Small Craft Advisory 1023, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
510 AM AKST Sun Dec 23 2018

.ANALYSIS AND UPPER LEVELS...

Weather across southern Alaska is generally quiet this morning. A
vertically stacked upper level low is positioned southeast of
Kodiak Island. This low sits at the base of a long wave trough
that encompasses western mainland Alaska, the eastern Bering and
the Gulf of Alaska. Increased instability through the Gulf,
combined with weak short waves passing through the flow of the
trough are creating showers through the Gulf and northern
coastline. On the backside of the trough, northerly flow persists
through the eastern Bering. A second upper level low sits east of
Norton Sound. Between these two systems, a deformation zone over
the western mainland is bringing light precipitation to the lower
Kuskokwim Delta and Bristol Bay. A deep vertically stacked low
spins southwest of the Shemya and continues to migrate southward.
A northwesterly oriented occluded front over the central Bering
and central Aleutians is tracking northeastward. The snow showers
and gusty winds have dissipated both in the area of the low and
the weakening front.

&&

.MODEL DISCUSSION...

Early on, models are in good agreement synoptically with the low
in the southwestern Gulf and the second south of the western
Aleutians. By Monday, models begin to diverge as unorganized low
pressure develops in the Gulf and a new North Pacific low begins
to advance towards the Aleutians. The EC is the fastest, and most
eastward solution with the NAM, GFS, and Canadian take a more
westward solution. Tuesday morning, models completely fall out of
step with the placement of the North Pacific low as it tracks
towards the Gulf. Models have been inconsistent from run to run
with this upcoming low with the outlier being the GFS.

&&

.AVIATION...
PANC...VFR conditions with light north to northeast winds are
expect to continue through the period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The low in the southwestern Gulf will continue to bring showers to
the southern Kenai Peninsula and Kodiak Island for much of today
but will then diminish tonight as the low weakens and pushes off
to the east. Southcentral Alaska will then see a brief period
without much precipitation that will also see temps become colder
again. Then on Monday night a front will push in from the south
and move in over Kodiak Island. Models are struggling to get a
good handle on this system which is resulting in low confidence in
the timing of the front as it pushes further north Tuesday. There
are also discrepancies with how far north it will move and how
much warm air it will bring up from the south. There is however a
general consensus that the majority of the precipitation will stay
along the north Gulf coast and not move too far inland.

&&

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

Snow around Bethel is expected to remain light through the early
morning, then diminish as the upper level wave dissipates later on
this morning. Light snow showers are also moving westward across
the Bristol Bay area. This may in part be the result of the
moisture meeting up with colder air near the ground with enhanced
lift making the snow shower activity more widespread. This too is
forecast to weaken with time but may take into this afternoon to
diminish altogether. The parent upper level low over the Bering
Strait is expected to drift northwestward away from the area. This
will allow weak ridging, or more aptly put, a lack of any
distinguishable upper level features, to result in clearing skies
Sunday night into Monday. Weak offshore winds should keep any fog
formation by Monday night on the patchy side, but it will still be
possible, especially towards the Lower Kuskokwim Valley.

Then late Monday night into Tuesday, an area of low pressure appears
likely to approach the Pacific side of the Alaska Peninsula
somewhere between the Shumagin Islands and Kodiak Island.
Regardless of track, this low will increase the offshore winds,
which will scour out any fog Christmas morning, and increase the
potential for snow across portions of the Lower Kuskokwim Valley
and Bristol Bay. The primary area of concern at this point is
along the Kuskokwim Mountains between Dillingham and Aniak. This
scenario also assumes the aforementioned upper level low over the
Bering Strait begins to migrate back towards the area. This would
enhance the lift in the same region, making for two separate
systems working together to produce snow. For now it looks to
start up Tuesday afternoon and persist into Tuesday night, though
whether it stays stationary or moves in any direction with time is
unknown as model discrepancies are very large by this time.

&&

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

Quieter than usual weather continues across the Bering this
morning as a low centered south of Shemya moves south and east
away from the Aleutians. Meanwhile a ridge of Arctic high pressure
will persist over the central Bering through Tuesday. This feature
is helping to shunt the train of Pacific storm systems southward.
The low currently south of Shemya will become absorbed by a
stronger low well south of Adak this evening. Because the ridge of
high pressure is in place, this storm will initially pass well
south of the Aleutians through Monday evening, with few impacts
other than keeping the colder northeast winds ongoing across the
eastern Aleutians. This low is thus taking the 50 miles further
south track mentioned in yesterday morning's discussion. So, other
than more Bering Sea-effect snow showers through Dutch Harbor over
to Cold Bay, there will be no direct impacts to those areas from
this storm.

On Monday night, the low will approach the Alaska Peninsula
somewhere east of Sand Point. The models all have the low moving
into this region, but where it goes exactly is highly uncertain.
For the most part however the low will be weak enough that any
precipitation on the Alaska Peninsula will be very typical for
that part of the world. Most of the models show a new triple point
low developing over the southern Gulf Tuesday afternoon resulting
in rapid weakening of the low south of the Alaska Peninsula.

Looking ahead, another strong low will be well south of Adak and
Atka by Christmas Night, and it may potentially impact portions of
the area by mid to late week.

&&

.LONG TERM FORECAST (Days 4 through 7)...

At the onset of the forecast period there are a series of surface
lows from the Gulf Of Alaska to the North Pacific to the Sea Of
Okhotsk. At 12z Wednesday, the first surface low is south of
Middleton Island, the second low is south of Adak and the third
low is near Kamchatka. Additionally, at the surface there is a
dome of high pressure over Central and Eastern Siberia. The GFS,
ECMWF and the Canadian all showcase these synoptic features
however, even at the onset of the forecast period the precise
location of the surface low with each storm is not in sync. As
time evolves and these storms progress eastward, the model
divergence increases. High confidence that the southern tier of
Alaska will be in a wet pattern. But due to the vagaries of the
global models, there is a hefty amount of uncertainty establishing
the start/stop times for the precipitation shields associated
with each cyclone. The two biggest items of concern are
determining whether these events will be straight snow or a wintry
mix. And how much additional snow will fall across portions of
Prince William Sound and Passage Canal which has been dumped on
the past couple of weeks; at sea level as well as in the higher
elevations.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales: 172, 174.
         Heavy Freezing Spray: 180, 181.

&&

SYNOPSIS AND MODEL DISCUSSION...RA
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JPW
LONG TERM...PJS