Small Craft Advisory 996, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 PM AKST Wed Dec 27 2017

.ANALYSIS AND UPPER LEVELS...
There is a flat upper level ridge over the northern Gulf and
southern mainland Alaska. Offshore flow exists at the low levels
with a surface ridge over the state. The offshore flow is
somewhat weak, and some fog and stratus exists over many of the
inland valleys. There is an upper level low over central Alaska,
and this is slowly strengthening and increasing southwesterly
flow over southern Alaska. A strong upper level ridge centered
over extreme eastern Russia extends over the northwest Bering Sea.
Underneath this ridge is an upper level low over the central
Aleutians that is moving towards the west. There is a fairly
strong front just to the southwest of the western Aleutians
associated with a low to the northeast of Japan.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement through Friday
afternoon. On Friday the models begin to diverge on the exact
location of surface lows along a front that stretches across the
Aleutians and southern Bering Sea. One of these is south of the
eastern Aleutians, and another near and/or south of the western
Aleutians. The main front itself, which is north of these lows, is
fairly well agreed on. Forecast confidence is thus a little above
normal.

&&

.AVIATION...
PANC...LIFR conditions in fog are expected to continue through
Thursday morning. There could be a few breaks late this
afternoon. Conditions should improve to VFR around noon Thursday
as more mid level moisture moves into the area and helps break up
the inversion, although there is a possibility this may take
longer to happen.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A very benign pattern will remain in place over Southcentral
through the end of the work week as weak high pressure over the
Gulf coast is replaced by southwest flow aloft on the periphery of
a closed upper low developing over the western mainland. This will
continue to promote fog as the primary forecast challenge in the
coming days. Patchy dense fog looks to linger over portions of
Upper Cook Inlet and the Copper River Basin through at least
Thursday morning as light winds persist beneath weakening high
pressure. Fog should begin to gradually diminish late Thursday
into Friday as the upper low becomes more established and mid
level cloud cover increases across the area. Meanwhile, weak
cyclonic flow over the Gulf will support some sporadic light snow
showers over the Gulf waters and for locations along the immediate
coast Thursday into Friday, with inland areas remaining dry.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Southwest Alaska will remain in a relatively quiet pattern for
the next couple of days in the absence of any strong surface or
upper level features. As time elapses an arctic trough will dig
into southwest bringing cold air south of the Brooks Range.
Compounding the matter, locations where the sky is clear expect
strong radiational cooling once the sunsets. For interior
locations, this forecast package went for min temperatures in the
single digits for Thursday morning and below zero Friday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The longwave pattern features a dome of high pressure over NE
Russia with a vortex southwest of Kamchatka. Several storms will
spin out of the vortex impacting the Aleutians. The first frontal
boundary will continue to approach the western Aleutians today.
The front will bring gale-force and storm-force winds to the area
along with a mix of snow and rain. Expecting a triple point low to
form bringing the unsettled weather further east this forecast
period into the Central and Eastern Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The models remain in great agreement in the general pattern of a
return to a much more active and warmer pattern for the South
Mainland into the new year. This will occur as the same pattern
than brought widespread warmth to this same area earlier this
month redevelops as a broad ridge from the coast of California
noses up into the Panhandle with a continued persistent trough
over the Bering. This will leave the Gulf/South Mainland under the
influence of a southerly jet that is expected to bring a series of
northerly moving systems into the South Mainland through the
middle of next week. The first (and likely coolest) of which is
expected to bring gale force winds from the Gulf into the Eastern
Bering for Sunday, along with snow, possibly mixed with rain to
the Gulf Coast. The active pattern is then expected to continue
into Mon-Tue a second stronger and warmer system moves into the
region. As is usually the case in the models as these large scale
pattern changes are occurring, model agreement in the exact track
of these systems is poor, leading to low confidence in the details
in the forecast. What is much more certain is a shift back to a
warmer and wetter pattern with the potential for more wind events.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 173-176 411-413.
         Storm 177 178.

&&

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