High Wind Watch 3, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
650 PM AKST Thu Jan 23 2025

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

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.

A look at satellite imagery shows ongoing amplification of the
upper level flow, which is the key to what will be significant
impactful weather over Southcentral Alaska over the next 3 to 4
days. A deep upper level low anchors a trough over the western
Bering Sea. A corresponding strong ridge is centered over the
Northeast Pacific extending northward to the eastern AK mainland.
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 are
riding the jet northward up the west side of the ridge, which is
amplifying out ahead of this low.

This sets the stage for upcoming weather. First, focusing on
precipitation, a short-wave over the eastern Bering will ridge
over top of the ridge this evening, leading to increasing
precipitation along south to west facing mountains of
Southcentral. This will be in the form of rain for lower
elevations where temperatures are well above freezing. The primary
area of impactful snowfall will be Hatcher Bass up to the
northern Susitna Valley - especially through the Broad pass area.
Precipitation will diminish behind this short-wave late
tonight/early Friday.

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 Saturday night. For
now, have capped wind gusts as around 70 mph for the Anchorage
Hillside and 45 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

&&

.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, High Wind Warnings remain in effect for the Eastern
Aleutians and Western Alaska Peninsula, where gusts between 60
and 80 mph have been observed today. A Coastal Flood Warning also
remains in effect for the Kuskokwim Delta coast.

The storm associated with the aforementioned warnings is
currently near the Pribilof Islands, with a broad swath of storm
force winds from the eastern Aleutians north into the southwest
coastline. These winds will quickly diminish by early Friday
morning (though conditions will remain breezy) as the system
rapidly moves inland. Behind this, and additional system will move
north and stall out to the south of the eastern Aleutians, with a
stationary front extending north 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 12 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...Light rain will move back over the terminal this evening
with VFR and possible periods of MVFR ceilings. A break in the
precipitation is possible Friday morning, though showers will
start moving in after 12Z with rain then expected to spread back
over the terminal by 21Z Friday. As precipitation picks up in
intensity, both visibility and ceilings will decrease by Friday
evening. Turnagain Arm winds will remain gusty as they continue
to bend into the terminal.

&&