Small Craft Advisory 1030, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 PM AKST Sun Dec 25 2016

.ANALYSIS AND UPPER LEVELS...
A zonally-oriented trough remains over all of southern Alaska
this morning. It is centered with an upper low that is oriented
over the Bering, and extends eastward over the remainder of
southern Alaska. This setup is drawing Gulf moisture northwestward
into the coast. The lion's share of the moisture remains over the
mountains, with only flurries or an occasional snow shower
reaching the lee-side communities, including Anchorage.

Warmer air associated with the influx of Gulf moisture continues
to very gradually push westward. Many higher elevation communities
in the mountains are above freezing now, with Palmer having gone
above freezing for the first time in a week earlier this morning.
For the Anchorage bowl and the northern Kenai Peninsula,
temperatures continue to gradually warm through the 20s. With the
front acting as the lifting mechanism for both the snow and the
strong winds moving northward towards Southcentral, winds have
been diminishing through the morning.

For southwest Alaska, the rain/snow line is roughly following the
surface freezing line. Thus, almost all coastal communities along
Bristol Bay are now in the mid 30s with rain. Warmer air also
extends northward along the Alaska range, with Iliamna and
Sparrevohn both in the upper 30s. Colder air remains further west
along the Kuskokwim Delta with Bethel and coastal communities
further west in the upper 20s to around 30.

Over the Bering, the nearly stationary low over the central Bering
continues to gradually weaken. However, a highly energetic jet
stream located south of the Aleutians is transporting wave after
wave of energy along the chain, one of which will eventually pick
up the low and move it south and eastward.

&&

.MODEL DISCUSSION...
The models remain in fairly good agreement short-term as the
atmospheric pattern remains nearly steady-state, with a large
stationary low over the Bering driving the weather. However, some
differences persist. The next low to develop in the Gulf is
forecast by all the models to move up the coast into Prince
William Sound. The question that each of the models seemingly have
a different answer to is what happens from there? While the models
linger the energy associated with the low in the Sound, where the
exact center of the low goes will determine how much precipitation
falls in the communities surrounding Prince William Sound. All of
the models except the GFS take the initial storm center eastward
towards the Copper River Delta by Tuesday morning.

Beyond that, the models diverge even more. The GFS takes the low
south out of Prince William Sound and redevelops it as it merges
with another wave of energy that tracks northeastward across the
Gulf into Yakutat. The GEM, EC, and NAM all show the low dying as
it moves into the eastern Chugach, but a remnant low or broad
surface troughing persists over Prince William sound into Wednesday
when a stronger branch of the polar jet dives southeastward and
takes the low along with it. The GEM keeps a much stronger remnant
low further west, eventually depicting precipitation as far west
as Anchorage, while the EC and NAM show a remnant low that is
weaker and much further east, with much less in the way of
precipitation in all locales. With such broad divergence in the
models, this part of the forecast is low confidence. The NAM/EC
solution was the preferred for this forecast cycle, being a mid-
way point between the other models, but also being the models that
are most in agreement with each other through the forecast period.

Further west over the Bering, all of the models are in much better
agreement with the smaller features. This is most likely due to
there being fewer smaller scale discrepancies in that area that
normally lead to the model divergence seen further east.

&&

.AVIATION...
PANC...Low level easterly flow across the Kenai and Chugach
Mountains will weaken overnight tonight. This will allow ceilings
to lower on Monday with an upper level trough moving through.
There may be some flurries or very light snow, but VFR conditions
are expected to prevail.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The warm air advection has peaked and expect temps to hold steady
or slowly fall from her on out. This will cause a transition from
rain back toward snow for sea level locations across the Eastern
Kenai Peninsula, including Eastern Turnagain Arm over to Whittier
as well as Moose Pass down to Seward. As the weather front loses
it's upper support, the channeled terrain gap winds will diminish
heading into the overnight hours. The bulk of the precipitation
will continue to fall over northern Gulf coastal communities.
However, as strong low level easterly flow diminishes and upper
trough tracks across the Kenai Peninsula expect to see more areas
of light precip across the Western Kenai Peninsula. Some flurries
or very light snow may also make its way northward to Anchorage
and the Mat-Su on Monday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The main forecast problem today for the southwest mainland remains
the duration of precipitation this evening, and the subsequent
development of fog. The front that stretches across much of the
Southwest mainland continues to fall apart, and as a result models
are showing a range of solutions in duration of showers and the
intensity. As a result, a blend of models was chosen. Broad moist
southwesterly flow across the region certainly makes the development
of fog more likely tonight and early tomorrow morning. However,
shear available near the surface and isolated disturbances aloft
may disrupt fog formation. As a result, currently just areas of
fog are forecasted, but fog could become locally dense tonight in
sheltered valleys.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The upper level trough that encompasses much of the Bering will
continue to be the main weather driver for the next several days.
Currently an upper level wave is poised to push from the Western
Aleutian to the central Aleutian Islands. Cold air advection and
downward vertical motion are expected to occur behind this wave,
and support broad gusty winds along the Aleutians through the
short term. Widespread showers are expected to occur due to the
persistent low, and broad cold air advection.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A big pattern change with possible warmer air and rain is on the
way for much of Southern Alaska. Beginning on Tuesday night, the
upper level low pressure currently responsible for active weather
over the northern Gulf region will still be in the area, but on
it's way out to the east. Isolated to scattered showers will
linger around the Gulf, while inland areas will not see much
outside of cloud cover. Winds will remain relatively light with a
weak outflow pattern along the coast taking hold. Out west, the
instrument of pattern change will be entering the Bering. A large
deepening low pressure system will be sending a warm front from
west to east, pushing onshore the mainland Wednesday night,
starting as snow, quickly changing to rain. These events set us
up for the extended forecast.

A big pattern change is progged by the models with the
aforementioned system. It will pump up a large ridge over the
Bering Sea first, then over Southcentral Alaska. Guidance is in
agreement with this longwave feature. The uncertainty comes with
the extent of warm air involved and it's push into the mainland.
The two camps of solutions at this point are as follows: one
bringing the warm front through Southcentral AK from southwest to
northeast which would bring much warmer air and rain, and the
other, shunting the warm air just to the south in the Gulf which
would severely limit the amount of moisture available, but
precipitation would stay snow rather than rain. The high-confidence
portion of the forecast would be pattern amplification, followed
by a strong cold-front passage. Guidance all hang on to an
amplified pattern for the remainder of the forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 119 174 176.
 		 Gale Warning 125 130 139 155 170 171 172 175 177 178 185.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...EN
LONG TERM...MTL