Small Craft Advisory 990, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...
High pressure has brought clear and cold temperatures to much of
the southern Mainland with generally winds throughout. The one
exception to this is dense fog and very low stratus that
developed over northern Cook Inlet and Knik Arm overnight as
strong stability developed over the only partially ice covered
Inlet. Gap wind prone areas of Prince William Sound also continue
to see gusty offshore winds due to a persistent pressure gradient.
Over the eastern Bering a pattern change is beginning to develop
as the persistent high pressure and northerly flow has given way
to southerly flow ahead of an approaching front. This front is
bringing rain and snow along with gusty southerly winds from the
eastern Aleutians northward into the Pribilof Islands as it tracks
eastward towards the southwest coast. Just west of this front a very
active pattern over the western Bering continues as a developing
storm force low is surging northward into the central Aleutians.

&&

.MODEL DISCUSSION...
Synoptic model agreement remains very good into Tuesday with high
pressure dominating over much of the Mainland. As a result the
higher resolution NAM and ARW were used for minor updates over the
Gulf and southcentral. The most significant of these updates was
regarding the fog/stratus in northern Cook Inlet which only the
WRF-ARW initialized reasonably well with, and was used heavily
for the short term forecast. Over the western Bering the NAM
continues to be a weak outlier, but otherwise model consensus in
positioning has improved significantly in the last 24 hours. The
models all appear to be catching up to the slightly faster and
deeper GFS solution, which was used once again for the morning
updates to increase winds over the western Aleutians/Bering.
Overall forecast confidence remains good with the exception of
the timing and density of Cook Inlet stratus and fog where
confidence is much lower.

&&

.AVIATION...
PANC...Dense fog and very low stratus will bring VLIFR conditions
at times to the airport through late this morning. Afterwards fog
and stratus will likely continue over northern Cook Inlet
throughout the day today, impacting approaches from the west. Low
stratus and possibly dense fog will then return to the terminal
area after 6z tonight. Overall confidence in the exact timing and
conditions in the TAF is low due to poor agreement from much of
the guidance.

&&

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

The main forecast challenge remains fog coverage, particularly
along Cook Inlet. While the sounding this morning indicates that
the atmosphere is dry, enough moisture is still available to
support shallow fog along the inlet. Meanwhile, across much of
mainland Southcentral, temperatures continue to plummet under
clear skies and fairly light winds. There still remains a
pressure gradient between the high over the mainland and general
low pressure in the north Pacific. However, offshore gap winds
are much weaker tonight and are expected to continue to slowly
diminish. The main forecast challenge remains the development of
fog during each night in the short term. Currently, models
indicate slightly less favorable conditions for fog development
overnight Monday night, with wind shear present in the boundary
layer. However, less dense fog is still expected to form over
northern portions of Cook Inlet.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The weather pattern continues to shift to a more active pattern,
as a front over the Eastern Bering begins to push inland over the
Southwest mainland today. This system will bring the next round of
precipitation to the Alaska Peninsula and Southwest Alaska today,
along with some gusty winds along the coast and Alaska Peninsula
as the weakening front passes. Precipitation will remain mostly
snow over the inland areas. There is the potential for a
rain/snow mix over the Bristol Bay coast with the slightly warming
southerly flow, as well as a transition to mostly rain over the
Alaska Peninsula. New snow accumulations are expected between 1 to
2 inches mainly along the Kuskokwim Coast, but will need to be
monitored as models are still struggling with the dynamics of this
system. Models are also trending toward warmer solutions which
could bring a change from snow to rain along the coast sooner than
expected.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The highest impacts through Tuesday surrounds the storm system
moving a front into the western half of the Aleutian Chain today.
Storm force winds are expected develop along the front this
morning across the Western and Central Aleutians with wind
direction along the front out of the east then veering to the
south behind the front. Winds also increase to storms across the
western waters with rain spreading north across the Bering. The
low center nears Shemya this evening and deepens on it's northward
track which keeps storm force winds through late tonight. Winds
taper to gales by Tuesday with a directional shift to the south
and west as the low center lifts north.

&&

.LONG TERM FORECAST (Days 3 through 7...Fri through Tue)...

A shift in the upper air pattern will be underway Tuesday night
and Wednesday as a high latitude blocking high over Alaska weakens
as a series of Bering Sea storms send energetic waves through the
ridge late week into the weekend. The first of these storms will
already be slowly weakening across the Western Bering Sea with the
associated remnant trough axis moving through Southcentral by
Thursday. Precipitation with this feature will be rather
disorganized as the weakened trough moves through the ridge axis.
However, a stronger storm force low moving into the Western Bering
Sea Thursday will send a much stronger occluded front into
Southern Alaska, spreading widespread precipitation (rain and
snow) across the southern mainland Friday and Saturday.
Thereafter, uncertainty grows with the exact storm track as the
baroclinic zone shifts south and another strong NPAC low moves
toward the Gulf of Alaska. By this time the forecast favors a
broad WPC ensemble blend approach.

Bottom line...the rather stagnant weather pattern will be
changing dramatically mid week and beyond, starting in the Bering
then moving east. Expect warming temperatures to near if not above
normal along with threats for snow and rain by Friday.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...EN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...JA