Area Forecast Discussions

919
FXAK68 PAFC 211341
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 AM AKST Tue Jan 21 2025

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

Active weather returns to Southcentral Alaska today with the
potential for (a) snowfall rates approaching 1 inch per hour in
Turnagain Pass, and (b) gusts as high as 50-65 mph along parts of
Turnagain Arm and Turnagain Pass.

A deepening area of low pressure is currently shifting northward
across the AK Peninsula with the attendant frontal boundary
extending southeast into the Gulf of Alaska. Satellite imagery shows
this front currently pivoting northward across the Gulf, currently
bringing rainfall and high-elevation snow to Kodiak Island.
Southeast winds will continue to increase in speed to gale-force
intensity (w/ storm-force gusts) in response to the front swinging
northward and the pressure gradient between the frontal trough and a
highly-amplified ridge axis off to the east.

Like the last several fronts in recent weeks, mild temperatures will
result in low-elevation rainfall and high-elevation snowfall for the
Kenai Peninsula and areas adjacent to the Prince William Sound. The
largest impact along the `lower` elevations, or essentially along
the road system, will be that of Turnagain Pass where somewhere in
the vicinity of a foot of snowfall is expected. Precipitation rates
will reach peak intensity this afternoon and evening, where rates in
excess of 1"/hr will be possible, and therefore the potential for
significantly reduced visibility at times.

The most uncertain and challenging forecast aspect is that of
precipitation type in the areas surrounding Turnagain Pass.
Confidence is reasonably high that the pass should remain snow, but
immediate lower elevations around the pass, as well as Portage and
Whittier present uncertainty. Recent fronts have had enough
intensity such that Whittier, and to a lesser degree Portage, have
maintained snow for longer than expected despite temperatures
warming above freezing. This will be a similar case. Should Whittier
remain mostly snow, several inches to a foot cannot be ruled out...
though temperatures will no doubt be at or above freezing. If enough
rain mixes in, amounts will be closer to 0.

Strong wind will also be a hazard today in addition to the snowfall.
The majority of strong wind on land will be confined to higher
elevations with the exception of Whittier-Portage-Turnagain Arm gap,
as well as the Anchorage Hillside. While wind gusts should remain
lower than last week`s windstorm, gusts in excess of 60 mph can`t be
ruled out for any of these locations. For Anchorage, confidence is
strong wind will remain confined to the Upper Hillside thanks to a
fairly strong down-inlet wind component. The trough passage will
occur around 9PM AKST this evening, of which when winds will likely
be the strongest for some of the lower elevations as we quickly lose
the down-inlet gradient and the SE winds out of the Turnagain Arm
may bend into town a bit.

The trough passage itself will also bring a chance of a rain and
snow mix to the lower elevations including Anchorage, as well as
the Mat- Su, though accumulation will be limited to non-existent.
The Alaska High Resolution Ensemble Forecast (AK-HREF) supports
this as probabilities of 0.5" snowfall are near 0 until reaching
the Talkeetna Spur Rd. To the north in Susitna Valley from
roughly just north of Trapper Creek to Broad Pass is most likely
to see accumulating snowfall through Wednesday between a
combination of this evening`s trough and then subsequent low-level
upslope snow coincident with further shortwave trough support.
The highest snow totals look to be Curry to K`esugi Ridge and the
Parks Highway on the northwest side of this ridge system.

Thompson Pass and Valdez are also likely to see accumulating
snowfall, with snowfall/precipitation quickly ending just north of
the Chugach Mountains. Convective snow showers moving from the
northern Gulf into the central and eastern Chugach are likely to
continue into Wednesday behind the front. Additional accumulation
should be fairly limited, though quick bursts of snow, and thus,
visibility-reductions will continue to be possible.

Following a brief lull Wednesday night, the tail end of a warm
front extending out from a low in the Bering will swing into
Southcentral Alaska and the Gulf. Broad lift owing to warm air
advection underneath shortwave troughing aloft should allow for
the development of loosely organized scattered-to-widespread
showers. Mild temperatures will once again result in low-elevation
rain and high-elevation snow, with snow levels gradually rising
through the day given the stream of warm southeast flow. At this
time it doesn`t appear this wave will produce significant and
widespread hazardous weather, though the possibility of any rain
on untreated ice- and snow-covered roads always presents the
possibility of additional slickness.

-Brown

&&


.SHORT TERM FORECAST SOUTHWEST ALASKA AND THE BERING
SEA/ALEUTIANS (Today through Friday morning)...


The active weather pattern looks to continue across Southwest
Alaska, the Bering Sea, Alaska Peninsula (AKPEN), and Aleutian
Chain through the end of the work week. The main culprit for the
active weather across Southwest, at least this morning through
Wednesday morning, is a North Pacific low which will lift
northward into the eastern Bering and over mainland Southwest
through Wednesday morning. Precipitation across the Kuskokwim
Delta and Bristol Bay will remain rain, but transition to a
rain/snow mix Tuesday evening and end as snow Wednesday morning as
cold westerly flow works in off the Bering.

The next system to follow is currently taking shape this morning
in the form of a Kamchatka low and front. The front is currently
pushing eastward across the Western Aleutians and western Bering
this morning as Shemya reports light snow. This front along with
precipitation, and strong gusty winds will continue eastward
across the Central Aleutians and will reach the Eastern Aleutians
and Pribilof Islands by Wednesday. Gusty southerly to
southwesterly winds and precipitation will work from the Eastern
Aleutians and Pribilofs to the AKPEN and mainland Southwest by
Wednesday night into the day on Thursday.

Meanwhile, another strong North Pacific low and its front will
lift northward to the Aleutian Chain by Thursday. A strong push of
warm southerly flow will accompany this system with moderate to
heavy rain likely across the Central and Eastern Aleutians for
Thursday. Storm force southerly winds are also likely across the
the North Pacific south of the Eastern and Central Aleutians and
northward into the Bering potentially reaching as far north as the
Pribilof Islands Thursday evening. Precipitation and gusty
southerly winds make it to the mainland Southwest by Thursday
afternoon with most precipitation falling as rain due to the warm
southerly flow. One thing to follow in addition to the
precipitation and gusty winds across mainland Southwest, is that
this setup could bring increased water level up Kuskokwim Bay that
could affect the communities of Kwigillingok and Kongiganak
Thursday evening into Friday. A strong cold air push will build in
behind this strong low pressure, now located near Saint Lawrence
Island, beginning on Friday with the Western Aleutians and Central
Aleutians seeing off and on convective snow showers through the
day. Stay tuned to the forecast as more details are fine-tuned
with regards to the track and impacts of the low traveling from
the North Pacific Thursday to Saint Lawrence Island Friday.

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

KEY POINTS:

- A highly amplified and potentially impactful weather pattern is
expected to affect much of the forecast area from Friday into
this weekend.

- Strong winds, heavy precipitation and very warm temperatures
will be possible over much of southern Alaska from Friday to
Monday as a strong frontal system moves across the region.

- Much colder air will move in from the northwest behind the heavy
precipitation early next week.

An extremely active, highly unusual and potentially highly
impactful pattern will be well underway at the start of the period
on Friday morning. A very strong longwave ridge is expected to be
in place over the Northeast Pacific on Friday morning, with the
crest of this ridge extending well up into Mainland Alaska. To the
northwest, a deep longwave trough with an embedded closed low is
expected to be moving up into the far northern Bering Sea and up
into far northeastern Russia. Two dramatically contrasting air
masses will be building under the main upper trough and Pacific
ridge. A very warm air mass will be in place under and towards the
upstream side of the ridge, including much of the southern
Mainland out to the eastern Aleutians. Meanwhile, a much colder,
Arctic air mass will be building under the longwave trough across
much of the northern and western Bering up into Siberia.

The gradient in terms of temperature and pressure fields between
the two synoptic features outlined above, both near the surface
and the upper levels, will be quite impressive for even Alaska
standards. For example, the highest surface pressure under the
ridge in the northeast Pacific could be as high as 1045 mb,
contrasting with a surface low moving into the Bering that could
deepen to around 980 mb or lower - a difference of at least 65
mb. A strong baroclinic zone (an area of strong temperature and
pressure gradients) will exist between the main high pressure and
the low/trough in the Bering Sea, initially in place Friday
morning along an axis from the central Aleutians to the coastline
of western Alaska. This effective frontal zone will slowly advance
southeast with time throughout the weekend into early next week,
and this will largely be the focus of the most inclement
conditions with this pattern. The strong pressure gradient setting
up along this boundary, plus at least two distinct stronger areas
of low pressure that will move north along it, will support
intense southwest winds, initially over the eastern Aleutians and
Southwest. Stronger winds could also affect parts of Southcentral
on Saturday and Sunday as the second low moves up from the Pacific
into Southwest, but this potential for strong winds will be
highly dependent on a still very uncertain low track.

An anomalous moisture plume, or atmospheric river, is also likely
to stream north along and ahead of the main frontal zone,
initially expected to be impacting parts of Southwest and much of
the Alaska Peninsula and eastern Aleutians on Friday. This deep
moisture tap will help support rather intense precipitation rates
for much of Southwest and later Southcentral compared to what is
typically possible for late January. Temperatures in many areas of
Southwest close to sea level will likely initially be warm enough
for rain at lower elevations, and this will likely also be the
case as the atmospheric river and front shift into Southcentral
between Saturday and Monday. There could be a transition over to
snow across both Southwest and Southcentral as cooler air begins
to move in behind the front between Sunday and Monday.

Lastly, it looks increasingly likely for a significant, sudden
temperature drop to advance west to east across the southern
Mainland early next week as the front moves through and as the
Arctic air mass cascades south and east. There is still a fairly
large spread in terms of when it looks like the colder air will
arrive for any given location, but there is a strong signal in
both deterministic and ensemble guidance that this cold air mass
will eventually make it into our outlook area early next week.
This could lead to more weather impacts due to the sudden switch
from warm temperatures and rain/snow to bitter cold. Stay tuned as
we follow this very active pattern and resolve details for
timing, precipitation amounts and wind magnitude for this setup
in the coming days.

-AS

&&

.AVIATION...

PANC... Northerly winds will continue to increase atmospheric
mixing through the day keeping VFR conditions through 08Z Wed.
As a front moves through the Gulf today, northerly winds change
to a more northeasterly/easterly direction mid- afternoon. Wind
gusts will range from 20 to 30 knots. Wind shear is likely from
this morning through 08Z Wed. Scattered rain rain/snow showers
are expected after 08Z Wed into 12Z Wed though VFR conditions
return afterward.


&&

$$