Small Craft Advisory 30, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...

An upper level col is located over the western Gulf this morning.
A col is the meeting point between two ridges to the north and
south, and two troughs to the east and west. It is characterized
by very light winds, and as such there remains very little
movement in the upper level flow this morning. Across mainland
Alaska, we continue to have clockwise, dry northeasterly flow
across the area, keeping things quiet, though there are still some
fog issues remaining near Anchorage and over Southwest.
Increasingly colder air continues to make its way southwest out of
the Yukon into the Copper River Basin.

Across Southwest Alaska and the Bering, the story continues to be
a sprawling vertically-stacked low centered near Adak, which has
been largely stationary for the past 24 hours or so. The low is
not a cold one by any means, and so temperatures everywhere west
of the Alaska Range continue to run at or above average. The
winds have been slowly diminishing with time as the associated
frontal systems fall apart. Precipitation associated with the low
for the most part has stayed over the Bering and from the Alaska
Peninsula west, though some light precipitation with the dying
front may impact coastal communities in Southwest Alaska today.

&&

.MODEL DISCUSSION...

The models are once again in very good agreement over the next
couple days, as the low over the Aleutians decays and merges with
additional impulses of energy moving towards it in the flow. Most
of the models are also picking up on a much weaker low developing
south of the Gulf, and moving northwest along a front to the south
of Kodiak. The NAM is quite a bit faster than the other models
moving that second low towards Chirikof Island, so it was
discarded after about 24 hours. Forecast confidence remains high.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Fog is present
along the mountains, and is expected to remain there until
dissipation later this morning. Dry, down-Inlet winds should
prevent fog development tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Tue and
Wed)...
The widespread fog/stratus that has plagued the Cook Inlet region
over the past few days is quickly diminishing this morning as cold
and dry air begins to work into the region. This surge of cold
air, along with the pressure gradient from a weak low in the
Northern Gulf will bring strong and gusty outflow winds,
especially to the Matanuska Valley, Valdez, and Copper River Delta
areas. This gradient will slowly weaken this afternoon, allowing
for winds throughout the area to diminish by this evening, but
these winds should provide enough mixing and dry air to prevent
the redevelopment of fog/stratus in Cook Inlet tonight. The most
noticeable impact of weaker winds and clear skies tonight will be
with overnight temperatures which are expected to fall to near-
below zero in many areas of Southcentral to near 40 below in the
Copper River Basin. This will however be a brief shot of cold air
as a westerly moving low in the Northern Gulf brings cloudcover
and warmer temperatures back into the region. This low will also
bring a chance of snow to the North Gulf Coast through Wednesday
night. For interior Southcentral, with easterly downsloping and
flow down Cook Inlet there is very good agreement that this will
system will bring increased clouds, slightly warmer temperatures,
but no snowfall expected.

&&

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

Much of the Kuskokwim Delta lies between a mid-level ridge
centered over Norton Sound, and a low over the Central Aleutians.
Closer to the surface, the pressure gradient is modestly tight,
which has allowed a continued transport of colder air from the
central portions of the state in an offshore flow pattern. This
has been fairly efficient draining down the valleys, as Sleetmute
is currently approaching -20, with Bethel and New Stuyahok is
just a few degrees above zero. This type of pattern looks to
remain in place through the next 36 to 48 hours, with perhaps just
a few snow showers affecting the Bristol Bay region, as
disturbances rotate northwestward around a central Aleutians low.

Things start to get a little more interesting by Thursday, as
another low moves into the southern Aleutian waters, sending a
surface front northward. Scattered precipitation will accompany
this front as it moves north through the area, while allowing the
winds to become more easterly north of Bristol Bay, and
southeasterly towards the AK Pen, allowing temperatures to
moderate some.

&&

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

The persistent area of low pressure which has taken up residence
just south of the Central Aleutians will continue to meander in
place through tomorrow morning, with vort maxes rotating around it
providing showery type precipitation to much of the Bering and
Aleutians. A stray thunderstorm or two can't be ruled out either
for locations from False Pass to Sand Point southward where
sporadic lightning strikes have been detected, within 15 miles of
these locations, and further south across the southern coastal
waters into the North Pacific. This has been the result of one of
the afore-mentioned vort maxes, with model progs showing decent
instability for this time of year. Any thunderstorm concerns look
to end by early afternoon though, as the disturbance lifts north
of the region with weak subsidence likely in its wake.

Heading into Wednesday and Thursday, the resident low will get
absorbed into a stronger system moving in from the west. This
system too looks to trek south of the chain, with the net affect
being winds increasing to gale force for the waters, and
precipitation coverage and intensity increasing once again as the
associated surface front lifts northeast across the region.

&&

.LONG TERM FORECAST (Days 3 through 7)...

As we get into the long term period on Thursday, the upper level
pattern over the state will be in the midst of a change. Thursday
will start off with the upper level high just offshore the Arctic
coast with the main upper level low just south of Dutch Harbor.
This will start the mild south-to-southeast flow once again over
mainland Alaska with east-to-northeast flow over the Bering Sea.
The jet stream will remain running west-to-east well south of the
Aleutian Islands and Gulf of Alaska. As the weekend arrives the
low in the Aleutians will remain in the general area, but the
upper level ridge will shift over Canada and a jet streak will
move south of the low, through the Gulf of Alaska and then around
the northern periphery of the ridge in the Yukon. The result will
be for a noticeable warm up in temperatures with the potential
for Chinook winds in Southcentral and/or Southwest Alaska over the
weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 127 130 170 171 177.
         Heavy Freezing Spray Warning 139 140 160 180 181.

&&

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