Small Craft Advisory 978, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 AM AKST Tue Dec 11 2018

.ANALYSIS AND UPPER LEVELS...

A long wave trough remains anchored across the Bering, stretching
from Kamchatka east to the western Gulf. South of this trough, a
170kt jet streak continues to provide a zonal flow across the
North Pacific and is suppressing future storm development
upstream south of the western and central Aleutians. A negatively-
tilted 495dm low situated over Bristol Bay is weakening as it
slides east-southeast and transitions to an open wave. Downstream
of this low, a shortwave trough is lifting north over the
Southcentral interior while a second shortwave ejecting from the
low moves into the central Gulf. A third shortwave continues to
track southeast from the central Aleutians after bringing a quick
burst of snow last evening to Unalaska.

At the surface, the shortwave moving onshore over Southcentral is
associated with a 960mb low situated north of Cordova. This
system is producing a mix of rain and snow along the coast
through Prince William Sound and areas of snow farther inland.
Another area of snow is falling north of Homer, across Cook Inlet,
and over the Anchorage Bowl as a weak surface trough extending
west from this low also lifts to the north. A second surface low
associated with the upper-level low over Bristol Bay is producing
snow and gusty northwesterly winds from Cape Newenham south across
the AKPEN along with strong gales over the open water. A trough
extending north of this low is the focus of widespread snow across
the lower YK Delta as moisture advecting from the west runs into
a cold air mass situated over the interior Southwest. Here, snow
has been falling throughout the overnight hours in an area from
Quinhagak to Bethel to Aniak.

&&

.MODEL DISCUSSION...

Models remain in good agreement through the next 48 hours
regarding the evolution of a low moving west into Prince William
Sound and a second low moving northeast from Bristol Bay. There
is also above average confidence of light accumulations of snow
across much of Southcentral through through tonight as the GFS,
NAM, and EC all show an occluded front across the southcentral
coast and a trough stretching along western Cook Inlet through
the Mat-Su Valleys. Farther west, there is good confidence in the
southeast movement of a developing low near Norton Sound into the
Bristol Bay region by midweek. This system will influence the
setup of a deformation zone over Southcentral (Cook Inlet and MAt-
Su Valleys) between it and a low in the Gulf on Wednesday with
another round of light snow likely.

The North Pacific low forecast to enter the Gulf on Wednesday is
also better resolved by the models, as the GFS and NAM now have
similar solutions with a 968mb staying in the southern Gulf. Most
of the impacts from this system, other than an uptick in the
offshore winds across Southcentral, look to affect the offshore
waters and the Alaska Peninsula moving into the latter half of the
week.

&&

.AVIATION...

PANC...Ceiling and visibility will drop to MVFR in snow this
morning. Periods of IFR conditions are possible through the
afternoon with reduced visibility due to bands of more moderate
snow. Ceiling and visibility improve tonight as the low lifts to
the north and the light snow comes to an end.

&&

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

The surface low came onshore near Cordova overnight and is
weakening as it moves northwest across Prince William sound. This
is bringing light snow to a good part of the Southcentral area
and will likely bring periods of snow to the region through
Tuesday night.

One of the bigger forecast uncertainties for today revolves around
the amount of snow which will make it into the Homer area. The
strong winds originating out of Kamishak Bay will bring Gale
force winds to Kachemak Bay. This pattern can sometimes cause
significant blowing snow conditions in Homer and the amount of
snowfall will dictate how low the visibility will get. While
temperatures currently are hovering near freezing, which will
keep blowing snow down, colder air will arrive with the winds and
make snow easier to blow around. The big reason that we are
leaning for less snow (and therefore not significant blowing
snow) is the track of the low. For it to really get the winds and
snow cranking at the same time, the low should track along the
eastern Kenai Peninsula into Prince William Sound and this one
came into the Sound from near Cordova. However, it is still worth
keeping an eye on conditions around Homer and Kachemak Bay today
even if it is not an ideal setup for a blowing snow event.

POPs were increased for Wednesday night in a swath from Seward
through Talkeetna, including the Anchorage and Matanuska Valley.
Both the NAM and GFS models are showing a deformation zone
developing which would bring in some snowfall. Kept the POPs right
at the 50 percent mark however due to the fact that deformation
zones are long and narrow and a small change in the track will
keep these areas precipitation free. The location this deformation
zone develops will be dependent on the cold upper level low
digging into the Bristol Bay region. Since this is a large feature
and models are more likely to resolve it well, in stark contrast
to most of the features we have been working with recently, it
increases the chances that the two models both showing it may be
right. We will have to wait for a few more runs to show whether we
expect this to turn into a snow event or just fizzle out.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Thursday Night)...

The low pressure system causing the banding that brought
widespread snow and blowing snow to the Kuskokwim Delta yesterday
and overnight last night is finally weakening as it shifts off to
the north and east. Expect winds to remain elevated across the
Kuskokwim Delta today, leading to localized reductions in
visibility in exposed open areas from the blowing snow. Otherwise,
things will continue quieting down in that region. The Winter
Weather Advisory for that area has been cancelled. Further east,
snow showers are continuing across Bristol Bay, every so often
mixing with freezing rain right at the coast. However, with cold
air quickly moving in, snow is expected from here on out. The
banding causing the snow will gradually drift northward through
the day today, becoming confined to just the Lower Kuskokwim
Valley by this afternoon. Light snow will persist in the Lower
Kuskokwim Valley into tonight, though snowfall rates should remain
light. Snow shower activity across the eastern Bering will
continue to impact coastal communities throughout the day into
Wednesday. A small Arctic low will develop over Norton Sound this
afternoon and will slowly drift southward into the Kuskokwim Delta
by Thursday morning. Snow shower activity with this low will
mostly remain along and off the coast, but when the center is
located just north of the Kuskokwim Delta on Wednesday, onshore
flow may enhance activity across the region. Any snow showers may
briefly be heavy, but will not be long lasting or result in
significant accumulations.

On Thursday afternoon, the center of the low will shift south into
Bristol Bay and then eventually dissipate as it moves into the
Aleutian Mountains/Alaska Range Thursday night. This will allow
for an offshore wind to develop as a strong low tracks well south
of the Alaska Peninsula late Thursday night. The offshore wind may
finally allow the clouds to clear out some. Behind the low, a
reinforcing shot of even colder air will make its way into
Southwest Alaska. Since through Thursday afternoon, cloud cover
is expected to keep temperatures relatively warm with the low
pressure system nearby; once the low dissipates and the clouds
follow, the clear skies will allow temperatures to plummet area-
wide. Thus, temperatures have been nudged up through Thursday with
highs in the single digits to the north and teens to the south,
and lows not too much lower than that. By Thursday night, lows are
expected to drop well below zero over the Kuskokwim Delta with
lows near zero further east. Highs will be in the teens to around
20 along the coast.

Looking ahead, Arctic high pressure building in the interior with
a storm track well south of the area will make for clear skies,
which will allow for some of the coldest temperatures of the
winter so far to move into all of Southwest Alaska. Lows Friday
night and Saturday night are expected to dip south of 10 below for
many areas away from the coast. Bundle up!

&&

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

Relatively quiet weather is expected for much of the Bering
through the short-term forecast period. A low along the west coast
will keep a steady stream of increasingly colder air moving south
across much of the Bering today right through Friday. Somewhat
warmer air will remain along and south of the Aleutians. That
gradient in temperature along the Aleutians will keep the main storm
track south of the chain. However, a series of small mesoscale-lows
are expected to form near the Aleutians today and Wednesday,
resulting in snow to the north of the chain, and a rain/snow mix
to the south. Any associated snow will be short-lived, but winds
may be gusty along with it.

A strong low will approach the western and central Aleutians
Thursday, but for now the worst weather is expected to remain
south of the chain. Any small northward shift in the track may
bring blizzard conditions to the central Aleutians, mainly from
Dutch Harbor west. Those in and around Dutch Harbor should monitor
future forecasts closely, primarily for the Thursday afternoon
and Thursday night time frame. All of the models currently track
the low well south of the Alaska Peninsula and Eastern Aleutians
on Friday, so a more significant northward shift in the track
would be needed for impacts to be expected there. Otherwise,
seasonably cold conditions will continue across the Aleutians
through Friday with snow showers in gusty north winds continuing
for most of the Bering.

&&

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

The long range forecast remains active across area waters as the
jet stream ushers storm systems along the Aleutian Chain to the
Gulf of Alaska. Inland locations across the southern mainland will
mainly be drier with cold air advecting the coolest temperatures
of the season for the end of the week. The next significant storm
system to monitor develops across the North Pacific Thursday night
bringing gusty easterly gales for the Aleutian Chain. This system
keeps a fast paced easterly track, but looks to move south enough
to keep significant impacts away from the Alaska Peninsula. Models
differ in timing and track on this system as it progresses
eastward, however they all lift it toward the Gulf Friday night.
The synoptic pattern amplifies over the weekend with a ridge to
the east and west of this storm system. This will likely slowly
the northward progress of the front through the weekend. There is
high uncertainty in the forecast this weekend into early next week
as models struggle with the evolution of this storm solution.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...KH