Small Craft Advisory 142, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
550 PM AKST Tue Feb 15 2022

.ANALYSIS AND UPPER LEVELS...

Areas of drizzle/freezing drizzle linger across Cook Inlet north
to the Matanuska Valley this afternoon as low-level moisture
remains trapped underneath a series of wave upper-level waves
moving across Southcentral from west to east. Satellite imagery
shows these features slowly retreating to the north as weak,
transient ridging attempts to build in ahead of the weather
system. As these waves lift north, any lingering drizzle should
begin to taper off, and this is evident in the 00z (3 pm AKDT)
observations around Anchorage indicating improved visibilities
across the area.

Farther west, a surface low near Unalaska is extending a warm
front across the eastern Aleutians and AKPen this afternoon.
Behind the front, temperatures are warming into the lower 40s from
Cold Bay to King Salmon, as a southerly flow ushers in warmer air
from the North Pacific. Rain is also falling along the AKPen. Out
ahead of the front, temperatures are around freezing, with a
wintry mix of precipitation beginning to develop across Bristol
and Kuskokwim Bays.

A storm-force low is also spinning up south of the Aleutian Chain.
While moving northward, this system is still too far south to
impact the western Aleutians.

&&

.MODEL DISCUSSION...

The biggest change over the past 24 hours is the depiction of a
more northwesterly track of the surface low currently over
Unalaska before shifting east and moving onshore over the
Kuskokwim Delta. Models are now hinting, that due to this more
northerly jog, slightly warmer air will likely advect across the
Southern Mainland through Wednesday in advance of this system.
Thus models, have all trended to transitioning precipitation
types, from Kuskokwim Bay east across Cook Inlet, the Anchorage
Bowl, and Prince William Sound. It also appears that with this
more northward track there is higher confidence in southeasterly
Turnagain Arm and Knik River Valley winds bending into Anchorage
and Palmer by mid-morning Wednesday, further complicating the
precipitation picture.

Models do have this system moving east quickly, with the shortwave
exiting Prince William Sound by Wednesday evening. The Canadian is
still the fastest solution, though, with other guidance about 3
hours slower in the timing. Even with this feature exiting, there
is no significant ridging or strong flow aloft in the wake of this
system. Thus, models continue to show lingering precipitation from
Cook Inlet north across the western slopes of the mountains.
Again, temperatures will be a factor in precipitation type.
However, guidance is hinting at slightly cooler air behind the
system, with any lingering precipitation likely falling as light
snow.

Models are in better agreement with the next wave moving across
the Southern Mainland for late Thursday into Friday. Again, this
feature looks a bit farther north than in previous runs. As such,
warmer air will inch farther north with more areas across
Southwest and Southcentral Alaska likely seeing a rain/snow mix,
or perhaps all rain.

&&

.AVIATION...
PANC...Predominantly IFR/LIFR ceilings are expected to remain
through the overnight hours with reduced visibility due to mist.
Snow, possibly mixed with some freezing drizzle, is expected to
develop early Wednesday morning as the next storm system
approaches. IFR conditions are expected to persist, with LIFR
conditions possible in heavier snow. Light northerly winds are
expected through the overnight and early morning hours. However, a
southeasterly Turnagain Arm wind is expected to move over the
terminal by 18z Wednesday. This wind shift should increase surface
temperatures and change precipitation over to rain from mid to
late morning through early afternoon before the wind relaxes,
allowing temperatures to cool back down to the freezing mark and
transition the precipitation back to a mix of rain/snow or all
snow before tapering off.

&&

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

An active wintry pattern will persist as a few frontal systems
combined with upper level troughs advance through Southcentral
and the Gulf of AK the next few days. Winter Weather Advisories
for Matanuska Valley effective from 3 am to 5 pm Wed and for
Susitna Valley from 3 am to 6 pm Wed for snow with expected total
snow accumulations of 4 to 7 inches for Matanuska Valley and 3 to
9 inches for Susitna Valley. Southerly winds advecting warm air
over the Gulf, including Kodiak Island, Kenai Peninsula, and
Prince William Sound, could favor mostly rainfall there and
mainly rain/snow mixed for parts of the eastern Kenai Peninsula
including Whittier and Portage. Part of this warming could drive
rainfall and rain/snow mixed over the Anchorage area by late Wed
morning into Wed afternoon, with rain/snow mixed expected for
Anchorage through Fri afternoon. Clearly, the P-type is a
challenge for several spots over Southcentral, but if southerly
winds continues at both surface and mid-level, then the potential
for rainfall is favorable especially across the Gulf and the
coastal areas.

As these expected fronts move across the western Gulf, winds along
Turnagain Arm will also pick up starting late tonight through
early Wed morning, and again on Thu morning through Thu night. The
first front approaching the region late tonight could bring 2 to
4 inches of snow for the Anchorage area, about 4 to 8 inches for
Matanuska Valley, and up to 10 inches of snow accumulation for
parts of the Copper River Basin. Heaviest snow will fall over the
Valleys and higher elevations. An upper-level ridge over the
region on Thu will provide a brief break in weather until the
second frontal boundary enters Southcentral Thu evening. The
latter system could bring another round of rain/snow mixed near
the Gulf coast and snow over much of Southcentral, with mainly
rainfall right along PWS and parts of Kenai Peninsula and the
southern Cook Inlet Thu and Thu night. A third system due Fri will
also bring another series of snowfall, but for now, will
concentrate on the first two expected systems that will keep
Southcentral quite busy the next couple days. Stay tuned!

&&

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

A surface low near St. Paul tracks into Southwest Alaska tonight
through tomorrow morning, bringing potentially impactful weather
to the region. Winter Weather Advisories are in effect for the
Kuskokwim Delta and Lower Kuskokwim Valley, where heavy snow is
expected. While snow totals remain the primary concern for the
Lower Kuskokwim Valley, the Kuskokwim Delta will likely see
additional hazards. As the surface low veers towards the Kuskokwim
Delta this evening, brisk winds could lead to blowing snow and
reduced visibilities. In addition, compressional warming will
likely produce a period of freezing rain west of the Kuskokwim
Mountains, especially as northerly winds reinforce cold air at the
surface. Further south in Bristol Bay, stout warm air advection
will keep most precipitation as rain. As described in the Special
Weather Statement for Bristol Bay, this could lead to heavy rain
and ponding water. Precipitation throughout Southwest will taper
off from south to north through Wednesday morning.

More precipitation arrives later on Wednesday, when an occluding
surface low in the Bering pushes its front into the Southwest.
Some uncertainty persists as models struggle to represent the
upper level shortwaves associated with these features, though
models are starting to come into greater agreement. The front
looks to push west to east starting Wednesday evening. While there
will be a quick break behind the front, precipitation will resume
shortly as the surface low itself pushes into Southwest. Overall,
precipitation is expected to linger through Friday. While
precipitation may start as rain in Bristol Bay, cold air wrapping
around the backside of the low will gradually transition rain to
snow.

&&

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

The forecast remains on track for the Bering and Aleutians. A
surface low tracking towards the Southwest will bring rain and
areas of gales across the Eastern Aleutians through Wednesday
morning. At the same time, a deep but weakening surface low
crosses into the Bering tonight. Models had previously struggled
with the track of this low but are slowly coming into agreement.
The low looks to track into the Central Bering, wrapping
widespread small crafts and areas of gales around it. Out head of
the low, its front brings rain as it tracks from the Central
Aleutians Wednesday morning to the Alaska Peninsula Thursday
morning. Winds will diminish and rain will gradually transition
into snow showers as the low occludes through Wednesday. Then, it
slowly crosses the Bering and moves into Southwest Alaska through
Friday.

A ridge builds Friday, bringing a brief period of calmer weather
as it tracks across the Bering. Behind it, more precipitation is
on the way as yet another surface low pushes its front eastward
into the Bering.

&&

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

Gulf of Alaska:

A front pushes into the Gulf Wednesday morning, bringing
widespread southerly small craft winds, 10 to 15 ft seas, and rain.
Winds largely diminish below small crafts, waves decrease below
10 ft, and rain tapers off around Thursday morning. Uncertainty
increases Thursday evening regarding the track and strength of an
incoming North Pacific low and its front. Winds will likely
increase to small crafts between Thursday evening and Friday as
the front tracks through the Gulf. However, the extent and
duration of small craft winds, as well as the potential for gales,
is difficult to determine at this time. Precipitation associated
with this front will likely fall as rain or rain/snow mix. By late
Friday evening, the low looks to move over the Northern Gulf,
bringing the potential for gusty, westerly gap winds and freezing
spray. Rain and rain/snow mix will transition to snow showers.

Bering Sea/Aleutians:

A busy pattern persists for the Bering and Aleutians through
Friday. On Wednesday, small craft winds and seas approaching 20 ft
are expected near the Eastern Aleutians as a surface low moves
from the North Pacific into Bristol Bay. Further west, a strong
North Pacific low tracks along the Aleutians Wednesday morning
through Friday. Rain or rain/snow mix falling ahead of the low
will transition into snow showers behind it. The low weakens
quickly, so while widespread small crafts are expected, gales will
be limited mostly to the Northern Bering and south of the
Aleutians. Seas 15 to 25 ft are expected near the low, with seas 8
to 15 ft elsewhere. There is some uncertainty as this low
interacts with a compact low moving across the Eastern Aleutians
and into Southwest Alaska on Thursday. However, both systems look
to remain fairly weak as they interact, with winds remaining small
crafts or lower. On Friday, yet another low pushes its front west
towards the Central Aleutians, bringing a broad swath of gales
and rain.

&&

.Long Term Discussion (Days 4 through 7: Thursday through
Sunday)...

An occluding North Pacific low tracks along the Aleutians from
Thursday through Friday evening. A triple point low forms near the
Eastern Aleutians Thursday morning, introducing uncertainty as
these two features interact. The EC suggests that the triple point
low will be absorbed into the occluding low, while the GFS and
GEM suggest that the two surface lows will rotate around each
other. How these lows interact has implications for winds and
precipitation. For example, the GFS brings the compact low
further north into the Kuskokwim Delta and Bristol Bay on
Thursday, suggesting greater snow amounts in those areas as
compared to the GEM. As the GFS brings the low further north, it
brings an increased potential for rain along Southern Bristol Bay
due to greater warm air intrusion inland. In contrast, the GEM
brings the compact low straight towards the Prince William Sound,
suggesting greater snow amounts along the North Gulf coast from
Thursday afternoon through Friday evening as compared to the GFS.
With winds, the GEM's stronger North Gulf low suggests that gap
winds will approach storm force early Saturday morning, as
compared to winds below small crafts with the GFS and EC.
Overall, details remain uncertain at this time for the front
tracking across southern Alaska from Thursday morning through
Friday, as well as for the various surface lows.

As this strong system moves across southern Alaska Thursday
through Friday, a ridge builds behind it. This heralds a brief
return to calmer weather and colder conditions through most of
Saturday. However, unsettled conditions resume as a strong low
near the Western Aleutians pushes its front across the Bering. The
front reaches Southwest Alaska late Saturday night into Sunday
morning. Confidence decreases at this point due to poor model
agreement. While details are uncertain, the position of the upper
level longwave trough over Alaska suggests that unsettled weather
is likely.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 111 145 152 155.
MARINE...Gale Warning 138 155 165 174-177 180 185 411 412 414.
		 Heavy Freezing Spray Warning 185 412.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
MARINE/LONG TERM...MK/KM