Small Craft Advisory 991, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
418 PM AKST Mon Dec 12 2016

.ANALYSIS AND UPPER LEVELS...
High pressure has brought clear and cold temperatures to much of
the southern Mainland. The one exception to this is dense fog and
very low stratus that developed over northern Cook Inlet and Knik
Arm overnight and lingered into the afternoon hours. The stratus
and fog actually extended south toward the city of Kenai, but only
made it as far south as the North Forelands. Gusty gap winds were
evident across the eastern Kenai Peninsula and along the North
Gulf Coast. Out west, a weak warm occluded front extending south
from a low northeast of the Pribilof islands to over the AKPEN,
brought rain to the Pribilofs and AKPEN. Mixed precipitation types
will be likely into the evening hours along the southwest coast as
the front heads north. A developing storm force low south of the
Aleutians is intensifying under a strong upper level jet and
heading north. The frontal system associated with this low is
spreading clouds and rain north to the Aleutians.

&&

.MODEL DISCUSSION...
Models are in fairly good synoptic agreement through the short
term (Wednesday). Afternoon forecast grids will be a blend of
current model runs and official.

&&

.AVIATION...
PANC...Low stratus/dense fog and the resulting LIFR/IFR and CIGS/VIS
will break up some late this afternoon and early evening.
LIFR/IFR conditions should move back in overnight. Conditions
will improve again Tue afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Ridging over the area will result in clear skies and cold
temperatures that will continue through Tuesday. This setup is
also conducive for fog for areas around Cook Inlet. With stronger
winds in the southern part of Cook Inlet there will be enough
mixing to inhibit fog for these southern locations. There will
also be higher pressure inland which will keep gap flow winds
going in the narrow gaps along the north Gulf coast. Tuesday night
a shortwave will move in over the area from the southwest but will
shear apart before making it over Southcentral Alaska with most of
the energy moving into the Gulf. This should keep the snow away
from Southcentral Alaska but more cloud cover will move in over
the area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Mon and Tues)...
A major pattern shift is upon us. Other than the high pressure
with dry and cold weather, we are about to shift to a more active
Bering storm track. The first in the series of "storms" is
already approaching the West Coast in the form of a weak upper
level wave. While very light precipitation is expected with this
system, it will open the door for some warmer air to start to move
in. This warm air moving over the cold surfaces will likely lead
to widespread, potentially dense, fog tonight along the Alaska
Peninsula and West Coast. A sharp ridge will then build in on Tue
ahead of the next system. As this ridge allows for more warm,
southwest flow and helps to stabilize the atmosphere, fog could
hang around much of the day. The next warm occlusion is making its
way toward the West Coast by early Wed. This one will have better
potential to bring some precipitation to coastal areas. With 850mb
temperatures soaring above freezing, the precipitation will likely
fall as rain.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Mon and
Tues)...
Several stout systems will impact the area the next several days.
The first is rapidly developing southwest of Adak and winds have
already gusted in excess of 50mph there. This system is forecasted
by some computer models to drop approximately 20mb in the next 12
or so hours. The strongest winds will pass through Atka later this
evening before shifting to the east and slowly waning. Seas will
respond and continue to build to over 25' by Tue afternoon. This
system will then slowly fall part as it drifts north over the far
Western Bering Tue through Wed. The next system will quickly
approach Attu and Shemya from the southwest by late Wed. Please
see the extended forecast discussion for more details.

&&

.LONG TERM FORECAST (Days 3 through 7...Fri through Tue)...
A shift in the pattern will be well underway Wednesday night and
Thursday as the high latitude blocking high currently over Alaska
weakens and shifts east. A weakening shortwave trough axis over
Southwest Alaska will be moving east into Southcentral. There is
uncertainty with the exact timing and strength of this trough, but
right now there certainly will be a chance for light "overrunning"
light snow across all of Southcentral as this system tracks
through Thursday and Thursday night. Meanwhile, a deep and near
hurricane force low will move into the Western Bering/Aleutians
Wednesday night. This will be the second in a series of strong
lows which will impact the region through early next week.

Sometime late Friday into Saturday, the occluded front associated
with the strong Bering low will merge with a subtropical warm
front then move into the Northern Gulf Coast Friday and Saturday.
There remains uncertainty with track and strength of this feature,
but there it is likely to bring widespread rain and snow to all of
Southern Alaska. As usual, amounts and the exact precipitation
type will be challenging, especially along the coasts where warm
air will have an easier time eroding the cold arctic air mass
currently in place. Thereafter, the storm track will remain in
place as an even deeper low moves into the Gulf of Alaska Sunday
night into Monday.

Bottom line...expect a dramatic uptick in weather activity as this
storm cycle unfolds late this week into the early parts of next
week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 174 175 176 177 178.
 Gale Warning 127 155 165 170 172 173 179 180 181 185.
 Heavy Frz Spray 126 127 129.

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...JA