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Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
446 PM akst Fri Dec 15 2017

Analysis and upper levels...

The atmosphere is becoming more dynamic again across southcentral
as a large Bering low shifts east along a very difluent upper
level North Pacific jet stream which is steamrolling northeast
into the Gulf of Alaska. A solid gale force (40 knots) front
associated with the aforementioned upper level jet was captured
moving across Kodiak into the western Gulf of Alaska at noon. This
has already brought over an inch of rain to parts of Kodiak
Island, and this rain will quickly spread north toward the Gulf
Coast later today. Over the Bering Sea, there is a well-defined
occluded low moving north across Bristol Bay toward Nunivak
Island. This is bringing strong gale force winds and rain/snow to
most of the southwest coast. There is another closed low center
just north of Adak Island supporting another region of gale force
winds.

&&

Model discussion...

The numerical models have now come into great agreement with the
strong front moving toward the north Gulf Coast this evening. Over
the past few days, there was quite a lot of disagreement among
the models with the potential for a closed low along the coast.
Some models (mostly the gfs) were very strong with the closed low
scenario whereas the European model (ecmwf) and Canadian were faster and had more
of a trough axis instead of a closed low...which also featured a
meso low moving north through Cook Inlet. Now all models are in
agreement with the latter solution, so winds are now expected to
come down through Prince William Sound, down Passage Canal, and
then down Turnagain Arm. Some winds will also come through
Anchorage from the southeast overnight as the front passes
through. In terms of precipitation, the regime is expected to
become showery behind the front given the rather impressive
instability, so models are not in good agreement with that and are
expected to remain so for the next couple days.

&&

Aviation...
panc...a front approaching the coast will bring threats for
flurries and possibly a little light rain this evening and early
tonight. After the front passes some southeast winds are expected
to impact the terminal with the chance for a mix of rain and snow
which may drop ceilings/visible to MVFR for a brief time. After that,
expect snow shower chances through the day Saturday as unstable
air moves over the terminal.

&&

Short term forecast southcentral Alaska (days 1 and 2)...
the strong front is pushing in to the Gulf of Alaska with a triple
point low starting to form south of Kodiak Island. A short wave is
ejecting out of the base of the low now over the Alaska Peninsula
heading toward Shelikof Strait then up Cook Inlet tonight. Rain
will spread from Kodiak east into Prince William Sound this
evening and mix with or change to snow in the northern sound and
especially the higher elevations tonight. The short wave rapidly
moving up the inlet will complicate the forecast, particularly
the wind. The Turnagain Arm and hillside wind should be
problematic. The current model solutions don't capture the
magnitude of the current wind, we're also concerned about another
shot of wind associated with the shortwave that is not captured
well overnight into tomorrow morning. We're still anticipating the
typical downslope drying across the coastal mountains as the
front moves through and with the strong wind again overnight and
tomorrow. Another concern is how much precipitation makes it into
the Copper River basin. At this point it looks like the downslope
drying across the chugach should prevent snow in Glennallen but
expect snow along the Alaska Range and Talkeetna Mountains in the
upslope flow. The Susitna Valley, once the flow becomes southerly,
should see a shot of snow along the Alaska Range and north of
Talkeetna, though not heavy.

&&

Short term forecast southwest Alaska (days 1 and 2)...

A strong low continues to move up the Kuskokwim coast this
afternoon, which is showing up nicely on the Bethel Doppler
radar, with a strongly occluded front extending east through the
lower Kuskokwim Delta. Strong wind gusts have been observed in
advance of the front, with winds quickly dropping off following
frontal passage allowing for some rain showers to develop or mix
in with the snow, as warmer air surges inland in the lower levels.

Meanwhile, robust cold advection in the mid-levels is quickly
moving inland south of these features, with plenty of instability
(relatively speaking) for some graupel or small hail to develop
based on the radar and observational data across eastern Bristol
Bay and adjacent locations. Additionally, numerical guidance
continues to show steep mid-level lapse rates approaching 9 deg.
C/km and total totals above 60. Vort maxes rotation around this
the trough axis that extends from a western Bering low to the
Kuskokwim low may provide enough of a burst in ascent aloft to aid
in updraft strength to possible allow a stray lightning strike or
two to develop. However, temporal and coverage uncertainties
precluded this being adding to the coastal land or marine
forecast zones.

As the western Bering low heads towards the akpen by Sunday
morning, the Kuskokwim low will rotate back southward with the
pair exhibiting a fujiwhara effect as they depart the region by
Sunday afternoon. Until then, showery precipitation will continue
to dominate your weather

&&

Short term forecast Bering Sea/Aleutians (days 1 and 2)...

Quite an interesting setup is in place as a 971mb low north of
Adak, which has been "wobbling in place" for the last 36 hours or
so has started its southeastward journey. Meanwhile, a 962 mb low
near Nunivak Island will halt its northward progression, and make
a u-turn as it starts to be influenced more by the incoming low.
These systems will rotate around each other in a unique fashion as
described above, with strong cold advection continuing along
their southern and western flanks. This has been quite evident in
the observational data, with cloud top cooling also noted on the
infra-red satellite loop. Models continue to show decent
instability for this time of year, and given the instability
parameters described about, an isolated thunderstorm can't be
ruled out. However, graupel or small hail is a distinct
possibility with any heavier shower that develops.

A transitory ridge will shift from west to east across the
Aleutians towards the Pribilof Islands for tomorrow, as todays
systems depart the region and a low rapidly deepens to 944 mb
across the western Bering by Sunday morning, with an attendant
cold front stretching south. This front will impact western
portions of the area with storm near storm force sustained winds
developing, and seas rapidly increasing in height given a
favorable fetch augmented by the baroclinic winds.

&&

Long term forecast (days 3 through 7...Mon through fri)...
the weather picture for next week is starting to become clearer.
There are 2 major features to focus on. The first will be a very
deep low over the western Bering Mon and its associated front. The
low will bottom out somewhere in the 945 mb range late sun into
early Mon. But its trek across the Bering will impact the whole
area to start the week. Along the front, storm-force winds and
moderate rain should be expected through the Aleutian chain as it
pushes from west to east. Behind the front, expect widespread
showers (transitioning from rain to snow) and gusty winds as cold
air wraps around. As the front impacts SW AK, it will start out as
snow with some gusty winds. This could create some difficult
travel conditions, so stay tuned to this portion of the forecast.
But with warm air and onshore flow, the snow should transition
fairly quickly to rain. Today models have come into better
agreement that this feature will hold together long enough to make
it into the Cook Inlet region. If a few ingredients hold together,
this could have the making of a decent snow event for the
Anchorage area. But there is still a lot of time and variables
that have to match before there is much certainty in this
solution.

The second major feature will be a building ridge over the North
Pacific to end the week. There is still abundant uncertainty as to
exactly where this feature will establish itself. On its western
edge, it will bring moderate to heavy precipitation and warm and
windy conditions. Underneath the ridge axis, it will likely be
cold and foggy. Stay tuned to the forecast to see exactly how this
pattern unfolds and what the sensible weather will be at the
surface as we move towards Christmas weekend.

&&



Afc watches/warnings/advisories...
public...none.
Marine...storm 119. Gales 120 125 150 155 165 170-181 185 351 352
411-414.

&&

$$

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