Small Craft Advisory 31, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 PM AKST Tue Jan 9 2018

.ANALYSIS AND UPPER LEVELS...
Most of the mainland part of the state is under upper level high
which is centered over the Seward peninsula and stretching over
the Kenai Peninsula into the Gulf of Alaska. The main upper level
low center is near the Dutch Harbor area which is bringing in
rather strong upper level southeast flow to Southwest Alaska as
these two centers are rather close together creating a tight
gradient over SW Alaska and the far eastern Bering Sea.
The surface low is weakening over the central Aleutians but will
persist for a number of days in that region as a low complex.
A weak surface low near Middleton Island in the northern Gulf is
bringing a few snow showers around Cordova and some gusty winds
through Thompson Pass.

&&

.MODEL DISCUSSION...
Models are in good agreement for only about two days before
substantial differences begin to emerge. By Thursday night the GFS
has been insistent on strengthening the wave moving into the Gulf
of Alaska from the south into a closed low which is about 10 mb
lower than the NAM and ECMWF wave. This will be the difference
between Gales and Storm force winds along the north Gulf coast.
The open wave with Gale force winds looks to be the most likely
solution and forecasts reflect that.
Models all are showing the large scale pattern shift brining in
warmer air this weekend but the details as to timing and track of
the main system remain a bit uncertain.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds will persist into
Wednesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Wed and
Thu)...
The cold "snap" will be quite short lived as the pattern
transitions quickly back to warm southeast flow on Wed. The Arctic
airmass that has intruded over the Copper River Basin will
continue to keep things gusty at the typical locations (Matanuska
Valley, Copper River Delta, Thompson Pass) this evening. As winds
relax and the boundary layer decouples overnight, many places
will see the coldest night in quite some time. The coldest spot
will be near Gulkana where morning lows on Wed will dip to near
40F below. Thankfully winds will subside through the evening as
upper level support moves away, so wind chills will not be so
severe. Some light snow showers along the north Gulf coast will
drift west towards Seward and the western Prince William Sound on
Wed. This moisture then gets acted upon by two different features.
A polar low drops south up against the AK Range while another
trough pushes towards Kodiak from the North Pacific. This will
produce some light snow accumulations for the higher elevation of
Kodiak Island and along the north Gulf coast Wed morning through
Wed evening.

Thu morning, another stronger front will move right through the
same area. This one will bring enough warm air to change Kodiak
City from snow to rain. It will also bring enough lift that we
have added some light snow accumulations to Kachemak Bay for Thu.
It is also possible for a few flurries to make their way into the
Cook Inlet region. The cross-barrier flow takes a while to
establish itself and become strong enough for the typical down-
sloping to dry things out. Until this happen, a few flakes could
fly during the early portion of the day. By later Thu, southeast
winds start to increase at all levels, so gusty conditions through
Turnagain Arm and warmer temperatures will return.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The Southwest Alaska coast will remain right on the eastern edge
of the Bering Sea trough through tomorrow, with weak upper waves
tracking from southeast to northwest. The vertical motion from
these is very weak, but low to mid level instability will provide
just enough lift to produce some showers. With fairly deep
southeasterly flow, the best chance of showers will be from
Dillingham north and west into the mountains. As you head inland
across Southwest conditions will become progressively colder and
drier. A series of lows tracking south of the Alaska Peninsula
Wednesday night through Thursday night will bend the flow more
toward the east, maintaining mostly dry and cold conditions.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
While one low weakens along the central Aleutians, an occluded
front extending from a deep low over the far northwest Pacific
will make its way toward the Aleutians. As the front elongates
along the Aleutian chain on Wednesday, expect multiple surface
waves to develop along the front. Ultimately, expect one of these
waves to develop into a closed low and set up shop not too far
from where the current low is located. Overall, weather will
remain fairly similar to today, with periods of rain and wind.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Heading towards the end of the week a familiar pattern begins to
setup with a longwave trough stretching through the Bering Sea and
ridging to the east over western Canada. This setup will bring
back warm southerly flow to the southern mainland for the weekend
and much of next week. While there is fairly good agreement in the
overall setup, the models are struggling with placement of lows
embedded within the longwave trough. Because of this, ensembles
were preferred in the long term over the deterministic runs.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 127 130 138 155 170-174 179 180 185 411-414.
         Heavy freezing spray 139 140 181.

&&

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