Gale Warning 402, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
529 PM AKDT Sat Apr 20 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

The steady stream of tropical moisture that has impacted portions of
the Gulf of Alaska and Kodiak Island will finally get pinched off as
the elongated trough begins to break down. While the heavier
rainfall across Kodiak has ended, light rain will linger a day or
two longer before finally coming to an end by early Monday. As such,
the Flood Watch was allowed to expire earlier this afternoon and any
additional rainfall should remain light enough that flooding is no
longer a concern.

The northwest to southeast oriented trough that has been parked
across the southwest Gulf to near the Pac NW will start to break
down today as a low on the southeast end of the trough lifts north
towards SE Alaska. The parent upper low south of Kodiak will also
start to lift north, helping to nudge the eroding frontal
boundary north towards the northern Gulf coast. Surface features
look rather messy the next few days with the main surface low
weakening, but new low forming near Kodiak tomorrow morning. This
will allow rain showers to linger through the day tomorrow until
the low moves into the AKPen. Will also see increasing showers
along the eastern Kenai and Prince William Sound as the surface
front and a shortwave trough lift north. Winds will remain gusty
today but will begin to steadily decrease as the gradient relaxes
over the Gulf.

A shortwave trough rounding the western periphery of the elongated
trough will dig into the southern Gulf by early next week with new
surface low quickly deepening over the Gulf. Behind the shortwave, a
ridging will build over the Aleutian Chain and eastern
Bering...helping to finally lift the main trough northward into
mainland Alaska. Depending on how this evolves, may see some showers
work up the Cook Inlet and into the Anchorage Bowl and Mat-Su
Valley by Monday afternoon and evening.

- PP

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA, THE BERING SEA/ALEUTIANS
(Days 1 through 3)...

A shortwave trough associated with the North Pacific low south of
the eastern Aleutians continues to move west near the Pribilof
Islands this afternoon. Precipitation mostly in the form of snow
will continue through this evening before the trough weakens later
tonight. The north Pacific low will continue to meander near the
Alaska Peninsula through Monday afternoon, allowing a chance of
precipitation for Southwest during that time.

Starting this evening, a building shortwave ridge near the
central Aleutians begins to move eastward towards mainland Alaska.
The ridge is expected to move over Southwest by Monday afternoon,
moving the North Pacific low northeastward towards Southcentral.
The ridge will allow for calmer conditions to persist over
Southwest through Tuesday, with the exception of some upslope
precipitation along the local mountainsides.

As previously discussed, the next area of concern will be the
formation of a low over the western Bering Sunday night. A
shortwave trough from the North Pacific and an upper-level low
from eastern Russia will start interacting early Sunday before
merging later that day. An occluded front associated with the low
will start moving eastward Sunday evening towards the western
Alaska coastline, providing widespread gale-force winds and
heavier precipitation for much of the central and eastern Bering.
Warmer air from the south should make most of the precipitation
rain along the Aleutians, while the Pribilof Islands will likely
get a rain/snow mix. Models are currently indicating an additional
front forming Monday night into Tuesday morning, providing
another round of precipitation and gale-force winds for the
western Bering.

The challenge at the end of the forecast period will be the
placement of the initial occluded front as it advances near the
western Alaska coast. Models are currently in disagreement over
the location of the ridge over mainland Alaska and how it will
interact with the Bering low and front. The Canadian and ECMWF
models bring the front over the coast as early as midday Tuesday,
while the NAM and GFS delay the frontal passage until Tuesday
night.

-BS

&&

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

A strong low in western Bering Sea will dominate the region while
a strong ridge remains stubbornly in place over the interior
Alaska and Canada through much of the forecast the week ahead. The
strength and placement of the ridge will influence the direction
and placement that the embedded shortwave troughs will flow into
the Gulf. Expect unsettled weather to continue through next week.
A front will lift through the northern Gulf midweek, bringing
light precipitation to higher elevations across southcentral due
to weak upslope flow and for Kodiak Island and Kenai Peninsula.

Subtropical moisture will be drawn into the Bering Sea low that
will occlude and spread heavy precipitation and gusty winds across
the Aleutians and the Pribilofs through midweek. While the low
circulation will remain over the Bering, the triple point of the
occlusion appears to be roughly around the Alaska Peninsula with
the front extending north along the SW Alaska coast. This front is
expected to lift towards Southcentral, but there is uncertainty
in how much it weakens before reaching Southcentral. Southeasterly
flow though could keep breezy conditions through Kamishak gap
Thursday.

Toward the later part of the week, the upper level low will begin
to tilt and nudge into the Gulf. Models begin to diverge in the
details for precipitation into the Gulf. A North Pacific low could
potentially bring the next round of relatively substantial
precipitation to the Gulf and Southcentral next weekend. There is
low confidence though due to strength of the ridge over the
eastern Pacific and the interior.

rux

.AVIATION...

PANC...VFR conditions and light winds will persist.

-CC

&&