Small Craft Advisory 96, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
449 AM AKST Wed Jan 30 2019

.ANALYSIS AND UPPER LEVELS...

An area of high pressure was centered over eastern Siberia into
the Bering Straight, and another area of high pressure was
building north from the western Pacific towards the Rat islands.
A trough of low pressure was sandwiched between these two features
as it stretched from the western Bering, though southwest Alaska,
and into south central.

Closer to the surface, a weakening surface low was centered just
north of the Rat Islands, with another low located along the
eastern Kenai Peninsula and a third low well southeast of Kodiak
Island.

&&

.MODEL DISCUSSION...

Model agreement continues to remain in excellent agreement through
the next few days, with the main trend noted being the Pacific
ridge building north across the Bering as it moves from west to
east. This is due to a deepening low pressure system moving into
the western Aleutians, and another low deepening in the eastern
Pacific to the southwest of Vancouver Island. This results in a
well-defined omega block pattern developing.

&&

.AVIATION...
PANC...VFR conditions should continue over the next several hours.
However, as waves of heavier precipitation retrograde from east
to west, some reductions in ceilings and visibilities to the MVFR
range are anticipated, with perhaps brief periods of IFR ceiling
at times if wet-bulb cooling can allow for snow to develop in the
heavier bands.

&&

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

A broad upper trough over Southcentral and a weakening surface low
near the southern Kenai Peninsula will continue clouds and periods
of mixed precipitation across the North Gulf, Kenai Peninsula,
Anchorage Bowl. Precipitation will lean more toward snow in the
MAT-SU Valleys and Copper River Basin.

An upper ridge builds in from the north tonight and in combination
with a Arctic trough dropping south along the ALCAN boarder
Thursday will usher in colder and drier air. In response,
clearing skies are expected Thursday along with cooler
temperatures. As cold air pours into the Copper River Basin and an
inverted surface trough settles over the North Gulf waters,
strong outflow winds will develop along the North Gulf Coast and
Western Prince William Sound Thursday. The strongest winds will
likely be near the mouth of the Copper River Delta and Thompson
Pass and Valdez areas. Gusty gap winds to a lesser degree are
expected over the eastern Kenai Peninsula, MAT-SU Valleys, and
Mountain passes. Strong winds developing on Thursday combined with
recent snowfall in Thompson Pass could combine to create Blizzard
conditions through the pass beginning Thursday morning and
continuing into Friday morning.

&&

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

Some light areas of precipitation continue to hang around
southwest Alaska. Light snow is falling from deformed band of
cloud cover associated with a trough over southwest Alaska.
Precipitation will taper off today as northeast flow takes over,
which should really dry out the area. It will also drop
temperatures, with the coldest being around -10F in some interior
areas. Patchy fog is also present this morning throughout much of
southwest Alaska. Fog should also be cleared out when northeast
flow increases later today, so today should be the last of it for
the most part. The next couple of days will see pretty quiet
weather with the colder, continental air moving in. Friday will
see the the arrival of the next system out of the Being Sea, more
on this in the next couple of days as the forecast is refined.

&&

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

The next major low is approaching the western Aleutians as
expected. It is still forecasted to bring storm force winds to
the western Aleutians/Bering later today and into tomorrow.
Meanwhile, the central and eastern Aleutians are generally under
an area of disorganized low pressure. One interesting feature on
satellite this morning is a compact low quickly moving east right
along the Aleutian Islands. It does not have much weather
associated with it. Over the next couple of days, a detached front
from the low currently moving into the western Aleutians will
progress east across the Bering Sea and Aleutian Islands. It will
approach the coast on Friday. This front will have a large swath
of precipitation with it, which will be the main feature
associated with it. Since the front becomes detached from the low
center, winds quickly diminish as the front pushes east. The
forecast for the movement of this front is being still being
refined so stay tuned.

&&

.MARINE (Days 3 through 5)...

The operational models all ping into back to back cyclones
developing south of the Aleutians, jumping the chain and then
bringing unsettled weather into the Bering. There are some minor
timing issues but any way you slice it, very confident that the
active weather pattern will continue for the western/central
Aleutians and Bering from Thursday through Sunday. Confidence is
high for a minimum of gale force winds, rough seas and more
precipitation with both storms. The surface low anchoring the
first front will be just south of Attu Island 12z Thursday. The
second surface low will be west-southwest of Adak 00z Saturday and
this low will rapidly accelerate north being west of the Pribilof
Islands by 12z Saturday.

.LONG TERM FORECAST (Days 3 through 7)...

The longwave pattern features a ridge from the Hawaiian Islands
extending into Southcentral Alaska with the longwave trough over
Central Siberia, Kamchatka and the Aleutians which is an atypical
orientation for a trough axis. There is a secondary dome of high
pressure aloft north of the Bering which is keeping things
interesting in the Aleutians/Bering. As time elapses, the dome of
high pressure to the north moves to the west and the upper level
ridge remains steadfast from the Gulf Of Alaska and into the
interior of Alaska. All the while, to the east of the ridge, an
arctic trough pushing south through the Yukon Territory will
advect a brutally cold airmass into eastern Alaska Wednesday
and Thursday. With a tight pressure gradient in place and the
placement of the ridge axis expect a surge of strong northerly
winds to Southcentral Alaska in many locations. Offshore flow is
expected from Seward to Cordova. The communities of Palmer and
Wasilla can expect the "Matanuska Wind" on Friday and possibly
Saturday. By Sunday, the ridge axis moves eastward enough to
change the wind pattern. For the remainder of the forecast period,
this ridge is going to have some impact over the eastern half of
the state of Alaska.

Out to the west, there will be a series of storms. That longwave
trough axis is going to spin up storm after storm. This pattern
doesn't show any chance of changing for the Bering/Aleutians
through Day 7 and arguably through Day 10.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...PS