Small Craft Advisory 909, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
328 PM AKST Fri Nov 18 2016

.ANALYSIS AND UPPER LEVELS...
Upper levels today show a cutoff high over the Arctic Ocean, with
a weak upper level low over Southcentral and a longwave trough
moving into the Western Aleutians. Ahead of the longwave trough,
the jet stream is building shortwave ridging in the Central
Aleutians, and a 150 kt jet streak moving northward is aiding in
the development of a surface low pressure system moving into the
Western Aleutians as well.

Over the mainland and Eastern Bering, two weak features are the
main weather drivers today. The upper level low over Southcentral
is driving a very weak trough moving onshore from the Northern
Gulf, bringing snow and rain showers to the windward side of the
Prince William Sound. This trough is also strengthening outflow
winds on the eastern side of the sound this morning, with gusty
winds blowing in channeled terrain such as the Valdez Narrows and
Thompson Pass. West of the Alaska Range, a weakening feature over
the Eastern Bering is brought precipitation and some surprisingly
strong winds to the Alaska Peninsula this morning, although it is
rapidly fading as it moves eastward. Some low clouds lingered
along the Kuskokwim coast early this morning, but further inland
it remained clear.

&&

.MODEL DISCUSSION...
Models have not been performing spectacularly well, although in
the east the problems are relatively minor. The main concern over
Southcentral is the timing of precipitation into eastern Prince
William Sound, and the potential for precipitation in Anchorage
overnight. Accumulations are expected to be low, so impacts from
these differences should be minimal.

However, in the west, the models are in total disagreement about
what's going to happen with the low currently moving into the
Western Bering. All models seem to show multiple low centers
forming, but as to which one continues to develop and move
eastward, and when, they cannot decide. The west forecast was
based on a blend of the GEM and NAM, with low confidence over the
next few days.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist for most of the
night. By tomorrow morning, downsloping will weaken and precipitation
is expected to move over the Chugach range and into the airport.
This is expected to fall as snow, and although it will be light,
will likely reduce the conditions to MVFR in showers. Reductions
in visibility should be fairly brief.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The model soundings for the Anchorage area continue to indicate
that the current airmass is relatively dry. However, steep
temperature lapse rates continue to persist, and as a result,
cloudy conditions and a chance at some passing light snow showers
remain possible. As the upper level low over the southern Kenai
continues to slowly track northeastward and into the Northern Gulf
coast by Saturday morning, light showers will continue across
Prince William Sound and the eastern Kenai Peninsula before
tapering off Saturday afternoon. Although snowfall will be
relatively light, snowfall will persist long enough for several
inches of accumulation in Prince William Sound and in the western
Kenai Peninsula. Snowfall is less certain for Anchorage, but high
resolution models are trending towards the chance for a few quick
snow showers especially Saturday morning.

Cold air will continue to filter into Southcentral as high
pressure remains established inland and low pressure remains
dominant over the Gulf through Saturday morning. Therefore, gusty
outflow winds will persist over Prince William Sound and through
local gaps across the area as the current pressure gradient
remains in place tonight and tapers off by Saturday morning. With
gusty winds and snow in place over Thompson Pass tonight, areas
of blowing snow are expected to reduce visibilities through
Thompson Pass tonight.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A front stalls along the Kuskokwim Delta overnight and diminishes
on Saturday. Snow develops along the coast this evening spreading
east toward Bethel by midnight. Snow accumulations will mainly be
along the coast up to one inch. Patchy fog can be expected along
the frontal boundary tonight and along the foothills of the
Aleutians and Alaska Ranges where the air mass remains more
stable. A dry offshore pattern ensues for the weekend as low
pressure across the Bering sends the next several systems on a
southeastward track along the Aleutians and Alaska Peninsula.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
An unsettled weather pattern continues across the Bering and
Aleutians through the weekend. Gale to storm force winds can be
expected along the Aleutian Chain with frequent directional shifts
as several surface lows traverse west to east. A ridge builds
across the western Bering on Sunday which slows the next low
pressure system heading up from the North Pacific Sunday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The active pattern will continue through next week with multiple
lows moving across the forecast area. However, the storm track
looks to stay south of mainland Alaska with lows moving across the
Aleutians and then into the Gulf. This setup will keep most of the
precipitation and warm air out of the mainland with coastal areas
seeing the majority of the precipitation. Although the models
disagree somewhat on the timing of the various lows, they are in
decent agreement with the position of the storm track.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 175 176 177.

&&

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...Pepe
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...DK