Small Craft Advisory 889, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
426 PM AKST Thu Nov 19 2020

.ANALYSIS AND UPPER LEVELS...
As discussed yesterday, a low developed south of Attu and has now
moved into the southern Bering Sea as a double-wrapped occlusion.
This low has an impressive amount of cold air aloft feeding into
it as evidenced by the cold-air cumulus clouds to the west and
south of it. The parent low has retrograded into eastern Russia,
but there is still a front that extends out of it over the Gulf of
Anadyr to Southwest Mainland Alaska and in the Gulf of Alaska.
This front is weakening as it moves into the ridge over the
interior of the state, but is still strong enough to bring some
light snowfall to the Kuskokwim Delta and Bristol Bay.

&&

.MODEL DISCUSSION...
Models are still struggling with the development and movement of
the lows, especially Friday night through Sunday. The large low in
the Bering Sea today is handled well until Friday night, but as
long as the NAM model is discarded after that time frame in the
Bering Sea region, there remains good enough model agreement for a
reasonable forecast solution. The biggest issue is in the western
Gulf of Alaska beginning Friday evening where a strong front is
moving into that area with the potential for low to develop along
this front. In this case, the GFS is being disregarded as it is
the only one that spins up a strong low off this front. The
remaining models have smaller lows develop along the front, but as
long as the focus remains on the front, and not the potential
individual lows, a forecast can be put together with reasonable
confidence.

The best news with the 12Z and 18Z model runs is that they are
converging on a solution for early next week. This would develop
a strong low south of the Alaska Peninsula and track it across
the southern Gulf of Alaska but not move it to the north Gulf
coast. This is slowly increasing confidence for the forecast for
early next week.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. The mid-level
clouds moving into the region should help keep fog out of the
airport through Friday.

&&

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

The majority of the energy associated with an upper level trough
and surface cold front continue to dissipate this afternoon. Snow
has transitioned to rain on Kodiak Island, with a rain/snow mix
across Prince William Sound.

As we have been tracking, our next area of focus is with the next
upper level trough and surface front moving into the western
Bering. Formation of a triple point low along the left exit region
of the jet streak is possible, however, models greatly disagree
if a low center eventually forms at the surface, including where
and when. What models are capturing very well is the placement of
the surface front. Due to this, the forecast was created utilizing
this feature. Expect a significant area of gale force winds
entering the western Gulf tomorrow morning and propagating towards
the northern Gulf Coast Saturday morning. Expect storm force
winds across the Barren Islands Friday night and across the
northern Gulf Coast Saturday morning as a barrier jet sets up.

Precipitation will follow this front along the same timeframe with
a wintry mix anticipated for coastal regions and higher
elevations. Due to downsloping, the Anchorage Bowl looks to
largely remain dry with this system. Expect an increase in local
effect winds for the Turnagain Arm and Matanuska Valley, as the
pressure gradient tightens between high pressure over the mainland
and low pressure in the Gulf. These winds should be strongest
Saturday morning before diminishing Saturday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Snow showers across interior Southwest Alaska will continue to
diminish overnight as remnants of a front weakens. The pattern
quickly shifts overnight into Friday morning as a low pressure
system moves into the eastern Bering. This frontal system will
bring strong winds and gusts to the Southwest coast and Alaska
Peninsula persisting through Friday evening. Expect widespread
snow to move over the coast starting tonight, spreading farther
inland throughout the day on Friday. Colder temperatures moving in
behind the front will allow for a chance of snow or mixed
precipitation along the Alaska Peninsula and Bristol Bay coastal
areas heading into the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A storm force low positioned between Dutch Harbor and the
Pribilofs this afternoon will keep an over the Bering and
Aleutians through the next few days. Winds will weaken to gale
force along the occluded front as it approaches the Southwest
coast overnight tonight into Friday. The strong winds will
continue throughout the day on Friday on the back side of this low
as cold air continues to be pulled into the region.

By Saturday morning, models over the eastern Bering and south of
the Alaska Peninsula continue to disagree on whether strong winds
will continue. Winds are forecasted to stay at small craft
advisory level or below in this package. However, winds could
increase if the outlying GFS solution wins out, which has an
intensifying closed low at the base of the trough over the
western Gulf on Saturday.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

Gulf of Alaska:

A lot of uncertainty remains during the 3-5 day timeframe. There
is moderate confidence a front associated with a strong low enters
the western Gulf Sunday afternoon. There is potential for storm
force winds over the Gulf waters beginning Sunday night and
lasting through Tuesday as the low tracks eastward into the Gulf.
There is still considerable uncertainty with the track and
strength of this low. Widespread gales are likely during this
time, however will need to continue to assess regarding the timing
and location of storm force winds.

Bering Sea:

Gale force winds are likely over the northern Bering waters
Sunday. There is potential for a storm force low to enter the
western Bering Sunday night. This low will spread gale force winds
across the central and eastern Bering through Tuesday. Model
solutions continue to struggle with strength and timing, however
there is high confidence in an active weather pattern over the
Bering through Tuesday.

&&

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

Beginning Monday, an upper level ridge over the central Bering is
flanked by two upper level lows - one over the Kamchatka
Peninsula and the other centered over the Gulf. As the Kamchatka
low moves eastward Monday into Tuesday, a more amplified pattern
will develop. Strong northwesterly flow is still possible over the
Alaska Peninsula late Monday into Tuesday as the pressure gradient
strengthens considerably. Meanwhile, ample cold air filling in on
the backside of the Kamchatka low will enhance winds across the
western Bering Tuesday. Model solutions (GFS/EC) suggest that a
second low develops over the central Aleutians late Tuesday into
Wednesday and lifts northward over the Bristol Bay area. As we
typically see with this type of pattern, there is potential for a
triple point low to form somewhere in the vicinity of Kodiak
Island late Wednesday. Depending on the track of this triple
point low, the middle of next week may present a more active
weather pattern across Southcentral.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm 131 132 150 413 414.
   Gales 120 125 132 138 139 155 160 165 170-175 179-181 185 351
352 412.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...KO