Small Craft Advisory 54, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
416 AM AKST Thu Jan 18 2018

.ANALYSIS AND UPPER LEVELS...
A decaying low continues to spins over Kodiak Island wrapping some
moisture into the region. Cold air has filtered across southwest
and any remnant moisture from this system will fall as snow north
of Shelikof Strait whereas temperatures over Kodiak are slightly
warmer and more conducive to rain or a rain-snow mix. Looking
upstream, a trio of lows is closing in on the Western Aleutians.
There is an occluded front north of Attu that extends east of
Atka but this boundary and the adjoining lows will weaken over the
Bering. The ominous low of the group is tracking northeastward
out of the north Pacific. A surface high that has been over
Siberia will begin to nose its way across the International
Dateline and into the Central Bering. The latest scatterometer
data clearly shows the anticyclonic moving into the Bering Sea.

&&

.MODEL DISCUSSION...
Models are in sync with large scale features through the weekend and
there is good run to run continuity. The primary challenge will
be pinpointing the temperatures as we head into a much cooler
pattern.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

The trend to colder temperatures has begun over Southcentral
Alaska with temperatures cooling through the weekend.
The low that pumped the warm air into the region is filling
rapidly over Kodiak Island where it is still producing rain and
snow showers.

There remains a weak upper level low over the Kenai Peninsula
this morning which will persist through tomorrow before heading
eastward Friday night into Saturday. Why this matters is that it
is going to keep some cloud cover over the region and prevent a
significant clearing trend. It also is keeping upper level support
from developing for outflow winds. Therefore the winds along the
north Gulf coast will be limited due to this. The surface pressure
gradient and cold air advection will be enough to produce some
gusty northerly winds, but they will not get overly strong
(compared to what they often do) as a result of no upper level
support for them at all. This upper level low will also bring some
scattered snow showers to the Prince William Sound area late
Friday into Saturday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
The low over Kodiak is expected to diminish and push off to the
east through tonight and as it does snow over southwest Alaska
will also diminish. Meanwhile, high pressure will build in from
the north that will bring cold northerly winds to the area. Friday
night and into the weekend this setup will bring temperatures that
stay below 0 F for most of southwest Alaska locations that are
not along the coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
The weakening low near the western Aleutians and its stalled front
that stretches over the eastern Aleutians will continue to bring
light showers to the area until high pressure forces them out
tonight into Friday. The high pressure will build in from the
north bringing colder air to the Bering and Aleutians. Friday
night into Saturday, another system will approach the western
Aleutians from the North Pacific but its progression will be
stalled by the high dominating the central Bering. This will keep
the majority of the precip from this system isolated to the
western and central Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast is still on track today with models in good
agreement showing a major pattern shift beginning this weekend. A
high amplitude blocking ridge will develop over the western Bering
which will cause a longwave trough to descend south across the
state by Sunday. The trough will drag brutally cold, eye lash
freezing air down from Siberia, where temperatures were recorded
as low as -80F earlier this week. This will lead to a significant
cold air outbreak over most of interior and western portions of
the state next week. Temperatures across interior sections could
drop into the -30s and -40s by Monday with locations along the
Bristol Bay coast dropping into the negative single digits or even
negative teens. Though this is not shaping up to be a record
breaking event, temperatures will be noticeably colder than it
has been all winter, especially when factoring in the expected brisk
northerly winds, which could drop wind chills as low as -50
across interior portions of SW Alaska.

Southcentral looks to escape the brunt of the cold air as the
Alaska Range will block the cold core to the north and west sides
of the range. In fact, a weak surface low looks to develop in the
Gulf by Tuesday which could bring periods of light/dry snow to
the mountains and communities along the Prince William Sound from
Tuesday through the end of the week, which should also pump in a
little bit warmer air from the south helping to moderate
temperatures across southcentral a bit. An upper level low looks
to drop out of the Arctic around mid week bringing another
reinforcing shot of cold air to the state. The low center should
track south across the western half of the state, which would
again concentrate the coldest air to SW Alaska. If the low tracks
more to the east, as shown in previous GFS runs, cold arctic air
could make its way into southcentral, though models seem to be
agreeing more on the western track of the low.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 178 180. Heavy Freezing Spray 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...KVP