Heavy Freezing Spray Warning 75, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKST Sat Feb 18 2017

.ANALYSIS AND UPPER LEVELS...
A broad upper level trough is located over the Bering Strait and
continuing southward across the Alaska Peninsula and well into
the Pacific Ocean. Areas east of this trough axis are getting weak
south to southwest flow aloft with areas to the west in generally
northwest flow. The jet stream is remaining well south of the
forecast area.

&&

.MODEL DISCUSSION...
Models are mostly in good agreement with the overall synoptic
pattern. The exception is with a strong low that will move into
the western Aleutians and Bering Sea early next week where the
Canadian model was dismissed due to a storm track that is 400
miles and 10 to 15 mb different from where the other three main
models are clustered. This is one of the those days where the
difference in the details is where the most significant portion of
the forecast decisions are made. Therefore the higher resolution
models are being used to capture the weak upper level disturbances
moving though Bristol Bay and Cook Inlet as well as some of the
outflow winds that should develop in the Gulf Coast region.

&&

.AVIATION...
PANC...The persistent and weak southerly flow up Cook Inlet is
conducive for keeping in the mostly MVFR conditions around the
airport through the night. There may be periods of IFR from some
low clouds or a short time where the lower cloud deck goes
scattered, but it should mostly be MVFR through Sunday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

Cold air advection at the lower levels of the atmosphere, cyclonic
flow, and embedded midlevel shortwaves moving from southwest to
northeast up Cook Inlet are a recipe for hit-or-miss snow showers
across most of the area today. This general pattern will continue
through the start of the week. There will be a few features that
should act to focus these showers over a certain area, and the
first of those features is a weak shortwave that comes through
upper Cook Inlet late tonight/early Sunday morning. This should
give an extra bit of lift that produces a more widespread - but
still light - area of snow showers around the Anchorage and lower
Mat-Su areas. In fact, as of this writing, snow shower activity is
already starting to increase over the southern Kenai peninsula
likely in response to this shortwave moving into the area.
Accumulations should be minor, but some locations could see an
inch or so by Sunday afternoon. Additionally, with cold advection
slowly getting more established, the patchy fog around Knik Arm experienced
this morning should not recur tonight.

During the day Monday, an upper low currently over the west coast
of Alaska will start to swing more toward the Gulf of Alaska as a
negatively tilted upper level trough. Models have all started to
show the lower-atmospheric response to this (in addition to the
ongoing cold air advection) in the form of a surface low spinning
up in Prince William Sound. Models are not currently showing
precipitation wrapping all the way back into the Anchorage and
Mat-Su Valleys from this development, but anytime we see a
negatively tilted upper trough there is the potential for the
moisture to come back further to the west than expected. Thus have
kept moderate chances for precip in the forecast, and will need to
monitor subtle changes in future model runs in case it starts to
look like upper Cook Inlet could see another round of light
snowfall before things dry out on Tuesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A weak trough will continue to produce light snow along the Alaska
and Aleutian Ranges tonight before moving off to the east on
Sunday. Otherwise, the main story will be clearing skies and
frigid temperatures into the beginning of the work week as colder
air filters into the region on the back side of the longwave
trough sitting over the western mainland.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Convective snow showers will continue across the central and
eastern Bering through Sun night as cold northerly flow continues
over the region. Attention will turn to a powerful low approaching
the Western Aleutians late Sunday into Monday. This will push a
storm force front into the Central and Western Aleutians, with
hurricane force gusts possible over the waters of the western
Bering. The front will then shift eastward towards Dutch Harbor
and the Pribilof Islands Monday afternoon into Monday evening
accompanied by heavy precipitation and gale force winds. Blowing
snow is not expected to be a significant concern across the chain
as snow quickly changes over to rain with the arrival of the
front, however it may eventually become an issue over the
Pribilofs where cold air hangs on a little bit longer.

&&

.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.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storms 175-178 411 413. Gales 173 174 412.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...AD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
LONG TERM...SS