Small Craft Advisory 636, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 PM AKDT Wed Aug 26 2020

.ANALYSIS AND UPPER LEVELS...

The pattern is slowly starting to change across the state. The 2
primary features this morning are a parent low just west of
Western AK and a vertically stacked low over the Northern Gulf.
First, the low out west continues to push precipitation and
moisture over SW AK. It produced some dense fog this morning over
Bristol Bay, which have now yielded to a fairly healthy band of
showers. The low over the Gulf of AK continues to produce showers
and some gusty winds over the coastal sites. This low will start
to lift inland this afternoon leading to more widespread showers.
Another weak wave continues to move over the Copper River Basin
towards the Susitna Valley courtesy of the broad cyclonic flow.
The rest of the Bering Sea remains quiet today with just some
patchy low stratus. By the time you get to the Western Aleutians,
the next front is just starting to push some steady SE winds into
that area.

&&

.MODEL DISCUSSION...Models remain in fair agreement through the
short term and forecast confidence is moderate to high through
Fri. Some of the finer details will center around heavier
convective showers and fog development Thu AM. Hi-res models and
hand edits were generally utilized for those minimal changes.
Please see the Long Term forecast discussion for more details on
model discrepancies coming into the weekend.

&&

.AVIATION...
PANC...VFR conditions should persist through most of the TAF
period. Ceilings could briefly reach into the MVFR range in any
rain showers. Southeast winds look likely this evening as a
coastal ridge pushes into the Turnagain Arm. The exact start time
could vary by an hour or two. These winds should be fairly short-
lived as the down-inlet gradient increases overnight.

&&

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

A wet pattern is on tap. The longwave pattern features a high
amplitude trough that spans from the Gulf Of Alaska to NE Russia.
There are multiple shortwaves circulating in the feature in the
middle to upper atmosphere. At the surface, there is a decaying
low near Montague Island, a surface low near Kodiak and another
surface low over Bristol Bay. This complex of disturbances is
going to bring additional moisture to portions of Southcentral
tonight and tomorrow. Friday a weak ridge will move into the
region with the ridge axis over Kodiak. It is a dirty looking
ridge but it should bring a brief break in precipitation activity
for Southcentral. Temperatures in the Mat-Su Valleys, the
Anchorage metro area and portions of the Kenai Peninsula will
increase a few degrees on Friday and Saturday.

&&

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

A vertically stacked low over Bristol Bay will move over the
AKPEN tonight and move southeast of the area. Showers will
continue for much of Southwest Alaska tonight and taper off early
Thursday morning over most areas. Showers will flare back up on
Thursday mainly over the higher terrain, however a weak area of
low pressure in the mid and upper levels will linger north of
Bethel, This feature will enhance showers Thursday mainly over
portions of the YK delta. Friday morning expect fog to develop
over portions of the YK delta with light winds and a ridge moving
over the area. Saturday a warm front will approach Southwest
Alaska this will bring increasing winds and rain.

&&

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

Low pressure over the eastern Bering Sea will continue to produce
showers near Cold Bay and Sand Point before tapering off Thursday
morning as the low pressure moves east. A transitory ridge will
cross the Bering and Aleutians tonight through Friday morning
bringing areas of stratus and patchy fog. Behind this ridge a low
pressure system west of Shemya will move eastward bringing
increasing winds and rain across the Western Aleutians through
Thursday. Friday and Saturday the winds and rain will spread east
through the central and eastern Aleutians. There are some model
differences as this low moves east the NAM, EC and Canadian models
were in best agreement with the GFS a bit farther south than other
models, leaned more towards NAM and EC solutions.

&&

.MARINE (Days 3 through 5)...

A North Pacific low moves across the Aleutians through Saturday.
Model confidence during this timeframe has remained consistent and
gale force winds are expected along the front in the eastern
Bering/Aleutians. Confidence begins to waiver Sunday morning. This
complex system spins up another low somewhere between 957 and 968
mb. Depending on the strength of this low will determine if winds
could potentially reach storm force or even hurricane force per
some guidance. As of now, high end gales are being forecasted.
Placement varies from over Dutch Harbor to just south of Saint
Paul with various locations in between not outside the realm of
possibility. The track of this storm through Monday remains highly
variable landing somewhere between the northeastern Bering or
crossing the Alaska Peninsula. All in all, there will be plenty
of refinement associated with this system as we go through the
week, but mariners in the area should pay close attention.

For the Western Gulf, timing of a front associated with a low
over the Aleutians is in better agreement today, expected Saturday
morning. Confidence is limited in regards to strength and
progression across the Gulf. Small craft to gale force winds are
possible as it propagates towards Prince William Sound late
Saturday into Sunday. Behind this front, confidence is reduced as
the track of the low, and its strength, remain in question. If
the storm tracks across the Alaska Peninsula, it could bring up to
storm force winds into the Gulf Monday.

&&

.LONG TERM FORECAST (Days 4 through 7/Sunday through Tuesday)...

A large area of low pressure is forecast to develop in the Bering
Sea on Sunday as the energy from the post-tropical remnants of
Typhoon Bavi merge with an upper level low in the Bering Sea.
There is a high degree of uncertainty in the exact placement and
track of this low. However, there is a high confidence that high
end gale force winds are to be expected with a moderate confidence
of storm force winds south of the low center.

Due to the high degree of uncertainty with respect to the track
of this low, an ensemble forecast methodology was used with
blending the ECMWF/GFS/GEM solutions.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 177 178 411.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...PJS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RC
MARINE/LONG TERM...CK/EJD