Heavy Freezing Spray Warning 193, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
628 PM AKST Sun Nov 28 2021

.ANALYSIS AND UPPER LEVELS...

An upper level low centered over the interior continues to keep
most of mainland AK in the deep freeze, with nearly everywhere
away from the immediate coastline (or out over water) still seeing
temperatures well below 0F. To the southwest, a complex low with
several centers at various stages of occlusion or formation spans
all the way from the western Aleutians to south of the Alaska
Peninsula. Out over the northern Gulf, a mesoscale low that has
aimlessly drifted in place for the past several days is finally
beginning to lose integrity as it swirls up towards the North Gulf
Coast just south of Cordova. Meanwhile, an extremely small scale
but surprisingly intense "Cook Inlet" effect mesoscale snow band
continues to sit just offshore along the Kenai Peninsula, except
for where it continues to bend into Homer along the leading edge
of westerly winds funneling into Kachemak Bay. This band managed
to produce extreme snowfall rates over a concentrated area this
morning, with well over 1 foot of new snow accumulation observed
up along the Homer Bluffs from late last night into this morning.

Looking at the details of the western system, there are currently
two distinct circulations embedded in broader cyclonic flow
extending well across the open North Pacific, southern Bering and
Aleutians. The first center located south of Amchitka is already
beginning to weaken as it becomes increasingly stacked/occluded.
Still, it continues to spread bands of rain across the Western
Aleutians, where temperatures remain well above the freezing mark.
Farther east, a much stronger and maturing low with central
pressure down near 955 mb is spinning out to the south of Dutch
Harbor. The extremely tight pressure gradient on the north side of
the low is driving winds gusting to near hurricane force at
times, with Nikolski observing wind gusts just over 70 mph
earlier in the day. The low is also bringing a band of moderate to
heavy snowfall across the Eastern Aleutians, including at
Unalaska, where conditions are deteriorating as winds gusting to
storm force at times are beginning to blow around the accumulating
snow and reduce visibilities. The attendant front with the
stronger low is advancing northeast towards the Alaska Peninsula
and Kodiak Island, with signs of a new low formation along it
perhaps just beginning to emerge on satellite imagery.

&&

.MODEL DISCUSSION...

The main point of disagreement continues to be related to the
short term and smaller scale details with the large and complex
system on track to move from its current location south of the
western Aleutians and up towards the Gulf and Kodiak Island. The
main struggle continues to be how models are (or failing to) lock
on to the very complex nature of how this system is evolving,
namely involving the continued cyclogenesis forming new centers
along the front associated with the entire system. This rapid-fire
formation of new centers is causing headaches abound for model
agreement and consistency with each new initialization, with the
consensus for the track of the collective center(s) continuing to
be a moving target over the next 24 to 72 hours.

As of this writing, newest data has shifted back in favor of a
more west track for the low, with increasingly better agreement
now appearing to align better for a track of the primary/largest
low, with 18z guidance generally taking it near or immediately
east of Kodiak Island by Tuesday morning, then north towards the
Kenai Peninsula and Prince William Sound by Tuesday evening. The
exception is the NAM, which has been discarded as a clear outlier
that has not been properly initializing the low based on
observational data and satellite trends. There are a number of
important changes to what is now expected in terms of more
impactful weather over the next couple days due to the track
shift and developing consensus for a more west track of the
primarily low across the Gulf for both domains. Most of these
changes involve when and what areas will see stronger winds, snow
and blowing snow during the next two to three days, mainly
involving areas near the Kenai Peninsula, Prince William Sound,
Kodiak Island and Bristol Bay. Please see the Southcentral and
Southwest AFD sections for additional details.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist for most of the
TAF period. Winds out of the north may begin to pickup slightly by
late on Monday morning, with gusts near 20 kt possible at times.
Ceilings will also likely err towards low VFR levels through the
period, with occasional MVFR ceilings possible.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 to 2: Tonight through
Tuesday)...

The low that has been lingering near Prince William Sound for the
past several days will move onshore and weaken into early Monday
morning. As it tracks northward, it will bring snow showers
across the Northeast Prince William Sound through the night.
Outflow winds through Seward, Whittier, and Kamishak Bay will
subside as the pressure gradient along the coast weakens. With
winds through Cook Inlet becoming northerly this evening, the snow
band near Kachemak Bay is expected to shift southward and taper
out through early tomorrow morning. For Thompson Pass, a Wind
Chill Advisory remains in effect as cold air funnels out of the
Copper River Basin and through the pass.

The major feature to watch will be the complex low tracking from
the North Pacific into the Gulf. While confidence and model
agreement have been steadily increasing over the past few days,
much uncertainty remains in the track of the low and the behavior
of any triple point lows that form along its front. With the
current forecast package, the low is expected to remain near
Kodiak as it tracks northward into the Gulf. This will lead to
stronger warm air advection and result in a quicker transition
from rain to snow for Kodiak Island. With the low nearer Kodiak,
stronger winds and blowing snow are possible. Uncertainty persists
with snow amounts as even small changes in temperature or how
quickly the snow transitions to rain can affect accumulations. A
Winter Weather Advisory has been issued for Kodiak and will be in
effect from early Monday morning through noon.

As the front tracks past Kodiak and into the Gulf, it is expected
to bring areas of moderate to heavy snow along the eastern Kenai
Peninsula and Prince William Sound from Monday through Tuesday.
Depending on how far north the front tracks, some coastal
locations could see a mix of snow with rain. Northerly flow behind
the low will result in strong winds through the Mat Valley, Cook
Inlet, and Shelikof Strait. Overall, temperatures should increase
through Tuesday with increasing sky cover and warm air advection
associated with the low.

&&

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

A general troughing pattern extending from the Alaska mainland to
the eastern Bering Sea will remain in place for tonight, with a
cold airmass that will maintain temperatures still well below
zero. Beginning Monday morning, a low south of AKPEN will help
draw moisture northwestward. A combination of an upper level wave
pinwheeling on the backside of the low with gusty winds at the
surface will cause blowing snow conditions to develop across the
northern Bristol Bay coast. Areas north and west of Dillingham
can expect to see the heaviest snow through Monday. Additionally,
blowing snow are also possible along the southern Bristol Bay
coast through Tuesday morning. Please see the Winter Weather
Advisory for more details. Snow will taper off with subsiding
winds by late Tuesday as the main low exits to the Gulf.

&&

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

A front working in concert with a low currently situated south of
Nikolski will continue to bring high end gales to low end storm
force winds and elevated seas across the Aleutians, Bering Sea,
and AKPEN through Monday evening. In addition to snow showers, these
expected conditions have prompted the issuance of a Blizzard
Warning for the Eastern Aleutians including Dutch Harbor along
with a Winter Weather Advisory for Alaska Peninsula until Mon
morning. Additionally, the latest operational model runs are
slowly coming into an agreement with the track and position of the
complex low as it migrates south of the Aleutians and into the
Gulf. Suffice it to say that an active weather pattern will
persist through early next week.

&&

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

Gulf of Alaska: There is a great deal of uncertainty in the track
of any feature that enters the Gulf through mid-week. An area of low
pressure is likely over the northern Gulf at the start of this
period, lingering through the end of the workweek. Elevated seas
and gale-force winds will be possible rounding the southern
perimeter of this low from Kodiak Island southeastward.

Bering Sea/Southwest: Elevated winds and seas will be possible
across the eastern Bering and southwest of Kodiak Island at the
start of the period but will be diminishing through Wednesday. The
next system is expected to be entering the southwestern Bering on
Wednesday. Models currently suggest this next system will quickly move
to the south-southeast of the Aleutian chain. However, a stronger
system is expected to move eastward across the Bering Thursday
into Friday. Southerly gales and elevated seas will lead this
front with west-northwesterly gales following.

&&

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

Models show a high amplitude ridge extending north-south across
the Bering Sea at the start of the extended period. This feature
will push east forcing the persistent long wave trough that has
been in place over the state eastward. This feature will also set
up the pattern for reinforcing the cold air over much of the area.

Guidance continues to indicate development of a strong Kamchatka
low. There is increasing confidence that this feature will drive
a warm front across the Bering as the core of the arctic airmass
over the state shifts east. This system may be the catalyst in
changing the overall upper-level pattern as it becomes more
progressive with a westerly steering flow. The front drives to the
west coast of the state by Friday as it begins to occlude.
Nonetheless, there may be some precipitation type concerns around
the AKPen and Bristol Bay as warmer air advects northward. Both
the GFS and EC then show the potential for a triple point low to
form in the western Gulf as the front moves to the interior of the
state. This new low would likely help draw moisture northward as
the front weakens and its associated shortwave moves east,
bringing the potential for light snow from the Kuskokwim Valley
east to the Copper River Basin through the day Saturday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...

PUBLIC...Blizzard Warning: 185.
Winter Weather Advisory: 161 171 181.
Wind Chill Advisory: 131.
MARINE...Storm Warning: 130 170 172-176 413.
Gale Warning: 119 120 127 131 132 138 139 150-165 171 177-180 351
352 411 412 414.
Heavy Freezing Spray Warning: 121 127 129-131 138 139 140 150
160 165 179-185 412 414.

&&

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...KC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MF
MARINE/LONG TERM...NR