Small Craft Advisory 1041, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 PM AKST Thu Dec 27 2018

.ANALYSIS AND UPPER LEVELS...
After a few quiet days, weather is finally returning to
Southcentral this afternoon. A strong low pressure system is
moving into the northern Gulf of Alaska, and continues on a
northeasterly track toward the AlCan border. Ahead of the low, an
inverted trough is bringing precipitation to the Southcentral
region, which is falling as rain along the coast and as snow
further inland. This trough is expected to develop into a
deformation band and remain fairly stationary over the Anchorage
area through the afternoon.

A vertically stacked low south of the central Aleutians is
responsible for another area of precipitation today. Most areas
have seen rain switching over to snow as temperatures dropped
below freezing earlier this afternoon, which will likely continue
to be the trend into the evening as the low moves eastward.

An upper level trough over the eastern Bering is dropping down
bringing colder air and northerly flow as it shifts east towards
the mainland. High pressure remains over the western Bering, with
a strong front approaching the western Aleutians.

&&

.MODEL DISCUSSION...
Model consistency continues to be lackluster this afternoon. The
current low moving into the Gulf continues on a track that is to
the northeast of what we saw yesterday. Nam and NamNest seem to be
initializing the best with current track so these were used for
the short term forecast in the east. Out west, the position of the
low heading toward the Aleutians is more consistent between
models, however the strength varies greatly between models. A
model blend was used for this afternoon's forecast, with a
slightly weaker trend on the winds coming into the Aleutians
tonight.

&&

.AVIATION...
PANC...Snow will taper off through the evening in the Anchorage
bowl, with slightly improved visibility. Expect fog to develop
later tonight and bring conditions to IFR. That said, there is
some uncertainty in vsbys as lingering midlevel clouds could
preclude fog formation.

&&

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

A surface low east of Middleton Island will continue to produce
widespread rain and snow with gusty outflow winds from Passage
Canal and Prince William Sound east overnight. An upper- level
shortwave along with a weak deformation zone nearly stationary
along the Cook Inlet continues to produce light snow from Homer
north through the Anchorage Bowl and lower Mat-Su valleys, where
in general one to two inches of snow has fallen. Both systems will
weaken as they progress to the northeast overnight, allowing the
snow to come to an end and the outflow winds to diminish. An
additional one to two inches of snow is possible from Anchorage
through the Mat-Su Valleys overnight. Farther east, rain will
change to snow as colder air wraps in the backside and
precipitation tapers off. Areas of fog are also possible along
northern Cook Inlet overnight into tomorrow morning as a weak
ridge builds in behind the trough.

The next storm system then tracks over Kodiak Island Friday and
along the eastern Kenai Peninsula Friday night. The surface low
will slowly strengthen as it tracks to the northeast and have
decent support from a negatively tilted, upper-level shortwave
and strong cold air advection. Another period of gusty winds
through the gaps and passes, along with a brief period of moderate
snow is expected late Friday through the first half of Saturday.
This system, however, will move through the region quickly, so
snow amounts overall will remain below advisory level, with
accumulations around 2 to 4 inches expected from Seward and Kenai
north across Anchorage and into the Mat-Su Valleys.

The active weather will continue into the weekend as a frontal
system approaches the Gulf Saturday night into Sunday. A deep
storm out along the Aleutians will help amplify the pattern, which
will ultimately lead to warmer temperatures and strong
southeasterly gap winds.

&&

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

An upper level trough extending across the Bering is anchored by a
low just south of Cold Bay. This low will continue to advance
northeastward into Southwest through the next 24 hours. The
associated warm front will bring a surge of moisture into the area
in the mid-levels of the atmosphere, while cold Arctic air
continues to stream in from the north at the surface level. This
will allow for accumulating snowfall across the northern Kuskokwim
Valley through Fri evening, with the greatest amounts expected
along the western side of the Alaska Range north of Dillingham to
Sleetmute. As a result, a winter weather advisory has been issued
for this area with this afternoon's forecast package. Looking
farther south over the AKPEN, it appears that enough warm air
associated with the warm front is in place to allow for mostly
rain or possibly a mix of both rain and snow for a short period of
time on Fri morning. As this low departs and moves eastward by
Sat morning, a brief ridge of high pressure will move in, allowing
for cold Arctic air to return. By Sat night, the pattern will
shift as a strong front with southerly flow arrives to the
Southwest coast.

&&

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

Several strong low pressure systems will impact the Bering and
Aleutians over the next few days. The first low is currently
centered just south of the eastern Aleutians and will move
northeastward into Southwest through the next 24 hours. As this
low departs, northerly flow allows for cold Arctic air to return
over the Bering through Fri morning. Meanwhile, a stout front
associated with a 940 mb Kamchatka low arrives over the western
Aleutians by late tonight. This front will stall out and quickly
occlude by Fri afternoon. In the wake of this front, a second low
develops south of Shemya at 956 mb Fri afternoon and strengthens
to around 945 mb as it crosses the western Aleutians on Saturday
morning. Winds and sea states are the main concern with this
system, with wide swaths of storm-force winds flanking the center
of the low. Another item of concern regarding this strong low is
the amount of cold air wrapping around the base of low, amplifying
a southwesterly low level jet over the western Aleutians. Right
now, model guidance places the strongest winds associated with
this jet just to the west of Adak. A slight easterly shift of the
low will place the jet directly over Adak. For now, have issued a
High Wind Watch for the Western Aleutians for late Friday night
through Saturday morning.

&&

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

A wet pattern is on tap for this forecast period. The forecast
period commences New Year's Eve day with a broad trough over the
Bering and Aleutians and a high amplitude ridge from the Eastern
Pacific to the Yukon Territory. Meanwhile at the surface at 12z
Monday, both the GFS and the ECMWF have a potent low west of Adak
and another low over SW AK/Bristol Bay with cyclonic flow
dominating the entire Bering and much of the North Pacific. At 500
mb at 12z Monday, both the GFS and the ECMWF have closed lows in
support of the surface lows with an amalgamation of embedded
shortwaves aloft.  Complicating matters, the GFS has an
additional closed low at 500 mb north of St Paul whereas the ECMWF
has a minor ridge axis. While the timing of the precipitation
shield may vary as it moves from west to east, what's noteworthy
is that warm air will be advected into the Aleutians, Southwest
Alaska and portions of Southcentral. By Tuesday, a new surface
low will spin up south of the AKPEN and this storm is headed to
the Gulf Of Alaska. By 00z Wednesday the GFS and the ECMWF progs
this new surface low 200 NM apart from the other at opposite ends
of the GOA. The timing of the sensible weather highlights are
dependent on the placement of the lows that develop and on the
ensuing track.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory For Snow: 152 161.
         High Wind Watch: 187 191.
MARINE...Gales: 119 130 131 170 172 173 174 351 412 414.
         Storm: 175 176 177 178 411 413.
         Heavy Freezing Spray: 160 180 181 185.

&&

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