Small Craft Advisory 10, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 PM AKST Wed Jan 2 2019

.ANALYSIS AND UPPER LEVELS...
There is a broad upper low over the Bering Sea into northeast
Russia. The Canadian west coast ridge has been temporarily
"squashed", and broad west-southwest flow exists from the
western Bering through the Panhandle. There are a number of
short-wave troughs associated with the Bering low. One of these
is moving slowly northeast through Southcentral this afternoon.
This trough is pushing colder air through southern Alaska. It is
also producing some snow, mainly north of Anchorage. A surface
low associated with this trough is over western Prince William
Sound and this is producing fairly strong winds over the northern
Gulf, southern Cook Inlet and Kodiak Island. Another trough is
over the eastern Bering Sea, with an associated surface low over
the north-central Bering with a front stretching east- southeast
to the Alaska Peninsula. There is another trough and surface low
just north of the western Aleutians.

&&

.MODEL DISCUSSION...
Through Friday afternoon, the numerical models are in good
agreement on the over-all pattern. They also agree fairly well on
the precipitation and surface features over Southcentral through
Thursday afternoon, with just some timing differences. The story
is a little different over the Bering, with the models still
struggling with a number of small scale features. These
discrepancies then slowly work their way east into the southern
Alaska mainland beginning Thursday. There are also some
differences on where they bring up a strong surface low into the
Aleutians/southern Bering on Friday.

&&

.AVIATION...
PANC...VFR conditions are expected, however brief MVFR conditions
are possible this evening in snow showers. Winds will taper down
this evening and then be light through the remainder of the TAF
period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level trough swinging from Southwest Alaska into
Southcentral Alaska this evening will continue snow showers over
the Susitna Valley, Talkeetna and Chugach Mountains and Matanuska
Valley. As the upper trough continues east into the Copper River
Basin, the snow showers will follow along with it, with the colder
and drier air pushing in behind it tapering off the snow showers
further to the west. The breezy west to southwesterly winds across
the area this afternoon and evening will gradually diminish
through the early morning hours as pressure gradients ease in
response to a small ridge building in behind the trough. Offshore
flow will increase Thursday night through Friday as a deep cold
pool becomes well entrenched in the Copper River Basin. A cold,
dry air mass will remain in place across Southcentral Alaska
through at least Saturday.

&&

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

After generally clear skies and colder air have moved in over
most of Southwest today, a weak shortwave trough ahead of a ridge
currently over the eastern Bering will move in overnight tonight,
bringing scattered snow showers to areas throughout the Kuskokwim
Valley through Thursday. As this shortwave continues to move
further inland to the northeast by Friday, the subsequent ridge
arrives for Saturday, allowing for a clearing trend as winds
remain predominately offshore. A few areas across Southwest may
see some potential fog development Fri night into Sat morning,
especially in places where the winds fall slack and skies scatter
out. Toward the end of the forecast period, southerly flow
returns as an occluded front associated with a Bering low
approaches the Southwest coast.

&&

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

A word to sum up the general pattern over the Bering would be
disorganized. Several areas of cyclonic flow are spinning adrift
of a weakening main low centered north of the Pribilofs. By
Thursday night, this low dissipates and moves north, making room
for an approaching Kamchatka low to move over the western
Aleutians. Another round of southerly flow is inevitable across
the western and central Aleutians as the leading edge of the front
accompanying the low traverses eastward, quickly occluding over
the eastern Aleutians by Sat afternoon. As the front reaches the
Pribilofs, blowing snow and possible blizzard conditions may
ensue for Saturday. Winds around the immediate low center will
reach gale-force with embedded swaths of storm-force winds seen
across the western Aleutians and Bering Friday night and
Saturday.

&&

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

Beginning the extended period forecast on Saturday evening, there
will be a strong storm force low over the western Bering Sea and
associated upper low with a developing high amplitude blocking
high over Eastern Siberia. Over Southern Alaska, there will be a
series of weak upper level waves merging into a consolidated
trough. With time, the upper trough over the mainland will deepen
in response to the building blocking high over the high latitudes,
so this will keep a storm track over the Bering Sea with cold and
dry high pressure forming over Southern Alaska. This pattern will
last through Tuesday until high uncertainty develops with the
development and placement of an arctic low over Alaska. The ECWMF
and GFS both show a low, but the GFS favors a quicker transition
to southwest progressive flow from the Pacific which would
eventually bring in more precip threats as early as Tuesday
whereas the ECMWF favors a much colder and longer lasting dry
spell, through at least Wednesday. Due to the anomalous ECMWF
upper level pattern, and overall high uncertainty with developing
arctic lows, this forecast will trend in more precip threats
beginning Tuesday. Precipitation threats will likely be all snow
given the cold air which will be in place, but for now, there is
no strong signal.

&&

.MARINE (Days 3 through 5)...

Beginning the extended period marine forecast, the main impacts
will be a developing storm force low over the western Bering Sea
on Friday. The low track is now highly certain with a general
track moving along and then north of Attu Island. The front will
extend northward into the Bering on Saturday and may reach storm
force as the low tracks north. With an easterly component to the
winds, heavy freezing spray may become an issue closer to the ice
edge.

Another powerful storm force low is on track to move into the
western Bering again on Sunday and Sunday night. Confidence with
this second low is quite good, and model trends have remained
consistent.

In the Gulf of Alaska waters, with the storm track over the Bering
Sea, there is high confidence that winds will remain well below
gale force through the period.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales...120 127 130 131 139 141.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP/KO
MARINE/LONG TERM...JA