Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
758 PM EDT Mon Sep 16 2024

Valid 12Z Fri Sep 20 2024 - 12Z Tue Sep 24 2024

...Overview...

Latest guidance continues to show a general theme of low pressure
systems tracking over portions of the North Pacific/Bering Sea
late this week through at least the weekend and then focusing more
into the Gulf of Alaska and vicinity early next week, as some
combination of dynamics over the mainland and separate energy
arriving from the Aleutians/Bering Sea yield a trough with
embedded upper low over and south of the mainland.  This would
produce the most active weather from the Aleutians and surrounding
waters to parts of the southern coast and Panhandle.  However
there is still a lot of spread and variability for individual
systems as well as for mainland flow aloft Friday into the
weekend, before a better consensus for troughing develops.

...Guidance Evaluation/Preferences...

The Friday-Saturday forecast over the northeastern Pacific has
become more uncertain with respect to wave development/track and
how much influence there may be on the Panhandle.  24 hours ago
the GFS was on its own in developing a stronger/northward wave
that would bring enhanced moisture to the region, with the machine
learning (ML) models also agreeing with the non-GFS majority. 
Through arrival of the 12Z ECMWF, there was a notable trend of the
UKMET/CMC (and the ensemble means in more muted form) toward some
variation of multiple GFS runs, while the ECMWF and 00Z/06Z ML
models (plus 06Z GFS) remained weaker and at least as far south as
Haida Gwaii.  Dynamics for this feature originate from initially
flat and progressive flow aloft, so predictability for details is
well below average.  At least given ensemble mean trends, it
seemed reasonable to adjust the forecast toward those means as an
intermediate approach to maintain some leeway pending future
shifts in guidance.  The new 18Z GFS remains rather strong but
with a track near Haida Gwaii, while the 12Z ML models have
trended better defined with the wave overall and some track it as
far north as between the Panhandle and Haida Gwaii.

Farther west, latest guidance diverges wildly for what has been
advertised as a leading Aleutians system.  Latest ECMWF/ECens runs
now hold it well back over the western Bering Sea with weaker
frontal waviness possibly breaking off farther east, while other
ensemble means and most ML models generally favor this system
lifting into the Bering Sea and weakening before arrival of the
second system.  This leaves the GFS as a deep extreme over the
Aleutians late week into the weekend, with the 12Z CMC the one
other solution that holds onto steady eastward progression of the
primary system.

The second Aleutians system has somewhat better continuity in the
ensemble means and to some degree the ECMWF/CMC and ML models. 
Note that the 12Z CMCens has shifted somewhat southward toward the
GEFS/ECens.  ML models support the idea of a fairly deep system
around Sunday albeit with some north-south spread, and like
yesterday the majority dynamical/ML cluster reaches near the
Alaska Peninsula by Monday and the Gulf of Alaska by Tuesday.  The
12Z GFS lost the system's dynamics well to the west so it differed
from other guidance, but the new 18Z GFS has returned to a more
plausible solution relative to the majority.

Over the mainland, guidance continues to struggle regarding the
relative strength of upper ridging that may build into
western/southern areas late week into the weekend versus troughing
that may dig in just to the northeast.  ECMWF/ECens runs have
consistently been stronger with the ridge while the GFS/CMC/UKMET
favor more troughing.  ML models have been mixed but overall favor
a solution between the two extremes.  With time a better consensus
develops toward mainland troughing, leading to some combination of
mainland dynamics and the approaching Aleutians system yielding an
upper low over the mainland and/or Gulf of Alaska by next Tuesday.
 

Today's forecast started with a selective model and ensemble mean
mix with a little manual editing to incorporate varying aspects of
continuity or trends depending on the system/feature.  The Friday
into early Sunday period started with a little more than half
total weight of the 12Z ECMWF/CMC with the rest means (12Z
GEFS/CMCens and 00Z ECens).  This yielded the desired intermediate
solution for northeastern Pacific low pressure while accounting
for majority trends (with a hint of continuity) farther west over
the North Pacific/Bering Sea and maintaining a solution reasonably
similar to continuity over the mainland.  By Days 7-8
Sunday-Monday, the blend increased ensemble mean weight to 70
percent while retaining 30 percent 12Z ECMWF which was the one
operational model acceptably close to the means in principle.

...Weather/Hazards Highlights...

A developing northeastern Pacific wave may bring some
precipitation to parts of the Panhandle Friday-Saturday but
confidence remains low for the details due to spread and
run-to-run variability for the system's latitude.  Regardless of
magnitude, relatively higher totals should be over the southern
Panhandle.  A leading system may bring some precipitation and
brisk winds to the Aleutians and vicinity around Friday-Saturday
but again with considerable uncertainty over details.  There is a
little more confidence (though still not above average in absolute
terms) for a potentially stronger system that tracks along or near
the Aleutians around Sunday and then continues near the Alaska
Peninsula and into the Gulf of Alaska by Monday-Tuesday.  This
system would bring wind and rain to the Aleutians/Alaska Peninsula
and surrounding waters followed by increasing precipitation
reaching the Panhandle.  How much moisture reaches the southern
coast will be more sensitive to the storm's exact track. 
Scattered rain/snow will be possible over the mainland, with
western/southwestern areas tending to be drier late this week into
the weekend with some degree of upper ridging likely to build over
the region.

Most of the southern part of the mainland into the Panhandle will
likely see below normal highs during the period.  Areas to the
north may see more of a mix of above/below normal anomalies, with
greater dependence on exact details aloft into the weekend. 
Eventual trend toward more agreeable mainland upper troughing by
the first half of next week may support a cooling trend by that
time if not earlier.  Expect morning lows to be somewhat more
above normal, but some modestly negative anomalies should spread
from the southwest into the Interior late week into the weekend.

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