Small Craft Advisory 663, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
447 AM AKDT Fri Sep 4 2020

.ANALYSIS AND UPPER LEVELS...

Yesterday evening, lightning strikes were detected over Point
MacKenzie and east of Cordova. There is a weak upper level low
over Southcentral with embedded minor shortwaves. Which is just
unstable enough to produce isolated showers and thunderstorms.
The radars at Kenai and Middleton Island continue to ping into
shower activity early this morning. There is a minor ridge over
Sand Point, the Eastern Bering and Bristol Bay. There is another
and more robust low near the Seward Peninsula in addition to the
closed low north of Attu Island. There is a 150 knot jet core also
near Attu.

&&

.MODEL DISCUSSION...

There has been some run to run continuity issues with the
NAM...GFS and Canadian. Very Confident that Saturday will be the
driest day for the Anchorage Bowl this holiday weekend. However,
there are some wholesale differences with the intensity and the
placement of the next low to hit SW AK and this will have
implications for what happens with the winds for Southcentral.

&&

.AVIATION...

PANC...At 853z the cig at 4,200 feet was observed at PANC. On the
Kenai radar there were still isolated showers over Point MacKenzie
and Turnagain Arm. Turnagain arm winds develop again today but
they don't look as strong as yesterday. So the current thinking is
that winds will be light and variable at PANC. High confidence
that the weather is going to be VFR on Saturday (all day).

&&

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

A broad cold upper low continues over Southcentral today and the
threat of showers. Cloud breaks combined with upper forcing may be
enough to kick off a few thunderstorms over the Kenai Peninsula,
Copper River Basin, and Susitna Valley this afternoon and early
evening. Drier and sunnier conditions are expected across the
region as ridging builds in on Saturday. A developing North
Pacific low and associated front heading toward the Alaska
Peninsula on Saturday will bring strong easterly winds to the
western Gulf and Kodiak Island with increased chances of rain
Saturday afternoon/night. There is still some uncertainty with the
speed and positioning of these features. So stay tuned for
updates.

&&

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

A brief period of ridging over the eastern Bering will lead to
clearer skies and warmer temperatures Friday and Saturday. The
exception to this is over the western Alaska Range, where an upper
level low over Southcentral could provide lift for showers over
the mountains on Friday. The pattern is expected to change first
over the AKPEN on Saturday as a tightly packed north Pacific low
moves into the area. There is some uncertainty in the model
guidance as to the strength and the east/west track of the low as
it pushes north into the area. Regardless, frontal precipitation
and wind is expected to reach the AKPEN Saturday afternoon. Rain
and gusty winds are expected to then spread across Southwest by
Sunday as the tightly packed low gets absorbed by a much broader
system in the north Pacific. The large low complex is then
expected to move north through Monday, with widespread rain
expected over southwest as it tracks north.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Thursday through Sunday morning)...

An elongated area of low pressure over the western Bering will
continue to bring widespread rain from Adak to Shemya through
Saturday before moving south within the broad trough present over
the north Pacific. Meanwhile, high pressure over the eastern
Bering should lead to calmer seas and clearer skies through
Saturday when a compact north Pacific low is expected to move over
the eastern Aleutians. There is still some uncertainty in the
track and strength of the initial, compact low and what influence
it will have as a broad region of low pressure moves over the
region on Sunday. Regardless, Gale force winds, higher seas and
widespread rain are expected over the central and eastern Bering
through Monday as the system tracks north.

&&

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

There is increasing confidence that a gale force low will move
northward across the central/eastern Aleutians on Sunday. Storm
force winds and waves are a possibility with this system as there
is still model discrepancy with the strength of the storm. Gales
are likely to continue with this low as it moves into the northern
Bering on Monday but should dissipate Tuesday. The front
associated with the aforementioned low will enter the northwestern
Gulf Sunday. As it propagates northward across the Gulf into
Monday, gale to storm force winds are expected before it
dissipates Monday night. Models are picking up on a small craft
advisory front entering the western Aleutians on Tuesday and
moving to the east into Wednesday.

&&

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

The amplified pattern will continue through Wednesday. A large
area of low pressure over the eastern Bering will bring widespread
rain and winds to much of the Bering Sea, the Aleutian Chain and
across the Gulf and Southcentral on Monday. Rain will continue
into Tuesday for much of the area as that low moves northward. An
upper level ridge continues to strengthen over the North
Pacific/eastern Gulf, at this time, which will begin to interact
with the trough digging down from the Bering and into the North
Pacific. This will set up an opportunity for an "atmospheric
river", which is a fetch of deep moisture. This will make its way
into the southwestern Gulf on Tuesday and should start
precipitation through Thursday. However, model discrepancies still
haven't resolved if the moisture will be over Southcentral or if
it will get shunted to the southeastern part of the state. All in
all, expect a wet and windy, fall-like pattern, as we move further
into September.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 352 150 155 178.

&&

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