Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 170037
AFDAFC

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

.ANALYSIS AND UPPER LEVELS...

Water vapor imagery indicates an upper level PV anomaly digging
southward from the northern Bering Sea to the Alaska Peninsula.
This upper level forcing caused widespread rainfall across much
of the forecast area today from the Alaska Peninsula to the north
Gulf of Alaska coast. To the west of this, high pressure is
dominating the western and central Bering and supporting a broad
stratus deck with occasional glimpses of blue skies. In between
these two features, a low level jet is bringing gusty gale force
winds across much of the Central Bering.

&&

.MODEL DISCUSSION...

In the short term, models are in fairly good agreement with the
GFS favored for the majority of the forecast. There are some
minor mesoscale differences between the models on the intensity of
the low level jet along Cook Inlet and through Turnagain Arm this
evening, but generally these differences are relatively small.
There are also the usual minor differences among the models in the
intensity and location of precipitation, but with fairly strong
forcing and consistency in models, these differences are fairly
small.

&&

.AVIATION...
PANC...CLoudy and rainy conditions across the Anchorage airport
will persist for much of the afternoon. There is a chance that a
weak Turnagain Arm wind will develop this evening, which should
help to lift ceilings and switch flow to a more southerly or
southeasterly direction. However, the timing of this switch is
still uncertain.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A typical cool and rainy August pattern will continue over
much of Southcentral through the weekend with a few breaks in
between. A cold upper level low dropping south out of the Bering
Sea is leading to broad moist southwesterly flow over the region
and widespread precipitation. Rain will continue overnight as the
upper low continues to dig south maintaining the overall
southwesterly flow into southcentral. Precipitation looks to taper
off some by tomorrow morning as the first shortwave moves off to
the northeast. Cold air aloft underneath the low will promote
instability leading to showery conditions, especially over the
mountains and the Prince William Sound. With enough daytime
heating, isolated thunderstorms may form over the Talkeetna
Mountains and the Copper River Basin tomorrow afternoon. The
pattern turns messy by Thursday night as several shortwaves
spinning around the base of the upper trough pass through the area
leading to continued chances of rain and showers over most of the
region throughout the day on Friday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An upper level trough slowly passing over the region will bring
continued cool, cloudy, and rainy conditions to the area east of
Bethel through Thursday. There is an area of clearing that is
developing along the southwest coast which, along with onshore
flow, will aid in the development of patchy fog/low stratus
overnight tonight. One limiting factor in this development is the
lack of extensive low clouds over the Eastern Bering which will
prevent any fog from becoming too dense or widespread. On Friday
a weak shortwave moving over the area will bring continued
cloudcover, however any rainfall from this should be light and
spotty in coverage.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
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. Most of the rainfall from this
system will be focused along a narrow area with the deformation
band, which the models are all in good agreement will setup from
just east of the Pribilof Islands southward into the AK Pen. As a
result small changes in the location of this band will have big
implications in how much rain will fall, but at this point there
is reasonable confidence that the heaviest rainfall will occur
along the Bering coast of the AK Pen. 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.

&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...ELN
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK
LONG TERM...KH



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