Small Craft Advisory 28, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
533 PM AKST Tue Jan 8 2019

.ANALYSIS AND UPPER LEVELS...

A weak upper level ridge remains in place over the eastern Bering.
Accompanying this ridge, persistent low level stratus and fog have
blanketed Southwest Alaska just behind a weak shortwave that is
bringing snow showers to areas across Southwest and the northern
AKPEN. On the west side of this ridge, a front associated with a
departing Kamchatka low continues to bring moisture-laden
southerly flow over the central Aleutians, extending as far north
as Saint Matthew Island. Behind this front, cold air that has
wrapped around the low continues to generate gusty westerly gales
over the western Aleutians. A notable feature that will likely
take center stage later in the week is a strong Arctic low
currently centered well north of the state in the Beaufort Sea.

&&

.MODEL DISCUSSION...

Models remain in generally good agreement for the near-term. Minor
disagreements arise by Thu evening regarding the position of a
North Pacific low as it approaches the western Aleutians.
Currently, the NAM/GFS solution is a bit slower than the EC. On
the contrary, model solutions agree with the location of the
strong Arctic low moving south and encompassing most of the state
by Fri morning.

&&

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

&&

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

A quiet dry weather pattern continues across Southcentral Alaska
for the next few days. However, there are a few features that are
worth discussing. First, radar shows some light snow ongoing
across the Southern Susitna Valley that is spreading northward
this evening. Any accumulations from this will be very minor, as
most of the forcing and the moisture is out over Southwest Alaska.
A few flurries cannot be ruled out this evening anywhere from
Anchorage north. This same area of snow is expected to merge with
an incoming Arctic upper low diving southward out of Interior
Alaska on Wednesday. The whole system will then track
southeastward across the Alaska Range into Southcentral on
Wednesday night. However, nearly all of the associated snowfall is
expected to remain on the far side of the Alaska Range. So by the
time it tracks across Anchorage and the Mat-Su, little more than
flurries are expected. A more notable impact by the passage of
this Arctic front/upper low will first be a ramping up of all of
the coastal gap winds, from Seward to Whittier and over to Valdez
and the Copper River Delta. Since the front will be tracking from
west to east, the strongest winds are expected through Whittier
and Passage Canal, with westerly wind gusts up to 55 mph
currently expected Wednesday night into Thursday morning. The
winds will only slowly taper down through the day on Thursday.

Behind the front follows the other big weather story, another
cold snap where all inland locations drop below zero. Temperature
readings as cold as 40 below are expected for the western Susitna
Valley, the Cantwell area, and the Copper River Basin around
Glennallen Thursday night into Friday morning. Another morning of
lows from 20 below to 30 below is expected for the Matanuska
Valley. Around Anchorage, look for lows to drop to near 10 below
on the west side of town to 20 below on the east side. Looking
ahead, the cold snap will have about that short of a duration,
however, with a big warm up expected this weekend.

&&

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

Warmer air from the Gulf will continue to flow northward ahead of
a weakening front over the AKPEN tonight into tomorrow, while
cold offshore flow dominates at the surface. This will bring snow
to the Bristol Bay and lower Kuskokwim Valley areas tonight into
Wednesday, with around 3 inches expected in Dillingham and closer
to 5 inches expected in Sleetmute. The heaviest rate will likely
occur this evening in Dillingham, then the area of heaviest
precipitation will gradually lift north past Sleetmute after
midnight. Lingering cloud cover behind the front will prevent
temperatures from dropping much during the day Wednesday, however
the Arctic high over much of the Interior will strengthen, and
offshore flow will continue to dominate. So, expect temperatures
to drop significantly overnight Wednesday into Thursday with lows
20 to 30 below in the lower Kuskokwim Valley and 10 to 20 below in
the Y-K Delta.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A front associated with a Kamchatka low will continue to bring
gales north of St. Matthew Island tonight, then winds will
diminish and the Bering Sea will be quiet until Wednesday night.
Ahead of the weakening front, expect warm air from the North
Pacific to stream northward, bringing snow showers to the Pribilof
Islands Wednesday into Thursday. A low east of Japan that can be
seen developing on satellite is expected to rapidly deepen, with
models depicting the center dropping more than 50mb over 24 hours.
This low is expected to bring storm, or even hurricane force
winds to the central Pacific, but by the time it reaches the
Aleutians it will have started weakening. Thus, hurricane force
winds are not expected, but storm force winds are expected to
arrive in the western Aleutians late Wednesday and persist through
much of the day Thursday.

&&

.MARINE (Days 3 through 5)...

Beginning Friday, there is high confidence that a weakening but
still strong North Pacific low will be centered just south of the
Rat Islands of the Aleutians. A wide swath of easterly gale force
winds will cover most of the Western Bering with an area of storm
force winds expected to be impacting the western third of zone
413 and the southern half of zone 411. The low is expected to move
to the west then slowly push southwest of Shemya Friday night.
This will bring storm force winds south across the western half of
zone 178, including Shemya and Attu Island Friday evening through
Saturday morning. As the low continues to push further west,
another, albeit weaker, low is expected to move north across the
central Aleutians. There is still a fair amount of uncertainty
with this system, thus lower confidence, but there is overall
agreement that northeasterly gale force winds will continue across
a good portion of the western Bering Sea, with the potential for
areas of southerly gales to develop east of the Pribilofs by
Saturday night. Additionally, with the next system pushing north,
this should keep cold arctic air, and thus heavy freezing spray
potential, confined to the Northern Bering Seas, mostly over zone
185 over the weekend.

Over the Gulf of Alaska, conditions are expected to remain rather
tranquil before a front sweeps north across the area on Sunday.
Right now, winds are expected to remain predominately below gale
force, though there is still a fair amount of uncertainty regarding
the timing and track of this system.

&&

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

The long term forecast begins Saturday with a big pattern change
in store for the southern mainland and Bering Sea. Brutally cold
arctic air under an upper level 'Polar Vortex' low will be
retreating north as the low is shoved north into the Arctic Sea by
a building inverted upper level ridge from the south. At the same
time, an upper level low will begin to build back in over the
Bering while a long wave ridge will build further east over the
US west coast. This will set the stage for deep southerly flow to
push north into the southern mainland by Sunday, bringing warm
air and plenty of moisture with it. Models depict multiple surface
features rotating around the base of the upper low into mainland
Alaska, though details surrounding these features are still murky
due to model inconsistencies. The first feature that both the EC
and GFS models are keying in on is a surface low (GFS) or short
wave trough (EC) moving north from the North Pacific and across
the AKPEN and southwest mainland around Sunday night. The track
with this system is vital to knowing the specific impacts. For
example, the GFS solution brings the low a bit further east over
the AKPEN, which is historically conducive for bringing a Chinook
wind into the Anchorage area. Meanwhile, the EC solution would
likely keep winds out of Anchorage and temperatures a bit cooler.

Regardless, there is high confidence that the pattern change will
bring much warmer and wetter weather to the region through at
least the middle of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Heavy Freezing Spray Warning 121 127 129 130 139 140 160
									  180 185.
		 Storm Warning 177 178.
		 Gale Warning 127 176 185.
FIRE WEATHER...NONE.

&&

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