Small Craft Advisory 850, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 AM AKST Tue Nov 7 2017

.ANALYSIS AND UPPER LEVELS...
A high amplitude upper level ridge is centered across the western
half of the state and extends north all the way up into the
Beaufort Sea. An upper level jet max is diving south through
Southcentral and is moving off to the east as the ridge axis moves
over central Alaska. This is acting to loosen the pressure
gradient across the region which is leading to weakening northerly
gap winds along the Gulf Coast and the Matanuska Valley. Skies
underneath the ridge are mostly cloud free leading to rapidly
falling temperatures, especially across the Copper River Basin,
where temps have dropped below zero.

Further west, a long wave trough over the Bering is pushing east
towards the coast, with the associated frontal boundary bringing a
large swath of cloud cover over Western Alaska. The Bethel radar
is showing an area of precipitation, mostly rain, beginning to
move ashore along the west coast.

&&

.MODEL DISCUSSION...
Models are again in good agreement with the overall synoptic
pattern through the short term. Noteworthy differences begin to
show by Wednesday night as an upper level shortwave moves across
the west coast. The NAM is a bit weaker than the GFS and EC
models, showing a less pronounced front, so the forecast hedged
more towards the stronger solutions.

&&

.AVIATION...
PANC...VFR conditions will prevail through the day. There is a
chance of fog developing over the Upper Cook Inlet and Knik Arm
tonight as flow weakens with the ridge axis moving overhead. Left
in vicinity fog for now as timing and extent are still uncertain.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
High pressure will build overhead today, leading to a gradual
slackening of the pressure gradient and allowing winds to die
down most places. With colder air pooled in the Copper River
Basin, winds will linger a little longer in parts of the
Matanuska Valley, Valdez to Thompson Pass area, and out of the
Copper River Delta.

Dry and cool weather will be the story through Wednesday night
as high pressure aloft is replaced by deep and fairly strong
westerly flow, which is often a dry pattern for Southcentral.
Temperature inversions will quickly steepen tonight as the surface
radiates out, so expect to see development of some fog,
particularly in the favored areas of northern Cook Inlet and the
Copper River Basin. A few passing clouds associated with a very
weak upper short-wave should keep it localized. Weak cold air
advection behind this short-wave will kick up winds once again
along the coast, though not as strong as the event which just
occurred.

&&

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

A front stretched along the Kuskokwim coast to the Alaska
Peninsula slowly moves east through the afternoon and dissipates
this evening as it reaches the Aleutian and western Alaska Ranges.
A mix of snow and rain is expected as the boundary moves along the
Kuskokwim Delta and western capes of Bristol Bay with rain across
the Alaska Peninsula. Snow will be light as the upper wave quickly
shears apart as it shoves into the ridge over the mainland. Patchy
fog may briefly develop during the morning and overnight hours as
the weak frontal boundaries slowly diminish. Another fast
propagating wave follows a similar path moving to the west coast
later tonight. A better chance for widespread precipitation in the
near term is Wednesday night into Thursday as the subtropical jet
amplifies a new ridge pattern across the Bering which kicks the
mid level low over Kamchatka into motion.

&&

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

A nearly zonal pattern sets up across the Bering through
Wednesday. This pattern advects a slightly cooler pattern across
the northern half of the Bering along westerly flow. A parent low
anchored over Kamchatka sends a few short waves from west to east
across the Bering bringing periods of isolated to scattered
showers through mid week. An active pattern shift begins Wednesday
night as the subtropical jet quickly amplifies over the western
waters. A warmer air mass will begin to modify the temperatures
across the western Bering and Aleutians on Thursday with drier
conditions and lighter winds along the center of the ridge which
nears the central Aleutians on Thursday. Winds are expected to
increase across the outer perimeter of the ridge on Thursday night
with the Kamchatka and next Pacific low actively trying to break
the ridge down.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The numerical models are in excellent agreement for the long term
portion of the forecast, therefore confidence is higher than
normal. The basic pattern is anchored by the long wave ridge along
about 170 degrees west longitude, and a long wave trough over
eastern Alaska into the Yukon territory. Several short wave
troughs top the ridge and drop northwest to southeast across the
state during the forecast period. The WPC guidance uses an equal
blend of the GFS and ECMWF, incorporating considerable amounts of
the deterministic solutions along with the ensemble means.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales: 185 352 411 412.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...BL