Prognostic Meteorological Discussion
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FXAK02 KWNH 272244

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
643 PM EDT Wed Sep 27 2023

Valid 12Z Sun Oct 01 2023 - 12Z Thu Oct 05 2023

...Storm track from south of the Aleutians to the northern Gulf of
Alaska to focus impacts from southern to Southeast Alaska this

...Pattern Overview and Guidance Evaluation/Preferences...

The focus during the first part of the period will be on a
deepened surface low pressure system as it moves into the Gulf on
Sunday. The associated cold front should move into western Canada
early next week but low pressure will be maintained over the Gulf
as additional shortwaves cross western Alaska and the Alaska
Peninsula into the Gulf Monday-Wednesday. Behind this, upper
ridging should build into the Bering Sea and eventually parts of
Mainland Alaska as another upper low moves into the Western
Aleutians by the middle of next week.

Recent guidance continues to show good enough agreement on the
overall upper pattern through the middle of next week, with some
typical spread in the details. Details and timing of individual
shortwaves rounding the Arctic upper low next week remain in
question but are smoothed out by a general model blend days 4-6.
There are some more noticeable timing/placement differences into
the Western Bering next week with the next system. GFS is by far
the strongest/deepest with this system, while the ECMWF is much
weaker/quicker. There is also a lot of uncertainty surrounding an
upper low meandering in the north Pacific through the period,
which some of the guidance wants to lift northward towards the
central/eastern Aleutians next Thursday. The GFS and CMC are most
pronounced with this, which seems related to a much
stronger/blockier upper ridge downstream over Alaska. The WPC
blend for today used a general model blend of the latest 12z
guidance through day 6. Transitioned towards a majority ensemble
mean blend by day 8, with some smaller contributions still from
the deterministic guidance just for some added detail. This
maintained good continuity with yesterdays WPC forecast as well.

...Pattern Overview and Weather/Hazards Highlights...

The deepening Pacific Storm will track south of the Aleutians into
the northern Gulf this weekend, offering possibly significant
maritime and coastal high wind/wave threats along with heavy
rains. The main impacts will spread from the Alaska
Peninsula/Kodiak Island through Southcentral to Southeast Alaska,
with models continuing to show a possible atmospheric river type
set-up. A cold frontal surge in the wake of the main system
passage, as aided by northern stream flow, may also then set the
stage for enhanced gap wind flow through Southcentral Alaska and
vicinity. The rest of Mainland Alaska should stay relatively
quiet/showery but there is potential for possibly more enhanced
precip across Western Alaska but highly dependent on a still
uncertain upper flow pattern. Precipitation chances will increase
with time and from west to east across the Aleutians as the next
system approaches from the West, and possibly even more so if
models trend towards a separate system lifting northward from the
north Pacific.

Temperatures across Alaska should trend cooler with time, with the
greatest anomalies across Southern/Southeast Alaska and the Brooks
Range. The North Slope, especially closer to the coast, should
stay near or above normal during the period, with far western
Alaska trending warmer as well by the middle of next week.


- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Sat-Mon, Sep 30-Oct 2.

Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at:

WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:

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