Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 180059
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 PM AKDT Mon Sep 17 2018

.ANALYSIS AND UPPER LEVELS...
A look at water vapor imagery shows a series of upper level lows
affecting mainland Alaska, with stagnant flow resulting in
persistent conditions. The two primary lows are located over the
Interior and the Gulf of Alaska with a secondary weaker one in the
Bristol Bay region. The deeper lows are both moving away from the
southern Alaska mainland with high pressure strengthening in the
vicinity of Cook Inlet. A third deeper low/trough is digging out
of far eastern Russia toward the west coast of Alaska, with a
surface trough pushing onshore. None of these features are
producing very much rain, with scattered to widely scattered
showers. However, cloud cover is fairly extensive across the
region, with some localized areas of sunshine.

Out west, a long-wave ridge dominates the Bering Sea with abundant
low clouds and fog. An intensifying low over the northwest Pacific
is tracking north-northeastward toward the far western Bering Sea.

&&

.MODEL DISCUSSION...
Models continue to struggle with handling of the upper low moving
out of Russia. There is a definite trend toward digging this low
southeastward toward Southcentral Alaska Wednesday night and
Thursday, with the GFS being the lone holdout to track the low
across northern Alaska. Still, uncertainty in timing and track of
the upper low remains high. Otherwise, models are in decent
agreement with large scale features. The biggest forecast
challenge is fog and cloud forecasts in this weak flow regime.

&&

.AVIATION...
PANC...Ceilings may drop to 5000 feet or below this evening
through overnight as very weak up Inlet flow develops. Ceilings
would then lift during the day Tuesday as low level flow shifts
to NW. In any case, expect VFR conditions and light winds to
persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

A complex, yet relatively benign upper-level weather pattern
reveals three areas of low pressure surrounding an upper-level
ridge parked over Southcentral Alaska. Although this ridge helped
scour out most of the lower clouds from the Susitna Valley east
through the Copper River Basin today, the respite from the
widespread cloud cover will be fleeting. A series of vorticity
maxima pinwheeling around the ridge coupled with plentiful
moisture parked underneath the ridge will help increase cloud
cover beginning tonight, especially along the Kenai Peninsula,
Chugach and Talkeetna Mountains, and across the southern portions
of the MAT-SU Valleys. It is also over these areas where enough
instability will be  present to produce a few isolated showers
overnight tonight and again tomorrow.

The best chance for measurable precipitation in the valleys and
across the Anchorage Bowl comes late Tuesday night into Wednesday
morning as an upper-level low currently spinning over eastern
Russia begins to dive southeast into central Alaska. As it does, a
shortwave will eject from the low and strengthen as it crosses the
Alaska Range and moves over the Susitna Valley. Widespread rain
is not expected, as moisture is limited with this system. But
given the increase in intensity as it moves east along with a
west-northwesterly flow (albeit weak), the ingredients are there
for scattered showers to develop along the upslope side of the
Chugach and Talkeetna ranges. This system weakens as it head
father east Wednesday night, but clouds and showers linger into
Thursday as the parent low continues its southward trek.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Thursday)

Showers will persist through the next few days. An inverted
trough will continue to extend north in the mid-levels of the
atmosphere from a weak low, which will slowly migrate from near
King Salmon towards Kodiak over the next few days. This inverted
trough is also getting a boost in the low levels from orographic
lifting and daytime heating along the Alaska range. A separate
area of showers is occurring along a trough axis aloft that
extends from a low over eastern Siberia to the western coast. This
feature will slowly slide south and east through Tuesday.

Showers will continue and expand in coverage Wednesday into
Thursday, as the Siberian low passes by just to our northeast with
disturbances in the northwest flow aloft, giving a boost in
ascent, aided by both a weak surface low in the same region and a
trough extending into the central Kuskokwim valley. Then, a
surface warm front will approach the coast on Thursday as a low
moves through the Bering.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Thursday)

Fog looks to be the initial forecast dilemma through Tuesday, as
an amplifying ridge builds across most of the Bering Sea and
Aleutians, with warm advection aloft passing over the cooler
maritime waters. This is almost ideal conditions for fog to
develop. Any showers through this time look to be confined to the
AKPEN, where northerly flow will allow for mainly diurnally and
orographically-driven showers to develop (aided by disturbances
rounding the ridge aloft), and the far western portions of the
chain. Here, a front extending south from a Siberian low will
allow sustain winds to reach gale force, with widespread rainfall
likely due to good low-level convergence, aided by disturbances in
the southerly flow.

The ridge looks to flatten for Wednesday into Thursday, as the
pair of Siberian lows described in the paragraphs above help
de-amplify it, with another low lifting from the North Pacific
into the Bering on its backside. This system will spread more
rainfall and small craft criteria winds and seas back into the
picture as it`s associated warm and cold fronts spread northeast
and east.

Aside from the initial set of gale force winds across the far
western portions of the chain, sustained winds and/or waves look
to remain in small craft criteria or below.

&&

.LONG TERM FORECAST (Days 4 through 7/Wednesday night through
Sunday night)...
To begin the long term forecast Thursday night, the models are
beginning to come into consensus with a further southerly track of
an Arctic trough approaching from the interior. This will allow
for cooler temperatures and the possibility of mountain snowfall
by Friday morning, especially from Talkeetna Mountains northward
where there is the best agreement for precipitation. Further south
over the Chugach confidence is much less that precipitation will
spread that far to the south.

Behind this trough, weak ridging moving over the South Mainland
should provide for a break in the active weather, however this may
be very brief for the southwest coast as a frontal system from
the Bering quickly moves in to the area to replace it. With the
weak nature of this ridging and poor agreement in how far east the
remnants of the Bering front will reach, forecast confidence for
the upcoming weekend is very low, especially in the rainfall
forecast around the South Mainland. The general trend then looking
into early next week will be for continued deteriorating conditions
for much of the South Mainland as the ridge axis shifts east of
the area and allowing for a stormy pattern over the Bering and
North Pacific to track into the area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 178.

&&
$$

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


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