Small Craft Advisory 809, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
642 PM AKDT Sat Oct 24 2020

.ANALYSIS AND UPPER LEVELS...
A broad upper level trough centered over the Eastern Bering is
brought widespread rain to areas of Southwest Alaska, as well as
much of the Gulf Coast this morning. Returns from Kenai radar for
much of the day also indicated widespread precipitation, however
very dry low levels and downsloping prevented the vast majority of
that precipitation from reaching the ground. Instead much of
Southcentral saw a generally cloudy but dry day, but with a few
passing flurries. Well south of the area near 35N in the Central
Pacific, a developing wave is present under a 130 kt jet streak.
This is expected to continue developing over the next 24 hours as
it quickly moves northward into the Gulf where it will be the
primary weather system of focus late Sunday-Monday. Over the
Central-Western Bering the weather is much quieter under broad
northerly flow and cold air advection, which is indicated by the
extensive shower activity and lack of fog or stratus.

&&

.MODEL DISCUSSION...
The lack of model agreement in the first 48 hours, especially on
the synoptic scale, continues to be a major forecast challenge
with the track/intensity of the approaching Gulf low for Sunday.
One area that the models continue to diverge significantly on is
when comparing the Gulf low track from the global GFS/EC and
mesoscale NAM starting Sunday afternoon. Overnight the NAM and GFS
continued to show wildly different solutions, while the EC was a
reasonable compromise between the two. Since 12z this morning
however the global GFS and EC have come much more in line with a
deeper low track skirting the Kodiak Island and Western Prince
William Sound coasts, while the NAM remains much weaker and
further east. Since the development of this system is still well
south of the area near the edge of the NAM domain, the current
forecast favors the global models which typically have stronger
performance in these types of situations. However to keep from
making major changes the forecast was only trended in their
direction. As a result forecast confidence generally remains low,
especially with precip type and any snow accumulations from Palmer
westward for Monday.

&&

.AVIATION...
PANC...LLWS will continue to occur over the terminal today before
beginning to diminish after 2z this evening. There is some
question as to exactly how quickly this will occur, but there is
good confidence in a general weakening trend between 0-3z.
Afterwards conditions should remain VFR with light north winds
until ceilings begin to fall below 5000 ft with a rain/snow mix
around 18z Sun. Afterwards there is a potential for accumulating
snow and IFR conditions late Sun-Mon, however that is still being
monitored as confidence is low.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Saturday
afternoon through Monday afternoon)...

A front over the central Gulf continues to gradually lift
northward bringing gale force winds to the northern Gulf waters
this evening. Precipitation is spreading inland with this front
and will continue to do so this evening with the greatest
precipitation expected to fall along the northern Gulf Coast. One
caveat continues to be the amount of dry air in place at lower
levels, so the onset of precipitation will likely be delayed,
especially for the western Kenai Peninsula and Anchorage Bowl.
Rain is expected along the coast while interior locations will see
snow.

For Sunday, the focus quickly shifts to the interaction between an
upper level trough over the eastern Bering and an inbound,
moisture-laden low from the North Pacific. Models have struggled
with these features over the past few days, however there is now
much higher confidence in this storm taking a more westerly track
and moving over the eastern Kenai Peninsula/western Prince William
Sound area (a GFS/EC solution). Additionally, one other challenge
has been the possibility for downsloping (leading to a reduction
in QPF values) however this is looking less likely. As a result,
this forecast package includes generally higher QPF values with
this next system for Sunday night into Monday.

The final challenge is precipitation type. For the Anchorage Bowl
and Mat-Su Valleys, current thinking is that precipitation will
start as rain with a transition to snow Sunday night. Accumulation
amounts still remain somewhat uncertain, but it is looking increasingly
likely that the greatest snowfall will be along the Anchorage
Hillside. As this system lifts inland and moves eastward,
widespread snow is expected over the Copper River Basin. Winter
Weather Advisories have been issued for Thompson Pass and the
Copper River Basin Sunday into Monday.

This forecast continues to evolve - please stay tuned as the
details begin to fall into place over the next 24 hours.

&&

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

Widespread showers will continue to move north and across
Southwest Alaska tonight. Rainfall observations from this
afternoon implied steady rain passing through Bristol Bay
including New Stuyahok and Manokotak, the Lower Kuskokwim Valley
and portions of Kuskokwim Delta. The arrival of a weakening low
over the central Bering will force more rain over the region
through at least Monday. As for the snow, much of it will fall
over the lower Kuskokwim Valley and near the Kuskokwim Delta
starting Sunday night. Colder temperatures aloft associated with
the weakening low from the Bering will cross the Kuskokwim Delta
Sunday night, which will increase the potential for snow showers
early in the week.

&&

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

The upper low has passed east of Pribilof Islands, and it's
slowly making its way through eastern Bering tonight and Sunday.
Small craft conditions were continued for another day for several
marine zones until winds and seas subside Sunday night. The
leading edge of a Northwest Pacific front will approach the
western Bering as early as Sunday morning. This could be a weak
frontal system, but will continue to monitor the situation
closely. As a result, it will be relatively quiet over the next
few days.

&&

.MARINE (Days 3 through 5: Tuesday through Thursday)...

The weather to watch on Tuesday will be an area of low pressure
located south of Prince William Sound. Uncertainty remains with
the strength and track of this system, however, models agree on
gale force winds across the central Gulf before the low moves
inland on Wednesday. Through Wednesday, calmer conditions will
provide for light winds across much of the Gulf. Over the Bering
Sea and Aleutians, winds and waves look to remain relatively calm
for the forecast period. There is potential for an area of low
pressure to skirt the chain beginning Wednesday afternoon,
however, model confidence is low at this time.

&&

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

A weak surface low lingers over the Northern Gulf on Tuesday and
tracks slowly to the east. Meanwhile, an upper-level ridge sets up
over the interior, beginning the flow of cold air towards
Southern Alaska, which will be quickly pushed out by a series of
arctic troughs moving across the state mid-week. While the
greatest impacts will remain well to the north, temperatures for
the Southwest and Southcentral regions will drop as the colder air
is advected in.

Late next week, there is better agreement between the Canadian and
ECMWF models across all parameters for a North Pacific surface
low that will pass just south of the Aleutians. The GFS seems too
slow and is quite a bit deeper in comparison, so less reliability
is placed on this solution. This system can potentially bring
strong winds to the coastal waters.

By Saturday, another system approaches the Aleutians from the
west. There is decent model agreement with this surface low. The
ECMWF seems fast and travels with an easterly path but
approximates a similar depth to the Canadian. The GFS, while much
deeper than the other two, shares a similar location and path
(southeasterly) with the Canadian.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 131, 141.
MARINE...Gale Warning: 119, 351.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...CK/KM