Small Craft Advisory 192, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
542 AM AKST Wed Mar 8 2023

.ANALYSIS AND UPPER LEVELS...

The amplified pattern continues over most of mainland Alaska this
morning. A strong ridge continuing to persist over the extreme
southwestern Gulf of Alaska as its axis extends north-northeastward
over Southcentral Alaska and up to the Arctic Ocean. Meanwhile,
an upper-level shortwave a little west of Nunivak Island
continues to move east-northeastward towards western Alaska. For
the Aleutians, mostly zonal flow with slightly building pressure
heights encompass most of the western/central Bering Sea and
western/central Aleutian Chain, while a stream of moisture between
the Gulf ridge and low pressure to the west in the North Pacific
is getting pulled northward towards the eastern Aleutian Chain and
extreme southern Alaska Peninsula. As a result of this, Cold Bay
currently experiences low ceilings, mist, and reduced visibility
to around 1 mile at times this morning.

&&

.MODEL DISCUSSION...

Guidance is in relatively good agreement when it comes to
placement and intensity of synoptic features through early
Saturday morning. There a few minor disagreements at the mesoscale
level however. One such point of contention is the evolution of
an eastward moving upper-level shortwave from the eastern Bering
Sea Wednesday morning through portions of the northern Susitna
Valley and northern Copper River Basin through Thursday morning.
The 00z Canadian model continues to hint at a stronger shortwave,
and thus, a little more precipitation, while other guidance is
not as strong with the wave. The other forecast challenge in the
short-term will be strength and duration of Southcentral Alaska
gap winds (Seward, Whittier, Valdez, and perhaps the Matanuska
Valley). All deterministic guidance is in general agreement with
onset of gap winds beginning during the day Thursday with upper-
level shortwaves driving southward down the eastern side of an
upper-level ridge centered in the North Pacific and extending
through western Alaska. The 00z NAM is the furthest west with
these southward driving waves and is the most aggressive with the
cold air advection, while other guidance is not as far west and
less robust with cold air advection.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist.

&&

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

The generous ridge of high pressure that brought plenty of sunny
skies the past few days will begin to flatten later today and push
westward as a low riding over the top will bring a shortwave
trough through Southcentral Thursday. Cold air advection and gusty
winds are expected to follow the trough. Precipitation associated
with this pattern shift appears to be more confined to the
interior mountains as it moves south and across the Copper River
Basin toward the Wrangell Mountains. No significant moisture is
associated with its passage.

Gusty winds can be expected through wind prone locations and
especially the gaps Thursday, ramping up through Friday. There is
enough uncertainty in the models to not have a clear picture of
the most likely scenario to expect. The main challenges are with
how much of an westward extent these stronger winds will have,
especially through the Matanuska Valley into Cook Inlet, and the
magnitude that can be associated from these winds. North to south
gaps are also much more likely to occur, primarily toward Valdez
and the Copper River Delta. The pressure gradient and cold air
advection will help accelerate winds through these gaps, but
models are struggling with core of this magnitude with NAM being
particularly bullish. This higher end potential has 850 mb winds
upwards of 80 kts while GFS is much lighter at half that
magnitude.

&&

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

Areas of rain and snow are ongoing this morning across the
Kuskokwim Delta and western parts of the Lower Kuskokwim Valley,
in tandem with a passing shortwave trough through the region. A
mix of rain and snow has been observed in Bethel and Lower Kalskag
with rain being observed in Aniak. This activity will shift
eastward through the day and eventually into Southcentral later
tonight as the shortwave exits the region to the east. A
developing ridge over the Bering will bring widespread dry
conditions back into Southwest as resultant offshore flow sees the
return of cooler temperatures over interior Southwest through the
rest of the week.

&&

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

A ridge of high pressure originally over the Gulf of Alaska is
gradually working its way westward across the Alaska Peninsula
this morning. High pressure is expected to expand across the
Bering through the end of the week with continued benign
conditions. Easterly winds and low stratus will be the more common
impacts into Friday.

Across the Aleutians, a warm front weakening to an inverted
trough is forecast to move from east to west across the Aleutian
Chain with rain and gusty winds from Unalaska today, and then to
Atka and Adak by Thursday afternoon. A weak trough over the far
western Aleutians may produce some widely scattered showers for
Attu and Shemya as well through tomorrow morning. By Friday, the
inverted trough moves back into the North Pacific with any
lingering shower activity over the Central Aleutians to quickly
diminish in coverage.

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)

The main question for the long-term continues to be in regards
to how quickly the upper-level ridge currently in place over
Southcentral breaks down as it interacts with an upper-level
trough setting up over the Alaska-Yukon border. Much of southern
Alaska will see northerly offshore flow heading into early
Saturday morning. As the aforementioned trough tracks across the
top of the aforementioned ridge, models are beginning to showcase
the trough strengthening and redeveloping over the Copper River
Basin. As the trough moves southward, expect increasing chances
for snow especially for areas south of Glennallen at higher
elevations. As the trough departs and moves into the Gulf, the
ridge is then displaced farther west. Colder temperatures will
arrive in Southcentral for Saturday and Sunday, particularly
across the Copper River Basin and northern Gulf coast as a cold
airmass spills in behind the trough. Some upper-level shortwaves
may work to enhance northerly winds through channeled terrain,
especially in areas such as Thompson Pass and the Copper River
Delta next weekend, though it will largely depend on the position
of these upper-level shortwaves.

Meanwhile, out west, calm conditions for the weekend and early
next week are expected as the upper-level ridge currently over
Southcentral shifts over the central Bering. Any moisture trapped
beneath the ridge of high pressure will likely introduce
persistent fog and low-level stratus for the Bering, Aleutians,
and Pribilofs. If the flow becomes more onshore in the Bering,
this could push the low-level stratus/fog inland into areas such
as Bristol Bay.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray: 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DAN
SOUTHCENTRAL ALASKA...VR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BL
LONG TERM...NSC