Small Craft Advisory 794, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
554 PM AKDT Mon Oct 19 2020

.ANALYSIS AND UPPER LEVELS...
A nearly stationary complex upper level trough over much of the
South Mainland is bringing rain and snow showers to the Southwest,
as well as steady snow from the Copper River Basin northward into
the interior. The snow over the Copper River Basin is from the
remnants of the cold upper low that brought the first snow of the
season to the low elevations of Southcentral yesterday. Over the
Gulf, a weak low moving into the Eastern Gulf is bringing offshore
flow and gusty outflow winds to much of the Gulf Coast.

Looking upstream, a high amplitude ridge is building from the
eastern Bering Sea toward western Alaska. Clouds and showers
persist over Southwest Alaska on the back side of the upper
trough. Weather across the Bering Sea and Aleutians is quite
benign under the influence of the ridge. Lastly, a short-wave
trough is digging southward out of eastern Russia, which will
lead to significant amplification of the upper level flow and
a more stormy weather pattern in the coming days.

&&

.MODEL DISCUSSION...
Models are in fairly good agreement with large scale features over
the next few days. In particular, they are locked on to the
general track of two deep lows developing south of the Aleutian
chain and Alaska Peninsula Tuesday through Wednesday. The 12Z NAM
was a bit faster in the northward progression of the frontal
system into Southwest Alaska Wednesday, but 18Z has come in line
with other solutions. With the more western system, the 18Z GFS
has shifted the low center further north toward the western
Aleutians and is quite a bit deeper (storm force) on Wednesday.
Will disregard this for now and continue with what had been a
consensus on a further south position and gale force low.

&&

.AVIATION...

PANC...While skies have cleared in the Anchorage area, radiational
cooling at sunset will quickly lead to formation of a surface-
based inversion. Thus, it appears likely that stratus and/or fog
will likely form tonight in Anchorage. The most likely scenario is
for a repeat of last night with an MVFR ceiling, but there
certainly could be lower clouds/vis in the vicinity. Low level
northerly winds will strengthen by late Tuesday morning, which
will clear out the clouds for good.

&&

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

A southward digging trough will continue to produce occasional
light snow over the Copper River Basin through the late evening
hours. Clouds will also linger from the Talkeetna and Chugach
Mountains east to the Al-Can border through tomorrow. A cold,
northerly flow in the wake of the trough will allow snow showers
to persist over the mountains through Tuesday. Valley locations
should remain relatively clear as drier air works in from the
interior. Low stratus, however, may hang around northern Cook
Inlet and the Kenai area late this evening through early Tuesday
morning before dissipating.

As the trough moves to the Gulf and a ridge builds inland, strong
northerly gap winds will develop along the coast in response to
both the increased thermal gradient and pressure gradient. Winds
speeds and gusts will first increase this evening along the
eastern Kenai Peninsula coast and spread east to Prince William
Sound and the Copper River Delta Tuesday. Winds peak for locations
like Seward and Whittier late Tuesday evening and continue through
late Wednesday for places like Valdez and the Copper River Delta.

A front is then expected to enter the western Gulf on Wednesday
with easterly gales and increasing chances for precipitation from
Kodiak Island and Shelikof Strait to the Barren Islands.

&&

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

Remnants of an elongated upper level trough will continue to
dissipate and move off to the southwest this evening as a ridge
builds in from the west. This should lead to showers dissipating
throughout the region by Tuesday morning, giving way to clear
skies and seasonal temperatures Tuesday and Wednesday for the
Southwest mainland. A strong front is then expected to reach the
southern AKPEN Wednesday morning, bringing strong winds and
moderate to heavy rain. By this time the upper level ridge is
expected to be centered near Norton Sound with the surface high
centered over the interior. These features should prevent the
progression of the front well into the Southwest mainland, instead
stalling and slowly weakening along the Southwest coastline on
Thursday. The main impacts from front will be strong winds and
rain from Cold Bay to Kamishak Gap as the front progresses north
Wednesday through Thursday afternoon.

&&

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

Surface high pressure and the associated ridge will allow for one
last day of calm weather across the region before two north
Pacific low pressure systems begin to impact the area Wednesday
morning. At this time a high amplitude trough and strong upper
level low center are expected to move southeast out of the
Kamchatka Peninsula. This will lead to the creation of two low
pressure systems in the north Pacific which will impact the region
through the end of the forecast period. On Wednesday morning the
eastern of the two lows is expected to be south of Nikolski, while
the stronger western low is south of Amchitka. Although the
eastern low is weaker, a strong pressure gradient will exist due
to the retreating high over the Alaska mainland. This should lead
to an area of storm force winds and rain along the front as it
moves from the central Aleutians Wednesday and into Bristol Bay
Thursday. Models remain in disagreement in regards to the western
low with some deterministic solutions showing gales over the
western Aleutians Wednesday before the low moves to the south. By
Thursday afternoon models come into better agreement that the
western low will absorb the remnants of the eastern low somewhere
near Adak. The resultant wind field should be widespread gales for
most of the Bering.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...

Gulf of Alaska:

A well developed front moves across the Gulf through Sat. The
track and development vary considerably, but widespread easterly
gale force winds and waves will spread across the Gulf through
Sat. The area of gale force winds retreat to around Kodiak Island
Sat, leaving small craft winds over the Gulf slowly diminishing
through Sat.

Bering Sea:

A North Pacific low and front moves into the Central Bering
through Sat. Track and development of the system vary
considerably, but overall area coverage looks reasonable, with
easterly gale force winds and waves spreading into the Bering.
High end gale winds expected in Bristol Bay near Alaska Peninsula
channeled areas. The winds wrap the low bring a second round of
gale force winds across the Eastern Aleutians Fri. The entire
system slowly diminishes to small craft winds Sat.

&&

.LONG TERM FORECAST (Days 3 through 7: Thursday through
Monday)...

At the onset of the forecast period there is a ridge axis that
spans from the East Pacific across Kodiak and north of the Brooks
Range. Additionally, there is a robust low over the Central
Aleutians. The leading edge of the precipitation shield will be at
the door step of Dillingham, King Salmon and Kipnuk. 48 hrs
later, the ridge axis is just east of Cordova and the upper level
low anchoring the aforementioned system is near the Pribilof
Islands. However, this pattern will result in broad southwest
flow over much of Southcentral and the precipitation shield will
have moved eastward. At this time, it is still too far out to tell
whether the front will bring any rain/snow to the Anchorage area.
The front will most definitely get sheared by the western Alaska
Range. Showers will linger over the Bering in association with the
remnants of the occlusion. In its wake, a new front will emerge
from the North Pacific. The precipitation shield will begin to
impact Attu Island and Shemya Saturday night.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 172.
 Gale Warning 127 128 130 155 165 170 171 173 174 175 176.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK/SB
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...MK/PS