Small Craft Advisory 824, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
431 AM AKDT WED OCT 19 2016

.ANALYSIS AND UPPER LEVELS...

The early morning weather map is highlighted by a strong area of
low pressure drifting south of the Alaska Peninsula. While the low
appears to have already reached the mature phase based off
satellite and surface observations, hurricane force wind gusts
have been observed through some gaps and passes of the Alaska
Peninsula. The strongest observed gust was 73 kt at King Cove.
Attached to this low is a potent frontal boundary boundary draped
across the northwestern Gulf of Alaska. As of midnight, a mix of
IR satellite imagery and scatterometer data shows a triple point
low developing near 54N 141W. This low has been rapidly moving to
the northwest and should move near Kodiak Island later today.
Looking out west, a weak area of low pressure has moved off the
Kamchatka Peninsula and is drifting toward Shemya and Attu. This
low has been weakening at a steady rate and old observations show
a nice swath of 25 to 35 kt winds associated with this low.
Looking at the upper levels, the highly amplified trough that has
been in place for the last several days has changed little in the
past 24 hours. Upstream upper level ridging over the northwestern
Pacific has begun to shift eastward which may be the feature that
will begin a pattern change.

&&

.MODEL DISCUSSION...

Models are in remarkably good agreement through the short-term.
There are, however, a few questions which could have some decent
impacts on the forecast in the coming days. The first is the
aforementioned developing triple point low. The GFS is trying to
keep this feature as more of an open wave while the NAM and GEM
are hitting this feature a bit more aggressively. Based on
observations this morning, it looks like the NAM has a better
depiction of this solution. The next question comes in the form of
a potential deformation band setting up over Anchorage and the
Mat-Su Valleys Thursday night into Friday morning. The GFS has
been hitting this deformation feature pretty aggressively the past
several days but to our amazement backed off substantially with
the 00Z run. The other 00Z guidance trended toward this
deformation zone setting up over east Anchorage with a couple
inches of snowfall across much of Anchorage and the Mat-Su
Valleys. Model disagreement makes that aspect of the forecast
quite challenging but chances for the first snow for parts of
Southcentral Alaska looks quite possible.

&&

.AVIATION...
PANC...VFR conditions will continue through Thursday morning with
light winds.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A rapidly weakening front will continue rain for Kodiak and the
north Gulf coast today, before a pattern changes comes on
Thursday bringing the first good chance for accumulating snow in
around the Cook Inlet region. Most of the mainland will see high
clouds overhead with no precipitation to speak of today into
Thursday. The gale force front draped from east to west across the
Gulf will fall apart today spreading light rain to coastal areas
as it weakens.

Late tonight a trough digging southward from the Bering Strait
drag arctic air through the southwest mainland and across the
Alaska Range. Moisture will be in place overhead from this
previous pattern/front when lift from the approaching trough
begins to develop precipitation from northern Cook Inlet into the
Susitna Valley. The potential snow looks to be a classic cold air
advection event from the west, though many places could start as
rain on Thursday afternoon. Colder arctic air will slowly filter
in from the west through the afternoon and evening, the rate of
which will affect how quickly rain turns to snow and ultimately
total accumulations. Expect a gradient of accumulation from west
to east, with highest amounts along the Anchorage Hillside and
upper elevations of the Chugach. Details of the evolution of this
system have yet to be resolved, but from this forecaster's synoptic
perspective the event looks to be a 1-4 inch type event. Stay
tuned...

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The strong north to northwesterly winds pouring through the gaps
and passes in the western half of the Alaska Peninsula will begin
diminishing later this morning as the surface low centered just to
the southeast of the Shumagin Islands tracks away to the south and
then southeast. Over mainland southwest Alaska, areas of light
rain and snow associated with the frontal system stretched from
Bristol Bay northeast and inland will taper off as the front
dissipates today. From tonight through Thursday night conditions
over mainland Southwest Alaska will remain mostly dry with
additional cold air aloft moving into the area accompanying an
arctic trough Thursday afternoon through Thursday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A frontal system out along the western Aleutians will continue
east to the central Aleutians this afternoon and eastern Aleutians
on Thursday before getting pulled into the large upper level
trough in the Gulf of Alaska. The next frontal system will
approach the western and central Aleutians from the south on
Thursday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
As the cold upper level trough moves over the Mainland on Friday,
ridging will build in from the Bering and the combination of the
trough, cold air, and the ridge will bring outflow winds to
Southcentral Alaska from late Friday into early next week.

The biggest question mark for the extended timeframe is what will
happen with a low moving into the Bering early next week. The
models all agree that a low will come in, but there are
significant disagreements on timing, intensity, and location.
One of those is a 40 mb difference of the low center between some
of the models leading to low confidence in how deep this system
will be. Because of these differences the ensembles were
preferred.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 120 130 131 132 138 155 160 165 170 172.

&&

SYNOPSIS AND MODEL DISCUSSION...MMC
AVIATION...DEK
SOUTHCENTRAL ALASKA...MTL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...DK/DEK

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