Small Craft Advisory 696, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 PM AKDT Tue Sep 15 2020

.ANALYSIS AND UPPER LEVELS...
The entire Bering Sea is under a broad upper level low. In the low
complex, the parent low is near Attu Island where the upper level
low and surface low are vertically stacked and slowly weakening.
A second surface low is near St Lawrence Island and while not
deeper than the parent low, it is stronger in the sense that the
pressure gradient around is tighter and the front which splits the
state in half from north-to-south emanates from it. This front
has a long fetch of moisture associated with it and is advecting
this moisture from around 30N in the North Pacific.

&&

.MODEL DISCUSSION...
The main model issue over the next few days revolves around
the low that will be near the Alaska Peninsula Wednesday night
and its track as it moves northward. The question is whether it
will remain in Cook Inlet cross the Aleutians Range before
reaching Cook Inlet. Taking the most recent runs as a whole, the
most likely track looks to be for the main low to jump the
Aleutian Range near Kamishak Bay right before entering Cook Inlet.
In this case it will still leave a residual lee side trough over
Cook Inlet which will impact the orientation of the pressure
gradient and therefore the winds. None of the models have the low
tracking on the east side of the Kenai Peninsula as was the case
in some previous runs and the farther west track with lower
central pressure remains the trend.

&&

.AVIATION...
PANC...Gusty southeasterly winds will persist through the evening
and then are expected to shift to the north around midnight.
Periods of rain will persist through the night, but visibility
and ceilings should remain VFR, though a short time of MVFR
ceilings cannot be ruled out early Wednesday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Tue
Night through Fri Afternoon)...

A N-S oriented front is beginning to stall out over the Talkeetna
and Chugach mountains this evening bringing light rain to the
region. A brief period of light Turnagain Arm winds might come
into west Anchorage this evening before down inlet flow develops
overnight and turns the wind south and out of Anchorage. Higher
winds are also likely through the Copper River Valley overnight as
the front breaks down. For Tuesday, light rain is expected over
once again over Chugach and Talkeetna mountains as the front
continues to break down. More persistent rain is expected over
the Prince William Sound. Southerly flow should keep skies mostly
overcast through Friday so day to day temperatures are not
expected to change by more than a few degrees with lows increasing
a degree or two each day.

Model guidance has finally begun to converge on a solution for
the north Pacific low expected to move over the AKPEN Wed night.
On Thu the low center is now expected to move over Southwest AK;
however, lee side troughing along the front is expected to produce
a mesolow over the western Gulf and Cook Inlet area. This should
lead to strong coastal ridging along the northern Gulf coast,
which will bring some strong winds through the major mountain
passes. Currently expecting strong winds over the upper Anchorage
Hillside and Turnagain Arm throughout the day, peaking in
intensity in the afternoon as the front moves in. Winds are also
expected to increase through the Copper River Valley, which will
peak in the afternoon Thu.

The other weather impact from this system is widespread, and in
some areas heavy rain. As the low slowly moves up the cook inlet,
a long, tropical moisture feed is expected to bring persistent
rain to the western Kenai Peninsula and Prince William Sound. Up
to 4 inches of rain may fall near Seward by Friday morning, with
places like Whittier and Valdez receiving between 2.5 and 3
inches. The Anchorage Bowl and Mat-Su Valley is expected to
remain mostly downsloped of heavy rain, but should receive on and
off light rain through Friday. Rainfall totals for these areas
are not expected to be high, measuring near and less than 0.25
inches by Fri morning.

&&

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

An upper level trough axis extends northwestward from the Alaska
Range through the Yukon Delta to an upper low centered near St.
Lawrence Island. This feature will continue to lift north and
away from the forecast area tonight, which will also lead to the
reduction of showery clouds this evening. Still, more clouds and
isolated showers from eastern Bering and south of Alaska Peninsula
will continue to make their way across the region through Wednesday,
before the next large scale system brings another series of
rainfall starting Thursday morning through Friday.

The rainfall observations over the Alaska Range and north of
Dillingham shows total rainfall reaching up to more than 0.20
inch, and up to 0.66 inch of rainfall over Kusko Delta. With the
expected series of rainfall in the coming days, the potential for
moderate to heavy showers is focused mainly along the Alaska
Peninsula and Bristol Bay through the Kusko Delta on Thursday. On
Friday, more widespread showers will continue spreading inland and
across the Alaska Range. Overall, the wet pattern will become
more pronounced for Southwest Alaska through the forecast period
with a few breaks of sunshine.

&&

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

The stacked upper low over western Bering will be the main weather
producer in the next couple days. Models shows a firm continuity
of this upper low slightly to the north of the Aleutians before
diving southeastward of Cold Bay, then curving northeastward
toward Bristol Bay on Thursday morning. Meanwhile, an emerging
upper trough near the western Aleutians will take days to develop
as it skirts along the Aleutians eastward between Thursday and
Friday, this system will bring more rain and moisture across the
region in the coming days. As a result, anticipate a lot of
rainfall over the area and at least small craft conditions through
the next few days.

&&

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

Lingering small craft winds and waves over Bristol Bay diminish
Fri night. A front South of the Aleutians moves into the Gulf of
Alaska for Sat. Confidence is good for small craft winds over and
South of the Aleutians through Sat night. A North Pacific low and
front move into the Western Aleutians and Bering through Sun.
While track and development of the low are mixed, confidence is
good for Gale force winds to wrap the low with local storm force
winds over the Aleutians through Sun.

A front stretches across the Gulf through Sun. Confidence is good
for small craft winds and waves with local gales around Kodiak
Island dissipating Sat. A second low on the front South of the
Alaska Peninsula moves across the Gulf through Sun. While track
and development vary, confidence is good for small craft winds and
waves to move across the Gulf through Sun.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Saturday will see a cool upper level trough entrenched over
western Alaska from the Arctic coast southward to the Alaska
Peninsula. With a cut- off low at the tip of this trough tracking
into the Southern Gulf of Alaska, it will keep a cloudy, cool and
showery regime over southern mainland Alaska for most of the
weekend.

On Saturday night, a large low will move into the western Bering
Sea and the south of the Alaska Peninsula by Tuesday. While there
is uncertainty to the exact track of this low, the general
pattern shows the strongest winds and heaviest rain will remain
over coastal and marine areas in the Bering Sea and Gulf of Alaska
for the first half of next week.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...CJ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...MK/EZ