Small Craft Advisory 611, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
437 AM AKDT Sat Aug 19 2017

.ANALYSIS AND UPPER LEVELS...

For the fourth day in a row, a shortwave associated with a jet
streak is enhancing the western periphery of the arctic trough
that is currently entrenched across the Alaska mainland. Within
this arctic trough is a rather potent closed low which continues
to advect colder air across most of Alaska. This "bowling ball
low" has been tracking across the Bering Sea coast with showers
evident on the Nome NEXRAD. Subsidence out ahead of the low has
brought a brief reprieve of showers to southwest Alaska but with a
very moist stable layer coupled with a slightly stable
environment, fog has been steadily developing overnight.
Southcentral Alaska on the other hand remains quite showery as a
strong shortwave on the southern end of the arctic trough
continues to move across the region. The same subsidence over
Southwest Alaska has begun to move over Southcentral Alaska which
should help reduce shower activity and coverage throughout the
day.

An area of low pressure associated with ex-tropical storm Banyan
currently located south of Atka looks to be weakening with a lot
of the upper level support shifting further east. However, recent
water vapor and infrared satellite imagery shows weak cyclonic
rotation developing along a frontal boundary associated with the
aforementioned low pressure system. While there is little sensible
weather associated with this system as it is well south of the
Aleutian Chain, that will change over the weekend as the developing
low quickly pushes into the Gulf of Alaska.

&&

.MODEL DISCUSSION...

Numerical guidance is in remarkable agreement once again this
morning. It's been surprising just how well the models have been
progging the individual jet streaks that continue to enhance the
arctic trough that has been over Alaska for over a week now. Since
the models initialized these features well, overall confidence
for the forecast package remains higher than normal. The only real
discrepancy of note is how the triple point low develops south of
Cold Bay over the next 18 to 24 hours. Model spread on low
placement is initially pretty large as they try to resolve the
development of the low but are in pretty good agreement with it
entering the western gulf Sunday morning. It looks like the most
recent model runs keep the low further west and south than the
runs last night which keeps interior portions of Southcentral
Alaska drier than previous runs. Basically, confidence is lower on
how much rainfall areas like Anchorage and Palmer will receive
while the entire northern gulf coast will see plenty of rainfall
on Sunday.

&&

.AVIATION...
PANC...VFR conditions are expected through the forecast period
with some showers developing toward Sunday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level trough over Southcentral Alaska this morning will
drift northeast today and exit the area this afternoon, resulting
in a decrease in shower activity today. The upper level low
centered over the Seward peninsula will move south for the next
couple of days, entering the Gulf just west of Kodiak Island
Sunday night. On Saturday night a system south of the Alaska
peninsula will get caught up in the flow of the upper low, and
will swing north into the Gulf on Sunday. This will become a
fairly strong system on Sunday as it swings north into the
northwest Gulf. Moisture with this system will push into the
Southcentral mainland on Sunday and Sunday night, bringing another
round of rain to the area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Expecting a fetch of moisture which is currently over the Bering
to get entrained into the back side of the midlevel low bringing
precipitation into the AK Peninsula tonight. A broad vortex over
western AK will continue to amplify enabling cooler air aloft to
work its way deeper into Southwest Alaska. Unsettled weather will
persist for the next few days as cooler air associated with the
trough moves southward. Between the embedded shortwaves and the
diurnal heating at the surface, the dynamics will be present to
support showers. Temperatures will hover near or slightly below
average for this time of year.

&&

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

South of Kiska, there is a closed low that is progged to move
eastward and slowly deepen. In its wake, expect ridging which
will keep a stable and relatively warm air mass in place. The
Aleutian Chain and portions of the Bering can anticipate more fog
and stratus. The ridge axis will persist into next week keeping
the areas of pesky fog in place.

&&

.LONG TERM FORECAST (Days 4 through 7)...

Beginning the extended period forecast Monday night, there will be
a closed upper low over the North Gulf of Alaska with a shortwave
ridge axis over the Eastern Bering Sea. Expect showery and mostly
cloudy/cool conditions to be over Southcentral with drier weather
over Southwest. Tuesday and Wednesday will be a transition period
as Southern Alaska moves into weak shortwave ridging as the upper
low weakens and moves east. Another trough over the Bering Sea
will move toward Southwest Alaska on Wednesday and eventually dig
south toward Kodiak Island through the remainder of the week. This
will keep a wet pattern in place across coastal regions with
clouds and rain chances for inland areas. The degree of downslope
flow for interior locations remains somewhat uncertain at this
point as the low track and placement will modulate that factor,
but it still looks wetter and cooler for most of interior Southern
Alaska for the foreseeable future.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MMC
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...JA