Small Craft Advisory 949, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
516 AM AKST Wed Dec 9 2020

.ANALYSIS AND UPPER LEVELS...

An upper-level trough pushes northeast across the Southern
portion of the state through Thursday afternoon. A band of snow
ahead of the trough with travel across the Kenai Peninsula towards
Anchorage and the North Gulf Coast, also hitting parts of the
Mat-Su Valleys as well. A ridge of high pressure will build
behind the trough and encompass Southern Alaska Thursday and
Friday. Thus, expect clear and dry conditions towards the end of
the week. Out west, a storm force low pressure system with
multiple embedded surface lows continues to bombard the Western
Bering as its front progresses towards the Southwest Coast
through Thursday night. This front will have occluded before
reaching the coast and is not expected to have significant impact
and will likely diminish before making it onshore. By the end of
the week, a second strong system out of the North Pacific is
expected to approach the Southern AKPEN.

&&

.MODEL DISCUSSION...

There is relatively good model agreement in the short term.
Solutions have come into better agreement on the orientation of
the upper-level trough moving northeast across the state today
and tomorrow. Decent model agreement as well for the upper-level
ridge that moves across Southern Alaska Thursday and Friday. By
Friday, models begin to disagree across for a seemingly strong
North Pacific low approaching the Southern AKPEN Friday afternoon.
There is some similarity on the general track but distinct
differences on speed, location, and strength. NAM/ECMWF puts this
low around 968 mb on Friday while the GFS is stronger (963 mb) and
the Canadian- NH is weaker (975 mb).

&&

.AVIATION...
PANC...VFR conditions will persist throughout today. A band of
snow showers are expected to pass through this evening, bringing
conditions back down to marginal. Conditions should improve by
Thursday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

A cold upper trough over Southwest Alaska moves slowly across
Southcentral tonight and early Thursday. Snow showers, perhaps
mixed at times along the North Gulf Coast, will remain fairly
persistent over the Southern/Eastern Kenai Peninsula and North
Gulf until trough passage tonight. Inland areas will see some
isolated snow shower activity today, becoming more persistent as
the trough moves through tonight. Drier conditions are expected
on Thursday/Thursday night with the onset of broad offshore flow
and weak gap winds over the North Gulf Coast and Eastern Kenai
Peninsula. Conditions may also become favorable for low stratus
and fog to develop along Turnagain Arm and Anchorage Bowl
Thursday night with generally light flow and low level temperature
inversions.

&&

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

Weak circulation over the Bristol Bay area this morning will
continue to bring light snow showers as the dissipating area of
low pressure moves east. A very cold airmass has begun to move in
from the north across the Kuskokwim Delta and Lower Kuskokwim
Valley and temperatures across much of the northern parts of
Southwest will not climb above single digits today. Overnight
temperatures will drop tonight to 10 to 15 degrees below zero
across the north while overnight lows will be in the single digits
below zero across the Bristol Bay region. Clear conditions are
expected for Thursday as an upper level ridge of high pressure
builds ahead of an advancing front. By Thursday evening, cloud
cover will spread in overhead as the leading front associated with
a storm force low in the Bering approaches the coast Friday
morning. In response to this approaching front, offshore winds
will pick up along the coast late Thursday. A North Pacific low
approaches the Alaska Peninsula Friday morning, bringing easterly
gale force winds. Models continue to struggle with this low, so
there is less confidence in track and strength at this time.
Precipitation with this system will spread northward across the
Alaska Peninsula and the Bristol Bay region Friday into Saturday.
Given southerly flow and a warmer airmass with this low, a
rain/snow mix, or all rain is the more likely form of
precipitation this weekend with snow accumulations expected at
higher elevations.

&&

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

Satellite imagery this morning shows a 971 mb storm force low
centered south of Shemya. Several smaller mesolows are embedded
within the leading front as it gradually moves eastward across the
Chain. Easterly to southeasterly storm force winds are expected
across the western and central Bering waters this afternoon as
the front advances eastward, reaching the Central Aleutians by
this evening. Given a much warmer airmass, precipitation is
expected to fall as a rain/snow mix initially with a quick change
to all rain along the Aleutian Chain. Across the northern Bering,
precipitation will like be all snow, with the Pribilof Islands
also likely to see more snow until a change to rain by late
Thursday. Overall, this is a pretty moist system, so expecting
higher QPF values with this front. Storm force winds are expected
to persist across the Northern Bering waters through Thursday
night before diminishing Friday morning. The weakened parent low
will remain in place over the western Bering, keeping snow showers
likely across the western Aleutians for Friday. Another low
approaches from the North Pacific Friday morning and tracks
northward toward the Alaska Peninsula and Eastern Bering. Models
continue to struggle with this low, so there is lower confidence
in the track and strength at this time.

&&

.MARINE (Days 3 through 5 - Friday through Sunday)...

Bering: A broad area of low pressure over the Western Bering will
produce some easterly Gales across the Northern Bering early on
Friday before weakening late day. Friday night into Saturday there
is model disagreement as to how quickly a strong low will
cross the Eastern Aleutians into the Eastern Bering, with risks
being towards stronger easterly winds than currently forecasted.
While the positioning of this low is uncertain heading into Sunday
there is general agreement on winds falling below gales across
most of the Bering.

Gulf of Alaska: A North Pacific low lifting towards the Eastern
Aleutians may generate some easterly Gales across the Southwestern
Gulf of Alaska on Friday before this system moves west of the
area and weakens. Later on Saturday into Sunday another low in the
North Pacific looks to lift into the eastern Gulf of Alaska.
Currently the winds look to remain below gale force across the
Western/Central Gulf, but if this storm takes a further westward
track there could be a period of Gales late Sunday before the
storm weakens.

&&

.LONG TERM FORECAST (Days 3 through 7)...

On Saturday we kick things off with a strong low just to the
south of the AKPEN. Most guidance points towards this low crossing
into the Bering Saturday morning and then eventually
weakening/being absorbed into a broader low complex. This storm
should bring a rain/snow mix to start, but a quick transition to
all rain for the eastern Aleutians and southern AKPEN.

Late Saturday another storm looks to lift out of the North
Pacific and into the Bering, ultimately aiming for the Northern
Gulf Coast by Sunday, but with jet support staying well south of
the area,this low looks to be weakening rapidly as it approaches
and it seems most of the precipitation impacts will remain
confined to coastal areas, with the interior staying quiet.

By Monday and Tuesday model agreement falls apart, with little
left in the way of coherent signal to work with. That being said
the primary jet looks to remain well to our south, so odds are the
pattern will remain relatively quiet across much of Mainland
Alaska, with the storm track generally remaining out over the
North Pacific.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm: 411-414, 173-176, 179, 185
		 Gale: 352, 130-131, 155, 165, 170-172, 177-178, 180-181
		 Heavy Freezing Spray: 130, 160, 180-181
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KM
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...BG