Small Craft Advisory 850, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 AM AKDT Wed Oct 31 2018

.ANALYSIS AND UPPER LEVELS...

A ridge of high pressure stretches from the western Gulf into the
eastern Bering. An inverted trough is extending through the
western Aleutians from an area of low pressure located well south of
Adak. The dry air associated with the area of high pressure was well-
captured by this morning's sounding from Cold Bay.

The subsidence behind yesterdays trough that crossed southwest
and southcentral Alaska was captured by the King Salmon, Kodiak,
and Anchorage upper-air flights as well. With this in mind, it's
no surprise that fairly decent fog and stratus coverage has been
observed across portions of the southwest and southcentral. This is
being locally aided by a moisture flux off the snow pack helping to
strengthen the inversions.

&&

.MODEL DISCUSSION...

Overall the NAM, GFS, and ECMWF models remain in good agreement
with the Canadian Global an outlier (similar to the last few
numerical runs) at both the surface and mid-levels. There are just
slight differences in location and placement of the pressure
fields. As such, forecast confidence is a little higher than
average. The main forecast challenge will be with the track, and
strength of a deepening area of surface low pressure as it treks
towards/across the central Aleutians, before getting absorbed by a
much stronger storm approaching the east central Aleutians.

&&

.AVIATION...
PANC...VFR conditions and light winds will predominately prevail
at the terminal over the next 24 hours. However, there may be some
brief reductions in visibility to 1/2 mile or lower with ceiling
near or under 300 feet. Confidence in this is moderate at best as
the fog/stratus will play tag with the airfield as it advects
down the Cook Inlet from the Knik Arm. Area web cams and
observations have already shown hints at this occurring.

&&

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

Generally dry and cool conditions will prevail across Southcentral
over the next couple of days. Upper ridging will bring relatively
stable conditions today giving rise to widespread stratus and some
fog over the Copper River Basin with patches of the same across
portions of the Anchorage Bowl/Knik Arm today. The stratus and fog
will mix out some in places in the Copper River Basin as pressure
gradients increase and produce locally gusty winds tonight and
Thursday. The threat of stratus and fog will likely continue into
Thursday across the Anchorage Bowl, particularly over north and
west Anchorage where fairly low visibilities may occur.

As alluded to, pressure gradients will strengthen tonight and
continue into Thursday. This will give rise to gusty outflow winds
across the North Gulf Coast, Thompson Pass, as well as, the
Matanuska Valley. A low skirting to the east across the southern
Gulf will bring gales to the offshore marine waters tonight and
Thursday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

A dry pattern is expected through Friday as a ridge shifts across
the eastern Bering to the central mainland. Low clouds and fog
this morning are expected to improve this afternoon. Patchy fog
is possible again Thursday morning as a stable air mass
establishes across the region, however dry offshore flow will be a
strong limiting factor. Light and variable winds this morning
will become east to northeast as the ridge center moves across the
interior. The next front advances across the Alaska Peninsula
Thursday night into Friday with rain and gusty east winds
developing along the boundary.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

An stacked storm system is organizing just south of the eastern
Aleutians this morning and will slowly progress northward into the
southern Bering this evening. Easterly winds increase to gale
strength for marines as the boundary swings north. The low shears
apart tonight as it slams into the ridge ahead of it. Another
rapidly organizing low in the North Pacific tracks south of the
central Aleutians today and turns northward, following a similar
track as the low today. However this storm system wraps up and
lifts north as a more powerful storm, with the surface low
expected to deepen to 960 mb or lower. The front with this system
moves to the eastern Aleutians and Alaska Peninsula Thursday
night. Gale force easterly winds will gust to storm force along
the boundary. There is still some uncertainty with the track and strength
of this system and should be monitored for updates.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The extended period begins Friday evening with an upper-level
ridge of high pressure over the mainland pinching off from the
main flow as it retreats to the north. This movement is in
response to the eastward progression of a mature low situated near
the AKPEN. An occluded front attached to the low lifts into the
northern Gulf from the southwest, with areas of easterly gales
moving toward the southcentral coast. The front is then expected
to weaken as it stalls along the coast for Saturday. Guidance
remains in decent agreement through Saturday, with the NAM, GFS,
and EC all suggesting this outcome. The outlier is with the
Canadian which tries to bring a weaker low along with the
occlusion through the Gulf to the southeast coast by Saturday.
Based on the overall consensus of the other models (deterministic
and ensemble), this is has been discounted in favor of the others.

Uncertainty ratchets up quite a bit after Saturday. The 06z and
12z GFS runs were on par with the EC runs and its GEFS
counterpart bringing the parent low into the Gulf on Sunday. The
EC was faster by about 12 hours with the track of the low toward
the southeast coast while the GFS weakened the low into more of
an open wave. The 18z GFS, however, has the more EC-like closed
low, but with the track hugging the southcentral coastline. So
while exact track and timing is still uncertain, there is high
confidence of inclement weather along the southern coast and
northern Gulf through Sunday.

Any remaining confidence in the models going into early next week
is then completely eroded by how the GFS and EC handle the upper-
level dynamics upstream over the Bering. The EC wants to build a
ridge between the aforementioned low and the long-wave trough over
the Bering. The ridge is then shifted east over the northern Gulf
and southern mainland for Monday and Tuesday as the Bering low
deepens and the ridge amplifies. The GFS, on the other hand, has
kept consistent with a zonal pattern, allowing the upper-level
shortwaves to progress east across the forecast area. The first
comes across the Aleutians beginning Saturday, reaching the
western Gulf on Monday and continuing to impact the southern
mainland through mid- week. The second impacts Southwest Alaska
late Monday with the third moving into the eastern Bering on
Tuesday. Overall, the long-term forecast beyond the weekend is
weighted more toward the GFS solution given that it is more in
agreement with ensemble guidance and consistent with previous
runs.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 155 165 170 171 172 174 179

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...TM