Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 160143

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 PM AKST Sat Dec 15 2018

A 960 millibar low sits about 1000 miles southwest of Anchorage
and is the big actor on the weather stage today. Most apparent was
the snow that impacted most of Southcentral overnight. To the
east of this low is another front that is swinging around counter
clockwise from this low. Warm, moist and fast moving air is
flowing out of the south along this front to the north. Storm
force winds are also noted with this front and are sliding north
of Prince of Wales Island. This low is also pushing an abundance
of clouds that stretch from the Alaska range, southeast to the
PacNW. Out west, and south of Adak is a second low that is
bringing gale force winds just along the southern periphery of
the AKPEN marine zones. Feeding both of these lows is a very
strong 180kt Pacific jet that is flowing along the 40th parallel.

Out west on the mainland, persistent and chilly northerly winds
are bringing the coldest temperatures of the season to the
region. Bethel dipped to -13, while Kalskag dipped to -18
overnight. Even light winds of 2 to 4 mph yielded wind chills
close to -25 or colder.


Another day has passed and models are again in great synoptic
agreement for the first 60 hours of the forecast. Aloft, there is
negligible differences in timing and strength of the low south of
Anchorage weakening and getting absorbed into the flow by late
tomorrow. The biggest differences arise from the low south of
Adak, and where surface lows will develop along a frontal boundary
that extends to the east. The overall trend is similar among all
guidance, but differences in timing, placement and intensity seem
to very by 6 to 12 hours and 10 or so MB.



PANC...VFR conditions will persist through the overnight hours
with minimal concerns. Expect a brief period of light snow as an
occluded front nears Sunday afternoon. The front looks like it
pushes quickly west of Anchorage, so think it will be a quick shot
of light snow. Have indicated VFR conditions with this for now,
but expect there might be a short window of MVFR ceiling and vis
as this passes through.


Southcentral will see a break in the snow tonight, before another
chance of snow returns on Sunday. A complex Gulf low moving north
Sunday afternoon will bring another shot of snow to Anchorage and
much of Southcentral.

This will be a two part system. First, the leading warm occlusion
will bring strong east-southeast winds to coastal areas. This
front will quickly slide from Prince William Sound across the
Chugach Mountains and into the Susitna Valley. Only around an inch
of new snow is expected across the Western Kenai up through the
Matanuska Valley with this fast moving front. However, for
Valdez, there is an impressive surge of moisture. With low
pressure holding offshore and keeping the gradient from the NE,
the cold air should hang on enough to produce over foot of snow
by Mon night.

Part two of the event looks to be the more impressive snow maker
for much of the Kenai Peninsula through the Anchorage Bowl. A
deep trough of cold air ejects a fast moving shortwave from SW to
NE up Cook Inlet. This should produce favorable snow conditions
(strong vorticity advection, cold air, moisture, and some modest
south to southwest low level flow). A surface low over the North
Gulf of AK will help to pull some of the previously mentioned
warm air into Western Prince William Sound. This will likely lead
to a wintery mix (or turning to all rain) for Whittier and parts
of the Portage Valley. The good news further inland is that it
looks like a lee-side trough should set up on the west side of
the Chugach Mountains. This should help to bring northerly flow to
the Anchorage area and reinforce enough cold air to keep the
precipitation all snow. This short-wave is moving from south to
north rather quickly and does not have near the instability of the
snow Fri night. So we have trimmed back the snow accumulations a
few inches with this package.

The good news for snow lovers is that cold air advection and a
weak surface low near the Gulf coast could cause snow to linger
into Tuesday for the Kenai Peninsula and Anchorage Bowl.


Light snow showers will linger tonight over eastern portions of
Southwest Alaska while the rest of the region will remain dry.
Light northerly flow will also dominate the area pulling down more
cold air from the north. On Sunday, an upper level low will move
over the area bringing back light snow showers that will clear out
Monday morning as the low shifts north. Meanwhile, along the far
western coast near Hooper Bay and Nunivak Island, snow showers
will move in with stronger winds causing areas of blowing snow
late Sunday night that will last through much of Monday. Tuesday
looks to see a reinforcement of cold air as northerly winds will
dominate Southwest Alaska.


A predominately northerly flow will dominate the Bering and
Aleutians through Monday. The low that is south of the western
Aleutians and tracking east looks to stay far enough south to have
minimal impact to the islands outside of some stronger winds late
tonight into Monday before the low turns south deeper into the
North Pacific. Around the Alaska Peninsula, northerly winds
combined with cold air advection has made for a good setup for
strong gap winds that will diminish slightly Sunday but still
linger into Tuesday. Additionally, snow showers look to develop
along the Alaska Peninsula and eastern Aleutians Monday and
combined with the winds could bring some blowing snow to these
areas into Tuesday. Also on Tuesday a front will move in over the
western Aleutians with a strong enough push of warm air to
transition snow over to rain.


.LONG TERM FORECAST (Days 4 through 7)...
The forecast period commences with the longwave trough extending
from western Alaska to the Gulf Of Alaska and all the way to the
Hawaiian Islands. Upstream, there is a high amplitude ridge that
spans from NE Russia to the Central Bering to the North Pacific.
Meanwhile at the surface, there are the remnants of a cyclone over
Prince William Sound and a mature low near Kamchatka with an
occlusion over the Western Aleutians. As the forecast period
elapses, the low near Southcentral will continue to decay and
the western storm will develop a triple point low and advance
eastward. Expecting another round of gusty winds and rough seas
with the Aleutian low. By 12z Friday, the models start to
significantly diverge at which time the confidence level in the
forecast package radically decreases.


MARINE...Gale Warning 119 120 130 131 132 136 138 150 155 176.
         Heavy Freezing Spray Warning 160 180 181 185.



MARINE/LONG TERM...PS is the U.S. government's official web portal to all federal, state and local government web resources and services.