Scientific Forecaster Discussion
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Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
458 PM akst sun Feb 19 2017
Analysis and upper levels...
the upper level map looks very similar to yesterday with the only
differences being the trough over the eastern Bering Sea is
slightly farther east and the ridge west of it is a little more
amplified which is bringing a jet streak into the western
Aleutians. Where this jet streak is correlates to the rapid
intensification of a surface low moving toward the western
Aleutians from the south. This low will be the big weather maker
for the Bering Sea area the next day or two. The one other
feature Worth noting in the Bering Sea region is a polar low that
has developed along the ice edge south of Saint Matthew Island.
These polar lows are small but can bring strong winds and heavy
snowfall to the area close to them. This low appears to be heading
southeast and will weaken as it moves between the Pribilof Islands
and southwest Mainland Alaska late tonight. Cold air advection
moving across the Alaska/Aleutian ranges will continue to move
into southcentral Alaska through tomorrow. As is typical, a
surface low is developing in the northern Gulf of Alaska in
conjunction with the cold advection. Weak upper level disturbances
continue to move up Cook Inlet and this will continue into
models are in very good agreement today. High resolution models
were preferred on both sides to try and catch the weak impulses
and small scale features like the polar low along the ice
edge...which is being picked up surprisingly well by the NAM.
panc...ceilings will keep conditions mostly MVFR around panc
through most of the evening and generally at or under 5000 ft for
the next day. Periods of light snow may drop visibilities down to
IFR temporarily through the night. Tomorrow should see slightly
improved conditions from today but there will likely still be
periods of MVFR ceilings.
Short term forecast southcentral Alaska (days 1 and 2)...
Snow has continued to linger across the Anchorage area, Matanuska
Valley and Susitna Valley as a potent shortwave continues to
propagate eastward. This shortwave is expected to continue
pushing eastward tonight but isn't expected to produce snowfall
over the Copper River basin. However, continued cold air advection
coupled with a negatively tilted upper level trough will likely
keep periods of light snow in the forecast for the aforementioned
locations through the overnight hours. The snow growth zone is
expected to become much drier tomorrow afternoon which should
allow snowfall to taper off during the day on Monday. Looking at
current radar images, it appears a band of snowfall has situated
over Fire Island. This band could shift eastward toward Anchorage
tonight as the upper level trough shifts eastward. Currently, we
are expecting up 2 inches of snowfall through Monday morning but
should the band move over Anchorage, higher snow accumulations
would be possible.
Otherwise, a colder and drier pattern will be situating itself
across southcentral Alaska during the first half of this upcoming
week. Expect low temperatures mainly below zero for much of the
region expect for the Anchorage Airport and parts of the Gulf
Coast. The coldest night will be Monday night and temperatures
will begin to return to near normal temperatures on Tuesday.
Short term forecast southwest Alaska (days 1 and 2)...
clearing skies and frigid temperatures will prevail over the
southwest Mainland through Tuesday morning as the longwave trough
over western Alaska slowly pushes eastward, with temperatures
generally remaining below zero with the exception of the Bristol
Bay coastline. The front currently poised to move into the western
Bering will reach the southwest coast during the day on Tuesday,
bringing potentially heavy snow and the possibility of blowing
snow lasting into Tuesday evening. This will also initiate a
significant warming trend heading into the middle of the week with
the passage of the front.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a period of stormier conditions is on tap for the Bering during the
first half of this week as a pair of strong North Pacific lows
track through the western Aleutians in rapid succession. The first
of these lows will track close to Shemya late tonight at around
954 mb, pushing a storm force front across the west/central
Aleutians accompanied by hurricane force gusts over the coastal
waters. Precipitation will begin as snow before quickly changing
over to rain, helping to mitigate any blowing snow concerns over
the western half of the chain. The front will then moves eastward
towards Dutch Harbor and the Pribilof Islands on Monday
afternoon, with winds generally weakening to gale force with some
storm force gusts by that time. A brief window of blowing snow
conditions will be possible over the eastern Aleutians during the
afternoon before the transition to rain. The worst conditions will
be over the Pribilof Islands, where colder temperatures will keep
precipitation as all snow for the duration of the frontal passage.
The blizzard watch has thus been upgraded to a warning. The
passage of this first front will allow conditions to improve
across the eastern Bering on Tuesday, however another strong front
will follow closely on its heels as another mid 950 mb low passes
west of Attu. This will return gale force winds and heavy rain to
the western half of the Bering by Tuesday morning.
Long term forecast (days 3 through 7)...
Midweek through the weekend forecast will be dominated by a ridge
that will nose in across southcentral and a trough that will dig
south across the Bering. Guidance remains in good agreement with
bringing a shortwave across the ridge and sliding southeast across
the Gulf of Alaska. There are select differences with the models
with how strong of a wiggle in the flow it will be and more
importantly how far south it will develop. The forecast today
sided closer with the surface placement and features from the ec
and GFS as the NAM seems to be the outlier in how fast it
transitions the low to Southeast Alaska. Mid week that will likely bring a
shot of snow back to the area. Ridgetop and higher flow is looking
to remain southwesterly, keeping downsloping chances down. Beyond
that time for southcentral, the trough over the Bering will slide
east and allow a front to stall out from the akpen and along the
Alaska Range. This will increase overall surface temperatures as
well as bring back the chances for mixed phase winter weather come
late in the week and again over the weekend.
public...Blizzard Warning 195.
Marine...storms 175-178 411-413. Gales 130 131 155 165 170-174 179