Area Forecast Discussion
Issued by NWS Anchorage, AK

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498
FXAK68 PAFC 151335
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
435 AM AKST Fri Dec 15 2017

.ANALYSIS AND UPPER LEVELS...
Showers are beginning to taper off across much of Southcentral
Alaska this morning as a disturbance along the North Gulf Coast
pushes inland and weakens, with the bulk of the remaining shower
activity limited to northeastern Prince William Sound from Valdez
to Cordova. In the upper levels, the western North America ridge
continues to break down as a weak trough lifts through the
northeastern Gulf, leaving fairly weak onshore flow focused over
the southern mainland. A sprawling low complex centered north of
Adak anchors a longwave trough encompassing much of eastern Russia
and the Bering Sea. An occluded front extends eastward from this
low center across the Bering and into the far southwestern Gulf,
with another deep low spinning up along the front just north of
Cold Bay. This front is spreading gale force winds and a mix of
rain and snow over much of the Bering from the Pribilof Islands to
the Akpen as it bears down on Greater Bristol Bay and the south
end of Kodiak Island.

&&

.MODEL DISCUSSION...
Models remain in excellent agreement through the weekend with
respect to the large scale pattern as the Bering trough shifts
eastward into over the Gulf and mainland Alaska. This will allow
the cooling trend to continue through Sunday and beyond, with
shower activity remaining mostly confined to coastal locations.
There is also reasonable agreement on the smaller scale features,
the most significant of which is the low progged to form along
the front over the western Gulf tonight. Stuck closer to the GFS
for this system as it still represents somewhat of a middle ground
solution to account for some of the subtle positional differences.

&&

.AVIATION...
PANC...The wind forecast will be a bit challenging this afternoon
as a Turnagain Arm jet aided by an approaching front is countered
by a strong down inlet pressure gradient. The down inlet
component is expected to win out and keep light northerly surface
winds at the terminal for the bulk of the afternoon, though gusty
southeast winds may still periodically make it into West
Anchorage. Given the expected northerly surface flow, low level
wind shear will develop by 21z as strong southeast winds move in
above the surface. Strong downslope flow should keep the terminal
dry through this evening, though some showers may still be
observed in the vicinity, with VFR conditions prevailing through
the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A strong front will push northeastward across the Gulf and
Southcentral Alaska today. Precipitation will spread over the area
with this system. Most of this will fall along the coast, with
some down-slope drying for many inland areas. A triple point low
will develop near Kodiak City early this afternoon, and work its
way into Prince William Sound (PWS) tonight. Down-slope impacts
will diminish as the low moves into PWS, and this will likely
bring snow to the Anchorage area and the Matanuska valley late
tonight and early Saturday morning. There is an upper level short-
wave that move through late Saturday morning into the afternoon
that will bring another good chance of snow. On Saturday and
Saturday night, the upper level low (currently centered over the
central Bering Sea) will push into southwest Alaska. This will
usher in a colder air mass. The cyclonic flow with this low will
keep chances of showers over the area Saturday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
A surface low pushes a front south to north through the afternoon
bringing a band of snow along the boundary as it moves across.
Temperatures moderate into the mid 30s along coastal Bristol Bay
and areas along the Aleutian range which will change snow over to
rain before the front passes to the north. Strong low level
southeasterly flow limits precipitation along west facing slopes,
therefore accumulations were adjusted to reflect highest amounts
inland. The air mass remains saturated near the surface overnight
Friday allowing for patchy fog to develop as temperatures cool
through Saturday morning. Areas along the Aleutian Range may
experience dense fog as steeper temperature inversions set up just
off the surface.

Sunday cold air advection returns across the southwest as the low
along the southwest coast dives toward the Gulf. Winds shift to
the northwest on the backside of the low with gusty conditions
along coastal locations as the pressure gradient is squeezed
between the exiting low and a ridge shifting to the east Bering.
Isolated to scattered showers continue across the region through
the weekend as weak upper level waves propagate from the west as
the trough slowly progresses to the Gulf.

&&

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

The low over the western Bering drives an active weather pattern
through Saturday night as it moves strong waves of energy through
the upper levels. This currently stacked system sends a stronger
wave across the eastern Bering today which closes off a triple
point low which then becomes the dominant center later today. The
upper low slowly tracks east through the weekend and meanders
slightly north on Saturday. Expect frequent directional wind
shifts as two surface low pressure systems remain active through
Saturday. Cold air advects along the backside low`s core with the
colder air moving off of Kamchatka. Upper level waves rotating
around the flow keeps scattered showers ongoing through the
upcoming weekend. Cooling temperatures aloft keeps showers
predominantly snow across the northern Bering Friday night into
Saturday with a mix of rain and snow south of the Pribilof Islands.

The next significant front moves to the western half of the Bering
late weekend. This system will bring moderate rain and gusty
southerly gales along its boundary as it swiftly makes progress
through early Monday.

&&

.LONG TERM FORECAST (Days 3 through 7...Sun through Thu)...
The long term will continue to see rather active weather, though
cooler (more seasonable) temperatures should move back in.
A fairly broad trough will slide through the area Sun into Mon.
There are several pieces of energy within this trough that could
produce precipitation. Let`s start where there is the most
certainty...the Gulf Coast. A potent surface front and persistent
onshore flow will continue to keep things very wet from Seward
over to Yakutat. Gusty winds will also accompany the front.
Precipitation should be mostly rain right along the coast, but
inland just a short ways, there will be enough cold air
(especially over the higher terrain like Thompson Pass) to see the
rain turn to snow. Where there is less certainty is across Cook
Inlet. There is likely shot for some snow Sat night into Sun
morning. There is an outside chance that this snow could linger
into Sun a bit longer resulting in a few inches of accumulation.

Meanwhile on Sun into Mon over the Bering, a deep "bomb" cyclone
will develop. This will bring storm-force winds and rain/snow to
much of the Central and Western Aleutians. In response to this
system, a sharp short-wave ridge will build over the Eastern
Bering. This short-wave ridge will push quickly over the mainland
on Mon. It will have a little cold air to work with over Western
AK, so expect some gusty northwest winds to push through the
typical spots (Barren Islands, Shelikof Strait). The front
associated with the 950 mb "bomb" cyclone will slam into Western
AK late Mon into Tue. Precipitation will start as snow, but should
transition to rain across the southern portion of the area. Stay
tuned to this system as some of the initial snow could be quite
heavy.

Model uncertainty starts to build as this front slides towards
South Central AK. Some models depict it bringing a healthy shot of
snow to the Cook Inlet, while others show it washing out before it
makes it that far east. Then uncertainty really begins to increase
towards the end of the week. All models hint at a resurgence of
the North American Ridge (think weather pattern for first half of
Dec), which would once again allow a deep fetch of moisture and
warm air to move over the state from the N Pacific. Exactly where
the ridge sets up will be critical for nailing down sensible
weather at the surface.


&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 161
MARINE...Storm Warning 119
 Gale Warning 120 125 130 131 132 136 137 155 165 170 172
  173 174 175 176 177 178 179 180 181 185 351
  352 411 412 414

&&

$$

SYNOPSIS AND MODEL DISCUSSION...CB
AVIATION...CB
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...MSO



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