Small Craft Advisory 665, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
455 AM AKDT Sat Sep 5 2020

.ANALYSIS AND UPPER LEVELS...

Snow was detected by a weather instrument at Eureka (which is part
of the Matanuska-Susitna Borough) for several consecutive hours
yesterday. In the past 24 hrs, a number of locations in
Southcentral and Southwest AK had minimum temperatures in the 30s.
Where the skies were clear, the temperatures plummeted. This
morning the upper level ridge axis spanned from Kipnuk to Kodiak.
There is a surface low south of Dutch Harbor and the longwave
trough has dug further south into the North Pacific. The core of
the jet is west of Attu Island with an estimated intensity of 130
knots. Also noteworthy is Super Typhoon Haishen which is near
Japan. A number of tropical systems in the West Pacific Basin
eventually reach Alaska, and Haishen's projected evolution is
being closely monitored.

&&

.MODEL DISCUSSION...

For initialization purposes, compared the surface low position on
ASCAT (Advanced Scatterometer) to the NAM, GFS, Canadian and
ECMWF projections for the same feature. They all hit the bullseye.
Model consensus emerged with the latest run. The major features of
the synoptic pattern are in lock step through 00z Tuesday.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist for the
aerodrome this TAF package. At this point in time, the "Turnagain
Arm Winds" will develop 6z Sunday but are not expected to impact
PANC. These local wind patterns will continue to be monitored.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weakening upper trough exiting Southcentral this morning will be
followed by ridging this afternoon. This will bring drier and
sunnier conditions across the region today. Though there will be
some localized low stratus and fog briefly this morning. A weather
front, associated with a storm force low moving toward the Alaska
Peninsula, will bring rain and windy conditions to Kodiak Island this
evening and spread increasing cloud cover over Southcentral.

The storm force low center weakens tonight and is replaced by yet
another low center which moves into the eastern Bering Sea Sunday
night. This center will bring gale to storm force winds over the
Gulf waters as its front sweeps north Sunday and Sunday night.
Additionally, pressure gradients will increase across the mainland
and give rise to strong gap flows, Turnagain Arm, Portage Valley,
Knik River Valley, and along the Copper River to name a few. Rain
will also spread north with the front Sunday night. With broad
southerly flow setting up aloft Sunday night, areas which would
be typically downsloped west of the Kenai, Chugach, and Talkeetna
Mountains will receive measurable rain.

&&

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

A narrow, negatively tilted ridge is expected to bring one last
day of sunshine and warmer temperatures to the mainland of
Southwest on Saturday. To the south, a compact storm force low is
expected to impact the AKPEN by midday on Saturday. Strong winds
and moderate to heavy rainfall is expected to push north towards
Bristol Bay by Sunday morning. As this system pushes north it's
expected to rapidly break down as it interacts with the terrain
and high pressure to the north. In its wake, a much broader region
of low pressure is expected to move slowly south to north over
the eastern Bering through Monday before making landfall somewhere
in the Y-K Delta region. This will bring widespread rain and
strong winds are expected over Southwest through Tuesday.

&&

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

The Bering Sea and Aleutian Chain is expected to transition into
an active weather pattern again through Tuesday with strong
winds, high seas and rain for much of the region. A broad longwave
trough is centered over the Bering with two low pressure systems
south of the Aleutians Saturday morning. Guidance has been
trending stronger throughout the week on the first, compact storm
force low expected to reach the eastern Aleutians/AKPEN on
Saturday. Storm force winds are possible in the waters north and
south of AKPEN before weakening Saturday evening as the storm
center moves over land. This system is expected to break down and
get absorbed by the much larger, second low pressure complex,
which will move over the central Aleutians Sunday morning.
Widespread Gales are expected Sunday from Bristol Bay, north to
the Y-K Delta and wrapping back around towards the Pribilof
Islands. On Monday, cold air wrapping around the low centered just
east of the Pribilofs should bring another round of Gales from
Cold Bay north into Bristol Bay. By monday evening winds and
waves across the region should subside as the low moves onshore
somewhere near the Yukon Delta.

&&

.MARINE (Days 3 through 5/Monday through Wednesday)...

A well-developed low over the eastern Bering Sea will exit
northward along the coast of western Alaska Monday through
Monday night. Widespread gales will be in place to start the day,
but winds will diminish through the period. Another smaller and
weaker low will enter the western Bering and Aleutians Monday
night into Tuesday. There is some uncertainty in depth and track
of this low. Expect at least small craft winds with some potential
for Gale force winds along the western Aleutians Tuesday through
Wednesday.

For the Gulf of Alaska waters, the effects of the Bering system
will linger through Monday, with a gale force front pushing
onshore along the north Gulf coast and Prince William Sound.
Conditions then look quieter until Wednesday when there is
potential for small craft or gale westerlies across the western
Gulf, though confidence is low.

&&

.LONG TERM FORECAST (Days 4 through 7/Monday through
Thursday)...

An active and generally wet pattern will continue through Wednesday
with a broad low over the Bering Sea moving up into the Norton
Sound by Monday evening. Models are in reasonable agreement that
the bulk of the heaviest precip associated with an occluded front
will end from SW to NE on Monday with showery/breezy conditions
likely lingering in the southwest and interior through at least
Wednesday morning. There is good model agreement that strong
southwest flow will continue towards the AKPEN/Southcentral until
midweek with several rounds of rainfall possible near the Prince
William Sound region. However, 12z guidance suggests the
"atmospheric river" event and associated moisture may be most
impactful closer to the AK Panhandle where the highest QPFs are
currently projected. Some model inconsistencies with this feature
remain, so this may need to be reevaluated for future updates.

The tap should firmly shut off to close out the week as the
longwave trough over the Bering Sea breaks down and as the
prominent Northeast Pacific ridge retreats into the the western
CONUS by Thursday. Much quieter and clearer conditions are fairly
likely region-wide Thursday-Friday as a "fair weather" ridge
develops along the Bering coast. Current solutions do lean towards
another wave developing near the AKPEN by Friday evening, so the
respite in the stormy fall pattern may be short lived.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm 130 155 165. Gale 352 131 132 136 137 138 139 412
413 414 150 160 170 172 179 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...MK/AS/SEB