Small Craft Advisory 147, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKST Wed Feb 12 2020

.ANALYSIS AND UPPER LEVELS...
Two large lows are impacting the forecast area today. The first
is in the Gulf of Alaska and has been the source of the wind and
snow over portions of Southcentral Alaska today. This low is in a
weakening phase and will move eastward as a ridge of high pressure
over the eastern Bering Sea and Southwest Mainland Alaska strengthens
and moves eastward.

The second low is approaching the central and western Aleutians
and is much stronger than the Gulf low. The surface low is in a
favorable region of the jet stream to keep it rather strong into
Friday as it moves slowly northward.

&&

.MODEL DISCUSSION...
Model agreement is much better today than the past few days with
no large issues into the weekend.

&&

.AVIATION...
PANC...Another shot of light snow is expected this evening but
northerly winds will be lighter than this morning and this will
keep the visibility and ceilings bottoming out at MVFR levels.
Northerly winds will allow dry air to move into the area Thursday
morning which should allow VFR conditions to persist over the
terminal for a while once this happens.

&&

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

The storm system over the Gulf will exit eastward tonight, with
precipitation diminishing across Southcentral. Snow probability
has been increased from chance to likely tonight for Anchorage and
the Matanuska Valley. A trailing short-wave trough will combine
with weak cold air advection and a little bit of upslope flow to
produce light snow, particularly near and along the mountains.
Strengthening offshore flow and cold air advection ahead of a
ridge building in from the east on Thursday will quickly bring an
end to any remaining snowfall. Gap winds will strengthen across
the western Gulf and Kodiak Island waters tonight, then along the
north Gulf coast and Prince William Sound Thursday in response to
the offshore flow and tightening pressure gradients between the
departing low and high pressure moving in.

Dry and colder conditions will settle in across Southcentral
Friday through Saturday as the upper ridge moves overhead. Another
front will will reach Kodiak Island Thursday night into Friday,
but zonal flow aloft will drive this eastward across the Gulf and
keep Southcentral dry. A broader upper trough will approach Friday
night through Saturday, with a series of weak waves ejecting out
of the trough and into Southcentral. Thus, expect increasing
clouds and chances of snow to return to Southcentral as we head
into the weekend.

&&

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

High pressure in Southwest Alaska and a low in the Gulf of Alaska
will create a tightening pressure gradient over the Kuskokwim
Delta and thus bring dangerous wind chills to the area overnight
Wednesday. Temperatures will recover as a warmer air mass ahead of
a low in the Bering moves in from the southwest and brings a
round of warmer air into the region. A frontal boundary associated
with a Bering low will move into the western AKPEN on Thursday
afternoon, bringing a chance for snow and rain with it. Much of
Bristol Bay will see snow on Friday morning with the front moving
through.

&&

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

A storm force low in the western Bering will be the primary
concern for the next 48 hours with strong winds across much of the
western Aleutians and western Bering. The strongest winds will be
Thursday and subside into Friday evening west to east. A front
associated with this low will swing through the Pribilofs on
Friday morning with gusty winds and snow. Precipitation will be
mixed throughout the Aleutians with temperatures in the 30s and
40s.

&&

.MARINE (Days 3 through 5)...

Bering/Aleutians: Look for an active pattern, with a series of
lows and fronts transiting the region. While there is some
question about where each low will track, generally expect a good
chance of gale force winds through the 3 day period. The strongest
winds look to be on Sunday, with a low along the western
Aleutians. There is chance of storm force winds near the low
center.

Gulf of Alaska: The Gulf will be on the periphery of the Bering
Sea storms, with the strongest winds across the southern Gulf.
It looks like winds will remain below gale force Saturday through
Sunday, with just a small chance of gales over the southern Gulf.
A more significant storm out west on Monday looks likely to bring
gales to much of the western Gulf on Monday.

&&

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

Beginning the extended period forecast on Friday evening, there
will be a stacked closed low over the Central Bering Sea with a
triple point low over the Gulf of Alaska, moving eastward. Onshore
flow across Southwest Alaska will bring in snow showers and more
cloud cover and warmer temps while arctic high pressure over
Southcentral continues to keep temps below average through the
weekend. A series of subtle low amplitude shortwave troughs will
pass through the Southern Mainland Saturday through Sunday night,
but there is low certainty at this juncture on track. For now,
clouds will likely be on the increase with precipitation chances
rising although the threat looks rather minimal, and this will
also favor coastal locations.

A big change is looking more and more likely beginning Sunday
evening and lasting through at least Tuesday. The flow pattern is
going to amplify dramatically, and will result in a potential
significant storm cycle across all of Southern Alaska with threats
ranging from heavy snow, coastal rain, strong winds, and more. At
this juncture details are much uncertain, but residents should be
aware and stay tuned as the details and storm track become more
certain.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 155.
MARINE...Storms 174-178 411 413.
         Gales 130-132 150 155 170-173 179 180 185 412 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...SEB/CK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LB
MARINE/LONG TERM...JA