Small Craft Advisory 1037, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 PM AKST Wed Dec 26 2018

.ANALYSIS AND UPPER LEVELS...
An impressive looking open trough on satellite over Southcentral
Alaska lifting northward this afternoon has brought very little
in terms of weather. Most inland areas have remained dry, with
some areas receiving a trace of precipitation throughout the day.
Even coastal areas have not received much precipitation. Dry air
aloft sits behind this trough across Southwest today, with just a
few flurries associated with low level moisture. The exception is
near the Aleutian and Alaska Ranges north of Iliamna where some
snow associated with the trough is still occurring.

The central Aleutians have been a bit more interesting as a weak
front associated with a low south of the Aleutian chain lifts
northward this afternoon. Steady snow showers throughout the day
have been observed from Dutch Harbor to Cold Bay as the front
progresses eastward. A second low forming south of the AKPEN is
moving northward. This feature hasn't impacted the area just yet,
but will be a feature to keep an eye on over the next few days.

There is an upper level trough over the northeastern Bering that
will be another feature that will play into the future forecast.
High pressure remains over the western Bering.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement on the synoptic pattern
in general, but differ on some important meso-scale features. For
the surface low pushing northward through the Gulf on Thursday,
the ECMWF is still the farthest east with this feature. The other
models have trended eastward over the last few runs, and the
confidence in the path of this low is increasing. The models also
disagree on a low south of the eastern Aleutians and Alaska
Peninsula for later tonight and Thursday. The GFS is the outlier,
with the surface low farther west than the other models. A strong
low approaching the western Aleutians on Friday and Friday night
also has some differences. The Nam is a definite outlier. The GFS
and ECMWF also disagree on strength and location, but the wind
fields are at least somewhat similar. The forecast will tilt
towards the GFS for this low.

&&

.AVIATION...

PANC...MVFR in snow are expected later this evening, which will
transition back to VFR later tonight. MVFR conditions in snow are
expected again beginning Thursday afternoon.

&&

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

A negatively tilted trough extending across the southern Kenai
Peninsula is slowly lifting north this afternoon as a shortwave
advances toward the region from the west. This trough is
producing a band of light snow from north of Homer across the
central Cook Inlet. Tonight, the snow will lift across the
Anchorage Bowl and over the Mat-Su Valleys with accumulations of
less than an inch expected. In the wake of this trough, a second
system will approach the Gulf from the North Pacific bringing a
strong push of warm air with it. Guidance has come into better
agreement with the track of this low, moving northeast through the
Gulf and making landfall east of Cordova tomorrow evening. Enough
warm air should move over Kodiak Island, eastern Kenai Peninsula,
and Prince William Sound to change any snow falling during the
onset of the precipitation event to a rain/snow mix and then to
all rain, limiting snowfall amounts in locations like Kodiak,
Seward, and Whittier.

Closer to the low, and the core of warmest air, precipitation
should fall primarily as rain. Locations along the lee of the
Kenai and Chugach will see less precipitation as the low advances
toward Cordova due to the development of a cross-barrier flow.
Increasing outflow winds will also become a concern tomorrow as
the pressure gradient between the low and cold high pressure over
the interior increases. Northerly winds will be strongest through
the evening hours in the gaps along Prince William Sound and over
Thompson Pass, where snow and blowing snow could reduce
visibilities to around one quarter mile at times.

A trailing shortwave will lift across southcentral late tomorrow,
producing another quick burst of snow for the Anchorage Bowl and
Mat-Su Valleys (1-2 inches) before weakening. The next system than
moves into the northern Gulf for late Friday with another
rain/snow mix along the coast and the potential for light snow for
the Anchorage area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Saturday)...

An upper level trough is currently in place over Southwest Alaska.
Anchoring this trough is a weak surface low positioned just south
of the Aleutians. At the surface, a weak ridge located over
Bristol Bay is generating offshore flow along the Southwest coast
this afternoon. Meanwhile, a disturbance over the Gulf is
producing onshore flow aloft and generating scattered snow showers
along the Kuskokwim Valley west of the Alaska Range, as some
light returns are visible on radar this afternoon.

By Thursday night, a deformation band sets up along the AKPEN as
the weakening low south of the Aleutians moves eastward into the
Gulf. Accumulating snowfall is expected along the AKPEN beginning
Thursday afternoon. Accumulating snowfall will also spread in
along the western side of the Alaska Range through Friday
afternoon.

As the upper level trough shifts east by Friday evening,
northerly flow will spread in across Southwest on the backside of
the trough. This will draw down colder Arctic air, bringing lows
down into the negative teens and twenties for Saturday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Today
through Saturday)...

Cold air advection persists over the Bering, thanks to continued
northerly flow. A weak area of low pressure south of the
Aleutians continues to slowly drift eastward and promptly
deteriorates by Fri morning as it is suppressed by the strong
northerly flow. In the meantime, however, the warm front
associated with this low has lifted over the Aleutians and into
the southern Bering, generating southerly flow aloft over the
large swath of northerly flow at the surface. This will continue
to lead to scattered snow showers over the eastern Aleutian chain.
Northerly flow picks up by Thursday evening as the departing low
moves into the Gulf of Alaska.

Following this low, a brief ridge of high pressure moves in, but
the main story for the Bering will be the stout front associated
with a strong surface low west of Attu Island, pushing across the
western Aleutians beginning on Thu evening. Some model runs
indicate the low center strengthening to sub-940 mb. This will
generate a pattern change across much of the Bering as southerly
flow with a swath of storm-force winds advances eastward through
Fri. By Sat afternoon, another North Pacific low moving up from
the south is centered between Shemya and Adak, bringing storm-
force easterly winds across areas of the western Bering while
strong gale to storm-force westerlies persist on the south side of
the western Aleutians. These strong winds will generate
increasing wave heights, possibly approaching 40 feet south of the
western Aleutians on Saturday.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The forecast period commences with a longwave ridge from the Gulf
Of Alaska to the Brooks Range with a deep upper level trough over
NE Russia and the Western Aleutians. Meanwhile at the surface at
12z Sunday, both the GFS and the ECMWF have a deep low between
Attu and Adak with a post frontal surface trough extending towards
Hokkaido Japan. Where these two models differ at 12z Sunday is
east of the parent low. The GFS has another surface low over
Bristol Bay and the ECMWF simply has weak troughing. Also the
ECMWF has a secondary low feature south of the chain which the
GFS doesn't have at all and then the GFS spins up a low by 18z
south of the ECMWF's position. This does not bode well for the
confidence level with the remainder of the longterm forecast
package since the divergence amplifies as time elapses. There is
good confidence that this will generally be a wet and windy
pattern for the Aleutians, but the timing of the sensible weather
highlights are a function of the precise track of the lows that
develop. This forecast package reflects a blend of operation and
ensemble data from the models.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 351 352 119 120 174 177.
         Storm 178.
         Heavy Freezing Spray 160 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...AH/BL
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
LONG TERM...PS