Small Craft Advisory 663, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 AM AKDT Mon Aug 27 2018

.ANALYSIS AND UPPER LEVELS...
The Norton Sound low has weakened and moved inland over the North
Slope while a large upper level area of high pressure remains
parked over the Gulf this morning. A weak upper level shortwave
has formed in the southern Bering over the Bristol Bay area and
its corresponding surface low has developed south of Sand Point.
Ahead of the surface low, a leading front is entering the western
Gulf. This has generated a few showers observed on radar this
morning over the northern AKPEN, Shelikof Strait, and Kodiak
Island. Behind the front, low level stratus clouds and patchy fog
are filling in along the coastal areas of Southwest. Lingering
rainfall continues to dissipate across the eastern coast of Prince
William Sound and the northeastern Gulf coast.

&&

.MODEL DISCUSSION...
Model solutions remain in generally good synoptic agreement
through the next 24 hours. While there was some uncertainty with
the timing of the upper level shortwave trough moving over
Southcentral this evening, all solutions now seem to be in good
agreement. Looking further out, models indicate a North Pacific
low approaching the western Aleutians Tue afternoon. There is some
disagreement on the exact position of this low as it enters the
Bering Wed morning. At this time, the GFS is showing a faster
solution with the low moving further east while the NAM and EC
solutions keep the low further west. The NAM/EC solution was
preferred for this morning's forecast package.

&&

.AVIATION...
VFR conditions will prevail for the forecast period as an upper
level trough moves east out of the area leading to drier
conditions. The main impact today will be gusty southeasterly
winds as Turnagain Arm winds combine with strong up-inlet flow.
Winds will diminish this afternoon as the pressure gradient
weakens. Looking to tomorrow night/morning, high pressure will
move into the area leading to light winds and clearing skies,
promoting potential fog development. Recent rainfall should
provide plenty of surface moisture though drier offshore flow may
inhibit widespread fog development over the region. For now,
there is higher confidence fog will develop over the Cook
Inlet/Knik Arm, possibly advecting inland, thus vicinity fog was
included in the TAF.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Wednesday)...
Even as the upper level low over the interior begins to lift
northward through the interior, a separate system will race
northward from the North Pacific into the Gulf today. The models
have been struggling with how far west to bring this low over the
last few days, but they now are coming into agreement that this
low will track far enough to the west to bring widespread rains to
Prince William Sound today. This resulted in some large changes
to the rainfall forecast over Kodiak Island as well as the Gulf
coast from Prince William Sound eastward, where widespread heavy
rain was added for this afternoon. The good news for areas further
west into Southcentral is that there remains good consensus that
most of the rainfall from this low will not reach as far west as
the Kenai Peninsula, leaving a cloudy but dry day for most other
areas today.

One forecast concern around Cook Inlet tonight will be for
the possible development of fog/stratus as winds diminish and
skies clear late tonight. This will also occur as offshore flow
begins to develop along much of the Gulf Coast behind the incoming
low, however without high pressure over the Mainland this is not
expected to bring significant down-inlet flow. Because of this
patchy fog and status was introduced from Cook Inlet northward
into the Susitna Valley for the next 2 nights, where generally
light winds will be present with increasing low level stability.
Other than this potential for patchy morning fog/stratus, most
areas will see a well deserved break in the active weather for
Tuesday and most of Wednesday as weak riding moves in. This will
allow for sunny skies to develop over much of Southcentral and
temperatures to rise back into 60's to lower 70's.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Today through Tuesday)...
Residual showers behind the occluded front that moved inland over
Bristol Bay yesterday continue to taper off early this morning
along the coast. A weak upper level trough currently over Saint
Paul will traverse the northern AKPEN this evening and move into
the Gulf. An upper level ridge of high pressure will fill in
behind this trough, providing generally fair conditions for Tue
though associated subsidence will likely allow for a low level
stratus deck and areas of patchy fog to persist along the coasts.
This area of high pressure will quickly move inland allowing for
another east-bound front to arrive over Kuskokwim Delta by Tue
evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Today through Tuesday)...
High pressure over the central Bering Sea will shift east as a
warm front strengthens into a gale force low over the northern
Bering Sea Mon night through Tue night. This low will impact the
entire Bering Sea and Aleutians, but the strongest core of winds
will generally be over the Bering Sea and Pribilof Islands.

&&

.LONG TERM FORECAST (Wednesday through Sunday)...
By Wednesday afternoon the next frontal system will be
approaching the Kuskokwim Delta coast with the associated strong
parent low located in the central Bering. With the this low
continuing to deepen, strong westerly gales associated with the
wrap around occlusion to its south and west are likely through
Wednesday night with the swath of gales tracking east following
the upper trough on Thursday. Additionally, strong west to
northwesterly winds are possible through the gaps and passes in
the western Aleutians and Alaska Peninsula Thursday night and
Friday as the upper trough swings through those areas.

Further inland, additional abundant rainfall will be the biggest
impact from the frontal system as well as the following upper
level troughs. The initial front will cross Southwest Alaska
Wednesday night and then move through Southcentral Alaska on
Thursday. An upper level shortwave trough with accompanying
moderate to heavy showers will push into Southwest Alaska Thursday
and Southcentral Alaska Thursday night to be followed by a
broader upper low/trough Thursday night through Friday. Snow
levels with the coldest portion of the upper level trough
additionally will briefly fall into the 4000 to 5000 ft range.

The general troughy pattern with cool breezy northwesterly flow
will continue through Saturday before a transient flat ridge
builds in from west to east. The next North Pacific frontal system
will cross the Aleutians around Sunday tracking northeast through
Bering on Monday and into Southwest Alaska Monday night or
Tuesday.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...KO/KVP
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO/JA
LONG TERM...JR