Small Craft Advisory 1043, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
537 PM AKST Fri Dec 28 2018

.ANALYSIS AND UPPER LEVELS...

A broad negatively-tilted upper level trough centered along an
axis extending from Norton Sound all the way down into the Gulf of
Alaska continues to drift eastward across the state today. A weak
upper level ridge is centered over the central Aleutians ahead of
a strong, deepening low approaching the western Aleutians.

At the surface, a 995 mb low centered over the Alaska Peninsula is
bringing a cold front into the western Gulf. Relatively warmer air
ahead of the front is moving inland over Southcentral. As a
result, precipitation is moving in from the south with radar
imagery showing bands of showers beginning to form over the
southern Kenai Peninsula. Precipitation is also lifting northward
over Southwest Alaska and the AKPEN, with temperatures warm enough
for rain over the southern AKPEN this morning. On the backside of
this low, cold Arctic air is descending, bringing strong
northerly flow over the eastern Bering and Southwest coast.

Lastly, a strong and well-defined front is visible on satellite
imagery this afternoon as the deepening low approaches the
western Aleutians. The front continues to traverse eastward over
Adak and is beginning to close in on Dutch Harbor. This has
resulted in strong, gusty southeasterly winds across the western
and central Aleutians.

&&

.MODEL DISCUSSION...

Models are in generally good agreement for the near-term synoptic
pattern for the next 24-36 hours, but continue to struggle with
the important details further out, where significant differences
arise.

Over the south central mainland, the models are consistent with
bringing the low up into Prince William Sound overnight into
Saturday morning and wrapping around strong northwest flow across
lower Cook Inlet and Shelikof Strait. The models are consistent
with this low decaying over the Prince William Sound area through
Saturday. At the same time, the models handle the incoming Bering
Sea storm similarly and push the front into the eastern Bering
Sea overnight while holding the cold air in place over the
Pribilofs. Where the models are struggling mightily is with the
next Bering Sea storm that is pin-wheeling around the anchoring
low over the western Aleutians. Here the models diverge in both
the placement and depth of this storm with significant differences
in the sensible weather, especially in the Pribilofs. This also
impacts how the models resolve bringing in the next storm across
the southwest mainland Sunday night into Monday morning. What is
consistent by Monday morning is a solution that supports warming
and chinook winds across the Chugach mountains.

&&

.AVIATION...
PANC...As the ground fog issue ends we await the onset of snow
moving up Cook Inlet from the south. The trough and low moving up
into south central Alaska tonight will kick out the fog, but usher
in snow. Initially this should hold in VFR then deteriorate to
MVFR due to CIGS and VSBY by 09-10Z. The low will stall in Prince
William Sound as the upper trough moves through Saturday
morning/afternoon, thus expect the MVFR to IFR conditions to
persist through Saturday morning and improving to VFR by around
21Z.

&&

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

A strong, upper-level trough moving east across the Bristol Bay
interior this afternoon will interact with a surface low lifting
north toward the Kenai Peninsula and its associated front sweeping
across the southcentral coast to produce widespread snow for most
of the region tonight into tomorrow. With low-level instability
ahead of the front and accompanying the low, precipitation from
the Cook Inlet to Prince William Sound will initially be on the
showery side. With cold air in place, precipitation type with this
event is expected to be all snow except for Kodiak island and the
immediate coastline. Snow showers will spread northward through
this evening to all but the Copper River Basin. A band of heavier
snow will then develop overnight as the upper-level trough moves
into southcentral Alaska and becomes negatively tilted. Snowfall
totals will be on the light side, with a general 3 to 6 inches
across the region as the trough slides east rather quickly. Strong
cold air advection behind the surface low will produce strong
westerly gusts across Kachemak Bay overnight tonight through
tomorrow morning. This, combined with lingering snow showers, will
reduce the visibility in the Homer area (especially along the
bluff) to as low as one quarter mile at times. A blizzard warning
has been issued for this region.

This system departs the region late Saturday; however, the
weather pattern remains quite active as a frontal boundary
originating from a deep Aleutian Low approaches the western Gulf
and Kodiak Island. Guidance continues to struggle with both the
overall frontal placement and with the position of the surface
lows. The general consensus, though, is for strong southeasterly
winds over the Gulf along the frontal boundary as well as the
increase of gusty southeasterly gap winds as a coastal ridge
and Cook Inlet trough develop ahead of the front. Warmer air
advecting north with this system will change snow to rain along
the coast. Strong downsloping winds along the lee of the Chugach
and Kenai Mountains will also allow temperatures to climb well
above normal from the western Kenai Peninsula north trough the
Mat-Su Valleys for the second half of the weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 2: Tonight
through Sunday)...

A very active pattern will continue for the southwest mainland
this weekend as a series of strong storm systems will impact the
area. First, an area of snowfall over the Kuskokwim River Valley
will continue pulling north and east tonight, bringing
accumulating snowfall from Sleetmute east to the western slopes of
the Alaska Range.

A deep upper trough will move in behind the departing system tonight
bringing very cold air down into the southwest mainland, with
temperatures dropping rapidly tonight, bottoming out tomorrow
morning in the negative single digits to even negative teens in
places. Skies will remain mostly clear Saturday before the next
system approaches Saturday evening. A warm front extending all the
way out from a strong low over the Western Aleutians will spread
inland Saturday night through Sunday morning. This front will
bring strong winds and periods of heavy snow, especially along the
coast and over the Y-K Delta Saturday night, leading to possible
blizzard conditions in these areas, thus a blizzard watch was
issued for the Kuskokwim Delta and Nunivak Island for this time.
Cold surface temperatures should remain entrenched over most
inland locations until Sunday afternoon when strong easterly
surface winds and overrunning warm air aloft mix out the
atmosphere, rapidly increasing the temperatures towards, or even
above, the freezing mark. By Sunday night, models are indicating
yet another system riding north across the AKPEN, bringing another
round of strong winds, precipitation, and an even warmer surge of
air to the mainland.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 2:
Tonight through Sunday)...

The Bering is heading into a very active storm pattern this
weekend, beginning tonight with a 945 mb low moving north and
crossing over the Western Aleutians. A combination of rapid
intensification and cold air wrapping into the system will lead to
very strong winds on the north and south side of the low.
Hurricane force wind gusts are expected in vicinity of Shemya and
over the open water, thus the high wind warning is still on track
for that area tonight. The low will stall out some on Saturday
and slowly weaken as it begins to retrograde back east across the
Western and Central Aleutians through the weekend, keeping strong
winds over the area.

The strong warm front extending out from the Western Aleutian low
will sweep east across the Bering and over the Pribilof Islands on
Saturday. Strong winds accompanied with periods of heavy snow will
lead to blizzard conditions over the Pribilofs on Saturday, thus a
blizzard warning was issued for that area. By Saturday night, a
secondary low looks to develop south of Dutch Harbor and move
north into the Bering. Models have been struggling quite a bit
with this system but seem to have around a 960 mb low placed
somewhere near or just east of the Pribilofs by early Sunday
morning. This will bring strong southerly flow to the Central
Bering with increasing temperatures through the area.

&&

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

The run to run consistency with the global models has been poor.
Generally speaking, anticipate a wet pattern for this forecast
period for the southern tier of Alaska with the possibility of a
rain/snow/freezing rain mix for Southwest Alaska. The forecast
period commences Monday Dec 31st with a broad trough over the
Bering and Aleutians and a high amplitude ridge from the Eastern
Pacific to British Columbia. Meanwhile at the surface at 12z
Monday, both the GFS and the ECMWF have a potent low now north of
the chain between Unalaska and Adak. And the GFS has a specious
cyclone near St Matthew Island that the ECMWF doesn't have at all.
South of the chain the models have an additional low but the
position of the GFS feature is more than 100 NM from the ECMWF's
position. Both models show this feature moving towards the Gulf Of
Alaska. The lack of consensus is echoed at 500 mb at 12z Monday,
which compounds with time. The timing of the precipitation shield
and the gusty winds is dependent the placement of the lows that
develop and on the ensuing track. There was a hefty amount of
blending and hedging this forecast period as a result of the
models' poor performance record.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...BLIZZARD WARNING: 121 195.
         WINTER STORM WATCH: 155.
         WINTER WEATHER ADVISORY: 152 161.
         HIGH WIND WARNING: 191.

MARINE...HURRICANE FORCE WINDS: 178 411.
         STORM: 130 175 176 177 412 413.
         GALES: 119 120 131 132 138 139 141 150 155 160 165
                170 172 174 179 180 181 185 352 414.
         HEAVY FREEZING SPRAY: 160 179 180 185 412 414.

&&

SYNOPSIS AND MODEL DISCUSSION...SA/KO
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...PS