Small Craft Advisory 998, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKST Thu Dec 15 2016

.ANALYSIS AND UPPER LEVELS...
The pattern shift is fully underway with the first front oriented
south to north over Southcentral Alaska with a small ridge over
Southwest Alaska and then the powerful low in the Bering Sea. A
strong upper level wave associated with the front will trigger
snow across Southcentral Alaska overnight while Southwest Alaska
will remain in warm southerly flow. The low in the western Bering
Sea is currently in the mid 940s mb and has a strong front ahead
of it moving into the eastern Bering Sea. The jet stream is
running south of the low and is still south of the Aleutian chain
where it will remain the next few days.

&&

.MODEL DISCUSSION...
As has been the case of late the models are doing reasonable as
far as being in decent agreement on any given model run. The
biggest challenge is the changes from run to run we have seen
over the past few days. The 12Z NAM has started a trend to bring a
period of downsloping over the western Kenai Peninsula and
Anchorage areas. This will be Friday afternoon and evening when
the warmest air aloft should be around Southcentral Alaska. This
pattern would keep colder air at the surface and is different than previous
runs. However the GFS is also starting to show this pattern and it
is likely it will pan out this way.

Out west the models are also changing significantly with each
model run. While it is certain there will be a series strong lows
tracking through the Bering sea the next few days. The one
currently in the Bering is well handled but the second one which
will develop rapidly on Saturday and move into the central
Aleutians is giving the models fits. What does look likely is that
it will deepen to around 940 mb by Saturday afternoon, but its
track is still uncertain.

&&

.AVIATION...
PANC...Light snow will begin to move into the area around PANC
this evening and persist throughout much of the night. This snow
will likely keep ceilings in the MVFR category while dropping
visibilities to VFR levels at times. Snow should taper off late
tonight through the early morning hours tomorrow. The
precipitation type is expected to remain all snow throughout the
event.

&&

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

Weak isentropic lift has developed as expected resulting in
lowering ceilings throughout the Cook Inlet region. With favorable
up-inlet 700-mb flow, snow duration and intensity should continue
an upward trend through much of the evening before ending by
Friday morning. Overall, still expecting around 2 to maybe 3
inches for most places in the Cook Inlet region. One point of
concern for potentially lesser amounts along the immediate western
slopes of the Chugach is the development of a lee trough
indicative of some downslope drying. This may curb snow amounts
slightly over East Anchorage for tonight.

Another system will be fast on the heels of this first one, so
expect very little break. In fact, models are all keying on
another area of isentropic lift right behind the first short-
wave. This will bring some additional light precip to the Cook
Inlet to Susitna Valley corridor late tonight through Friday. Flow
will begin to back toward the southeast and strengthen ahead of
the next upper trough and surface front, so areas in the lee of
the Kenai and Chugach will dry out. A steady supply of warm air
advection, combined with downslope warming, will lead to a mix of
precip types Friday through Friday evening. Although there may
initially be some freezing rain or even rain as air above the
surface warms, confidence continues to increase that it will not
be significant or impactful. What will be far more impactful is a
period of cold advection snow on the back-end that will likely
bring wet snow to the Upper Cook Inlet region from late Friday
evening through early Saturday. As warmer air surges up the inlet
briefly on the heels of this front, wet snow may end as a mix of
rain and snow before ending, with high temperatures possibly
reaching the mid-30s for Anchorage and the Mat-Su Valleys Saturday
afternoon before temperatures return below freezing through
Saturday evening.

&&

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

The remnants of the front currently out in the Bering sea will
move onshore southwest Alaska tomorrow (Friday) morning. Precip
type will be a challenge over the Kuskokwim Delta and interior
portions of the Bristol Bay zone where a rain/snow mix will be
the standard. Meanwhile, coastal parts of the Bristol Bay zone
will be warm enough for rain. There won't be a huge amount of
precip that falls from this weakening front, so impacts are
expected to be minor. The interior valleys of the lower Kuskokwim
valley will remain the coolest, and some isolated pockets of
freezing rain can't be ruled out, but generally the atmosphere
should remain just cool enough for snow. By Saturday, winds will
decrease and become a bit more offshore, which will allow some
cooler air to move back into the area, switching any remaining
showers safety back into snow.

&&

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

The strong 949 mb low in the western Bering sea has entered the
weakening phase, and will continue to lose strength as it heads
to the northeast. Satellite scatterometer data confirm a fairly
large area of near-hurricane force winds south of the western
Aleutian islands earlier this afternoon. The remnants of this
strongest part of the storm are passing over the western Aleutian
islands at this time, with winds expected to peak this evening
before diminishing. The weather front associated with this storm
is also beginning to weaken as it moves through the Pribilofs and
eastern Aleutians this evening.

The next major low moves into the area on Saturday, with the
central Aleutians in the crosshairs of this storm. There is still
a lot of uncertainty about track and also intensity of the storm,
but given the 200 kt jet stream south of our area that is helping
to feed the storms in our region lately, a central pressure in
the mid 940 mb range is definitely within the realm of
possibility. Confidence is high that the central Aleutians will
see at least a brief period of very strong winds from this storm,
but certain model solutions show a stronger and much longer
lasting scenario. This will be monitored closely over the next
12-24 hours of model runs, as hopefully the available guidance
will begin to converge on a solution.

&&

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

An approaching zonal jet will slam into the the ridge over
mainland AK on Sat, causing the ridge to weaken and move rapidly
east. This will bring a moderating trend to temperatures to all of our
area of responsibility, as well as increased precipitation
chances. 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...Storms 175 176 177 178 Gales 150 155 165 170 172 173 174
179 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...CC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AD
LONG TERM...DS