Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 171210

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
410 AM AKDT Thu Aug 17 2017


Infrared satellite imagery has been our best friend tonight
because it has allowed us to track a cold pool that had been
situated over Anchorage during the evening hours. After several
rounds of moderate to heavy rainfall accompanied by thunder and
small hail, the cold pool associated with the upper level low over
Southcentral Alaska has finally shifted off to the north. While
Anchorage has seen improving conditions, rainfall continues
across portions of the Matanuska and Susitna Valleys early this
morning. Water vapor imagery has also been useful because it has
allowed us to track the jet streak that continues to dig out the
upper level trough that extends across the Alaska mainland into
the North Pacific. Needless to say, the trough over the state of
Alaska is rather impressive with the upstream ridge over the
Bering Sea being equally as impressive.

Attention is beginning to turn towards ex-tropical storm Banyan as
it begins it`s extratropical transition and moves toward the
Central Aleutians. There should be little impact to the Aleutian
Islands in terms of wind and rain as much of the cold air is tied
to the arctic trough currently encompassing the Alaska Mainland.
With little cold air to ingest, this cold core "extratropical
low" will have a hard time gaining strength.



Models are in good agreement through this afternoon but are
struggling how to resolve the low pressure system approaching
Mekoryuk and how it will dissipate over Bristol Bay later today.
This should have only minor impacts on the overall forecast with
the main focus being the arctic trough entrenched across the
Alaska mainland. With the trough well entrenched across the state,
it looks like continued rain showers will be common as mid-level
and upper-level instability should help maximize precipitation
processes. Ex-tropical storm Banyan will remain a problem child
until the extratropical transition is complete and it begins being
picked up by the upper air network. Exact placement of this
system as it moves toward the Aleutians is rather difficult to
determine but the arctic trough over the mainland should actually
deter the system and keep it from impacting the mainland.



PANC...VFR conditions and light winds will continue through
tonight with the possibility of a few stray showers throughout the
day. MVFR ceilings are expected Friday morning as stratus over
Cook Inlet develops and moves into the airport.


The upper level trough over southern Alaska will remain in place
for the next couple of days while slowly weakening. The flow aloft
will periodically switch between southwest and west as a series of
vorticity maximums sweep through the area. The over-all result of
this will be cloudy and showery weather. The shower activity will
increase with each vort-max and decrease in between them.
Temperatures will be a bit below normal, especially the daytime


An upper low over the Bering Strait will continue to eject minor
shortwave disturbances southeastward along the western periphery
of the associated trough, and over much of the southwest Mainland
through Saturday morning. This pattern will keep precipitation
over much of the southwest Mainland into the weekend, with a small
break/decrease in shower coverage Friday night into Saturday
morning. Foggy conditions are also expected over parts of the
Kuskokwim Delta and Bristol Bay areas, as persistent onshore flow
keeps a healthy supply of moisture flowing over the area.


Rain and gale force northwesterly winds will continue streaming
from the Central Bering into the Eastern Aleutians/AK Pen as a
deformation band passes through this morning. Most of the
rainfall from this system will be focused along a narrow area with
the deformation band over the eastern Bering and stretching
southeastward into the Alaska Peninsula. A second wave of
precipitation will quickly follow a similar track from the
northern Bering, tracking into the Pribilof Islands tonight and
stretching southeastward into the Alaska Peninsula through friday

Also starting on Friday, the extra tropical remnants of Tropical
Storm Banyan will have weakened substantially as it moves into the
Central/Eastern Aleutians. There is very good confidence that
this system will have weakened to small craft or less by the time
it approaches the chain, with no significant impacts expected to
the area.


.LONG TERM FORECAST (Days 3 through 7)...

An active wet pattern continues in the long range forecast
Saturday through late next Tuesday. A merger of two low pressure
systems is anticipated for this upcoming weekend. A broad trough
deepens over the western coast and shifts east through Saturday.
At the same time a fast paced North Pacific low races along the
Aleutian Chain, taking a northward turn Saturday night. The merger
of these two systems will likely bring moderate to heavy rainfall
along the gulf coastal areas and Southcentral. Models differ in
timing and track of this merger, therefore a broad brushed
solution of the ensembles of the ECMWF and GFS were used to update
the inherited forecast. For the Bering and Southwest mainland, a
short break with drier conditions settles in Sunday as a ridge
builds behind the eastward moving trough over the mainland. On
Monday rain moves back into the southwest coast from a fast diving
low off of Kamchatka, and another North Pacific front moves into
the western Bering/Aleutians along an eastward progressive
subtropical jet streak.


MARINE...Gale...155 170 171



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