Small Craft Advisory 63, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 PM AKST Sat Jan 19 2019

.ANALYSIS AND UPPER LEVELS...
Southcentral and Southwest mainland Alaska remain in a relatively
quiet pattern this afternoon. The fog that has been hanging
around stretches from Palmer along the western Anchorage Bowl
down through Turnagain Arm, but has decreased in coverage from
what was seen yesterday. The Cook Inlet has seen a lot of drying
this afternoon, resulting in less foggy conditions. Increased
offshore flow continues over Southcentral. A highly amplified
ridge over the eastern Bering is bringing high pressure,
resulting in calm and mostly dry conditions for the AKPEN and
Southwest mainland. A few light snow showers were seen across the
Kuskokwim Delta due to a weak surface low. Things are a bit more
interesting out west with a low sitting west of Shemya and a
stronger low south of Adak lifting northward. A front stretching
across the western and central Aleutians has brought precip and
strong gale force winds along the frontal boundary.

&&

.MODEL DISCUSSION...
Models are in fine synoptic agreement into the middle of next week
and then degrade quickly. For the closer term forecasts the main
questions are to the speed of the upper level wave moving from the
Gulf into Southcentral Alaska Sunday evening into Monday. The NAM
has slowed considerably from the 12Z to 18Z model runs with this
wave and is now slower than the GFS or EC. This will be something
to continue to watch. Farther west there is a strong low near the
960 mb level which will move from the Pacific to the central
Aleutians on Sunday night. Models are overall strengthening the
front associated with this low and we are now expecting storm
force winds in the central Aleutians with this front. It should
also be noted that there is a nearly 250 mile difference in
placement of this low on Sunday night between the EC and the
Canadian models with the GFS and NAM in between those positions.
While the elongated structure of the low will help to mitigate the
sensible impacts of this difference, it could make a large
difference in how this low develops and tracks though the middle
of next week.

&&

.AVIATION...
PANC...Fog will remain the main challenge through Sunday afternoon
as clear skies (above the fog) will keep the inversion around and
the radiative cooling process going. A short wave moving over the
area from the northwest this afternoon has the potential to
decrease the fog some if the vertical mixing increases. However, at
this time it looks that in spite of some breaks, it will remain
more on the foggy side at the terminal into Sunday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Sunday night)...

A slow transition in the synoptic pattern is expected through
Monday with the jet stream pushing a warmer North Pacific system
toward the southern mainland. In the meanwhile, Sunday will be dry
and cold along with patchy fog along the Inlet and valleys. Skies
will be mostly clear for viewing the lunar eclipse for much of
Southcentral...the exception to this may be across the Copper
River Basin. The next front races up from the North Pacific
Sunday night, however its progress slows while moving to the gulf
as the associated surface low stalls just south of the eastern
Aleutians.

Current forecast trends were kept with the boundary moving across
the southern Gulf and Kodiak Island by Monday morning.
Precipitation along the front may start as snow, then changes
quickly over to rain with this warm core system. The front pushes
north across the Kenai by Monday early afternoon and continues
north spreading rain to the northeast Gulf waters and Prince
William Sound. Snow is expected for coastal locations as the cold
air mass persists under offshore flow at the surface. Snow will
be light along the western Kenai with this first frontal push of
the week, as low level easterly flow limits accumulations due to
downsloping over the mountains.

&&

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

Cold and dry conditions will persist across much of Southwest
Alaska through the remainder of the weekend as a northeasterly
offshore flow dominates. The exception to this will be clouds
sliding north to south over the YK-Delta overnight into early
Sunday as a weak surface trough rotates around a dome of high
pressure anchored over the central Bering. The patchy fog that
developed yesterday along the Alaska Range and lifted to a low
stratus layer today may reform again tonight with no surface wind
flow to drive out the lingering low-level moisture.

The pattern then begins to change and become more active from
south to north late Sunday into Monday as a strengthening North
Pacific low moves toward the central Aleutians, pushing the
Bering high north. An occluded front stretched east of the low
will drive across the AKPEN Sunday night then lift slowly north
into Bristol Bay Monday. Precipitation along the front will begin
as snow, but transition to rain along the southern half of the
AKPEN as warmer air moves in from the south. As the front slows
over Bristol Bay, an upper-level vorticity maximum will move over
the Southwest helping to enhance snowfall, especially from
Dillingham north into the Kilbuck Mountains where light
accumulations are possible. As this frontal system weakens, a
second front (and potential triple point low) pinwheeling north
from the parent low moves toward the AKPEN late Monday and across
the Southwest Tuesday. This system will bring another round of a
rain/snow mix along the AKPEN and snow across the Southwest, with
light accumulations again for Dillingham and points north.

&&

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

A low west of Shemya will continue to weaken tonight as it lifts
to the north and west. High pressure over the central Bering is
keeping conditions rather quiet with only a few bands low stratus
from Nunivak Island to the Pribilofs. For the central Aleutians,
clouds and winds will be on the increase tonight as a deepening
North Pacific low approaches from the south. This system will
deepen into a storm-force low tomorrow. The band of storm-force
winds accompanying the low will lift from the Pacific side to the
Bering side of the central Aleutians Sunday night as the low
reaches the chain then begins to drift slowly back to the west
toward Atka. Widespread gales will also spread north across the
central and western Bering Sunday night through early Tuesday.
These winds will also impact the Pribilofs where blowing snow
could be an issue late Monday into Tuesday as snow along the
occluded front lifts into the area. Cold air will be in place over
the Aleutians during the onset of this event, so expect
precipitation to begin as snow. However, as warmer air quickly
lifts up from the south, precipitation over the Aleutians will
transition to mostly rain by early Monday. Although weakening
slowly, this low will remain fairly stationary through Tuesday.
Therefore, expect precipitation (mainly snow showers) and
widespread gales to to linger across the Bering into mid-week.

&&

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

The marine outlook is fraught with uncertainty through this period
with significant model uncertainty. Most of the models have a low
somewhere over the central Aleutians on Tuesday morning, but there
are several hundred miles difference in the centers between the
western-most EC and the eastern-most Canadian models at that time.
This low remains more-or-less stationary into Wednesday and acts
as an anchor low as a series of additional waves of low pressure
move northward across the eastern Aleutians/Alaska Peninsula
through Thursday. Each of the lows has the potential to produce
gales and perhaps even some storm force winds in localized areas,
but with large model uncertainty Wednesday and Thursday in both
timing and storm track, where and when those strong winds will
occur cannot be said at this point. The Gulf will be relatively
quiet, though any fronts or lows that take a more eastward track
my produce gale force winds at times across the western Gulf as
well.

&&

.LONG TERM FORECAST (Days 3 through 7: Tuesday through Saturday)...

Model uncertainty with the track of individual lows and as a
result, any hazardous weather associated therewith is high today.
A few things can be said regarding the large-scale weather
pattern, however. An upper level trough over the central Bering
will shift west and consolidate into an upper level low that will
persist over the western Bering through Saturday. Further east,
an upper level ridge over the western half of North America will
amplify through the week. With much of mainland Alaska in between,
this will set the stage for the pattern to become increasingly
active as the upper level winds turn predominately out of the
south. The southerly winds and frequent bouts of precipitation
should return Southern Alaska to a notably warmer and cloudier
weather pattern from Tuesday through Friday. The upper low shifts
further west towards Kamchatka, which in turn will shift the
storm track slowly westward in time perhaps allowing for quieter
weather for the weekend. A quieter weather pattern may start to
allow temperatures to moderate cooler back towards seasonal
normals, but until then warm and wet will be the weather theme.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storms 172-174. Gales 130 155 165 170 171 175 177
179 411-414. Heavy freezing spray 160 180 185 411.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...JW