911
FXAK02 KWNH 242356
PMDAK

Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
756 PM EDT Tue Sep 24 2024

Valid 12Z Sat Sep 28 2024 - 12Z Wed Oct 02 2024

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


...Overview...

A trough axis is forecast to be in place across the eastern Bering
Sea and western Mainland to Alaska Peninsula as the medium range
period begins Saturday. Shortwaves within the western trough are
forecast to combine and create an upper low centered over Bristol
Bay or so early next week, directing moisture into Southcentral
for some heavy precipitation over the weekend. While details of
Pacific surface lows are still uncertain, recent models suggest a
midlatitude low may turn northward into the eastern Pacific near
Southeast Alaska and spread some moderate to heavy precipitation
there early next week. Troughing should move eastward as next week
progresses, and will have to monitor Arctic energy moving toward
northwestern Alaska near the middle of next week.


...Guidance Evaluation/Preferences...

Recent model runs agree that troughing over the western Mainland
will be in place Saturday, and energies within it should combine
to form a broader upper low centered atop Bristol
Bay/Kuskokwim/Alaska Peninsula areas over the weekend. Among the
12Z guidance, models initially have pretty minor spread with the
details aloft and with associated surface lows. The 12Z CMC was
the strongest with an early week secondary low near the eastern
Aleutians/western Alaska Peninsula, but a model blend handled the
systems reasonably well.

The dominant forecast issue continues to be the
track/depth/existence of a potentially deep low pressure system
initially tracking across the midlatitude Pacific and then lifting
into the Northeast Pacific into early next week. Some model runs
over the past couple of days have shown a deep low turning north
more quickly toward Southcentral Alaska, including the 06Z GFS
with a central pressure into the 950s. However, recent 12Z
deterministic guidance (ECMWF, GFS, and CMC) happens to agree
fairly well on a track of this low farther east before turning
northward into the northeast Pacific Monday-Tuesday (nearing
Southeast Alaska), with central pressures that vary somewhat but
certainly stay in the 970s or higher. The 18Z GFS came in
differently again, with a low that is slower to turn north, but
weaker. Ensemble member low plots indicate that any of these
solutions are plausible as they show ample spread with the track
and central pressure of the low. The means continue to try to
combine the two lows, for more uncertainty.

Models generally show troughing moving eastward into Tuesday and
beyond, while there is some signal for Arctic troughing to move
toward northwestern Alaska. The details will have to be determined
in future forecast cycles.

The WPC forecast began with a blend of the 12Z deterministic
guidance favoring the 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...

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. Recent
forecasts show that Southeast Alaska may see notable precipitation
as the low track may be closer to that area, so precipitation
amounts trended up in this forecast, but without high confidence.
There may be an uptick in precipitation across western parts of
the state by the middle of next week ahead of possible Arctic
energy.

In a broad sense, cooler than normal temperatures are likely over
western Alaska with warmer than normal temperatures over northern
and 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

$$