Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
430 PM akdt sun Oct 4 2015
Analysis and upper levels...
The upper level flow over southern Alaska has become zonal today
with the 250 mb jet stream flowing over the Aleutian Islands.
A cut off low remains well south of the area near 40n 150w.
The location of the jet stream is conducive for the development
of the surface low in the Bering Sea. This surface low can be
seen developing on the satellite data in the western Bering Sea
and will move into the northeastern Bering Sea Monday. Weak high
pressure at the surface is present over the southwest Mainland and
southcentral Alaska today. This high pressure is also bringing in
low level southwest flow which is helping to keep these areas
moist. Therefore there was widespread low stratus and fog this
morning. Further east the flow is a little more offshore which has
allowed some clearing in the eastern part of Prince William Sound
and eastward along the coast.
models continue to be in good synoptic agreement through the
middle of the week. There has been some small variation as to the
timing and exact track of the low moving into the Gulf of Alaska
Monday night into Tuesday but that looks to have mostly resolved
with the latest model runs.
Short term forecast southcentral Alaska (days 1 and 2)...
A weak surface ridge centers over the Copper River valley tonight
with nearly zonal flow aloft. This pattern presents an increase in
offshore flow along the coast and down the Cook Inlet...elsewhere
expect a relatively weak flow pattern which brings back areas of
fog tonight. Persistence of this ridge pattern can be expected
into Monday across southcentral. Quick weather pattern changes are
anticipated across the Gulf on Monday with two upper level storm
systems from the Bering and the North Pacific heading toward the
Mainland. On Monday morning winds increase across the Gulf...Kodiak
Island and the Cook Inlet...as a surface low near the Kuskokwim
Delta tightens the gradient. A system moving from the North
Pacific is expected to push an upper level disturbance into the
southern Gulf on Monday which brings rain across the southern
central Gulf waters.
A front in the eastern Bering swings north and east Monday night
bringing widespread rain along a boundary from the Kenai to the
Gulf through Tuesday afternoon. Gusty winds can be expected across
the boundary as it pushes north toward southcentral. Winds across
the Turnagain Arm will also increase late Tuesday morning as the
pressure gradient tightens with the frontal approach. Winds were
increased for the turnagain and higher elevations forecast on
Tuesday...also gale strength winds were added the marines near the
Short term forecast southwest Alaska (days 1 and 2)...
southwest Alaska will have a short reprieve from rainfall tonight
as a weak area of ridging moves eastward through the region.
Precipitation tonight should be limited to mostly isolated
showers along the Aleutian/Alaska Range and the northwestern
Kuskokwim coast. A front associated with a rapidly developing low
currently in the far western Bering will move rapidly eastward to
lie along the southwestern Mainland by Monday afternoon and move
inland by Tuesday afternoon. This system has a decent moisture tap
from the North Pacific...so widespread rainfall is expected along
the front. As the low deepens...winds to gale force are expected
along the Kuskokwim Delta and the western convective available potential energy of the Bristol Bay
region. As the low moves north of Saint Matthew Island Tuesday
evening...the front will begin to weaken in place over
southwestern Alaska with winds starting to diminish along the
Areas of fog are expected to redevelop this evening along the
Kuskokwim Delta and Bristol Bay as strong onshore flow and the
weak ridging will allow for fog development. Fog is expected to
dissipate over the region as the front begins to impact the coast
late tomorrow morning.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
winds and seas will be increasing over much of the region tonight
as the low currently between Shemya and the commander islands
rapidly develops tonight as it moves eastward into the central
Bering Sea. The front associated with this low will bring gale
force winds and widespread rainfall as it moves swiftly to the
southwest Alaska coast by Monday afternoon. The low is expected
to move to near the Pribilof Islands by midday Monday and then to
near Gambell by Tuesday morning and become vertically stacked.
While the cold air advection on the backside of the low is not
overly impressive...it should be strong enough to allow for a
swath of gale force winds through the central Bering Sea and
Aleutian chain behind the front in the northwesterly flow.
Long term forecast (days 3 through 7)...
deep layer southerly flow along with a decaying front extending
from a low over the northern Bering will bring rain to much of the
southern Mainland Tuesday night into Wednesday before attention
turns to a deep low set to approach the western Gulf during the
latter half of the week. Guidance has trended slightly faster with
this system over the past 24 hours as it is now expected to pass
in the vicinity of Kodiak Island Thursday night...but the overall
consistency in keeping the intensity around or below 970 mb lends
increasing confidence that a period of strong winds and heavy rain
will impact portions of the Gulf and Gulf Coast through Friday as
an associated front moves over the northern Gulf. Models have also
begun to key in on a potentially stronger system to impact the
Gulf during the weekend as a wave over the central Pacific
entrains moisture from the remnants of what is now tropical storm
oho and then rapidly deepens and rides northward towards Alaska on
the western periphery of a Stout Ridge over western North America.
Confidence in this scenario is much lower given the typical model
struggles involving remnant tropical systems...but this does hint
at the potential for a very rainy and windy end to the week for
the Gulf region. Meanwhile...a showery regime will continue across
the Bering for much of the week in broad cyclonic flow.
Marine...131 165 170 175 177 179 180 181.
Synopsis and model discussion...ez
southwest Alaska/Bering Sea/Aleutians...rf