Small Craft Advisory 672, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
806 AM AKDT Mon Sep 7 2020

.ANALYSIS AND UPPER LEVELS...

This morning's water vapor analysis shows a deep tropical feed of
moisture, emanating from the northwestern portions of Hawaii,
having made in-roads into southcentral. This helped foster some
heavier rain rates early this morning, but nothing too out of
hand. An approaching surface front has also helped the winds
become gusty across the Alaska Peninsula, southwest alaska, and
especially portions of Southcentral. Thus far, the highest
measured winds have been across the Hillside and south Anchorage,
where Upper DeArmoun and Paradise Valley, where 73 and 80 mph
gusts were reported respectively.

Looking at the mid to upper levels, a ridge of high pressure
stretched from the West Coast northwest ward, as it was trying to
build across the eastern Gulf. To the west of this feature was a
fairly deep trough that stretched from the Chukchi Sea south
through the Central Aleutians, and down towards the central
Pacific.

&&

.MODEL DISCUSSION...

The models remain in very good agreement through 72 hours at the
surface and aloft. The main forecast challenge will be the timing
of various disturbances crossing southwest and southcentral Alaska
on the backside of an eastern Gulf ridge of high pressure that
will build across that region.

&&

.AVIATION...
PANC...LLWS will continue to be an issue through the morning,
while slowly decreasing with time through the remainder of the
morning. However, gusty winds in the 30 to 40 knot range remain
likely through much of the forecast period. Some patchy low
stratus may try to make a run at the field from the Cook Inlet
from time to time. Otherwise, ceilings and visibilities look to
remain largely in the VFR category.

&&

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

A weather front will move rapidly east across Southcentral
this morning. Rain will transition to showers as the front
continues east. However, there will be potential for moderate
to locally heavy rain with upslope over the Susitna Valley
and along the Gulf Coast today before tapering off. Although
the more intense gap winds have/are peaking this morning,
strong pressure rises on the backside of the front will keep
fairly brisk gusty winds (mostly southerly) across Southcentral
into Tuesday morning. A few areas, such as the Copper River,
will see southerly winds linger on into the late afternoon
hours on Tuesday.

&&

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

Unsettled weather is expected to continue Monday across Southwest
as a broad but strong low pressure system moves from Nunivak
Island Monday morning towards the Seward Peninsula Monday night.
This storm track means that strong south/southwesterly winds and
rain are expected across the region on Monday. Rain should be on
the lighter than that seen on Sunday as the region is now in the
cold sector of the storm and cut off from the tropical moisture
feed that was present on Sunday. Southwesterly winds from 25 to 45
kts over the eastern Bering and Bristol Bay could also bring high
surf along the coast from Kipnuk to Naknek through Monday
evening. Water levels are not expected to exceed 1 to 2 feet above
astronomical high tide; however, coastal erosion is possible due
to rough seas during high tides.

By Monday night the winds will begin to decrease across the region
but southwesterly flow is likely to continue the pattern of
overcast skies with showers possible over areas of high terrain.
For Wednesday high pressure will begin to build in from the west.
This will mean mostly sunny skies and drier weather for the AKPEN
and Bristol Bay regions while the Y-K delta and Middle Kuskokwim
are expected to stay mostly cloudy due to the advection of low
stratus from the Bering.

&&

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

A broad 990 mb low over Nunivak Island Monday moring is expected
to continue moving north toward the Seward Peninsula throughout
the day Monday. This low will continue to produce gale force
northerly gusts from the Pribilof Islands up to Saint Matthew
Island and southwesterly gales from north of Dutch Harbor to
Bristol bay. Seas in and west of Bristol Bay could be as high as
15 feet. Winds and seas are expected to slowly subside into
Tuesday morning as the low moves out of the region. A high
pressure ridge is expected in the wake of this storm, which will
lead to clearer skies and calmer seas over the central and eastern
Bering through Wednesday.

Ridging over the western Bering on Monday will keep winds on the
lighter side before the next front moves in early Tuesday morning.
The parent low pressure system is expected to stay off of the
coast of the Kamchatka Peninsula through Wednesday as there is
little upper level support to move it into the region. As a
result, the impact to the western Bering will be a gale force
front that moves in Tuesday morning, pushing slowly east over Adak
Wednesday morning where it will become stationary and weaken.

&&

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

High pressure exits the Eastern Bering by Fri. A Western Bering low
and front move to the Central Bering for Fri. While track and
development of the low varies slightly between models, confidence
is good for widespread Southerly gale force winds and waves with
the front diminishing to Easterly small craft winds in the Central
Bering by Fri.

For the Gulf of Alaska waters, a front moves into Southeast Alaska
for Fri. Confidence is good for Southerly small craft winds and
waves with the front diminishing by Thu. High pressure moves
across the Gulf through Fri. Confidence is good for Westerly gale
force winds through the Barren Islands dissipating Thu. A new
front approaches the Southwest Gulf Fri.

.LONG TERM FORECAST (Days 4 through 7/Wednesday through
Saturday)...

The recently discussed "atmospheric river" event will be in the
process of closing out by Wednesday as the jet axis shifts east
and as a sizable shortwave traverses the state from west to east.
Areas of rain are possible for particularly western slopes of the
Alaska, Talkeetna and Wrangell ranges with this wave as the flow
becomes more westerly and then northwesterly by Thursday. Given
any higher rainfall totals from the previous days for the
southeast region, any additional precip on Wednesday ahead of the
main shortwave near the eastern Chugach Mountains or Copper River
Basin could extend or aggravate higher levels on area waterways.

By Thursday-Friday, agreement remains good for a window of drier
weather for the mainland with a shift to northwest flow and
ridging forming over the eastern Bering. A brief period of enhanced winds
through the Kamishak Gap into the Barrens is possible late
Wednesday to early Thursday, but otherwise calmer low level winds
can be anticipated for the latter half of this week. Some global
solutions show additional midlevel waves riding in with the
synoptic flow through parts of the interior, but there will be
considerably less moisture for these perturbations to work with by
this point. Therefore, both Thursday and Friday should be calmer
and drier all in all.

Model discrepancies become larger going into the weekend,
especially for a small low lifting up from the North Pacific. The
GFS shows this feature making it into the AK Gulf where minor
coastal and marine impacts would be possible with the
ECMWF/Canadian instead showing the low weaker and stalling out
well to the southwest. There is much better agreement for a
deeper/closed low to track through most of the Aleutians between
Thursday and Saturday with the usual impacts (fog, low stratus and
showers) expected around this time frame.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...HIGH SURF ADVISORY 155 161
MARINE...GALE WARNING 119 125 130 138 139 160 165 178 180 411 414
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...MK/AS