961
FXAK68 PAFC 241455
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
555 AM AKST Fri Jan 24 2025

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through
3/Today through Sunday Night)...

Key Point:

* Flood Watches have been issued for most of Southcentral Alaska
  for Friday morning through Monday Morning. A long duration
  rainfall, heavy at times, will fall on a frozen ground, limiting
  absorption of water into the ground. There is high potential for
  widespread areas of ponding/standing water, especially for low
  lying or poor drainage areas. Flooding along area rivers and
  streams is also possible.

* An approximation of Storm Total Rainfall Amounts Friday morning
  through Monday Morning:

  -Western Kenai/Anchorage/Matanuska Valley/Kodiak Island: 1 to
   2.5"

  -Susitna Valley/Eastern Kenai/Prince William Sound: 2 to 6"

* Gusty southeasterly winds will accompany the rainfall, strongest
  through Turnagain Arm into parts of Anchorage, the Knik River
  Valley into Palmer/Wasilla, and along the Copper River. Wind
  gusts could reach 70 mph along the Anchorage Hillside. The
  combination of wind and rain could bring down additional trees,
  particularly in areas where the ground is not frozen or where
  trees are already weakened from the recent windstorms.

* Snow levels will rise Friday through Saturday, reaching as high
  as 3000 feet in the Mat-Su and 3000 to 6000 feet Saturday
  afternoon from Anchorage down to the western Kenai. The only
  areas along the road system that could remain mostly snow
  throughout this event are the north end of Broad Pass up to
  Cantwell; the top of Thompson Pass; and Paxson up to Isabel
  Pass.

* Colder air will move into the region sometime Monday, possibly
  changing precipitation to snow. At a minimum, there is good
  potential for temperatures to drop below freezing early next
  week, causing any standing water to freeze.

Satellite imagery this morning shows a well amplified longwave
trough / ridge pattern with the upper-level trough anchored over
eastern Russia and the ridge centered over the northeast Pacific.
Both features are anomalously strong for this time of year. The
result, a strong jet stream (150 kts+) and atmospheric river can
be found in between the low and ridge, from the Aleutians
northeastward to Interior AK. A low amplitude short-wave and
surface low continue to move with the jet northward up the west
side of the ridge, which is amplifying out ahead of this low.

An initial shortwave trough continues to move east this morning,
with the more widespread precipitation of yesterday now mainly
confined to the mountains and favored upslope areas of the
northern Susitna Valley and Copper River Basin, a result of the
stout southwesterly flow. Temperatures overnight have remained
above freezing for most locations, with the exception of mountain
passes and parts of the Copper River Basin. Snow levels this
morning are sitting around 1,800 ft. However, in locations where
the winds have temporarily diminished, such as Turnagain Pass,
temperatures are also closer to freezing.

The current forecast remains on track. As such, there are no
changes to the rest of the forecast discussion from yesterday
evening.

As the upper level ridge continues to amplify, the atmospheric
river will reform and reposition on Friday, tapping into deeper
moisture and warmer air from a low well down in the Pacific (~40N
Latitude). Rain will gradually overspread Kodiak Island/western
Gulf and most of Southcentral during the day Friday. There will be
little change in the pattern through Sunday - with periods of
rain across lower elevations of Southcentral through this entire
time period. Of note, there is a lack of strong dynamical forcing
aloft, with just some weak low amplitude waves transiting the west
side of the upper ridge. However, the moisture content of the
airmass will be extremely high, so it will take very little
forcing to produce rain. What looks more impressive is low level
forcing, with warm air advection and strong (southerly) upslope
flow.

For this forecast package, have tried to hone in on approximate
storm total rainfall Friday through Sunday night, which will be
significant for all areas - likely setting new January records for
rainfall. While snowpack is quiet meager at lower elevations of
Southcentral, the ground is frozen. This means water will not be
absorbed and will tend to drain to lower spots and sit on top of
the ground. This will likely lead to widespread standing water and
flooding of low lying areas - from roads to yards to rivers. With
snow levels expected to rise quite high on Saturday (as high as
3000 feet for Hatcher Pass and in the 3000 to 6000 foot range for
Anchorage and the western Kenai) snowmelt will also contribute to
runoff and lead to rising water levels in area rivers/streams.
Bottom line: this is a rare event for January which will likely
cause significant impacts.

As for the wind, relatively lower pressure will persists to the
west from the eastern Bering Sea to Southwest AK as weak lows
transit northward through that corridor. This will lead to gusty
southeasterly gap winds over the next few days. As is usual, the
strongest winds will be found along Turnagain Arm into Anchorage,
Knik River Valley into Palmer, and along the Copper River. Of some
concern, the warm air advection will lead to development of a
temperature inversion, which is conducive to mountain waves along
the Front Range Chugach in Anchorage. This event does not look
nearly as strong as the January 12th event. However, winds could
still be quite strong Friday night through Sunday morning. For
now, have capped wind gusts as around 70 mph for the Anchorage
Hillside and 50 mph for lower elevations of Anchorage. Will
continue to evaluate stability profile and mountain wave potential
to determine if winds could be a little stronger than that.

Forecast confidence for what follows this rain event is low.
Model spread is quite large for early next week with progression
of the upper trough. Two potential scenarios are much colder and
mostly dry or somewhat colder with snowfall. In either case, the
colder temperatures will lead to freezing of any water sitting on
the ground.

-SEB/TM

&&


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

Very active weather will continue through the weekend as two
air-masses continue to collide over the central Bering. The
current weather pattern is an anomalous ridge over the north
Pacific and a deep and very cold upper level trough of low
pressure over the northern Bering and Siberia. For now, most of
Southwest Alaska is in the warm sector of the storm, with the cold
northerlies currently from Atka west. This cold airmass will
gradually push east through the coming days, bringing a return of
snow and drastically colder temperatures by Monday. For now
though, a Coastal Flood Warning remains in effect for the
Kuskokwim Delta coast until 9 AM this morning.

As the previously mentioned storm has started to move inland,
sustained winds are quickly diminishing but will remain breezy
over the next several hours. Behind this, and additional system
will move north and stall out to the south of the eastern
Aleutians, with a stationary front extending north and east into
the Kilbuck, Ahklun and Kuskokwim Mountains. To the east and west
of this front will live two very different weather regimes.
Currently, the stationary front is expected to extend from
Unalaska to the previously mentioned mountain ranges. To the west
of this line, persistent north winds and snow are expected. Snow
totals from late Friday night into Sunday will be in the 8 to 14
inch range, with some locally higher amounts possible.
Accumulation at sea level can be tricky to forecast, but
temperatures right off the surface for Unalaska and Nikolski will
be very cold, so heavy snow at 35 degrees is very possible, in
addition to north winds. To the east of the front, strong
southerlies will persist, with widespread rain across Bristol Bay
and the eastern portions of the Lower Kuskokwim Valley. For the
Lower Kuskokwim, slight deviations in the frontal location will
dictate the rain and snow transition area.

For Sunday and Monday, the cold air will begin to progress farther
to the east. How this transition occurs remains uncertain,
particularly if there will be enough moisture left to see a quick
shot of snow for Bristol Bay. A much colder airmass will follow
the transition, resulting in temperatures dropping below zero.

&&

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

Key takeaways:

- Deep southerly winds and heavy precipitation shifts away from
   the southern coast towards the Panhandle.

- Significant pattern shift spreads colder temperatures across
   the forecast area early in the work week.

On the upper level Alaska Weather map, a long wave trough
continues with an Easterly track over Mainland Alaska by the end
of the forecast period. A number of shortwaves provide the energy
for changes. The ECMWF anchors the strengths and trajectories
through the week, as the GFS becoming more of an outlier. The
UKMET and Canadian models clusters closely with the ECMWF by
Thursday. The strong warm air intrusion across the Southcentral
diverts Eastward with the changing pattern. Coming in from the
Northwest, colder air begins to sweep into Western and Interior
Southcentral Alaska through Thursday.

On the surface, a well developed low and front muscles across the
Interior, pushing the heavier rainfall and gusty winds over
Prince William Sound into the Southeast Panhandle through midweek.
Locally heavy snow moves in from the Southwest and West and along
the Alaska Range across the Canadian Border by Thursday. Out
West, gusty winds and broad areas of snow move from the Central
Aleutians and Bering over the Alaska Peninsula through Tuesday. A
developing low over Kamchatka brings another round of snow and
gusty winds across the Southern Bering and Aleutians into the
North Pacific by Wednesday.

&&

.AVIATION...

PANC... VFR to MVFR conditions continue into Friday morning with
rain expected to resume by 00Z Saturday. As precipitation picks
up in intensity, both visibility and ceilings may decrease to IFR
Friday night into Saturday. The Turnagain Arm Jet will also be
active, bending north over Fire Island and the Terminal area.
Expect a layer of low level wind shear. With prevailing winds of
30 to 40 kts from the SW, shifting to the SE at 15 to 25 kts
below 400 ft.



&&

$$