Small Craft Advisory 1036, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 AM AKST Wed Dec 26 2018

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low spins in the southeastern Gulf this
morning. The low sits at the base of a long wave trough that
encompasses much of western mainland Alaska, AKPEN and Gulf.
Within the flow of the trough, there are several short waves
present, one of which is over the AKPEN with a vertically stacked
low at the base. The jet stream is in a meridional pattern with
the ridge sitting south of the AKPEN and and the trough extending
into the eastern Gulf. With the surface low sitting in the left
exit region of the jet and a band of vorticity extending from the
low center, a deformation zone has established itself over Kodiak
Island. This deformation zone combined with the influx of
moisture is bringing precipitation to the northern Gulf and the
Kenai Peninsula. Water vapor satellite imagery shows the edge of
the band well, extending from Bristol Bay over Kodiak Island.
Southcentral however, remains dry as a deep layer of dry air and
easterly winds are creating challenging conditions for the
precipitation to reach the surface. In the west, instability is
increasing throughout Bristol Bay along the deformation zone. The
instability with the addition of moisture from the Gulf is
initiating snow bands through the Lower Kuskokwim Valley. The
Bering is still sitting under high pressure which is keeping the
cold air in place with persistent east to northeasterly flow.

&&

.MODEL DISCUSSION...

Models continue to agree early on through Thursday morning
regarding a North Pacific low moving into the southwestern Gulf
Thursday and a low pressure system south of the central Aleutians.
As the low enters the Gulf, the NAM is the most northern track
(closer to Kodiak Island) and the EC is the most southern. The GFS
is splitting the center while the Canadian is off in the middle
of the Gulf. By Thursday afternoon, models diverge even further
with the NAM and GFS setting up over the northwestern Gulf south
of the Kenai Peninsula, and the EC and Canadian moving into the
northeastern Gulf. Friday, the models all agree that the system
will move inland over Prince William Sound. While the models all
take this low on different paths, the meteorological outcomes are
not going to be too significantly different. With this system, a
GFS/NAM combination was used. Regarding the low in the North
Pacific south of the chain, models continue to split with the NAM
being the fastest solution and the GFS being the slowest.

&&

.AVIATION...

PANC...VFR conditions with light north to northeast winds will
prevail through tonight. A weak shortwave trough lifting north
over the area this evening, however, will bring a chance for
brief light snow showers.

&&

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

The old front has stalled over the southern Kenai Peninsula and is
bringing rain and snow to the area. The front will continue to
diminish through today and eventually be pushed north as a short
wave moves in from the south this afternoon. As it does there is a
chance for areas around Cook Inlet and north into the Mat-Su
Valleys to see some showers. Tonight a new system will approach
the Gulf from the North Pacific bringing a strong push of warm air
with it. However, models are still struggling somewhat with the
track of the system as it approaches the north Gulf coast Thursday
afternoon which will play a factor in what areas see the most
precipitation and what type of precipitation will fall for those
areas. Most of the guidance seems confident that the Anchorage
area will see some snow Thursday afternoon/evening but the amounts
will be a challenge until the models get a better handle on the
low position.

&&

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

Just when there was the potential for some active weather across
Southwest Alaska, it turns out things are going to remain
relatively quiet for a while longer. King Salmon radar is showing
weak echoes associated with a snow band that extends north from
near Iliamna into the Lower Kuskokwim Valley. Radar returns and
observations so far have been underwhelming to say the least,
thus, the forecast for today has lowered snowfall totals in that
area to an inch or two. Any snow showers associated with the band
will lift northward this afternoon and dissipate tonight.

Utilization of the models shows a weak front expected to impact
portions of Bristol Bay starting late tonight and into Thursday
morning. These snow showers will be enhanced by an approaching
upper level low. Even though the surface front is likely to
dissipate as it moves inland during the day, the approaching
upper level low should help to produce scattered to widespread
snow showers over most of Southwest Alaska Thursday into Thursday
night.

Lastly, an upper level shortwave circulating around the low will
move into the Kuskokwim Delta late Thursday night, though the
upper level front will extend much further south from there.
Meanwhile, a surface low is expected to develop on the south side
of the Alaska Peninsula Thursday night, which will be forced by
the same upper level front. This feature will help to enhance the
moisture transport into Southwest Alaska during the day Friday.
Thus, the combination of the approaching upper level low and
shortwave may result in the development of steadier snow across
Bristol Bay and the Lower Kuskokwim Valley. Chances of snow have
been raised to likely for the day on Friday, though amounts for
now are generally looking to be light, in the 1 to 3 inch range.
Much colder air associated with the upper level low itself returns
to Southwest Alaska behind this feature Friday night.

&&

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

Inherently quiet weather continues across much of the Bering Sea
this morning as continuous north to northeasterly winds persist.
As we have been seeing for a couple weeks now, this is resulting
in widespread stratus with the flow remaining generally cyclonic,
as the cold air moves over the relatively warm Bering, resulting
in the cloudiness. This pattern is likely to persist right through
Thursday night for most of the area. A generally weak, slow moving
low south of the central Aleutians is expected to continue moving
east with time, brushing the Eastern Aleutians and Alaska
Peninsula by Thursday night. In those areas, there may be brief
periods of steadier snow showers, but no significant weather is
expected from it.

All of that will change late Thursday night into Friday across the
western Aleutians. All of the models are showing a very strong,
moisture-laden storm system ramping up over the Northwest Pacific
and moving into the Western Aleutians early Friday morning and the
Central Aleutians Friday afternoon. Very strong winds look to be
the key hazard with this, as otherwise relatively warm air should
keep most areas that get precipitation with this feature as mostly
rain. The models have been very consistent with the development of
this strong front and associated lows behind it, though as usual,
the timing and consistency of those models remains very
changeable. That said, it now appears at least storm force winds
are likely across a large area including the Western and Central
Aleutians, which given current model trends may go higher with
time. Interests in this area should keep close watch to future
forecasts as this system has the potential to require future
watches and warnings to need to be issued.

&&

.LONG TERM FORECAST (Days 3 through 7...Fri Dec 28 through Tue Jan
1)...

The extended forecast still features a very active weather pattern
across the entire area. Models continue to advertise a fast-moving
SW-NE storm track from the North Pacific into SW AK. The major
shift from yesterday is that the ECMWF has backed off on how
strong it builds the ridge over the ALCAN Border by early next
week. Yesterday, the EC and GEM (Canadian) were advertising a
strong high-amplitude ridge building along the ALCAN Border. This
would have set up a very meridional pattern going in to next week.
However, the EC has now trended towards the GFS which would
yield a colder forecast with several modest storms moving quickly
through the pattern.

With that much uncertainty between models and even from run to run
within a model, the forecast for next week is highly uncertain at
this time. The few things we can lock onto though are late this
week and into the weekend. The deep, cold trough digging down from
the Bering Strait into the North Pacific will be sure to push some
snow showers over the Alaska Peninsula. These will favor the north
side of the chain. For the marine zones south of the peninsula,
expect gusty winds through bays and passes into very early Sat
morning. There is also high certainty with a deep (possible
mid-940 mb range) low developing over the Western Aleutians Sat
morning. The associated front will bring a healthy area of gale-
storm force south to southeast winds. It will also push a good
amount of warm air up into the chain. So while precipitation could
start as heavy snow, it will quickly transition into heavy rain.

Along this front will be the primary forecast challenge. There
will be triple point lows forming along this occluded boundary.
But trying to nail down exact timing, placement, or strength of
these lows is a fool's errand at this time. Each one of these
systems will have the potential to bring a surge of warm air into
the mainland with it. However, we will not start investigating any
specific system until the models have come into better agreement
regarding the upper levels (ALCAN ridge) first.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 119 120 131 132 173 174.
         Heavy Freezing Spray 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/JER
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...MSO