Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 251228
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
428 AM AKDT SUN SEP 25 2016

.ANALYSIS AND UPPER LEVELS...
The weakening upper level trough that has brought widespread
clouds and showers throughout the northern Gulf and southern
mainland this weekend continues to slowly weaken as it lifts to
the north. Instability from the cold air aloft allowed isolated
thunderstorms to fire again on Saturday evening along the North
Gulf Coast, but these have mostly dissipated since. Over the
southwest skies have cleared dramatically behind the trough axis
with diminishing winds, which may lead to the development of
fog/stratus for low lying areas this morning as conditions remain
moist from recent rains. Over the northern/western Bering a front
associated with a low over eastern Russia is beginning to move
eastward towards the mainland. This low is expected to move
through the eastern Bering beginning this evening, bringing with
it rain and gusty winds to the southwest coast and AKpen.

&&

.MODEL DISCUSSION...
The models remain in very good synoptic agreement into early next
week, especially with regards to the strength of the front moving
through the Bering today. Over Gulf and Southcentral, the main
challenge continues to be with the timing and intensity of
convection firing under the cold upper level trough, and as a
result the coverage of showers throughout the area continues to be
fairly broad. Otherwise forecast confidence remains high into
early next week.

&&

.AVIATION...
PANC...Showers will continue through at least mid-evening as upper
trough axis stalls just to the west with light winds through the
lower atmosphere. Though went with VFR cigs predominating, cigs could
drop to MVFR or even high IFR in times of more persistent shower
activity at any time today. Will need to watch for potential
clearing and low stratus/fog toward the end of the TAF period
Monday morning. For now, opted to leave out as not convinced
clearing will occur for such formation.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

Widespread showers are occurring across Southcentral this morning
as the upper level trough approaches the region, along with cooler
air aloft. Convection is heaviest along the North Gulf Coast near
Cordova. The combination of warm ocean temperatures and cooler
fall temperatures is helping to especially steepen lapse rates in
these areas early this morning with several lightning strikes
occurring. Showers will continue across much Southcentral today
as the upper level trough slowly pushes eastward throughout the
day today. By Monday morning, the upper level trough will push to
the southeast into the Gulf of Alaska and weaken. This will help
to support drier conditions across the region. Surface low
pressure will linger in the Gulf of Alaska through Tuesday, which
should support light offshore flow.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An upper level trough positioned over the Lower Kuskokwim Valley
continues to track eastward accompanied by scattered showers along
the Alaska Range before exiting the Southwest Alaska region by
Sunday evening. Upstream a weak ridge moves into the area by
Sunday morning along the Kuskokwim Delta resulting in drier
conditions as this feature slips into the lower Southwestern Gulf
region by Monday. There will be a slight chance for fog developing
with the clear skies, and radiational cooling this morning over
the Kuskokwim Delta as this upper level ridge moves eastward. By
Sunday evening a weak low pressure system drops off the Kamchatka
Peninsula into the northern Bering. As this low tracks into the
Kuskokwim Bay it brings a swath of rain, and stratus to the
Kuskokwim Delta coastline by late Sunday evening. This synoptic
feature tracks to the Alaska Peninsula by Monday bringing the same
weather scenario to the aforementioned region, and the Bristol Bay
interior.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The Bering Sea/Aleutians remains under southwesterly flow as a 996
mb low drops south off the Kamchatka Peninsula into the northern
Bering this morning. This will strengthen the pressure gradient as
this system moves along the Southwest Alaska coastline. Therefore,
look for a window of gale-force southwesterly winds along the
Central Aleutians from Sunday evening through Monday morning as
this system tracks into the eastern Bering. This synoptic feature
brings rain/stratus from the Pribilof Islands, Eastern Aleutians,
and the Alaska Peninsula through Monday morning. Meanwhile, a weak
ridge builds into the Western Aleutians/Bering by Monday afternoon
before the next new system brings a weather front into the
aforementioned region by late Monday evening.

&&

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

The extended period will be a more progressive pattern than
currently in place with a transition from a low amplitude longwave
pattern to a much more amplified longwave pattern. A series of
very potent shortwaves will move along the periphery of an arctic
trough anchored over far eastern Russia and significantly amplify
through trough as each wave passes. Models, however, are struggling
a lot with how much jet energy will be associated with each shortwave
which looks to have significant impact on how the longwave pattern
(and therefor surface pressure fields) evolve. The GFS is by far
the most aggressive with a jet streak near 100 kt at 500 mb and
150 kt at 250 mb developing a very strong closed low over the
western Bering Sea. This solution is by far the outlier as most
other guidance, including ensembles, keep the upper low much
further north over Kamchatka/Siberia and weaker. All of the
aforementioned solutions amplify the longwave trough significantly
more which correlates to longwave ridging building over the
mainland of Alaska. This should allow the Alaska Mainland to
remain largely on the dry side through much of the upcoming week
before rain chances increase by next weekend as a potent front
pushes across the Bering Sea.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales...170 172.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...EN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...MC



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