Gale Warning 673, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
527 AM AKDT Wed Oct 30 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Friday night)...

The storm system which brought widespread snow to Southcentral the
last couple days is now exiting across the Northeast Pacific. A
trailing neutrally tilted short-wave is tracking eastward across
Southcentral, with light precipitation out ahead of it from
Cordova northward to the Copper River Basin. Winds across
Southcentral, Kodiak Island, and the Gulf coastal waters are
diminishing with the retreating low, though gusty gap winds
continue along the coast. Seas also remain elevated due to the
strength and duration of winds behind the low yesterday through
last night. As an example of this, Bouy 46080 in the northwest
Gulf is reporting 22 ft seas as of 4am. Surface temperatures have
largely remained elevated overnight due to the presence of wind.
However, as the wind dies down, temperatures are now quickly
falling through the 20s for inland locations.

There are two remaining winter products, both of which have been
extended until 10am this morning: a Blizzard Warning for Thompson
Pass and a Winter Weather Advisory for the Copper Rover Basin. The
worst conditions have passed, but it will take a bit longer for
snow and wind to completely end as the aforementioned upper level
short-wave exits eastward today.

A short-wave ridge will build in from the west today and move
overhead of Southcentral tonight. This will provide a brief break
in the weather, with mostly sunny skies across the region today.
The leading edge of long-wave trough over the Bering Sea will then
reach the Gulf and Kodiak Island late tonight along with a frontal
system and developing triple point low. These features will be
affected by a fast moving deep low crossing the North Pacific.
Amplification of the flow out ahead of this low will help to
strengthen the triple point low as it passes just south and east
of Kodiak Island Thursday morning. As a result, have trended
toward a larger area of Gale force winds on the back side of the
low. Still, this low is compact and much weaker than the previous
storm system, so impacts will be more localized. Furthermore, the
amplification of a short-wave ridge aloft, will cause the trough
to dig southeastward across the Gulf, leaving a weaker trough to
cross Southcentral Thursday night into Friday. The end result will
be a light precipitation event for Southcentral Thursday through
Thursday night. Warm air will stream northward into Southcentral
ahead of the trough and low, with snow levels as high as 2500 feet
for the Cordova/Valdez areas and into the southern Copper Rover
Basin. Thus, expect more areas to see rain from this system.
Inland, snow accumulation will generally range from an inch or
less up to 3 inches. However, it does look favorable for a
deformation band to set up over northern Cook Inlet in the
vicinity of Anchorage and the northwestern Kenai Peninsula
Thursday afternoon/evening. As a result, there could be locally
higher amounts - but nothing remotely close to what we saw with
this past storm.

Attention will then quickly shift to the much larger and stronger
low in the North Pacific on Friday. Model spread with the track
of this storm has been large, varying from Bristol Bay to the
southern Gulf. The most likely solution and what the current
forecast represents is a track in between these two extremes.
Based on this, expect the greatest impacts of wind and precipitation
along the Gulf coast and Kodiak Island. The closed low aloft
should largely prevent much (if any) precipitation from spreading
inland across Southcentral. This storm will bring another round
of warm air, so the bulk of precipitation will likely be in the
form of rain except for high mountain snow.

-SEB

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA AND THE BERING
SEA/ALEUTIANS (Today through Friday night)...

Two major storm systems are poised to impact the region through
the end of the work week. The first will bring gusty winds to the
Western and Central Bering/Aleutians today, as well as rain, snow
and potentially freezing rain to Southwest Alaska. The second
storm system will transit across the Eastern Aleutians, Alaska
Peninsula, and Bristol Bay on Friday, bringing strong winds and
moderate to heavy precipitation. While no watches, warnings, or
advisories are in effect at this time, we are closely monitoring
the second storm system as changes in its track could lead to high
winds over land.

Diving into the details... a sub-960 mb low is currently
approaching the Western Bering/Aleutians. Cold air wrapping around
this occluding low will produce a broad swath of gales across
much of the Western and Central Bering/Aleutians through tonight.
Out east, the front associated with this low is beginning to push
onto the Southwest coastline, with precipitation beginning to be
picked up by radar. Snow is expected across much of Southwest
Alaska, with a transition to rain or mixed precipitation later
this evening for Bristol Bay from about Dillingham south. Forecast
snow amounts have remained largely unchanged since the previous
forecast package: about 2-4 inches for most communities across
Southwest Alaska, with all rain expected from about Naknek/King
Salmon and south.

The biggest uncertainty for today involves the transition zone,
which will likely include Aleknagik, Dillingham, and Togiak. As
warmer air moves in aloft with this front but temperatures near
the surface stay right around freezing, confidence is low as to
what precipitation types will occur. Current thinking is that
precipitation in this area will start as snow, may switch over to
a short period of freezing rain in the late afternoon/early
evening, and then either stay all snow (if temperatures are cool
enough) or transition towards rain (if enough warm air advection
occurs). With the lower levels of the atmosphere right around
freezing, this will be a challenging aspect of the forecast.
Either way, any freezing rain that may occur is not expected to
last long or produce much ice accumulation.

By Friday, attention shifts to another sub-960 mb low moving in
from the North Pacific. Models have had a pretty consistent
representation of this storm over the past couple days, lending
high confidence to its strength, potent moisture, and relative
track. Still, slight deviations with the track could change where
the strongest winds or heaviest precipitation are expected. For
now, there is excellent model consensus that sustained winds with
this low will be storm force, with hurricane force gusts possible.
If the low track stays along or south of the Eastern
Aleutians/Alaska Peninsula, the worst of these winds will remain
offshore. However, if the low track shifts northward, it's
possible that the Eastern Aleutians could see some of these high
winds. The other thing to watch for will be the moisture content
of this storm. Periods of moderate to heavy precipitation are
likely, especially on the Pacific side of the Alaska Peninsula.

-Chen

&&
.LONG TERM FORECAST (Days 4 through 7: Saturday through
Tuesday)...

For the extended outlook, a broad upper level low will move over
the Bering Sea with smaller, fast moving shortwaves rounding the
trough as it progresses eastward Saturday through Monday. The
surface low will move into Bristol Bay with associated fronts
progressing through Bristol Bay, Alaskan Peninsula and eventually
out into the Western Gulf of Alaska. Model guidance for the upper
level features varies with the favored Canadian, ECMWF and UK
suggesting the Bristol Bay route and less likely GFS and AI models
favoring a more southward track into the Gulf of Alaska. Either
scenario will bring possible gale force winds and widespread
precipitation across most the upper Aleutians, AKPEN and into the
Western Gulf through Sunday with conditions improving on Monday.
This system will eventually combine energies an approaching North
Pacific system over the Central Aleutians on Tuesday.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through at
least 12Z Thursday. An upper level short-wave trough and surface
occluded front will then approach from the south, with light snow
spreading up Cook Inlet and arriving at the terminal between 12Z
and 16Z. Low level southeast flow will strengthen across Kenai and
Chugach Mountains. It won't be terribly strong, but should be
enough to keep ceiling/vis from dropping real low. Thus, have
indicated MVFR conditions in the TAF.

&&