Small Craft Advisory 962, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 AM AKST Wed Dec 5 2018

.ANALYSIS AND UPPER LEVELS...

A large vertically stacked low remains nearly stationary as it
spins over the Aleutians between Atka and Dutch Harbor. The low is
weakening, as any temperature advection near the center of the low
has long since ended. Nevertheless, winds of 25-35 mph out of the
south continue over the eastern Aleutians and Alaska Peninsula,
while an area of gales continues on the cold side of the low over
the central and western Aleutians. The low is tapping some
subtropical moisture, and directing it northward into Bristol Bay
and the Lower Kuskokwim Valley this morning. It still has some
decently warm air with it, and as such moderate rain with
temperatures in the lower 40s continues over portions of Bristol
Bay. Sub-freezing temperatures are being scoured out over the
Lower Kuskokwim Valley as most areas have risen into the mid 30s
and changed over to rain. The Susitna Valley is on the eastern
edge of the plume of moisture and precipitation over Bristol Bay.
Thus, Skwentna and portions of the northern and western Susitna
Valley are occasionally getting snow, but most inland areas east
of there are otherwise seasonably cold and dry. Scattered rain
shower activity continues along the coast through Prince William
Sound, with a snow/wintry mix being reported in Valdez. The
strongest winds near mainland Alaska are over Kodiak and Bristol
Bay generally out of the southeast at 15-25 mph with higher gusts.

Through the upper levels, the upper level low and associated
troughing is slowly beginning to move eastward as a westerly jet
streak approaches the upper level low south of the western
Aleutians. East of the trough is a continuation of the typical
strong ridging that has been very common across southern mainland
Alaska so far this winter. This pattern continues to bring mild
conditions with above average temperatures across the mainland,
resulting in more precipitation-type issues and forecast
challenges determining where the rain-snow line will be.

&&

.MODEL DISCUSSION...

Unsurprisingly, model agreement has taken a turn for the worse
regarding how this large low in the Bering ultimately meets its
demise as it moves eastward south of the Alaska Peninsula on
Thursday. All of the models suggest a triple point low feeding off
an upper level shortwave rounding the base of the longwave trough
will develop and track to near Kodiak Island late Thursday night
into Friday. However, how quickly this low moves, how strong it
will be, and where its eventual track takes it are questions that
simply cannot be answered at this time. The models are still
trying to pin down how the energy transfer from the Aleutians low
to the new low occurs and how deep the new low can get. There is
somewhat better agreement on the frontal position and increasing
southeasterly winds developing across the northern Gulf Thursday
night, but the maximum strength of those winds and where they
occur will be highly dependent on the low track. The relatively
dry weather over Prince William Sound and the coast of the Kenai
Peninsula will end Thursday as the front and associated winds move
into the coast, but disagreements between the models are still
large regarding where the heaviest rains will be seen. At this
point nearly all of the north Gulf Coast looks likely to see at
least some period of heavy rains until the models better decide
where the plume of moisture will set up. This pattern favors a
continuation of downsloping conditions further inland through
Friday, which once again will favor windy conditions along
Turnagain Arm and the Hillside on Friday.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. Wind shear
will diminish this morning as the stronger winds coming across the
Chugach diminish. Note the Turnagain Arm wind is likely to persist
south of Anchorage for much of the day into the night with
variable strength.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Thursday night)...

Deep southerly flow up the west side of a high amplitude ridge
will transport a series of short-waves northward across the Gulf
to Southcentral over the next couple days. These waves will
primarily travel overhead of existing surface fronts, generally
following the same general lines of longitude. In other words,
there will be north-south oriented areas of precipitation. The
first will be aligned over the far western Gulf, including Kodiak
Island. The second will be across the central Gulf up to eastern
Prince William Sound and into the Copper River Basin. Areas in
between, including the population centers of the Kenai Peninsula,
Anchorage, and the Mat-Su will remain mostly dry. Precipitation
will primarily be in the form of rain along the coast and snow
inland and in the mountains.

All model guidance is now pointing to an intensifying low moving
up from the Pacific Thursday night, with a track toward Kodiak
Island. This will bring widespread strong winds and heavy
precipitation to the Gulf waters and Gulf coastal zone. Have gone
max gales with this storm, but there is a good chance it may be
strong enough to produce storm force winds over the northern Gulf.
The track of the storm favors western Prince William Sound/eastern
Kenai Peninsula for some of the heaviest precipitation with strong
east to southeast upslope flow. Warm air accompanying the low
means most of this will be in the form of rain. Meanwhile, the
strong low level flow means inland areas will remain drier thanks
to downslope flow.

&&

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

While the most dynamic synoptic feature move through the area on
Tue, the area will remain quite wet through the middle of the
week. A potent 85 kt upper-level (30,000') jet continues to push
very moist air into the region from the North Pacific. This
moisture is caught between a building mid-level ridge over the
ALCAN border and the massive low over the Central Aleutians. The
net effect is a band of moderate rain across parts of Bristol Bay.
This band is nosing all the way up into the Lower Kuskokwim Valley
where temperatures are cold enough (at higher elevations too) to
turn things over to snow, or at least of a rain snow mix. We do
expect several inches of wet snow to accumulate across the Lower
Kuskokwim Valley. While the airmass is still relatively warm
overall, some colder air is wrapping back into the area. This is
making for a rather tricky forecast as places on the west side of
the precipitation band could trend towards snow while places in
the middle of and east of the band will generally be rain.

As we go into tonight, the first piece of precipitation will move
off to the north. However, another developing wave will almost
immediately take its place. This one will hug a bit closer to the
Alaska and Aleutian Ranges, but will still highlight rain to the
south and better chances for snow to the north.

A deeper trough of cold air will develop off the YK-Delta coast on
Thu. This will cause a new surface low to develop near the Seward
Peninsula. This feature looks to track a band of snow into the
Delta by Thu evening into Fri morning.

&&

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

The broad, warm low will continue to weaken as a new front slides
in from the west. First, the sprawling low center just west of
Dutch Harbor continues to keep cyclonic (counter-clockwise) flow
across the area. This means warmer, more steady rain in southerly
flow on its eastern flank, while colder, showery, and gusty
conditions persist to its west. This low will continue to fill
today as it drifts east.

It will open the door for a new warm front to push into the
Western Aleutians starting this afternoon. This will cause winds
to shift to more of a southeasterly direction initially. However,
those winds will be short lived as a stronger upper-level
shortwave will dive in right behind this front late tonight. This
shortwave will usher in some potent cold air under gusty westerly
winds and change any rain back over to snow. This frontal boundary
then elongates over the Western and Central Aleutians through the
end of the week.

&&

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

The numerical guidance continues to come into better agreement,
which shows a wet and stormy pattern looks to continue through
early next week for the region. The first system of significance
will be a rapidly strengthening low that moves north to near
Kodiak Island, as a piece of energy from the current central
Aleutians low phases with energy ejecting north in the front edge
(of the trough) southerly flow. The strengthening low will also be
positioned close enough to the rear entrance region of a 70 knot
upper jet to our north/northeast, and in the left exit region of
an incoming 130 knot jet streak. Although these two jets are
somewhat more displaced that one would normally prefer to maximize
a coupled jet dynamics, they appear close enough to have some
degree of interaction, increasing the amount of lift available for
the developing storm.

Adding to the mix is a strong mid-level disturbance dropping
southeast to near Nunivak Island. This system will bring down a
shot of much colder air from the Arctic, which looks to encompass
the eastern two-thirds of the Bering/Aleutians and southwest
Alaska through Friday, spilling into Southcentral on Saturday,
and the Copper Basin (at least the northern part) on Sunday. With
colder air in place, some wintry type precipitation will likely
develop. It'll be interesting to see how much of the tropical
moisture affecting the coastal locations gets advected into this
colder airmass, as current indications are moisture values being
400 percent above normal or more.

Then, the Bering trough will get kicked eastward for the weekend
as both the Nunivak disturbance and a strong storm near the
Kamchatka Peninsula move east. This transient trough will absorb
another disturbance moving into the Gulf from near the tropics,
helping to bring added warmth and moisture into yet another
developing cyclone that rapidly deepens as it tracks from the
eastern Gulf towards Prince William Sound (PWS) in the Friday
afternoon into Saturday time frame. Then, as the PWS low wobbles
in place as it slowly fills, a third and fourth storm eject
northward while phasing, reaching eastern PWS for the end of the
weekend.

This whole entourage of systems finally gets shoved east as a
pair of systems move into the western Bering and Aleutians. The
first feature of interest here is a strong front attached to
another Kamchatka low, as it rapidly sweeps eastward across the
Bering/Aleutian chain during the weekend. Following quickly on its
heels is a strong storm that enters the picture south of Adak on
Sunday, riding the island chain eastward to Kodiak by Monday. Both
of these systems will have at least sustained gales, with the
latter likely being of storm force strength and widespread
precipitation.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning: 177 178 185 412 413.

&&

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