Scientific Forecaster Discussion
fxak68 pafc 191356
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
456 am akst sun Feb 19 2017
Analysis and upper levels...
a progressive pattern change is beginning across our portion of
the hemisphere this morning. An upper level trough is over much of
the Mainland and eastern Bering, steering snow showers across the
Kenai Peninsula and into the Anchorage bowl and Susitna Valley.
The main upper ridges are over the western Bering and Canadian
rockies. At the surface, weak low pressure is over Prince William
Sound as a large area of high pressure over the Bering is pushing
east. A front associated with a Kamchatka low is moving across the
western Aleutians. The triple point from this low is forming well
southwest of Attu and will become a major player later today.
models agree that the Prince William Sound low will slowly drift
east, allowing the Bering ridge to move across the Mainland
through Tue, ending precipitation in its path. The eastward
progression of what had been a blocking ridge will then allow the
North Pacific triple point low to strengthen significantly. The
associated front will then move across the Bering through Tue. By
this time yet another equally strong North Pacific low will move
to the western Aleutians by Tue night. Through Tue night the
forecast leans toward a NAM solution in order to pinpoint wind
speed and precipitation potential around all of the lows.
panc...weak upslope and cold advection are interacting with a
saturated, unstable atmosphere, causing nearly continuous light
snow this morning. The snow will diminish to snow showers by
midday. With lots of low level moisture in place there is
potential for a brief period of fog as snow ends, so lower
ceilings/vis may linger longer than currently indicated. A weak
low over Prince William Sound may cause additional snow shower
activity tonight which would lower conditions to MVFR once again.
Short term forecast southcentral Alaska (days 1 and 2)...
snow showers out ahead of a 700 mb short-wave and along a narrow
axis of isentropic lift (from the interior Kenai to Anchorage,
the western mat valley and on up the Susitna valley) will shift
northward today, weakening along with the forcing mechanisms.
Showers will linger longest across the Susitna Valley due to
persistent southerly upslope flow on the leading edge of the
Arctic trough sitting to the west.
A short-wave digging down the back side of the Arctic trough will
help kick it eastward into tonight through Monday night. Cold air
advection will be strongest across the western Gulf, so have
continued the trend of strengthening winds. With cold air advecting
in from the west, a Lee side trough will set up over Cook Inlet
and the Susitna Valley, maintaining light winds and allowing the
abundant low level moisture to remain in place. As colder air
filters in from the west and the upper trough becomes negatively
tilted, this should provide just enough upward vertical motion to
produce some light snowfall tonight into Monday. However, expect
very little in the way of accumulation.
As the trough continues eastward across southcentral and the Gulf
Monday and Monday night, tightening pressure gradients and cold
air advection will lead to gap winds across western Prince
William Sound and the northern Gulf Coast. Meanwhile, colder
temperatures will settle in across the region.
Short term forecast southwest Alaska (days 1 and 2)...
clear, cold, and light winds will be the predominant conditions
for the next couple days. Very early today some low clouds and
flurries will be left over along the Alaska/Aleutian ranges, but
will dissipate fairly quickly this morning. The Arctic air
entrenched over the Kuskokwim Delta will bleed into Bristol Bay
today, keeping temperatures generally below zero across most of
the Mainland except for eastern Bristol Bay which could get above
zero today. Generally light winds will persist, aiding in
radiational cooling, which will allow overnight lows to again fall
into the negative teens and twenties the next two nights.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a shifting pattern will bring a strong front and low pressure
system into the western Bering tonight and central Bering on
Monday. Instability showers will continue over the eastern Bering
today, before tapering off this evening. A warm/occluded front
will begin to impact the western chain this evening with storm
force winds and gusts approaching hurricane level. Precipitation
will begin as snow, but quickly change to rain as the front
overtakes the area. Once the front reaches the central Bering on
Monday afternoon, winds and snow could combine for a period of
blizzard conditions for the Pribilof Islands Monday night.
Long term forecast (days 3 through 7)...
the long term forecast starting Tuesday will continue the active
trend. The western weather pattern will be dominated by a cluster
of lows that will slide north across the Kamchatka peninsula.
Beneath these systems and at the surface will be low pressure
systems that will slide north - pulling a warm front with it. Each
successful low will drive more moisture and warmth north,
eventually eating away at the cold h85 temperatures that will
settle in through this weekend. By Wednesday evening the first of
these lows will slip north across Norton Sound. This low will pull
a weakening front across southcentral late in the week. The
forecast confidence in specifics drops mid week for a few reasons.
All guidance agrees on a low spinning along the front that will be
driven east along the akpen and south of Kodiak. It is at this
point that the biggest differences, and ultimately the drivers in
our weather, appear. The GFS drives the low well south of the Gulf
of Alaska while the ec and Canadian skirt Prince William Sound.
GFS ensemble members do not give too much help as the spread is
rather large into the early week. Wednesday the GFS and ensembles
keep SC dry while the ec and Canadian bring in some snow.
Regardless, an active pattern will continue through next weekend
with details to be appearing through the week.
public...blizzard watch 195.
Marine...heavy freezing spray 130 138 139 141 165.
Storm Warning 175 176 177 178.
Gale Warning 130 131 138 170 172 173 174 185.