Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 230021
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
421 PM AKDT Sun Oct 22 2017

.ANALYSIS AND UPPER LEVELS...
A deep upper level trough has carved itself out over the Bering
Sea and into the North Pacific, reaching as far south as 35N. The
center of the upper level arctic low is centered over the Y-K
Delta with a broad area of cold air advection over the Bering
Sea. Closer to Alaska, there is a trio of low amplitude shortwave
troughs embedded within the cold air. The first is a very subtle
feature lifting northeast across Cook Inlet this afternoon. This
one, however, is weak and rather moisture starved. A more
substantial shortwave is developing south of the Alaska Peninsula,
and there is another on satellite imagery developing south of
that feature around 43N. Rounding out the synoptic features...there
is an upper level low just west of the Kamchatka Peninsula, and
this will eventually merge with the remnants of Typhoon Lan and
deepen into a strong storm over the Bering Sea.

In terms of impacts for the mainland, the biggest story remains
convectively enhanced snow showers, which have been focused over
the Bering Sea and Southwest coast, some of which have been quite
heavy along the AKPen. However, a batch of instability driven
showers are also showing up on both the Kenai and Middleton radar
sites, likely enhanced by the subtle shortwave trough lifting
through Cook Inlet.

&&

.MODEL DISCUSSION...
The numerical weather models remain consistent in depicting the
general synoptic pattern through late week, but are in overall
terrible agreement and consistency regarding smaller scale details
associated with the pattern. These details revolve around shortwave
trough features embedded within the larger scale upper level
trough. It seems most of the low level forcing with these subtle
lows is associated with instability/convective driven ascent given
the very steep low level lapse rates. Moreover, the complex wave
mergers as these lows move north toward Southcentral further muddy
the details. For this forecast, a blend of the NAM and ECMWF were
favored for snow potential across Southcentral. The GFS was an
outlier with a much deeper, slower, and farther west track with
the second low moving towards Southcentral Tuesday.

Out across the Bering Sea, the models are in excellent agreement
with the developing near hurricane force low over the western
Bering Sea, so a blend of all the models was favored.

&&

.AVIATION...
PANC...VFR conditions will dominate through the TAF period with
just some passing mid level clouds in the vicinity. Winds are
expected to remain less than 10 kts.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The longwave pattern featuring a sprawling Arctic low over
western Alaska with southwesterly flow into Southcentral will
remain in place through Tuesday morning. Upper-level disturbances
will continue to rotate around the wave and up Cook Inlet, which
will keep chances for snow in the forecast. Models are notoriously
bad at handling individual waves in this pattern, so timing of
snowfall continues to be a forecast challenge. With the exception
of a few light flurries scattered over the area, the next shot of
organized precipitation looks to affect the Anchorage/Kenai area
Monday afternoon/evening. The wave responsible for this low is
currently south of the Alaska Peninsula. As this feature moves to
near Kodiak Island this evening, rain/snow will spread over the
lower Cook Inlet area and Kodiak Island. By Monday morning,
precipitation will move to the southern Kenai Peninsula with
mainly snow falling over land and some mixed precipitation over
the waters. Monday evening, the wave will move into Southcentral,
spreading light snow over the area. Lower-level support is weak
at this time, so expect snow accumulations to be fairly minimal.
Between one and two inches are possible along the Anchorage
Hillside and higher elevations, while under an inch, if anything,
is expected in the Anchorage Bowl.

The snow should taper off Monday night before the next low quickly
moves into the area Tuesday morning. The dynamics associated with
this system are much more impressive than Monday`s weak low, so
expect Tuesday`s to be a much more impactful system. Models are
having difficulty coming to a consensus on the track of the low as
it tracks to the northern Gulf Tuesday afternoon, so confidence
in the exact timing and placement of Tuesday`s snow is fairly low.
However, confidence is rising that this system will bring
snowfall that will impact portions of the Kenai Peninsula,
Anchorage, and Mat-Su Valleys on Tuesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
A North Pacific low developing south of the Alaska Peninsula will
track north tonight spreading snow across the eastern areas of
Bristol Bay. The track of this system is expected to track north
across the Aleutian Range through Monday afternoon as it weakens
into a trough. A winter weather advisory was issued during the
afternoon package for areas east of King Salmon as heavy snow is
expected to accumulate along the Aleutian Range. The next front to
move into the southwest reaches the coast Tuesday. Southeasterly
gusty winds increase along the coast Tuesday afternoon as the
frontal boundary moves across the eastern Bering. Precipitation
will likely start as snow then quickly change over to rain as a
warm air mass advects inland, however the Kuskokwim Valley should
be monitored for changes as the cold pool along the Western Alaska
Range may bring precipitation challenges late Wednesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A strong extra-tropical low (being fed by warmth and moisture from
Typhoon LAN) tracks northward tonight, pushing a frontal system
across the western Aleutians. Heavy rain and gusty southeasterly
winds develop overnight across Shemya along the boundary. The high
wind watches for the western and central Aleutians were upgraded
to warnings during the afternoon package as confidence increased
for gusts between 70 to 80 mph to occur along the boundary Monday
through Monday night. Storm force winds with hurricane force
gusts are expected across the marine waters surrounding the
western and central Aleutian islands along with seas between 40
to 50 ft. The low center picks up eastward momentum by Tuesday
morning and pushes the frontal boundary into the eastern and
northern Bering. As it tracks to the east the low slowly fills,
however this system remains strong enough on its track to bring
max gale and storm force winds to the rest of the Bering through
Wednesday. Moderate to heavy rain will accompany this system,
reducing visibility at times.

&&

.LONG TERM FORECAST (Days 3 through 7)...
An active pattern is expected to continue during the long range
forecast through the end of next week. The focus remains on an
upper level disturbance tracking over the northern Gulf and
Southcentral Alaska Tuesday evening into Wednesday, as well as
the very deep low with tropical connections tracking across the
Bering and Southern Mainland through the end of next week.

An upper level wave tracking further inland over Southcentral
Tuesday night, along with a surface low over the northwestern
Gulf, can bring another shot of snow for the southcentral area
through Wednesday. Highest snowfall totals are currently expected
along the Eastern Kenai and areas surrounding Prince William sound
Tuesday afternoon though Wednesday morning. Models are still
struggling with the overall timing and track of the low, but have
started to show a trend toward tracking it over the western
Gulf/eastern Kenai. Confidence is still not high in this scenario,
but if models continue to trend in this direction it will increase
the potential for the above mentioned snowfall to occur.

The leading frontal system ahead of the rapidly deepening low over
the NW Pacific will track eastward through the eastern Aleutians,
Alaska Peninsula, and Southwest Mainland Tuesday into Wednesday.
This will begin to usher in warmer temperatures over the eastern
Bering and southwest mainland by Wednesday morning, and build over
much of the southern Mainland through the end of the week. This
system will also bring some moderate to heavy precipitation to
these areas, as well as strong gusty winds. Rain is expected to be
the dominant precipitation type along the Aleutians and Alaska
Peninsula given the warming southerly flow and heavy marine
influence. The southern Mainland on the other hand will start off
as snow Tuesday afternoon and transition to rain or a rain snow
mix along the Gulf and Southwest coast Thursday. Models remain in
good agreement through the later half of the week regarding this
system, but confidence begins to wane as the models begin
struggling more with the track and timing as the low center tracks
into the eastern Bering on Thursday.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 187 191. Winter Weather Advisory 161.
MARINE...Storm Warning 173 174 175 176 177 178.
  Gale Warning 185.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...RD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...TP



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