Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
754 PM EDT Sun Oct 20 2024
Valid 12Z Thu Oct 24 2024 - 12Z Mon Oct 28 2024
...Overview...
The primary storm system of interest will be low pressure tracking
from near the Alaska Peninsula as of early Thursday and likely
settling over or just south of the southern Gulf of Alaska through
at least the weekend. This system will support a period of
enhanced rainfall along the southern coast and Panhandle along
with areas of strong winds especially over open waters. Farther
north expect mean troughing aloft over most of the mainland
through the weekend, aside from a ridge possibly extending into
the eastern mainland around the end of the week. Latest guidance
brings an Arctic upper low closer to the northern coast than in
some prior runs. Meanwhile low pressure and associated frontal
system reaching the western/southern Bering Sea by Friday may
split, leaving behind more persistent Bering Sea low pressure as
upstream dynamics arrive. By next Sunday-Monday a lot of spread
develops in the guidance regarding flow details from the eastern
Bering Sea into the mainland and Gulf of Alaska.
...Guidance Evaluation/Preferences...
A composite of 12Z operational guidance reflected consensus well
during the first half of the period, early Thursday into early
Saturday. During this time frame the guidance shows better than
average clustering and continuity overall for the system tracking
from the Alaska Peninsula into or just south of the southern Gulf
of Alaska. By early Saturday some notable spread develops for the
exact depth of the storm, with the ECMWF/CMC deeper than other 12Z
and prior solutions. The new 18Z GFS has a low-confidence wave
development that ultimately leads to an even deeper main low by
Saturday. Gradually stronger trends in the ensemble means at
least support a model average that is deeper than recent machine
learning (ML) models. Elsewhere, latest CMCens/ECens means
support the ECMWF/CMC idea of the Arctic upper low reaching a
little farther south than the 12Z GFS during the early-middle part
of the forecast. The new 18Z GFS adds a new wrinkle of the upper
low staying well to the west. There is good agreement for the
system reaching the western/southern Bering Sea by Friday with a
leading wave breaking off into the North Pacific by Saturday
(ultimately getting absorbed into the southern periphery of the
northeastern Pacific circulation).
Differences rapidly increase by Sunday-Monday. The most prominent
issue is regarding the strength of a ridge that reaches around the
eastern Bering Sea by Sunday and whether this ridge ultimately
builds into the mainland to replace the initial mainland trough,
or flatter and more progressive flow dominates. There is a portion
of the ML spread that even offers the potential for more
troughing. In recent runs the GFS has been showing the strongest
ridging, with the 06Z run particularly extreme versus the full
ensemble envelope. The 12Z GFS was just a little weaker with the
ridge while the new 18Z run has continued the gradual weakening
trend. Not surprisingly the GEFS hints at the GFS scenario but to
a less pronounced degree. On the other extreme, the 12Z CMC keeps
what ridging it has west of the mainland the whole time, keeping a
trough over the mainland. The 12Z CMCens/ECens means both favor a
compromise, while ECMWF/CMC runs both show progressive flow across
the North Pacific/southern Bering Sea into the Northeast Pacific.
ML models also show a lot of spread with no particular clustering
evident. Preference is for an intermediate solution that in
particular leans away from the GFS.
As for other aspects of the forecast, the late-period flow details
will influence what becomes of the southern Gulf system. A
relative majority of models/means would suggest the surface low
will not get pulled southward to the degree of recent GFS runs
(and the 12Z GEFS at least waited an extra day before tracking
more south versus the 12Z GFS). Also, Mainland Alaska surface
high pressure by next Monday would be weaker than the GFS per
consensus. The Arctic upper low may linger just north of the
mainland into the start of next week, though the ensemble means
say that it may not be as far southwest as the 12Z ECMWF/CMC. Low
pressure should continue to drift over the Bering Sea as upstream
dynamics reinforce it, with flow details to the east determining
whether another wave may break off from the associated frontal
system (tracking into or near the Gulf), or a larger scale system
tracks northeastward over the Bering.
Based on the array of available guidance for late in the period,
the Sunday-Monday part of the forecast transitioned toward 40-50
total weight of the ensemble means (12Z GEFS/CMCens and 00Z ECens)
with the GFS getting phased out and most of the remaining
operational input consisting of the 12Z/00Z ECMWF runs (the split
helping to cancel out some less confident details in one run or
the other).
...Weather/Hazards Highlights...
Flow around deepening low pressure expected to track from the
Alaska Peninsula into or just south of the southern Gulf of Alaska
during Thursday into the weekend will likely support a period of
enhanced precipitation along the southern coast and Panhandle,
along with increasing winds/waves offshore. The eastern
Aleutians/Alaska Peninsula may also see brisk winds behind the low
late this week. Highest precipitation totals should extend from
the Southcentral coast into the Panhandle, with activity trending
lighter from west to east during the weekend. Some areas of light
snow may be possible over the east (ahead of the mean upper trough
axis) and north (to the south of an Arctic upper low). The
Aleutians may see one or more periods of rain from frontal systems
during the period.
The majority of the state should trend toward below normal highs.
The North Slope may remain above normal through the period, though
latest guidance trends for close proximity of an Arctic upper low
would yield more moderate anomalies. Expect warmer anomalies for
min temperatures, starting out mostly above normal on Thursday
followed by a mix of above/below normal readings to the south of
the persistently above normal North Slope.
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