Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 250648
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...RESENT
National Weather Service Anchorage AK
1048 PM AKDT Thu Sep 24 2020

.ANALYSIS AND UPPER LEVELS...

Southcentral Alaska remains in a sloppy pattern with the
weakening complex low in the Gulf continuing to pump moisture
into the area. There is not much downsloping left with the weak
flow aloft so a lot of this moisture is able to precipitate out
inland on the lee side of the mountains. While there is not in the
way of higher surface pressure in western Alaska, northerly flow is
bringing in drier air from the interior of the state and is keeping
skies much clearer over there today. A cold upper level low is
just west of the Bering Strait and colder air is streaming into
the Bering Sea with northerly flow and associated cumulus clouds.
The weak low near Adak should drift south of the Aleutians and not
be much of a factor to the area.

&&

.MODEL DISCUSSION...
Models have come into good agreement with the movement of the
small lows embedded in the larger Gulf low complex. The next
significant system will be moving into the Gulf early next week
and models are rapidly converging on a common solution to this one
as well. There will be some details of importance that will need
to be refined as this storm develops, the confidence is increasing
that this will become a major low for the Gulf and surrounding
areas.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Ceilings
should settle in the 3500 to 5000 ft range overnight, though MVFR
ceilings are possible overnight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: This
afternoon through Sunday)...

A large area of low pressure across the Gulf continues to spin waves
northward into Southcentral. This trend will continue through
Friday. While the more persistent rainfall will occur around the
Prince William Sound area much of the rest of the area will likely
have periods of rain through Friday including the Anchorage and
MATSU valleys. The most impressive wave is currently a low pressure
spinning just west of Juneau and will move north and west towards
Southcentral tomorrow with best chances of rain Friday afternoon
through Saturday morning. Saturday a low pressure east of Kodiak
Island will move west into Southwest Alaska. This will drag a weak
front across the northern Gulf enhancing coastal precipitation from
Kodiak Island to the Prince William Sound. As this front weakens a
coastal ridge will briefly develop Saturday afternoon through Sunday
morning which will increase winds across the Turnagain Arm down
through Portage and the Passage Canal. These winds are expected to
turn southward into the Cook Inlet, therefore, not expecting any
impacts to Anchorage or the hillsides.

On Saturday evening into Sunday, models are now in good agreement
with a strong low moving into the Gulf with storm force winds and a
good swath of moderate to heavy rain. GFS hints at some Hurricane
force winds, but uncertainty at this point if that will happen. We
went with a blend of models leaning more towards the NAM solution.
Most impacts are expected to be offshore in the Gulf, but some
coastal locations may feel some impacts from winds and rain from
Kodiak Island through Prince William Sound.

&&

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

A low in the Gulf is continuing to bring generally cool,
northerly flow to much of Southwest Alaska as has been the case
the past several days. Scattered showers remain possible for the
immediate Kuskokwim Delta coast and areas near the Alaska Range.
This pattern will persist through Saturday afternoon, at which
time the aforementioned weak low will track inland over the
Bristol Bay region, spreading showers from east to west into early
Sunday morning. Drier air will again build into Southwest Alaska
for the remainder of Sunday, with partly cloudy skies expected.
Temperatures will trend downward over the coming days, falling
below freezing overnight across interior areas. They will be
slightly higher near the coast, moderated by increased cloud cover
and higher sea surface temperatures.

&&

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

An upper low centered over the Arctic is leading to the
development of a number of weak lows/troughs within its broad
circulation. Thus, showers will continue over much of the region
through the weekend in a generally northerly flow, although
slightly drier air will be in place over the central/eastern
Aleutians into Saturday. Slightly cooler temperatures in the 40s
are expected.

&&

.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...EZ
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...MK/BG/SEB


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