Small Craft Advisory 970, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
536 AM AKST Tue Dec 6 2022

.ANALYSIS AND UPPER LEVELS...

A negatively-tilted upper trough extends southeastward from
Siberia to over Southcentral Alaska. Upper level ridging now
extends northeastward across the Gulf into Yukon Territory. The
jetstream remains south of 50N west of 155W and then extends
northeastward into Yukon Territory.

Closer to the surface, areas of snow are moving northeastward
across Southcentral and the northern Alaska Peninsula.

&&

.MODEL DISCUSSION...

Forecast confidence remains high with respect to the big synoptic
picture but lower for the fine mesoscale details. The biggest
forecast challenge continues to be snowfall amounts across
Southcentral as a small shift in the low track over the Gulf can
significantly impact snowfall accumulations. A more near-shore
track would result in higher snowfall amounts while a more off-
shore track would result in lower amounts. Mesoscale models show a
consistent trend of a mesoscale deformation band forming across
the Anchorage Bowl extending southwestward cross the central Kenai
Peninsula on Wednesday. If this deformation band forms, it means
significantly higher snowfall amounts for these areas. Where this
deformation band forms significantly impacts the snowfall amounts.

&&

.AVIATION...
PANC...VFR conditions are expected with temporary LIFR conditions
through 16z as snow showers and heavier snowbands can
significantly reduce ceilings and visibility. Conditions will then
remain LIFR to IFR for the remainder of the TAF period as snow
bands increase in frequency.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Thursday night)...

A fairly complex pattern is leading to forecast uncertainty today,
with good potential for moderate to heavy snow across portions of
Southcentral today through Wednesday. A high amplitude long-wave
trough centered over the eastern Bering Sea will progress east-
northeast into Southcentral and the Gulf of Alaska today. A host
of embedded short-waves will move through the trough, with some
rotating northward around the eastern side of a vertically stacked
low spinning over Siberia, some right into Southcentral, and some
digging southeastward across the Gulf of Alaska. Meanwhile, a
surface low near Kodiak will deepen ahead of the trough and track
across the northern Gulf. An increasing negatively tilted trough
aloft combined with bottom up development of the low over the Gulf
will lead to a deformation band of heavy snow extending north-
northwest from the low center. However, the complex movement of
the upper level short-waves is leading to uncertainty in track of
the surface low and exact location of the heavy band of snow.

Based on model trends overnight, it is looking like the deformation
band will form along the western Kenai Peninsula midday then shift
north and eastward this afternoon and evening where it will linger
through early Wednesday morning. Based on this solution, expect
the heaviest snow from the western interior Kenai Peninsula north
to Anchorage - and possibly continuing into the southern Mat-Su.
Have issued Winter Weather Advisories for the western Kenai
 and Anchorage. Any small shift in the surface low track and
location of the deformation band will significantly affect the
snow forecast, so stay tuned to the latest forecast updates.

To further complicate matters, the 12Z Anchorage sounding is still
quite warm in the low levels. During periods of light precipitation
areas observations indicate the presence of light rain or drizzle.
Do expect steady cooling of the atmosphere as the upper trough
moves in, with low level cold air advection lagging behind tonight
through Wednesday. Despite this, expect the bulk of precipitation
to fall as snow - and this does not affect the likelihood of
heavy snow underneath the deformation band. Areas along the Gulf
coast are warm enough for plain rain, with mostly snow across the
remainder of Southcentral.

An upper level closed low will form over the northwest Gulf by
Wednesday morning and sink southward into the central Gulf. This
will bring and end to the deformation band. However, cold air
advection and northerly flow (and some weak cyclonic flow aloft)
will cause light snow to linger along the north to west side of
mountain ranges through Wednesday night. Meanwhile, strong and
gusty gap winds will develop across the western Gulf and Kodiak
Island tonight, then across Southcentral mountain ranges tonight
through Wednesday. A reinforcing shot of Arctic air will arrive
Thursday while low pressure lingers over the eastern Gulf. As a
result, expect persistent gap winds through the end of the week
along with steadily dropping temperatures. By Thursday night, much
of Southcentral will be back in the deep chill, with sub-zero
temperatures likely inland and single digits for much of the
remainder of the area.

&&

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

Precipitation moves off to the east through the morning hours.
There may be some periods of lingering snowfall in places but
overall, conditions should begin to dry out. There are a series of
surface lows moving across the Bering Sea into Tuesday night,
pulling a colder airmass towards Southwest Alaska. Anticipate
temperatures dropping back into the teens and 20's by Thursday.
Additionally, winds will pick up along the Alaska Peninsula late
Tuesday night and persist into Wednesday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

Gale force winds across the Bering Sea are diminishing to small
craft today. There will still be some enhanced winds south of the
Alaska Peninsula that will hold on into Wednesday morning. A
couple of surface lows traversing across the Bering are advecting
a colder airmass southwards, but for the most part these systems
are relatively non-impactful. Benign conditions are expected over
the next couple of days. The next front is expected to move into
the southern Bering Wednesday afternoon. The front is weaker, with
areas of small craft winds and light precipitation.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

Long term model guidance continues to show significant
discrepancies with placement of key synoptic features over the
weekend. Ensembles are initially in somewhat good agreement to
start the day 4 period on Friday. Weak ridge of high pressure will
be making its way into Southwest Alaska from the eastern Bering
while a trough of low pressure over the eastern Gulf of Alaska
slowly exits the region to the southeast. Thus average to above
average confidence exists in decreasing precipitation over
Southcentral along with decreasing temperatures over Southcentral
on Friday. This should also see any precipitation over the Bering
and Southwest also diminish on Friday with temperatures from the
Aleutians into Southwest remaining generally unchanged.

Saturday into Sunday is where confidence begins to drop off more
dramatically. The GFS ensemble brings a developing low across the
Aleutians, much further south than the EC, while amplifying a
ridge into Southeast Alaska. The EC ensemble paints a less
amplified pattern, with a weaker trough skirting the Aleutians and
a weaker ridge of high pressure over the Gulf of Alaska. The
implications being much stronger northerly flow with the GFS and
the potential for widespread gales through the Aleutians, Alaska
Peninsula and into the Gulf of Alaska by Sunday. The EC would
promote milder winds over the Aleutians on Saturday, but does
agree on enhanced drainage winds into the Gulf of Alaska on Sunday
as low pressure begins to build offshore of Southeast Alaska.
Ultimately, both ensembles have a more amplified trough building
into the eastern Gulf of Alaska by Monday with high pressure
ridging into the Central Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 101 121.
MARINE...Storm Warning 130
Gale Warning 120 131 132 136 137 138 150 155 170 173 185 351 352 411.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ED
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
LONG TERM...BL