Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 140200
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 PM AKST Wed Dec 13 2017

.ANALYSIS AND UPPER LEVELS...
The upper level pattern consists of a now familiar amplified
north-east Pacific upper ridge axis with a deep trough extending
from the Bering Sea south into the north-central Pacific Ocean. A
powerful but weakening low amplitude shortwave trough has now
moved inland along the North Gulf Coast with the associated
dry slot bringing a bout of sunny skies to parts of Cook Inlet
and Anchorage. Gusty southeast winds persist across some favored
gaps near Anchorage and across the Matanuska Valley and the
surface low tracks into the Lower Kuskokwim Valley. Out west,
there is an upper level deformation band associated with the
aforementioned shortwave trough which is bringing a mix of rain
and snow to parts of interior Bristol Bay and the Kuskokwim
Mountains. Even farther west over the western Bering Sea, there is
an occluded front moving east which is associated with a broader
scale low with a series of smaller embedded mesoscale features.
This will be the next major weather maker heading into the
weekend.

&&

.MODEL DISCUSSION...
Models are in excellent agreement with the synoptic pattern
heading through the weekend, but they are in terrible agreement
with the small scale features which will eventually form into the
more important large scale features. The first area of agreement
is with a series of smaller lows which are expected to move into
the Bering Sea...then merge into a much stronger gale force low.
The solutions vary from a deep low centered over the Pribilof
Islands (GFS/NAM) to a more elongated front (ECMWF and now the 18z
Canadian Regional). This will then effect the position of a basal
low which is expected to move into the Gulf of Alaska sometime
Friday into Friday night. The GFS eventually develops a 957 hpa
low west of Kodiak Island while the ECMWF has a weak fast moving
trough axis pushing through the Gulf. The differences are
dramatic with the associated impacts equally dramatic. For now, a
middle ground approach will be taken for the latter feature while
a blend to the GFS and ECMWF was favored for the Bering Sea.

Closer in time, the models are now once again trending west with a
low moving up toward the North Gulf Coast Thursday with the
chances for precipitation increasing across the Copper Basin and
westward along the Chugach Mountains.

&&

.AVIATION...
PANC...Southeast winds are expected to continue through late this
evening then gradually diminish overnight. VFR conditions are
expected to persist through the TAF period. There is a chance for
some 3500-5000 ft CIGS Thursday morning through afternoon as a low
tracks to the east.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
As the front pushes north and breaks apart, winds should diminish
around Southcentral Alaska tonight. The front moving off will also
help to clear out precip for western portions of Southcentral
Alaska. For places around eastern Prince William Sound and north
through the Copper River Basin, precip will continue especially
along the north Gulf coast. Right along the coast it will fall
primarily as rain with mainly snow further inland.

A low moving through the eastern Gulf tonight will enhance precip
for these areas as it approaches the coastline. However, there is
some discrepancy amongst the models on how far west the precip
will get. For now the most likely scenario looks to be keeping the
majority of new precip over eastern Prince William Sound and east
of the Talkeetna Mountains. Thursday night Southcentral Alaska
will clear out most of the precip before the next system
approaches on Friday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
(Tonight through Friday night)

Precipitation continues to wind down this evening, as an area of
low pressure departs the region to our north. However, a residual
trough axis will continue to slowly lift across the region in its
wake, extending from a system over eastern Siberia. Some wrap
around moisture from the low will interact with this trough axis,
allowing showery type precipitation to continue, especially on
the south and west facing slopes of the mountains through Thursday
evening.

The weather becomes much more interesting for the the Thursday
evening through Friday night time frame, as all the models
continue to struggle with their solutions of where a pair of
surface low pressures form, and how it affects the surface winds
and associated frontal position. For now we used a GFS/ECMWF
combination since the NAM has trended towards that solution with
support from the Canadian models. In short, the general consensus
is for a strong low to form N across the AKpen or Bristol Bay
region, with another strong low further west. A strongly occluded
front will lift northeast from the peninsula with widespread
precipitation developing along it, some of which may be moderate
in intensity at times. Snow looks to occur north of a Cape
Newenham to Port Alsworth line, with a mix or changeover to rain
south of there. Some strong winds will also precede the front near
the coast during this time.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
(Tonight through Friday night)

An area of disturbed weather, with several embedded areas of low
pressure will continue to develop/consolidate into a strong low
pressure system over the next 48 hours. A strong cold front was
progressing eastward across the western portions of the Aleutians,
with enough cold air aloft interacting with the warmer waters to
produce isolated thunderstorms just south of the chain. This looks
to continue through Thursday night, and model solutions continue
to show instability remaining if not building during this time.
Thus, a slight chance for thunderstorms was added to the forecast
for locations from Adak westward, and across the associated
waters.

As this occurs, another strong low will develop over the eastern
Bering Sea, with the two lows connected by a surface trough. The
cold front from the initial areas of low pressure will strengthen
while beginning an occlusion process as it treks through the chain
and up the peninsula. Widespread precipitation will develop along
and ahead of it, with some strong winds possible as it crosses the
region. Precipitation will become more showery following the
frontal passage.

&&

.LONG TERM FORECAST (Days 3 through 7...Sat through Wed)...
"Uncertainty" is the key word in this timeframe. The weather
models not only continue to conflict with one another, they also
conflict with themselves on nearly each successive run. The result
is overall low forecast confidence in most of this period. The
general theme to the pattern will be a broad trough sliding
further east over the Eastern Bering. This will be possible
because the stout North American ridge that has been locked in
place over the West Coast of the Lower 48 will finally give-way a
little bit. As the trough slides east, its southeast quadrant will
interact with an intense sub-tropical jet. This generally leads
to strong triple-point low development. However, because of the
vast uncertainty on where exactly the trough sets up, and more
importantly, where the low center is within the trough, it is
extremely difficult to bite off on any one solution. Late Friday
night, there is agreement that another wet and windy front will
push into the north Gulf coast. It is early Sat that the GFS is by
far the most bullish in carrying a mid-950 mb low right over
Kodiak Island. This scenario would lead to a similar set- up to
what we have seen several times this week with heavy rain along
the north Gulf coast and strong winds from S/SE to N/NW. The NAM,
EC, and Canadian are a bit less intense and further east with this
feature. The net effect will be another round of rain and wind
for much of the Gulf. However, exactly where this low tracks and
how strong it is will have implications on snow versus rain for
certain areas and how strong the barrier jet might be.

The one area where this is good certainty is with a "bomb" cyclone
over the Western Bering late in the weekend into early next week.
Cold air over Siberia will work in tandem with a strong 300 mb
jet to rapidly deepen a surface low pressure center. Models all
prog it to near 950 mb or slightly lower. This will produce storm
force winds and seas that may build to over 30 feet.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 150-170 172 174 176 179 180.

&&
$$

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



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