Small Craft Advisory 678, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
424 PM AKDT Wed Sep 9 2020

.ANALYSIS AND UPPER LEVELS...

A ridge remains anchored over the eastern Pacific, extending into
the southeastern Gulf. Broad, southwesterly flow along the back
side of the ridge continues to push moisture from the north
Pacific over the western and northern Gulf, with areas of low
clouds, fog, and rain from Valdez and Cordova east. Over the
interior, there are a series of westerly shortwaves. The first of
these is moving across northern Susitna Valley and Copper River
Basin this afternoon with clouds and showers. The second is
weaker and is moving into the Kuskokwim Valley from the Y-K Delta.
This feature is also enhancing cloud cover and shower activity
across the Middle Kuskokwim Valley. The third shortwave can be
readily seen on infrared and water vapor imagery and is dropping
southeast out of the Bering Strait.

For the Aleutians and Bering, a ridge is in place over the
southern Bering, eastern Aleutians, and southern AKPEN. Farther
west, a Kamchatka low is sweeping a gale-force front over the
western Bering and Aleutians. The front is positioned just east of
Shemya and has stalled as it runs up against the ridge
downstream.

&&

.MODEL DISCUSSION...

Models remain in good agreement through the short term forecast
period. The are some discrepancies by day three regarding the
position and strength of the deepening Kamchatka low northwest of
Attu. However, there is good clustering of this system within the
guidance, and the sensible weather - widespread rain and gales
both along the front and wrapping underneath the center of the low
- would not be too varied given the spread in solutions.

For Southcentral and interior Southwest, guidance is still keying
in on the advection of cooler air aloft with the passage of a
shortwave overnight Wednesday into Thursday morning. The main
forecast challenge with this system will be the strength of
developing outflow winds and areal coverage of cloud cover, both
of which will influence low temperatures over portions of
Southcentral Thursday and Friday mornings.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through the
forecast period.

&&

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

Primary forecast focus is on colder temperatures and gap winds.

The first in a series of shortwave troughs will move across the
area tonight. This trough will enhance shower activity mainly from
the Susitna Valley eastward to the Copper River Basin. Some
instability is expected through this evening so a couple isolated
thunderstorms possible across the Copper River Basin where best
heating and instability will exist. This shortwave will begin to
usher in colder air tonight and tomorrow with lows dipping into
the mid to upper 30s across the Anchorage Bowl and MAT-SU valleys
with a few spots near or below freezing. Winds overnight across
the Anchorage bowl should keep temperatures above freezing.
Additionally, cloud cover may also keep temperatures a few degrees
warmer so not expecting a widespread freeze tonight. Gap winds
will begin to increase tonight and strengthen tomorrow as the
pressure gradient increases between high pressure over Southwest
Alaska and lower pressure to the southeast. These winds will
affect Seward, Whittier, Valdez, the Copper River Delta and
adjacent coastal waters. Tomorrow night a weaker shortwave trough
will move across the area, but this wave is expected to be drier
with a few showers expected in the Copper River Basin. Friday
morning is expected to be the coldest morning with temperatures in
the mid 20s to low 30s in most valleys, so a freeze warning has
been issued. Gap winds will begin to weaken by Friday afternoon,
except for the Copper River Delta where they will stay gusty
through Saturday morning. Another cold morning on tap for Saturday
with lows in the 20s to 30s in most valleys. Otherwise dry with
clearing skies expected for Friday and Saturday with nice weather
expected into the weekend as high pressure takes hold over the
area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Tonight
through Saturday)...

An upper level short-wave which crosses Southwest Alaska has now
exited eastward, leaving behind an upper-level ridge currently
over the eastern Bering Sea. This ridge will slowly push eastward,
moving overhead of Southwest Alaska on Friday and remaining in
place through Saturday. This will lead to cooler and drier
conditions for much of the area, particularly for the Kuskokwim
Valley where temperatures will bottom out in the upper 20s
tonight and Friday night. By Saturday, the current ridge will
weaken, but a new ridge will form ahead of a deep low moving into
the western bering Sea and move over Southwest Alaska. Thus, the
cool and dry weather will persist into the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Tonight
through Saturday)...

A vertically stacked low and associated surface occluded front
over the western to central Bering and Aleutians will continue
to push up against the west side of the ridge sitting over the
eastern Bering Sea. Therefore, the front will make little
additional eastward progress and wind and rain will remain
west of Adak through Thursday. The remainder of the are will
remain relatively clear and dry, with perhaps some patchy fog
under the influence of the ridge sitting overhead.

A second low will develop and intensify over the northwest
Pacific Thursday through Thursday night, absorbing the current
low by Friday. A short-wave dropping out of Russia will help
amplify the upper level flow, leading to a stronger and deeper
low than the current one. This in turn will lead to formation of
another blocking ridge downstream over the eastern Bering Sea
which will cause the low to stall just north of Attu late Friday
night, similar to the previous pattern. Generally expect more
widespread impacts from the rain and wind with this second storm.
However, the bulk of rain and wind will remain west of a line
from Saint Paul to Atka through Saturday afternoon. The ridge
will then shift east over Southwest Alaska Saturday night,
allowing the front to make more eastward progress across the
central Bering and Aleutians.

&&

.MARINE (Days 3 through 5, Saturday through Monday)...

A Western Bering low remains nearly stationary while its front
slowly moves across the Bering through Mon. The track and
development of the low vary, but confidence is good for gale force
winds and waves to wrap the low and extend along the front
through the central Bering on Sat, then diminish to small craft
winds with the front across the eastern Bering for Sun and Mon.

High pressure spreads across the Gulf through Mon with mostly
Northerly winds over the Gulf. Weak troughs ride over the high
through Mon.

&&

.LONG TERM FORECAST (Days 4 through 7, Saturday through Monday)...

The pattern is expected to remain amplified through the weekend
with a ridge over mainland Alaska and a trough situated over the
western half of the Bering. This will keep conditions cool,
clear, and dry across Southwest and Southcentral Alaska through
Monday. By Sunday night, an occluded front associated with a ~982
mb low northwest of Attu will sweep across the eastern Bering and
approach the Southwest coast with a triple point low potentially
developing over the eastern Bering. By Monday, the front is
expected to move onshore with rain and gusty southerly winds for
Southwest Alaska as the low lifts north. The front and the
associated upper-level shortwave then continues east, as the ridge
retreats, toward Southcentral with increasing chances of stronger
winds and widespread rain by Tuesday. The entire longwave trough
over the Bering moves east as well, resulting in a more pronounced
south to southwesterly flow extending from the eastern half of
the Aleutians to the Gulf of Alaska. this looks to usher in a
period of unsettled weather across much of the region by mid-week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...FREEZE WARNING 101, 111.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SEB
MARINE/LONG TERM...MK/TM