Small Craft Advisory 46, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
514 AM AKST Mon Jan 17 2022

.ANALYSIS AND UPPER LEVELS...

The weather remains quiet across Southcentral for the time being.
Overnight, mostly low stratus and areas of fog were the main
issues. A weak upper level shortwave in the flow aloft was passing
over the western Alaska Range this morning with mainly high level
clouds out ahead of it. However, it seemed to be doing minimal
impact to what was happening at the low levels with low stratus
and fog as Kenai was at a mile or less much of the night and
Anchorage as well down to as low as a quarter mile this morning.
A strong upper level ridge of high pressure is currently spinning
over the North Pacific along with a corresponding surface low with
its center just south of the Gulf of Alaska. Also of note, is the
upper level shortwave trough that brought the snow to
Southcentral yesterday continues moving off to the southeast
across Southeast Alaska. Gap winds continue through the typical
areas due to the pressure gradient still present. Whittier as of
this writing was still gusting upwards of 40 kts due to the
pressure gradient banked up against the mountains. The Valdez
Narrows area was also gusting up to 27 kts overnight. Other than
that, cooler temperatures have returned with most areas in the
teens and 20s except for the Copper River Basin where there were
single numbers to near zero degrees.

Weather remains fairly quiet across Southwest as the subsidence
side of the aforementioned upper level shortwave passes through.
As a result, nighttime microphysics imagery showed some clearing
of the clouds overnight, though, some low stratus is around for
some locations. Dillingham was one of the areas that had a low
stratus deck overnight. Some flurries or light snow showers were
being observed mostly along the Kuskokwim Delta Coastline this
morning underneath the low stratus across the area. Further
inland, including Bethel was partly to mostly cloudy.
Temperatures across much of Southwest were single numbers
especially where it was clear and teens with 20s on the Alaska
Peninsula.

A rather large 970 mb low pressure system spinning south of the
Aleutians continues to meander in place this morning. Its front
remains draped across the Bering with northeasterly flow north of
it across the northern Bering. This is leading to wave heights
of 20 to as high as 30 feet (Buoy 46070) in some locations due to
the persistence of the flow. Heavy Freezing Spray continues near
the ice edge due to this as cold air comes off the ice. ASCAT
Scatterometer data also confirms high end gale force winds to 45
kts for this area with storm force winds but outside the area of
responsibility. This system also continues to bring unsettled
weather to much of the Aleutians and Bering with Dutch Harbor
observing rain for much of the night.

&&

.MODEL DISCUSSION...

Models remain in generally good agreement out through Tuesday
morning before diverging. Models are diverging on an upper level
shortwave trough that will be moving northeastward along the
Alaska Peninsula and then moving over or near Kodiak as it heads
towards the Gulf of Alaska. The NAM keeps the energy with this
upper level shortwave stretched out. The Canadian-NH is doing
something similar to the NAM with stretching the energy but in an
entirely different orientation. The GFS keep a more consolidated
and concentric look with the energy more tightly together. The EC
keeps it an open wave and moves it through Southcentral much
faster. Therefore, end result being model guidance is in a tizzy
on what happens with this wave which will then in turn impact
sensible weather details as to who may see snow and who doesn't
towards mid week. This includes for out across Southwest. Other
than that, model guidance remains in rather good agreement on
another low moving north towards the Western Aleutians taking the
other ones place. Therefore, not much change for this area.

&&

.AVIATION...

PANC...Fog has settled over the terminal along with the low
ceiling yielding to 1/4 mi visibility and ceiling around 200 ft
placing the terminal at LIFR conditions. The fog and low stratus
is expected to linger around the terminal for much of the
morning. Some guidance does show the fog lifting during the
afternoon/evening with VFR conditions through Tuesday morning.
However, there is high confidence in the fog forecast with low
confidence on when a return to VFR conditions will resume. Best
chance for VFR conditions will be during the afternoon hours
before fog settles back in after sundown. Light and variable
winds are expected to persist throughout the TAF period.

&&

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

Quiet weather is on tap across Southcentral Alaska today as an
upper level ridge transits the area. Areas of low clouds and fog
will persist, especially across the western Kenai Peninsula,
Anchorage, and the Copper River Basin. The low level atmosphere
has stabilized as indicated by the 12Z Anchorage sounding.
However, it is weakly unstable with shallow inversions. Thus,
this does not look like a scenario where we would expect widespread
dense fog. More likely, there could be some brief and localized
areas of dense fog. The sun should be able to work on the weak
layer of stability, resulting in some diminishing of the areal
extent of fog/stratus this afternoon, before expanding again after
sunset. Elsewhere across the region, expect some sunshine with
seasonable temperatures.

As the ridge slides eastward tonight, expect increasing high clouds
to move up from the south. This will be followed by a series of
weak short-wave troughs. Forecast confidence in the track of these
short-waves is quite low. However, generally expect light snow
spreading northward across the Gulf of Alaska and into
Southcentral Tuesday through Tuesday night. It's difficult to
nail down snow accumulation at this point, but all solutions are
pointing toward little or no accumulation (dusting to one inch)
for inland areas of Southcentral. Kodiak Island and the Gulf coast
will be impacted by a stronger short-wave and surface low moving
up from the northeast Pacific Tuesday and crossing the central
Gulf Wednesday. Expect some light snow accumulation on Kodiak
Island Monday night into Tuesday before precipitation changes to
rain. The eastern Kenai Peninsula and Prince William Sound areas
would likely then see some snowfall Tuesday through Wednesday.
The low will remain over the Gulf, so forcing generally looks
weak and this looks like a minor snowfall. The upper level short-
waves and surface low look like they will exit eastward Wednesday
night, bringing an end to precipitation across the region.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 2: Today
through Tuesday)...

A weakening, stacked low near the Western Aleutians is pushing its
front towards the Southwest. Ahead of the front, fog banks are
advecting inland from the Kuskokwim Delta and Bristol Bay coasts.
In the Lower Kuskokwim Valley and interior Bristol Bay, plentiful
moisture from recent snowfall is also contributing to valley fog.
The fog should dissipate by late morning to early afternoon as
winds begin to pick up ahead of the front. Cloud cover increases
through Monday, though snow chances remain low until the front
reaches the Kuskokwim Delta and Bristol Bay coasts Tuesday
morning. Then, snow will slowly spread inland. Currently, about 1
to 3 inches of snow is forecast through Tuesday, with highest
amounts near the coast and over the Kuskokwim Mountains. Since
snow amounts and winds will remain relatively low, any blowing
snow is expected to have minimal impacts on visibility.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 2:
Today through Tuesday)...

Unsettled conditions continue for the Bering and Aleutians as
several lows and fronts impact the region. A weakening, stacked
low near the Western Aleutians is currently pushing its front
across the Central and Eastern Aleutians, bringing rain to
locations such as St. Paul Island and Dutch Harbor. This front
will continue tracking north towards St. Matthew Island and east
into the AKPEN and Southwest, though rain and rain/snow mix
will diminish as the front weakens.

Late Monday night to early Tuesday morning, a compact low enters
from the North Pacific, grazing the Western Aleutians before
overtaking and absorbing the stacked low. Its front stretches
almost parallel to the Western and Central Aleutians, bringing
light to moderate rain over the area through Tuesday.

&&

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

Gulf of Alaska: A North Pacific low will send a front into the
southern Gulf on Thursday with easterly winds up to gale force as
it lifts north towards the North Gulf Coast. A narrow barrier jet
may develop as the front nears the North Gulf Coast. Also, easterly
gale force winds are possible near/along the shore on Friday.
Winds will flip to southerly behind the front for the remainder of
the Gulf on Friday, with seas building up to 20 ft. Confidence is
high for generally benign conditions on Wednesday, with more
uncertainty for the strength and timing of the front on Thursday
and Friday.

Bering Sea:
Expect steady easterly gales across the central/western Bering
Wednesday through Thursday. On Friday, the low will shift
northeast towards the Alaska Peninsula and Bristol Bay as the
Siberian high pressure retreats north into the Chukchi Sea. This
will allow winds to weaken and become more northerly, diminishing by
Friday morning. Forecast confidence is moderate to high through
Friday.

&&

.LONG TERM FORECAST (Days 4 through 7: Thursday through
Sunday)... Uncertainty picks up right away at the start of the
extended range, with the primary model discrepancies stemming from
the interaction between two separate upper level lows. The first
low will start out as a positively-tilted Arctic trough extending
between the North Slope and Seward Peninsula. Most solutions show
the base of this trough close off and become detached from upper
level steering flow, with models struggling with the weak steering
and resulting trajectory for the low’s path through Friday. At
the same time, a deeper and nearly stacked low over the North
Pacific will pivot north into the eastern Aleutians and Alaska
Peninsula by Friday morning as a new longwave ridge rapidly begins
to take shape upstream over the Gulf and southern mainland.
Models depict varying degrees of phasing between this second low
and the aforementioned low aimlessly drifting into the northern
Bering, and the run-to-run consistency for this interaction is
notably poor. Regardless of how exactly this element of the
pattern plays out, it appears quite likely for a surge of tropical
warmth and moisture to drive north around the developing ridge
into much of Southcentral and Southwest from Friday to Saturday.
This will support increasing potential for periods of snow or
mixed precipitation at the end of the week as temperatures warm to
or perhaps above freezing for inland areas, with heavier amounts
of mainly rain possible along the North Gulf Coast and southern
Alaska Peninsula. Deep southerly flow and above normal
temperatures will likely continue through the end of the weekend
with all models and ensemble means keeping the strong longwave
ridge anchored over the Gulf and up along the AlCan border.
Farther west, another low could lift north into the eastern Bering
on Sunday, bringing gusty winds and more rain/snow to the eastern
Aleutians, Alaska Peninsula and Southwest.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 411 412 176 177 178 185.
Heavy Freezing Spray Warning 185
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MV
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
MARINE/LONG TERM...MK/MF