Small Craft Advisory 905, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
511 PM AKST Wed Nov 25 2020

.ANALYSIS AND UPPER LEVELS...
A high amplitude trough remains in place from eastern Russia to
mainland Alaska, anchored by a closed low over the far northwest
Bering Sea. A broad 100-150kt zonal jet streak extends along and
south of the Aleutian Islands. Arctic air continues to poor out of
Russia and across the Bering Sea and North Pacific. This pattern
has produced and will continue to produce fast moving deep storm
systems across the Alaska region.

The feature of most interest today is a negatively tilted short-
wave trough lifting northward across the southern Alaska mainland.
A triple point low (with estimated central pressure of 975mb) is
tracking northward up Cook Inlet, with an occluded front pushing
onshore of Southcentral. As of midday, this was leading to
widespread strong winds, with storm force winds along the north
Gulf coast and Prince William Sound, and hurricane force wind
gusts in Turnagain Arm and across the Kenai and Chugach Mountains.
Precipitation, heavy at times in the Prince William Sound region
is falling primarily as rain at/near sea level, with snow in the
mountains. However, colder air is advecting in behind the low and
front, causing precipitation over Southwest Alaska to change to
all snow.

Meanwhile, a series of weaker upper level short-waves are tracking
eastward across the Bering, Aleutians, and Alaska Peninsula. With
cold air aloft and still warm sea surface temperatures, the
overall environment is fairly unstable. This is evident in the
cold cloud tops seen in IR Satellite imagery. As a result, there
are widespread rain and snow showers and gusty winds across the
region.

&&

.MODEL DISCUSSION...
In general, the models continue to struggle with the exact track
and depth of deep lows over the next few days. They are grouped
together, but when you are focused on where the core of strongest
winds will track, it creates a challenge for honing in on potential
impacts. As far as handling of specific systems, models have come
into much better agreement with a deeper low crossing the Gulf
Thursday and moving into Prince William Sound Thursday night.
Thus, the wind and precipitation forecast will be overhauled to
reflect this common solution. Out west, there is a larger spread
with two lows tracking to the Aleutians on Thursday. The track
along the central Aleutians Thursday night will be key to whether
Adak and Atka experience hurricane force winds. After this
critical juncture, models are in good agreement as the system
continues eastward to the Gulf and Southcentral this weekend.

&&

.AVIATION...
PANC...The early part of the TAF forecast is quite challenging.
Southeasterly Turnagain Arm winds will be in full force this
afternoon into evening. These winds will result in a jump up in
temperature, so that if any precipitation were to fall, it would
likely fall as rain or a rain/snow mix. At this point, low level
flow will still be fairly strong across the Kenai and Chugach
Mountains, so any precipitation that falls should be very light.
Winds will then shift around to S to SW as colder air moves in
this evening. Expect at least a brief period of light snow
between 06Z and 12Z to reduce ceiling/vis, though exact timing is
uncertain. Then expect rapidly improving conditions after 12Z,
along with diminishing southerly winds.

&&

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

Heavy periods of snow will pass through Matanuska Valley
especially over the Hatcher Pass this evening. Meanwhile, the rain
and snow will continue over much of the Prince William Sound as
cloud remnants from a weakening frontal zone treks inland and
northward tonight. The next active developing storm from the
southwestern Gulf of Alaska will bring another series of snow
showers over Kodiak on Thanksgiving morning, then across Prince
William Sound Thanksgiving Day and Thursday night. A shortwave
from the North Pacific will follow right behind the weakening
system on Thursday night, which could reinforce the potential for
a chance of snow Friday morning over Prince William Sound.

Another major developing storm from the Bering Sea will again
bring another series of snow over much of Southcentral Alaska.
This feature will impact the region with mix rain/snow between
Friday night and Saturday night, with strong winds up to 35 mph.
For now, will monitor the future development of this latter
system, so stay tuned for more updates.

&&

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

Temperatures across much of the coastal areas, moving inland, have
begun to drop as cold air advection moves over the area from the
Bering Sea. This has transitioned rain to snow for much of the
region. Temperatures will continue to drop for the next few days,
with all precipitation staying as snow. Windy conditions have
started to taper off as the front has moved out of the area, and
calmer conditions should prevail after midnight.

Snow showers will continue Thursday morning into Friday with coastal
areas anticipated to receive the most snowfall. Friday afternoon,
a front associated with a strong low pressure system over the
central Aleutians will bring the next chances for snowfall to the
area. Limited confidence in model agreement on the initial
location of the low keeps the timing, placement and track of this
front uncertain. There is confidence, however, that snowfall will
occur south of Bethel, across Bristol Bay, the AKPEN and inland
areas nearby Friday afternoon through Sunday morning.

&&

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

Westerly flow continues across the Bering this afternoon. This
will continue through tomorrow morning, until the next low
pressure system begins to move across the western Aleutians in the
afternoon. This is a strong system, bringing a large swath of
storm force winds across the Aleutian Chain with hurricane force
winds anticipated from Adak to Atka. A High Wind Watch remains in
effect from Adak to Atka for Thursday evening through Friday
morning. This system will bring another round of rainfall for the
Chain, as temperatures will approach the mid-40s. After this,
expect a lull in the weather before the next system approaches the
western Aleutians Sunday morning.

&&

.MARINE (Days 3 through 5/Saturday through Monday)...
A large and deep storm system will make its way from the Bering
Sea into the Gulf Friday night through Saturday, with widespread
gales across the region storm force winds likely along the leading
frontal boundary as it crosses the Gulf. As this system weakens in
the Gulf Saturday night, another storm force low will track south
of the Aleutian chain. As this storm approaches the Gulf on
Monday, the low will start to take a more northerly track. Thus,
there is potential for another round of storm force winds across
the Gulf waters as the leading front crosses the Gulf on Monday.
The forecast will stick with gales for now on Monday, but suspect
as we get closer models will settle on the stronger solution.

&&

.LONG TERM FORECAST (Days 4 through 7/Sunday through Wednesday)...
A seasonably strong upper level jet combined with progressive
upper level flow will persist through at least the first half of
next week. There is seemingly no end in sight to the supply of
large deep storm systems traversing the Alaska region. Starting
out on Sunday, one storm will be in the weakening phase over the
northern Gulf and Southcentral Alaska. Another storm will be
strengthening as it tracks just south of the Aleutian chain.
Expect widespread strong winds and moderate to heavy precipitation
along the Aleutians and Alaska Peninsula as this storm continues
eastward Sunday through Sunday night. As this low continues to
deepen, a short-wave ridge will strengthen out ahead of it, which
will then cause the storm to curve northward as it approaches the
Gulf. As a result, expect precipitation and wind to overspread
southern Alaska. Based on a storm track in the vicinity of the
Alaska Peninsula and the western Gulf, warm air will surge
northward toward Southcentral. Thus chances are this will be
primarily a rain event along the coast, with potential for enough
warm air to spread inland and cause precipitation to change to
rain there as well. The effects of this storm look to linger
through Wednesday, with colder air moving in behind it. Stay
tuned, as it's too early to nail down specific impacts from this
storm.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Hurricane force 175 173 174 176 177.
         Gale 119 120 130 139 155 170 172 178 351 411 413.
FIRE WEATHER...NONE.

&&

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