Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 121357

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKST Wed Dec 12 2018


A negatively titled trough persists across the Bering and western
Gulf with the trough axis shifting slowly eastward over the
previous 12 to 24 hours. The core of the jet stream (with a jet
max near 180kts) remains over the North Pacific. The southern
track of the jet stream is helping keep the stronger and more
organized surface features south of the Aleutian Chain and AKPEN.
In the mid-levels, an elongated 491dm low is anchored over the
northern Bering. A series of shortwave troughs are pinwheeling
around the underside of this low. One shortwave is positioned near
the Anchorage Bowl with an associated weak surface low bringing
areas of light snow from Cook Inlet north to Talkeetna. The most
recent scatterometer pass also indicates an area of westerly gales
from Augustine Island to Middleton Island. A second, shortwave
with a sheared vorticity lobe stretches from Bristol Bay east
across Kodiak Island. This feature is bringing light snow showers
from Cold Bay to just south of King Salmon and is helping
reinforce the cold air advection over Southwest Alaska where
overnight temperatures have dropped into the single digits. A
third, fast-moving shortwave with an associated 994mb surface low
is tracking east toward the Pribilofs with a small area of gales.



Run-to-run consistency between the various guidance remains an
issue; however, there is still a general consensus regarding the
main weather features and associated sensible weather. This is
seen in the 06z guidance where the NAM, GFS, and EC are in
agreement with both the low over western Prince William Sound and
the trough over the Cook Inlet this morning weakening as the
upper-level shortwave lifts north today. The 06z NAM and GFS
guidance then have a second shortwave over Kodiak Island lifting
northeast along the eastern Kenai Peninsula for late Thursday. The
NAM solution is farther east and the GFS faster and farther west
when compared to the EC, and Canadian. The EC and Canadian draw
the center of this shortwave nearer to Cook Inlet while the NAM
and GFS progress the feature farther east toward Price William
Sound. Hence, the timing, intensity, and track of this shortwave
(and the its associated surface low) will have implications on
wind speed and precipitation amounts through the region. Given the
overall uncertainty, but the relative agreement between the GFS
and NAM, a blend of both solutions was used to find a middle
ground. This is also an issue with the next surface low moving
into the western Gulf for Friday into Saturday. Here, the GFS and
Canadian are in general agreement with the placement of the
frontal features and center of circulation with the GFS a bit
father south and the EC lagging even father behind. Again, a blend
of of the NAM and GFS was used placing the low well southeast of
Kodiak with a trough along the southcentral coast and a front
stretched across the central Gulf.

Farther west, models are in better agreement with the track and
evolution of a low tracking southeast from Norton Sound on
Thursday. Guidance also shows the development of a Kamchatka low
beginning Thursday, however there is still quite the spread on
location and strength of its front over the western Aleutians and
its influence on a secondary North Pacific low approaching the
Aleutians by the end of the week.



PANC...MVFR to IFR conditions in snow are expected to continue
through the majority of the TAF period. A few VFR breaks are
possible during breaks in snowfall. VFR conditions are expected to
return to the airport complex later tonight as the trough lifts
north. Brief gusty south winds are still possible through mid-
morning, but in general winds will be light.



The 500 mb trough extends over northern Cook Inlet and the Copper
River Basin this morning with a a weak meso-low at the surface in
northern Cook Inlet. This meso-low has kept the light snow in the
region through the night and will be slow to dissipate today
which should keep periods of light snowfall going for the
Anchorage area northward through much of the day. However, there
is not a lot of moisture available so snow accumulation should
still be rather low. Snow chances in Southcentral will persist
through Wednesday night as another upper level short wave moves
over the region.

Another shot of snow will move into the area Thursday night into
Friday as another wave moves across the area from the southwest.
This one will be a little different as the main area of lift will
be just to the east of the short wave and this feature is the key
to how far east the snow will develop. The NAM has it the farthest
east which would bring the precipitation mostly to Prince William
Sound and keep Anchorage and the Mat-Su dry. The GFS also is
showing this feature, but has it weaker and on a more westerly
course which could just clip Anchorage and the Matanuska Valley.
The ECMWF is not showing this feature at all which would bring
the snow over the Anchorage and Matanuska Valley on the more
westerly course of the wave. The farther east solutions of the
NAM and GFS were favored as the structure of the low looks more
conducive for this to push farther to the east and the forecasts
reflect this. However, this will need to be re-evaluated which
each successive model run the next two days.


through Friday Night)...

An area of low pressure currently centered in western Norton Sound
will slowly drift southward down the coast until Thursday night,
when it will be absorbed into a much bigger low south of the
Aleutians. While the center remains north of the area during the
day today, onshore flow, cloud cover, and somewhat warmer
temperatures (if teens can be considered warm) are expected for
the Kuskokwim Delta today, with snow showers likely along the
coast. The snow showers will be drawn eastward by the low from the
very unstable atmosphere over the Bering, which is supplying the
moisture for the snow showers. On Thursday, the low will track
west of Cape Newenham, which may draw the same snow shower
activity northeastward into coastal Bristol Bay before being
absorbed. Other than the departing snow showers drifting northward
over the Lower Kuskokwim Valley this morning, a dry stretch of
weather is expected area-wide through Friday Night.

The primary weather story otherwise will be the influx of
increasingly colder air into Southwest Alaska from the north, with
light offshore winds or calm winds expected at the surface. With
very notable clearing behind the snow showers over the Kuskokwim
Valley currently, snow shower activity has been limited to
coastal locations. Thus, with the expectation of clearer skies
across the area than previous forecasts, it`s increasingly likely
that the power combo of ingredients for bitterly cold
temperatures: cold air in the mid-levels of the atmosphere, a snow
pack over the area, clear skies, and light winds will become more
common with time. Thus, expect a strong cooling trend over the
next few days as interior locations approach 20 degrees below zero
Friday night into Saturday morning. Depending on the track of a
strong low that will pass well south of the Alaska Peninsula
Friday, helping draw even colder air from Interior Alaska
southward, wind chill concerns may increase this weekend.


through Friday Night)...

A weak low over Norton Sound this morning will slowly drop
southward through Thursday night, helping locally enhance the snow
shower activity continuing across much of the rest of the Bering.
The snow showers are the result of cold Arctic air moving over the
relatively warm waters of the Bering. A strong low will approach
the central Aleutians Thursday. While nearly all of the impactful
weather associated with the low will remain south of the chain, it
will absorb the weak low off Cape Newenham Thursday night, and
will lead to gusty easterly winds over much of the Aleutians
through Friday night. Increasingly colder air over the northern
Bering will cause freezing spray and heavy freezing spray concerns
near St. Matthew Island to continue for the foreseeable future.
The snow shower activity will continue over much of the Bering
into the weekend.

Elsewhere a weak front associated with another small low near
Kamchatka will cause there to be rain and snow showers over the
western Aleutians through Friday night.


.LONG TERM FORECAST (Days 4 through 7)...

The forecast period commences with the longwave trough extending
from Western Alaska to the Gulf Of Alaska, meanwhile the ridge
will be over the Central Aleutians. On the 15th of December at
12z, the GFS at 500 mb has a closed low near the Brooks Range,
another in the Gulf of Alaska and a third one southwest of Attu
Island. The ECMWF is pretty similar although the feature south of
the Western most Aleutians is depicted as more of an open wave.
Each upper level disturbance has multiple shortwaves embedded with
the cyclonic flow. At the surface, the GFS and ECMWF have
rendered some pretty significant differences with the placement of
the surface lows early in the forecast period. Generally
speaking, expecting a progressive pattern this forecast period.
However, given the vagaries of the models hedged towards NCEP WPC
guidance for the pops.


MARINE...Gales 119 120 130 132 150 155 165 170 176 177 351 352



LONG TERM...PS is the U.S. government's official web portal to all federal, state and local government web resources and services.