Small Craft Advisory 914, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
541 AM AKST Sat Nov 28 2020

.ANALYSIS AND UPPER LEVELS...

The upper level jet extends from the Kamchatka Peninsula to
Southeast Alaska with the main jet core to the south of the
Aleutian Chain. This pattern is anchored by two occluded lows that
are readily apparent on satellite, one to the south of the AKPEN
and the second over the western Bering. The western low has lost
most of its upper level support and baroclinic energy and is
expected to weaken as it moves south into the north Pacific. The
much stronger eastern low that brought hurricane force gusts to
Nikolski yesterday will be the primary area of focus for todays
forecast package.

The deformation band from the primary low continues to bring gusty
winds and snow to Bristol Bay and the Kilbuck Mountains. On the
backside of the low, cold northerly winds are funneling through
the western AKPEN, producing snow showers tin areas like Cold
Bay. The occluded front just off the coast of Southcentral this
morning can be seen on both the Kenai and Middleton Island radars.
With the east-southeast orientation of the front, the western
Kenai, Anchorage Bowl and Mat-Su Valleys have remained downsloped
as of this morning. At the surface, winds have been increasing
through Portage Valley through the overnight hours with a peak
gust of 60 kts this morning along with reduced visibility and snow.

&&

.MODEL DISCUSSION...

Models have come to pretty good agreement in regards to the front
over the Gulf coast today. Discontinuities become apparent this
evening in regards to the low track moving north over the Gulf
Coast. After 36 hours guidance indicates either a broad region of
low pressure from Kodiak to Cordova or two low centers over the
northern Gulf. Minor differences in these solutions will result in
precipitation differences across Southcentral that will need
refinement in future forecast packages.

&&

.AVIATION...
PANC...VFR conditions will persist. A few light snow showers are
possible throughout the day that may temporarily lower ceilings to
MVFR; however, these are expected to be short lived.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Today
through Sunday Night)...

A large area of low pressure over the southwestern Gulf south of
Kodiak Island is the primary weather maker around Southcentral
this morning. It's associated leading front is currently moving
into the north Gulf Coast, causing the active weather over the
area. Blowing snow conditions have been occurring over Turnagain
Pass and Portage Valley, where a Winter Weather Advisory for
blowing snow remains in effect until 9am this morning. Barrier jet
winds are impacting the north Gulf Coast into Prince William Sound
as well, producing storm force winds. No other hazards are
actively in effect. Some of the moisture from the front is trying
to infiltrate westward across the mountains, but most of that
moisture is evaporating before it reaches the ground as virga.

The front will weaken through the day today, but southerly
onshore flow across the Gulf will keep the upslope component
continuing to wring out mountain snows and valley rain or a mix of
the two near the coast. The center of the low itself will move
into the northern Gulf this evening, then will track into Prince
William Sound tonight. This will enhance snowfall rates in the
coastal mountains, including Thompson Pass. There is some
discrepancy on the track of the low by Sunday morning, as most of
the model solutions have a secondary low forming over southern
Cook Inlet, and absorbing most of the energy from the first low.
This scenario would help to keep Anchorage, the Mat-Su, and
western Kenai Peninsula dry. Meanwhile, the snow will be steady
along the coast and through the mountains. This includes the
Seward Highway. The center of the low centered relatively close by
south of Seward Sunday morning should help to keep the winds much
lower across the eastern Kenai Peninsula by Sunday morning, so
the blowing snow threat will decrease with time, despite the
continuing falling snow. Forecast uncertainty increases by Sunday
night as the guidance struggles on where this new low slowly
tracks. While the general direction will be eastward back towards
the Middleton Island area by Sunday night, another option includes
the low opening up into a trough over Prince William Sound. This
second solution would keep the snow ongoing in all of the
surrounding mountains until full dissipation Monday morning.

For inland locations, most of this weekend is looking dry, as any
Gulf moisture making it inland will struggle to produce much snow.
That said, isolated snow showers can't be ruled out for Anchorage,
the Mat-Su, and Western Kenai Peninsula. Somewhat better chances
of precipitation are in the forecast for the Copper River Basin,
mainly from Glennallen south, as the southerly onshore flow
associated with the aforementioned Gulf low should be more
successful and getting more moisture inland in that area. Chances
are lower in the Cook Inlet region as a dry offshore flow is
expected to be predominant.

Temperature-wise, most of the area will stay at or above normal
this weekend, though Sunday night into Monday could offer a brief
return to colder, more normal temperatures. This will be brief,
however, because the next low in this parade of storms looks to
bring much warmer conditions and rain chances to much of
Southcentral next week.

&&

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

A cold but generally benign weather pattern is expected for
Southwest Alaska over the next 3 days. Current radar imagery and
observations show snow and blowing snow reducing visibilities in
Bristol Bay as a low pressure system continues moving
northeastward along the AKPEN. Snow should gradually taper off
this morning and allow visibility to improve, thus the Winter
Weather Advisory is set to expire at 9am AKST. Conditions will
become colder and drier as brisk northeasterly flow will bring in
continental polar air from the Alaskan interior Sunday and
Monday. Patchy fog is expected along the coast of the Kuskokwim
Delta and Bristol Bay Saturday night as MOS guidance shows little
to no dewpoint depression for various locations along the
Southwest coast.

&&

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

A fairly active weather pattern is expected to continue over the
next 3 days. Winds are gradually decreasing while seas are
gradually abating today as the storm system that has transited
through the Bering/Aleutians over the past couple of days exits
the region into the Gulf of Alaska. However, the break in the
action will be short-lived as a second storm system moves eastward
across the North Pacific. There is high confidence the center of
this storm system will remain well to the south of the Aleutian
chain, though the northern edge of this storm system will produce
storm force winds across the Aleutians Saturday night through
Monday afternoon. The western Aleutians will experience these
storm force winds first, as this low moves eastward winds will
gradually strengthen over the eastern Aleutians and weaken across
the western Aleutians. A GFS/NAM blend was the preferred solution
with the NAM being favored for the position of the storm center
while the GFS was favored for the intensity forecast. Over the
past couple of weeks the GFS and NAM have outperformed the rest of
guidance in these respective areas with the NAM having the best
handle on the position of synoptic scale features and the GFS
having the best handle on the intensity of these synoptic scale
features. High seas of 20-30ft are expected along the Pacific side
of the Aleutian chain of the western and central Aleutians Sunday
afternoon through Monday afternoon, these seas will gradually
abate thereafter.

&&

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

Gulf: A weak low lingers in the Northern Gulf through Monday
afternoon before moving onshore and dissipating. On Tuesday, a
series of shortwaves troughs move across the Gulf towards the
North Gulf Coast as a second weakening low approaches Kodiak
Island from the south. Winds will be primarily southeasterly on
Monday, transitioning to southerly by Tuesday afternoon as the
low moves north. Storm force winds may result as the front pushes
the North Gulf Coast, with a barrier jet likely to set up late
Monday night. Winds should begin to dissipate by Tuesday evening.

Bering: A strong North Pacific low pushes towards the western Gulf
over the course of a few days. This results in widespread gales
across the Southern Bering and possible storm force winds along
the central/eastern Aleutian coasts Monday into Tuesday. On
Tuesday the low is positioned further east and lingering gales and
small craft winds will be confined to the Southwest coastal
waters. Additionally, flow will be primarily northerly on Tuesday
with colder air being advected along the coast, thus, freezing
spray is likely, with heavy freezing spray possible along parts of
the immediate coast. High pressure sets up over the Central Bering
Tuesday afternoon.

&&

.LONG TERM FORECAST (Days 4 through 7/Tuesday through Friday)...

A strong front pushes the Southcentral Coast late Monday night.
With this, widespread precipitation is likely, with the heaviest
along Western Prince William Sound and Eastern Kenai. Further
inland, snow is likely through Wednesday morning before becoming a
rain/snow mix in the lower elevation. Significant warm air
follows this front, thus, precipitation along the coast should
start as a snow or rain/snow mix Tuesday morning and transition to
all rain by Tuesday afternoon. Rain will likely linger through
Thursday as a second low pushes the coast from the Southern Gulf.
Precipition transition back to snow by Friday with colder air
behind the low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 125, 161.
MARINE...
Storm Warning 119 172 174 177-178 411.
Gale Warning 125 128-130 123 139-140 150 155 160 165 170 173
175-176 180-181 351-352 413-414.
Heaving Freezing Spray 160 180 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CJ
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
MARINE/LONG TERM...KM

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