Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 291231
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
431 AM AKDT THU SEP 29 2016
.ANALYSIS AND UPPER LEVELS...
An amplified ridge builds over southwest Alaska extending
northward into the Chukchi Sea as it gradually moves eastward this
morning. This synoptic feature is reflected at the surface with a
1027 mb high pressure over the Lower Kuskokwim Valley extending
eastward along the Alaska Range. This is resulting in weak onshore
winds accompanied by marine layer stratus, and areas of patchy to
dense fog. Meanwhile, upstream there is a closed low over the
Kamchatka Peninsula with an associated trough extending southward
through the Western Aleutians into the North Pacific Ocean. The
surface analysis is showing a 979 mb occluded low over the
Kamchatka Peninsula with jet support from a 120 knot southwesterly
jet core. An associated weather front stretches through the
western/central Bering Sea into the Central Aleutians producing
gale force southerly winds. Water vapor imagery is depicting a
nice comma cloud over the Central Aleutians with moisture
streaming over into the lower Bering and the Pribilof Islands.
Farther west a warm conveyer belt is streaming through the
Eastern Siberian region with the trailing weather front
approaching the Western Aleutians this morning.
The models are in decent agreement with the GFS/NAM and the
Canadian GEM being the preferred models this morning. The ECMWF
was the outlier as it pushes the North Pacific Low faster eastward
along the Aleutian Chain before the system moves into the Bering
Sea by Friday evening. The forecast confidence is above average as
we head into the weekend.
PANC...VFR conditions and light winds are expected to persist
through Friday morning.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Clear skies dominate once more across Southcentral this morning,
with an amplified ridge pushing northwards from the Pacific. The
exception to that is along Cook Inlet where a sharp inversion
near the surface is helping to trap moisture and support fog in
Homer and Kenai. Another dry day is expected across much of
Southcentral, with only a few cirrus clouds expected. As high
pressure becomes further established, expect temperatures to
continue to be cool in the overnight hours over the next several
days, with patchy frost. Also, as high pressure builds in across
the interior today, the contrast with the gulf trough will be
enough to support offshore northwesterly flow along coastal gaps.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An impressive ridge of high pressure will remain situated across
Southwest Alaska through this evening before it begins to displace
eastward as a deepening low and upper level trough moves into the
Bering Sea Friday morning. Much of the mainland will remain on the
dry side through the short term forecast (through Saturday morning)
but rain chances will increase along the west coast Friday evening
and afternoon. In addition to the increasing rain chances, strong
winds approaching 50 mph will likely be observed overnight Friday
into Saturday along the west coast as the front strengthens as it
moves toward the coastline. Stronger gusts approaching 60 to 65
mph are not out of the question for immediate coastal locations
like Cape Newenham.
The Alaska Peninsula will likely feel the brunt of this developing
low with winds gusts of 70 mph expected over many locations and
possibly even a few stronger wind gusts possible Friday afternoon
into Saturday morning. Some uncertainty is introduced into the
forecast due to warm air being advected north ahead of the warm
front which could create a nice stable layer that prevents the
core of winds from fully mixing down. However, in previous cases,
a south to southeast warm air advection event has produced
noticeably strong wind gusts at both Cold Bay and Port Heiden.
Mountain waves may be induced depending on stability at mountain
top level which also may bring stronger isolated wind gusts across
the Alaska Peninsula. A high wind warning/watch was discussed
overnight but due to lack of confidence and expected coverage of
winds exceeding 74 mph no watch or warning was issued.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A gale force front is currently moving across the western Bering
and will continue to push eastward throughout the day. The main
focus of the forecast package will be a low pressure system that
will quickly spin up south of Adak and Atka this afternoon and
evening. An influx of energy in the form of a jet streak will help
wrap up a pretty potent shortwave and help close off an upper
level low over the North Pacific. A surface low will develop in
response to this new upper level feature and a very potent frontal
boundary will develop over the eastern half of the Bering Sea on
Friday. It looks like a nice swath of 40 to 45 kt winds will be
observed Friday night and Saturday with a nice area of 50 to 55 kt
winds south of the Alaska Peninsula and near Bristol Bay. Stronger
gusts are possible with such a dynamic and developing system but
winds are not expected to be much stronger than 55 kt. Seas will
build to 24 to 26 ft in response to a nice southerly fetch near
the Alaska Peninsula and 16 to 20 ft in the eastern Bering Sea.
Coastal flooding is not anticipated along the west coast of Alaska
due to an unfavorable fetch orientation.
.LONG TERM FORECAST (Days 3 through 7)...
Forecast thinking remains largely unchanged for the extended term
beginning Saturday as a Bering Sea front brings a return of
precipitation to the Southwest mainland, while a high amplitude
ridge continues to yield clear skies across Southcentral AK. Model
agreement is slightly improved today regarding the evolution of
the system moving through the Bering late in the week and how that
ultimately impacts the mainland ridge. The front is still expected
to enter the southwest Gulf of AK and then be shunted towards the
southeast by a developing North Pacific wave this weekend, with
high pressure weakening but still remaining in place over
Southcentral AK as the ridge begins to break down. This lends
increasing confidence that the majority of Southcentral AK will
stay mostly dry well into next week, with outflow winds and
freezing overnight low temperatures remaining the primary forecast
concerns. Meanwhile, the Bering will remain in a much more active
pattern as various waves continue to rotate around the the
periphery of the deep Siberian low, with a weak disturbance on
Sunday followed by potentially stronger systems on Monday and
again towards the middle of next week.
MARINE...Gale 160 165 170 172-177 179-181 185.
SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC