Hurricane Force Wind Warning 1, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
437 PM AKST Thu Jan 4 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Saturday evening)...

A frontal boundary continues to lift north across Southcentral
continuing the unsettled pattern. The front is producing rain and
snow showers along the southern and eastern side of the Kenai
Peninsula, with rain occurring at lower elevations and snow
above. Coastal communities such as Seward and Whitter will
continue to see a snow/rain mix this evening, becoming mostly snow
as the temperatures cool overnight. Up to six inches of
additional snow accumulation is expected along the Seward highway
from Seward to Girdwood overnight, with areas such as Turnagain
Pass and just north of Seward to see those higher values. The
period of strongest snowfall intensity will likely be tonight near
midnight, becoming lighter but continuing through the day Friday,
finally diminishing Friday night. Anchorage, Western Kenai, Mat-
Su, and the Copper River Basin will remain largely sheltered from
the impacts from the passage of this front. Kodiak Island remains
rainy, but will see a break by late Friday afternoon and overnight
Friday into Saturday morning.

The break will not be long for Kodiak nor the Southcentral
Coastline as a new frontal boundary will extend from the low in
the southern Bering across into the Gulf of Alaska. This front
will sweep northward similarly to the last boundary, reaching
Kodiak Island first by late Saturday morning. For Kodiak City,
precipitation will likely begin as rain, with a transition
possible to a rain/snow mix by Saturday night. For mainland
Southcentral, an initial push of light snowfall will begin across
the southern and eastern sides of the Kenai Peninsula by late
Saturday morning. On the western side of Kenai Peninsula, Homer
and Seldovia will see a similar onset time of snowfall, with
higher accumulation rates expected on the southern side of
Kachemak Bay. Persistent snowfall is expected to continue into
Sunday where the axis of moisture aligns with the flow into
Western Prince William Sound, increasing snowfall potential in
that area. This system does seem to have the potential to linger
significant snow showers across this area into the beginning of
next week. Just the period from Saturday afternoon to Sunday
evening, the higher elevations along western Prince William Sound
may see one to two feet of new snowfall. Further down during this
period, Seward and Whitter may see an additional 1-4 inches of
accumulation with 2-6 inches possible along the Turnagain Pass
corridor. It is currently unclear how much snow makes it through
the downslope hole and enters Anchorage. As the axis aligns with
Turnagain Arm by early Sunday afternoon, this seems like the best
time for some light Anchorage snowfall, however confidence is
currently not high.

-CL

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3/Tonight through Sunday)...

Key Points:

* Current deep low in the Bering Sea will continue to weaken and
  exit the area through friday night.

* Another powerful storm system will move into the Bering Sea
  Friday night through Saturday. Similar in strength to the
  previous storm, expect widespread impacts from strong winds
  along with rain and snow.

A large vertically stacked low centered about 85 nm north of
Unimak Pass continues to slowly weaken as it tracks slowly
eastward. Gusty winds and showers still persist across much of
the Bering Sea, Aleutians, and Southwest Alaska. Gale force winds
can still be found along and south of the Aleutian Chain. The low
will reach Bristol Bay Friday morning, then dissipate as it heads
inland Friday afternoon. The upper level low will open up into a
trough late tonight, with a series of short-waves then tracking
inland across Southwest Alaska on Friday. This will lead to more
widespread shower activity, primarily in the form of snow. The
best chance for some light accumulation will be Dillingham
northward to Wood-Tikchik State Park and east to Koliganek and
New Stuyahok. Precipitation will then taper off across Southwest
Alaska Friday night as the short-waves exit to the north and
east.

There will be a brief break in winds and precipitation behind
the low. However, attention will quickly shift to a rapidly
deepening low over the North Pacific on Friday. The set-up looks
quite impressive, with interaction of the Polar and Sub-tropical
jets leading to initial intensification of a low crossing the
North Pacific; a low position between the left exit region and
right entrance region of two jet streaks; and the low bottoming
out in the 940s as it crosses the central Aleutians Friday
night/early Saturday. This last point is a key contrast with the
previous system which bottomed out over the North Pacific and
tracked into the Bering Sea as a fully occluded vertically stacked
low. Thus, generally expect stronger winds with this next storm
system and much warmer air to stream northward out ahead of it.
Both operational and ensemble model guidance have been rock solid
in the depiction of this storm. However, operational runs have
trended deeper over the past 24 hours. Based on the pattern
considerations above, the latest forecast fully buys into the
idea of a low bottoming out near or just above 940 mb.

As is always the case, small changes in the track of the low or
speed of the low will affect which areas see the strongest winds.
For now, have gone ahead and issued High Wind Watches for the
Central to Eastern Aleutians (including Adak, Atka, Nikolski, and
Unalaska/Dutch Harbor) for Saturday. Winds ahead of the system
Friday night will be strong, but as is typically the case, the
the strongest winds will be behind the low on Saturday in the
area of cold air advection and along the bent back occlusion.
The other likely threat, similar to the previous storm, will be
Blizzard conditions along the Kuskokwim Delta coast and Nunivak
Island - so have gone ahead and issued a Winter Storm Watch. Other
potential hazards, but less likely, include high winds (75 mph or
greater) along the southern Alaska Peninsula and a period of snow
and blowing snow in the Dillingham area before it warms up. Stay
tuned as we continue to track this storm and fine-tune the weather
impacts.

The storm will lift northward across the Bering Sea and very
slowly weaken saturday through Sunday night. It will be much like
the wake of the current storm, with showers and gusty winds
lingering across the region, but diminishing with time.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday through Thursday)...

Most of the active weather for the long term forecast period will
take place along the western portion of the state. A longwave
Bering low persists through the early portion of the week,
allowing for precipitation to occur along the frontal boundary as
it extends into central Alaska. Southcentral and western Alaska
will have near or just below freezing temperatures early week, so
mixed precipitation is possible. Some gale-force winds will be
possible as the system moves northward. The trough associated with
the Bering low will generate a couple of shortwave lows that have
the potential of providing precipitation to southcentral Alaska
in the form of snow and a rain/snow mix.

Late Tuesday into Wednesday, a building ridge will begin to
settle into central Alaska, as the main Bering low is forced
northward, providing calmer weather for that portion of the state.
There is some inconsistency with the placement of the ridge, but
most of central Alaska is expected to be covered through the end
of the forecast period. There is model agreement that a new low
pressure system will deepen as it makes its way across the western
Aleutian Islands into the Bering Tuesday- Wednesday, but the
track of the storm relative to the western Alaska coast will be
dependent on the strength of the central Alaska ridge. Strong
gale-force winds with storm-force gusts are possible as the system
traverses the Aleutian chain.

BS

&&

.AVIATION...

PANC...Elevated snow showers will continue to move across
northern Cook Inlet and the Anchorage Bowl this evening and
overnight. Easterly low/mid- level flow should result in
downsloping dry conditions with VFR CIGs tonight into tomorrow
morning, though some MVFR CIGs could move through with the showery
activity. There remains some potential for marginal LLWS to
develop near the 2000 ft threshold as southeast winds increase to
around 30 to 35 kts near and above this layer through tonight, but
may fall just underneath inclusion for TAF package.

-MJB/PP

&&