Small Craft Advisory 25, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKST Mon Jan 6 2020

.ANALYSIS AND UPPER LEVELS...

The Alaska Mainland remains firmly entrenched into a deep arctic
air mass as a 506 dm 500 hpa closed low remains in place, centered
over the Alaska Range. 850 hpa temps are still quite cold, around
-17 to -23 C across the Southern Mainland. Temps are running well
 below average across the region, with Skwentna and Sleetmute
 taking the top cold spots in Southern Alaska at -39 F and -42,
 respectively, this morning. Mostly clear skies are promoting
 these temps, but there are some local areas of fog and low
 stratus across parts of the AKPEN, Kenai Peninsula, and around
 Anchorage.

Over the Bering Sea, a second arctic trough aloft is centered over
the northern Bering Sea with a mesoscale surface low centered just
northeast of the Pribilof Islands. Most of the Bering water is
still covered by convective snow showers given the cold air aloft
over warmer Bering waters. Finally, over the open North Pacific
south of the far western Aleutians, there is a bomb cyclone
developing southwest of Attu/Shemya Islands. This developing warm
seclusion will wrap up well west of the western Aleutian Chain,
but the strong storm force occluded front will impact Shemya
today.

&&

.MODEL DISCUSSION...

The numerical models are in excellent agreement with the general
synoptic pattern through Wednesday. Forecast confidence is high up
to this point as cold temperatures will remain in place over the
Mainland. Discrepancy does develop over the Bering and Aleutians
as a strong front moves east as a series of triple point lows
spin-up along the front. However, the general idea is all the same
and the sensible weather impacts are relatively high confidence,
as well. The GFS and NAM Nest were favored models.

&&

.AVIATION...
PANC...Fog remains the big challenge around the airport and
likely will through this afternoon. As has been the case, it will
likely be fluctuating between periods of VFR and LIFR.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Mon and
Tue)...
The pattern remains mostly unchanged featuring small lows in the
Gulf of AK bringing quick bursts of snow with very cold air over
much of Southern AK. First, the next in a series of fast moving
lows will track into the Cordova area this morning. This could
bring some light accumulations to Cordova through McCarthy, but we
are not expecting more than about an inch. This system will
reinforce the offshore pressure gradient leading to continued cold
and dry conditions as air is pulled down from the Interior through
Southcentral AK. This will also keep breezy winds in places like
Whittier, Valdez, and Resurrection Bay. Another weak wave will
transit the Northern Gulf on Tue. This system looks to stay away
from all land areas. Thus, expect the very cold conditions to
remain through the week with the only interruption to these
temperatures being the fog and stratus hanging around Cook Inlet.

By late Tue into Wed, the cold core upper level low responsible
for this airmass finally shifts SE off the mainland and into the
Gulf. This set-up bears watching as it could mix down much
stronger winds through Thompson Pass, Valdez, and the Northern
Gulf. As of the current forecast models, this system would not be
the classic (NE-SW) orientation and would not have the upper level jet
support to bring the strongest winds to these locations. However,
it certainly looks to take advantage of the thermal gradient more
than we have seen in the past few days.

&&

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

An upper-level shortwave and associated weak surface low east of
the Pribilofs continues to slowly push southeast this morning. As
this features moves toward the AKPEN, areas of low stratus and
scattered snow showers will persist south and west of King Salmon
through Monday evening. Elsewhere across the interior, high
pressure will keep arctic air anchored in place through the next
couple of days with only isolated areas of fog lingering in the
Kuskokwim Valley and along the Alaska Range. By Wednesday,
temperatures will begin to moderate as a warm front advances east
across the Bering. By Thursday morning, this front looks to bring
some snow (with a possible changeover to rain for the southern
AKPEN) and an increase in southeasterly winds to coastal
Southwest Alaska.

&&

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

A storm-force low well southwest of Shemya this morning will
sweep a front over the western Aleutians late Monday into Tuesday.
As it does, high-end gales will develop from Atka west along with
an initial burst of snow changing over to rain as warmer air
advect over the western Chain. By early Tuesday, a triple-point
low forms along the occluded front and lifts north, just east of
Atka. This will push precipitation in the form of snow changing
to rain as well as gusty southeasterly winds across the central
Aleutians through Tuesday. The low then weakens south of the
Pribilofs as the front stalls over the eastern Aleutians and
southern AKPEN Tuesday night before both dissipating by Wednesday.

A second gale-force front then lifts to the western Aleutians late
Tuesday into Wednesday as another low spins up south of Shemya.
This system will lift north Wednesday, pushing its front north and
east to Saint Matthew Island and the Southwest Alaska coast by
early morning Thursday. This second system will also usher in
another shot of warmer air, with any snow at the onset of
precipitation quickly changing to rain for much of the southern
Bering and eastern two thirds of the Chain.

&&

.MARINE (Days 3 through 5: Wednesday through Friday)...

Gulf of Alaska:

A bit more activity is expected, particularly on Thursday over the
Gulf of Alaska, as a low tracking across the southern Gulf causes
widespread easterly gales over much of the Gulf, with localized
storms possible through the windier gaps and passes, and possibly
through Shelikof Strait. Otherwise, generally quiet conditions are
expected on Wednesday ahead of the low, and on Friday some
localized gales are possible through the Barren Islands, but
conditions should be quieting down and improving as high pressure
builds.

Bering Sea:

A very active period is expected across the Bering. On Wednesday a
low approaching the western Aleutians may produce gales in that
area, mainly on the Pacific side of the islands. Its leading front
will spread gales across the central Bering Wednesday morning, and
will gradually dissipate as it moves to the eastern Bering
Wednesday evening. The trailing front crosses the central
Aleutians Wednesday night, which may produce a few gales in that
area. The low will rapidly weaken as a much stronger one follows
behind it from the North Pacific. Storm force winds are possible
with the low, specifically through the central Aleutians
(Adak/Atka) Thursday night. Then on Friday morning the storms
shift westward as the low center tracks over the western
Aleutians, just east of Shemya. Meanwhile the leading front will
produce widespread gales over much of the rest of the Bering
through Friday. Storms on westerly winds will wrap around the
south side of the low over the western Bering and Aleutians
through Friday night.

&&

.LONG TERM FORECAST (Days 3 through 7: Wednesday through Sunday)...

The main theme of the long-term forecast will be a warming trend
in the temperatures. The arctic cold air mass over much of the
mainland will begin to shift eastward into Canada, as the storm
track moves into the Bering. This will allow a series of lows and
associated fronts to move into the Bering which will slowly but
surely dislodge the cold air in place over mainland Alaska,
especially over Southwest Alaska.

Starting on Wednesday, a weakening, fully occluded low will
meander across the western Aleutians, as its leading front moves
into the eastern Bering. Cloud cover and a few snow showers are
possible Wednesday night into Thursday along the coast, but the
front will be largely dissipated, so any threats of snow will be
low. A secondary triple point low will develop south of the Alaska
Peninsula Wednesday night, which will track east across the
southern Gulf on Thursday. Impacts should largely be confined to
the Kodiak Island area, as the track keeps any snow further south.
The cold air in place across Southcentral should be pulled south
into the Gulf behind the low, so the gap winds will likely be
strong on Thursday.

Another much more powerful low will move into the western
Aleutians Thursday night into Friday, causing widespread gales and
storms across the chain. There is some difference in the track of
the low in the models, but all are expecting the center of a
strong low to be over the western Aleutians east of Shemya by
Friday morning. The low will continue intensifying as it moves
north of the chain through the day, thus storm force winds are
likely across all of the western Aleutians during the day Friday.
Its leading front will move into Southwest Alaska Friday night,
with an air mass warm enough that rain may not be out of the
question, especially along the coast. The front moves inland on
Saturday, with mixed precipitation or snow possible all across
Southwest Alaska. A triple-point low will develop over the western
Gulf Saturday evening, but where this low develops is highly
uncertain. Its development and the strength of the front will
determine how much, if any, moisture makes it over the Alaska
range to affect the Cook Inlet region Saturday night into Sunday.
The models all agree that the Arctic air mass will persist over
the Copper River Basin through Sunday, so expect the gap winds,
particularly through Thompson Pass and the Copper River Delta, to
remain elevated.

Even if the front dissipates before any moisture reaches
Southcentral, the much warmer air mass and cloud cover are
expected to make it into the Cook Inlet region. This will
allow temperatures to return at least to normal (20s for highs,
teens for lows), if not warmer this weekend. Meanwhile,
temperatures returning to normal could happen in Southwest Alaska
as early as Friday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 176-178 351 411. Heavy Freezing Spray 121 127 129
130 139 160 180 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...Ahsenmacher
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...JPW