Small Craft Advisory 57, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
619 AM AKST Thu Jan 20 2022

.ANALYSIS AND UPPER LEVELS...

A complex upper-level pattern dominates the region as a broad
longwave trough extends south across the Bering and Aleutians and
into the North Pacific. Downstream, an upper-level ridge
stretches up from the Northeast Pacific across Southeast Alaska
and into the Canadian Arctic. Several shortwaves propagating
through this pattern are helping to draw warm, moist air
northward from the tropical Pacific. The strongest of these
shortwaves sits atop a 965 mb low just south of the eastern
Aleutians. Out ahead of the low, a stream of tropical moisture is
evident on water vapor imagery, feeding a front pushing across
the Aleutian Islands and Alaska Peninsula, bringing mixed
precipitation to the region. Microwave passes show 1.5 inches of
precipitable water (200 to 300 percent of normal) streaming north
along this front. This warm, southerly flow has also built a ridge
which has kicked yesterday's snow-producing shortwave in
Southcentral out eastward. As a result, precipitation has ended in
all but the eastern portions of the Copper River Basin, while low
stratus lingers around Cook Inlet in its wake. Stratus also
covers much of Southwest Alaska, producing some flurries in
isolated locations.

&&

.MODEL DISCUSSION...

Models remain in good synoptic agreement through the short term.
There is also increasing agreement with regard to the timing and
placement of upper-level shortwaves and surface lows tracking
northward through the broad southerly flow. There are still some
slight differences in the placement of these features, however.
The most significant impact will be their influence on the
precipitation type forecast across Southern Alaska. The biggest
challenge will be timing the change over from snow to rain, as
well as the areal coverage of any freezing precipitation for both
Southwest and Southcentral Alaska for Friday into Saturday.

&&

.AVIATION...

PANC...MVFR cigs are expected to begin to lift late this morning,
giving way to VFR cigs higher than 5000' by this afternoon as
northerly flow develops. Ceilings will lower again early Friday
morning with periods of snow accompanied by MVFR cigs/vsby
possible by late Friday morning.

&&

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

The weak system that brought snow to a large portion of
Southcentral yesterday has moved eastward and out of the area.
This morning arrives with a mix of either low stratus or clear
skies for much of the region. All this will give way over the
course of the day to the next system which is moving quickly into
the area from the south-southwest. This system has a lot of warm
air with it and will turn precipitation for all coastal areas to
rain by tomorrow.

There are two big forecast challenges with this system. The first
is how much downsloping will occur on the lee side of the
mountains. At this time, it looks like the area from Anchorage
northward through Talkeetna will mostly downslope out. The best
time for precipitation in these areas will be Friday afternoon as
an upper level wave embedded in the southwesterly flow moves
through.

The second big forecast challenge is with precipitation type
Friday into Saturday. The most likely area for there to be
freezing rain will be in the Copper River Basin. The Edgerton
highway looks to be in a spot where the cold air will remain
trapped at the surface while the above freezing air aloft moves
overhead Friday afternoon through Friday night. This area may
stretch northward to include the Glennallen area Friday night. By
Saturday, the southerly winds will have kicked in and that will
bring temperatures at lower elevations of the Copper River Basin
above freezing.

The surface flow from Anchorage through the Mat-Su valleys will
remain northerly until Friday evening when there will be a
pronounced southeasterly push. This will bring Turnagain Arm winds
into the Anchorage Bowl and cause temperatures to jump above
freezing after being mostly kept at bay for most of the day on
Friday. In fact, much of Southcentral Alaska will see warming
through the night Friday night due to southerly surface winds
becoming more widespread.

&&

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

A North Pacific low is pushing its front into Southwest, marking
the beginning of a period of precipitation and warmer temperatures.
As the front tracks north, several inches of snow are expected
across Southwest Alaska. Falling snow in combination with
northeast winds will lead to the potential for blowing snow and
reduced visibilities. As such, a Winter Weather Advisory remains
in effect for areas north and west of Dillingham. Elsewhere in
Bristol Bay, there is the potential for a wintry mix. The
precipitation type forecast remains challenging with the question
of how far inland warm air tracks and how long colder air at the
surface holds out. For now, the expectation is that snow will
quickly transition to rain south of King Salmon. However, as the
front stalls over Dillingham and King Salmon, there could be a
potential for a mix of snow and freezing rain in those areas. The
current thinking is that precipitation will be mostly snow, with
little to no ice accumulation expected.

Another North Pacific low and its front enters the Southwest
Friday morning, allowing warmer air to surge further north. Areas
near Iliamna could see a brief period of snow mixed with freezing
rain before warming of the column switches precipitation over to
all rain. From there, precipitation will gradually transition to
rain from south to north. Stay tuned as we continue to refine the
forecast.

&&

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

A rather static weather pattern will continue this week as a pair
of North Pacific lows and their associated fronts cross north of
the Aleutians and into the Bering Sea. The first low south of the
eastern Aleutians today pushes a front across the eastern
Aleutians and AKPEN, bringing snow at first, transitioning to
rain today on warm, southerly flow. Easterly gales through the
eastern Aleutians and AKPEN today shift to northerly and diminish
by Friday evening as the dual lows cross the Aleutian Chain. For
the central and western Aleutians, a cold, Arctic airmass
streaming across the warmer waters of the Bering produces
persistent scattered snow showers on northerly flow through the
weekend.

while
rain will be predominant in the Eastern Aleutians.

&&

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

Gulf of Alaska...A front along the North Gulf Coast dissipates
Sunday. A new front moves to the Northern Gulf by Monday. Forecast
confidence is mixed with the timing and strength of the second
front. Gusty winds over the Gulf diminish late Saturday. Seas to
17 feet subsiding. Gusty winds ahead of the second front increase
to widespread gales across the Western Gulf and Alaska Peninsula
Sunday and Monday. Seas building to 20 feet over the Western Gulf
for Monday.

Aleutians / Bering...A well developed Central Bering low moves
through the Bering Strait by late Sunday. Weak low pressure forms
over the Western Bering Sunday. Gusty winds over the Central
Bering, diminish Sunday. Seas to 12 feet, subsiding. A North
Pacific low moves across the Eastern Aleutians Sunday, and into
Bristol Bay by Monday. Widespread gales over the Alaska Peninsula
and Bristol Bay for Sunday, diminishing Monday. Seas to 20 feet on
the Pacific side of the Alaska Peninsula Sunday and Monday,
subsiding.

&&

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

Model agreement is decent at best, but there are still quite a
few discrepancies in the long term. Guidance stays relatively
consistent for upper level ridging across the eastern Gulf,
extending northwards into the Mainland and tracking eastward
towards Canada. Longwave troughing dips down over the Aleutians, a
surface low developing at the base of the trough. This may create
a wet pattern across Southwest Alaska as well as along the
eastern Kenai and Kodiak Island as we enter early next week. It is
still too early to determine precipitation type at this point,
and will rely fairly heavily on how much warm air is able to make
its way over land. As it stands now, guidance is pointing towards
strong southerly flow for Southcentral, which may allow
temperatures to remain above freezing through early next week. The
back side of the trough will likely see a decrease in
temperatures as flow transitions to northerly, namely along the
Bering Sea and Western and Central Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 141.
         Winter Weather Advisory 161.
MARINE...Gale Warning 119 120 130-132 138 139 150 155 165
                      170-177 179 351 352 411-414.
         Heavy Freezing Spray Warning 179 185 412 414.

&&

SYNOPSIS AND MODEL DISCUSSION...CQ
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
MARINE/LONG TERM...KZ/KM