Small Craft Advisory 1017, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 AM AKST Fri Dec 23 2022

.ANALYSIS AND UPPER LEVELS...

At upper levels, a large blocking ridge continues to build over
the Chukchi Sea while extending over northwest Alaska. A trough
from the Arctic is dropping south across southeast Alaska and
approaching Southcentral, bringing a very cold airmass with it. A
low near the western Aleutians is exiting into the North Pacific.

At the surface, cold and clear conditions persist across most of
southern Alaska. A front extends from the eastern Gulf of Alaska
east to Kodiak Island and the Alaska Peninsula. Another front
extends from the eastern Aleutians east across the Bering Sea.
Precipitation with the fronts have been quite variable across both
the east and west domains. The eastern Aleutians are primarily
seeing rain, while Kodiak Island and the Pribilof Islands have
been getting rain, snow, and even a rain/snow mix at times.

Strong winds are a continued theme across both forecast domains.
Widespread easterly gale-force winds blanket the Gulf, Alaska
Peninsula, eastern Aleutians, and Bering Sea. Gusty offshore winds
persist along the Bristol Bay and Kuskokwim Delta coasts. With
the deep Arctic trough approaching Southcentral, gusty
northeasterly winds have started developing across portions of the
Mat-Su Valleys, northeast Prince William Sound (including Valdez,
Thompson Pass, and Cordova), and the Copper River Basin. Various
locations within these areas are already seeing reduced
visibilities from blowing snow as the strong winds transport
existing snow from the ground. Blizzard Warnings remain in effect
today for Valdez, Thompson Pass, and the Matanuska Valley. The
Anchorage area is seeing an uptick in gusty winds, though they are
not expected to get as strong as the winds in the aforementioned
areas. Frigid temperatures coupled with strong winds are promoting
very low winds chills across much of inland Southcentral,
especially for the Copper River Basin and through Thompson Pass
where Wind Chill Advisories remain in effect today.

&&

.MODEL DISCUSSION...

Guidance remains in decent agreement with larger scale features
through approximately Saturday afternoon. Thereafter, models
struggle to resolve the exact evolution of a surface low that
forms south of the Alaska Peninsula (AKPen) earlier that morning.
There is general consensus for the low to eventually push into the
Bering, but the timing, track, and strength of the low differs
amongst the models. Thus, precipitation and wind forecast
confidence decreases beyond Saturday afternoon mainly for Kodiak
Island, southwest Alaska, and the eastern Bering/Aleutian region.

&&

.AVIATION...
PANC...VFR conditions will persist through this TAF period.
Northerly winds are increasing this morning and are expected to
peak early this afternoon through this evening. An Airport
Weather Warning remains in effect for northerly winds gusting to
35 kt or greater. Gusty winds should be relatively continuous, but
there may be brief periods where winds die down.

&&

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

The forecast remains on track for a "bora" windstorm across
Southcentral through Friday night, with widespread gusty winds,
and with very cold air temperatures and wind chills returning
later today. The arctic trough responsible for the impactful
weather has moved south over Southcentral this morning. This
feature continues to look a bit stronger and has a more southern
progression than in previous model runs. As such, wind gusts from
Palmer and Wasilla to Valdez have been slightly stronger than
initially advertised, and the morning forecast package reflects
forecasted wind gusts of up to 75 mph today for the Matanuska
Valley and up to 65 mph for Valdez and Thompson Pass. The
greatest impacts from this event will come from reduced
visibilities due to blowing snow and hazardous travel conditions
due to significant drifting of snow. Although the Matanuska
Valley and Valdez area will see the strongest winds with this
system, gusty winds will also develop across a larger portion of
Southcentral gusty winds expected over the northern Copper River
Basin (along the Alaska Range), through Thompson Pass/Valdez, the
Susitna Valley, north to west Anchorage and the western Kenai
Peninsula from Nikiski south to Homer.

The gusty winds will begin to slowly diminish Friday night.
However, winds will remain elevated through Saturday due to the
tight pressure gradient between the arctic high and a surface low
spinning up near Kodiak Island, moving northwest across the AKPen
on Saturday. This area of low pressure could also bring light
snow across the southern half of the Kenai Peninsula later tonight
and across Kodiak Island through Saturday.

A second area of low pressure will lift north across the eastern
Gulf of Alaska, pivoting toward eastern Price William Sound for
Saturday. This low is expected to be a compact low, keeping winds
strong along the north Gulf coast. This feature will also spread
snow from Cordova and the Copper River Basin Saturday west to the
eastern Kenai Peninsula late Saturday night or early Sunday
morning. Snowfall is expected to be light across all areas due to
the antecedent colder and drier airmass.

A much larger and stronger storm system is then expected to move
toward the southern Gulf for Sunday with its occluded front
extending northward across the northern and western Gulf. The front
will bring precipitation to Kodiak Island Sunday and along the
north Gulf coast by late Sunday. The airmass out ahead of this
will start out cold enough for snow, but there likely will be a
transition to rain along the immediate coast Sunday night into
Monday as warmer air moves in with the front.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Sunday)...

A rain/snow mix will continue for the Alaska Peninsula through
this morning as easterly to southeasterly upslope flow continues.
A robust shortwave will eject westward from an upper-level trough
located over Yukon, Canada this afternoon/evening. This will
result in producing a strong low pressure system over the Alaska
Peninsula. Models are having a hard time handling the exact track
of the low, and will therefore, impact where the highest winds,
precipitation, and blowing snow concerns will be. This forecast
package reflects precipitation impacting the Alaska Peninsula
tonight, then working its way north into the Bristol Bay region
for Saturday, and finally reaching the Kuskokwim Delta coast for
Sunday. As the aforementioned low tracks northeastward along the
Chain, northerly winds will increase region wide. With this, the
potential for blowing snow coincided with reduced visibilities
becomes possible along the Bristol Bay coast and portions of the
Alaska Peninsula. Due to elevated frigid northerly winds, wind
chill concerns also arise as this system progresses. As a result,
a Wind Chill Advisory has been issued for the Middle Kuskokwim
Valley starting tonight and expiring Saturday afternoon. Be sure
to check back in with the forecast as models get a better handle
on this feature and the possible impacts it may have.

&&

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

Active weather will continue for the Bering and Aleutians for the
next several days. A Flood Warning is still in effect for
Unalaska/ Dutch Harbor until 11:45 AM this morning as scattered
showers continue in the area. The low pressure responsible for
the rain across the eastern Aleutians the past few days continues
to elongate and weaken as it slowly drifts northwestward. The
Pribilofs will continue to see scattered rain and snow showers
today before tapering off in this afternoon/evening as multiple
shortwaves traverse westward over the central Bering. A robust
shortwave will eject westward from an upper-level trough located
over Yukon, Canada this afternoon/evening. This will result in
producing a strong low pressure system over the Alaska Peninsula.
Models are having a hard time handling the exact track of the
aforementioned low, but colder air and strong winds are likely to
occur with snow being the main precipitation type for this
system. Depending on where the low tracks and how strong the
winds get, the potential for blowing snow coinciding with
reducing visibilities is possible for the eastern Aleutians and
Pribilof Islands. Be sure to check back in with the forecast for
updates as the evolution of this system becomes more clear.

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

Looking ahead in the extended forecast, the latest model runs and
ensemble guidance are in fairly decent agreement with the
evolution and progression of the synoptic features. An upper level
wave over northeastern Alaska will retrograde southwestward into
a closed upper low. Compared to previous runs where the center of
the low was over the eastern Bering, the models now track it to
the western Bering instead. The low will remain quasi-stationary
early next week, but models significantly diverge from Tuesday
onward. Meanwhile, the upper low will push storm systems from the
Pacific into the Gulf of Alaska. While specific details still need
to ironed out, most of the coastal areas will see some wintry mix
and inland areas will stay on the drier side. A general troughing
pattern associated with the aforementioned low out west will
result in an unsettled pattern for much of the Bering Sea and
Aleutians Islands heading into late next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning: 111, 131.
		 Flood Warning: 185.
	     Wind Chill Advisory: 131, 141, 152.
MARINE...Storm Warning: 130, 139.
 		 Gale Warning: 119, 120, 125, 127, 128, 131, 132, 138,
		 140, 150, 160, 165, 179-181, 351, 352, 411-414.
		 Heavy Freezing Spray: 127, 128, 130, 139, 140, 180,
		 181, 185, 160, 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AF
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...NS
LONG TERM...MF