Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
758 PM EDT Tue Sep 10 2024

Valid 12Z Sat Sep 14 2024 - 12Z Wed Sep 18 2024

...Bering Sea storm expected to bring areas of heavy rain, strong
winds, and coastal flooding to portions of the mainland this
weekend...


...Overview...

Most guidance shows a fairly progressive and energetic pattern
during the period, with at least one and possibly two significant
storm systems affecting portions of the mainland.  The leading and
more confident storm should track over the Bering Sea this weekend
and is most likely to reach near the northwestern mainland by
early Monday. Expect this storm to bring areas of heavy rainfall
to some western and southern areas, along with strong winds over
the west plus coastal flooding concerns along the southwestern
coast.  Guidance suggests another developing system may track
somewhere between the Bering Sea and North Pacific during the
first half of next week, with corresponding uncertainty in effects
on the mainland.  The current majority cluster shows a Bering Sea
low track but generally at a lower latitude than the first system.
 Another low emerging over the far northwestern Pacific may begin
to influence weather across the western Aleutians by next
Wednesday.

...Guidance Evaluation/Preferences...

For the leading storm that should be deepest over the Bering Sea
this weekend and trend weaker as it nears/reaches the northwestern
mainland, there are still some questions regarding
detail/track/timing but with a reasonable majority scenario to
provide a coherent forecast.  Among dynamical guidance, the
ensemble means have been showing a fairly consistent track and the
12Z ECMWF/UKMET/CMC offer the closest comparisons in principle. 
GFS runs have been a lot more inconsistent.  The 12Z version takes
longer to deepen the system and then tracks the low farther south
than other models/means.  The new 18Z GFS offers yet another idea,
placing more emphasis on trailing dynamics aloft and yielding a
much weaker low than other guidance.  The most common theme in
latest runs is for a depth reaching into the 960s mb, most likely
around late Saturday or Saturday night.  The 12Z ICON reaches the
950s mb though.  Looking at latest machine learning (ML) guidance,
the 00Z/06Z runs offered potential for some elongation to exist
over the Bering Sea as of early Saturday before the dynamics aloft
and surface low pressure consolidate farther eastward.  New 12Z ML
runs seem to be trending toward the non-GFS dynamical models with
a better defined system already as of 12Z Saturday, albeit with
some continued north-south track spread thereafter.

The second potential system has a lot more spread/model
variability and thus much lower confidence in specifics.  Latest
ensemble means (with the ECens being relatively more consistent
with the signal in recent runs) suggest that western Pacific low
pressure will track northeastward across the Aleutians and into
the Bering Sea in association with a phased upper trough, with the
surface low reaching near the south-central western coast by early
next Wednesday.  The past couple ECMWF runs are closest to this
idea, while the CMC has switched from a Bristol Bay track in the
00Z run to a slower/northwest track near St Lawrence Island in the
12Z run.  The 12Z ICON is between the 12Z ECMWF/CMC at the end of
its run late Tuesday.  In contrast, latest GFS runs area tracking
a southern tier shortwave ahead of any northern stream dynamics,
leading to a weak North Pacific wave that has little impact on the
state.  00Z through 12Z ML models add to the intrigue, with most
of them favoring a defined wave across the eastern
Aleutians/Alaska Peninsula--but latest AIFS means have tilted
farther north closer to the dynamical means.

For the system nearing the western Aleutians by next Wednesday,
operational models generally side with latest ECens means while
the GEFS/CMCens means are slower.  However the 12Z/18Z GFS runs
represent a slightly slower trend versus earlier runs that strayed
a little faster than most other solutions.  ML models encompass
this east-west spread among the dynamical models/means.

Overall guidance preferences were for the non-GFS majority with
the leading Bering Sea storm and for the ECMWF/CMC (more the
former) plus 12Z CMCens/00Z ECens means for the second system, as
the 12Z GEFS was weaker than the other means.  This led to a
starting blend consisting of the 12Z ECMWF/CMC/UKMET from early
Saturday into early Monday, followed by a quick transition to half
ECMWF/CMC (35/15 weight) and half total CMCens/ECens means by
Tuesday-Wednesday.

...Weather/Hazards Highlights...

Expect strong low pressure tracking across the northern Bering Sea
during the weekend to bring areas of enhanced precipitation to
western areas, and then west to east across southern areas in
association with the storm's frontal system and embedded southern
coast wave. The Day 3-7 Hazards Outlook depicts the areas most
likely to reach hazardous criteria for rainfall, from portions of
the Seward Peninsula south to the eastern Alaska Peninsula on
Saturday and then along/just inland from the Southcentral coast on
Sunday.  Meanwhile higher elevations will likely see snow but with
less pronounced totals.  This storm will also produce strong winds
over the Bering Sea and into parts of the mainland, but with
speeds currently forecast to remain below hazardous criteria. 
However the long fetch of southwesterly flow across the Bering Sea
will bring a threat of coastal flooding, especially to the
southwestern coast of the mainland.  By Monday, lingering low
pressure over or near the northwestern mainland and shortwave
energy aloft may support persistence of lighter precipitation.

Confidence is below average regarding the details of the next
potential storm during the early-middle part of next week. 
Current guidance preference would bring the low track through the
Aleutians and southern Bering Sea, then into the southwestern
mainland--south and somewhat weaker versus the leading system. 
This would confine the majority of precipitation to the
southwestern mainland and southern coast.  Possible alternatives
include a farther south track that would reduce effects across the
southern mainland, or less likely a farther north track.

The forecast pattern will favor daytime highs ranging from mostly
below normal across the south and Panhandle to a mix of
above/below normal readings over the interior and mostly above
normal closer to the northern coast.  Morning lows should be
mostly above normal through the period, aside from some modestly
below normal readings over the southwest interior on Saturday and
in parts of the Panhandle on multiple days.

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