Winter Weather Advisory 74, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
516 AM AKST Thu Dec 23 2021

.ANALYSIS AND UPPER LEVELS...

A ridge of high pressure remains draped across the western
Aleutians extending into Southcentral. A 155 kt westerly jet
streak is placed over the Southwest Alaska, while a 120 kt
northerly jet streak is located over the eastern Gulf. With the
main flow over Southwest Alaska being southwesterly, moisture and
warmer air is being pulled from the North Pacific to the
Southwest and Aleutians. The 00Z Anchorage sounding shows a very
dry layer in the lower to mid- levels of the atmosphere, with a
small fully saturated layer remaining next to ground level. This
accounts for the lower-level clouds in the area this morning as
the moisture is trapped in the lower levels. Over Southcentral, a
lee- side trough has developed and a low has positioned itself in
the Gulf. This aforementioned lee- side low is beginning to pull
winds from the north and cause small craft winds through the
Barren Islands as seen on the Scatterometer this morning. The draw
of northerly winds along with cold air advection is beginning to
dry and scour out low-level clouds over Southcentral. Satellite
imagery is starting to show clearing over Palmer, Homer, and
Kodiak. Radar is showing returns over eastern Prince william Sound
and Cordova as this fast moving precipitation moves farther southeast
this morning.

&&

.MODEL DISCUSSION...

The models remain in good agreement on the large-scale features
impacting southern Alaska. The primary point of disagreement
continues to be precipitation type with the next front expected
to move into Southwest Alaska as early as Friday afternoon. Any
mixed precipitation type for this next system should be a short
transition as the atmospheric column of air is not as cold as the
last system, and the turning of winds becoming more southerly
will draw warmer air to the areas. Another forecast challenge is
the timing of when snow may begin in Anchorage. Snow will have to
saturate a dry atmosphere and forecasting when this could exactly
happen is difficult. However, overall forecast confidence is
good.

&&

.AVIATION...
PANC...VFR to MVFR conditions are expected to persist as
lingering moisture will result in a period of lower ceilings.
However, as strong northwesterly winds develop aloft behind the
low, that should help to start clearing the terminal and return
the area to VFR conditions.

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

Very strong west to northwest flow over top of a strong subtropical
ridge sitting over the North Pacific combined with surface pressure
gradients will lead to widespread gales across the Gulf coastal
waters through this evening, with smaller areas of embedded storm
force winds. The more westerly direction (as opposed to the more
frequent northwesterlies so far this winter) will allow winds to
move into much of southern Cook Inlet and continue eastward along
Kachemak Bay and the Homer area. The upper level ridge will
rapidly amplify over the Bering Sea today through tonight.
This will shift flow over Southcentral from westerly to north-
northwest, helping to drive arctic air from Interior Alaska
southward across Southcentral and into the Gulf of Alaska. Thus,
also expect strengthening gap winds along the north Gulf coast
and Prince William Sound. Short-waves riding over top of the upper
ridge will enhance subsidence and winds along the coast. Winds
will diminish Friday-Friday night as the upper ridge builds
eastward over Southcentral and the Gulf.

Meanwhile, the aforementioned upper level short-waves and westerly
flow will produce some localized areas of snow along west facing
mountains today. As the flow veers toward the north tonight,
subsidence and cold and dry air advection will bring an end to
the snow. The next chance for precipitation will come Friday night
through Saturday night as a series of short-waves track over the
ridge. At this point in time, none of these short-waves look
particularly strong and they will be moving quite fast. Therefore,
if it does precipitate, it will be brief and light in nature. As
the heights rise aloft, low level temperatures will warm - with
850 mb temperatures likely rising above freezing by Saturday night
for all except the Copper River Basin and northern Susitna Valley.
This is the perfect recipe for freezing rain if it were to
precipitate. Chances are though, it will remain dry for most of
the region for Christmas Day.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3:
Thursday through Saturday)...

The Alaska Peninsula remains relatively calm this morning with
diminished winds and dry conditions extending from Port Heiden to
Cold Bay. Satellite imagery shows a band of low-level stratus and
fog stretching from Iliamna to Port Heiden due to southwest winds
pushing a warmer air mass into the Aleutian Range. Further north
and to the west, radar is showing an area of light snow from east
of Bethel into Aniak and down to Kilbuck. A subtle shortwave
disturbance is skirting the region within broad northwesterly flow
aloft. This activity is not expected to result in any meaningful
accumulations and should gradually diminish later this evening.

After an extended period of rain and freezing rain on Wednesday,
Thursday will prove less eventful across Southwest. However, a low
lifting across the Bering will bring another warm front into
Southwest on Friday with a warming trend and another chance for
freezing rain. There are some discrepancies within model guidance
as to timing of the warm front and precipitation into Southwest,
but general thinking is for precipitation to move into Southwest
Friday during the morning hours. Precipitation preceding the
arrival of warmer air will pose at least a small window for
potential freezing rain, and roads and surfaces could become slick
through the morning hours. Steady west to southwest winds
accompanying the front will help to push colder temperatures out
of the region and any freezing rain should transition to all rain
by late afternoon if not sooner. The best threat of freezing rain
will exist over the Kuskokwim Delta but the threat may also
present itself further south into Bristol Bay. A second round of
showers is then forecast on Friday night into Saturday morning as
shortwave trough digs southeastward and into the Gulf of Alaska.

&&

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

Generally benign conditions are ongoing across the Bering and the
Aleutian Chain this morning. A North Pacific High remains south
of the Eastern Aleutians and the Alaska Peninsula with a shortwave
ridge expected to translate across the Pribilof Islands into the
Kuskokwim Delta later this evening. As a result, southerly winds
will remain at or below 20 knots with minimal shower chances. A
steady stream of southwest flow across the Aleutians will help to
keep temperatures above freezing for the next few days. The next
shortwave trough will track through the Bering on Friday with a
swath of 30 to 35 knot winds forecast to lift over the Central and
Eastern Aleutians before diminishing near Nunivak Island Friday
night. A second, yet stronger upper low, will lift out of the
North Pacific and into the western Bering on Saturday. Widespread
gales are forecast to develop from the Aleutians northward toward
Saint Matthew Island. This area of winds should exit the Bering
Saturday night as the associated upper low moves into the Chukchi
Sea.

&&

.MARINE (Days 3 through 5 - Saturday through Monday)...

Bering...A well developed North Pacific low intensifies as it
moves to North Kamchatka Sunday, then weakens as it continues into
the Chukchi Sea by Monday. A front extends across the Western
Aleutians through Monday. Forecast confidence is moderate for the
low tracks through Monday. Gusty winds increase to widespread high
end gales with areas of storm force winds across the Western and
Central Bering late Sunday, diminishing Monday as it moves into
the Chukchi Sea. Seas building to 22 feet over the Bering Sunday,
subsiding Monday. A new North Pacific low approaches the Western
Bering late Sunday. Widespread high end gales with areas of storm
force winds spread to the Central Bering by Monday. Seas building
to 34 feet over the Western Bering, and to 22 feet for the Pacific
side of the Aleutians through Monday.

Gulf of Alaska...Low pressure continues over the Eastern Gulf
through Monday. Forecast confidence is moderate with pressure
differences over the Gulf. Gusty Westerly winds spread across the
Southwestern Gulf through Monday. Gap winds from the Alaska
Peninsula increase to gales into the Gulf for Monday. Seas
building to 12 feet over the Southern Gulf by Monday.

&&

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

Global models share very similar large scale synoptic solutions
consisting of the Omega Blocking pattern and the onset of a few
strong Northwest Pacific and Siberian lows transiting across the
Bering into Southwest Alaska through the middle of next week.
Multiple fronts will bring gales and potentially near storm-force
winds across the Bering/Aleutians through much of the long term.
Considering the warm tropical moisture and warm southwest winds
pulling behind the fronts, the potential for rainfall is favorable
over the Aleutians/Bering, with rain/snow mix more inland of
Southwest Alaska from the expected frontal systems. P-type
forecast could be a challenge, but as solutions continue to
improve over time, perhaps we'll see better P-type agreements in
future model runs. Over the Gulf and Southcentral, latest solutions
suggest very light snow to an absolutely quiet pattern there
until Wednesday when a transitional front turned-into an open
trough shifts over the western Gulf, leading to increased NW winds
along the Shelikof Strait and Kamishak Bay through the Barren
Islands. The potential for gap winds is favorable as well over
Southcentral coastal areas should weakening fronts from Southwest
Alaska drives past the Alaska Range into Southcentral. For now,
will monitor these episodes closely.

The upper ridge axis will provide mostly clear skies and light
winds at night as it drifts from Southwest to Southcentral through
Wednesday. The potential for widespread foggy conditions should be
closely monitored as well especially when there's subsidence air
passing through the entirety of the forecast period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 131, 351.
	     Gale Warning: 119-125, 127, 129, 130, 132, 136-139, 141,
		 173, 175-177, 352, 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...NS
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BL
MARINE/LONG TERM...MK/CB