Small Craft Advisory 858, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
415 AM AKDT THU NOV 3 2016

.ANALYSIS AND UPPER LEVELS...

A very powerful near 200 knot upper level North Pacific jet streak
stretches from the southern Kamchatka Peninsula to 40N 160W (or,
about 1000 miles south of the Alaska Peninsula). Cold arctic air
originating from Eastern Siberia is pouring out through the Bering
Sea into the North Pacific, and this thermal gradient is what is
supporting the intense jet streak. At the exit of the jet streak,
across the Northeast Pacific ocean, is an upper low moving into
the Southeast Panhandle of Alaska with a series of trailing low
amplitude shortwave troughs. Water vapor and IR satellite imagery
depicts the complexity of the wave pattern embedded within the
broader synoptic scale trough as these features interact and merge
with one another. Another shortwave trough has formed along a
secondary weak baroclinic zone over the Eastern Bering Sea,
enhanced by a strong low level shearing flow. An increasingly
strong offshore pressure gradient across Southcentral has ended
shower activity across the North Gulf Coast.

&&

.MODEL DISCUSSION...

The numerical models are in excellent agreement with the general
large scale synoptic pattern heading into next week, but are in
much lesser agreement with smaller scale shortwave troughs
embedded within the broader scale long-wave trough. There has been
a rather large change in the models with respect to a second low
in the Northeast Pacific which had been progged to move toward the
North Gulf Coast late this week into the weekend. Now, that
feature is progged to be much weaker and will take a track east of
the ALCAN. This will allow for more weak troughing to move across
Southern Alaska. This is where forecast confidence drops heading
into the weekend. While the details are subtle, the differences in
phasing lows and moisture moving in from the Northeast Gulf of
Alaska lend rather low confidence in potential light snow chances.
The NAM and Canadian GDPS models are more bullish in generating
light snow across much of Southcentral Friday night into Saturday
while the ECMWF and GFS are much drier. There is also low
confidence with a consolidating low over the Bering, but this is
of less impact due to its weak strength with the main story being
showery weather due to marine instability. The ECMWF and GFS were
favored models on both sides, but the uncertainty with
precipitation was taken into consideration with some credence
given to a potential NAM/Canadian GDPS scenario, so low end
slight/chance precip chances were added for this forecast package.

&&

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

&&

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

A fairly strong low over the eastern Gulf moves north and weakens
through tonight bringing increased chances of snow to the
southeast Copper River Basin. Pressure gradients will increase
along the north Gulf coast and the eastern Kenai Pen this morning
giving rise to to gusty outflow winds. These conditions are
expected to weaken this afternoon/evening as pressure gradients
relax. Broad cyclonic flow aloft will push weak waves across the
Gulf and Southcentral Alaska through Friday night. A complex area
of low pressure consisting of several weak low centers will become
established across much of the north Gulf Friday. The result will
be increased chances of snow inland with a mix of rain and snow
along the coast as we head into the weekend.

&&

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

Flow over southwest Alaska will weaken as the low over the Bering
weakens and elongates. As this happens shower activity will also
diminish but will still linger along the coast. Interior locations
will see less cloud cover and colder temperatures overnight
allowing for the possibility of fog formation with the highest
chances being tonight. On Friday night, showers will then spread
back inland as a shortwave rotating around the main Bering low
swings in over the area.

&&

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

The low over the northwest Bering will slowly track to the
southeast as it weakens and will make it to the Eastern Aleutians
Saturday morning. This will change the strong westerly flow over
much of the Bering and Aleutians to a weaker northerly flow. It
will also allow for continued shower activity over the area with a
mix of snow and rain showers, that will be mostly rain during the
day and mostly snow overnight. The exception is the Western
Aleutians that will see enough cold air advection to stay snow.

&&

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

A broad vertically stack low over the Bering will continue to
weaken through the day Saturday as it shifts south across the
eastern Aleutians and Alaska Peninsula. A developing low south of
the Aleutians will continue east along 47N with minimal impacts
to the forecast area. By Sunday afternoon or evening a
strengthening low will reach the western Aleutians with an
associated weather front positioned through the Central Aleutians
extending into the lower Bering Sea. This system tracks east along
the Aleutian Chain before exiting into the North Pacific Ocean
Tuesday keeping the Bering Sea and the Aleutians under a showery
weather regime.

Meanwhile, the Southcentral region continues with generally drier
conditions and high pressure in the interior. However, things
will be changing along the northern Gulf Coast and in the lower
southeastern Copper River Basin by Saturday as a low pressure
system tracks to the Yakutat area and lifts moisture into the
Copper River basin. With temperatures below freezing, look for
chances of snow for that aforementioned region through the weekend.
By Sunday evening a storm force low develops in the central Gulf
before tracking to the northern Gulf, and dissipates Tuesday evening.
This synoptic scenario brings gale to storm force southeasterly to
easterly winds from Monday morning through Tuesday. Look for
widespread rain to occur mainly along the northern Gulf and the
coastal communities.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 175 176 177 178 411 413.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...PD/JR