Small Craft Advisory 64, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 PM AKST Sun Jan 20 2019

.ANALYSIS AND UPPER LEVELS...
The big weather driver of the day remains to be the vertically
stacked low south of the central Aleutians lifting northward this
afternoon. Its frontal boundary, which has already brought gale
force winds, is pushing towards the AKPEN. Snow showers have
switched over to rain for Dutch Harbor as this front moved over
the area. High pressure, associated with an upper level ridge,
remains over most of the Bering is keeping clear and dry
conditions over much of western Mainland. A meso-low spinning in
the Gulf with a precipitation band is causing showers along the
coastal areas, while interior locations are remaining dry. The
Anchorage Bowl is still seeing foggy conditions, while the Copper
River Basin remains under a 2000 ft stratus deck this afternoon.

&&

.MODEL DISCUSSION...
The short term model agreement has improved from yesterday. This
is especially in regards to the large low just south of the
central Aleutians. Yesterday's model runs had a discrepancy of
nearly 250 miles on the low center between the ECMWF and Canadian
(with GFS and NAM in between those positions). Today the models
are almost identical with not just the initialization, but in its
movement the next two days.

The next big model discrepancy to watch is in the Wednesday timeframe
where the GFS is the model bringing the most warm air northward
toward Southcentral and Southwest mainland Alaska and is also the
quickest to do so. The NAM and ECMWF are hinting at the
possibility of the warmest air going east of Southcentral Alaska.
However, regardless of the model the odd do look like the chance
for warm downsloping winds to reach inland areas (such as
Anchorage) is increasing.

&&

.AVIATION...
PANC...Fog will continue to be in and out through the night near
the airport. It is possible that it will dissipate for a while
this evening before reforming later in the night as it did over
this past day. The pattern begins to change tomorrow which should
bring the end to the fog issues for a while.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Tuesday night)...

Another cold night is expected across Southcentral with patch fog
mainly along the Inlet. Scattered showers across the northern
gulf will spread inland across to the eastern areas of the Copper
River Basin late this evening as a weak surface low across the
western gulf sends a front inland. Otherwise, relatively benign
conditions expected with increasing clouds overnight as the gulf
low pushes inland across the Kenai and dissipates.

Further south, a front moves up to the southern Gulf and spreads
precipitation across Kodiak between late evening and midnight.
Precipitation type should change fairly quickly over to rain as
this system pushes a warmer air mass from the North Pacific.
Precipitation will be less organized as the front moves across the
Kenai late tonight into Monday due to the boundary running way
ahead of the driving low and another triple point low developing
along the eastern Gulf. However, the eastern Kenai, western Prince
William Sound should see a few inches of fresh snow as the warm
subtropical air mass overruns the cold dome which only slowly erodes.

Another warm front follows on Tuesday moving up to the western
Gulf during the morning and then advances quickly north and
weakens during the afternoon.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2/Sunday night
through Tuesday)...
High pressure, with clear skies and cold temperatures continues
for southwest Alaska. However, this will change over the next
couple of days with the arrival of a frontal system from the
south early tomorrow. This system will bring cloud cover and a
wind shift, which will warm temperatures. Valley locations inland
will stay cold for another day or so as warm air is expected to
override cold air in place, leaving some pockets of cold air. The
frontal system should kick out any remaining cold air by midweek.
Also expect rain/snow with this system, with precipitation
starting as snow, then transitioning to a mix on Tuesday for
coastal areas as warmer air takes over. Blowing snow with some
gusty winds will also be possible along the coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2/Sunday
night/through Tuesday)...

The storm force low in the north Pacific continues to approach the
Aleutian Islands with the storm force winds expected to impact the
central Aleutians early tonight. Widespread gale force winds are
expected to continue in the Bering Sea for the next couple of days
as this low becomes the main weather feature. Rain and snow is
also expected with this low. The rain/snow line currently lies
right along the Aleutians and will progress north through Tuesday
to lie just south of the Pribilofs. This means mostly rain for the
Aleutians and mostly snow for the Pribilofs with this system. Gale
force winds and snow will create the potential for blowing snow in
the Pribilof Islands. Visibility could be reduced to one half mile
at times, so keep an eye on the forecast for more information as
this forecast continues to come together.

&&

.MARINE (Days 3 through 5)...
(Wednesday through Friday)

The marine outlook remains challenging as model continuity remains
modest. The general trend over the last 24 hours has been for a
northwestward shift in the surface and upper air pattern, as a
ridge of high pressure builds northwest across the east central
Gulf, then across Southcentral Alaska into the North Slope. This
will also allow a series of lows lifting across the western and
central Aleutians on Wednesday to retrograde towards the western
Aleutians and Kamchatka Peninsula by Friday. How fast this occurs
and where the low pressure centers will be is yet to be
determined.

However, there has been some improvement with this since
yesterday. The GFS and Canadian models have similar forecast
solutions, where as the European model (ECMWF) seems somewhat in
agreement, but to a much lesser extent. This causes surface low
strength differences of up to 20 mb. Overall, the most likely
scenario at this point is for each system and their associated
fronts to bring sustained gale force winds with some gusts perhaps
approaching lower-end storm force. Some of these fronts may reach
the western Gulf before weakening as the bump into the downstream
ridge.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Model uncertainty remains moderate in the Wednesday through Friday
timeframe, before the numerical forecasts continuity degrades
more towards the weekend. In general the surface and upper pattern
favors a ridge of high pressure gradually building northwestward
from the eastern Pacific, into the east central Gulf, continuing
towards the North Slope and eastern Siberia. This would allow for
moderating temperatures as we head through the period with some
precipitation type issues for Southcentral possibly developing,
due to warmer air aloft advecting over the colder surface
conditions, as a weakening front from the Bering lows tries to
push into the region before weakening somewhat while slowing down.
Should this occur, the over-running warm air around 850 mb would
cause at least some precipitation type concerns, most likely to
develop in the late Wednesday through Thursday timeframe.

Meanwhile, a series of lows will continue to lift north across
the central Aleutians before stalling as they build into the
building ridge, causing each low to rotate around each other in a
Fujiwhara-like fashion early on. As the ridge continues to build
towards the weekend, its associated effect will be to shift this
area of disturbed weather further west towards the western
Aleutians, extending northwestward towards the Kamchatka
Peninsula.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm 173. Gales 155 165 170-172 174-179 185 411-414.
Heavy freezing Spray 139 160 180 185 414.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ/AH
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...PD