Small Craft Advisory 643, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
513 AM AKDT Sat Jul 27 2024

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

A look at the upper level pattern shows closed upper level lows
over Mainland Alaska and the far western Bering Sea, with zonal
flow to the south across the Aleutians and southern Bering Sea
eastward to the Gulf of Alaska. Focusing in on Southcentral, a
short-wave south of the upper low over the Interior is rotating
eastward across Southcentral early this morning, producing showers
from the Cordova-Valdez areas to the Copper River Basin and
surrounding mountains. Partial clearing behind the short-wave has
led to some patchy fog, but it should burn off fairly quickly
during the morning hours with the arrival of sunshine. Gusty gap
winds and strong southerlies in Cook Inlet have largely died down
overnight with the slackening of pressure gradients.

The aforementioned short-wave will be slow to exit the Copper River
basin, with clouds and showers lingering through much of the day,
especially along and near the Wrangell Mountains and Alaska Range.
Meanwhile, a weaker short-wave will drop down the back side of
the low into Southcentral triggering some showers back to the west
over the Susitna Valley. A few showers are possible this afternoon
along the western Chugach and northern Kenai Mountains, though
deep westerly flow will keep them away from the population centers.
Much of the region outside of the Copper River Basin and northern
Susitna Valley will experience warmer and drier conditions today
under partly to mostly sunny skies.

The upper low and accompanying short-waves will exit eastward
toward the Yukon tonight, with showers tapering off across
Southcentral. A series of short-waves embedded in the fast zonal
flow will then transit from the Bering Sea to the Gulf. The first
won't do much more than bring some increasing clouds and patchy
light rain to Kodiak Island this afternoon and evening. The
second, higher amplitude short-wave will be accompanied by a warm
front tracking out ahead of a low moving into the Bering Sea.
The airmass behind this warm front is quite warm and moist, with
850mb temperatures on the order of 8 to 14 degrees Celsius and
Precipitable Water values of 1.5 to 2". This will bring widespread
light rain to Kodiak Island on Sunday. The short-wave will exit
eastward across the Gulf Sunday afternoon and evening, taking most
of the forcing for precipitation with it. However, some of the
low level moisture will continue northeastward to the Kenai
Peninsula, with light rain likely for the southern Peninsula by
Sunday afternoon. For the rest of Southcentral, Sunday will start
out dry with some sunshine, but clouds will increase as the day
goes on.

The second upper low over the western Bering Sea this morning
will march eastward across the Bering Sea and reach Southwest
Alaska Sunday night. Similar to Friday, short-waves will eject
out of the upper trough into Southcentral Sunday night through
Monday, leading to widespread rain. The rainfall distribution
looks somewhat similar to this past event as well, with deep
southerly flow favoring Prince William Sound and the northern
Susitna Valley for some of the higher rainfall amounts. Forecast
models vary on the details of the upper low and associated short-
waves Monday through Monday night, though there is good agreement
on a general track from Southwest Alaska to Southcentral and the
Gulf. As a result, confidence is high that clouds and showers
will linger through at least Monday night and perhaps longer.

-SEB

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3: Today through Monday)...

An occluded low is now moving a ways west of the Pribilofs this
morning as its front extending to the southeast works its way
from the eastern Aleutians up into the AKPen. Rain and areas of
low stratus/fog will continue to advance northeast along and
behind the front as warm Pacific air moves into the southern
Bering southeast of the low center. Meanwhile, a fast-moving
shortwave trough will speed northeast from the open Pacific up
past the western Aleutians this morning, then into the central
Bering Sea. This influx of upper level forcing will maintain or
slightly strengthen the low and it's main frontal feature as they
both continue northeast throughout the day today, with a second
area of rain and brisk westerly winds expected to follow near
the trough along the Aleutian Chain by this evening. Out across
Southwest, a transient, flat ridge moving in ahead of the next
Bering system will provide some reprieve from the wet and cloudier
pattern today, with little more than a few lingering showers near
the Middle Kuskokwim Valley anticipated through this evening.

Looking towards Sunday and beyond, a complex and much more active
setup is taking shape, and some key changes in this forecast
package have been made to reflect rapidly gaining support for
what could become a much stronger system by late summer standards.
On Sunday, a deep upper low will move out of Kamchatka, arcing
south and east into the central/northern Bering. This upper low
is now looking more likely to interact favorably with the remnant
surface center drifting close to St. Matthew Island by Sunday
afternoon. A substantial deepening of the surface low into the
980s mb range now appears increasingly likely up until it moves
onshore near or just north of the Kuskokwim Delta on Monday
morning. A corridor of westerly winds increasing up to 30 kts
has been introduced for much of the eastern Bering Sea out to the
Southwest coastline for Sunday night into Monday now that a broad
consensus for a much stronger low to develop has quickly
materialized.

There is still some uncertainty for the maximum intensity of
winds near this low, and gale force sustained winds will be
possible as the low reaches peak intensity from Sunday night into
Monday. The highest chance for sustained gales will be focused
just south of the low, including near or just south of Nunivak
Island out to the Kuskokwim Delta Coast. Seas up to 15 feet could
also develop near this corridor, spreading east with the stronger
winds into the Kuskokwim Delta and Bristol Bay areas during the
day Monday. Those with marine interests (especially near the
Southwest Coast and AKPen) should monitor the forecast closely
for updates as we follow this potential for an unseasonably
strong system early next week.

AS

&&

.LONG TERM FORECAST (Days 4 through 7: Tuesday to Friday)...

An upper level trough digging across Southwest Alaska Monday will
continue to push east through late Monday into Tuesday. This
feature, while slightly different with each forecast model, looks
to bring widespread rain to Southwest Alaska late Sunday through
Monday and to Southcentral Alaska early Monday through early
Tuesday.

Unsettled conditions with periods of showers are then expected
to persist for Southern Alaska Tuesday and Wednesday as the
upper level low remains situated over mainland Alaska. Gusty
northwesterly gap winds are possible across the Alaska Peninsula
on Tuesday as the low departs to the east over the Northern Gulf
and a weak transient surface high builds in over the central
Aleutians. Another system emerges from the North Pacific and
impacts the Aleutians with periods of rain on Wednesday. Overall,
temperatures will remain below average for the long term period
with persistent cloud cover and rain.

-DD

&&

.AVIATION...

PANC...Gusty southerly winds will die down early this morning.
VFR conditions and light winds will persist for the rest of the
TAF period.

&&