELS...

The storm force low remains well on track to the south of Kodiak
Island this morning. The storm's warm front continues to push
north, impacting the Gulf coast and Bristol Bay with widespread
rain shown on the Middleton, Kenai, and King Salmon radars. Storm
intensity peaked around 12am with Amatuli Island measuring gusts
up to 75 mph. Sitkinak Island also measured gusts up to 75 mph
Tuesday evening. Bouy 46080 to the east of Kodiak City measured
peak wave heights of 23 ft. (rain totals)

The synoptic pattern continues to amplify, allowing the parade of
lows to continue to travel north from the central pacific. The
trough will continue to take on a negative tilt today as a ridge
build over western Canada. Low level water vapor imagery shows
dry, hot air building over the western United States with a long
moisture fetch on its windward side. At the base of the trough the
next low that will impact the western Gulf again Friday morning
can be seen forming on satellite imagery. Over the western Bering,
high pressure continues the pattern of calm winds.

&&

.MODEL DISCUSSION...

Models remain in very good agreement on the strength and track of
the low impacting the western Gulf, AKPEN and Bristol Bay today.
There are slight differences between the models in regards to the
next system impacting the western Gulf and Akpen Thursday and
Friday. Slight variations in the location of the warm front will
lead to differences in rain totals for the coastal Kenai Peninsula
and Kodiak Island. By the weekend models are in good agreement
that the jet will become more zonal. This will transition the
storm pattern to be out of the west/southwest, rather than the
southerly storm track that has been observed over the past week.
As such, the GFS and EC are in good agreement that the next system
will follow westerly track to the south of the Aleutian Chain on
Saturday before turning northwest and moving towards the Cook
Inlet on Sunday.

&&

.AVIATION...
PANC...VFR conditions will persist through the TAF period. A storm
in the Gulf will lead to northerly winds Wed until a front moves
over Anchorage Wed night. This will bring Turnagain Arm winds and
rain into the terminal.

&&

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

A storm force low complex south of the AK Pen winds and moves
north to over Bristol Bay this evening. Its associated front,
currently extending east across Kodiak Island and offshore Gulf
marine areas, will move north across Southcentral late this
afternoon/early this evening. Winds are expected to diminish
across the Gulf waters. Strong gap winds across Turnagain Arm and
Portage Valley will increase significantly this evening and
continue strong overnight tonight. As the front lifts north gusty
southeast winds will bend into the Anchorage Bowl and the gusty
winds in the Matanuska Valley will favor a southeast wind out of
the Knik River Valley. Gusty south winds are also expected to
develop along the Copper River this evening with pressure rises on
the backside of the front. Most of the aforementioned winds will
peak overnight tonight, but will remain fairly gusty through
Thursday. Moderate rainfall will spread to the southern and
eastern Kenai Peninsula this morning and to the northern Gulf this
afternoon. Rain will become showery in these areas after frontal
passage.

The next storm force low complex heads north Thursday night and
will bring another round of moderate to heavy rain to Kodiak
Island and storms to the Gulf waters. Winds across Turnagain arm
and through Portage Valley, and over higher elevations near
Anchorage, are expected to ramp up Thursday night and continue
strong into Friday. A change in the orientation in the pressure
fields Thursday night will favor a Matanuska wind vice a Knik
wind. Southerly winds are also expected to diminish along the
Copper River.

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

The first strong low to impact the area is approaching the
southern side of the Alaska Peninsula southwest of Kodiak Island
this morning. The center of the low will make landfall near
Chignik this evening, then will move to the waters in between the
Pribilofs and Nunivak Island by Thursday evening, then will be
absorbed into a second low Thursday night. The leading front is
moving into the Kuskokwim Mountains, but as the front tries to
cross the mountains, it will weaken substantially as it leaves
much of its moisture behind. Gale force winds ongoing across
Bristol Bay this morning will shift westward with the front, with
the strongest winds shifting to the typical mountain gaps,
especially the Kamishak Gap, which will persist continuously over
the next 3 days. Downslope winds will keep the rain off and on for
most of Bristol Bay, with upsloping north and west of Dillingham.
But for much of the rest of the day, the rain will be scattered
for much of the area. Overnight, the center of the low sitting off
the coast of Kuskokwim Bay will stall out, so upslope
precipitation will continue in the Kuskokwim Mountains, while much
of the rest of the area stays under cloudy and windy conditions.
This will keep low temperatures well above seasonable averages.

On Thursday, as the first low drifts northwestward away from most
of the area, the second low will make its presence known
increasingly as the day progresses. Rain chances will increase
associated with the second low across Bristol Bay Thursday
afternoon into Thursday night. The best chances of precipitation
so far for the Kuskokwim Delta will be on Thursday night as the
second low spreads rainfall northward with the help of the first
low. Gap winds will also re-increase rapidly late in the day
Thursday into Thursday night with gusts to 70 mph expected through
Kamishak Gap and Lake Iliamna Thursday night.

On Friday, the second low will cross the Alaska Peninsula and move
into the Bristol Bay waters in the morning. The low center will
stall out over the Bristol Bay for much of the day, until a
secondary wave moves in and finally makes the low resume its
northwesterly track up the coast. Thus, expect Friday to be the
wettest day for most of Southwest Alaska outside of the strong
downslope areas. The Kamishak Gap winds will also shift from
easterly to southeasterly, so the strong winds will extend into
the Lower Kuskokwim Valley Friday afternoon and gradually diminish
Friday night.

&&

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

Two storms are expected to track through the Eastern Bering from
southeast to northwest along the coast the next two days. This
will keep the eastern half of the Bering very active through
Friday. For the western half of the Bering, expect northerly
winds continuously over the next few days. The cool, but not
particularly cold air mass moving down out of Siberia will keep
instability showers also continuously through the Central and
Western Aleutians. Temperatures will be seasonable to slightly
below average through Friday.

&&

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

Bering/Aleutians

A storm system will move northwestward across the AKPEN into the
eastern Bering Sea on Friday. Winds will remain under gale force
over the open water. There is a chance of gale force winds along
Cape Constantine and Cape Newenham due to the localized funneling
of winds. Confidence is low in the exact forecast of these winds
as there are significant differences in solutions with regards to
the placement and strength of the low center. The general trend is
this storm system will move into the northeastern Bering Sea and
weaken while remaining stationary through Sunday.

Gulf of Alaska

A storm system will move northwestward through the western Gulf
and cross the AKPEN into the Bering on Friday. Widespread gale
force south-southeasterly winds associated with this storm system are
expected in the Gulf on Friday. These winds will ease to below
gale force Friday night and on Saturday. However, a second storm
system will move into the southern Gulf on Sunday. Confidence is
very low in the forecast for Sunday as there are significant
discrepancies between solutions on the placement and intensity of
the low center.

&&

.LONG TERM FORECAST (Days 3 through 7/Friday through Tuesday)...

The pattern remains highly amplified as a deep negatively-tilted
upper trough extends across the Bering/Aleutians and the AKPEN
while an upper ridge extends across the west coast of Canada. A
strong storm system will move northwestward from the Gulf of
Alaska into the eastern Bering/Aleutians. This low will
potentially bring 2-3" of rain to Kodiak and the southern Kenai
Peninsula. Widespread gale force winds are expected in the western
Gulf on Friday as this system moves over Kodiak and the AKPEN.
This storm system will move into the northeastern Bering Sea and
weaken into a trough through Sunday. A second storm system will
move into the southern Gulf of Alaska on Sunday, though forecast
confidence with respect to this second system is low as there are
significant differences between solutions in regards to the
placement and intensity of this low. The overall trend is this low
will move into the northern Gulf/southern Alaska mainland and
weaken into a trough while remaining nearly stationary, however
there is a high degree of uncertainty where this will occur due to
the significant model differences. A third storm system will move
into the western Bering/Aleutians Tuesday afternoon, though
confidence is very low as there are significant discrepancies in
solutions with respect to the placement and intensity of this low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm 130 352.Gales 119 120 125 127 130-132 138-140 150
                             152 160 165 170-172 180 351 414.
FIRE WEATHER...NONE.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
Issued by National Weather Service Juneau AK
1228 PM AKDT Wed Sep 30 2020

AKZ101-111-011400-
Anchorage-Matanuska Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, and Chickaloon
1228 PM AKDT Wed Sep 30 2020

...STRONG WINDS ACROSS TURNAGAIN ARM AND THE ANCHORAGE BOWL THIS
EVENING THROUGH THURSDAY NIGHT...

A series of storm force lows and fronts head north and bring
bouts of strong winds across the Anchorage Bowl and Turnagain Arm.
The first of these two storms will produce strong easterly winds
with gusts in the 60 to 70 mph range along Turnagain Arm and
higher elevations this evening. Lower elevations will experience
southeast winds of 20 to 45 mph. Winds with this first system
will peak overnight tonight, but will remain fairly gusty through
Thursday. The second storm moving north Thursday night into Friday
will bring another round of strong winds across Turnagain Arm and
higher elevations.

Please check www.weather.gov/anchorage for updates.

AKZ171-011400-
Kodiak Island-
Including the city of Kodiak City
1228 PM AKDT Wed Sep 30 2020

...HEAVY RAIN TO CONTINUE THROUGH THE WEEK...

Heavy rain will continue for Kodiak Island through the end of the
week. The eastern side of the island has reported 3 inches of
rain over the past 2-3 days and another 3-4 inches is expected
through the end of the week. This will increase the potential for
small stream flooding. Additionally, heavy rain falling on already
saturated soils will increase the chance for mudslides in areas
with steep and unstable slopes.

Please check www.weather.gov/anchorage for updates.

AKZ125-011400-
Western Prince William Sound-
Including the cities of Whittier, Seward, Girdwood,
and Moose Pass
1228 PM AKDT Wed Sep 30 2020

...HEAVY RAIN TO CONTINUE THROUGH THE WEEK...

Locally heavy rain is forecast for Seward and surrounding areas
with totals of between 4 and 7 inches by Saturday morning. This
will lead to sharp rises along the Resurrection River and other
small streams in the Seward area. The Resurrection
River is expected to crest Friday into Saturday up to action stage.
Other small streams in the Seward area are likely to rise
approaching bankfull levels by the end of the week as well.

Rain will cause considerable ponding of water in low lying areas,
near culverts, and along small creeks and drainages, a portion of
the runway at the Seward airport may also become covered with
water from the Resurrection River. Significant flow through the
Lowell Creek Diversion tunnel is also expected.

Please check www.weather.gov/anchorage for updates.