Small Craft Advisory 712, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
532 PM AKDT Sun Sep 20 2020

.ANALYSIS AND UPPER LEVELS...

A trough continues to extend from the Chukchi Sea southward
through the central Aleutians. A fairly deep (for this time of
year) 979 mb surface low anchors the base of this tough, and
was located halfway between Adak and the Pribilof Islands this
afternoon. To its east, a flattening ridge is still present
across southwest Alaska and the Alaska Peninsula, with a weaker
low also located across the eastern Gulf.

Local radar imagery showed a few showers affecting the Kuskokwim
Delta, and some more convective showers across Prince William
Sound and the central and northeastern Gulf, where a random
lightning strike or two has occurred. In between these two areas
is a convergence band that bisected the northern half of the
Cook Inlet where somewhat heavier and more persistent rainfall
was occurring.

&&

.MODEL DISCUSSION...

The models remain in very good agreement over the next few days,
leading to higher than average forecast confidence. The main
challenge is with the hi-res wind forecasts, as they're showing
gales along the Gulf Coast versus storm force sustain winds in
the boundary layer wind fields. Given the strength of the low
tracking into the Gulf, with barrier jet dynamics developing, it's
a good bet the hi-res will eventually show storm force winds once
we get inside a 36 hour forecast...but that'll be something to
keep an eye on.

&&

.AVIATION...

PANC...VFR conditions with light winds will persist through the
forecast period. A persistent convergence boundary is aiding in
shower development across the Cook inlet, with some low clouds
around 1000 feet developing there and at the base of the Chugach
Mountains on the east side of town. These clouds may offer very
brief periods of scattered to broken conditions at that level.

&&

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

An upper level shortwave will continue to slowly move east across
Southcentral tonight and tomorrow morning. This wave will produce
scattered showers across the area with the more concentrated
showers across the Susitna Valley and Talkeetna Mountains.
Meanwhile a vertically stacked low pressure system over the
northeastern Gulf will move north and weaken on Monday. This low
will produce rain from Cordova to the Prince William Sound and up
into the southern parts of the Copper River Basin through tomorrow
with most of the rain near the coast and higher elevations. As we
head into Monday afternoon and evening just a few showers expected
north of the Chugach Mtns and into the interior. By late Monday
into Tuesday another low pressure system moves into the Gulf.
This will bring Gale to Storm force winds as a barrier jet forms
across the northern Gulf. The heaviest rain will be from Kodiak
Island to the eastern Kenai and Prince William Sound where 2 to 4
inches of rain are possible Tuesday into Wednesday. Seward is
expecting around an inch of rain or slightly higher. North of the
Chugach Mtns, most of the area will be dry with lingering clouds
slowly thinning late Tuesday into Wednesday as an upper level
ridge builds across Southcentral.

&&

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

A low tracking over the eastern Aleutians is bringing generally
easterly winds across much of Southwest Alaska. Its front is
expected to remain mostly offshore, however brief rain is likely
along the Bristol Bay coast this evening through early Monday
morning. Winds will slowly shift to northerlies as the
aforementioned low moves into the Gulf, bringing cooler
temperatures toward mid-week. High temperatures will remain in
the 40s over much of the region, though cloud cover will somewhat
moderate the lows, keeping them above freezing outside of higher
elevations. Showers are possible Wednesday near the Alaska Range.

&&

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

A broad low pressure system is tracking southeastward across the
Bering. It will move across the eastern Aleutians, entering the
North Pacific by early Monday morning. Gusty northwesterly winds
and widespread showers are expected on its backside through
Wednesday, however winds will slowly diminish over that timeframe
as the low moves farther away into the Gulf. Showers will taper
off generally west of Atka, which will be more under the influence
of a ridge centered near Attu.

&&

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

The large and complex low over the Gulf on Wednesday will break
down and move onshore over Southeast Alaska on Thursday. There is
high confidence that winds will remain calm over the Gulf through
Friday. Northerly winds over the eastern Bering will subside
throughout the day Wednesday and return to calm conditions
Thursday morning. Models have come to better agreement that the
next storm system will stay south of the Aleutian Chain, resulting
in higher confidence of calm conditions for the Bering through
Friday.

&&

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

The broad longwave trough is expected to continue the fall pattern
for most of the AOR with colder temperatures coming into the
Bering on Friday. The main weather story for the start of the
forecast period is a broad low pressure complex over the Gulf.
Multiple meso-low centers are expected to revolve around one
another in the Gulf through Friday as the system weakens. This
will bring rain to coastal areas of Southcentral, while a series
of upper level vortices provide chances for rain to inland areas
Thursday and Friday.

A slight pattern change will occur by Friday afternoon as a polar
trough drops in over the Bering Strait and into the central Bering
Sea. 850 mb temperatures ranging from -5C to -10C are expected to
move over the Bering through Sunday. This may bring some snow or
mixed precipitation types to the Bering and western Aleutian Chain.
For Southwest this could mean high temperatures in the upper 20s
to low 30s on Friday. Highs should rebound into the 40s by
Saturday as the next storm system moves into Gulf. Model consensus
has improved considerably since yesterday's deterministic runs.
Solutions have trended more toward yesterday's EC run, with a
much weaker system moving quickly east to the south of the
Aleutian Chain on Friday before moving north into Gulf on
Saturday. Guidance still has considerable spread in intensity and
timing of this feature, but it should return heavier rainfall to
coastal areas of Southcentral on Saturday and Sunday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 119 120 130 131 132 138 139 160 170 171 172
173 174 175 176 177 179 180.
FIRE WEATHER...NONE.

&&

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