Small Craft Advisory 953, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
413 AM AKST Thu Dec 10 2020

.ANALYSIS AND UPPER LEVELS...

The upper level trough over Southern Alaska exits to the
northeast today and high pressure sets up over the Southern
Mainland through Saturday. Thus, drier and clearer conditions are
expected across Southern Alaska through tomorrow.

A deep, vertically stacked storm force low still resides near the
Western Aleutians through the Friday night. Multiple shortwaves
along the front will continue to track across the Bering, exiting
to the north by Friday morning. Widespread precipitation and
strong winds are expected across the Bering as a result. By
Saturday morning, the low will dissipate and be absorbed by a
second system entering the Southern Bering. A second system
currently in the North Pacific is making its way towards the
Southern AKPEN and will likely cross over into the Bering by the
Saturday. The associated front is expected to diminish as it
reaches the Southwest Coast, thus, precipitation will be confined
to the Southern AKPEN and parts of Bristol Bay Friday evening into
Saturday.

&&

.MODEL DISCUSSION...

Model agreement is relatively good on the synoptic scale with the
exception of a strong North Pacific low approaching the southern
AKPEN by Friday night. Models have been struggling with this
system over the last few model cycles as it progresses north.
While they are agreeing that it crosses the southern AKPEN and
enters the Bering on Saturday, the specific track and speed
differs across all solutions. There is also a decent size spread
in strength with the lowest at 967 mb (GFS) and the highest at 976
mb (Canadian-NH). Agreement is fair for the storm force low west
of the Aleutians. The trend to stronger winds tomorrow continues
across all models. The GFS wants to form a second closed low
within one of the embedded shortwaves traversing across the Bering
tomorrow night, but it seems to be an outlier.

&&

.AVIATION...
PANC...MVFR to IFR conditions this morning with the development of
some low stratus and should improve to VFR by this afternoon.
Conditions may drop to IFR again tonight with the possibility of
reforming low stratus.

&&

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

A slowing moving upper trough progresses eastward across
Southcentral today. The potential for isolated to scattered snow
showers will continue from the Anchorage Bowl to the MAT-SU
Valleys until the trough passage late this morning. Higher
pressure and cooler temperatures then spread across Southcentral
tonight and Friday. The focus then will then shift to local gap
winds across the north Gulf Coast and Eastern Kenai Peninsula/Western
Prince William Sound tonight and Friday. Cooling temperatures
will produce shallow temperature inversions and favor stratus and
fog development in spots. The most likely areas will be in inland
valleys, Knik Arm/Turnagain Arm, and Anchorage Bowl tonight and
Friday. A weather front heading north into the Western Gulf will
bring increased chances for rain and snow to Kodiak Island Friday.

&&

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

An upper level ridge will bring generally quiet conditions to
Southwest Alaska through Friday. Gusty northeasterly winds will
likewise bring cold temperatures, with highs remaining in the
single digits today away from the coastline. A front will lift
northward across the AKPEN on Saturday. However, much of the
precipitation looks to remain on the windward side of the Alaska
Range.

&&

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

A strong front continues to track across the central Bering and
Aleutians. While the strongest (hurricane-force) winds will remain
over open waters, storm force winds are now occurring over the
Pribilofs. Snow this morning combined with these winds will lead
to periods of significantly reduced visibility. Rain is expected
to mix in by early afternoon, cutting into the accumulations.
However the gusty winds will continue through the evening (see
Winter Weather Advisory). Strong southerly winds centered near
Unalaska will likewise persist through the morning. Minor snow
accumulations are possible early, before precipitation changes to
rain. Winds will generally diminish through Friday as the
associated low weakens over the western Bering. A second, more
compact low will approach the eastern Aleutians on Friday bringing
strong easterly gales and moderate mixed precipitation to the
region. The low will track northwestward into Bristol Bay by
Saturday, bringing similar conditions to the Pribilofs.

&&

Marine (Days 3 through 5/Sat through Mon)...

Bering/Aleutians:
Forecast confidence is low for Saturday through Sunday, with
guidance showing large discrepancies in the placement and
intensity of synoptic scale features. In particular, expect a low
to track up from the Pacific and approach the Alaska Peninsula
friday, but unsure of how quickly this low will lift northward
into the Bering Sea. Bottom line, expect widespread gales across
this region at some point during these two days, with potential
for a small area of storm force winds south of the Alaska
Peninsula. Confidence falls even further on Monday, so it's hard
to say where storms will be positioned at that point in time.

Gulf of Alaska:
There are discrepancies in the placement and intensity of synoptic
scale features, however most guidance agrees on a deeper low
tracking from the northeast Pacific into the eastern Gulf. Thus,
winds will likely remain below gale force across the western Gulf
waters.

&&

.LONG TERM FORECAST (Days 4 through 7/Sun through Wed)...

The pattern is expected to become more amplified during the day
4-7 period. Teleconnections support a weakening of the jetstream
winds which will help the Rossby Waves amplify. This trend is
reflected in both the GFS and ECMWF deterministic models as well
as GFS and ECMWF ensembles, however there are some differences on
where these trough and ridge axes are placed and oriented between
both the GFS and ECMWF. A few areas of consistency in the long
range prognostics are a ridge axis over the Gulf of Alaska on
Sunday. There are differences on the orientation of this ridge-
axis but due to the common theme of this ridge axis being present,
lighter winds and clearer skies are possible across the northern
Gulf. Another area of congruence is the ridge axis extends over
Southwest Alaska Sunday night which could result in clearer skies
and colder temperatures for the Kuskokwim Delta and Kuskokwim
Valley. GFS and ECMWF ensembles show signs of a Rex Block
developing over the Bering Sea on Sunday and persisting into
Wednesday as easterly 200 mb zonal winds in the polar jet region
will help weaken the polar jet and allow for the potential block
to form. Ensembles have the upper trough associated with this Rex
Block over the Bering, thus a more unsettled weather pattern is
to be expected. Monday through Wednesday there are many discrepancies
at both the surface and the upper levels which results in a lower
confidence forecast. However, one feature that seems consistent
to point out on Wednesday is the upper level ridge that extends
northwestward from the North Pacific into Bristol Bay. This ridge
seems plausible as latest guidance shows the Madden Julian
Oscillation (subseasonal phenomena that involves strong tropical
convection that propagates eastward from the western Indian Ocean
to the eastern Atlantic) propagating eastward to near 120E, which
supports the Rossby Wave orientation to have this upper level ridge.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 195
MARINE...Storm: 411-414, 179, 185
	  	 Gale: 351-352, 132, 138, 150, 155, 160, 165, 170-172, 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...AP
MARINE/LONG TERM...ED