Small Craft Advisory 956, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
400 PM AKST Tue Dec 12 2017

.ANALYSIS AND UPPER LEVELS...
The train of lows continues with deep southerly flow from the
eastern Pacific up to the southern mainland of Alaska. A weak
low is lifting northward across Southwest Alaska today, with a
secondary low nearly stationary south of Cold Bay. Temperatures
have warmed enough across Southwest Alaska to produce mainly rain.
Showers persists along the Gulf coast behind a front which moved
inland this morning. Otherwise, dry and warm conditions continue
across Southcentral.

Satellite imagery shows signs of a frontal wave several hundred
miles south of Kodiak Island. With weak upper level support and
low level baroclinicity expect the process of cyclogenesis to be
slow.

Broad northerly flow and cold advection across the Bering Sea and
Aleutians continues to produce snow showers. However, a pattern
change is in store with a strong jet over the northwest Pacific
beginning to drive eastward south of the Aleutian chain. A deep
mature low north of the jet is slowly making its way toward the
western Aleutians.

Ultimately, the eastward progression of this low/trough will
cause the high amplitude trough over the north central Pacific
to lift northward and lead to a pattern change across the entire
Alaska region as we head through the remainder of the week.

&&

.MODEL DISCUSSION...
Models have finally converged on a similar solution with the
developing low over the northeast Pacific today and its track to
Southwest Alaska. There are still some minor differences in
intensity and track of the front across the Gulf, but forecast
confidence has greatly increased. However, confidence quickly
falls behind this system, with yet another frontal wave or low
tracking northward across the eastern Gulf as the upper trough
begins to lift northward on Wednesday. For now prefer the
ECMWF/GFS/NAM idea of a weak fast moving low and will disregard
the very different Canadian solutions which depict a much slower
and deeper low. The end result of this feature will be to prolong
and enhance precipitation in eastern Prince William Sound and the
Copper River Basin.

Model differences also develop as trough moves to the Bering Sea
and Aleutians the next couple days, with favorable conditions
for development of multiple surface lows rotating around a broad
upper center. Bottom line, forecast confidence drops off
considerably as we head through Wednesday and beyond. As far as
the plan for the forecast, will focus on depicting key aspects
of the pattern change and not worry too much about location of
individual features at this point in time.

&&

.AVIATION...
PANC...The big question early on is whether Turnagain winds will
bend into the airport this afternoon/evening on a consistent basis
or just an occasional gust. Pressure gradients look very marginal.
Have stuck with a bit of wind in the forecast, but it may stay
just to the south over Fire Island. Otherwise, with strong
downslope flow VFR conditions will dominate through the TAF
period. With a marginally deeper and faster moving low headed
inland across Southwest Alaska tomorrow and a much stronger front
crossing Southeast winds will definitely move into the airport
Wednesday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The pattern over Southcentral Alaska continues with the next front
moving north through the southern Gulf. It will make it to the
north Gulf coast early Wednesday morning bringing more precip to
the coast. Lower elevations look to see mostly rain from the
front.

Some showers will sneak past the coastal ranges bringing rain and
snow to inland locations, with the further inland areas seeing
more snow than rain. Winds will also accompany the front with the
strongest winds through east-west oriented gaps. After the front
pushes inland, flow will remain out of the south which will result
in continued showers along the coast.

&&

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

Broad southerly flow over the region continues to transport
moisture and predominantly rain showers (some snow showers along
the Kuskokwim Delta coast) over parts of Southwest this
afternoon. A strengthening shortwave moving north from the Pacific
will plow into Bristol Bay and the Lower Kuskokwim Valley tonight
and tomorrow, bringing rain with a mix of wet snow to those
regions. This system exits tomorrow evening and will be a series
of very weak and subtle shortwaves which will keep snow chances
over the region through Thursday as colder air slowly infiltrates
eastward behind each successive shortwave.

&&

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

A low over Bristol Bay will lift north tonight, keeping an
enhanced northerly pressure gradient over the central Bering
Sea/Eastern Aleutians and the Pribilof Islands. The gradient will
begin to weaken tomorrow with a gale force front entering the far
western Bering Sea this evening and moving east slowly through
Thursday. The air mass should be cold enough to mainly support
snow since it will quickly occlude tonight, although some rain
will mix in at times as the front tracks east.

&&

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

The weather pattern will remain active through day 7 (Tuesday).
We expect a continuation of the train of lows moving into the Gulf
of Alaska and Bering Sea. The models continue to struggle with
this pattern and coming into agreement on the location and
intensity of these storms for the entire extended forecast period.

Beginning Thursday, a strong low will move to the western Alaska
Peninsula area, though where exactly is still uncertain. It will
then push a front into the Gulf of Alaska. The ensemble mean
looks to be the best path forward at this point. By Friday the
deterministic operational model runs differ significantly with
respect to where the low center will be. This storm should be in
the Eastern Bering Sea by Friday, but here again, the models
diverge with the placement of the low center. This has a
significant impact on the sensible weather over the southern
mainland and the Bering Sea/Aleutians. As we progress through the
weekend the models do come into agreement bringing the strong
large low to the Prince William Sound region by Sunday thus
finally breaking down the blocking ridge over the west coast of
North America. The big take away from this will be a trend to
cooler temperatures as we near the weekend and into early next
week. Beyond Sunday the models once again diverge and have
significantly different solutions Monday and Tuesday, thus overall
confidence in the extended forecast is on the low side. The
overall approach to the extended forecast will be to nudge and
trend toward the ensemble solution, but in general we will not be
making any big changes to the forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 120 125 130 131 178.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...SA