Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
646 PM EST Thu Nov 28 2024

Valid 12Z Mon Dec 02 2024 - 12Z Fri Dec 06 2024

...Wet/snowy pattern for the southern coast and Panhandle next
week...

...Overview...

Most guidance shows a blocky large scale pattern consisting of a
southeastern Bering Sea upper low as of early Monday wobbling back
into the central Aleutians and then eastward to a position near or
south of the eastern Aleutians/southwestern Alaska Peninsula by
next Friday, while an initial col region over the mainland
transitions toward more ridging that extends northwest from the
Panhandle and western Canada.  The main upper low and various
other shortwave impulses will likely support multiple surface lows
between the Aleutians/North Pacific and the Gulf of Alaska, but
with below average confidence in specifics for most of them. 
However the overall pattern should support a prolonged period of
wet/snowy weather along the southern coast and Panhandle.  Expect
most of the mainland to lie within the gradient between high
pressure over the Arctic and surface lows to the south.

...Guidance Evaluation/Preferences...

The most notable difference aloft arises during the first half of
the week with the southern Bering Sea upper low.  There is a
considerable majority cluster consisting of the 12Z
ECMWF/UKMET/CMC and latest ECens/CMCens means that depict greater
retrogression into the central Aleutians versus recent GFS runs
through 12Z which drop it more southward.  Latest machine learning
(ML) models also point toward the ECMWF cluster.  This difference
has implications at the surface as well.  The ECMWF cluster
reflects a surface low associated with the upper low (of varied
strength, ECMWF on the weak side and CMC strongest) with a
separate zone of other surface lows from the mid-latitude Pacific
northeastward.  In contrast, the GFS scenario wraps mid-latitude
Pacific low pressure into the upper low to yield a deep storm
system near the eastern Aleutians.  The new 18Z GFS looks like it
is starting to nudge toward the majority in that it does not drop
the upper low as far south and retrogrades a little more, though
it still wraps Pacific low pressure into the feature as in
previous runs.  Preferences side with the majority ECMWF cluster
overall.

Farther east, the specifics of surface lows tracking into the
northeastern Pacific toward the Alaska Peninsula/Gulf of Alaska
have minimal confidence due to the small scale of supporting
shortwave impulses.  There are at least hints of a couple defined
waves/frontal systems tracking toward the southern coast in the
Monday-Wednesday period, with the favored blend reflecting these
features.  The 12Z UKMET strays on its own with a wave nearing the
southern Panhandle by the end of its run early Wednesday.  It is
likely that details will change over future forecast cycles.

During the latter half of the week there is some convergence in
the guidance toward a mean upper low reaching near or a little
south of the eastern Aleutians/Alaska Peninsula, with
corresponding surface low pressure.  The ensemble means have been
fairly consistent and agreeable with the overall evolution by late
in the period but of course there is still considerable spread and
variability among the operational models for surface specifics
from the North Pacific into and near the Gulf of Alaska. 
Preference by this time frame transitions toward primary emphasis
on the larger scale mean system rather than on any potential waves
to the east.

Finally, guidance shows an emerging western Pacific system whose
influence could reach into the western Aleutians by next Friday. 
However most solutions keep it fairly suppressed, with only recent
GFS runs showing meaningful effects reaching the Aleutians.  The
new 18Z GFS has trended southward. 

The early part of the forecast started with a blend of half ECMWF
(split between 12Z and 00Z/28 runs) and the rest 12Z CMC and UKMET
based on preferences near the Aleutians and to reflect the most
common themes farther eastward.  As solutions converge somewhat
over the Aleutians and North Pacific around/after midweek, the
rest of the forecast added some 12Z GFS into the operational model
part of the blend while increasing ensemble mean input (12Z
GEFS/CMCens and 00Z ECens) to 30-40 percent.  CMC input was nudged
down some by next Friday as the model's Aleutians upper low became
a western extreme while dampening the Panhandle/western Canada
upper ridge some versus most other solutions.  The resulting
forecast maintained reasonable continuity for the large scale
pattern while refining details for surface systems.

...Weather/Hazards Highlights...

The overall pattern will be favorable for periodically enhanced
precipitation along and near the southern coast (including the
Alaska Peninsula) and Panhandle next week, with southerly mean
flow aloft between an Aleutians/North Pacific upper low and
Panhandle/western Canada upper ridge as well as surface systems
tracking toward the southern coast.  A lot of the specifics in
terms of timing and intensity will take additional time to resolve
though.  Coastal and nearby low elevations should tend to see
rain, with snow elsewhere.  Milder conditions over the southern
Panhandle will likely yield broader coverage of rain with snow
confined to higher elevations.  The one detail with some degree of
confidence is a fairly coherent signal for more intense
precipitation over the southern half to three-fourths of the
Panhandle early next week.  Therefore the Days 3-7 Hazards Outlook
depicts a heavy precipitation area for this activity from Sunday
(late in the short range period) through Tuesday.  Rainfall may
produce enhanced runoff and rises on area rivers.  Most of the
mainland north of the Alaska Range should remain dry next week. 
One or more of the potential surface lows could generate areas of
brisk to strong winds but with minimal confidence in details at
this time.   

The northern three-fourths of the state will start next week on
the cold side with below normal readings, including some overnight
lows in the -20s over eastern interior areas.  Aside from valley
locations where cold air typically persists longer, the overall
pattern will support a pronounced warming trend with time.  Most
areas should see above to well above normal temperatures by next
Friday, with perhaps parts of the North Slope still a little below
normal.

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