Small Craft Advisory 815, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
455 AM AKDT SUN OCT 16 2016

.ANALYSIS AND UPPER LEVELS...

Upper level troughs are over the eastern half of mainland AK and
draped across Kamchatka into the western Bering this morning. A
massive upper level ridge is centered over eastern Russia as well
as the eastern Bering. A tight northerly gradient persists over
western AK into the AKpen, and the near zonal jet remains just
south of the forecast area. At the surface, a strong low pressure
center is over the northern Gulf and a weaker low is just south of
Attu. A decaying front extends from the western Bering to Dutch
Harbor, and a cool high remains over much of the mainland. The
tight pressure gradient between the mainland high and the Gulf low
is keeping strong wind blowing out of channeled terrain near
Valdez, and through the Matanuska Valley.

&&

.MODEL DISCUSSION...

Models agree that the low in the Gulf will drift to near Homer
later Mon and Mon night. This will bring plenty of unstable
moisture-laden air into the northern Gulf, Prince William Sound
and the Kenai Peninsula through Mon night. The gradient
responsible for gusty winds will also remain in place during the
period.

The decaying front over the Bering continues to dissipate through
tonight. The low south of Attu will move east and affect the
western and central Aleutians through Sun before moving into the
Gulf on Mon. The forecast utilizes a NAM-GFS blend through the
period.

&&

.AVIATION...
PANC...VFR conditions with N winds up to 10 kt will continue this
morning as the pressure gradient continues to flow down inlet.
These winds will begin to increase this afternoon as a wave moves
into the western Gulf, with the strongest winds developing this
evening with gusts up to 25 kt that will persist into Monday
morning.

&&

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

Offshore winds will be trending upward over the next 12 to 24 hours
as an area of low pressure in the Gulf of Alaska continues to
strengthen and move north and west. Meanwhile, higher pressure
still resides along the western half of Alaska with cooler 850 mb
air spreading pushing south across Southcentral Alaska. The result
will be increased pressure gradients and stronger gap flows
tonight and into Monday morning. Gap flows will relax some Monday
afternoon with weakening pressure gradients.

In addition, precipitation confidence remains high along the
north Gulf coast and Prince William Sound areas. The greatest
impacts from snow and wind look to be in Thompson Pass where a
combination of winds and snow will make for reduced visibilities
through the pass. Coastal areas of the eastern Kenai Peninsula
will see mainly a mix of rain and snow with snow expected inland
and higher elevations. Some light snow or snow showers could make
it over the Kenai and Chugach Mountains today.

&&

.FIRE WEATHER (Matanuska Valley)...

Northeast winds of 20 to 35 mph with gusts to 45 mph can generally
be expected today with higher gusts to near 60 mph possible
tonight. Northeast winds should slowly diminish beginning by mid
morning Monday with wind speeds dropping 15 to 25 mph by late
Monday afternoon.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Dry offshore flow will persist through Monday night as an arctic
trough digging over the eastern half of the mainland retrogrades
westward to the vicinity of Bristol Bay and the Alaska Peninsula.
Temperatures across southwest AK will continue to steadily trend
downward during this time as the coldest air mass so far this
season filters into the region with the arrival of the trough.
Gusty north to northeast winds will also develop by this
afternoon in response to a tightening pressure gradient between
the digging trough and the strong high over Eastern Russia.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A decaying frontal boundary will bring light showery conditions to
portions of the central Bering through early Monday, with a
passing North Pacific low also bringing a chance of rain to the
Western and Central Aleutians before it moves away from the area.
Otherwise, gusty north winds and gradually clearing skies will
prevail across the eastern half of the Bering under the influence
of dry northerly flow in advance of the approaching mainland
trough. Winds will continue to increase through Monday as the
trough moves into the region, with showers also returning to the
area as the accompanying cold air aloft moves over the relatively
warm waters of the Eastern Bering.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long range forecast beginning Tuesday will be a slow
transitioning pattern with a persistence forecast through the end
of the week keeping dry and cool conditions inland and rain
across the area waters. Models have been struggling on the how to
resolve the track and the speed of the low currently diving into
the western Gulf. Starting Tuesday, this trough will dive into the
North Pacific, cut-off, and bring a rapidly developing low likely
as far north as Kodiak Island, but prevent it from moving much
further into the Gulf. There are some differences in the models
in the placement, especially from the latest runs of the EC that
have been further north than any other solution, and was treated
as an outlier. There is enough agreement in the strength of the
front however to bring gale force winds and rain throughout the
Gulf for mid-week, but this will need to be fine-tuned as
agreement in low placement improves. Later this week, a low over
the northwest coast and another low from the North Pacific will
slowly break down the upper ridge center over the Bering Strait
through the end of the next work week. The ECMWF and ensemble mean
were used to update the forecast as it was favored for it's run to
run consistency.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 120 127 130 131 139 172.

&&

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...LIW/BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...KH/DEK