Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 151400
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 AM AKST Sat Dec 15 2018

.ANALYSIS AND UPPER LEVELS...

IR satellite and radar imagery shows a clearly defined trough
continuing to lift to the north and west across the interior of
Southcentral Alaska this morning. Along this boundary, areas of
light stratiform snow are falling from the windward side of the
Alaska Range north through the Susitna Valley. Behind the trough,
a moist, unstable airmass is resulting in widespread snow showers
from Kodiak City and Seward east across Prince William Sound.
Along the immediate coast enough warm air has moved in to allow
for a mix of rain and snow. The instability has also resulted in a
few lightning strikes over the coastal waters south of the Kenai
Peninsula and Prince William Sound overnight.

Over the southwestern Gulf, a storm-force low is continuing to
slowly drift to the northeast, swinging an occluded front over
the eastern Gulf. The latest scatterometer pass denotes a large
swath of storm-force winds along the front over the central gulf
with widespread gales south of Kodiak Island and the AKPEN
wrapping around the backside of the low. This is also confirmed by
the numerous buoys reports of peak gusts between 40 and 50 knots.

A weak stationary trough near Nunivak Island is producing a few
snow showers along the immediate coast from Hooper Bay south to
Cape Newenham. For the interior of Southwest Alaska, it is
another cold night with overnight lows between 5 and 25 below
zero. The exception to this is around Bristol Bay where increasing
clouds overspreading from the northward moving trough have kept
temperatures in the teens above zero.

Farther west, northerly flow continues across the southern Bering
and Central and Eastern Aleutians. A weak polar low is also
spinning up off of an occluded front from a Kamchatka low and is
now drifting east across the central Bering. A developing North
Pacific low southwest of Shemya is also bringing clouds and a
warmer southerly flow to the western Aleutians.

&&

.MODEL DISCUSSION...

Forecast confidence is still above average given the good model
agreement through the weekend. Guidance continues to have similar
timing and intensity for the low south of Kodiak, as well as a
low traversing south of the Aleutians this weekend. As for
precipitation over Southcentral with the next system Sunday,
guidance is falling in line much like the previous event with the
NAM more bullish on precip/snow amounts compared to the GFS and EC
with the hi-res models somewhere in between. Guidance then begins
to break down after late Sunday with respect to the track of the
low as it moves near Prince William Sound and where it eventually
comes ashore. This will impact overall snow amounts across
southcentral, especially in the lee of the Kenai and Chugach once
again. The GFS and Canadian increase the southeasterly flow as a
stronger, closed low moves across the Kenai Peninsula. This would
increase the downslope potential when compared to the NAM solution
of a more more northerly track into Prince William Sound. As per
the previous storm, a middle ground approach was taken with this
upcoming system, but with a heavier weight toward the GFS. Models
also vary quite a bit on the intensity and location of the next
surface low entering the southern Gulf from south of the AKPEN.
Although there is disagreement on the finer details, guidance
suggests this feature will continue to the east and impact the
Alaska Panhandle before curving northwest into the northern Gulf
for early next week.

&&

.AVIATION...

PANC...Lingering snow showers through early morning (16z) may
briefly lower vis and cig conditions to MVFR. Otherwise, VFR
conditions with light northerly winds are expected through the
day.

&&

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

A low resides just south of the Gulf of Alaska with a strong front
situated east-to-west across the Gulf. This front will bring Storm
force winds to the Gulf offshore waters and numerous Gale
Warnings to coastal waters.

Snow showers will continue over coastal mountains today, but inland
areas will see a break in precipitation as the deformation axis
that was over southcentral yesterday has moved westward to the
Alaska Range and then next front is still in the Gulf. There may
some short breaks in the cloud cover, but enough high clouds are
between these systems to keep things from really clearing out.

There is a bit of warm advection with this low which will turn
the precipitation type to rain for some of the coastal locations.
Kodiak Island (lower elevations) has already seen this change over
but will also switch back to snow overnight Saturday night as
warmer air moves to the Prince William Sound area Sunday. This
warmer air will not penetrate inland enough to turn rain to snow
except perhaps near Portage on Sunday and Sunday night, but this
is still uncertain.

Sunday and Sunday night will bring more snowfall back to inland
areas of southcentral Alaska as the front in the Gulf, which will
be considerably weaker by then, pushes inland as the cold upper
level low in Southwest Alaska sends in another strong short wave.
While some downsloping may slightly mitigate the snowfall, the
tightly packed isotherms at 850 mb signify the downslope will be
minimal and the exact timing of the snow and amounts are the only
things to be hammered out as it looks likely this snow will occur.
This snowfall on Sunday night could make the Monday morning
commute in the Anchorage and Mat-Su regions difficult as there is
a potential for advisory level snowfall to occur with this system.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Friday
Saturday through Monday)...

Its a tale of two air masses today across the southwest mainland.
A deformation zone is developing over eastern portions of the
Kuskokwim Rive Valley and AKPEN out ahead of a strong low in the
gulf. This is leading to a somewhat narrow band of snow forming
from south of King Salmon north towards Sparrevohn. Radar imagery
shows snow already falling over King Salmon, Igiugig, and Iliamna.
This band will slowly drift west then stall and diminish by the
evening as it runs into cold and dry air to the west. Total snow
amounts are expected to remain below 6" for most lowland and
populated locations. Further west over the Kuskokwim River Delta,
cold arctic air will funnel down from the north under persistent
northerly flow. Low temperatures in this area are still on track
to fall below -20F Sunday morning, with temps during the day
struggling to make it into the negative single digits. The
persistent, though relatively light, northerly winds will add an
extra bite to the air with windchills approaching -40F in places.
Cloud cover from the low in the gulf will help to keep the coldest
temps west of the Kuskokwim River Valley and AKPEN, but as the
low drifts east on Monday, the cold air will push east covering
most of the southwest mainland, with nearly the whole area below
0F by Monday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3/Saturday through Monday)...

The Bering and Aleutians will be under a pretty stagnant pattern
through at least early next week as a deep and cold upper low
leads to persistent northerly flow over most of the region. Today,
a strong North Pacific low several hundred miles south of Kodiak
will lead to gale force northerly winds along the Eastern AKPEN
and south into the North Pacific. Winds will die down through the
day as the low tracks east. Northerly flow will slacken some as
ridging builds east into the Central Bering tonight. Another
North Pacific low will move into the area Sunday but will stay
south of the Aleutians as the upper low over the Bering acts as a
buffer. Of note, heavy freezing spray will be an issue for
mariners in the Northern and Eastern Bering near the coast as
bitterly cold air streaming off the mainland will combine with
small craft winds. Main areas of concern will be in Bristol Bay
and south and west of the Kuskokwim Delta coast.

&&

.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 upstream
there is a high amplitude ridge that spans from the Western
Bering to the North Pacific. As the forecast period elapses,
a vortex of bitterly cold air will descend from Siberia and the
longwave pattern will become more amplified. The deterministic
models have depicted a series of lows impacting the Aleutians, the
Gulf of Alaska and the Prince William Sound. Tuesday 12z the GFS
and the ECMWF have a surface low in the Gulf Of Alaska, a dome of
high pressure south of Adak and a potent low southwest of Attu
Island. By Wednesday, the global models have already diverged
substantially. This forecast package reflects a blend of WPC
guidance and some hedging between operational models.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning...119 120 131 132 150 155 176.
 Storm Warning...351 352.
 Heavy Freezing Spray Warning...160 180 181.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...PS


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