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fxak68 pafc 220142 

Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
442 PM akst Tue Nov 21 2017

Analysis and upper levels...
early this afternoon a weak front stretches along a line from
just south of Talkeetna to the southern tip of the western Kenai.
The upper trough driving this feature is swinging eastward from a
parent low over the Chukchi Sea. An upper shortwave ridge is to
the east of the trough with nearly zonal flow to its west over the
Bering. The next impactful low to monitor is quickly approaching
the western Aleutians from the North Pacific.


Model discussion...
models are generally in good agreement on the synoptic pattern
through the short range forecast into early Thursday evening. Good
model consistency is noted along the last several runs, therefore
only minor updates were needed for the afternoon package.


panc...VFR conditions are expected through the taf package,
however there is a chance for brief MVFR to develop along a weak
front projected to move across the Anchorage bowl this evening.
Winds will generally be a light northerly flow through Wednesday


Short term forecast southcentral Alaska (days 1 through 3)...
a weak shortwave trough will continue pushing east across
southcentral this evening bringing a quick shot of snow to most
locations on the west side of the chugach and Kenai Mountains,
though no more than an inch of accumulation is expected. The
trough weakened considerably as it came over the Alaska Range
today and will fall apart even more as it moves over higher
terrain to the east, meaning areas along the Prince William Sound
and Gulf Coast should see little if any snow from this system.
High pressure will build in again behind the departing trough
tonight and tomorrow bringing another shot of cold air advection
and strengthening northerly gap winds. The orientation of the
upper level flow and pressure gradient will favor areas along the
coast including Valdez/Thompson Pass, Whittier, and Seward, where
winds could gust as high as 35-50 mph peaking tomorrow afternoon.

The flow will rapidly shift tomorrow evening as another low over
the western part of the state moves in. A front extending from the eastern
Bering across the akpen will sweep NE over Kodiak and the
southern Kenai bringing another round of rain and snow to those
areas. High pressure over northwest Canada will block the low from moving
further north and push the center to the south and east over the
Gulf, keeping the area mostly dry through the rest of the week.


Short term forecast southwest Alaska (days 1 and 2)...

The forecast period commences with a polar low over the Chukchi
Sea with the remnants of the frontal boundary moving into western
Alaska. In the wake of the frontal passage, ridging will build
into southwest Alaska tonight resulting in a lull between
cyclones. Wednesday a mature low will have emerged into the
central Bering and the models have converged on a solution that
bring this storm to southwest Alaska late this forecast period.
Early Thursday the surface low will begin to dive southeastward
from Nunivak Island to Bristol Bay. Widespread precipitation and
coastal impacts anticipated with this storm.


Short term forecast Bering Sea/Aleutians (days 1 and 2)...

The progressive pattern for the Bering and Aleutians continues.
A low that is currently south of Attu Island is progged to
rapidly deepen and push north and east. This storm is projected to
evolve into a significant storm that will harness a widespread
area of cold air advection. The attendant cold front will migrate
eastward impacting the Aleutian chain and the central Bering. The
operational runs of the GFS and the NAM have the surface low
deepening more than 20 mb in 24 hrs. Anticipate strong winds,
rough seas and wintry precipitation. Early Thursday, the surface
low will begin to dive southeastward from Nunivak Island to
Bristol Bay.


Long term forecast (days 3 through 7)...

The forecast heading into the weekend will see the exit of a
strong low into the Gulf of Alaska and eventually the Pacific northwest of
the lower 48. Friday, winds along the akpen should begin to abate
and the concerns for gusts reaching warning level will wane. This
weekend will see a transient Bering Sea ridge build north of the
central Aleutian chain and slowly slide to the east. Low pressure
lifting from the western Pacific will drag a front across the
Bering and merge with a different area of low pressure Sunday.
This will reintroduce precipitation across much of the southwest.
However, with strong southerly wind transporting warmth aloft,
problems with precipitation type arise during this timeframe. The
Friday and through the weekend forecast for southcentral remains
pretty basic. Offshore flow will dominate the entire period and
skies should become relatively cloud free. Temperatures will
slowly drop this weekend with the clearing skies and limited wind
overnight. Chances for snow are bleak through at least Monday.
Guidance takes a turn come next week and becomes challenging. The
long range forecast trended to more of an ensemble weighting as
the Standard suite of deterministic models provided a wide range
of forecast possibilities.


Afc watches/warnings/advisories...
public...cfw and west-southwest 155 and 161, npw 181 and 195.
Marine...storms 165 170 179 180 181 185 411-414. Gales 120 130-132
138 150 155 160 172-178.



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