Small Craft Advisory 961, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 AM AKST Thu Dec 14 2017

.ANALYSIS AND UPPER LEVELS...
A subtle pattern change is underway this morning as the persistent
ridge over the CONUS and western Canada shifts slightly eastward
and an upper level trough lifts into the Gulf from the southwest.
In terms of sensible weather impacts, conditions are much the
same as in recent days. Scattered rain showers are present across
much of the southern mainland in broad onshore flow, though many
locations are mostly dry in between systems. More steady rain is
beginning to spread back into eastern portions of Prince William
Sound as the latest embedded wave tracks through the eastern Gulf.
A sprawling low pressure system is nudging eastward over the
Western Aleutians, with an occluded front located between the
central and eastern parts of the chain and widespread rain/snow
shower activity over the western half of the Bering.

&&

.MODEL DISCUSSION...
Models are in good synoptic agreement through the end of the week
as the ridge breaks down. This will allow a colder southwest flow
regime to develop as the Bering trough shifts eastward towards
the southwest mainland. Models have also come into somewhat better
agreement with some of the details of the broad low complex as it
shifts slowly eastward through the Bering. The largest
differences arise with a strong low progged to move into the
western Gulf late Friday night. Nearly every solution suggests a
much stronger system than was depicted 24 hrs ago, however there
are still significant discrepancies with respect to low placement
and intensity. Favored the 18/00z GFS solution for the morning
package as it represents a decent middle ground approach at this
time. An additional overnight trend worth watching is that the
precipitation associated with the wave tracking over the eastern
Gulf has been inching westward with each successive model run,
suggesting the potential for some light snow to make it into
Anchorage later this afternoon. Will continue to monitor this
trend today in ensuing runs.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through early
this afternoon. Light snow showers will spread into the terminal
from the east around 00z, lowering cigs below 5000 ft and possibly
to MVFR at times. Conditions should then improve overnight as the
disturbance responsible for the snow showers weakens and departs
the region.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A fairly strong low over the northern Gulf will move into Prince
William Sound this afternoon. Precipitation will continue to push
west and north today, with rain along the coast and snow for most
inland areas. The numerical models are now in fairly good
agreement that precipitation from this system will likely spread
as far west as Anchorage. The 850 mb temperatures will be cold
enough along the western fringes of this low that the
precipitation type for Anchorage probably will be snow. This
system will then push northeast and break apart tonight as the
flow aloft becomes southwesterly. The next system will move into
the area from the southwest on Friday. The front will reach Kodiak
Island late tonight and the Kenai Peninsula by late morning. Winds
will be quite strong over the Gulf and through Turnagain as the
front approaches. By late afternoon the numerical models begin
struggle, mainly with where/when a possible triple point low will
develop along the front.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The southwest mainland will be in a col today as the next low
develops across the western Bering. A triple point low develops
along the frontal boundary as it crosses the Alaska Peninsula,
then quickly races toward the southwest coast tonight. The next 30
hours brings a challenging forecast for precipitation types and
amounts. Generally increased winds and precipitation along the
front and added blowing snow along the Western Capes.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The storm system currently over the western Aleutians will slowly
lift north and advance eastward through Saturday bringing snow and
rain along the southern Bering with snow across the Central and
northern waters. The main challenge with this system is a triple
point low which is expected to develop late today just south of
the eastern Aleutians. This low tracks north crossing to the
eastern Bering overnight. Gale winds increase rapidly along the
associated frontal boundary with moderate to heavy rain and snow.
Winds taper down Friday night, however precipitation continues
through the weekend as low pressure remains dominant.

&&

.LONG TERM FORECAST (Days 3 through 7...Sat through Wed)...
"Uncertainty" is the key word in this timeframe. The weather
models not only continue to conflict with one another, they also
conflict with themselves on nearly each successive run. The result
is overall low forecast confidence in most of this period. The
general theme to the pattern will be a broad trough sliding
further east over the Eastern Bering. This will be possible
because the stout North American ridge that has been locked in
place over the West Coast of the Lower 48 will finally give-way a
little bit. As the trough slides east, its southeast quadrant will
interact with an intense sub-tropical jet. This generally leads
to strong triple-point low development. However, because of the
vast uncertainty on where exactly the trough sets up, and more
importantly, where the low center is within the trough, it is
extremely difficult to bite off on any one solution. Late Friday
night, there is agreement that another wet and windy front will
push into the north Gulf coast. It is early Sat that the GFS is by
far the most bullish in carrying a mid-950 mb low right over
Kodiak Island. This scenario would lead to a similar set- up to
what we have seen several times this week with heavy rain along
the north Gulf coast and strong winds from S/SE to N/NW. The NAM,
EC, and Canadian are a bit less intense and further east with this
feature. The net effect will be another round of rain and wind
for much of the Gulf. However, exactly where this low tracks and
how strong it is will have implications on snow versus rain for
certain areas and how strong the barrier jet might be.

The one area where this is good certainty is with a "bomb" cyclone
over the Western Bering late in the weekend into early next week.
Cold air over Siberia will work in tandem with a strong 300 mb
jet to rapidly deepen a surface low pressure center. Models all
prog it to near 950 mb or slightly lower. This will produce storm
force winds and seas approaching 35 feet.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None
MARINE...Gale Warning 119 120 130 131 132 150 155 160 165 172 174
					  176 177 178 179 180 185 351 352 411 412 414

&&

SYNOPSIS AND MODEL DISCUSSION...CB
AVIATION...CB
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...MSO