Small Craft Advisory 938, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
746 PM AKST Tue Nov 27 2018

ANALYSIS AND UPPER LEVELS... The upper level pattern is rather
simple over the state today with a trough extending over the
Bering Sea and Alaska Peninsula and the upper level high well east
of the state. This is bringing generally southerly flow over
southern mainland Alaska and west- to-northwest flow over most of
the Bering Sea and Aleutians. The subtropical jet stream at 250 mb
is running well south of the area, but there is a jet streak
around 100 kt over the Western and Central Aleutians which is
helping to spin up a surface low tracking over the Western
Aleutians this afternoon. This low will track over the Eastern
Aleutians tonight and then south of the Alaska Peninsula tomorrow
afternoon. The main surface low for mainland Alaska is centered
just north of Kodiak Island this afternoon with a trough extending
to the northwest over mainland Southwest Alaska. This pattern is
helping to produce the stratiform precipitation in Southwest
mainland Alaska and showers over Southcentral Alaska.

MODEL DISCUSSION...
Models are in good synoptic agreement and are making some
refinement on mainly the track of the low tracking across the
Aleutians tonight through tomorrow morning. The next item to
really look at is the developing south of the Central Aleutians
Friday and moving through the Bering Sea through Saturday. The
models started developing this low on the model runs yesterday
after not having it at all before that. Models all recognize it
and its tropical origins (currently tropical depression Man-yi
south of Japan). What has changed from yesterday is the low is
tracking farther east and is about 25 mb weaker than they showed
yesterday. What this all means is that there will be a strong low
in the Bering Sea Friday into Saturday, but the track and actual
strength is still very much up in the air.

AVIATION...
PANC...VFR conditions and light winds will persist.

SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
(Tonight through Friday)

More winter-like weather make an appearance over the next few
days, with much colder air and increasing snow potential in our
future. Heading through tonight, a Kodiak low is expected to
travel up the Cook Inlet, before moving ashore while stalling near
Palmer while weakening. Showers will continue through the night,
before changing over to snow tomorrow as an upper trough crosses
the region, allowing for the cold air to make inroads to
Southcentral. Once the trough moves north of the area,
precipitation chances will diminish. Right now it looks like the
changeover from rain to snow should occur between 10 AM and 2 PM,
lasting into the evening. At this time precipitation amounts look
to be very light, with the lower elevations seeing less than an
inch in most locations. The cold air will continue to strengthen
its grip across the region on Thursday, although the core of the
cold air will remain over west of the Alaska Range. Still, we
expect 850 mb temps to drop down into the -10 to -12 deg. F range.

Our attention then turns to a system moving in quickly on its
heels for Thursday and Friday. This system is currently affecting
the west central Aleutians, and is forecast to be located near
Kodiak Thursday morning, reaching Prince William Sound by Friday
morning. This is a favorable track to put the deformation zone
over the Kenai Peninsula, Anchorage Bowl, and Mat-Su valleys.
Precipitation with this storm on Friday looks to all snow, except
for coastal locations near the Sound.

SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Tonight
through Thursday)...

A deformation zone currently over interior Bristol Bay and the
Lower Kuskokwim Valley will drift northeastward overnight and
slowly dissipate Wednesday. This feature is responsible for the
snowfall ongoing across those areas, and why a Winter Weather
Advisory remains in effect through tonight. Behind the snowfall,
much colder air will move into Southwest Alaska. Thus, making for
some of the chilliest days and nights of the season Wednesday
through Thursday. A few of the models are bringing a compact low
south of the Alaska Peninsula Wednesday night, which may bring a
period of light snow south and east of King Salmon. A
warm/occluded front barreling across the Bering will reach the
Kuskokwim Delta on Thursday afternoon. Initially, this feature is
unlikely to contain enough warm air to change the precipitation
type from snow. The Western Capes west of Dillingham and the
Kuskokwim Delta may see a period of snow and blowing snow, which
may produce visibility issues into Thursday evening.

SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Tonight
through Thursday)...

An active and progressive weather pattern persists as two
significant features traverse the area. The first is a compact low
racing eastward across the western and central Aleutians. As it
approaches the Central Aleutians tonight, the center of the low
is expected to track south of the island. This will put Unalaska
under an intense band of occasionally heavy snow starting after
midnight. A rare Winter Weather Advisory is in effect through 10am
Wednesday for 4 to 8 inches of snow. In addition to the heavy
snow, gusty winds will contribute to reduced visibilities. The low
and associated snow move into the North Pacific Wednesday morning.
Behind the low, reinforcing cold air will turn continuous snowfall
to a more snow showery regime. A strong warm front will move into
the western Aleutians Wednesday morning. It will initially be warm
enough for rain to be the dominant precipitation type with strong
winds gusting up to 50 mph expected. The front will gradually
weaken as it advances towards the Southwest Alaska coastline by
Thursday afternoon.

.LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)...

The longer term forecast has a bit more clarity this evening, with
many of the models coming into better agreement. Thus the forecast
this evening had a bigger hint of deterministic forecasting,
versus the cautious ensemble approach. This weekend, ridging
across the Gulf of Alaska will slide into Canada as the (present)
upper level low across the western Bering begins to weaken as it
slides east. Present indications are that southcentral will see
another return to southerly/southeasterly flow as we head into the
early work week next week. It's not looking to be as warm of a
jump as we've seen this past week, but it is still quite early and
guidance could further support this, or dive one direction or the
other. The southwest and Bering will see the bulk of the activity
as a storm will move into the Bristol Bay region Saturday,
followed by a second system that will shoot north across the AKPEN
on Sunday. Temperatures across this region will drastically
change from this mid week's forecast. Precipitation type will be
the biggest challenge as we move from Friday and into Saturday as
the cold air is shoved north, while warmer air allows rain to make
it's way to the ground, north of the Alaska Range.

AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 161 and 185.
MARINE...Gales 130 132 150 155 172-178 185 411-413.

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