Storm Warning 92, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
524 AM AKDT Fri Sep 25 2020

.ANALYSIS AND UPPER LEVELS...

Looking at the current satellite and water vapor imagery, the jet
stream remains far off to the south of Alaska with the core of the
energy being aimed at southern British Columbia and the northwest
United States. As a result, a broad longwave trough is currently
in place across much of Alaska with little waves of upper level
energy embedded within this longwave trough. No significant upper
level shortwaves are present. This is resulting in low ceilings
currently across the Anchorage Bowl and the Kenai Peninsula with
occasional showers from time to time. More rigorous showers were
observed south of Cordova moving westward towards Prince William
Sound this morning. Patchy fog has also been observed around the
Anchorage Bowl especially Birchwood where visibility dropped to
1/4 to 1/2 mile at times. Gulkana and some areas in the Copper
River Basin and Palmer in the Matanuska Valley also observed some
time periods of fog. With the persistence of the pattern in place,
expect this to linger around for a time this morning.

Further west, the weather remains quiet with general northerly
flow across Southwest keeping the area mainly dry. The only area
where reflectivity was showing up on current radar was the Bristol
Bay area from Iliamna to King Salmon where a light brief shower
could occur. Over the Bering, cool air continues to flow
southward keeping the region quiet with just cumulus clouds
covering a decent portion of the area especially the western and
central Bering. A weak low south of Adak will continue moving
southward and away from the central Aleutians this morning and not
be much of a factor.

&&

.MODEL DISCUSSION...

Model guidance remains in agreement on a broad longwave trough
over much of Alaska and the Bering for the next two days. However,
there are very subtle shortwaves rotating around a dissipating low
currently in the Gulf. Due to how subtle these upper level
shortwaves are it will be challenging to forecast exactly how each
one will affect Southcentral and when any rain will occur.
Basically, expect the cloudy and wet pattern to continue with the
occasional showers. Confidence continues to improve and increase
with the significant low that will move up into the Gulf of Alaska
Saturday through Sunday as model guidance continues to come
together on a solution. The main issue remains the exact placement
of the low which have impacts down to the local level on what
occurs. This will continue to be fine-tuned as we get closer.

&&

.AVIATION...
PANC...LIFR to IFR ceilings have moved over the terminal
overnight. The threat for patchy fog is also possible across the
terminal this morning with nearby areas reporting fog. This is
expected to continue through the late morning before lifting up to
MVFR conditions for the late morning into the afternoon. Any
showers that move over the terminal late morning into the
afternoon hours have the capability to give a better chance for
the MVFR conditions for ceiling or even VFR conditions for a time.
The risk for low ceilings to maybe IFR again exists for Saturday
morning after the showers end.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Sunday Night)...

A weak, diffuse low in the Gulf continues to direct Gulf moisture
and associated rainfall into Southcentral this morning. The
easterly flow is mostly favoring the mountains, Prince William
Sound, and the western Susitna Valley. An occasional spritz of
light rain has been making it over the Chugach to wet the roads as
well this morning. The primary weather concern this morning is
patchy dense fog that has been developing and dissipating across
portions of Anchorage, the Glenn Highway, and the Matanuska
Valley. Anywhere light rain develops, the fog situation improves.
Expect the patchy dense fog to persist into the mid-morning,
dissipating by midday. Otherwise, today will be similar to
yesterday, though with additional dry time. Nevertheless,
scattered showers will persist throughout the day. Tonight, the
weak low in the Gulf drifts west into southern Cook Inlet. As it
passes, precipitation chances will increase for tonight from
Anchorage south, then increasing everywhere on Saturday as the
low and associated front move north into the coast.

Late Saturday afternoon into Saturday night, all eyes turn south,
to a strong North Pacific low that will be moving into the
southern Gulf. The latest model trends have shifted the track of
the center further west since yesterday, and most of them now
strengthen the low into the lower 960s mb area as it approaches
the Gulf. This means the low is now forecast to produce at least
storm force winds both ahead of the low along the coast with the
barrier jet and the offshore southern Gulf behind the low as a
cooler air mass works in behind the storm. The low center will
move into the northern Gulf, somewhere near Middleton Island on
Sunday, then drift into Prince William Sound Sunday night. Along
this track, periods of heavy rain are likely in Kodiak Saturday
night into Sunday. The strong easterly flow north of the low will
also produce a few inches of rain across most Prince William Sound
locations as well. There looks to be a brief period the winds
through Turnagain Arm will increase Saturday afternoon and evening
with the leading front, but fortunately the strong low staying
east of the area, heading into Prince William Sound should mean
the winds across the Hillside will stay well below damaging
levels. This of course assumes the eventual track of the low
remains where it is. Thus, right now the primary threats
associated with the low will be very strong winds across the open
waters of the Gulf, and heavy rain over the eastern Kenai
Peninsula, Prince William Sound, and Kodiak Island.

The weakening low will gradually drift into mainland Southcentral
Sunday night, but most of the rainfall will remain along the
coastal mountains. Looking ahead, the pattern remains active with
another strong front expected Tuesday night. As far as
temperatures go, this active pattern and storm track will keep
temperatures much warmer than average, especially with overnight
lows. Each low and front from Saturday through the upcoming week
will bring a reinforcing warm air mass into the area, which should
mean even lows in the 30s will be few and far between except over
the Copper River Basin.

&&

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

The low over the Gulf of Alaska is continuing to bring northerly
flow and cool temperatures to the Southwest. Scattered showers
will remain possible with this pattern through Saturday afternoon.
After this time, the low will weaken as it moves inland, bringing
showers across the Alaska Peninsula and further northwest towards
Bristol Bay and the Kuskokwim Valley. By Sunday afternoon, drier
conditions should prevail, with a slight increase in temperatures
for the area as these conditions are shunted into the area by a
low in the Gulf.

&&

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

A low in the northwestern Bering, with shortwaves surrounding it,
will continue to bring showery activity to the area. Winds should
not exceed small craft criteria as this system propagates into
the North Pacific late Saturday. Saturday morning, a low moving
north into the Gulf of Alaska will cause northerly flow on the
backside of this low, into the north and eastern Bering. Expect
small craft criteria winds with this feature and an increase in
precipitation through Sunday as it moves into the central Bering
and Aleutians.

&&

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

There has been a large change in the forecast for this weekend.
It now looks like a large deep storm system will develop over
the Pacific and track right into the heart of the Gulf of Alaska
Saturday night through Sunday. Expect widespread gale to storm
force winds across all of the Gulf waters, with a chance of small
areas of hurricane force winds. This storm will rapidly weaken as
it pushes onshore along the north Gulf coast Sunday night/Monday
morning, with winds coming down fast across the Gulf. However, a
second storm system will follow not far behind. This storm will
take a farther west track toward the Alaska Peninsula. There is
quite a bit of uncertainty in the exact track and strength, but
would expect a good chance of gale to storm force winds from the
Gulf to Alaska Peninsula and Bristol Bay.

Looking west to the Bering Sea and Aleutians, a polar shortwave
drives southward across the eastern Bering into the North Pacific,
with gusty small craft northerlies Sunday through Monday. A
low will then pass south of the western Aleutians Monday through
Tuesday. For now the forecast reflects small craft winds, but
there is potential for gales right along the western Aleutian
chain.

&&

.LONG TERM FORECAST (Days 4 through 7/Monday through Thursday)...
Expect active weather with large and strong storm systems during
the extended forecast period thanks to a highly amplified pattern.
Beginning on Monday, a deep low which moves into the Gulf Sunday
will rapidly weaken and push onshore along the north Gulf coast.
This will likely bring a quick shot of moderate to heavy rain to
the Gulf coast and Prince William Sound. Meanwhile, the next
short-wave rounding the base of the trough over the North Pacific
will cause the downstream ridge to amplify. This will shift the
jet and storm track westward. As a result, expect the next storm
system to approach the Alaska Peninsula and Kodiak Island Monday
night through Tuesday. While there is some uncertainty in
strength and track of this system, all solutions indicate a large
storm system affecting the southern Mainland of Alaska Tuesday
night through Wednesday night. Widespread strong winds and heavy
rain are likely. This low then exits into the Bering Sea while the
next storm system moves up out of the Pacific. The spread in
solutions is even larger with this low, but chances are it will
also impact the southern Alaska Mainland.

Out west, the remnants of Tropical Storm Dolphin will transition
to an extratropical low and pass just south of the western
Aleutians Monday through Tuesday. This will bring a quick shot
of rain and wind. Otherwise, with the primary storm track much
farther east, conditions across the western Bering and Aleutian
Chain will remain relatively quiet.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MV
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CK
MARINE/LONG TERM...MK/BG/SEB