Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
717 PM EDT Thu Sep 19 2024

Valid 12Z Mon Sep 23 2024 - 12Z Fri Sep 27 2024

...Heavy rain for portions of the southern coast and Southeast
early next week...

...Overview and Guidance Evaluation/Preferences...

An active pattern will continue over the high latitudes with Fall
in full swing. Upper pattern favors strong ridging/positive height
anomalies in the mid-latitudes north of Hawai'i and splitting
ridging over the Bering, giving way to a deep but progressive
upper low early next week. This will scrape the Aleutians on a
path toward Kodiak Island and into the Gulf of Alaska as the upper
trough axis settles over the mainland, bringing at least a couple
days of heavy rain to portions of the southern Coast and into the
Southeast. By later next week models diverge a bit on a potential
system lifting northeastward from the Pacific into the Gulf of
Alaska, as well as the next system upstream entering the Bering
Sea.

Updated 12Z deterministic guidance remains in good agreement for
the start of the period early next week, which led to a simple
composite blend for the updated WPC forecast of the
GFS/ECMWF/CMC/UKMET. Expect for a deep upper-level low and
accompanying strong (sub-980mb) low pressure system at the surface
to have passed over the Aleutians/Alaska Peninsula and into the
Gulf of Alaska by early Monday, tracking by Kodiak Island and then
off the Kenai Peninsula by Tuesday. Guidance begins to diverge
Wednesday as the models differ on exactly how additional
upper-level energies over the Gulf will evolve. The initial
upper-trough remains amplified over the Gulf in the latest
ECMWF/GFS while the CMC and UKMET as well as the ECens and GEFS
means favor a secondary incoming shortwave, with an accompanying
surface low pressure system lifting northward across the
northeastern Pacific towards the Gulf of Alaska. Upstream, the
ECMWF/CMC favor a slower progression of a new system over the
Bering with an upper-trough moving in from eastern Russia in the
latter part of the period, while the GFS is quick to favor
shortwave energy amplifying a trough over the Aleutians. The
ensemble means keep upper-troughing to the north/west similar to
the ECMWF/CMC, though the GEFS does favor an axis a bit further
eastward over western coast. Included an increased contribution
from the ECens/GEFS means to complement the ECMWF/CMC for the
latter half of the period, with the deterministic guidance
generally fitting within the envelope of the ECens/GEFS means.
Over the mainland/interior, troughing will maintain itself through
the week.


...Weather/Hazards Highlights...

A strong low pressure system passing from the Aleutians/Alaska
Peninsula into the Gulf of Alaska just south of Kodiak Island and
the Kenai Peninsula will be the focus early next week, with at
least a couple days of very heavy rain favored from around Prince
Williams Sound into the Southeast. A strong influx of moisture
from the tropical Pacific may lead to daily rain totals of several
inches (perhaps in the 4-5" range per the latest GFS/ECMWF) for
favored coastal/upslope locations Monday-Tuesday. The deep low may
also lead to some gusty winds, including for favored gap areas
both along the southern coast and west into the Alaska Peninsula,
but these will likely be coming down in strength by Monday as the
system begins to weaken. Another system lifting northeastward into
the Gulf from the Pacific may bring in another surge of moisture
and additional heavy rainfall for the latter half of the week,
though with greater uncertainty on timing and amounts.

For the interior, areas precipitation are expected throughout the
week with upper-troughing in place, though totals should be light.
Periods of light snow will become more common for some of the
lower elevations as temperatures follow a downward trend through
the week. Temperatures in general will be below normal for highs
and slightly above normal for lows, owing to abundant cloud cover.
Much more of the state will see lows below freezing, creeping
closer to Southcentral/Anchorage.

Putnam

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