Small Craft Advisory 975, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 PM AKST Wed Dec 16 2020

.ANALYSIS AND UPPER LEVELS...

An upper level shortwave stretched across the northeastern Gulf
coast will continue to lift to the northeast. Farther west, a
broad upper level trough extends over the western Bering with
multiple upper level lows in place. A North Pacific low is visible
on satellite imagery centered south of the Alaska Peninsula and
gradually tracking toward the northeast. High level cloud cover is
already spreading across the northern Gulf with this low. Low
level stratus and patchy fog has spread across much of Southwest
Alaska as well as Southcentral in the wake of the departing
overnight snowfall.

&&

.MODEL DISCUSSION...

Model solutions are struggling to resolve the exact track of a
surface low located south of the Alaska Peninsula as it moves
northeastward into the Gulf through Thursday night. While the
differences in track are small, there could be big differences in
the forecast, especially with regard to precipitation type for
Kodiak Island as the low passes overhead through Thursday.
Meanwhile out west, NAM/GFS has fallen into agreement regarding
the strength and track of a storm force low over the central
Aleutians. Later in the week, models seem to have a good handle on
a broad complex low across the Bering through the weekend.

&&

.AVIATION...

PANC...Low level stratus in the wake of exiting precip will likely keep
cigs MVFR-IFR through tonight. Improving conditions expected
Thursday with northerly flow helping to clear out lingering
moisture.

&&

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

Low stratus or patchy fog is expected to linger around Anchorage,
the northern Cook Inlet and Knik/Turnagain Arms through tomorrow
morning. This is the result of residual moisture left behind the
departing system in valley locations. Colder and drier northerly
flow will increase at the low levels and should help to clear
things out Thu morning.

Close monitoring continues for the low moving northeast towards
Kodiak Island this afternoon. Models continue to struggle with the
exact strength and track of this low which will determine smaller
scale details including precipitation type for Kodiak Island and
the potential for blowing snow through Thompson Pass. For now, the
GFS was favored, with some of the Canadian models and the EC lending
credibility to this solution. Temperatures look to remain cold to
start for Kodiak tonight, with snow or a wintry mix being the
precipitation type to start. Warming associated with the passage
of the low will change this over to rain tomorrow morning, before
changing back over to snow or a wintry mix tomorrow evening. All
in all, expect precipitation type to vary from tonight through
tomorrow night for Kodiak City. Outflow winds will increase in
strength due to the tightening pressure gradient as the low
approaches the north Gulf coast, and a strengthening temperature
gradient, as cold air accumulates in the Copper River Basin and
warm air approaches with this low. We have issued a Winter Storm
Watch in effect from Thursday evening through Friday afternoon for
blowing snow in Thompson Pass. Wind gusts could reach 50 mph with
visibility dropping to a quarter mile or less at times.

After this, the NAM and GFS are trying to spin up the possibility
of another low forming south of Kodiak Island, and taking a
similar track as the one preceding it. This solution was not there
yesterday, and confidence does not accompany it at this time. We
will continue to monitor the potential formation of this low with
future forecast updates.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Model solutions place a low near Kodiak further south today than
yesterday, therefore, precipitation for today should be confined
to the AKPEN in the form of light snow. A couple of weak upper-
level shortwaves move southwest to northeast across the Southwest
Mainland bringing the chance for some light snow into Bristol Bay
and the Kuskokwim Valley Thursday and Friday. Periods of patchy
freezing fog may linger across the Mainland through tonight,
especially across the Kuskokwim Valley. Meanwhile, a strong low
is tracking eastward across the Southern Bering through Saturday
morning until it reaches the Bristol Bay coast. As it reaches the
coast, expect light snowfall along the Southwest Coast beginning
Friday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A strong low moves eastward across the Southern Bering towards
Bristol Bay through Saturday morning. Areas of gales south of the
low will track across the Aleutian coasts as this system pushes
east. Additionally, a band of storm force winds are expected to
pass over Adak on Thursday. Widespread light precipitation will
impact the Bering and Aleutian Chain as the low passes. This will
mostly be a rain/snow mix or all snow, with periods of rain south
of the Chain tonight. Model agreement for this system has
increased since yesterday, with all models taking an easterly
track just north of the Aleutian Chain. Discrepancy lies in the
speed of the system, with the GFS/NAM tracking it faster. GFS has
been the most consistent, thus, a GFS/NAM solution was used.

&&

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

Gulf of Alaska

There is moderate confidence that low pressure will persist over
the Prince William Sound through the forecast period. Model
discrepancies remain in regards to the strength of the westerly
winds to the south of the low center. Small craft to low end gales
are possible. There is high confidence a front will reach the
western Gulf on Monday with small craft to gale force winds
likely.

Bering Sea:

Generally benign conditions are expected throughout the Bering on
Saturday. On Sunday a broad, strong and complex low with multiple
centers is expected to move into the western Bering. Areas of gale
force winds are possible near the individual low centers Sunday
and Monday; however, confidence in the exact location of the
stronger winds is low at this time.

&&

.LONG TERM FORECAST (Days 4 through 7/Sunday through Wednesday)...

A west to east pattern change of warmer and wetter weather appears
to take place on Sunday as a broad and strong low pressure system
moves into the western Bering. Ridging along the storms front is
expected to bring much warmer temperatures and rain to areas of
the Aleutian Chain and Southwest Alaska through Monday. The finer
details of the forecast are messy due to the complex and broad
nature of the front and the low itself. Guidance indicates the
presence of many small low pressure systems and which one will
become dominant over time will be a forecast challenge. For
Southcentral, weather appears to be quiet through Monday morning
other than a few areas of snow near the coast. Guidance then
agrees that broad southerly flow will take hold of the region
through Wednesday. Generally speaking this pattern would mean rain
for lower elevations of coastal areas with snow likely inland and
over high terrain. The details of the rain/snow line is highly
uncertain at this time.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 131.
MARINE...Storms 175 177. Gales 119 120 130 170-174 176 352 413.
FIRE WEATHER...NONE.

&&

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