High Surf Advisory 1, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
604 PM AKST Sat Jan 11 2025

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Monday night)...

Key Points:

* A strong storm moves into Southcentral late tonight with
  widespread strong winds and precipitation expected.

* High Wind Warnings are in effect for late tonight through Sunday
  afternoon for Anchorage, Portage Valley and East Turnagain Arm,
  and Cordova. The highest winds are expected over the Anchorage
  and Eagle River Hillsides.

* The Mat Valley remains in a High Wind Watch as conditions are
  looking less favorable for high winds. However, gusts up to 75
  mph are not out of the question.

* Coastal areas will see periods of heavy rain at lower elevations
  and heavy snow in the mountains. With multiple inches of rain
  likely, this could lead to flooding along area streams,
  especially across the eastern Kenai Peninsula where a Flood
  Watch remains in effect through Monday morning.

* Inland areas will see much less precipitation, though most areas
  will still see periods of precipitation accompany the winds.

* Much colder air will move in aloft Sunday night into Monday,
  causing any remaining precipitation to change to snow. All signs
  point to a shift in the overall pattern, with a return to more
  seasonable temperatures and chances of snow for Southcentral
  Alaska next week.

With the first of two strong storms departing the Southcentral
region this afternoon, attention now turns to the second larger
and stronger storm moving in later tonight. The overall forecast
is on track and confidence in the greatest impacts of wind and
heavy precipitation has changed little. High Wind Watches for
Anchorage, Portage Valley and the East Turnagain Arm, and Cordova
have all been upgraded to High Wind Warnings. A High Wind Watch
continues for the Mat Valley. A Winter Storm Warning continues for
Thompson Pass for heavy snow while a Winter Weather Advisory has
been issued for the Richardson Highway north of Thompson Pass for
freezing rain, sleet, and snow. No other substantive changes have
been made to the forecast.

Please see this morning's discussion below for additional details
as forecast reasoning has not changed:

One of the biggest threats is for a damaging wind storm in the
Anchorage area late tonight through early Sunday morning. Forecast
soundings show a definitive critical layer, with a wind shift
from southeast to south or south-southwest above mountain top.
This can be conducive to formation of mountain waves and rotors.
Model soundings vary more on the atmospheric stability. The
occluded front crossing the western Gulf should bring some warm
air northward, but not a lot. Additional warming will occur as air
sinks off the mountains, though this could also be limited by a
moist atmosphere. In any case, these phenomena, which can lead to
very strong and erratic winds, are notoriously difficult to
forecast because they are very sensitive to the atmospheric
conditions for which we have limited ways to monitor when dealing
with conditions above the surface.

At the time when the potential for mountain waves looks greatest
(early Sunday morning), the low will be positioned over the
southeastern Bering Sea, with the occluded front over the western
Gulf. With much lower pressure at the south end of Cook Inlet,
this will lead to a strong down-inlet pressure gradients which
will keep the Turnagain Arm jet south of Anchorage. This gradient
combined with a lee side trough just west of the mountains (due to
the downslope flow) looks like it will induce a fairly robust
west to northwest wind at the surface for areas along Knik Arm and
down into north to west Anchorage. With nothing to drive the
winds inland at this point, the threat of high winds will be
highest along and right near the Chugach Mountains, including East
Anchorage and Eagle River. By the time the surface low gets far
enough north to induce the Turnagain Arm jet to bend northward,
winds will already have started to weaken. Thus, the threat of
high winds for north to west Anchorage (and areas further from the
mountains looks minimal). Confidence in reaching at least 75 mph
or greater along the Upper Hillside is high.

Stay tuned to your local forecast for any additional details on
what will surely impact travel across the region and could lead to
damage and/or power outages in some locations. The impacts from
this storm will last all the way into Monday, with gusty southerly
winds and snow showers lingering, but with improving conditions
through the day.

SEB/BL

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3: Today through Monday night)...

A large storm-force low pressure system enters the eastern Bering
Sea tonight. Strong southeast winds will spread into Southwest
Alaska through Kamishak Gap into the greater Bristol Bay region
along with rain and snow moving back across much of the area as
well. Northerly gale force winds set up across the Pribilof
Islands and eastern Aleutians, bringing in a colder air mass from
the western Bering Sea. Confidence has increased that the
Pribilofs islands will be impacted by snow on the western edge of
precipitation associated with the low. Snow showers, combined with
gusty winds, are expected to reduce visibilities to below one
mile at times, with potential for visibilities dropping as low as
one-half mile.

Farther north, 4 to 8 inches of snow accumulation is expected for
Nunivak Island and the Kuskokwim Delta coast from Kipnuk north
Saturday night into early Sunday morning, for which a Winter
Weather Advisory is now in effect. Confidence has also increased
for blowing snow potential for these areas as well, with
visibilities dropping to one-half mile or less at times. Blowing
snow is also possible for additional portions of Southwest Alaska
as the low lifts northward, bringing a push of southwesterly winds
along with snow showers, particularly for locations farther
inland across the Kuskokwim Delta. However, any significant
visibility reductions would likely be isolated to a small area for
a short time. A Coastal Flood Advisory is also in effect for the
Kuskokwim Delta coast, particularly between Kipnuk and
Kwigillingok coincident with high tide Saturday, Sunday, and
Monday evenings.

Out west, a compact gale or storm force low looks to track near
the western Aleutians. With a cold air mass moving into the
western Bering Sea, there is also potential for blowing snow for
those islands Sunday evening, but there is still significant
uncertainty in the low's track. The cold air mass spreads across
the Bering Sea and into Southwest Alaska with the northward exit
of the large low Sunday night, dropping temperatures into the 20s
for Monday and into the teens Monday night. Expect a general
drying trend across the region as well for the beginning of next
week.

Eovino/Quesada

&&

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

On the Alaska Weather map, an elongated upper level trough
stretching from the Russian Far East across the Aleutians and
Bering. This trough gets a little boost from passing shortwaves to
reorient over Southwest Alaska and the Gulf of Alaska for
midweek, before continuing its Eastward track into Canada by
Thursday. An upper ridge building across the Western Aleutians
Tuesday moves to extend across the Gulf of Alaska and Western
Alaska into Northeast Russia through the end of the week.
Deterministic GFS and ECMWF carry the features and changes well
through Thursday with the Canadian models joining up into the
weekend.

A decaying surface low on the Southcentral coast spreads locally
heavy rains along the coastal locations, with locally heavy snow
further inland and over higher elevations through Wednesday. A
second low over the Shumagin Islands brings mostly rain with snow
over Bristol Bay from the Eastern Aleutians, Alaskan Peninsula and
Kodiak Island through midweek, before moving over the Kenai
Peninsula before dissipating over Prince William Sound Thursday.
Bands of snow progress across the Bering through Thursday. A well
developed North Pacific low and fronts spreads snow changing to
rain across the Western and Central Aleutians Wednesday and
continues over The Bering, Southwest Alaska and the AKPEN by
Friday. Gusty winds with gale force gusts work their way from the
Central Aleutians over the AKPEN and Bering through Friday.

&&

.AVIATION...

PANC...VFR is generally set to prevail through much of the TAF
period. While west winds remain prevalent this afternoon, a brief
period of gusty southeast winds out of the Turnagain Arm cannot be
ruled out as today's trough lifts north. As the leading frontal
boundary crosses the Gulf, it will again cause winds to bend down
Cook Inlet toward lower pressure, leading to another wind shift
back to west to northwest and redevelopment of low level wind
shear. Tonight's system will be much stronger than the current
storm with a much stronger Turnagain Arm Jet. In addition,
conditions look favorable for mountain waves to form along the
Front Range Chugach, leading to strong erratic winds. While these
likely won't reach the terminal, it does look like a good set-up
for extremely strong low level wind shear. At some point tomorrow
morning, strong and gusty southeasterly Turnagain winds will then
bend into the terminal. There is uncertainty in timing and
strength, as the Turnagain jet will begin to weaken before bending
northward into Anchorage.

&&