Small Craft Advisory 10, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 PM AKST Tue Jan 3 2023

.ANALYSIS AND UPPER LEVELS...

A long-wave trough is carved out over Alaska and the North Pacific
with a zonal subtropical jet stream centered well to the south at
around 35N latitude. As a result, there are countless weak lows
and short-waves embedded within the long-wave, creating quite the
forecast challenge. The one strong feature of note is large gale
force low over the central Gulf of Alaska. It is tracking west-
northwest on a trajectory toward the northwestern Gulf, with winds
and precipitation spreading rapidly toward the Gulf coast and
Kodiak Island. Weak short-waves out ahead of this low are transiting
Southwest Alaska, bringing areas of light snow. Otherwise, quiet
conditions prevail across the remainder of southern Alaska. Out
west, the Bering Sea, Aleutians, and southern Alaska Peninsula
remain in a showery regime, with weak short-waves helping to focus
shower activity. A series of somewhat stronger out of phase
short-waves and multiple surface lows are strung out along the
central to western Aleutian chain. This is producing locally
stronger winds and heavier, more persistent snow showers.

&&

.MODEL DISCUSSION...

Models are locked on to the Gulf low as it passes just south of
the Kenai Peninsula tonight through early tomorrow. However,
differences develop with the remnants of the low as it rapidly
weakens and heads across Southwest Alaska. This creates some
uncertainty in which areas will see snow on Wednesday, especially
for Bristol Bay coastal cities/villages. Model spread grows fairly
quickly as we continue through Friday. This will be less of an
issue for southern Alaska forecasts, with very weak upper level
short-waves ensuring low impact weather. Out west will be a
different story, with chaos in the many lows transiting the
Aleutians and southern Bering Sea. There is extremely low
confidence in any single solution, so will attempt to depict the
most likely category of wind and broad-brush the precipitation
forecast. Despite the many model differences, the one commonality
is trending toward stronger lows and more widespread snow as we
head through the remainder of the week.

&&

.AVIATION...
PANC...Gusty northerly winds will develop tonight as a strong low
crosses the Gulf. As the low passes just south of the Kenai
Peninsula early tomorrow, strong easterly winds will begin to
cross the Chugach and Kenai Mountains, which should lead to a
brief period of low level wind shear. Some light snow is possible
on the leading edge of the system before strong downslope kicks
in. At this point in time the airmass will be cold enough for
snow. Any snow that does fall should be very light, so VFR
conditions will likely prevail. Worst case, if heavier, steadier
precipitation makes it over the mountains there could be a very
brief period of MVFR conditions.

&&

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

A strong low moving into the Gulf of Alaska this afternoon will
push a front to the north Gulf coast this evening as it tracks
northwest to the Barren Islands. This will produce a round of
heavy precipitation for the coast, as well as strong, gusty winds
for the eastern Kenai Peninsula tonight, with storm force gusts
possible through the Portage Valley. A quick round of light snow
should be possible as the leading edge of the front begins to push
over the coastal mountains while low-level flow is still
northerly over Cook Inlet tonight. Accumulations will be very
light, likely up to only a few tenths of an inch. Then, the low-
level flow turns southeasterly Wednesday morning, causing a wind
shift at the surface over the Kenai Peninsula and Anchorage Bowl.
Cross-barrier flow and downsloping will lead to temperatures
warming above freezing as a result, with chances for
precipitation dropping. However, overrunning precipitation will
be possible for the Hillside. This is likely to remain snow as
snow levels rise to only around 1000 ft, though any precipitation
making it over the lower elevations would fall as rain.

The low weakens rapidly as it crosses the Aleutian Range into
Southwest Alaska on Wednesday, causing the strongest winds to
subside by Wed afternoon. Weaker surface winds and partly clearing
skies will allow temperatures to cool Wednesday evening, turning
any precipitation back to snow for inland locations. Southeasterly
flow will continue to promote upslope showers along the north
Gulf coast. Inland, weakened cross-barrier flow will contribute
to an increased likelihood of snow showers Wednesday night as
various shortwaves continue to rotate across Southcentral.
However, there is a great deal of uncertainty in the timing and
placement of these shortwaves, so pinpointing any particular area
of greatest likelihood is not possible at this time.

Precipitation chances decrease on Thursday as drier air and
weak ridging moves in the from east, and by the evening
hours, even the coastal showers begin to wane. Clearer skies
contribute to a cooling trend Thursday night, and by Friday,
Southcentral is most likely to be cold and dry once again.

&&

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

Most of the snow showers since this morning has tapered off
across the Southwest mainland, with some showers lingering along
the coastal mountains. With a moist boundary layer from the recent
precipitation, low stratus will return this evening for the
Kuskokwim Delta and Bristol Bay. Areas of fog were added into the
forecast package for these regions tonight, with conditions
improving by Wednesday morning.

The next storm system will cross the western Gulf and into the
Alaska Peninsula (AKPEN) Wednesday afternoon. Another round of
widespread snow showers is likely over the Bristol Bay region from
late Wednesday to Thursday. The upper level support has weakened
a bit with the deformation band now shifting further northeast,
especially areas encompassing Koliganek to King Salmon to Iliamna.
There is also some uncertainty to the extent of the precipitation
reaching north and west of Dillingham, but snow accumulation will
be generally light with 1-2 inches possible between Wednesday
night and early Thursday. Otherwise, most of the snow showers will
be favored for the upslope areas along the Aleutian/Alaska Ranges
and local mountains. The low is expected to weaken and lift out
of the area for drier conditions as a ridge of high pressure
builds overhead by Friday.

&&

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

An elongated trough of low pressure extending from Kamchatka to
the western Bering Sea will continue to be the main weather
driver. A weakening frontal boundary will bring snow showers and
small craft winds across the western Aleutians tonight. For the
rest of the week, model guidance continue to struggle with
multiple low centers rotating around the aforementioned trough.
The current forecast package has high end small craft to low end
gale winds and seas of 10-15 feet for the western Bering and
Aleutians between Wednesday and Thursday. Cold, northerly flow
over the relatively warm waters meanwhile will maintain occasional
snow showers for the rest of the Bering Sea and Aleutian Chain. A
ridge of high pressure builds across the central and eastern
Bering for late Thursday and Friday, while a North Pacific low
approaches the Chain sometime on Friday.

$$.LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)...

The general consensus in the long-term points to a longwave trough
pattern with a series of weakening low pressure systems moving
along the northeast Pacific and the Bering Sea, with high pressure
building over Siberia and Interior Alaska. There is quite a bit
of disagreement this far out in time, with the GFS being the
biggest outlier currently, with a deeper low dragging south of the
Aleutian Chain. As it stands now, precipitation impacts look to
be primarily along the coast, with the bulk of it along the
Southeast Panhandle. But due to inconsistencies among the models,
confidence remains on the lower end. Further inland is likely to
see a drier trend as high pressure lingers over the Interior. The
Aleutians may see more unsettled weather, especially with the
further north GFS trend, later in the week. Overall, patterns look
to be relatively climatologically normal.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 119 120 125 130 131 138 139 351 352.
         Heavy Freezing Spray Warning: 185.
FIRE WEATHER...NONE.

&&

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