Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 150025
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 PM AKDT Thu Sep 14 2017

.ANALYSIS AND UPPER LEVELS...
The deep long wave trough continues to encompass the Bering Sea
anchored by the upper level low over Wrangel Island. Embedded in
the base of this trough is a surface low south of the Pribilof
Islands rapidly moving to the Bering Sea coast. This low is
pushing a front into the Bristol Bay area today spreading rain and
areas of fog along and inland of the coast. The front has swept
through Kodiak Island and is pushing to the North Gulf Coast. The
strong gradient along the coast is resulting in the strong cross
barrier down slope winds across the Chugach Mountains, thus the
typical rain shadow is evident on the radar and satellite
imagery.Further out west is the next storm on tap for the Bering
Sea and Aleutians now south of Shemya.

&&

.MODEL DISCUSSION...
In the short term the models are in good agreement. As we move out
to late Friday and Saturday the models continue to struggle to
reach a consensus for the track of the next storm that will impact
the Aleutians and Bering Sea. The ECMWF has been the most
consistent with this storm and the Nam and GFS have been coming
around to the ECMWF solution over the past several model runs and
continue to do so. The models are in good agreement on the
intensity of the storm, but the challenge is where precisely will
the low center track. The latest runs have continued to move
toward the ECMWF solution. This is critical as to whether the
strongest band of wind hits St George, or both St George and St
Paul, or entirely misses the Pribilof Islands late Friday into
Saturday.

&&

.AVIATION...
PANC...Expect the VFR conditions to persist with showers in the
vicinity through this evening. The gusty Turnagain Arm wind is a
little tricky this evening as it appears to have gone aloft late
this afternoon which is causing some low level wind shear as a
result. While these winds are expected to diminish this evening
there is still a chance they will come back to the surface and
cause some on-and-off periods of gusty SE winds.
The Turnagain Arm winds should pick up again tomorrow afternoon.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level shortwave tracks from the south to north through
the Southcentral Alaska area tonight before exiting the region by
Friday afternoon. This synoptic feature will be accompanied by
rain with with gusty southeasterly Turnagain Arm winds, Knik
winds, and the Copper River basin winds before the gradient begins
to weaken. Meanwhile, a storm force low is tracking near the
Pribilof`s with an associated weather front approaching the lower
Southwestern Gulf by late Friday afternoon. As this system moves
eastward look for a triple point low to develop over Kodiak
Island by early Saturday morning. The front with this system will
track northward into the northern Gulf by Saturday afternoon.
This will bring widespread moderate to heavy rain at times through
the Southcentral region. As the pressure gradient strengthens, a
barrier jet develops along the northern Gulf with strong gusty
southeasterly to easterly winds into the Prince William Sound area
before spreading into Turnagain Arm and higher elevations as we
head into the weekend. The Anchorage Bowl looks to have drier
condition resulting from the cross-barrier flow. Therefore, look
for mostly wet conditions in general.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Showery and cloudy conditions will slowly diminish through the
tonight period over much of the southwest mainland as the upper
level trough pushes further eastward and the surface front
continues to weaken. A stronger front and low pressure system
comes quickly on the heels of the above mentioned system. This
strong system will swing a front over the mainland on Friday
bringing another round of rain and gusty southeasterly winds to
the area. The front will exit on Saturday leaving the area in a
moist and showery cyclonic flow as the parent low tracks into
Bristol Bay. Strong westerly winds are expected to pick up along
the Alaska Peninsula Friday afternoon as the low traverses the
eastern Bering Sea through Saturday evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A strong Bering Sea low continues to develop over the central
Aleutians/Bering this evening with a leading gale-force front
tracking over the Eastern Aleutians/Bering during the tonight
period. This leading front will spread rain and gusty south to
southeasterly winds over the eastern half of the Aleutians and
Bering through Friday afternoon before tracking inland.

The focus then shifts to the parent system which will continue to
intensify further over the central and eastern Bering Friday.
Colder air begins to filter in on the back end of the system
Friday morning, allowing the low-level jet to sink and increase
the gusty winds at the surface. By Friday afternoon, hurricane-
force gusts are expected to develop along the western periphery of
this system, but there is still some uncertainty as to where the
core of the highest winds will track. With the increased
probability of the strongest winds being in proximity of the
Pribilof Islands, in particular St. George Island, we issued a
High Wind Watch for this area from Friday evening through
Saturday morning. Winds over the eastern Bering will start to
diminish as the system slowly weakens and tracks into the Bristol
Bay area on Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term outlook will feature a somewhat complicated pattern
beginning next week with various model solutions and little
consensus, keeping overall forecast confidence past day 4 low at
this point. To start, a strong low pressure system originating in
the Bering Friday/Saturday will move into the Gulf of Alaska and
then east into Southeast Alaska while weakening by early next
week. On Monday, models are showing a weak shortwave trough
dropping out of the Arctic, riding the back end of the upper level
trough over the state, which will lead to more unsettled weather
for southwest and southcentral Alaska keeping temperatures cool.
By midweek, a low pressure system supported by an active sub-
tropical jet forms south of the western Aleutians then heads east
skirting the island chain. Models diverge greatly at this point
with the GFS taking the system too far south to impact the area,
while the EC takes a more northerly trajectory bringing a front
with heavy rain up through Southwest and eventually into
Southcentral by Wednesday evening. Since the EC has been more
consistent over the past week with handling a more amplified flow,
the forecast trended towards the later solution.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Watch 195.
MARINE...Gale 150-160 171-177 180-185.
         Storm 165 170 179.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...KVP



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