Small Craft Advisory 870, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKST Sun Nov 12 2017

.ANALYSIS AND UPPER LEVELS...
The large high is persisting over the eastern Aleutians into the
Bering Sea both at the surface and aloft. A trough associated with
a strong low over the Arctic extends into the central interior
and is moving over the top of the Ridge and diving south into the
Gulf of Alaska. A trough extends from a low off the southern
Panhandle northwest to Prince William Sound. As the trough dives
south to the North Gulf Coast the surface low will reestablish.
Deep cold air is persisting over the eastern interior. On the west
side of the high is yet another low now well south of the western
Aleutians.

The passage of the trough has mixed the atmosphere enough over
southwest Alaska to improve ceilings and visibilities over the
Kuskokwim Delta. As this trough passes into the North Gulf Coast,
the pressure and thermal gradients will intensify resulting in
increased gap winds.

&&

.MODEL DISCUSSION...
The models are in excellent agreement through the short term
leading to high confidence in the forecast for the next couple of
days. Of course the finer details that are not well resolved by
the models remains the challenge in determining the magnitude of
the expected winds through Thompson Pass and Valdez.

&&

.AVIATION...
PANC...VFR conditions will persist through Monday afternoon. The
challenge is when will the north wind kick up again and at what
magnitude. Once the upper trough dives into the Gulf of Alaska we
expect the north wind to pick up this evening then drop off
toward the morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Mon and
Tue)...
The big story across the area will be strong offshore winds. A
sharp ridge of higher pressure will build quickly over the Eastern
Bering tonight into Mon. The eastern periphery of the ridge will
basically form almost a due north to south line from Prudhoe Bay
through the Prince William Sound. This will allow several pieces
of energy to move through this area. It will also allow some even
colder air to start to fill in to the Copper River Basin. These
forces combined will bring some very strong winds across Southern
Alaska, especially to the typical north to south oriented gaps
(Thompson Pass, Seward, Matanuska Valley, etc.). The best chance
for these winds to reach to the 75 mph warning-level will be
during the day Monday through Thompson Pass into Valdez. The
upper-level support aligns over the area as the cold air builds
near the surface north of the Chugach Mountains. A High Wind
Watch remains in effect to highlight this potential. The key
take-away with this wind event is the long duration. While there
may be minor fluctuations in the speeds, winds will remain strong
across much of the area into Wed.

Precipitation will be on hold for most locations in this pattern.
The lone exception will be this evening across the eastern-most
portion of the Copper River Basin where a little light snow is
falling. After that, the cold, dry offshore flow will make for
continued cold temperatures with clear skies.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
Ridging sits over the west coast where it will remain into Tuesday
when a front approaches from the Bering. The front will push in
over the Kuskokwim Delta Tuesday night with precip beginning as
snow. However, there will be a strong enough push of warm air to
transition the snow to rain along the coast. The front will then
move over the rest of southwest Alaska on Wednesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
High pressure over the eastern Bering will slowly track east
through the week. To the west of the ridging a system is moving
toward the western Bering with its front moving through the
Western Aleutians. The front will move over the Central Aleutians
on Monday and then the Eastern Aleutians and Pribilof Islands
Monday night. This system will be warm enough for precip to fall
as rain. Behind the front, high pressure will build in over the
area from the west.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Wednesday, there will be a
shortwave trough moving across the Bering Sea and a shortwave
ridge axis aloft moving over Southcentral. A deep low over the
British Columbia coast will still have an influence over the area
as offshore winds along the North Gulf Coast continue to pour into
the Gulf of Alaska. The upper trough will begin to shear out as
it moves inland across Southwest Alaska, but it should have enough
moisture to squeeze out some snow over Southcentral sometime on
Thursday. This trough will be quickly followed by another low
amplitude shortwave trough in northwest flow which should bring
back more dry and cold northerly flow to Southern Alaska Friday
into Saturday. This will be a brief repeat of the offshore flow
pattern we have seen the past week or so.

By late Saturday into Sunday, the global models continue to hint
at a potent shortwave trough crossing over the Bering Sea and
moving over top the semi-permanent high pressure ridge over the
Bering. This potential storm could be a significant weather maker
for Southern Alaska for the weekend, but this is still quite a
ways out to get into specifics, or even if it will happen. But,
for now, the threat exists for an active weather pattern by the
weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Watch 131.
MARINE...Gale Warning 120 121 126 127 128 130 131 132 138 150.

&&

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...JA

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
928 PM AKST Sun Nov 12 2017

AKZ171-140100-
Kodiak Island-
Including the city of Kodiak City
928 PM AKST Sun Nov 12 2017

...RESUSPENDED VOLCANIC ASH FROM KATMAI VOLCANO...

Strong northwesterly winds are producing areas of resuspended
volcanic ash from Katmai Volcano, in particular across Shelikof
Strait to the west side of Kodiak Island. Strong winds are
expected to continue through Monday, so areas of haze and
suspended ash may remain visible.