Small Craft Advisory 1000, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
406 PM AKST Thu Dec 28 2017

.ANALYSIS AND UPPER LEVELS...
An upper-level ridge remains intact over Eastern Alaska, but is
greatly encroached upon by a large low over Northern Canada and a
low over the Seward Peninsula, which is held in place by a stout
high over Eastern Russia. The main Pacific storm track remains
well south of mainland Alaska but close enough to the Aleutians to
move a storm force low toward the central islands.

At the surface, the big bubble isn't exactly trouble-free.  High
pressure near the AlCan border is holding near 1040 mb. Fog and
stratus are the challenges for many locations in Southwest,
Southcentral and the Copper River Basin. In the West, clouds
versus temperatures will remain an ongoing challenge.

&&

.MODEL DISCUSSION...
Models initialized well but quickly move out of step in handling
the North Pacific surface features approaching the Aleutians and
AK Pen. In the West, forecast trends have been following closer to
the ECMWF. Today, no one model is a clear winner but forecasts
were weighted toward the ECMWF in deference to previous trends
with no great amount of confidence.

In the East, wind was in general agreement with both NAM and GFS.
However, precip trends were not in alignment. This led to a slower
approach for the weekend arrival of a front in the Gulf.

&&

.AVIATION...
PANC...Fog will continue to lurk in the Anchorage bowl and is
expected to move back into the terminal this evening. However, as
usual, the exact timing and just how dense it becomes remains
somewhat uncertain. For this forecast package, trends from the
past two nights will be favored and will introduce IFR fog
conditions this evening before it clears back out again by
Friday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The potential for fog/stratus will continue to be the primary
early term forecast concern in an otherwise benign weather
pattern. Of particular concern will be for the redevelopment of
fog around Northern Cook Inlet into the Anchorage Bowl and
Matanuska Valley that saw dense fog develop under a similar
scenario last night. This fog mostly dissipated under thin layer
of stratus this afternoon, however as this layer is quickly
diminishing fog is expected to redevelop this evening after
sunset. At this point it does not appear that widespread dense fog
will occur again due to some improvement today with slightly
stronger down-inlet flow, however this also wouldn't be completely
out of the question and will need to be monitored this evening.
The good news is there is reasonable confidence that this will be
final night of fog/stratus in this pattern as a front moving
through the Gulf beginning Friday night should provide enough
mixing and cloudcover to prevent more redevelopment.

This Gulf front will then be the primary weather maker into this
weekend as it spreads snow mixed with rain along with gale force
winds northward. There however remains significant uncertainty in
the models in how quickly the precipitation shield will spread
into the North Gulf Coast, possibly as fast as midday Saturday
(from the GFS) or as slow as Sunday (from the NAM). See the long
term discussion for some more details on this front, but the
general trend will be for deteriorating conditions for most of the
area over the course of this weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Surface high pressure will continue, with weak offshore flow
tonight and Friday. The upper level low centered over the Seward
Peninsula will slowly drift north over the next couple of days.
This will allow southerly flow aloft to begin bringing weather
systems towards southwest Alaska beginning late Friday. A weak
warm front will push into the area late Friday afternoon and
evening. This will bring a chance of some snow to the western
Bristol Bay area. Also, this will switch the winds to east rather
than north/northeast, and the winds will increase. A more
substantial front (occlude) will push into the area Saturday,
bringing chances of precipitation to all of the southwest, as well
as stronger winds and warmer temperatures. Some areas in Bristol
Bay could see temperatures warm enough for a mix of rain and snow.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A strong front over the Aleutians will slowly push northward
tonight, and then stall briefly on Friday as a triple point low
develops south of the eastern Aleutians and then approaches the
eastern/central Aleutians. There are enough model differences in
the handling of this low that the forecast confidence is not very
good on the exact location of this low, but models agree fairly
well on the slow northward progression of the front. Yet another
triple point low is likely to form south of the Alaska Peninsula
on Saturday, but again the numerical models are not in agreement
on this development.

Over much of the Bering Sea, easterly to northeasterly flow will
continue through Saturday with high pressure to the north and the
low pressure systems to the south. Warmer air will slowly work its
way northward over the central and eastern Bering, while cold air
slowly works its way southward over the western Bering.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A pattern shift for the southern tier of the state is still on
track to begin this weekend, ushering in warmer and wetter weather
for the New Year. Models have been consistent in strengthening the
ridge over the Panhandle, allowing a broad longwave trough to set
up over the western portion of the state. This should allow the
synoptic flow to become more southerly through the Gulf and
Southwest Alaska. The first front to encroach upon the mainland
will appear on Saturday, bringing southeasterly flow into the Gulf
and precipitation to the Kodiak area. Models have not been very
consistent on the speed of this front, either run-to-run or
between solutions. All runs seem to be slowing this feature,
which seems likely, since it usually takes some time for fronts
to make their way northward with a blocking ridge over the
mainland.

Right now, this weekend's weather looks like it will remain snowy
for the Kenai Peninsula, with a mixed bag for precipitation over
the Southwest. Southeasterly winds will be gusty as the front
moves onshore, causing downsloping and little precipitation in the
usual leeside areas. By the middle of next week, the longwave
pattern will have completely transitioned to a longwave trough and
southerly flow over the mainland, which means a stormier pattern
through the end of the week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 150 155 160 165 170 171-176 179 180.
         Storm 177 178 411.
&&

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