Small Craft Advisory 706, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
455 PM AKDT Sun Sep 16 2018

.ANALYSIS AND UPPER LEVELS...

A vertically stacked upper-level low sits at the base of a long
wave trough which is draped over the western Gulf. With little
upper level support, the instability that once trailed the low
pressure and associated front is weakening. A diffused front
located over Southcentral Alaska and the western Gulf has produced
a line of rain along the boundary. While the rain has been light
and scattered in nature, the southern Kenai Peninsula has
experienced more steady rainfall starting during the evening hours
yesterday. Radar does show some areas of heavier rain south east
of Middleton Island and banded moisture throughout Southcentral.
The rain in the west is dissipating, straying from the record
rainfall experienced in King Salmon yesterday. As the trough in
the east persists, zonal flow continues in the Bering Sea with
widespread stratus and fog present throughout the area.

&&

.MODEL DISCUSSION...

Good agreement in the models persists through Monday evening as
the trough and associate low move south and east. Early Tue the
GFS creates a double barrel low with a new closed low pressure
center developing in the southwestern Gulf. The EC, NAM and
Canadian models lack the secondary low and push further east with
the original system and a low center with slight variation. This
discontinuity persists through the day with the GFS continuing to
be the outlier.

&&

.AVIATION...
PANC...VFR conditions with periods of light rain will continue
into the early evening hours as a the remnants of a frontal system
weaken overhead.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Tonight through Wednesday)...

A weak upper level trough over Southcentral has largely stalled
out this afternoon as far as forward progression goes. The trough
is in the process of dissipating in place. The result has been
that any shower activity over the area has been very light and
scattered in nature, essentially amounting to sprinkles for most
of the day today. This is expected to continue for the next couple
days, with a bit more atmospheric moisture around but with nothing
to force any stronger, faster-moving areas of showers to develop,
move through, then clear out. Over the Gulf, an area of low
pressure now centered well east of Kodiak Island is expected to
move southeast away from mainland Alaska. In turn, that will take
the steadier rains currently affecting the Homer area and much of
the rest of the southern Kenai Peninsula and move them southward
into the Gulf tonight as well. With very little else in the way of
upper level or atmospheric phenomena expected to impact the area,
a continuation of the current conditions of mainly cloudy skies
and areas of sprinkles is expected to continue into Tuesday. On
Wednesday, ridging developing out ahead of an upper level wave
moving across Interior Alaska should help to break the area out of
the cloudy pattern somewhat as the upper level winds increase
enough to move the weather along. There is a good amount of
disagreement in the models as to whether the feature stays up
north, or continues moving southeast, which will impact the late
workweek time frame. More on that below.

The Turnagain Arm winds may brush the south and west side of
Anchorage tonight with 10-20 mph winds, but they will diminish
Monday morning.

&&

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

The shower activity over Southwest Alaska continues to wane this
evening as an upper-level low slides from the AKPEN into the
western Gulf. A few showers will linger in far interior sections
of the Southwest overnight as a second upper-level low drops
southwest from central Alaska before quickly shooting northwest
as it phases with a stronger low over eastern Russia. Although
precipitation will end over the remainder of the southwest
overnight, an onshore flow will keep clouds over the region. A
weak shortwave will advance toward the coast tomorrow. This system
will bring another round of scattered showers through Monday
evening. A weak ridge will slide over the area Tuesday morning
before quickly departing as the next system drops in from the
northwest. A third wave will approach the west coast Tuesday night
and move across Southwest Alaska Wednesday. There is still some
disagreement between models in the exact timing and intensity of
the wave (and how it interacts with an upper-level low dropping
south from the northern Bering). No matter the solution, the
expectation is for yet more clouds and rain across the Southwest
and AKPEN for mid-week.

&&

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

Low pressure in the northern Bering and high pressure centered
south of the central Aleutians is producing a broad westerly flow
that will continue through Monday. Accompanying the westerly flow
is a large canopy of low stratus and fog stretching from the
eastern Bering west to Shemya. The pattern becomes a bit more
amplified beginning Monday night as an upper-level low over the
Chukchi Sea drops south into the northern Bering. Upstream, a
ridge axis over the central Aleutians will stretch northward
across the Bering and into eastern Russia. The ridge should allow
the fog and low stratus to persist across the central Bering
through Tuesday before it weakens and shifts south. Farther west,
a gale- force front will approach the western Aleutians late
Monday. This system will then weaken on Tuesday as it moves east
across the central Bering and Aleutians. A second north Pacific
low will move towards the western and central Aleutians Wednesday
with small-craft winds and the chance for rain as the previous
system moves across the eastern Bering towards the west coast of
mainland Alaska.

&&

.LONG TERM FORECAST (Days 4 through 7/Wednesday night through
Sunday night)...
Continued model differences and poor run-to-run consistency lead
to overall low forecast confidence in the late next week through
next weekend. There are some commonalities in the long-wave
pattern which allow some generalities about the type of weather
one would expect, but it is not possible to nail down any details
about timing/location of precipitation. Beginning Wednesday night,
the key feature is an upper level low/trough which is riding over
top of a long-wave ridge centered over the Bering Sea and north
central Pacific. Models disagree about whether this will amplify
and dig southeastward across Southwest and Southcentral Alaska or
remain further north over northern Alaska. There is a trend in
guidance toward the more amplified solution. Based on this, will
maintain a fairly broad brush chance of precipitation over the
southern mainland, especially for inland areas. Cooler temperatures
would accompany the trough, so if it does in fact dig southward
would expect snow on the mountains. If the trough ends up staying
further north then would expect drier and warmer conditions,
especially for Southcentral Alaska.

Meanwhile, for the Aleutians and Bering Sea it looks like a fairly
wet pattern as systems move up from the northwest Pacific and over
the fairly flat ridge late next week. All models agree on
amplification of the northwest Pacific trough and downstream
bering ridge as we head through next weekend. That would lead to
continued wet conditions for the Bering and Aleutians, possibly
approaching Southwest Alaska Sunday depending on exact amplitude
of the ridge. East of the ridge, would expect mostly dry
conditions with seasonable temperatures for Southwest and
Southcentral Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 178.

&&

SYNOPSIS AND MODEL DISCUSSION...RJA
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...SEB