Small Craft Advisory 729, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
525 AM AKDT Tue Sep 25 2018

.ANALYSIS AND UPPER LEVELS...

A long-wave trough stretches over the eastern Bering Sea over the
central Aleutians this morning as the front in the Gulf of Alaska
has dissipated. This trough is vertically stacked and is being
amplified by high pressure building over the western Bering Sea.
At the upper levels, the jet stream is mimicking the location of
the trough which is supporting the southwesterly flow pushing
higher wind speeds aloft toward the Al-Can boarder. With high
pressure building, much of Southwest Alaska is experiencing calmer
winds, and satellite and surface observations show patchy skies.
Over the Kuskokwim Delta, cold air aloft moving inland is causing
convective style cellular storms and rainfall which can be seen
by radar.

At the surface, the front that brought significant rainfall to
Southcentral has moved inland and a drier airmass is advecting
into the area. Rain showers are dissipating but scattered showers
still remain in some locations around Prince William Sound. A
packed pressure gradient along the North Gulf Coast and
southwesterly flow are keeping gusty winds in the mix throughout
Cook Inlet, the Anchorage Area, Turnagain Arm and Mat-Su.

This afternoon, some areas along the Turnagain Arm and
Anchorage Hillside experienced 75+ mph winds with wind damage and
power outages reported throughout Southcentral. In addition to
the high winds, as the front moved inland, increased instability
caused several thunderstorms to pop up throughout Southcentral
causing lighting and hail to fall throughout the area.

&&

.MODEL DISCUSSION...

Models are in good agreement through Wednesday as quiet weather
going to dominate most of southern Alaska. Models do begin to
deviate on Thursday in regards to a Kamchatka low that is
entering the Western Bering Sea. While the general idea is
consistent, models are struggling slightly with the placement of
the low. A secondary conflict arises near the AK PEN where another
system is moving in. Models are not coming to a single solution
in regards to a North Pacific low that is well south of the area.
The EC is a much faster solution with the development occurring
Thu morning and the NAM not creating this second low at all.
Models however, are similar in the associated frontal motion and
its movement into the area.

&&

.AVIATION...
PANC...VFR conditions will persist today. Gusty southerly winds
over the airport complex will diminish by late morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Low level instability and upslope flow will continue to produce
showers along southwest facing slopes this morning, primarily
along eastern Prince William Sound, the Talkeetna Mountains and
the Alaska Range. Low level flow will then slacken considerably
this afternoon as a short-wave trough over Southwest Alaska exits
northward. This will bring an end to the showers as well as to
the gusty southerly surface winds still affecting some areas. With
light winds and mostly clear skies tonight, the forecast remains
on track for one of the coldest nights of the year thus far.

As we head to through mid week, a strong blocking ridge sitting
over the east Pacific will build northward toward Alaska. A
stationary front which is currently strung out along the top of
this ridge will lift northwestward across the Gulf and Kodiak
Island Wednesday night, then continuing northward toward
Southcentral Alaska Thursday. With abundant tropical moisture and
south to southwest flow aloft this looks quite favorable for rain,
so have trended strongly upward with PoP/QPF in the latest
forecast. There is some uncertainty in the strength of the ridge
and orientation of the ridge axis which will determine whether
rain spreads across all of Southcentral Alaska.

&&

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

The surface low near Norton Sound will continue to decay.
Lingering showers are possible today for portions of Southwest as
the residual cyclonic and onshore flow persists. However, in the
wake of the frontal passage this forecast period will generally
be dry and mild. A ridge continues to move east across the
Aleutians/Bering Sea which will facilitate the drying trend.
Nocturnal fog development is also possible. Wednesday a low south
of the chain with an inverted trough will begin to steer a fetch
of moisture into the AKPEN.

&&

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

A shortwave ridge will quickly push across the region bringing a
round of mild weather. There is a mature low upstream near
Kamchatka. The models bring this boundary and later a reinforcing
impulse into the Aleutians. Expect gale force winds and rough
seas with this batch of unsettled weather. Wednesday a low south
of the chain with an inverted trough will begin to usher a fetch
of moisture into the Eastern Aleutians and the AKPEN. The models
have all trended towards an uptick in wind velocities resulting in
gale force winds later in the forecast period.

&&

.LONG TERM FORECAST (Days 4 through 7: Wednesday Night through
Sunday)...

Wednesday night will start out with a front associated with a
North Pacific low moving northwestward, spreading rain across
Kodiak and the Alaska Peninsula. The models are trending a bit
further northeast with the northern edge of the precipitation
shield as compared with yesterday's runs. As such they're now
suggesting the rain spreads across much of Bristol Bay, and
eventually northwestward across the Kuskokwim Delta, though the EC
and GFS are about 18 hours apart as to when that rain makes it
into the Kuskokwim Delta. Thus, the differences between the models
still leave significant uncertainty as to the specifics of when
and how much rain falls in any one place in Southwest Alaska. On
Friday through Sunday, the trailing front behind the parent low
lingers along or off the coast, gradually drifting westward away
from mainland Alaska by Sunday. The presence of the front should
mean a sustained period of gusty southeast winds across Southwest
Alaska Friday and Saturday.

For Southcentral, the front and associated precipitation over
Kodiak will continue there into Friday, though the heaviest rain
should be winding down by Friday morning. During the day Thursday,
the rain will overspread the southern half of the Kenai Peninsula
and up the coast to about the Seward area before beginning to
diminish and retreat southwestward Thursday night. High pressure
over the interior will hold strong and keep Anchorage and points
north and east dry throughout the entire long-term forecast
period, though Thursday now looks to be a cloudier day than it
looked like in the models yesterday.

Temperature-wise, the entire long-term forecast will feature a
warming trend. Behind the front, much warmer air will move into
Southern Alaska Friday through Sunday, which will help support
well above average afternoon highs, though the longer nights and
mainly clear skies will still support some chilly nights for
lower elevation locations as inversions likely set up each night.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 411 412 150 155 177 178.

&&

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