Gale Warning 603, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 PM AKDT FRI OCT 14 2016

.ANALYSIS AND UPPER LEVELS...

An upper level ridge is centered near the Bering Strait and extends
over all of mainland AK and eastern Russia today. A broad trough
over Kamchatka extends to the eastern Bering, with an upper low
near Vancouver island. The nearly zonal jet runs from Japan to
northern California, with a cyclonic curve around the low near
Vancouver island. At the surface, a weak low is near Shemya,
another is south of the Gulf of Ak, and a third is aimed at
Vancouver island. High pressure continues to build across the
mainland part of the state.

&&

.MODEL DISCUSSION...

Models are in good agreement through Sat morning before the
solutions begin to show variation. This means the Shemya low will
be absorbed into the Bering trough this afternoon, the low south
of the Gulf will continue to drift southeast,the low approaching
Vancouver island will move inland later today, and the relatively
cold air beneath the mainland high will remain in place through
the weekend. On Sat morning a low will develop over the northern
Gulf that will wobble in the area through Sat night. By early Sun
it will bring a surge of moisture into the northern Gulf. This
will begin to interact with the cold air flowing over the
mainland, especially near Prince William Sound and its immediate
surroundings. The forecast leans upon the GFS solution which is
more aggressive than the nam in bringing precipitation to the
coast by early Sun.

&&

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

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A broad area of low pressure will remain situated across the Gulf
of Alaska with multiple low centers through the short term
forecast. This will allow for offshore flow to persist across the
region and allow much of the area to remain on the dry side. In
addition...the offshore flow will continue to keep fog at bay as
winds mix down drier air aloft. On Sunday, the bulk of the energy
in the Gulf of Alaska will consolidate into one low center south
of Prince William Sound and will couple with an upper level
easterly wave which will increase precipitation chances along the
northern gulf coast. Models are struggling considerably with the
timing of this feature which create issues when trying to pinpoint
both precipitation type and how much rain/snow is expected to
fall.

At this time, it looks like there is a high likelihood of 1-4
inches off accumulating snow through Thompson Pass Sunday evening
through Tuesday morning. Valdez, on the other hand, will find it
harder to come by snow as temperatures hover above freezing during
the overnight hours as offshore winds continue to keep temperatures
near 34-36 degrees at night. Only a trace of snowfall is currently
forecasted for Valdez. The Copper River Basin may see snow upslope
along the Wrangell Mountains and the coastal range but no
accumulating snowfall is expected to make it into the heart of the
valley near Gulkana or Glennallen. No snow is forecasted for
Anchorage or the Mat-Su Valleys at this time as the easterly wave
should remain east of the area.

Strong offshore winds will increase on Sunday in response to the
energy in the gulf consolidating into a single low center. This
will increase the pressure gradient across the region and winds of
30-45 mph will be seen through Thompson Pass and the Copper River
Delta while Valdez will see winds 20 to 35 mph. There could be
some visibility reduction through Thompson Pass from blowing snow
with strong winds coinciding with snow fall. These conditions will
continue to be assessed to see if any products are warranted.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The Southwest can expect a persistent pattern through the weekend
as ridging settles in from the northern coast. Temperatures trend
down this weekend as a cold air mass filters from the Arctic. Dry
offshore flow will be gusty on Sunday with a tightening pressure
gradient between the gulf low slowly tracking east while the ridge
sags south.

&&

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

A showery pattern is expected to continue across the central and
southern Bering as a weak frontal boundary remains along the
central waters. The surface low near the Pribilof Islands will
diminish overnight which will bring improved visibilities. A new
surface low develops tonight across the western waters just south
of Kamchatka. This development provides enough surface convergence
to initiate showers along the western waters through the weekend.
Another upper level trough moves a wave into the western Aleutians
late Saturday with north and eastward progression into the Central
Bering for Sunday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A dramatic pattern change is in store for the Southern Mainland by
the end of the weekend, as a series of arctic troughs merge and
form into a deep pinched off upper low near Kodiak Island and the
Northwestern Gulf Sunday night/Monday morning. This system will
help filter in Cold arctic air into the Copper Basin, down Cook
Inlet, and across Southwest Alaska, as the surface low move across
the Northern Gulf. This will also act to increase the offshore
pressure gradient through favored coastal gaps Sunday through
Monday. Strong dry offshore flow will keep inland locations dry
with the main brunt of moisture moving up Monday into Tuesday.
This is where uncertainty develops as the amount of warm air aloft
and strength of the surface low is very uncertain which will
eventually effect precip type and distribution along the coast.
There is a possibility of accumulating snow and accompanying gusty
northerly winds through Thompson Pass. The Eastern Copper Basin
will also see accumulating snow threats along the Wrangle
mountains, but predominant cross-barrier flow will keep inland
locations west of the Basin rather dry.

By Tuesday, the arctic trough will retrograde west while
simultaneously digging south. This will allow for warmer air to
move toward the Gulf which will change any surrounding locations
seeing a mix of of rain and snow to all rain. Tuesday night, A
North Pacific low will merge with the arctic trough and deepen
into a strong gale force low. This will keep wet weather mainly
along the coast as most of the energy will remain well south of
the mainland. Temperatures will moderate to normal inland as
clouds move in and the cold arctic feed is cutoff. Come Wednesday,
models are showing another Arctic trough crossing the bering
strait and digging across mainland Alaska through Friday morning,
bringing another push of colder arctic air to the Southern
Mainland.
&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...ELN
SOUTHCENTRAL ALASKA...MC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...TP