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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
118 PM akdt Monday Sep 15 2014

Analysis and upper levels...
the alaskan domain remains under the influence of a sprawling
longwave trough centered over the eastern Bering Sea. The strong
low that has dictated regional weather conditions for more than a
week is now substantially weakened...in fact the upper circulation
has largely weakened into an open wave that is now rotating
northward toward southern Alaska. A round of steady rain moved
through south central Alaska earlier this morning...with the
majority of rain again falling along the coastlines. Drier and
colder air aloft has now worked into the area...and after a brief
lull the deep instability in the atmosphere is starting to be
realized in the form of showers and locally heavy downpours over
the Kenai Peninsula heading northward toward Anchorage and Prince
William Sound. At the surface...the remnants of the original low
pressure center are gradually moving inland across southwest
Alaska. This arrangement...coupled with weak pressure rises moving
in from the south...is creating a favorable scenario for moderate
strength south to East Gap winds to develop in the usual channeled
terrain areas of the southern Alaska coastline.

West of the Alaska Range...winds across the Mainland are
generally lighter and shower activity more widespread but less
intense across. With the leftovers of the low making slow progress
to the north- northwest...overall wind flow is gradually becoming
more onshore. An area of fog/stratus is evident on satellite
imagery out over Bristol Bay...which should make a gradual
movement toward the coast this evening and overnight. Out over the
Bering a weak low is moving along or just north of the chain. This
low is approaching Dutch...but is a fairly soft-hitter as far as
Bering Sea storms are concerned...and is not expected to bring
major impacts.

&&

Model discussion...
the models are all in reasonable synoptic agreement throughout the
short term (84 hour) forecast period. The low currently
approaching the far western Aleutians will slide eastward near or
just south the Aleutians. As this storm approaches the waters
south of the Alaska Peninsula around midweek it will absorb the
leftover energy in the Bering Sea and result in a significant
storm in the western Gulf by the 2nd half of the week. As
expected...there have been timing and strength adjustments with
each model run...though the European model (ecmwf) has been more erratic/variable
than the American models. Thus prefer to lean fairly heavily on
the GFS/NAM solutions.

&&

Short term forecast...

Southcentral Alaska...
moist southeast flow will gradually become more southerly as the
upper trough and surface low remnants continue to shift off to the
northwest tonight. This will keep most areas with the exception of
the Copper Basin in a rather cloudy and showery pattern for the
next few forecast periods. There may be an improvement in
conditions Tuesday afternoon as weak high pressure builds across
the area...but this will be short-lived as the aforementioned
strong storm taking aim at the western Gulf will usher more
widespread steady rain into the area by late Tuesday night or
Wednesday morning. One of the big remaining questions with the
next storm is how quickly the lower-level flow switches to the
southeast and opens up a downslope hole on the inland side of the
coastal ranges. This should safely happen by Thursday
morning...but Wednesday and Wednesday night could be pretty
wet...especially for the western Kenai Peninsula...before the
downsloping kicks in. Meanwhile...the coastline will certainly see
more significant rainfall totals...although one big difference
from the previous storm will be a more east-west orientation of
the front. Thus expect the eastern Kenai/western Prince William
Sound area to see higher overall totals than the Valdez/Cordova
corridor.

Southwest Alaska...
showers will continue across most of the area through at least
Thursday morning as the remnants of the current decaying low
linger in the area. Fairly light winds gradually veering to more
southerly...especially across Bristol Bay overnight...will likely
bring some of the stratus or fog in from the ocean...with a
saturated boundary layer and the lengthening period of darkness adding
to the likelihood of ceiling and visibility challenges over the
next 24-36 hours.

Bering Sea/Aleutians...
most shower activity will remain over the eastern half of the
region...as weak high pressure west of the Aleutians slowly
strengthens through the end of the week. Winds will remain on the
light side...especially by relative standards...as the multiple
disturbances moving through the area will largely remain south of
the Aleutian chain. By the end of the week...energy will
consolidate around a strong new low in the northeast Pacific. This
will strengthen northerly winds across the Bering and usher in a
surge of seasonably cool air.

&&

Long term forecast...
the longwave trough currently extending from the Bering Sea to the
southwest Mainland will shift eastward to the western Gulf by the
latter part of the week with an embedded upper low remaining in
the vicinity of Kodiak Island through the weekend. This will be
accompanied by a moderate strength surface low that develops south
of Kodiak Island and moves northward into the Gulf on
Thursday...bringing more rain and gusty winds to the Gulf Coast
region through Friday. The wet pattern will then persist through
the weekend as the upper low gradually weakens over the western
Gulf...with rain along the coast and more showery conditions farther
inland. Meanwhile...the eastern Bering will also remain showery as
northerly flow is established in the wake of the trough and
cooling temperatures aloft help to generate instability over that
region.

&&

Afc watches/warnings/advisories...
public...none.
Marine...none.
Fire weather...none.
&&

$$

Amend/cumulonimbus Sep 14

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