Small Craft Advisory 580, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
515 AM AKDT Sun Jul 29 2018

.ANALYSIS AND UPPER LEVELS...
Water Vapor and IR satellite are showing an expansive low pressure
system over the Bering early this morning with two closed low
centers. One low center is just west of the Pribilof Islands and
is slowly shearing apart, while the other low is strengthening
just south of Adak. A blocking ridge remains just to the east of
the low, spanning across the eastern Gulf and Panhandle with
northward extent into the interior. The loss of a strong jet
stream influence and low level jet allowed a few breaks in the
clouds across Southcentral overnight, mainly along inland
locations and the western Kenai Peninsula. The most impactful
weather is along the Aleutians and Alaska Peninsula where a merger
of two surface lows is bringing gusty winds and rain.

&&

.MODEL DISCUSSION...
Models are in excellent agreement through Tuesday night as they
are now in alignment with the progression of the Bering low. The
energy of the low just west of the Pribilof Islands will be
absorbed by the developing low south of the chain. All models are
in lock step with this low crossing to the southern Bering today
and continuing on a slow northerly track through Tuesday night.

&&

.AVIATION...
PANC...VFR ceilings and visibilities will likely be lowered to
MVFR at times through Monday morning as lower clouds stream north
from the inlet, however there is too much uncertainty for timing
it into the TAF at this time. Tempo groups for MVFR conditions are
included for the morning package. Winds will mainly be light and
variable as a weak Gulf ridge is expected to be the dominating
force.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
High pressure will build over Southcentral Alaska through Monday.
A weak vort-max will slide through Prince William Sound this
morning and then northeast through the Copper River basin around
noon. This will produce some showers for these areas. Plenty of
low level moisture will linger over the northern Gulf and the
surrounding land areas. Inland areas will see warmer daytime
temperatures and less cloud cover as the high pressure builds.
Kodiak Island will be on the very western edge of the ridge, and
thus will have some influence from the Bering low. This will
bring a series of weak upper level systems, that will keep
conditions cloudy and occasionally wet.

&&

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

There is a strong low that is near the Central Aleutians. There
are multiple shortwaves embedded in the cyclonic flow and being
ejected towards Southwest Alaska. This setup will result in waves
of precipitation and cloudy skies this forecast period. This low
will be a resident of the Central Bering most of the period.
Additionally, this low already has a tight gradient associated
with it. This wind field coupled with the terrain of the Aleutian
Range will result in localized gap winds. Gale force winds are
also possible west of the Kuskokwim Delta and Etolin Straight this
forecast period.

&&

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

The low near Atka is still on track to deepen a little more today
and then the track will recurve northward jumping back into the
Bering Sea. A tight pressure gradient is expected the next 48 hrs
across most of the Chain and over the Central/Eastern Bering. The
models are even pinging into pockets of gale force winds east of
the Pribilof Islands and north of Unalaska. Rough seas and periods
of widespread precipitation will continue through the forecast
period. Early Tuesday, a new low will be closing in on the Western
Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...

To start the month of August, an upper level ridge will be
centered over eastern and southeast Alaska, continuing the above
average temperatures seen over the past week there, while an upper
level trough will be located over the eastern half of the Bering
Sea. This persistent pattern will begin to change as upstream
energy coming off of Russia will nudge the trough/ridge couplet to
the east. The upper low will begin to lift NE by Wednesday
pushing a front across SW Alaska into Southcentral, though it
looks like most of the precipitation will be focused north of
Anchorage. An impressive closed high will begin to build north
just south the the Aleutians by Thursday, which will help amplify
an anomalously strong upper jet over the Bering. This will lead to
stronger winds developing over the Bering and Aleutians from mid
week on.

Right now, it looks like an upper trough over the Chucki Sea will
help to flatten the ridge, allowing a secondary low to ride south
and east over the top of the ridge and into mainland Alaska by
Thursday. This system looks to bring a good punch of moisture and
cooler air to the area and should reach southcentral by Thursday
night. Past Thursday, there is some disagreement among models as
to how far north the ridge builds, but it does appear the ridge
should be far enough east and north to bring the chance of drier
weather to the western and central parts of mainland by the
weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 170 and 180.

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...KVP