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FXAK02 KWNH 232355
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
755 PM EDT Mon Sep 23 2024

Valid 12Z Fri Sep 27 2024 - 12Z Tue Oct 01 2024

...Heavy precipitation likely across favored portions of
Southcentral this weekend...


...Overview...

As the medium range period begins Friday, initial upper troughing
and a surface low causing precipitation over Southeast Alaska will
move away, while a trough axis sets up over western Alaska.
Shortwaves within the western trough are forecast to combine and
create an upper low centered over Bristol Bay or so and direct
moisture into Southcentral for some heavy precipitation over the
weekend. Details of Pacific surface lows supported by the low
aloft are still uncertain. East of the trough, upper ridging will
generally prevail until the trough starts to move eastward into
Tuesday.


...Guidance Evaluation/Preferences...

Recent model runs agree that low pressure over southern parts of
the Panhandle will be weakening by Friday and then moving away
from the state. Troughing over the western Mainland will be in
place, and on the southern side a shortwave or compact upper low
is forecast to shift eastward across the southern Bering Sea
Friday, which possibly combined with other energies (like a
shortwave diving south through the western side of the trough like
the 12Z ECMWF showed) should form a broader upper low centered
atop Bristol Bay/Kuskokwim/Alaska Peninsula areas over the
weekend. Models have some variety in the details aloft and with
associated surface lows for typical spread that a model/mean blend
handled reasonably well.

The dominant forecast issue continues to be the
track/depth/existence of a potentially deep low pressure system
initially tracking across the mid-latitude Pacific and then
lifting into the Northeast Pacific into early next week. Many
models were quite strong with this surface low yesterday and with
today`s 00Z/06Z runs. However, 12Z operational runs greatly backed
off on the potential for a strong low, or at least delayed it from
reaching near Alaska until Tuesday or Wednesday. The 12Z ECMWF did
not strengthen the low much at all in the midlatitudes and keeps
it weak as it turns north early next week, possibly directed east
by the other low that it has south of Kodiak Island. Meanwhile the
12Z GFS shows a couple of different surface lows turning north
into the northeast Pacific that end up combining by Tuesday for a
strong low (low 970s). The CMC indicates one low that is deep, but
slower than the GFS where it does not even get near Alaska by the
end of the period next Tuesday. Ensemble member low plots show
ample spread with the track and central pressure of the low, as
the ensemble means generally end up combining the two lows. At
least with the ensemble means there is some agreement with each
other despite the disagreements overall. There is support for
heavy precipitation over the weekend in parts of Southcentral with
the initial low features, which does not rely on the track of the
uncertain Pacific low. However, there is certainly spread in how
much QPF may reach the Panhandle early next week and with winds,
depending on the Pacific low`s strength and track.

The WPC forecast began with a blend of the deterministic guidance
favoring the 12Z GFS and ECMWF, but reduced the proportion of
deterministic runs gradually in favor of the ensemble means,
reaching over half means by the end of the period as spread
increased especially with the Pacific low. There is support for
heavy precipitation over the weekend in parts of Southcentral with
the initial low features, which does not rely on the track of the
uncertain Pacific low. However, there is certainly spread in how
much QPF may reach the Panhandle early next week and with winds,
depending on the Pacific low`s strength and track.


...Weather/Hazards Highlights...

The initial northeast Pacific/near Panhandle or Haida Gwaii system
may bring increased precipitation continuing into early Friday
before moving away. Farther west, light to locally moderate
precipitation is likely in the Aleutians to Alaska Peninsula late
week with upper and surface lows in the vicinity. As these pivot
east and tap into more Pacific moisture, precipitation totals will
increase over the weekend, especially in favored areas like the
terrain on the west side of Cook Inlet and the southern terrain of
the Kenai Peninsula to Prince William Sound, with rain and/or snow
depending on timing and elevation. Beyond that time, the forecast
gets quite uncertain depending on the evolution of surface lows in
the northern Pacific. Will have to await better model agreement
before determining if Southeast Alaska will see significant
precipitation or not. The central and northwestern mainland may
see some light and scattered precipitation late this week,
followed by a drier trend over the weekend, but then perhaps
expanding in coverage early next week.

In a broad sense, cooler than normal temperatures are likely over
western Alaska with warmer than normal temperatures over eastern
Mainland Alaska under the trough to ridge pattern respectively.
40s to low 50s are forecast for highs in most lower elevation
areas. Expect greater coverage of above normal morning lows, with
most of the below normal readings confined to the southwest and
the Panhandle. Cooler than average temperatures are expected for
the Panhandle.


Tate


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

$$