Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
157 am akdt Tuesday Jul 28 2015
Analysis and upper levels...
The region remains dominated by a broad upper level trough
stretching from the Beaufort Sea into the North
Pacific...encompassing all of the Bering Sea and southern Alaska.
The anchor for this trough is a tight closed low over the Beaufort
Sea...with a secondary closed and weakening low over the northern
Gulf Coast. Two weaker shortwave troughs in northerly flow are
tracking south along the western periphery of the trough...one
over the Aleutian Islands...and another currently moving over the
Seward Peninsula near the Bering Strait. Inland...considerable
clouds and moisture continues to linger across the region...with
an ill-defined but moist deformation zone over portions of the
Susitna Valley into the Matanuska Valley. This is supporting a
stratiform rain shield...and extensive clouds that will linger
into the day today.
With the low over the northern Gulf rapidly
weakening...this has resulted in a rather weak surface pressure
gradient...largely dominated by a very weak thermal low across the
interior...and a secondary low pressure center over Cook Inlet.
With time...weak upper level ridging will build in across
southcentral Alaska...with troughing remaining over the eastern
Bering and far southwest Alaska. This will promote continued
shower activity...especially in the afternoon periods.
Temperatures will gradually warm...and clouds will slowly become
less widespread as the atmosphere slowly dries out heading into
the late week.
The models are in excellent agreement with the large scale upper
air pattern through the short term period. There are a few small
minute detail differences with respect to how the models weaken
and move the Gulf low east in the short term...but those do not
have impacts for the region. A weak closed low migrating through
the North Pacific...which was being handled differently by the
various models last night...are now in good agreement and have
trended toward the GFS solution which brings that low toward the
far southwest Aleutian chain. The largest discrepancies...once
again...remain with the amount of precipitation...cloud
cover...and temperatures across the region. No model...in that
regard...was favored as they all tend to struggle with
precipitation processes in these patterns...largely due to the
extensive clouds and lack of strong forcing.
A persistent pattern will remain in place across the
region...with a slow modulation toward slightly warmer and
drier...but definitely not toward any critical levels as the
pressure gradient remains weak...and substantial low level
moisture and clouds remains persistent until the latter week.
Wet thunderstorms will remain a threat...mainly across the
mountains...but will remain isolated in nature.
Short term forecast southcentral Alaska (days 1 through 3)...
This morning will start out with extensive cloud cover across most
areas...as well as a region of stratiform rain stretching from the
southern Susitna Valley into the Matanuska Valley. Some of this
rain will make it into Anchorage as well. Once again...daytime
heating will result in enhanced afternoon showers...especially
over the mountains...with a few isolated thunderstorms over the
Alaska Range and Talkeetna Mountains. By Wednesday and
Thursday...showers will gradually become less widespread...with
decreasing instability as weak ridging aloft begins to develop.
This will allow for a slow gradual warming and diminishing clouds
heading into the latter week. Local patchy fog will continue to
remain a threat during the overnight and morning hours given the
amount of low level moisture.
Short term forecast southwest Alaska (days 1 through 3)...
Weak disturbances aloft moving across southwest Alaska will keep
the area under the threat of showers through Thursday. The most
impressive of these disturbances will begin impacting the region
today as it moves south from the Seward Peninsula. Showers will
be enhanced along the higher terrain...with isolated thunderstorms
forming in the afternoon across the lower Kuskokwim valley and
Bristol Bay region. On Wednesday...this low will begin to
strengthen as it moves into the eastern Bering...with a continued
threat for showers and isolated thunderstorms during the daytime
and early evening periods. Ridging aloft will develop on
Thursday...with decreasing instability and diminishing shower
Short term forecast Bering Sea/Aleutians (days 1 through 3)...
A Stout shortwave trough moving across the central and eastern
Aleutians this morning will leave showers across those areas in
its wake through Tuesday night before the overall broad scale
trough moves south of the Aleutian chain...leaving a broad region
of weak surface pressure over most of the Bering. This will keep
drizzle...clouds...and fog across the area heading into late week.
A weak low will skirt across the far southwest Aleutians on
Wednesday...but this will largely only bring brief easterly winds
to the periphery of the coastal water zones. Finally...a weakening
upper level disturbance will attempt to dive south over the
Pribilof Islands on Wednesday and Wednesday evening...but this
system will be weakening rapidly as it moves south...only bringing
a glancing shot of showers to the Pribilof Islands.
Long term forecast (days 4 through 7)...
There are a few key features that deterministic guidance are
having a tough time resolving over the past couple of model runs.
While models start off in decent shape on Wednesday with an
elongated trough encompassing all of Alaska...the way the pattern
begins to shift over the weekend becomes quite complicated. It
appears that this upper level trough will begin to push south of
the Alaska Mainland as high pressure over the Bering Sea begins to
nudge inland and eventually build across the interior of Alaska.
This will keep shower activity common across southcentral Alaska
while the Bering Sea and southwest Alaska will be largely on the
dry side thanks to the subsidence associated with the high
pressure building across the area. As the upper level trough drops
south...two scenarios will emerge. The first being a concentric
upper level low that pushes into the North Pacific with ridging
pushing into southcentral Alaska. The other scenario being the
concentric low staying in the Gulf of Alaska and keeping rain
showers in the forecast through the extended period. After looking
in depth at the progressiveness of the upper level pattern...have
trended a touch toward the drier and warmer solution for the time
being. Models have been Flipping back and forth a lot in the past
several days so the forecast confidence is below normal.
Synopsis and model discussion...ahsenmacher
southwest Alaska/Bering Sea/Aleutians...ahsenmacher