Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
652 PM EST Wed Nov 24 2021
Valid 12Z Sun Nov 28 2021 - 12Z Thu Dec 02 2021
...Arctic cold over much of the Interior through the weekend
followed by moderation over the east...
...Increasing precipitation along the southern coast and Panhandle
early-mid week...
...Overview and Guidance Evaluation/Preferences...
In principle the past day of guidance has remained fairly
agreeable and consistent with the large scale forecast that
involves a developing eastern Bering Sea/western mainland upper
trough helping to direct a consolidating North Pacific storm
northeastward into the Gulf of Alaska, followed by the system
weakening in place as upper troughing (with one or more embedded
closed lows possible) persists. This storm may produce strong
winds over some areas and will likely bring an episode of moderate
to heavy precipitation to the southern coast and Panhandle. This
pattern that evolves after an initial deep upper low is centered
over the south-central mainland early Sunday should keep western
areas cold while allowing for some moderation to the east next
week. Farther west, a weak system should quickly drop
southeastward over the western Bering Sea/Aleutians around Tuesday
followed by a building upper ridge that reaches the same region by
early next Thursday.
Over the past day the models have generally trended closer
together for the North Pacific into Gulf evolution, especially by
way of the GFS trending south of the Alaska Peninsula and the 12Z
ECMWF nudging back to the west some by Tuesday. Detail
differences remain though. These issues include initial low
pressure just south of the Aleutians on Sunday (no clear
clustering per se at the moment) and storm track/possible frontal
waviness into the Gulf (12Z CMC becoming an eastern extreme with
the primary low, 12Z GFS stronger with an extra wave that the 18Z
run dampened somewhat). From Sunday into Tuesday a 12Z model
composite represented the most common ideas of guidance well. The
main refinement from continuity was a somewhat north/northwest
surface low position by early Tuesday.
This blend also worked well elsewhere early in the period and then
incorporating some of the 12Z GEFS/00Z ECMWF means with the GFS,
00Z/12Z ECMWF, and decreasing input of the CMC accounted for the
typically increasing detail spread by Wednesday-Thursday. There
are various possibilities for exactly how flow within the western
mainland upper trough may evolve later in the period while
throughout the forecast there is some continued ambiguity over how
much connection there will be between upper troughing west of the
Canadian Archipelago and the mainland mean trough. Also spread
increases for the weak system reaching the western Bering Sea by
Tuesday, with the 12Z CMC at least looking rather extreme with its
eastward progression given the persistence of the pattern to the
east. Timing differences for the upper ridge reaching the Bering
Sea/Aleutians by day 8 Thursday are fairly typical for that far in
advance.
...Weather/Hazard Highlights...
Through the weekend expect many areas to see very cold
temperatures, especially over the southwestern mainland where
readings may be 20F or more below normal. Similar anomalies may
extend eastward across the south-central Interior. The Panhandle
into far southeast corner of the mainland as well as parts of the
north could see slightly above normal lows though. The transition
of the upper pattern from a mainland upper low toward a
north-south trough aligned over the western mainland should
support a moderating trend over eastern areas (with pockets of
moderately above or below normal readings) while keeping the west
well below normal.
Weakening low pressure over the Gulf of Alaska this weekend may
maintain light to moderate precipitation over the southeastern
coast and Panhandle. A wave tracking well to the south may bring
some moisture into the southern Panhandle as well. Intensifying
North Pacific low pressure should bring precipitation and brisk to
strong winds across the Aleutians (winds initially from the east
and then turning northerly/northwesterly) and possibly into the
Alaska Peninsula and Kodiak Island early in the week. A storm
track into the Gulf should increase precipitation from near the
Kenai Peninsula through the Panhandle Tuesday onward. While not
perfect yet, today's guidance has improved its agreement versus
yesterday with respect to coverage and magnitude of locally
moderate to heavy totals. The weak system dropping through the
western Bering Sea/Aleutians may produce an area of organized but
generally light precipitation around Tuesday or so. By next
Thursday southerly winds may strengthen over the western Aleutians
ahead of a storm emerging from Asia.
Rausch
Hazards:
- Heavy precipitation across portions of mainland Alaska, Mon, Nov
29.
- High winds across portions of mainland Alaska, Mon, Nov 29.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Tue-Wed, Nov 30-Dec 1.
- Much below normal temperatures across portions of mainland
Alaska, Fri-Mon, Nov 26-Nov 29.
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