Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
757 PM EDT Sun Sep 15 2024

Valid 12Z Thu Sep 19 2024 - 12Z Mon Sep 23 2024

...Overview...

Most guidance suggests a stormy period for the Aleutians, with a
couple systems that may produce some enhanced rainfall/winds which
then may reach into the Alaska Peninsula and the southern
coast/Panhandle by the weekend and early next week.  Over the
mainland, expect a lingering diffuse mean trough aloft as of
Thursday (supporting weakening Gulf of Alaska low pressure) after
supporting a stronger storm beforehand.  Then it becomes more
uncertain how flow will evolve over the mainland, with
current/recent solutions varying considerably over how much
ridging or troughing may evolve.  This forecast problem will also
play a role in the evolution of the expected Aleutians systems as
they track eastward.

...Guidance Evaluation/Preferences...

Across the North Pacific and adjacent areas, as of Thursday expect
low pressure to be organizing between Kamchatka and the western
Aleutians while a wavy front extends well eastward across the
North Pacific.  By Friday-Saturday, latest GFS runs are
essentially on their own in developing well-defined low pressure
along this front and tracking it to near the Panhandle.  The
overwhelming cluster of most other dynamical and machine learning
(ML) guidance favors weaker/more sheared dynamics that would keep
any frontal wave weak and suppressed, likely south of Haida Gwaii.
 The 12Z GEFS mean does not have a defined surface low either,
though it lifts its overall front farther northeastward for a time
(though joined to some degree by the UKMET by the end of its run
early Saturday).  With typical spread, consensus shows the leading
upstream system tracking near or just south of the Aleutians
Friday-Saturday--but perhaps a little weaker than 00Z/12Z ECMWF
runs per other dynamical/ML models.  This system may stay defined
into Sunday but then the question mark becomes whether it
continues into the Gulf of Alaska or gets merged into the rapidly
approaching second Aleutians system.  Most guidance aside from
ECMWF runs would suggest the second system could be a little
deeper than the first.  Meanwhile a fair amount of track/timing
spread develops with the second storm.  There is a relative
majority of solutions clustering around the Alaska Peninsula
(Bristol Bay to south/east of Kodiak Island), while minority
options are farther northwest (latest CMCens means, ECMWF AIFS
means, small number of ML models) or slow (GFS runs). 

The mainland forecast starts out rather agreeably with a broad and
diffuse mean trough aloft, but models/means gradually diverge with
time.  Recent ML guidance generally favors some degree of
southeast-northwest oriented ridging that reaches into at least
the southwest mainland by Saturday, which is closest to ECMWF/CMC
runs and the ECens mean, versus GFS runs that drop troughing or
even an upper low into the the southwestern mainland.  However
there is a lot of spread beyond that, as 00Z ECMWF/CMC runs and
the 12Z CMCens mean end up being quite strong with the upper ridge
in contrast to 12Z ECMWF/CMC/UKMET runs that favor more troughing
over the northeastern half or so of the mainland.  In fact, the
12Z ECMWF/CMC ultimately drop the northern tier upper low into the
southwestern mainland to yield a feature by next Monday close to
the position depicted by some other guidance bringing in the upper
low from the Bering Sea/Aleutians.  Thus, broadly speaking there
is somewhat more confidence in an upper low being near the
southwestern mainland by next Monday than in how it gets there. 
Given the spread and variability of guidance, prefer an
intermediate solution that offers some mainland troughing per
continuity but also brings ridging into the southwest for a time
as seen in the majority cluster.

Guidance preferences away from various aspects of the GFS led to a
Thursday-early Saturday blend starting with half ECMWF runs
(00Z/12Z) with the remainder composed of the 12Z CMC/UKMET,
followed by a shift to 40 percent ensemble means (12Z GEFS/00Z
ECens) and lingering input from the two ECMWF runs and 12Z CMC by
Sunday-Monday.  At that time the operational model component was
split among half 00Z ECMWF and the rest 12Z ECMWF/CMC, to temper
the influence of mainland troughing in the latter solutions.  This
approach yielded some aspects of continuity along with refinement
for the Aleutians storms as well as mainland flow aloft.  Today's
guidance shows a better signal for a cold front to develop/push
into the mainland with portions eventually becoming stationary
late in the period.

...Weather/Hazards Highlights...

Weakening low pressure over the Gulf of Alaska should support
lingering light to moderate precipitation over the Panhandle into
Thursday.  Meanwhile areas of rain and higher elevation snow will
be possible over much of the mainland late this week as some
degree of upper troughing persists.  From Friday onward, two storm
systems may track along or near the Aleutians to produce areas of
enhanced rain/winds/waves with effects from these systems possibly
extending into the Alaska Peninsula and then the southern
coast/Panhandle by the weekend and early next week.  The best
signal for highest precipitation totals by Sunday-Monday is over
the Southcentral coast and Panhandle.  Note that there is still
uncertainty for storm track, especially with the latter system.  A
less probable but still possible farther north track would
likewise spread significant weather farther northward than
currently forecast.  Also worth noting, GFS runs would bring an
earlier surge of moisture to the Panhandle toward the end of the
week but for now there is minimal support for that scenario. 
Precipitation details over the rest of the mainland from the
weekend into early next week will depend on uncertain specifics of
flow aloft.

Southern tier locations including the Panhandle will tend to see
below normal highs during the period.  Areas to the north may see
more of a mix of above/below normal anomalies, but with a possible
cooling trend north and west depending on how much upper troughing
evolves over the mainland by the weekend.  Expect morning lows to
be more above normal aside from some modestly negative anomalies
progressing from the southwest through 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