Small Craft Advisory 11, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
502 AM AKST Thu Jan 4 2018

.ANALYSIS AND UPPER LEVELS...
The upper levels show a rather meridional set-up across the state
this morning. The sub-tropical jet remains very active south of
the Aleutians at 125 kt. It turns sharply through the Central
Aleutians and is helping to support the parent upper-level low
near Shemya. The low remains in the base of a developing broad
longwave trough. The jet then climbs a shortwave ridge over SW AK
before diving back down into a trough over the southern Gulf of
AK at about 95 kt.

Closer to the surface, the low that brought snow to much of South
Central AK on Wed is falling apart southeast of the Prince William
Sound. Some lingering showers are holding on, but the radar trends
continue to show activity waning. Satellite shows some low
stratus/fog all the way from Bristol Bay through the Matanuska
Valley. It is moving in and out of various airports this morning,
but has not become widespread in nature. Further west, a
stretching warm occlusion across the Aleutian Chain is providing a
perfect spot for triple-point low development. These lows will
bring primarily gusty winds and moderate rain as they usher in
much warmer air than the Bering has seen the past week. All of
these smaller circulations will eventually be absorbed by the
parent low over the Central Bering as we head towards the weekend.

&&

.MODEL DISCUSSION...
Models are in decent agreement over western AK and better
agreement in eastern AK. In the east, the remaining areas of
uncertainty are the potential for fog development from the Cook
Inlet up through the Matanuska and Susitna Valleys. There are some
lingering snow and rain showers hanging on near Prince William
Sound and the southern Copper River Basin. These should continue
to taper off, though the exact timing remains somewhat uncertain.
Out west, several pieces of energy will move up from the North
Pacific and into the Bering. Models have been fairly steady
depicting which low will be the dominant feature as we head
towards the weekend. However, as there are several triple point
lows in the mix, models have struggled to lock on to the strength
and exact placement of each of those. We leaned towards the GFS
tonight to keep the forecast consistent and start to add some more
detail. Recent runs have shown a trend towards a compact low
developing near Dutch Harbor tonight into Fri morning. If this
holds true, a high wind warning (75 mph) may be needed for the
Alaska Peninsula.

&&

.AVIATION...
PANC...Fog has moved in and out of parts of the Anchorage Bowl. If
it were to develop over the terminal, it could hang on through
the morning hours. By afternoon, winds become northerly and should
further dry things resulting in VFR conditions for the remainder
of the period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level low centered over Prince William Sound will move
south and exit the area today. Some snow showers are possible this
morning, with dry conditions expected across Southcentral Alaska
by this afternoon. Upper level high pressure will build into the
area today and tonight. Offshore flow will develop at the lower
levels, with the usual winds through gaps and passes. For inland
areas that are not windy (such as the Copper River basin and the
Anchorage area), some fog is possible early Fri. The upper high
will retreat north of the area Friday night as a trough approaches
from the southwest. The associated front will reach Kodiak Island
Friday afternoon, with a surface low developing near Kodiak by
late Friday night. Some moisture ahead of the front will bring
precipitation into the Gulf coast Friday night, along with
increasing easterly winds.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Isolated showers are possible early in the period along with
patchy fog. Remnant moisture and general onshore flow will leave
the entire area across the Delta and Bristol Bay vulnerable to
fog. Otherwise, conditions will be mostly dry with the exception
of some snow showers lingering along the AK Range. As time
elapses, the next front will approach the area from the Eastern
Bering. This storm will bring warmer temperatures to the region.
Pre-frontal wind gusts will develop first along the coast by late
morning before slowly push inland by Thursday evening. The AKPEN
will get hammered tonight and Friday as a slug of energy pushes
inland. The strongest winds will materialize along the gaps
between the volcanoes and the Aleutian Range. A high wind watch
has been issued for the Cold Bay and King Cove zone commencing
late tonight and into Friday morning. Precipitation associated
with the front will stay offshore until Friday. The thickness
values associated with this storm suggest possibly wintry mix at
first changing over to all rain.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A series of lows will impact the region. A 960 mb low southwest
of Adak will push north and bring unsettled conditions to the
Central Bering. A second low south of Adak will move towards
Unalaska and produce strong winds and rough seas. The pressure
gradient on the storm tracking towards Dutch Harbor has raised
concerns about the potential for hurricane gusts for Friday. There
is high confidence in widespread storm force winds and moderate
confidence in stronger winds for gaps and passes. In association
with the aforementioned lows, warm air is advecting into Western
and Central Aleutians. Moderate to heavy rain is also expected
with this rather warm system for early January. By 18Z Friday the
surface low will be between King Cove and Unalaska steering that
gradient eastward. In its wake, the models are pinging into a
trough digging across the Aleutians. Overall, a very active
pattern is anticipated this forecast period and the primary
sensible weather highlights will be gusty winds and wave heights.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The most impactful weather this weekend will be centered around
the Gulf as a weak low near Kodiak Island moves northward and
dissipates near the Eastern Kenai Peninsula. This is a feature
that the models have consistently been trending weaker with since
Monday, and is now expected to remain below gale force and bring
precipitation generally to the Gulf Coast. Depending on the exact
low track, there is an outside chance snow could move into
Southcentral on Sunday as downsloping no longer is expected to be
significant along the Chugach Mountains. For now this will remain
as just a chance of snow, however if the models begin trending
further west (such as the latest GFS) these chances may need to be
increased.

Much quieter weather is then expected to move over the southern
mainland for early next week as confidence remains very good in
widespread ridging, producing clearing skies and cooler
temperatures. This ridging will move over the area as storminess
returns to the Aleutians/Bering, beginning with the gale force low
projected to move into the area late Sun-Mon. This pattern is
expected to continue into the middle of next week as a broad
trough sets up over the Bering with ridging over the South
Mainland.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Watch 181.
MARINE...Storm Warning 150 155 160 165 170 172 180 181 185 414.
         Gale Warning 132 171 173-179.

&&

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