Small Craft Advisory 51, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
757 AM AKST Tue Jan 15 2019

.ANALYSIS AND UPPER LEVELS...A broad upper trough remains in place
over the Bering. A vertically stacked low embedded in the trough
continues to sit nearly stationary in the southern Bering Sea.
Strong north winds and cold air advection on the backside of the
low is bringing snow to the central Aleutians and cold air
wrapping around the low is bringing showers to the eastern Bering.
An occluded front associated with the low extends along the Bering
coast of Southwest Alaska toward the AKPEN. At the surface, high
pressure to the east is enhancing the pressure gradient, causing
strong south winds and warm air advection over the AKPEN and
Southwest. This is facilitating precipitation mainly in the
mountains and coastal areas, while downsloping is keeping most of
the Bristol Bay and inland Kuskokwim Delta area dry. Looking
farther east, a low is shifting from the north Pacific into the
Gulf. This has brought a transition from south flow to general
east flow in the Gulf, thus conditions in Southcentral are
trending colder than yesterday.

&&

.MODEL DISCUSSION...Models are resolving current weather well and
are generally in good agreement for the short term. HiRes
guidance was used for wind fields with this forecast package.
Models start to diverge with the setup of a north Pacific low
Thursday. The trajectory in the GFS and NAM is farther north than
the EC, thus GFS and NAM indicate gales in the eastern Aleutians
while the EC is not quite as strong. Friday into Saturday, another
potential gale force low will develop, and shift northward,
according to the GFS and EC. The CMC does not have this feature,
and decided to lean towards GFS and EC based on ensemble
solutions.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. A rain/snow
shower may develop near the terminal Wed morning, but this should
not have any impact on the conditions at the airport.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Showers have diminished and skies have cleared out for much of
Southcentral Alaska ahead of a new low moving into the Gulf
today. This low will bring precipitation back to the north Gulf
coast with rain along the coast and snow at higher elevations. The
shortwave associated with the low will track inland tonight
diminishing some of the precipitation. However, another low south
of the Alaska Peninsula will help bring in a stream of moisture to
the western Gulf following the first low. The models are having
some difficulty handling this setup leading to uncertainty as to
where the majority of the precipitation will be Wednesday into
Thursday. Most of the guidance has either the southern part of
Kenai Peninsula or Kodiak Island receiving the most precipitation
with most of those favoring Kodiak Island for the majority of
that time period.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Monday
through Thursday)...
The frontal system along the coast this morning will drift
offshore and dissipate today with the gusty east to southeasterly
winds diminishing somewhat through the day and then shifting more
northeasterly tonight. One weak easterly front wave will rotate
across the Alaska Peninsula tonight followed by a slightly
stronger one midday Wednesday which will extend some light
precipitation up into the Bristol Bay area. A more consolidated
wave will rotate through Wednesday evening through Wednesday night
with a small lee side surface low forming along the north side of
the Alaska Peninsula and precipitation (mostly snow due to the
northeasterly flow) extending north up to the Kuskokwim Delta. A
deformation band will move slowly from east to west across
Southwest Alaska Thursday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3/Tuesday through Friday)...

The remnants of a once strong low remain near the Pribilof
Islands this morning. A deformation axis is also pushing westward
and interacting with the remnant low, with a focused area of rain
pushing over the Eastern Aleutians this morning and moving west
during the day. This region will remain the focus of active
weather for land areas as colder air behind the low moves into the
deformation axis over the Eastern Aleutians. Rain will change
into snow threats and accumulating light snow Wednesday continues
through Thursday night as a series of upper waves retrogrades
westward over the region. Beyond that, expect scattered rain/snow
showers to impact most of the Bering and Aleutians as broad
northerly flow encompasses the entire region. A gale force warm
front will reach the Western Aleutians Thursday morning then stall
and weaken Friday.

&&

.MARINE (Days 3 through 5...Thu through Sat)...
The main features across the marine waters to start this time
will be a front over the Western Bering and a filling low south of
Kodiak Island. Confidence is high that the front will contain at
least small craft level southerly winds, but they could reach low
end gales.

By Sat, things start to get a little more interesting. Models all
depict a new low developing near the Central Aleutians. However,
they vary wildly in the strength and placement of this feature.
It does look like the system that develops out there will bring a
return to gale force winds by late Sat. However, it would be
nearly impossible to say exactly where or how strong the strongest
winds will be. For the Gulf of AK, models also show a low
developing just north of 50N. This will certainly shift flow to
become easterly across most of the eastern waters. At this point
winds should be able to achieve at least small craft levels. Some
of the forecast models depict a stronger low that would bring a
closed circulation with gale-force winds.

&&

.LONG TERM FORECAST (Days 4 through 7...Fri through Mon)...
This time period will commence with relatively quiet weather
across much of the state. A negatively-tilted ridge of high
pressure will push into the southern mainland from SE Alaska. This
will keep the predominant flow out of the south and help to keep
temperatures at or above seasonal normals. For SW AK, it does look
like you all will see the best chance for wet weather as some of
the moisture ferried up by the southerly flow pushes over the
Aleutian Range. With warmer air, it is tough to say exactly what
type of precipitation will fall, but for now, it is looking like a
mix of rain and snow. Over the Bering, one front will decay and
elongate over the Eastern Bering while another develops in the
Western Bering.

Model uncertainty starts to grow quickly over the weekend. The 2
key features will be a cut off low over the Northern Interior and
a trough over the Gulf of AK. Models differ in how fast they want
to move the cut off low north through the state. When they do this
will dictate how north the trough and associated upper level jet
can move back over the southern mainland. At this time, we will
likely keep a broad brushed approach to PoPs, except for areas
along the coast where wet weather seems more certain. It also
looks reasonable at this time that the next active weather period
will come from the Gulf of AK. This means temperatures will
continue to be mild with the threat of rain mixing with the snow.

The Bering will continue to repeat a similar pattern to what we
have seen the past few weeks. One parent low will slowly stall and
fill over the Western Aleutians. As cold air wraps around it and
jet activity provides upper level support, a few new surface lows
could spin up quickly over the Central Aleutians. While some of
these systems could be moderately strong, they will only be able
to maintain that intensity for a 24hr period or so as they quickly
move north away from support.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 130 412
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MM/JA
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
MARINE/LONG TERM...MSO