Small Craft Advisory 1001, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
427 PM AKST Fri Dec 16 2016

.ANALYSIS AND UPPER LEVELS...
A strong low in the Bering Sea is the most prominent weather
feature on today's map. The front extending out of this low is
moving through southwest mainland Alaska this afternoon and into
southcentral Alaska tonight.
The jet stream is quite strong but running south of the Aleutian
Islands. However it is in a favorable position to rapidly
intensify a developing low southwest of Attu. This low is the one
still causing problems with weather prediction models as it swings
though the central Aleutians into the southern Bering Sea.

&&

.MODEL DISCUSSION...
Models guidance is good enough to give us reasonable confidence
for the main forecast questions for southern Alaska for the next
day or two. The biggest question still lurking is how strong winds
will be in the central Aleutians where there is currently a High
Wind Watch for Saturday morning through Sunday morning. Models
all show some very strong winds near the area but whether the
swath of strongest winds goes over Adak and Atka or passes south
of that region is still a bit uncertain and the answer will vary
greatly with the model used. Right now the ECMWF track looks to be
the most likely.

&&

.AVIATION...
PANC...Conditions should remain VFR through the evening hours
with just a chance of a snow shower temporarily lowering ceilings
to MVFR levels. After midnight there will be some steady snow
moving into the area that will bring conditions down to MVFR to
IFR levels. There was earlier concern that there could be some
rain or freezing rain this evening but that looks less likely and
all the precipitation should be snow.

&&

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

Relatively little change to the ongoing forecast for the next
storm that will impact the Southcentral region tonight through
much of Saturday. One of the primary forecast challenges for today
and tonight is precipitation type. Models indicate a deepening
layer of above-freezing temperatures moving northward across the
Kenai Peninsula up to Anchorage and southern portions of the Mat-
Su. Downslope flow will also contribute to warming. Light surface
winds, mostly out of the north, will keep temps at or below
freezing for most of the Cook Inlet region, which is usually a
good setup for freezing rain. In this case, expect the warmest
areas to remain dry. A rain-snow line will make northward progress
on the Kenai Peninsula and along the Gulf coast, but do not expect
freezing rain to be an issue.

Downslope drying ends tonight as the region comes under influence
of a slow-moving and increasingly negative-tilted mid-level
trough. This will allow for development of a low-level convergence
zone and possible mesoscale banding off of Turnagain Arm, all
while 850-mb colder air is moving up-inlet. The air mass may be a
bit too warm for snow as precip first begins, but if the intensity
of the precip does in fact pick up quickly, as expected, this
will cause a rapid turnover to all snow. Have continued with
snowfall amounts, with generally 2 to 5 inches in the Anchorage
and Matanuska Valley. Heaviest snow amounts will largely be
dependent upon which locations see the longest duration of low-
level convergence and larger-scale deformation.

Lingering snow showers (except rain showers along the coast) gradually
wind down through Saturday night. The very active pattern
continues through the weekend into early next week as a strong
front associated with a powerful Bering cyclone will then push
north to the North Gulf Coast by Sunday evening. A triple-point
low may develop along the North Gulf Coast Sunday night and weaken
as it stalls in western Prince William Sound Monday. This may
result in additional snow for inland areas at that time.
Meanwhile, strong winds to storm force are possible along the Gulf
coast in the typical barrier-jet region Sunday afternoon.

&&

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

The remnants of the front from the Bering Sea will continue to
move through the area overnight tonight. Temperatures will cool a
bit behind the front, but will be slow to drop too low as
moderating onshore winds continue. Nevertheless, precipitation
should change back to snow at most locations overnight but will
not result in much accumulation at all. The exception will be
over the eastern portions of the lower Kuskokwim valley where
steady snow will continue overnight and some areas may see upwards
of 4 inches of snow before things wrap up Saturday morning.

By Sunday afternoon the remnants of another strong front from the
Bering Sea will move toward the area, but will be much more of a
glancing blow as most of the energy will stay south of the area
and slide into the gulf of Alaska. This will result in lower
chances of precipitation, but will also allow cooler temperatures
to be pulled back down into the area from interior Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

The region is under the influence of broad cyclonic flow
associated with the weakening storm now located over the far
northern Bering Sea. This means gusty winds are slowly coming down
at most locations, but this will be short lived as the next storm
is already starting to form southwest of the western Aleutians.
Satellite imagery shows the atmospheric wave interacting with the
strong 200 kt jet stream has begun to form a low pressure center.
We are still expecting this low to strengthen very
rapidly...likely dropping around 50 mb in 24 hours before reaching
a peak intensity in the low 940s mb range by tomorrow (Saturday)
afternoon northwest of Adak. The rapid intensification of this
storm is somewhat unusual, and for this reason we expect very
strong winds to be realized on both the leading edge of the storm
as well as on the more typical west/back side. It looks like the
central Aleutians are the location most in the crosshairs of the
worst part of this storm, though it still looks like the absolute
worst conditions will stay just south of the islands and out in
the North Pacific. Models have drifted a bit northward today with
the expected low placement, which increases the likelihood of
damaging wind potential for Adak and Atka Saturday evening and
overnight. A high wind watch remains in effect, and future model
runs will be watched closely for the possibility of continued
northward movement of the storm, which would likely result in the
watch needing to be upgraded to a warning overnight tonight.

&&

.LONG TERM FORECAST (Days 3 through 7...Sun night through Fri)...

Fast progressive flow will develop across the forecast area late
this weekend through the end of the workweek. The initial low near
Kodiak Island Sun night will swing a front into the north Gulf
coast on Mon. Snow will accompany this front, mainly over the
coastal mountains with some bleeding inland.

The next low over the central Bering Sun night will move to the
north Gulf coast by Wed. This low will then decay in place through
the rest of the week. Rain and snow will affect the eastern
Aleutians and Southwest Ak through Wed, and then snow is expected
again along the coastal mountains of Southcentral through the end
of the week. Some snow will also cross the coastal mountains
during the middle of the week.

The final low will develop Wed over the extreme western Bering
and reach the Pribilofs by the end of the week. This will bring
rain and snow to the Aleutians and Bering during the second half
of the week. Some strong wind will also affect the island with
this system.

The run to run consistency demonstrated by the models is beginning
to improve. Confidence is now moderate with the timing of the
lows, as well as the location of precipitation.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Watch 187.
MARINE...Hurricane Warning...176 177 Storm Warning...172 173 174
175 178 Gale Warning...120 130 131 132 138 139 150 155 160 165
170 171 179 180 185.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...CC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AD
LONG TERM...DS