Small Craft Advisory 789, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 PM AKDT Wed Oct 18 2017

.ANALYSIS AND UPPER LEVELS...
The main feature on water vapor satellite is a complex upper low
spinning in the eastern Gulf of Alaska. A very zonal jet stream
lies south of the Aleutian Chain, driving the predominant storm
track from the northwest Pacific straight eastward into the Gulf.
While most of the active weather associated with the Gulf system
is out of our area, the main impact on the mainland has been to
draw a colder air mass in from the interior of the state, while
increasing winds along the Gulf coast through favored gaps in the
mountains. Much of Southcentral Alaska is clear as cold advection
dries the air mass out, but the upper trough is currently keeping
a mid-level deck on the west side of the Alaska/Aleutian Range.
Much of the Bering Sea is engulfed in instability showers as
arctic air streams over the warmer water. The next system to
progress across the Chain is beginning to affect the Western
Aleutians.

&&

.MODEL DISCUSSION...
Guidance in the short term is in great agreement with the
evolution of the synoptic flow. There are differences with low
positions within the complex Gulf circulation but effects on our
forecast are the same in any solution. The biggest differences in
the models come late Thursday into Friday as the upper level
trough captures some energy from the Gulf over the eastern portion
of the Copper River Basin. Some guidance shows a prolonged light
snow for the McCarthy area while other are much drier. High
resolution guidance will be utilized for this forecast package to
capture outflow winds along the north Gulf coast a little better.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Much of Southcentral AK will settle into a dry northwest flow
pattern through Thursday evening as a large low over the
southeastern Gulf departs towards the Panhandle and a weak upper
trough pushes across the region from the west. This will leave
gusty outflow winds as the primary short term forecast challenge
as most of the area remains precipitation free. Upper level
support for these winds looks to be somewhat lacking, leaving the
surface pressure gradient and cold air advection from the
interior as the primary drivers of this event. This should keep
outflow winds more on the modest side, although strong gusts can
still be expected through the typical coastal gap flow areas
including Valdez, Whittier, and Seward. Winds should begin to
diminish overnight Thursday as an upper low over the Beaufort Sea
pivots to the western mainland and upper level flow shifts around
to the southwest. Meanwhile, the exception to the dry conditions
expected across most of the area will be the eastern portion of
the Copper River Basin, where the combination of the upper trough
approaching from the west and a weak surface low over the
northeastern Gulf should be sufficient to produce a period of
light snow Thursday into Thursday night. Minor accumulations will
be possible within Wrangell-St. Elias National Park including the
McCarthy area through Friday morning.

&&

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

Northwesterly flow will continue to filter cold air into the
southwest mainland through the next couple of days. Gusty
northwesterly winds over the Alaska Peninsula will taper off
through tomorrow. The southwest mainland area will remain fairly
dry through Friday, with only a couple of exceptions. Unstable
northwesterly flow banking up against the Kuskokwim Mountains and
Alaska Range will squeeze out a few showers in the vicinity of the
mountains as the air is orographically forced upward. In
addition, cold air flowing over the relatively warm ocean waters
will continue to steepen lapse rates and support some isolated
showers along the Kuskokwim Coast and Nunivak Island. Otherwise,
expect mainly dry weather with a continual cooling trend to
slightly below normal temperatures by Friday. Slightly more active
weather will return Friday evening as an arctic low will push
southward into the Kuskokwim Delta, bringing a better focus for
some more organized precipitation.

&&

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

The main feature driving the weather over the Bering Sea and
Aleutians will be a northwest Pacific low tracking east towards
the western Aleutians tonight. As the low moves near Shemya
tomorrow morning, it will be caught up in the base of the
downstream longwave trough over mainland Alaska, preventing any
northward movement into the Bering Sea. As has been the pattern
for the last week or so with a zonal storm track over/south of the
Aleutians, the majority of the impacts of this low and it's
associated front will remain confined to the Aleutians and
southern Bering Sea. Expect a swath of small craft to gale force
winds and rain over the western/central Aleutians and southern
Bering Sea tonight through Friday. As the low departs to the
Northeast Pacific Friday, widespread northerly flow will resume
over the Bering Sea, with increased chances of showers as
instability increases.

Meanwhile further east, small craft force cold northwesterly
winds over the eastern Bering Sea will weaken overnight as
pressure gradients slacken. The area will remain relatively quiet
under a col Thursday before broad northerly flow quickly resumes
Friday.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday night through
Wednesday)...

An active pattern is expected through the long range forecast
through the middle of next week. The main challenge surrounds a
storm system which slides east into the Gulf this weekend
bringing the potential for snow across Southcentral into early
next week.

For the start of the weekend, cold air advection keeps
temperatures below normal across the southern mainland into
Saturday morning under dry conditions as low pressure tracks south
of the Alaska Peninsula to the Gulf. Weak disturbances across the
Gulf will initiate rain showers along a surface low near the
northeast coast heading into Saturday morning, with a few snow
showers moving inland over the eastern Copper River. For locations
along the Bering, including the southwest coast, precipitation
chances increase through the upcoming weekend as an upper low
pressure system in the eastern Bering slowly propagates south
toward Bristol Bay. A colder air mass advecting from the northern
Bering changes precip type mainly to snow except along the
Aleutians where a mix of rain and snow prevail under moderate
temperatures.

The biggest challenge of the upcoming forecast is the storm system
which tracks into the gulf on Saturday. This system is currently
expected to lift a frontal boundary to the northern waters by
Saturday afternoon ahead of a surface low. This low moves across
the central Gulf Saturday night and deepens to 970 mb. This warm
core system will spread rain across the Gulf on Saturday, with
showers along the Gulf coast starting initially as snow mixing
with or changing over to rain through Saturday night. The upper
level low associated with this front begins to lift north towards
Sunday morning, which brings the potential for snow to develop
across inland areas of Southcentral. Models continue to struggle
from run to run on the timing and track of this system, therefore
only small changes were made to increase precipitation chances for
the latter part of the weekend. With high uncertainty during the
mid range forecast, kept changes minimal through the middle of
next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 130-132 150 175-178 352 411 413.

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...RD
LONG TERM...KH