Heavy Freezing Spray Warning 101, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
536 PM AKST Mon Dec 19 2016

.ANALYSIS AND UPPER LEVELS...
An upper low extends from northwest alaska to the main upper low
near Atka. An upper level trough is pushing from the North Gulf
Coast inland into the Copper River Basin. Broad southwest flow is
pushing moisture into the Bristol Bay region. A deformation band
extends across the Kuskokwim Delta under the upper trough and is
producing snow as it slowly moves south to the Kilbuck
Mountains. Cold air aloft is wrapping around the main low near
Atka resulting in very unstable conditions that have produced
isolated thunderstorms north of Unalaska. Another upper level low
is forming south of Prince William Sound that is tracking to the
northeast.

&&

.MODEL DISCUSSION...
The model struggles continue. The GFS and Canadian models were
preferred for the superior handling of mesoscale conditions across
south central Alaska. At the synoptic scale the models are in fair
agreement bringing the low from the Gulf to the North Gulf
Coast while the low in the Bering Sea tracks across the Alaska
Peninsula to south of Prince William Sound Tuesday.

&&

.AVIATION...
PANC...Snow under the trough stalled over the region will likely
keep CIG and VIS down into the early evening than rapidly lift as
the deformation band dissipates. The uncertainty is with respect
to how fast the band dissipates, thus when the conditions return
to VFR. There is slight chance for another shot of snow and MVFR
conditions again around 12 to 18Z.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The deformation zone that has been over eastern parts of the Cook
Inlet area, and given Anchorage snow, will diminish early this
evening. However, a shortwave will then move in from the east
keeping light, but diminishing, snow over the area tonight. The
low in the northern Gulf will continue its northerly track
bringing more snow to areas around eastern Prince William Sound,
with rain mixing in right along the coast. The heavier
precipitation will diminish by Tuesday morning.

By Tuesday afternoon a trough will setup over western Alaska
bringing broad, unstable, southwesterly flow to Southcentral
Alaska. Multiple lows and shortwaves will push through this flow
keeping scattered to numerous showers over much of the area into
Wednesday morning. Meanwhile, the trough axis will push east over
the Cook Inlet area into Wednesday bringing dry air into the area
from the west.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The deformation band along an upper level trough draping over the
west coast will bring a band a steady light snow from just south
of Bethel northeastward into the Kuskokwim Valley. Moisture and
dynamics are both fairly weak with this snow, so most areas will
see less than an inch of snowfall tonight. Ahead of this band skies
are clearing over much of Bristol Bay, which has allowed for
patchy fog and stratus from the Bering Sea to spread inland with
weak onshore flow. This fog will begin to break up on Tuesday as
the deformation band will shifts to the east in advance of the
upper level trough by the afternoon. As this occurs, light
snowfall will spread into Bristol Bay by Tuesday evening. The trough
will then slide to the east, allowing clearing skies with falling
temperatures beginning in the Kuskokwim Delta late Tuesday, and
spreading into Bristol Bay for Wednesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The low over the South Bering has weakened to gale force, allowing
winds and seas to begin to subside throughout the region. Further
to the north, the same deformation band associated with the
snowfall over the Southwest Mainland today has extended as far
west as the Pribilof's, bringing rain showers to that area that
will diminish this evening. Starting Tuesday, the Bering will exit
the Bering into the North Pacific as it slowly weakens, allowing
for extensive rain that have spread throughout the Aleutians to
begin to clear from west to east. As this occurs, widespread
northerly flow and cold air advection showers will develop over
the Central-Eastern Bering.

&&

.LONG TERM FORECAST (Days 3 through 7)...
On Wednesday night and Thursday the trough over the Gulf of Alaska
will move east, ushering in a fairly dry period for the southern
portion of the State. A flat ridge over the Bering Sea and
Aleutians will still allow some showers over that area. A low will
move into the Aleutians and southern Bering on Friday and open up
into a trough, which will mostly dissipate as it hits the western
mainland early Saturday. A large upper low will then move into the
Bering Saturday and Sunday, then settle in along the west coast of
Alaska on Sunday and Monday. The models are in better agreement
today than they have been the last few days, but ensemble guidance
was still preferred by WPC, mainly the 06z GEFS and 00z ECE
(ECMWF Ensemble).

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 132 150 155 172 174 176 180.

&&

SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...BL