Small Craft Advisory 1006, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
408 PM AKST Tue Dec 18 2018

.ANALYSIS AND UPPER LEVELS...

A negatively tilted long wave trough spreads over western Alaska,
the Alaska Peninsula, eastern Aleutians and into the western
Gulf of Alaska. This trough has an upper-level low anchored at
the base near Kodiak Island. Northwesterly flow on the backside of
the trough is creating gale force and some storm force winds
through the gaps in the Alaska Peninsula and Kamishak Bay. Cold
air advection is continuing over the Bering and western mainland.
Offshore flow is continuing dry conditions over western mainland
which is keeping the area dry. A compact surface low is passing
through the central Bering and moving over the Alaska Peninsula
near Cold Bay. The low is brining gusty winds and snowfall to the
Pribilofs and the central Aleutians. A front is moving over the
western Aleutians which is consistent with an arm of the jet that
is pushing over the western and central Aleutians. While most of
the jet is flowing well below the Aleutian chain and the Gulf, a
smaller, isolated segment is over the Kenai Peninsula to the
western Alaska Range. This segment of the jet is associated with a
deformation zone over the Kenai Peninsula. Southerly flow is
brining moisture to Southcentral saturating the atmosphere. A meso
low has developed in Prince William Sound today. The combination
of this low with the deformation band is allowing for snow showers
to persist through Southcentral.

&&

.MODEL DISCUSSION...
Models all have a similar synoptic scale pattern through Thursday.
Differences are mainly in the placement of individual pressure
centers. So they have similar handling of Gulf winds and wind
fields over the Bering Sea. For now will be leaning toward the
high resolution NAMNEST to capture local effect winds.

&&

.AVIATION...
PANC...periods of MVFR/IFR visibilities and ceilings are expected
with snow showers through this evening. Generally expect snow to
diminish but some MVFR ceilings will linger into Wednesday
morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The band that brought snow across Anchorage today has pushed off
to the west and is draped across northern Kenai Peninsula and
southern Susitna Valley. Tonight the center of the main band
should stay primarily over the same area but spread out and bring
lighter snow to locations over the western Kenai and Anchorage.
However, guidance has been struggling to handle this feature and
the possibility remains that the band could push back over
Anchorage and bring more snow than forecasted. Early Wednesday
morning the front that is approaching the north Gulf coast will
push into Southcentral Alaska spreading more snow to most
locations with the heaviest along the coast and lighter amounts
inland. The front will have pushed through by Wednesday evening
and then snow will diminish throughout Southcentral Alaska through
Thursday allowing for colder temperatures across the area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Dry and cold conditions continue across much of the Southwest with
cold advection and offshore flow. Temperatures should continue to
run well below zero in the interior. Temperatures will likely be
warmer over the Delta as low stratus and some flurries continue
through tonight. An occluded front currently over the western
Aleutians will weaken by the time it arrives near the west coast,
but may still bring a shot a snow to the immediate coast late
Wednesday and Thursday.

&&

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

A fairly compact surface low moves rapidly south across the AK Pen
this afternoon and should enhance the northwesterly flow over the
western AK Pen briefly this evening. The surface low will leave an
elongating deformation zone in its wake allow for moderate to
heavy snowfall to continue overnight. In addition to gusty winds
reducing visibility, snow accumulations will also be great enough
for a Winter Weather Advisory. Please see the advisory for more
details. The strong northwesterly flow across the AK Pen will
shift east tonight with ridging building in to the west ahead of
the occluded front. The gale force front with with some force
gusts will weaken late tonight through Wednesday as a frontal wave
develops along its boundary south of the Aleutians and moves
east. This will tend to shear the front apart allowing the
northern portion of the front to weakening as it tracks toward the
Southwest Mainland and keeping fairly good precipitation
potential south along along the Aleutians in the vicinity of the
developing wave.

.MARINE...
Gusts out of bays and passes between False Pass and Sitkinak
Island will continue through the overnight hours. Strong cold
advection will push gusts to the 50-55 kt range. This winds will
quickly abate on by late morning Wed as the flow becomes more
westerly.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Friday's setup is looking similar to the model forecasts
yesterday with a low south of the AKPEN moving into the southern
Gulf, bringing gusty offshore winds to the north Gulf coast and
favorably aligned terrain gaps through Saturday afternoon. A
trough rotating through the Gulf Saturday night and Sunday may
bring some precipitation to coastal areas then lift north on
Monday bringing a chance of snow to the Copper River Basin.

There is more confidence with a front moving over the western
Aleutians and Bering on Friday due to better model agreement
today. Strong winds will remain possible at times in the western
Bering through Sunday. A deep low on Sunday will move north over
the western and central Aleutians, which models have come in
better agreement on since yesterday. Better confidence in this low
brings increasing chances for Gales over the Aleutians and Bering
through early next week. The exact intensity and placement of
this low has decent agreement, but is less certain as it moves
eastward through Tuesday. Stay tuned over the next few days as
this feature continues to be looked at.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 181.
MARINE...Gale 131 132 137 138 150 155 165 170 175 176 177
		 Storm 130.
		 Heavy Freezing Spray 130 138 160 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/BC
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...AH/JR