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Southcentral and southwest Alaska forecast discussion 
National Weather Service Anchorage Alaska
434 PM akst sun Dec 28 2014

Analysis and upper levels...
complex low system over the western Bering Sea with a very high
amplitude ridge over Mainland Alaska. Jet rounds the ridge over
the Mainland with speeds up to 125 knots. This flow is tapping
moisture from 35n which explains the warmth of the air mass. A few
upper level disturbances will be moving north along the West Coast
of Alaska between the ridge and low complex in the western Bering
Sea.

At the surface...precipitation in aligned from the North Pacific across
the akpen north through the Bering Strait along that boundary
between ridge and trough. Areas of heavy rain are anticipated over
Kodiak Island and portions of Bristol Bay. Cold air in place over
portions of Bristol Bay has led to some periods of moderate
snowfall at times. Precipitation will continue to stream north
over western Alaska this evening with some areas switching back
and forth between rain and snow depending upon precipitation
intensity. The Kuskokwim valley will likely remain as snow through
much of tonight. A stronger warm push on Monday will result in a
majority of western Alaska experiencing rain. A stacked low is
over the western Aleutians rotating around the parent low just off
the coast of the Kamchatka peninsula. With the boundary in
place...strong winds are present over the akpen and along the
Aleutian Range. Some strong gap flows have developed over south
central Alaska as well. Winds will continue as boundary remains
and then strengthens Monday afternoon and evening.
Therefore...a couple high wind watches have been issued.

&&

Model discussion...
overall...models are in quite good agreement through the first
few days of the forecast...through Wednesday. The outlier is the
Canadian regional model at this time. The Big Ridge is
anticipated to slide east over the next 24 hours as a couple of
upper level disturbances ride up the back side of the ridge along
the West Coast of Alaska. A much stronger wave moves into the
eastern Bering Sea Tuesday and flattens the ridge over the
Mainland Wednesday.

Of course...details of the forecast are a bit different. One of
the big differences is the structure of the jet as it swings into
the Gulf of Alaska Monday night and Tuesday. Depending upon the
cohesiveness of the jet structure...the winds through the gaps
along the Aleutian Range and southcentral Alaska will peak over
High Wind Warning. All models move the surface feature at the same speed
currently. They also have trended a bit farther west this morning. At
this time...due to this structure difference and western
position...high wind watches will be issued.

The eastern domain favored the NAM and GFS in the short term. The
western domain preferred a blend of European model (ecmwf) and GFS.

&&

Short term forecast southcentral Alaska (days 1 and 2)...
an upper level disturbance along the frontal band from the Gulf of
Alaska into southwest Alaska with some jet support to bring a
good amount of rainfall to Kodiak Island. As the
front...baroclinic zone...moves north to the Kenai Peninsula the
precipitation will spread over the Kenai Mountains. Due to the
strong south southeasterly flow at middle levels...western Kenai will
be relatively dry outside of Kachemak Bay. In addition...the
strong middle level winds leading to lots of wind shear over the area
through tomorrow. The temperatures at lower levels is not as warm
with this first wave tonight into tomorrow morning...so expect
snow at higher elevations especially Turnagain Pass. A second
stronger short wave trough moving north through the North Pacific
Monday to swing into the eastern Bering Sea Monday night. This
will push the front farther north and to the coast with
precipitation. The surface low Monday night will be in the eastern
Bering with a strong gradient wind through the Barren Islands to
Kamishak Bay. This wind will lift north to Anchorage late Monday
night. Combined with the influence of the Turnagain Arm...a high
wind watch will be issued. The front will move through the area
Tuesday with a short wave trough moving through southcentral
Alaska Tuesday night with precipitation spreading inland.

&&

Short term forecast southwest Alaska (days 1 and 2)...the active
weather will continue. A seasonably strong jet around 110 to 120
kts will remain centered over the area as a synoptic trough digs
even deeper with cold air spilling into the North Pacific. A
couple of big differences in the days ahead compared to Sunday.
First thing is that most of the cold air has been scoured out at
this point. Almost all places have turned over to rain and should
stay rain for the next few days with the possible exception of the
deeper valleys of the lower Kuskokwim valley. Some places could
see a rain snow mix during the slightly cooler overnight
hours...but with continued warm air advection...there is little
chance of significant accumulation. The next difference is in the
winds from sunday's system compared to the one the arrives late
Monday. The low today is rather diffuse and moving right through
Bristol Bay. The one that forms tomorrow is further to the west
with a bit stronger development. This will make for a slightly
more unstable atmosphere which means some of the higher winds
aloft could mix down towards the surface. To address this
possibility we have issued a high wind watch for the Bristol Bay
area.

&&

Short term forecast Bering Sea/Aleutians (days 1 and 2)...the
potent low over the central Bering that produced near-storm force
winds earlier today will drift north out of the area by Monday
morning. The pattern will continue to be dominated by the strong
southerly flow over the eastern Bering. This will keep the
strongest winds over the West Coast of the Mainland while broad
north to west flow continues across the rest of the Bering. A mix
of rain and snow showers will be in on and off with weak cold air
advection associated with the north to west flow. By middle- day
Tuesday another Stout low rolls out of the northwest Pacific. It will
bring another round of near- storm force winds into the western
Aleutians.

&&

Long term forecast (days 3 through 7)...
a highly amplified flow pattern will be in place by the beginning
of the extended period Tuesday afternoon...with a strong low
working its way northward while beginning to weaken...and a
strong North American ridge extending through much of the eastern
Mainland. Precipitation will be widespread across most of southern
Alaska...with warm air resulting in mostly rain and rain showers.
Rain will be heaviest across the Gulf Coast into Prince William
Sound through Wednesday morning...when showers begin to taper off
as the front pushes further inland. Temperatures are expected to
rise well above average across most inland locations through middle week
in the form of a weak Chinook. The trough over the eastern Bering
will push eastward over the Gulf by Friday morning...bringing a
brief reprieve to the amplified ridge pattern and warm
temperatures. Colder and drier conditions will set up over much of
the Mainland into the weekend...as ridging over northern Alaska
helps offshore flow become more dominant over the area.

&&



Afc watches/warnings/advisories...
public...high wind watch 101 161
marine...storm 130 131
gale 119 120 125 132 136 137 138 139 150 160 180 185
fire weather...none.

&&

$$

Synopsis and model discussion...jn
southcentral Alaska...jn
southwest Alaska/Bering Sea/Aleutians...mso
long term...tp

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