Small Craft Advisory 709, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
431 PM AKDT Sat Sep 19 2020

.ANALYSIS AND UPPER LEVELS...

A shortwave ridge axis extends from the north Pacific, through
southwest Alaska, up through the center part of the state. This is
in between a trough that extends from eastern Siberia through the
central Aleutians, and an area of low pressure across the
southern Gulf. Area radar imagery shows showers with this low
moving northward through the Gulf Coast, with other showers trying
to move into coastal locations of southwest Alaska as the
aforementioned ridge breaks down.

&&

.MODEL DISCUSSION...

The models are generally in good agreement at the surface and
aloft over the next few days, leading to higher than average
forecast confidence.

&&

.AVIATION...

PANC...VFR conditions along with southerly winds occasionally
gusting into the 20 to 25 knot range will persist through this
evening. MVFR ceilings are possible after 8z due to an increase in
shower activity around the terminal. There is also some indication
in the forecast guidance that IFR ceilings may develop over the
terminal after 14z. Confidence in this is low, however, and is
currently not included in the current TAF package.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Sat
afternoon through Tue afternoon)...

An upper level trough to our west will move in this evening,
bringing increasing chances for rain across Southcentral. The
greatest amount of precipitation will be along the northern Gulf
coast and the eastern Kenai Peninsula. There are some showers
lifting northward this afternoon over Prince William Sound and
across the Susitna Valley, but as this trough moves in, rain is
expected to become more widespread. Sunrise on Sunday morning may
reveal sights of termination dust across the higher elevations of
the Chugach and Talkeetna Mountains, as cooler temperatures aloft
allow for overnight precipitation to fall as snow.

As the trough exits to the north Sunday afternoon, conditions
should generally improve over the Susitna Valley, western Kenai
Peninsula, and Anchorage Bowl, though a stray mountain shower
cannot be ruled out. Additional rainfall is likely for the Prince
William Sound area as well as across the Copper River Basin
through Sunday night.

By Monday, the attention shifts westward, where a large low
developing over the Bering moves toward the southeast and lifts a
front across Kodiak Island early Monday. Abundant moisture and
gusty easterly winds are likely, with gale force winds expected
along the extent of this front across the Gulf waters. While the
low center remains well south of Southcentral over the Gulf, the
associated front will bring gusty winds and rain to the northern
Gulf coast as it lifts northward Tuesday morning. Once again, the
bulk of precipitation is expected to fall across Kodiak Island and
the northern Gulf coast, while interior areas of Southcentral
including the Susitna Valley and northern Copper River Basin
should remain relatively dry through Tuesday.

&&

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

Generally quiet conditions are expected through the weekend. Weak
shortwave troughs are currently producing isolated showers across
portions of the Kuskokwim Delta and Bristol Bay. These will
gradually taper off overnight under a weak upper level ridge.
Combined with weak southerly flow, patchy fog and/or low stratus
is possible near the Kuskokwim Delta coast. A similar pattern is
expected for Sunday with weak shortwaves propagating over
Southwest Alaska bringing isolated showers. A front associated
with a low over the eastern Aleutians will bring gusty easterly
winds to much of the region by Monday. As such, conditions will
remain dry in the lee of the Alaska Range and temperatures will
drop slightly, with lows well into the 30s over interior areas.

&&

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

A broad low will move southeast across the Bering through the
weekend. Its associated front will bring steady rain and gusty
southerly winds across the Aleutians, while strong northwesterly
winds and showers will develop on its backside. Cold air
advection is starting to become more robust as the fall season
approaches. The low will become vertically stacked near Unalaska
on Sunday evening, thereby weakening. A cool, northwesterly flow
will develop is its wake ahead of a developing ridge over the
western Bering. Thus, showers will continue through early next
week.

&&

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

Gulf of Alaska: A broad and strong storm system is expected to
remain nearly stationary and weaken over the southern Gulf through
Thursday. There is high confidence in Gale force winds over the
northern Gulf coast with potential for storm force gusts if a
secondary low is to develop in the lee of Kodiak Island on Tue. By
late Wednesday night the low is expected to weaken, which will
bring winds down to at or below small craft criteria.

Bering Sea/Aleutian Islands: There is high confidence that the
broad low pressure system over the Gulf will bring moderate to
strong northerly winds over the eastern Bering. The winds will be
strongest (gale force) through the terrain gaps from Nikolski to
Sand Point through Thursday morning. By this time winds should
calm down to well below gale criteria as the low to the east
breaks down. Over the western Bering, there is high confidence
that high pressure and calm winds are expected to persist through
Wed night. On Thursday confidence decreases significantly in
regards to the next system, with models showing a 20 mb spread.
If the stronger solutions comes to fruition, gale force winds are
possible moving east across the Aleutian Chain Thursday night into
Friday.

&&

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

A broad and strong low pressure system is expected to be centered
over the Gulf at the start of the forecast period. The main impact
from this system will be for the Gulf coast and Kodiak Island,
where strong winds and rain are expected to persist through
Wednesday afternoon. For Southcentral it should be mostly dry,
with seasonal temperatures and the occasional shower as weak
shortwaves move around the low center. On the backside of the
low, strong winds and increasingly cool, northerly wind are
expected to persist over the eastern Bering and coastal areas of
Southwest through Thursday night. By Friday morning, an arctic
trough is expected to move in from the Bering Strait, which will
cool off the Bering considerably through Saturday.

The main forecast question is in regards to the next system
forecast to move over/near the central Aleutians Thursday night.
Forecast confidence decreases considerably due to a large model
spread in both the location and strength of this system. The GFS
and Canadian models have a storm force low clipping the central
Aleutians on Friday as it rapidly moves east into the Gulf by
Saturday morning. The primary difference between these solutions
and the EC, which has a considerably weaker system, is whether or
not jet remains zonal or becomes slightly meridional. If the jet
is to become meridional, then the storm will have a better chance
of tapping into the strong temperature gradient N/S of the
Aleutian Chain to produce a stronger storm. Regardless, model
agreement and forecast confidence increases once again Saturday, with
a strong system moving into the Gulf.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 160 172-175 177 179 411-414.
Storm Warning 176 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...CJ