Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 161147
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
347 AM AKDT Wed Aug 16 2017

.ANALYSIS AND UPPER LEVELS...

Water vapor imagery this morning shows a potent jet streak
transiting the Bering Strait and moving into the eastern Bering
Sea. By the look of recent satellite imagery, it appears the upper
level trough that encompasses the entire state of Alaska has begun
to amplify in response to this jet streak. The trough that
encompasses Alaska is an extension of the polar jet stream that
continues to filter in colder air across the majority of the
state. The subtropical jet, which has made several appearances to
Alaska through the summer months, remains well south in the North
Pacific due to the impressive polar jet. This pattern does not
look to change anytime soon as continued disturbances from the
arctic will only act to reinforce the current synoptic pattern
that has been observed for the past week or so. In the mean time,
high pressure over the Western Aleutians has actually brought
about some clearing skies with fog not being an issue due to the
very dry surface layer. Looking back toward the mainland, a rather
impressive frontal boundary has started to bring rainfall to the
Bering Sea coast with additional showers developing as far east as
the Cook Inlet Region due to some instability ahead of the main
trough.

&&

.MODEL DISCUSSION...

Models remain in very good agreement through much of the short-
term. Numerical guidance, from both the 00Z and 06Z runs, has
initialized the jet streak over the Bering Strait rather well
which helps increase confidence in model output. While models
handle the evolution of the longwave pattern rather well, a series
of disturbances embedded within the upper level trough over Alaska
are hard to resolve. This will keep some areas more rainy than
others through the forecast period but should have little impact
otherwise. One thing we will be watching is Tropical Storm Banyan
that was located at 31.2N 163.4E at 09Z Wednesday. This system is
expected to become extratropical by Friday as it moves toward
Amchitka and Adak. Models notoriously struggle with extratropical
transitions and thus are showing a wide variety of solutions.
Overall, this system is expected to have little impact to the
region thanks to the high pressure over the Western Aleutian
Islands.

&&

.AVIATION...
PANC...Rainfall will move into the area between 15Z and 16Z.
ceilings will drop to MVFR with the possibility of brief IFR
ceilings in heavier embedded showers. Rainfall will aper off
and ceilings begin to improve around 06Z to 09Z Thursday but
showers will remain in the area.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level trough over Southwest Alaska will move into
Southcentral today. Moist southerly flow aloft will bring rain to
much of the area, with the exception of the Copper River basin
which will be mainly dry and breezy. The rain will change to
showers tonight as the main moisture stream pushes to the south.
Showers will continue through Thursday as fairly strong cyclonic
flow aloft remains over the area. By Friday morning showers will
diminish, as the trough begins to lift out and the flow aloft
becomes more westerly.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An upper level trough has started to work its way into the
southwest Mainland from the northern Bering this morning, as well
as an upper level low tracking through the Alaska Peninsula and
Bristol Bay area. This upper level forcing working in tandem with
the onshore flow supplying ample amounts of moisture to the area,
has started to support the development of widespread rainfall
across the Bristol Bay and Kuskokwim Delta this morning. The upper
trough will remain over much of the southwest Mainland, with a
series of embedded shortwaves tracking through the area and
keeping showers in the forecast through Thursday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
High pressure in the west Bering continues to support relatively
drier conditions and less cloud cover in the region, with broad
northwesterly flow across much of the Bering. However, with
slightly more stable conditions over the western Bering, some
patchy fog is still expected over the area. The low level jet
between this high pressure system and the trough over the
Kuskokwim Delta coast will gradually increase through this
afternoon and bring gales to the Central and Eastern Bering.
Fairly drier conditions are expected across much of the Bering,
with the exception of the eastern Bering as an upper level trough
dives southward.

&&

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

An active wet pattern continues in the long range forecast
Saturday through late next Monday. 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.


&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale...155 170 171 179.
FIRE WEATHER...NONE.

&&

$$

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



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