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000
FXAK02 KWNH 182356
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
755 PM EDT Thu Apr 18 2024

Valid 12Z Mon Apr 22 2024 - 12Z Fri Apr 26 2024

...Stormy pattern expected over the Aleutians/Bering Sea next
week...


...Overview...

Latest guidance seems to be converging toward the idea of mean
ridging aloft persisting over most of the mainland next week, with
some lingering uncertainty over whether weak shortwave energy
initially over/near the southwestern mainland may pass through or
around the ridge.  The southern part of the elongated upper
trough/low should support a wave tracking toward or a little north
of Haida Gwaii with some potential influence on the southern
Panhandle.  Meanwhile the models and ensembles are stabilizing for
the general idea of a slow moving deep-layer low over the
Aleutians into southern Bering Sea.  The overall system should be
strongest during the first half of the week while it is over or
near the western Aleutians, with gradual weakening thereafter.
Areas of strong winds will accompany the system while one or more
leading fronts will help to enhance precipitation from the eastern
Aleutians into the Alaska Peninsula.

...Guidance Evaluation and Preferences...

Both the dynamical models/ensembles and ECMWF-initialized machine
learning models (MLs) continue to show difficulty with the finer
details of the initial elongated upper low trough extending over
and southeastward from the southwestern corner of the mainland.
There is still a mix of solutions for the southwestern mainland
energy, ranging among progression across the mainland/weakening in
place/lifting northward around the ridge.  Meanwhile there is a
lot of latitude run-to-run variability and spread for the
remainder of the system that tracks across the Northeast Pacific.
With the MLs not really favoring one portion of the dynamical
guidance spread, preference sides with an intermediate track
toward or a bit north of Haida Gwaii.  This would be a compromise
between the southern 12Z and northern 00Z ECMWF runs.  GFS runs
lean on the southern side while the 12Z CMC is close to the 00Z
ECMWF.

Guidance is making progress in stabilizing the solution for the
strong storm likely to be near the western Aleutians early next
week.  Latest trends are gradually increasing confidence toward
the deeper half of yesterday`s spread, with recent GFS/ECMWF runs
generally waffling among the mid 960s - low 970s mb for greatest
depth around late Monday-Tuesday and 00Z MLs ranging between the
upper 960s and 970s.  The UKMET is in this range while the CMC is
on the weak end.  The manual forecast depicts a 972-975 mb system
valid 12Z Tuesday to provide a deeper trend from continuity but
accounting for ongoing variability.

Beyond Tuesday, the ensemble means maintain good agreement and
continuity that the overall area of low pressure should wobble
very slowly eastward over the Aleutians/southern Bering Sea.
Operational model runs lean a bit west of the means late next
week, with the CMC tending to be on the western side of the
envelope at most valid times.  MLs span most of the model/ensemble
mean spread but with an average closer to the means.  The mid-late
week forecast for specifics at the surface and aloft have lower
predictability, given potential input of dynamics from Siberia
and/or the western Pacific.  There will be various possibilities
for specifics, whether maintenance of the initial surface low or
redevelopment in some form, along with multiple fronts whose
character may change from day to day.  Consensus suggests the most
likely low track will be across the southern half of the Bering
Sea by mid-late week.

Aside from aforementioned issues with the ultimate path of initial
southwestern mainland shortwave energy, model/ensemble guidance is
generally better clustered today toward the mainland mean ridge
aloft holding onto a southeast-northwest orientation through late
next week.  This is in contrast to yesterday`s GEFS/CMCens that
depicted broader southwesterly upper flow by next Thursday.

Based on guidance preferences and data currently available
locally, today`s forecast started with a blend of the 12Z/00Z
ECMWF, 12Z/06Z GFS, and a small weight of the 12Z CMC for the
first half of the period.  This approach, with a little editing,
yielded good definition for the deep Aleutians/Bering Sea storm
along with the desired intermediate track for the weak system over
the Northeast Pacific plus a tempered depiction of initial
southwestern mainland energy aloft.  The multi-model blend already
accounted for positional spread over the Aleutians/Bering Sea
fairly well, so days 7-8 Thursday-Friday added only 20-30 percent
total weight of the means (12Z GEFS/CMCens).

...Weather/Hazards Highlights...

The strong storm near the western Aleutians early next week will
be the dominant system of interest through the period, though it
should gradually trend weaker after Tuesday.  A leading front will
likely focus an axis of anomalous moisture into the eastern
Aleutians by Monday, with this area highlighted for heavy
precipitation in the Days 3-7 Hazards Chart.  As this and possibly
additional fronts push eastward, the precipitation emphasis will
likely shift into the Alaska Peninsula and surrounding areas by
mid-late week.  Some of this activity may also be heavy but
confidence in specifics is lower at that time frame.  Meanwhile
this storm should also produce areas of strong winds across the
Aleutians and Bering Sea.  Forecast wind speeds are currently
below hazard criteria and thus no high wind area is depicted.
Highest wind speeds should be early in the week, ahead of the
leading front and just to the south of the surface low center.
Surface low/front details over the Aleutians/Bering Sea become
more uncertain later in the week but there is a good signal for
the general persistence of low pressure and potentially additional
fronts.  Some lingering precipitation is possible along the
southern coast early in the week due to the initial shortwave
over/near the southwestern corner of the mainland.  The weak
system tracking across the northeastern Pacific may bring a period
of mostly light precipitation to the southern Panhandle.  This
part of the forecast has low confidence though, as the southern
side of the guidance spread would be far enough south to keep the
Panhandle mostly dry.

Most of the state should see above normal temperatures during next
week, with the highest anomalies tending to be over the Interior.
To the north of a front settling over the central mainland, cold
high pressure over the Arctic may keep temperatures near to
somewhat below normal across the North Slope.  Locations near the
Southcentral coast and northern Panhandle may also see near or
slightly below normal readings.

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

$$