High Wind in Alaska, December 16 to 18, 2016

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 AM AKST Fri Dec 16 2016

.ANALYSIS AND UPPER LEVELS...
The active pattern in the flow aloft is beginning to take shape as
a mature Bering Sea storm acts to reposition the longwave trough
further to the east this morning. The amplified upper level ridge
of the past couple days is sliding to the east in response to the
new low pressure system. The jet remains south of the Aleutians,
but is very energetic, a 160+ kt core coming off of east Asia.
Southcentral Alaska is barely on the edge of the Bering system. A
shortwave trough lifting overhead, ahead of the incoming front,
brings light snow to much of the area. Temperatures have moderated
greatly from the past few days as the source for this air mass is
the northern Pacific and not the central interior. Winds are
relatively light for the southcentral while the 950 mb low in the
Bering is bringing gale to storm-force winds to the Bering, almost
reaching hurricane force at times.

&&

.MODEL DISCUSSION...
The next event to evaluate guidance is frontal boundary/triple
point low moving into the region from the southwest. Guidance has
been pretty locking into this solution for a couple days, but
slight differences remain. The biggest differences have to do with
the weak triple point that will be forming, while most have some
lowering of pressure in the Prince William Sound area, the general
trend has been to lower pressure in the Cook Inlet region as well,
which would have an effect of downslope flow holding on longer.
The other change in the past couple days is the upper trough
is now progged to not just shear out over the northern gulf, but
go negative tilt and wrap some energy back around over the
western Chugach. The consistent theme among models is a period of
moderate snow late tonight into Saturday morning.

&&

.AVIATION...
PANC...light winds and VFR conditions should persist. Wind shear
will be the main forecast problem as Turnagain Arm winds get going
toward the afternoon hours. However, a stronger down-Inlet
gradient could bend the winds further down this Inlet and not
manifest as wind shear overhead.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The short-wave trough which brought snow to much of Southcentral
overnight into early this morning will exit to the east today.
Isentropic lift and low level warming will continue behind the
trough as a short-wave ridge moves in behind it. Thus, expect
areas of light precip to persists, especially from Cook Inlet
up to the Susitna Valley as well as across Prince William Sound.
Meanwhile, southeast winds will increase as the next trough
approaches from the west, helping to dry out areas in the lee
of the Kenai and Chugach Mountains.

One of the primary forecast challenges for today and tonight is
precipitation type. Models indicate a deepening layer of above
freezing temperatures moving northward across the Kenai Peninsula
up to Anchorage and southern portions of the Mat-Su. Downslope flow
will also contribute to warming. Light surface winds, mostly out
of the north, will keep temps at or below freezing for most of the
Cook Inlet region, which is usually a good set-up for freezing
rain. In this case, expect the warmest areas to remain dry. A
rain-snow line will make northward progress on the Kenai Peninsula
and along the Gulf coast, but do not expect freezing rain to be
an issue.

Attention then shifts to the next trough moving through Southcentral
overnight tonight through Saturday which has potential to produce
heavy snowfall in the Anchorage and the Matanuska Valley. The
latest guidance all indicates the short-wave becoming negatively
tilted and rotating back toward Anchorage early Saturday morning
while low level southwest flow ushers colder air back up the
Inlet. The air mass may be a bit too warm for snow as precip first
begins, but if the intensity of the precip does in fact pick up
quickly, this will cause a rapid turn-over to all snow. Have
trended toward higher snowfall amounts, with generally 3 to 5
inches in the Anchorage and Mat Valley. If the trough does in
fact wrap right back into this area and the changeover totals
could easily be higher than this, especially along and near the
mountains.

&&

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

A remnant occluded front associated with a weakening storm force
low over the Western Bering will continue to race east and weaken
today and tonight. Coastal locations will be warm enough to change
to rain (remaining a rain/snow mix towards Hooper Bay). Interior
locations will likely remain snow although some sprinkles/light
rain will be possible. A pocket of isolated freezing rain can't be
ruled out across parts of the lower Kuskokwim Valley, but it is
more likely precipitation remains in the form of snow or briefly
turns to rain during the day. The front will clear eastward by
Saturday morning with colder air moving in behind it, along with
rain and snow showers which will be becoming isolated/scattered by
Saturday evening. Another occluded front will move into Southwest
Sunday morning, bringing with it more snow and rain threats as it
moves inland Sunday.

&&

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

The deep low over the Western Bering remains storm force this
morning as buoy obs and ASCAT wind passes revealed a core of 50-55
knot winds this morning. The associated occluded front is moving
into Southwest Alaska and will quickly weaken. The story quickly
turns to a developing hurricane force low which will undergo
intense bomb cyclogenesis and move toward the Central Aleutians
Friday night and Saturday. Storm force winds will move into the
Bering Sea as the warm front lifts north, but the main core of
powerful hurricane force winds are expected to remain south of
the Aleutian chain. For such a powerful low, the greatest impacts
(seas 40-53 feet) and hurricane force winds will remain relegated
to the marine waters near the low.

&&

.LONG TERM FORECAST (Days 3 through 7...Sat night through Wed)...

A series of lows will move rapidly east across the forecast area
during the period. The initial low pressure system in the eastern
Bering will swing a front up against the north Gulf coast on Sun,
bringing accumulating snow mainly to coastal areas of Southcentral
Alaska through Mon. The next low in the series will move across
the western Aleutians Mon with rain and snow, the eastern Bering
and Akpen on Tue with rain along the Aleutians and snow inland,
and reach the north Gulf coast Wed. This will bring more snow to
Southcentral by midweek. For the Bering, strong winds will be
associated with the lows as they transit the region.

Because of the run to run inconsistency demonstrated by the models
recently, confidence is low with the timing of the lows, as well
as the location of precipitation.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Hurricane 176 177.
         Storm 173 174 175 178.
         Gale 155 165 171 172 179 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...MTL
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...DS