Small Craft Advisory 958, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
419 AM AKST Wed Dec 13 2017

.ANALYSIS AND UPPER LEVELS...
The pattern remains largely unchanged over the region this
morning as deep southerly flow between the North American ridge
and the Bering trough keeps warm, wet, and windy conditions
focused over the southern mainland. The latest in the long series
of systems embedded in this southerly flow is currently moving
over Kodiak Island, with an associated front approaching the Gulf
coast. In terms of sensible weather impacts, this is promoting
what have now become very familiar conditions across the region,
with rain focused along the coast and interior areas staying drier
as a result of downslope flow in the lee of the coastal mountain
ranges. Strong gap winds are also present through favored areas
such as Turnagain Arm. Meanwhile, scattered snow showers prevail
in northerly flow over the central Bering, with a weak front
bringing rain and gusty winds to the Western Aleutians.

&&

.MODEL DISCUSSION...
Models are now in good agreement for the system currently
impacting the Gulf and also for the large scale pattern in the
coming days as the ridge finally begins to break down with the
Bering trough shifting eastward towards the mainland. There are
still plenty of small scale differences as this slight pattern
change begins to take shape, with the NAM being a notable outlier
among the 00z guidance with respect to the evolution of the Bering
surface low by Thursday night/Friday. For the morning package,
used the NAM early on for the current system and then transitioned
to a GFS/EC blend to capture the broad strokes of the coming
pattern change.

&&

.AVIATION...
PANC...Low level wind shear will continue to impact the terminal
through late morning as a front pushes inland along the North Gulf
Coast. Southeast winds will then move into the terminal around 20z
as the down inlet pressure gradient weakens and reverses. This
will also be the most likely time for showers to develop in the
vicinity of the terminal as downslope flow briefly subsides. VFR
conditions are expected to persist through the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The strong front over the north Gulf coast will work its way
inland today. The bulk of the precipitation with this system will
fall along the coast and coastal mountains. However, there is
enough moisture and dynamics with this system that some
precipitation is also expected at many inland locations. The air
mass remains quite warm, with temperatures 15 to 20 degrees above
normal. This also means that the precipitation will fall as rain
at most of the lower elevation locations. Weak anticyclonic flow
aloft will push into the area tonight behind this system, leaving
just a few showers over the area. The eastern Copper river basin
will remain in fairly deep moisture and precipitation will be slow
to move out on Thursday. A subtle shift in the pattern will begin
on Thursday as the strong southerly flow aloft moves east, and
the flow becomes more southwesterly. A weak system will cross the
Gulf from southwest to northeast on Thursday. A fairly strong
front associated with a low in the southeast Bering Sea will
approach Kodiak late Thursday night as the southwest flow aloft
increases in strength.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Wednesday and
Thursday)...

A low centered near Kodiak Island early this morning will track a
bit west of due north into interior Southwest Alaska later this
morning, then move to near Norton Sound late this afternoon. On
this track, precipitation will continue across the area through
the day today. It should remain snow across the Kuskokwim Delta,
though much less total precipitation is expected there today.
Meanwhile for Bristol Bay it will be all rain. As cooler air
works in behind the center of the low, any rain that develops in
the Lower Kuskokwim Valley this morning should mix with snow
during the day. Any steady precipitation will diminish to flurries
and isolated snow showers tonight as weak high pressure builds in,
along with some slightly cooler air. Thursday and much of Thursday
night will be quiet weather-wise in between systems, though
mainly mountain snow showers will still be around. The next front
with a renewed slug of warm air moves into Bristol Bay late
Thursday night into Friday morning, with snow at the onset
transitioning over to rain.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Wednesday
and Thursday)...

A front moving into the western Bering this morning will be the
start of somewhat of a pattern change through the region as it
allows for the return of warmer air. The cold air mass currently moving
through the central Bering will slide eastward into coastal
mainland Alaska, and the associated northerly winds will diminish
as weak high pressure sets up. Meanwhile the western Bering front
will slowly press eastward, reaching the central Bering by
Thursday morning, as multiple weak areas of low pressure develop
along the baroclinic zone and help gradually advance the warmer
air mass eastward. The strongest of those lows will take advantage
of plentiful jet energy across the north Pacific to deepen as it
crosses the central Aleutians somewhere between Adak and Dutch
Harbor Thursday evening. The models are having a very difficult
time handling this, so uncertainty with the track and any impacts
in any particular location are still quite high. As the low
approaches the Pribilofs Thursday night it will become cut off
from the jet energy feeding it and will stall, with track
uncertainties only further increasing from there. The impacts from
the low through the central Bering, Pribilofs and the central and
eastern Aleutians will be widespread precipitation, mainly in the
form of rain, gusty winds, and a long duration as the low stalls.
Meanwhile the warm front will continue eastward across the Eastern
Aleutians and Alaska Peninsula Thursday night, reaching Southwest
Alaska and Bristol Bay by Friday morning.

&&

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

The weather pattern will remain active through day 7(Tuesday). We
expect a continuation of the train of lows moving into the Gulf
of Alaska and Bering Sea. The models continue to struggle with
this pattern and coming into agreement on the location and
intensity of these storms for the entire extended forecast period.

Beginning Thursday a strong low is moving to the western Alaska
Peninsula area, though where exactly is still uncertain, pushing a
front into the Gulf of Alaska. The ensemble mean looks to be the
best path forward at this point. By Friday the deterministic
operational model runs differ significantly with respect to where
the low center will be. This storm should be in the Eastern
Bering Sea by Friday, but here again, the models diverge with the
placement of the low center. This has a significant impact on the
sensible weather over the southern mainland and the Bering
Sea/Aleutians. As we progress through the weekend the models do
come into agreement bringing the strong large low to the Prince
William Sound region by Sunday thus finally breaking down the
blocking ridge over the west coast of North America. The big take
away from this will be a trend to cooler temperatures as we near
the weekend and into early next week. Beyond Sunday the models
once again diverge and have significantly different solutions
Monday and Tuesday, thus overall confidence in the extended
forecast is on the low side. The overall approach to the extended
forecast will be to nudge and trend toward the ensemble solution,
but in general we will not be making any big changes to the
forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None
MARINE...Gale Warning 119 120 125

&&

SYNOPSIS AND MODEL DISCUSSION...CB
AVIATION...CB
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JPW
LONG TERM...SA