Small Craft Advisory 706, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 PM AKDT Fri Sep 18 2020

.ANALYSIS AND UPPER LEVELS...

The low which brought abundant rain and wind to southern Alaska on
Thursday has made its way north, reaching the Seward Peninsula.
The main impacts to both the Southwest and Southcentral are now
from multiple shortwaves south of the low center, traversing from
the Bering Sea across the southern mainland. Coupled with atmospheric
instability created when colder air aloft moved in behind the low,
these short-waves are maintaining showery and gusty conditions
across the region today.

Out over the eastern Aleutians an upper-level shortwave with a
weak surface rotation is traveling quickly eastward toward the
Gulf of Alaska. Impacts from this compact low are minimal, with
some small craft winds, low clouds and light rain in the Dutch
Harbor area. Lastly, a low near the Kamchatka Peninsula is
strengthening as it tracks toward the Aleutian chain. This will
be the next big storm system and focus of the forecast for the
next few days.

&&

.MODEL DISCUSSION...

Models are in good overall synoptic agreement for the next few
days. There is some disagreement with the interaction of a short-
wave trough (or multiple short-waves) approaching Southcentral
Saturday night and the interaction with the low over the Gulf at
this time. All solutions indicate that some moisture will be
pulled northward and a deformation band of heavier precipitation
will form. However, the location of this band varies from Prince
William Sound all the way west to the Anchorage area. Thus,
confidence in the precipitation forecast for these areas is low.

Meanwhile, models have locked on to a solution with the strong
low tracking eastward along the Aleutian chain this weekend,
then moving into the Pacific and Gulf early next week.

&&

.AVIATION...

PANC...VFR conditions will persist. Gusty winds look like they
will begin to die down some time Saturday morning as the
influence of the low over Northwest Alaska finally wanes.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Fri
afternoon through Mon afternoon)...

An upper level low in place across much of the state will allow
for some unsettled weather to continue over Southcentral. With a
weak trough in place over the area this afternoon, showers are
moving in across the northern Gulf and coastal areas of Prince
William Sound. Meanwhile, the upper level low to our west
continues to move eastward, and will bring a much stronger trough
to the area by Saturday morning. While models struggled a bit over
the last few days with the track of this low, there has been much
better agreement today with a more eastward track, though the low
center will remain over the Gulf. By Saturday night,
precipitation will fill in across Southcentral with the greatest
amount expected along the northern Gulf coast. As the low departs
on Sunday, conditions will improve with just a few lingering
showers. The next system to impact Southcentral will arrive from
the west Sunday night. This looks to be a much stronger low with a
leading front bringing winds and rain to Kodiak Island Sunday
night before lifting to the northeast by Monday. Another bout of
gusty southeasterly winds and rain is looking increasingly likely
for the start of the next workweek.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Monday)

Cold air advection will continue for another day or two in
response to a shortwave and an upper level trough running through
Southwest Alaska from the north of Yukon Bay. This pattern will
yield to cooler overnight temperatures, with isolated showers
passing through the region over much of the forecast period. As a
result, anticipate a relatively quiet forecast period with only
minimal showers over the area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Monday)

A highly anticipated circulation from the Northwest Pacific will
bring gales Saturday and Saturday night over much of the Bering
Sea, including western and central Aleutians. At the same time,
the gusts will potentially build up until it reaches severe gale
conditions between Sunday and Sunday as the center of the low
pressure system passes southeastward and beyond Dutch Harbor.
Hazardous wave heights will potentially reach up to 20 to 25 ft
along the west and south of the Aleutians west of King Cove,
meanwhile, light to steady stratiform rain will pass through much
of the forecast area through the forecast period. Overall, wet
pattern will predominate local conditions over the area in the
next coming days.

&&

.MARINE (Days 3 through 5/Monday through Wednesday)...
A very large storm system will make its way from the vicinity of
the eastern Aleutians/southern Alaska Peninsula on Sunday night
into the Gulf of Alaska and northeast Pacific by Monday night.
Widespread gales are likely across much of the region from the
eastern Bering Sea to the Gulf of Alaska Monday through Tuesday.
There is also potential for a secondary surface low to form over
the Gulf sometime Tuesday. If this does occur, then would likely
be at least a small area of storm force winds. The system does
look like it will begin to weaken on Wednesday and exit toward
Southeast Alaska. Thus, winds should finally begin to die down.

&&

.LONG TERM FORECAST (Days 4 through 7/Tuesday through Friday)...
The main focus to start the extended period on Tuesday will be a
very large storm system tracking from the Bering Sea to the Gulf
of Alaska and northeast Pacific. Based on this storm track, rain
and wind will clip Bristol Bay and then impact Kodiak Island and
the Gulf coast. Further north areas of Southwest and Southcentral
Alaska will remain mostly dry with seasonable temperatures. There
is some uncertainty in low position over the Gulf and in whether
a secondary low will form and enhance winds/rain somewhere in the
Gulf. However, do expect this system to weaken and exit eastward
Wednesday through Thursday. In general, after the impacts of that
system diminish, the weather looks relatively quiet across the
forecast area through Thursday.

As we head to Friday there is fairly large model spread in the
details of specific storm systems, but there are some commonalities
in the upper level pattern. It looks like an Arctic trough (or
multiple short-waves) will dig south across the Bering Sea and
into the north Pacific, causing the upper level flow to amplify.
This would then favor cyclogenesis over the Pacific south of the
Alaska Peninsula, with a storm track into the Gulf, which would
bring a return to storminess over the Gulf on up into Southcentral
Alaska. Stay tuned.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 170 172-178 411-413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...KM