Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 230039

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 PM AKDT THU SEP 22 2016


A longwave trough over mainland Alaska continues to shift east
this afternoon, pushing the main front in the Gulf onshore over
southeast Alaska. Winds and rain over Prince William Sound and the
Copper River Delta are weakening through the day as the front
departs to the east. The most impactful feature left over our area
is a low pressure system that developed along the front yesterday
and pushed northward inland tonight. This low has brought an
impressive slug of precipitation over the northern Gulf Coast and
the Cook Inlet Region last night through this morning. The rain
associated with this feature has tapered off into showers through
the early afternoon as the low and its associated moisture moves
further inland to the north. Out west, the vertically stacked low
that has persisted over the Bering over the past few days will
continue to weaken as it finally moves inland tonight near Bristol
Bay. Northerly cold air advection around the low is allowing for
fairly unstable conditions over the Bering Sea and Bristol Bay as
sea surface temperatures below the cold air remain relatively
warm. This is supporting ongoing convective showers over the
waters and the Aleutian Islands.


Models are in very good synoptic agreement as the weather pattern
becomes more benign through the weekend. As colder air moves
southward around the base of the longwave trough and over the
warmer waters of the Bering Sea and Gulf of Alaska, models
struggle a bit with the amount of instability depicted through the
weekend. With all else being equal, the higher resolution models
were used for the forecast.

.AVIATION...Gradually improving conditions will be the situation
for PANC. Winds will remain gusty with showers around this evening
but both winds and showers will diminish as the night progresses.
Ceilings are expected to remain VFR but may occasionally drop
below the 5000 FT level this evening and overnight.


After the current low pressure system and front pass this evening
the area will be left in an unstable, showery regime. As the upper
trough moves away this evening the best chances for rain showers
will be in the upslope flow along the north gulf coast, as well as
the northern Susitna Valley. A cold core associated with the upper
low will migrate from the Alaska Peninsula into the northern Gulf
tonight. The cold core aloft in combination with climatologically
maxed sea surface temperatures will result in widespread
instability bringing numerous showers and even scattered
thunderstorms. Currently the majority of strikes is along upslope
areas of the northern gulf, but coverage should increase as
instability does.

Friday and Saturday the same conditions hold, however the highest
confidence in focus for lift is still going to be upslope flow. No
major vorticity maxima are progged, and any that do happen to be
in the flow will be sub-model resolution. The takeaway is that we
are in a pattern conducive for showers but nailing down their
location will be tough more than a couple hours out.


Widespread shower activity will continue to be the primary
weather concern into the weekend across most the Southwest
Mainland as the Bering low makes gradual eastward progress along
the coastline an eventually inland over the Bristol Bay area. The
most intense shower activity is expected to remain along the
coast through tonight in closer proximity to the low, however
showers will begin to diminish in coverage across the Kuskokwim
Delta by Friday as the low pushes inland over Bristol Bay. This
will serve to enhance showers over interior Bristol Bay and the
Kuskokwim Valley through Saturday. With the continued marginal
area of instability over the Eastern Bering, the potential for
some isolated thunderstorms remains in the forecast mainly for
the coastal areas along Bristol Bay and Alaska Peninsula.


The story remains largely the same across the Bering through
Friday as the broad low pushing eastward towards Bristol Bay
continues to promote widespread showers and gusty winds. Winds
will predominantly be around Small Craft Advisory level for most
areas through Friday, although some gale force gusts will be
possible in the vicinity of the Pribilof Islands on the backside
of the low. Showers will begin to diminish from west to east
across the western half of the Bering tonight into Friday night,
as ridging builds in behind the departing low. Some locally gusty
gap winds are expected along the Eastern Aleutians and Alaska
Peninsula in dominant Northwesterly flow Friday afternoon through
Saturday morning. Areas of marginal instability surrounding the
Eastern Bering will keep the potential for some isolated
thunderstorms to continue developing along the Bering and Pacific
side of the Eastern Aleutians through Saturday.


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

Beginning the extended period forecast Saturday evening, there
will be an elongated arctic trough extending from the North Slope
through southern Alaska with a shortwave trough over Siberia
moving toward the Bering Strait. Flat ridging will be over the
Bering Sea. Leftover remnant showers will consolidate along the
Alaska Range and east as the slow moving digging trough supports
the development of a weak lee low over the Susitna Valley. Most
precipitation will be in the form of rain at lower elevations, but
some mixed snow may be possible by Saturday night near the Alaska
Range and Talkeetna Mountains as low as 3000 feet. On Monday, the
shortwave moving off Eastern Siberia will weaken then dig into
southwest Alaska before taking a track into the southwest Gulf of
Alaska. This will keep moisture and precipitation over southwest
Alaska with Southcentral staying dry, aided by some offshore flow
as the previous trough moves east.

Thereafter, much uncertainty develops with the flow pattern as it
becomes more progressive with a series of Siberian shortwave
troughs moving into the Bering. Ridge amplitude is somewhat up in
the air as the models struggle with potential phasing with a weak
migratory NPAC trough. The phasing solutions show the amplification
of a midweek trough moving off Siberia with a decent slug of
moisture tracking into southwest Alaska late Wednesday and
possibly again on Friday. For now a conservative ensemble approach
was taken due to the poor model trends and the large changes from
yesterday. This forecast will continue to keep Southcentral
largely shielded by the Alaska Range with most precipitation
chances remaining over the Bering and southwest Alaska. Some
models are showing a return to storminess by the weekend, but that
is much too far out to be of concern just yet.


PUBLIC...FLOOD Advisory 145.



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