Small Craft Advisory 466, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 PM AKDT Wed Jun 13 2018

.ANALYSIS AND UPPER LEVELS...
An upper level low is lurking just north of the Arctic Coast with
a trough extending out of it over the Alcan border. Weak high
pressure is to the southwest of the trough bringing in a large
area of somewhat clear skies from the Cook Inlet region westward
across the northern Bering Sea. The remainder of the Bering Sea
is covered in the typical summer stratus.
South of the Aleutian Islands the jet stream is interacting with a
strong low which is still intensifying. This low is moving
northward and will become the main weather feature for most of
the forecast area from Thursday through a good part of next week.

The only areas of weak instability today are in the Copper River
Basin and over the Kuskokwim Valley. As the instability is weak,
only showers are expected in these areas and no thunderstorms.

&&

.MODEL DISCUSSION...
Models have come into good synoptic acquirement with the movement
and strength of the large low that is tracking into the area for
the latter half of the week. The GFS was preferred in the Bering
Sea region as the 12Z NAM is about 150 miles farther northeast
with the low compared to where the GFS, ECMWF, and Canadian
models have it Friday night. There is a good chance the 18Z NAM
will shift closer to the other models, but it is still worth
watching to see if they can all come into closer alignment for
this large storm.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through
Thursday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
Be sure to get out and enjoy the nicer weather over the next 2
days as forecast confidence remains very good in a pattern shift
towards southwesterly flow and a much wetter pattern by the end
of this week. Until then, a weak ridge of high pressure will bring
generally clear and dry conditions to most of the mainland
through Thursday, with chances for precipitation limited to
isolated showers along the Talkeetnas and Alaska Range.

Beginning on Friday, the pattern change will begin to be noticed
as the first in a series of frontal systems moves into the Gulf
for the morning, which will then spread northward into the Gulf
Coast and Southcentral by Friday evening. This will bring with it
widespread rains not only to the Gulf Coast but into Southcentral
along with gusty winds out of Turnagain Arm and along the Knik
and Copper Rivers. With deep/moist southwesterly flow however,
these winds are not expected to produce downsloping for most
areas. So, expect a wet Friday evening for most areas outside of
the Copper River Basin.

&&

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

The dry, northerly flow responsible for the tranquil weather
continues across most of southwest Alaska will continue through
Thursday evening. By that time, clouds and rain will begin to
overspread the Alaska Peninsula as a strengthening north Pacific
low south of the eastern Aleutians swings an occluded front over
the area. The clouds and precipitation move north over the rest of
southwest Alaska Friday and linger through Saturday as the low
tracks slowly into the eastern Bering. Gusty easterly winds are
also possible Thursday night through Friday night as the pressure
gradient from this system enters the region, especially in gap
locations east of the Alaska Range.

&&

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

Low stratus and patchy fog will remain rather persistent over the
southern and western Bering as high pressure continue to dominate
the weather pattern over the Bering. This ridge begins a retreat
toward eastern Russia Thursday as a developing north Pacific low
makes its way closer to the central Aleutians. This system will
intensify as it heads north, swinging an occluded front across the
eastern Aleutians and Alaska Peninsula late Thursday. Gale-force
winds will also develop along the Pacific side of the eastern
Aleutians Thursday as the system approaches. The surface low
begins to fill in and weaken Friday night before crossing the
Aleutians and entering the Bering Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7...Sat through Wed)...
The long term pattern continues to look like a major shift away
from the mostly sunny and mild start that we have seen for June.
The general idea of the broad longwave trough establishing itself
over the Eastern Bering holds true and all models remain in near
"lock-step" with the synoptic set-up. Now, there is starting to be
some more clarity with individual waves of energy and when the
heaviest bouts of rain will be.

Sat will start for most of the region with the initial front
departing to our north. With potent southerly flow continuing to
pump moisture into the mainland, all areas will remain highly
vulnerable to showers. The best chance for these will be along
southerly facing terrain and the north Gulf Coast. The next best
defined shortwave will move quickly from south to north across the
Gulf of AK late Sun night into Mon morning. This will bring
another round of moderate rain from the Gulf Coast into the Cook
Inlet. This batch of rain will eventually move into SW AK and the
Susitna and Copper River Valleys later Mon. It will also stir up
another round of gusty southeasterly winds. West of the trough
axis over the Western Bering, flow will remain out of the north.
This will keep things quite dry across that area, but we can never
rule out widespread low clouds developing as June is a
climatological peak for marine stratus.

Beyond Mon, uncertainty starts to creep back in regarding
individual waves of energy. But the overall cloudy and showery
pattern will hold over most of southern AK through the week. There
are some signs of hope by the end of the week as the parent low
ejects northward and ridging tries to nudge in from the N Pacific.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 172 174.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...MSO