Small Craft Advisory 114, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
513 AM AKST Mon Feb 5 2018

.ANALYSIS AND UPPER LEVELS...

Ridging originating from a persistent upper level high over
northwest Alaska continues to weaken this morning, giving way to a
ridge featuring much warmer conditions over the Gulf. As the more
moisture-laden airmass creeps its way northward, the end of cold
and clear weather will come along with it. Meanwhile out west, an
upper level low centered over the Western Aleutians near Amchitka
continues to support a feed of moisture over the central and
western Bering, much if which is emanating from a strengthening
low headed for the Alaska Peninsula and Eastern Aleutians. This
low and its highly anticipated development of a triple-point low
to its east over the western Gulf, will be the primary players
for mainland Alaska's first widespread precipitation event in
quite some time.

Much of the rest of mainland Alaska will get one final night of
clear, cold, quiet weather. Nearly all inland locations are at or
below zero, while coastal communities are broadly in the teens and
20s. The winds remain a factor through Thompson Pass and the
Valdez Narrows, courtesy of a mass of built up cold air in the
Copper River Basin that has nowhere to go except through those
outlets. However, many of the other gaps, including the Matanuska
Valley, Broad Pass, and Seward, are getting a break and seeing
relatively light winds.

Transient fog has developed around the Anchorage area this
morning, and it is forecast to be variably dense through the
first few daylight hours before dissipating.

&&

.MODEL DISCUSSION...

The models are in surprisingly good agreement given the general
complexity of the upcoming weather across Southern Alaska. They
all depict the low currently approaching the Alaska
Peninsula/Eastern Aleutians to continue to track northward across
that area today and to the east of the Pribilofs Monday night.
Meanwhile, all generally agree on the track of the secondary
triple point low tracking east of Kodiak Monday night, then
turning eastward around Middleton Island Tuesday afternoon. Thus,
broad agreement lends lots of confidence to the forecast.
Unfortunately, with the next round of precipitation headed for
Southern Alaska, many forecast details are not anywhere near as
clear-cut. Some of the models indicate there will be downsloping
cutting into snow totals for areas like east Anchorage and the
Matanuska Valley, while others don't have nearly as much
downsloping. There remains some uncertainty as to how quickly the
system can moisten up the atmosphere, as it remains still very
dry in most levels this morning.

&&

.AVIATION...

PANC...Shallow fog has developed over the Anchorage area this
morning. It appears to be transient, but will briefly bring
conditions to as low as LIFR when it's at its densest. It will
dissipate late this morning. Snow moves into the area early
Tuesday morning, and will be heaviest through the morning commute
before tapering to off-and-on snow showers for the rest of the
day.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The longwave pattern features ridging that spans from the North
Slope southward over the East Pacific. Meanwhile, a new system
will continue to move towards Kodiak eventually displacing the
ridge to the east. This morning across the Mat-Su Valleys, expect
single digit or sub-zero temperatures while Glennallen and Copper
Center are near negative 40. Anchorage is chilly but fog has
developed over portions of the metro area. Today a frontal
boundary will lift north through the Gulf of Alaska. Kodiak City
can expect rain today but as the system tracks northeastward and
towards Middleton Island the thermal profiles for locations in
Kenai indicate a snow event which will begin this evening. Tuesday
snow will fall inland reaching Turnagain Pass and Anchorage. For
now, expecting snow to begin in Anchorage early Tuesday morning.
The deformation band associated with this low shears apart and it
is unlikely that Anchorage will receive much more than an inch.
But any amount of precipitation is potential for commuting
impacts. The Chugach Mountains near Prince William Sound and the
Kenai Mountains will be the big winners from this event. Towards
the end of the forecast period an inverted trough develops at the
surface over Prince William Sound which justified keeping higher
pops late in the period for PWS.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Monday through Tuesday)...
The prolonged period of dry weather will come to an end across
Southwest Alaska as a gale force low pressure system lifts
northward across the southern Alaska Peninsula and into the
eastern Bering Sea today. Snow and blowing snow will develop
across much of Bristol Bay this afternoon as an occluded front
approaches from the south. Strong low level southeast flow will
enhance snowfall along the hills and mountains north and west of
Dillingham. While the winds will advect warmer air into the
region, the bulk of much warmer air will follow the triple point
into the Gulf of Alaska. With cold antecedent conditions, much of
the warmer air will initially ride over the cold dome in place,
enhancing lift and snowfall. Temperatures look cold enough to
remain all snow, with the exception of along the Alaska Peninsula
south of Pilot Point. Northerly winds out ahead of the front
combined with the cold surface temperatures will lead to very low
visibilities in blowing snow from Dillingham north and west, so a
Winter Weather Advisory has been issued for this afternoon and
evening.

The front will elongate and weaken as it continues north and west
to the Kuskokwim Valley and Kuskokwim Delta tonight, so generally
expect lighter snow accumulation. The front will stall in the
Kuskokwim Valley which may lead to a longer period of light snow.
Blowing snow can also be expected along the Kuskokwim Delta,
though do not expect visibilities to fall as much as in Bristol
Bay due to the lighter nature of the falling snow. A few showers
will linger through Tuesday, though generally expect to dry out.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Monday through Tuesday)...
The Eastern Aleutians is in a sweet spot of deformation on the
back side of the aforementioned low, so steady rain will continue
through the morning hours. Colder air will wrap in behind the low
this afternoon and evening which will lead to a transition to a
rain/snow mix as showers linger. Further west, a series of short-
waves ejecting out of the North Pacific trough, will affect the
Aleutians the next couple days with stronger winds and rain. There
is quite a bit of uncertainty in the storm tracks, but it does
look like the lows will consolidate into a single deeper low
tracking into the Bering Sea by mid week.

&&

.LONG TERM FORECAST (Tuesday evening through Sunday night)...
Our extended forecast begins mid-week with a weak low pressure and
front in the Gulf, bringing snow to the north Gulf Coast, and
exiting to the east. For interior locations this means clouds and
possibly some very light snow, followed by clear and cold
conditions for later this week. Out west, the next active feature
of the pattern is a longwave trough, weakening and broadening
over Kamchatka. Both it and the blocking ridge are decaying from
their current state. A frontal boundary and deep southerly flow
will exist across the Bering Sea.

Into the longer term forecast, guidance quickly devolves into a
jumbled mess. As the two previous anchors in the longwave pattern,
the blocking high and Kamchatka trough, break down, guidance
struggles with a very active east Asian jet, and where exactly to
direct that energy. The big differences in the deterministic
models have to do with how they handle the blocking high pressure.
The ECMWF re-establishes it over the Bering Strait region, keeping
Southcentral AK in a cold outflow pattern, while the GFS wants to
build in a ridge from the sub-tropics getting us into a chinook
pattern. Literally opposites. The extended guidance from WPC will
utilize an ensemble blend for the forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 161.
MARINE...Gales: 119 127 131 138 150 155 160 165 180 351 414.
         Heavy Freezing Spray: 127 128 139 160.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...PJS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SEB
LONG TERM...ML