Small Craft Advisory 769, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
440 PM AKDT Sun Oct 7 2018

.ANALYSIS AND UPPER LEVELS...
The weather pattern over the state and nearby North Pacific
waters is rather complex today with a strong upper level wave
over Southcentral ALaska which is detached from the surface low
which is in the Gulf of Alaska. In the Bering, a moderately strong
low is near the Central Aleutians with a Gale-force front ahead
of it moving through the Eastern Aleutians. This low will anchor
itself in the central Bering Sea the next few days and become a
major steering component for other systems this week. What also
makes this low system challenging is the presence of a short wave
low embedded in the front south of the Aleutians. While this wave
will not amount to much, it may impact the way a triple point low
farther south develops and moves toward the Gulf Monday night. In
addition to all this, the remnants of former typhoon Kong-rey can
be seen on satellite tracking well south of the Western Aleutians
near 50N 170E. The interplay of this former-typhoon and the
anchor-low in the Bering Sea will be the crux of the forecast
challenge for the middle of the week. More information on this in
the model discussion section below.

&&

.MODEL DISCUSSION...
Models have struggled with the upper level wave moving up Cook
Inlet and across Prince William Sound today. This feature moved
faster than even yesterday afternoon's models showed and it was
not until overnight, when it was almost model initialization time,
that the NAM and GFS finally hit upon it. The good news is that
now that this feature is adequately resolved, the other features
in the short term look to be handled very reasonably. There are
two items to watch through the middle of the week that may cause
model difficulties. The first is a triple-point low that will
develop south of the Alaska Peninsula Tuesday and move over Kodiak
Island by early Wednesday. While this low looks to be resolved in
a similar manner across models, its anticipated structure may
change as it gets closer.

An even bigger challenge will be in the Tuesday through Wednesday
night time period when the remnants of former typhoon Kong-rey
arrive on scene. This low will be strengthening again in its
extra-tropical phase, but the anchored low in the Bering Sea is
cutting off the cold air source which will limit the amount it
strengthens. The amount it strengthens will have an impact on the
exact track it takes, as the stronger it becomes, the farther
west it should track. Therefore there is quite a spread on tracks
with the Canadian model being 15 mb and 275 miles farther west
than the NAM on Tuesday night. At this time, the GFS and ECMWF
models have a very similar solution in both track and strength (in
between the Canadian and NAM) which takes it through Shelikof
Strait and the into Cook Inlet Wednesday. We are adjusting
forecasts toward this solution for the afternoon forecast package.

&&

.AVIATION...
PANC...The biggest question concerning the TAF will be whether
some fog or stratus develops late tonight. The rain today and a
light southerly wind expected most of the night will keep the
lowest levels of the atmosphere mostly saturated which does favor
the development of fog and/or stratus, so some was introduced
into the forecast.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
A few showers will linger through the early evening hours for the
MAT-SU Valleys south and east to Prince William Sound as an
upper-level shortwave continues to slide east. A low situated in
the northern Gulf will weaken as it slides toward the Alaska
Panhandle with small-craft winds diminishing by midnight. As a
ridge builds in from the west tonight and remains in place Monday,
the mainland will begin to see some drying and clearing
conditions. Patchy fog across the Cook Inlet north into the MAT-SU
Valleys is also possible tonight through Monday morning. In
addition, the building ridge will help establish offshore flow
with brisk gap winds overnight through Monday evening, especially
along the east coast of the Kenai Peninsula, Passage Canal, and
areas from Valdez on east.

A triple-point low will move into the western Gulf late Monday
bringing rain and gales along its occluded front. This system
will slow its forward progress, quickly weaken, and transition to
an open wave as it enters the northern Gulf and approaches the
Southcentral coast with the majority of precipitation and
easterly wind gusts confined to the coast. A second, more
organized low (the extra-tropical remnants of typhoon Kong-rey, to
be specific) then follows the first with rain overspreading
Kodiak Island and gales developing across the western and northern
Gulf by late Tuesday. A 140kt jet streak over the western Gulf
will allow this low to deepen as it moves into Shelikof Strait
Tuesday night and into Cook Inlet by Wednesday. As it does,
widespread rain will fall across coastal locations along the
associated warm front. A southeasterly flow aloft, however,
should limit rainfall for locations in the lee of the Chugach,
Kenai, and Talkeetna Mountains due to downsloping conditions. In
addition, the deepening low will act to tighten the pressure
gradient, resulting in rapidly strengthening southeasterly gap
winds for Turnagain Arm, Passage Canal, and the Copper River
Basin. For Anchorage, winds along the hillside will increase
beginning Tuesday afternoon. Gusts of 45 to 55 mph are possible
overnight Tuesday through the first half of Wednesday before
steadily diminishing Wednesday evening as the low weakens and
tracks inland.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Wednesday)...
Scattered showers occurring across the area this afternoon will
diminish after sundown. During the overnight, nearly calm winds
inland after a day with onshore flow is expected to lead to a new
round of fog across much of the lower elevations of Southwest
Alaska tonight. Winds will slowly increase along the coast tonight
ahead of the next front. The front is a slow mover, and as such
most of the day Monday looks dry across all of Southwest Alaska
with high pressure in place inland. On Monday night, a low moving
northward along the Alaska Peninsula will help the front spread
inland, bringing rain to Bristol Bay and the Kuskokwim Delta,
along with breezy southeast winds. Those winds should help keep
the lee of the Aleutian and Kuskokwim Mountains relatively dry. On
Tuesday morning the front moves inland and the low diminishes,
which with the diminishing winds should mean most of Southwest
Alaska sees widespread rains during the day.

Another much stronger low, the remnants of former Super Typhoon
Kong-Rey will also approach the area south of the Alaska Peninsula
Tuesday night, bringing another round of heavier rain to Bristol
Bay and the Kuskokwim Valley, with the rain holding off in the
Kuskokwim Delta until the day Wednesday. The center of the strong
low looks to track up Shelikof Strait and into Cook Inlet, so
being on the cold side of the center, Southwest Alaska will get a
cold rain with gusty winds throughout the day Wednesday as the
storm tracks inland. With most of the warm air shunted well off to
the east, there is some potential for higher elevation snow with
this storm as well.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)
There are two big low pressure systems that will impact the
Bering Sea and Aleutians through Wednesday. The first is the low
currently centered west of Adak. It is producing gale force winds
and steady rains around it as it tracks northward towards the
western Bering. Its warm front to the east is also producing gale
force winds, which will impact the central Aleutians and the
Pribilofs during the day Monday with strong winds and soaking
rains. Meanwhile the low in the western Bering will become nearly
stationary before beginning a slow eastward drift Monday night and
Tuesday. The warm-turned-occluded front associated with the low
will move into Southwest Alaska on Tuesday. The much weakened low
will once again stall over the central Bering during the day
Wednesday, causing widespread showers over much of the Bering Sea.

On Tuesday afternoon the second stronger low, the remnants of
former Super Typhoon Kong-Rey, will approach the southern Alaska
Peninsula. The low itself will be strengthening as an
extratropical low, causing widespread gales and even some storm
force winds Tuesday night and Wednesday morning across the waters
between the Shumagin Islands and Kodiak Island. Small craft to
minimal gales are expected in Bristol Bay during the day Wednesday
as the center of the low tracks up Shelikof Strait.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term, beginning Wednesday night, has a pattern with a low
over the Bering and ridging east of the Gulf. This setup will last
through the rest of the week and will result in multiple
shortwaves rotating around the Bering low that will swing in over
the southern mainland. More rain is expected with each wave moving
through. This pattern looks as though it could break up over the
weekend but there is a fair amount of model disagreement for this
timeframe. With the wide spread of operational guidance, the
ensembles were preferred.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 132 170 172>175 179 185.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...DK