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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
434 am akdt Tuesday Oct 25 2016

Analysis and upper levels...

A deep cyclone is moving north across the southern periphery of
eastern Siberia. The associated high end gale force front
(verified by surface observations and ascat satellite data) is
tracking east across the eastern Bering Sea/Aleutians and the
Pribilof Islands. Strong shortwave ridging remains over southern
Alaska. An offshore pressure gradient across southcentral is
keeping outflow winds going as a shallow cold air mass is
maintaining the pressure gradient. However, with northerly flow
aloft weakening, the pressure gradient will also begin to weaken
today. With time, the strong front will quickly weaken and stall
out over southwest Alaska and Kodiak Island as high pressure aloft
shifts into the Yukon Wednesday.


Model discussion...

The models are in excellent agreement with the large scale pattern
heading into the beginning of the weekend. However, they are in
lesser agreement with a developing cutoff low south of Kodiak
Island Wednesday and Wednesday night. The GFS is stronger and
farther north than all other models. The impact would be storm
force winds across coastal waters south of the eastern Aleutians.
Precipitation would also spread farther north along the Gulf
Coast. The European model (ecmwf) was a good middle ground solution among the GFS
and Canadian model which keeps gale force winds and a farther
south track of the surface low/front.



For Anchorage...VFR conditions with light winds will continue
through Wednesday.


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

Outflow winds along Prince William Sound and the north Gulf Coast
will finally die down today as the upper ridge moves overhead of
southcentral. An occluded front moving onshore of western Alaska
today will stall west of the Alaska and Aleutian ranges tonight
as the upper level trough is forced northward up the west side
of the ridge. A little precip may spill over the mountains to
Kodiak Island, but the primary change in weather for southcentral
will be a shift to southerly flow as the ridge slides eastward.
This will lead to increasing high clouds and slowly moderating

Meanwhile, a short-wave crossing the North Pacific today will
interact with a triple point south of the Alaska Peninsula tonight
leading to cyclogenesis. There is some uncertainty in the track
of this low as it passes south of Kodiak Island Wednesday and
Wednesday night. However, there is high confidence that this will
bring stronger winds and rain to Kodiak Island and much of the
Gulf. The northward extent of the low will be the key factor to
whether some of this rain and wind reaches the Kenai Peninsula
and north Gulf Coast. Inland areas of southcentral will surely
remain dry.


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

The Bering front moves into the eastern waters this morning
spreading rain and strong gusty winds to the Kuskokwim Delta and
Alaska Peninsula. A strong low level jet Ushers a warmer Pacific
air mass across the southwest through the afternoon; therefore,
precipitation across inland areas should fall mainly as rain for
areas west of a line from Sleetmute to Iliamna. The front weakens
as it progresses east of this line tonight, but it maintains
enough organization to develop light rain and snow along the
Aleutian and Alaska Range through early Wednesday morning. Patchy
fog develops behind the front tonight as wind tapers down and a
shallow cold air mass advects in creating a weak inversion just
off the surface.

On Wednesday into Thursday, a low pressure system develops south
of the Alaska Peninsula. Gusty winds increase across the Alaska
Peninsula and Bristol Bay as the low deepens and lifts north
toward Kodiak Island. This system keeps precipitation chances in
the forecast through mid week with easterly waves rotating toward
the Bering. There is some uncertainty in how warm the air mass
becomes, bringing challenges precipitation type; however, downsloping
on the Lee side of the mountains should keep accumulations light.


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

The front across the central Bering moves into the eastern waters
by late morning bringing rain and storm force winds through the
afternoon. South winds shift out of the west and taper down
behind the front, but gale strength winds remain widespread
through mid week as strong shortwaves rotate in from a new low
center developing within the dominant low over eastern Siberia.
Isolated thunderstorms are possible today across the western
Bering and Aleutians as an unstable air mass advects in from the
North Pacific. The next surface low to monitor for storm force
winds, approaches the western Aleutians Wednesday evening. This
feature is expected to deepen rapidly as it nears Shemya and then
weaken as it crosses into the Bering.


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

Overall there has been very little change in the long term
forecast thinking since yesterday, with an upper level ridge
building over the southern Mainland and a broad upper level
trough tracking over the Bering Sea and Aleutians by the end of
the week. The ridging over southcentral and the Gulf will keep
most of the active weather confined to the Bering or North
Pacific. This will allow for the current sunny, dry, and offshore
flow pattern to persist into late this week.

Models remain in good agreement that the the blocking ridge will
begin to shift to the east, allowing the amplified trough to
become much more broad, and possibly bringing more active weather
into the Gulf and possibly into southcentral. However, there are
still some significant differences in the models as to how much
development will take place in the Gulf, as well as with a
developing low over the Bering on Friday. As a result, these
inconsistencies in the models have kept forecast confidence in
the long term forecast low from the weekend through next week.


Afc watches/warnings/advisories...
Marine...155 165 170 172 176 177 178 179 180 181 185 411 412 413



Synopsis and model discussion...ja
southwest Alaska/Bering Sea/

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