Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
417 PM akdt Sat Aug 29 2015
Analysis and upper levels...
a pattern adjustment is underway this afternoon. An upper level
trough that extends from eastern Russia to The Rockies contains a
cut-off low over Barrow that is beginning to dive southeast to the
Gulf. Another large cut-off low is between the southern Kamchatka
peninsula and the western Aleutians. A North Pacific ridge is
still poking into the southern Bering. At the surface...a broad
area of low pressure is over the northern Gulf...with another low
west of Attu. Another weakening low is near Barrow. A surface
ridge has retreated into the North Pacific with only the extreme
northern quadrant still nosing into the Bering...while another
high from eastern Russia is diving into the northern Bering. A
weather front is bringing some rain to portions of southwest Alaska
and Kodiak Island.
models remain in excellent agreement and confirm the pattern
shift. The upper low over northern Alaska will dig into the base of
the trough by Monday and temporarily consolidate over the
northeastern Gulf Coast. This digging will be accompanied by two
waves that will sequentially tighten the surface pressure gradient
across the southern half of the state. The first will be tonight
into sun...and the second stronger wave Sun night into Monday
morning. The initial wave will push the Bering/southwest Alaska front
into the North Pacific.
While this is happening...high pressure nosing into the southern
Bering will merge with the high moving off the russian coast later
on sun. This ridge of high pressure will then dominate the eastern
Bering and then move into southwest Alaska Sun night into Monday. By Monday
afternoon the pressure gradient over southwest Alaska will weaken...but
be slightly more persistent over southcentral...at least until
high pressure and cooler air on northwest flow begin to filter
across the Alaska Range later on Monday and into Tuesday.
In the far west the Kamchatka/Attu low will move north and bring a
front across the western Aleutians sun and the western Bering on
Monday. Any eastward progression of this feature will be limited by
the strength of the high that develops over the Bering.
In order to better capture details of the wind field the forecast
utilizes the higher resolution NAM in the east and Gem/ec in the
Short term forecast southcentral Alaska (days 1 and 2)...
the northerly offshore flow will continue as we head into the
early part of next week. We will see cooler temperatures combined
with strong gusty gap winds associated with upper level
disturbances moving through the region. The first disturbance will
drop from the north later tonight resulting increased pressure
gradients. By Sunday night the next disturbance will follow the
same track as the previous and exit the southcentral region into
the northern Gulf by Monday. This synoptic feature will further
enhance the strong gusty northerly winds. Look for the same
scenario with strong gap winds through Whittier...Seward...Valdez
..Thompson Pass and the Copper River Delta through Monday before
the weakening of the pressure gradient. The other area of concern
will be the Barren Islands and the northern Kodiak Island region.
They will have strong gusty northwesterly winds as the colder air
filters into the region. The Barren Islands are expected to have
gale force winds by the late evening as the shortwave moves
through and will be accompanied by the shortwave pushing through
the region. Kodiak Island will be in the same strong gusty gap
wind regime before diminishing by late Monday.
Short term forecast southwest Alaska (days 1 and 2)...
as the upper low over the state elongates to the south it will
keep north to northwest flow over the southwest through Monday.
This region will also be squeezed between the low and ridging over
the Bering keeping gusty winds...especially through channeled
terrain and along the coast. The winds will then weaken Monday as
the low pushes off to the southeast. Shortwaves circulating around
the low will also keep showers over the southwest that will
diminish as ridging over the Bering pushes in over the area into
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the main impact for the Aleutians is the low to the southwest of
the western Aleutians and its associated front that stretches
across the central Aleutians and much of the southern Bering. The
front will continue its slow track east bringing rain to the
islands and then diminish as it is being held up by ridging to
the east. The low itself will slowly track north and bring another
shot of rain to the western Aleutians Sunday afternoon as it passes.
Long term forecast (days 3 through 7...Tuesday through sat)...biggest
change for this time period from the previous days is that it
looks like the northern energy will keep the ridge suppressed a
bit longer. Rather than seeing the ridge be over the southwest and
southcentral portions of the state on Tuesday...there will still
be some pieces of energy moving through from the northwest. This
will keep some gusty winds...especially through favored areas like
Seward...Whittier...and the Barren Islands. It will also keep
temperatures on the cooler side for another day. Places further
inland where the winds die off first will likely be the coldest
spots on Tuesday morning. By Wednesday...models still indicate
that an extension of the North Pacific ridge should build over
southwest and southcentral. This will result in abundant sunshine
and should allow temperatures to moderate a bit going through the
Over the Bering...typhoon atsani remnants will stay anchored in a
broad trough over the western Bering. This will keep warm moist
air flowing into the eastern Aleutians and Pribilof Islands
leading to widespread fog and the chance for rain. By
Wednesday...a weak surface low develops over the central Aleutians
along an old boundary. Models are now in much better agreement in
carrying this low quickly northward toward the Bering Strait.
While not a deep low by any means...it could be just enough to
help to tick winds up to the small craft level. It will also bring
a fairly wide swath of rain from the eastern Bering to the West
Coast by Wednesday morning.
The latter half of the week will feature the warm front over
southwest Alaska falling apart as it tries to make it across the Alaska
Range. But the ridge and dry over the Mainland could be enough to
keep things dry across much of the area. Even if a few showers do
develop across the Kenai and Chugach Mountains into Friday...no
one system looks impressive at this time. The flow becomes more
zonal by next weekend...which could open the door for more weak
systems from the Bering.
Marine...gale 121 127-138.
Synopsis and model discussion...ds
southwest Alaska/Bering Sea/Aleutians...dk