Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 150158
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 PM AKST Tue Nov 14 2017

.ANALYSIS AND UPPER LEVELS...

There has been little change with the upper level pattern over the
past 24 hours as a deep trough remains over eastern Alaska and the
Yukon with a ridge axis over the eastern Bering Sea/Southwest
Alaska. Farther west, there is an upper trough over the far
western Bering Sea and an associated warm front over the central
Bering Sea. Northerly flow aloft and cold air packing along the
mountains continues to push air mass through mountain gaps with
favored locations like the Matanuska Valley, Valdez/Thompson
Pass, and Eastern Turnagain Arm and Seward. Several peak gusts
this morning have been at or above 50 knots both through Thompson
Pass/Valdez and the Matanuska Valley.

&&

.MODEL DISCUSSION...
The models are in excellent agreement with the large scale general
pattern through Saturday. They have also come into very good
agreement with an elongated shortwave trough which is expected to
move through Southwest Alaska Wednesday before tracking east into
Southcentral Wednesday night into Thursday. However, there are
some slight differences with the orientation/strength of the
upper trough with the NAM, Canadian Regional and GFS favoring a
slightly stronger/slower upper trough progression whereas the
ECMWF is weaker/faster. This has an impact on snowfall potential
across inland areas of Southcentral including Anchorage. For this
forecast, a blend of all model solutions is favored which keep in
light snowfall accumulation potential in the forecast for Thursday
morning.

Later in the extended period forecast, much model uncertainty
remains with a potential strong low which could impact all of
Southern Alaska late weekend (see extended forecast discussion
for more details).

&&

.AVIATION...
PANC...Gusty north winds will persist through tonight then become
light and variable Wednesday morning. VFR conditions are expected
to persist through Wednesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Wed and
Thu)...
Offshore winds will slowly taper off as eyes shift towards snow
potential across the area Thu morning. First off, the wind event
has likely peaked or is peaking and should continue to taper off
through the overnight into Wed morning. However, there will still
be some rather gusty winds through the typical spots like the
Matanuska Valley, Whittier, and Valdez tonight. Blowing dust will
also continue with these winds, especially through the Copper
River Delta and the Matanuska Valley.

After this, attention shifts to a potential quick shot of snow for
early Thu morning. Models continue to show a progressive west-east
moving front coming from SW AK across the AK Range and into
Southcentral. There is still some uncertainty with this system as
models have shown a couple variations that would lend to slower
timing and/or varying snow amounts across the area. For this
forecast package, we have trended things toward a bit slower
timing (Thu early to mid morning across Anchorage) and a bit
higher snow amounts. As the front sweeps across the area and into
the Copper River Basin, it looks like the best chance for a couple
of inches will be along the Anchorage Hillside and toward Gulkana.
This system seems very reminiscent of the snow that fell across
the Cook Inlet on the 21st of Oct dropping anywhere from 1 to 3
inches. Thursday morning commuters should monitor the forecast
closely.

&&

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

A cold front currently located just west of the coast late this
afternoon will continue to move inland tonight. A weak surface low
will develop along this front in the Bristol Bay region. However,
model consensus is for an offshore flow to be maintained, allowing
colder air to continue to slowly advance towards the coast from
inland locations. Ahead of this front, a wedge of warmer air in a
southwest flow aloft will move over a relatively shallow layer of
colder air below it along the Southwest Alaska coast.

While this is not the most ideal setup for freezing rain with ice
accumulations, some brief periods of freezing rain are expected
to develop. As the front crosses the region, warmer air from the
waters will make more of a push inland where precipitation will
change to more of a rain/snow mix. For locations east of an Aniak
to King Salmon line, look for precipitation type to be predominately
snow, where 2 to 5 inches may accumulate. Lighter snow amounts
will occur farther west due to the mixed bag of wintry precipitation.

As the front reaches the Alaska Range by Thursday morning, an
upper-level disturbance will dive southeast across the region
shortly after frontal passage, with additionally showery type
precipitation developing area-wide and gusty winds towards the
coast.

&&

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

The main focus looks to be increasing wind speeds across the
eastern Bering Sea, with pretty decent gap wind development south
of the AKPEN following frontal passage. Speaking of this front,
the latest observations had it along a line from just west of
Nunivak Island to west of Dutch Harbor late this afternoon. This
feature will continue its eastward journey over the next 36 hours,
clearing the area by early Thursday morning. Brisk northwesterly
winds will develop with gale force winds very likely Wednesday
evening into Thursday morning. Significant gap wind gusts will
develop south of the peninsula, with communities along it seeing
gusts near 55 mph.

Another disturbance will also dive southeast across northern and
eastern portions of the Bering behind the initial front, with gale
force winds expected along northwestern and northern portions of
the Bering Sea Wednesday night into Thursday as well.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Yet another outflow event will develop along the North Gulf Coast
on Friday as an Eastern Gulf low intensifies and weak ridging
begins to move through the Mainland from the Bering. Just ahead of
this ridge, a shortwave will quickly push through the Southwest
coast, bringing light accumulations to many areas of Thursday,
which may also produce areas of blowing snow as gusty
northwesterly winds quickly follow it.

Certainty in the forecast then begins to quickly break down this
weekend as significant spread in the track of a rapidly deepening
low moves from the Bering into either the Gulf or North Pacific
(depending on which model guidance you look at). The EC has been
the most consistent with this low, bringing a rapidly deepening
low into the central Gulf by Sunday with widespread snow and wind
to the Southern Mainland. The GFS on the other hand was very
sporadic overnight, but is beginning to becoming more consistent
in a track much further to the south, providing only a glancing
blow of winds to the gulf and little if any precipitation to the
Southern Mainland. Due to this large spread in the low track and
expected impacts, little if any change is being made to the
extended forecast today awaiting better model consensus. This is a
system that bears watching however, as trends in either direction
will result in significant changes to the weekends forecast.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 125 127 150 155 172 185.

&&

$$

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



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