Small Craft Advisory 881, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
533 AM AKST Fri Nov 17 2017

.ANALYSIS AND UPPER LEVELS...
Southern Alaska is in the process of transitioning back to a
drier offshore flow pattern this morning as a pair of upper level
disturbances depart into the Gulf and help to dig a trough over
the eastern half of the mainland. Areas of low stratus or patchy
fog linger across some portions of Southcentral AK including the
upper Cook Inlet region as the first shortwave which brought
widespread snow to the area yesterday moves into the northeastern
Gulf near Yakutat. The second shortwave which tracked rapidly
through Southwest AK last evening has now moved into the far
southern Gulf, ushering in strong northwest winds from the coastal
waters of the western Gulf to the Pacific side of the AK
Peninsula. Farther west, more benign conditions reside across most
of the Bering as another ridge builds into the region. Meanwhile,
a front associated with the latest Kamchatka low is beginning to
move over the western tip of the Aleutian chain.

&&

.MODEL DISCUSSION...
Models show good agreement through the short term as the synoptic
pattern briefly re-amplifies and reverts to more of an outflow
wind dominated regime yet still remains fairly progressive into
the weekend. The ridge building over the central Bering will
gradually shift east across the mainland through Saturday night,
setting the stage for the next system to drop into the area from
the Bering Strait region on Sunday (see the Long Term discussion
below for details on this system). Generally went heavier on Hi-
Res guidance early in the forecast to capture the next round of
outflow winds and then broadened the approach to more of a
blended solution to account for lower confidence later in the
weekend.

&&

.AVIATION...
PANC...IFR ceilings are expected to remain over the Upper Cook
Inlet region through late Friday morning before lifting around 20z
as stronger northerly flow develops. VFR conditions are then
expected to persist through Saturday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A few lingering areas of snow over the eastern Copper River Basin
should end this afternoon. Low status and patchy fog is expected
to linger from Cook Inlet/western Kenai Peninsula to the Anchorage
Bowl into the afternoon hours today. Status and fog potential over
the Matanuska Valley should continue until winds pick up early
this afternoon. The amplifying upper ridge over western Alaska in
combination with increasing pressure and temperatures gradients
across Southcentral will give rise to strong outflow/gap
conditions across much of the Gulf Coast today. Winds will likely
peak late this evening then slowly diminish through Saturday.
northwest gales with storm gusts over the western Gulf today will
slowly diminish tonight. A weather front approaching from the
west Saturday night will bring increasing chances of snow to the
Cook Inlet/Mat-Su corridor and rain/mix to Kodiak.

&&

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

A strong and very potent storm system will be developing Saturday
through into Sunday and impacting all of Southwest Alaska. The
parent low, currently near the Kamchatka Peninsula, will take
advantage of numerous ingredients to become a powerful storm this
weekend. Unseasonably warm air over much of the western and
central Bering is streaming north on the west side of a large area
of high pressure. The storm is gathering cold air over Siberia. As
the center of the low moves southeastward over the Bering Strait
on Saturday, it will undergo rapid cyclogenesis as a very
favorable jet and impressive temperature gradient feed the storm.

As the low center tracks towards Southwest Alaska, the very warm
air over the Bering will override the shallow cold air across the
Kuskokwim Delta and Bristol Bay. The warm air should have no
problem quickly scouring out the cold air across much of the Lower
Kuskokwim Delta and near the coast of Bristol Bay, thus places
such as Bethel, Dillingham, and King Salmon should see a period of
rain Saturday evening in Bethel, and overnight Saturday night in
Dillingham and King Salmon with rising temperatures overnight.
The rain and warmer temperatures will limit snow accumulations.
Cold air on strong, gusty winds rushes quickly into the area
behind the low, possibly turning all rain back over to snow before
ending Sunday morning.

Further east towards the Lower Kuskokwim Valley and locations
north and east of King Salmon such as Iliamna and the Lake Clarke
National Park are expected to stay all snow as the warm air
struggles to make it that far inland. Thus, impressive snow totals
are possible. This will be especially true as the cold air wraps
around the north side of the low, dropping temperatures and
raising snow ratios making for a very powdery, fast-accumulating
snow. The snow will continue through much of the day Sunday,
ending late afternoon. A Winter Storm Watch is in effect for
those locations for upwards of a foot of snow possible. Very cold
air follows the storm for all of Southwest Alaska.

&&

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

The current strong winds south of the Alaska Peninsula are
diminishing this morning and all eyes turn to the next storm
system developing this weekend. Very warm, moisture-rich air is in
place over much of the central and western Bering. Meanwhile, cold
air over Siberia is on a collision course with the warm air,
setting the stage for the development of a strong storm.

As the low tracks towards Southwest Alaska Saturday, the cold air
from Siberia on the west side of the low and on the east side of
the strong high will race southward and replace the warm air
across the Bering from Siberia. 50 to 60 mph winds with gusts as
high as 85 mph over a large swath of the eastern Bering Saturday
night through much of the day on Sunday are possible following the
cold frontal passage. A High Wind Watch is in effect for the
Pribilofs.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Expect near to below normal temperatures across southern Alaska,
with periodic snow across the Southwest. Fronts will dissipate by
the time they reach Southcentral Alaska. Expect continued gap
winds along the north Gulf coast.

The forecast begins Saturday evening with a transition in the
upper level pattern taking place. A cold outflow pattern with gap
flow along the north Gulf coast will briefly give way to a front
and low pressure system sliding into the western Gulf Sunday
morning. The front will bring rain to Kodiak, and snow to the
Kenai Peninsula, potentially as far north as a line from Skwentna
through Anchorage/Palmer. The most likely place to see
accumulating snow would be from Kenai through Homer. Going into
the extended forecast, we return to a more familiar outflow
pattern. However there are differences between what we've been
seeing to the upcoming pattern.

The upper jet looks to be more zonal in the upcoming pattern while
the low-levels remain conducive for weaker outflow and gap winds.
The jet stream looks to be very active and progressive with
multiple embedded with a storm track from the northwest Pacific
through the southern mainland/Gulf of Alaska. Expect three west to
east moving fronts to approach the mainland and weaken through
next week. Each of these systems will bring cold air from the
Arctic on the backside, which will prevent the progressive pattern
from moving into the Interior.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 152 161. High Wind Watch 195.
MARINE...Gales 127 128 130 131 132 138 150 155 160 175 185 351 352 411
412 413.

&&

SYNOPSIS AND MODEL DISCUSSION...CB
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...TP