Gale Warning 832, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
548 AM AKST Thu Dec 21 2023

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

Quieter weather will prevail the next couple of days before a
major storm system brings strong winds and heavy precipitation to
coastal areas beginning Friday night and likely continuing through
the weekend. First off, a long-wave trough is now centered over
mainland Alaska, the Gulf of Alaska, southward into the northeast
Pacific. A vertically stacked low is spinning just east of Middleton
Island, with light precipitation spreading onshore from the
Cordova area northward into the Copper River Basin. Gusty gap
winds have developed on the back side of the low, namely for
Seward/Resurrection Bay and Whittier/Passage Canal. Snow has
backed as far west as Whittier and is being blown around by 45 mph
wind gusts, leading to low visibilities. However, this should be
very short-lived as the precipitation shield shifts eastward this
morning. The eastward progression of the long-wave trough will
push this low onshore just east of Cordova this afternoon, with
light snow spreading northward across much of the Copper River
Basin. Precipitation will then diminish tonight through Friday
morning as the trough exits to the northeast. The rest of
Southcentral will remain dry. Areas of low clouds extend from the
Matanuska Valley southward to the western Kenai Peninsula, caught
below a subsidence inversion (as seen in the 12Z Anchorage
sounding). While there may be some improvement during the day as
cooler/drier air advects in from the northwest, expect the
inversion to hold and thus maintain areas of low clouds in the
region.

All attention will then shift to a deep low tracking south of the
Aleutian chain today. Short-waves will dig down the back side of
this low into the North Pacific as the low passes south of the
Alaska Peninsula on Friday. This will lead to rapid amplification
of the upper level flow and force the storm system to begin to
lift northward toward southern Alaska. Model guidance has been
quite consistent and is in good agreement with the evolution of
this system Friday through Friday night, so forecast confidence is
fairly high. With the latest forecast updates this morning, have
trended stronger with winds and increased precipitation amounts.
Kodiak Island will start as snow Friday morning, but expect a
fairly quick transition to rain Friday afternoon.

Snow showers will develop well out ahead of the front across the
eastern Kenai Peninsula and Prince William Sound Friday as an
upper short-wave rounds the low center and tracks quickly
northward. Steadier and heavier precipitation will then Friday
night into saturday morning. Strong easterly low level flow will
lead to the heaviest precipitation in upslope flow across western
Prince William Sound and inland to the Seward Highway corridor of
the eastern Kenai Peninsula. This same flow will lead to much
lighter precipitation initially for the Cordova/Valdez areas
while downslope flow will lead to dry conditions for most of the
western Kenai Peninsula northward to Anchorage and the Mat-Su.
Marginally warmer air will arrive with the front sometime late
Friday night or early Saturday, leading to a transition to rain
for areas near sea level along the coast, with a mix of rain and
snow likely spreading inland across the eastern Kenai Peninsula.

Impacts across the coastal waters will be significant, with
widespread gale to storm force winds and seas on the order of
15 to 20 feet for all except the inner waters. The front will move
through quickly, so the worst conditions won't last all that long.

Forecast confidence takes a precipitous Saturday and beyond.
The GFS was the first model to pick up on a new low forming over
the northeast Pacific as a short-wave ejects out of the highly
amplified long-wave trough - and tracking this low northward into
the eastern Gulf and then northwestward toward Southcentral as the
trough takes on a negative tilt. All of the model guidance now has
some indication of a new low, but timing/depth/track vary
significantly. This will have huge ramifications on the forecast
because the presence of such a low has good potential to bring
even stronger winds and heavier precipitation to wherever is in
its path. The low will also affect the evolution of other
features, including whether the initial front pushes onshore of
Southcentral Saturday or stalls along the coast. This in turn
affects how much warm air moves into Southcentral and the
resultant precipitation type as well as whether precipitation
spreads inland beyond the coastal mountains. Stay tuned as there
winter weather and travel disruptions are likely to impact much
of Southcentral this weekend.

&&

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

Several major updates have been made to the forecast overnight.
Overall, the low currently moving towards the Western and Central
Aleutians is expected to remain a bit further south, increasing
confidence that precipitation for the Eastern Aleutians will
remain all snow. Winds have also increased with this system,
leading to expanded/extended gale and storm warnings for the
marine forecast areas, as well as increasing confidence in the
likelihood of extreme freezing spray as we head into the weekend.
Three new warnings/advisories are currently in effect:

* Winter Weather Advisory for Eastern Aleutian Islands: Snow and
  blowing snow expected beginning noon today.

* Blizzard Warning for the Bering Sea coast of the Alaska
  Peninsula from Friday morning through Saturday

* Blizzard Warning for the Bristol Bay coast from Friday afternoon
  through Saturday afternoon

Onto the details: Fog and lingering areas of snow, rain/snow
showers are all that remain of the broad low that has been
influencing weather across Southwest Alaska, the Bering Sea, and
the Aleutian Islands these past few days. The next weathermaker, a
~965 mb low, can be seen on satellite as it makes its way towards
the Western and Central Aleutians. Precipitable water values as
high as 1 to 1.25 inches accompany its front, hinting at the
moisture this system will bring to Alaska. As mentioned in the
summary above, the latest model runs have been showing a slightly
stronger low with a track that's slightly further south. These
seemingly minor differences have led to quite a few changes with
the forecast.

The biggest impact has been to the Eastern Aleutians, where a
Winter Weather Advisory is currently in effect. With the low and
its attendant warm air remaining further south, there is
increasing confidence that precipitation will stay all snow for
the Eastern Aleutians. This brings the potential for higher snow
amounts, as well as reduced visibility in blowing snow. Because
temperatures will be right around the freezing point, there is
quite a bit of uncertainty with snow amounts and how far
visibility will drop. Current thinking is that strong winds will
lead to significant visibility reductions in blowing snow,
especially at elevation. As for snow amounts, the current forecast
is actually on the lower end when it comes to what operational
models are putting out. We're leaning on lower snow amounts
because sea surface temperatures are quite warm (around 5 degrees
Celsius), and this marine influence will likely lead to denser
snow and lower amounts. As for the Central Aleutians, temperatures
climbing into the mid 30s suggest that snow amounts will be much
lower, with little reduction in visibility due to blowing snow.

Shifting to where the Blizzard Warnings are in effect along the
Alaska Peninsula and Bristol Bay coasts: this is mostly in line
with forecast trends from yesterday. Confidence is highest in the
winds, which will be strengthened by: (1) a tightening pressure
gradient as the low center tracks south of the Alaska Peninsula,
(2) strong cold air advection around the backside of the low, and
(3) a robust upper level shortwave moving south towards the Alaska
Peninsula. The limiting factor for this set-up will be snow.
Models continue to show differences with where the deformation
(snow) band will set up in Southwest Alaska. If snow ends earlier,
conditions could improve earlier than what's currently in the
forecast. Stay tuned as we update the forecast, and be sure to
stay weather-aware when planning potential holiday travel late
this week into the weekend.

-KC

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...

A broad upper level longwave trough extends across most of the
Bering into Western Alaska through the period. A number of
shortwaves rotate through the pattern and distort the trough
through midweek. Most of the active weather will be occurring over
the Gulf of Alaska with areas of precipitation, locally heavy
across coastal Southcentral Alaska. Precipitation starts from the
Southern end of the Eastern Kenai Peninsula across Prince William
Sound into the Copper River Valley. Locally heavy snow or mixed
rain, with rain along the coast. Snow should occur further inland.
While there is some risk of downslope drying, the troughs and
weak fronts pushing over the Southcentral portions of the state
may have some warm air aloft and bring local areas of freezing
precipitation Sunday, ending Christmas Day. A second front moving
into the coastal zones repeat the cycle Tuesday into Wednesday.
Gusty winds up to gale force are on tap for the marine areas
through the entire forecast period.

Most of the Bering Sea and Aleutian Chain will be within the
broad trough through the long-term period. Unsettled conditions
are expected throughout the Bering Sea and Aleutian Chain through
Wednesday. However, precipitation should be more showery in
nature. Where the uncertainty here is exactly where North Pacific
lows track before they enter the Gulf. Depending on how far north
of south they track in proximity to the Aleutian Chain and Alaska
Peninsula will determine if the Aleutians could see enhanced
precipitation from fronts or if they remain showery. As for
Southwest Alaska and the AKPEN, precipitation from two fronts
lifting into the Gulf Sunday and Tuesday especially for the AKPEN.
Conditions look to remain showery for Monday as northwest flow
kicks in along with cold air advection behind the fronts.

Kutz

&&

.AVIATION...

PANC...Reduced ceilings are expected early this morning. On
satellite imagery there is an area of fog and low stratus near
PANC. Wind over the terminal to remain fairly light and out of
the north. Amendments will be made as needed.

&&