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FXAK02 KWNH 222355
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
754 PM EDT Sun Sep 22 2024

Valid 12Z Thu Sep 26 2024 - 12Z Mon Sep 30 2024

...Overview...

Latest guidance continues to show a mean upper trough/low setting
up near the western coast of the mainland with the arrival of a
Bering Sea system and incorporation of lingering west coast
energy.  Some degree of troughing should also extend south into
the Pacific from late week onward.  Meanwhile, upper ridging in
various forms should prevail over eastern and northern parts of
the mainland through the period while an upper low/trough will
eject out of the Northeast Pacific/Gulf of Alaska late this week.
A storm system ahead of this ejecting trough may affect parts of
the southern Panhandle late this week.  Then flow ahead of the
Bering Sea system may increase precipitation along the southern
coast by the weekend, followed by still-uncertain details of
potentially very strong Northeast Pacific storm development that
could bring heavier precipitation and strong winds to the southern
coast/Panhandle.

...Guidance Evaluation/Preferences...

Recent model runs have continued to display significant
north-south spread for the northeastern Pacific system that may
affect the southern Panhandle, with some adjustments among
individual solutions.  System evolution and track remain sensitive
to exactly how initial North Pacific shortwave energy interacts
with upper low/trough lingering over and just south of the Gulf of
Alaska.  Ensemble means have tended to be on the northern side of
the spread with a track into the southern Panhandle, with the 12Z
CMC similar to that cluster.  GFS runs had been rather far south
but the 12Z/18Z runs have nudged up toward Haida Gwaii.  The 12Z
ECMWF (north of its 00Z run) is between Haida Gwaii and Vancouver
Island--a track favored by most of the machine learning (ML)
models.  Prefer an intermediate track near Haida Gwaii, near or a
fraction north of the GFS.  This represents only a slight
southward shift from continuity, with trends running a little
faster versus yesterday as well.

The system tracking into the Bering Sea continues to display
fairly typical spread and variability for the type of system and
time frame.  Today`s runs show a little more southwestward
elongation of upper dynamics, pulling the surface system a bit
southwest of continuity (and the average of ML models).  The
latest GFS is a little stronger than most previous runs, bringing
it more in line with established consensus.

The dominant forecast issue from the weekend into the start of
next week continues to be the details of potentially strong
development of low pressure (possibly reaching at least into the
960s mb) initially tracking across the mid-latitude Pacific and
then lifting into the Northeast Pacific/Gulf of Alaska.  In a very
broad sense the operational guidance has been showing such a
signal for development over the past couple days but with a lot of
spread for track and timing.  Recent CMC/ECMWF runs have generally
been more consistent than the GFS (though the 12Z/21 ECMWF was a
weak exception).  Today`s guidance generally delays the evolution
by about a day.  This yields an extra day of the initial Bering
Sea system lingering over the Bristol Bay/Alaska Peninsula region
before deepening Pacific low pressure becomes the dominant feature
south or east of Kodiak Island by Monday.  Corresponding to what
is still considerable ensemble member spread, the means are still
somewhat ill-defined with how this evolution takes place.  However
the remarkable clustering of the means (GEFS/CMCens/ECens plus the
ECMWF AIFS mean) at the surface by day 8 Monday provides some
degree of confidence in surface low existence and a favorable
pattern for increasing precipitation along the southern coast and
Panhandle.  ML models have varied a lot as well, though new 12Z
runs seem to be showing a little more of a deepening signal
overall.  Uncertainty/predictability at that extended time frame
favors a conservative deterministic forecast as a starting point,
about three-fourths toward the ensemble means relative to the
operational dynamical models (though as noted below, requiring
about a half model/half mean blend due to position differences).
Improved clustering and continuity going forward would provide
support for a deeper solution in the official forecast.

Forecast considerations favoring an intermediate solution for
significant features led to the first part of the forecast period
starting with an evenly weighted composite of the 12Z
GFS/ECMWF/UKMET/CMC.  The blend started to incorporate a little
ensemble mean guidance (12Z GEFS/CMCens and 00Z ECens) by day 6
Saturday, quickly settling at nearly half means thereafter with
lingering input from the 12Z GFS/CMC and more so 12Z ECMWF (since
its surface low was a little closer to the means).

...Weather/Hazards Highlights...

A rapidly moving northeastern Pacific system most likely tracking
over or near Haida Gwaii may briefly increase precipitation/winds
over parts of the southern Panhandle late this week.  There is
still a fair amount of latitude uncertainty for the system`s track
so confidence remains low to medium for the magnitude of effects.
Farther west, the system initially tracking across the Bering Sea
will spread light to locally moderate precipitation (mostly rain)
into the Aleutians and Alaska Peninsula late this week.  Leading
flow may start to enhance rain and high elevation snow from the
eastern Alaska Peninsula to Kenai Peninsula and nearby areas
during the weekend.  The southern coast and Panhandle may see a
more significant increase of precipitation by Sunday-Monday in
association with low pressure reaching toward/into the Gulf of
Alaska, while potential also exists for this low to be quite deep
and produce strong winds.  Continue to monitor forecast closely as
details become better refined in coming days.  The central and
northern mainland may see some light and scattered precipitation
late this week, followed by a drier trend.

The forecast continues to show consistently below normal high
temperatures across southern/southwestern regions and the
Panhandle, while upper ridging will likely promote more areas of
above normal highs (with just some pockets of slightly below
normal) over central and northern regions.  Expect greater
coverage of above normal morning lows, with most of the below
normal readings confined to the southwest and the Panhandle.

Rausch

Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html

$$