Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
756 PM EDT Sun Oct 01 2023
Valid 12Z Thu Oct 05 2023 - 12Z Mon Oct 09 2023
...Multi-day heavy precipitation storm threat from Southeast to
southern Alaska, with freezing rain and snow potential into the
Interior...
...Overview...
Guidance continues to show an evolving pattern that should favor a
period of heavier precipitation across southern portions of the
state including the Panhandle, especially late this week and to a
less pronounced degree into the weekend and early next week.
Models and means as a whole suggest that a system over the
Aleutians as of early Thursday should merge/interact with a
leading system tracking north toward the Alaska Peninsula,
ultimately yielding a slow-moving trough/upper low over or near
the southwestern mainland. A downstream mean ridge should extend
into the southeastern mainland from the Panhandle and then western
Canada. Flow ahead of the Alaska Peninsula system will provide
the primary surge of moisture late this week while details of one
or more trailing Northeast Pacific lows, and thus precipitation
coverage and amounts, become more uncertain thereafter. The
primary feature of interest to the north will be an upper trough
extending into the mainland from an Arctic upper low, with some
debate over its progression.
...Guidance Evaluation and Preferences...
Models and ensembles generally agree that the initial Aleutians
system should shear out as associated dynamics merge or otherwise
interact with the system tracking northward to the Alaska
Peninsula. By Friday-Saturday some typical spread develops for
low track, with the 12Z UKMET straying to the western side of the
envelope while the 12Z CMC eventually extends the track farther
northwest than the model/mean majority. Otherwise predictability
and confidence in specifics decrease considerably as surface low
details will depend on smaller scale shortwave evolution. There
is a loose signal for a frontal wave possibly tracking into the
Gulf of Alaska around Saturday, with enough solutions in common
for a blend to show a hint of this feature, but predictability is
low. In addition the operational models show a seemingly random
array of possibilities for how one or more separate waves may
break off from an initial western Pacific system. Lows on the
northern side of the spread could brush the Aleutians and/or track
into the northeastern Pacific/Gulf of Alaska, per the 00Z CMC/06Z
GFS, while a farther south track would have minimal effects for
the state and surrounding waters. The ensemble means provide a
more coherent and stable forecast at the surface and aloft later
in the period, emphasizing low pressure a little east of Kodiak
Island on average while depicting some elongated North Pacific low
pressure that could feed into the mean area of low pressure south
of the mainland.
Farther north, guidance has varied quite a bit for specifics of
the initial Arctic low/mainland trough. After the 00Z CMC lifted
it out much faster than other guidance, it still appears to be on
the shallow/fast side. Otherwise latest ECMWF runs show longer
persistence of the trough than the GFS, with the respective
ensemble means leaning toward their operational counterparts in
slightly weaker form. An intermediate approach appears reasonable
for this feature.
Forecast considerations led to starting with a blend of more 12Z
GFS/ECMWF emphasis relative to the CMC/UKMET early in the period,
followed by incorporating more 12Z GEFS/CMCens and 00Z ECens input
with time so that the means comprised 60-75 percent of the
forecast by days 7-8 Sunday-Monday. Remaining model input
transitioned to splitting the ECMWF among its last two runs to
offset some detail uncertainties while holding onto some of the
12Z GFS and phasing out the CMC.
...Weather and Hazards Highlights...
With relatively greater confidence than the remainder of the
forecast, expect a strong surge of moisture (possible atmospheric
river) during the latter half of this week into Southeast Alaska
as a warm front lifts through the Gulf of Alaska in advance of a
system tracking northward to the Alaska Peninsula. This moisture
should produce heavy rainfall and mountain snows from the
Panhandle back to portions of the southern coast. Details for the
Alaska Peninsula low and any additional potential systems reaching
into or just south of the Gulf of Alaska become more uncertain
after Friday. Thus confidence decreases with respect to
precipitation coverage and amounts, but the overall pattern seems
to offer a continued potential for enhanced rainfall and mountain
snows extending across the broad southern mainland tier, including
Southcentral and the Alaska Range as well as the Panhandle.
Lingering cold daytime temperatures across the southern tier may
pose a threat for impactful freezing rain into the Interior with
snow over other areas. For the five-day period, the highest
precipitation totals will likely be over the Panhandle and
southeastern coast.
Between low pressure over and east of the Alaska Peninsula and
high pressure forecast over or north of the North Slope, a broad
area of the mainland into the eastern Bering Sea should have a
fairly tight gradient supportive of brisk to strong winds
especially late this week. High temperatures will tend to be
below normal but with some moderation possible over southern areas
with time. Locations near the northern coast should see above
normal highs on most days. Min temperatures will likely trend to
near or above normal levels from the weekend into next week after
starting out below normal over Interior/eastern areas late this
week.
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