Small Craft Advisory 951, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKST Sat Dec 1 2018

.ANALYSIS AND UPPER LEVELS...

As the seismic activity in Southcentral slowly winds down this
morning, the weather is beginning to ramp up. A broad upper level
low continues to churn over the Bering Sea this morning with a
large upper ridge centered over the eastern mainland. A highly
amplified southerly jet is pushing across the AKPEN into western
and northern mainland, helping to drive a strong surface low and
associated frontal system across the western and southern parts of
the state. Strong southerly flow at the surface is pumping in
warm moist air into southwest and southcentral AK, with above
freezing temperatures leading to precipitation falling as rain all
the way up into the lower Kuskokwim River Valley. One exception
is in the Northern Susitna Valley, where temperatures are still
below freezing and steady snowfall has been observed for most of
the night. As the frontal system approaches Southcentral and the
Cook Inlet corridor, downslope and gap winds are beginning to
increase, especially in the Turnagain Arm. Radar imagery indicates
low level drying over portions of the Western Kenai Peninsula and
the Anchorage Bowl.

Further west over the open Bering and Aleutians, cold air being
drawn south from Siberia on the backside of the upper low and is
leading to drier conditions, with open cell cumulus observed on
satellite imagery. A compact surface low is seen approaching the
Central Aleutians from the North Pacific as it gets pulled along
the base of the upper low to the north.

&&

.MODEL DISCUSSION...
 Models are in good synoptic agreement this morning with only
minor location and timing differences. As always, the devil is the
details and models are struggling to come to a consensus on how
far north and quickly warm air gets pulled into Southcentral and
how fast the front pushes through the area this afternoon and
evening. This will make precipitation type the primary challenge
for the forecast period.

&&

.AVIATION...
PANC...Another challenging TAF this morning as a front moves
through the area. Any snow that does develop will quickly
diminish as a southeasterly Turnagain Arm wind kicks in. Wind
shear was the main concern as gusty surface winds persist out of
the southeast with southerly flow aloft. VFR conditions will drop
briefly to IFR as the front moves through. VFR conditions are
expected to return behind the front.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A front is pushing through Southcentral Alaska from the southwest
and has brought a strong push of warm air with it that has
transitioned precipitation on the western side of the Kenai
Peninsula to rain. Areas further north are still seeing snow but
many will also change over to rain this afternoon. Current
guidance favors this transition happening without freezing rain
being a factor, but there is still a chance for a brief period of
freezing rain to occur. Another wave will quickly follow the
first but this time with a cold push that will bring snow back to
most locations north of Cook Inlet. Southcentral Alaska will then
see a brief reprieve before another system approaches from the
south Sunday. This system will be reinforced by an upper level
trough and bring snow to many areas around Southcentral Alaska.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Saturday
through Monday)...
The low train continues for southwest Alaska. The second
forecasted low is pushing its frontal system through southwest AK,
with the low center moving northeast through the eastern Bering
Sea, just off the coast. Weather is as forecasted, with the
frontal system warming temps and bringing rain showers to the
area. Storm force winds are still expected in the eastern Bering
today behind the front. This low will push through today and a
brief break in the weather will occur around mid day.

Things are not done though as the next system is quickly on its
heels. The next system will move in tonight, again bringing gusty
winds to the Bristol Bay area. Temps will remain warm enough that
precip will again be largely rain. Weather will then quiet for a
couple days, though late monday the southwest coast will see an
increase in wind speeds with the outer bands of a much larger low
skirting along the Aleutian Island chain.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through
3/Saturday through Monday)...
Gale and storm force winds will be present today in the eastern
Bering and along the eastern Aleutians in association with the
frontal system that is now pushing through southwest AK. These
higher winds are a post frontal push, so once they subside today
the eastern Aleutians will get a brief reprieve in the weather.
The next low in this active pattern is already moving into the
central Aleutians out of the north Pacific. It will bring another
round of gales for the eastern Aleutians later today and tonight.

The rest of the Bering will largely see cold weather and isolated
to scattered snow showers that are circulating around a broad low
located east of Kamchatka. In fact, it is this broad low coupled
with a ridge over southeastern AK that is driving the track of
these repeated lows over the eastern Aleutians. Out west, largely
zonal flow for the western Bering and Aleutians for the next
couple of days. Sunday into Monday a large low pressure system
will move out of the north Pacific to be located on the south side
of the Aleutians. This system will be the next major player for
the Bering and will drive the weather for the region early next
week.

&&

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

The forecast period commences with a longwave trough across the
Western Bering and a ridge axis over the Yukon Territory, the
Gulf Of Alaska and SW AK. There are several potent shortwaves embedded in
the upper level trough. At the surface, there is a deep mature
low near Adak. At the onset of the forecast period, there is good
agreement with the global models regarding the surface low south
of the Aleutian Chain. The consensus and the synchronicity is a
welcome change in model performance. As time elapses, expect this
low to push eastward across the Aleutians and into SW AK late in
the forecast period. A second low will form over the Sea of
Okhotsk and take a similar path. The gustiest winds are expected
with the leading low which has the tightest pressure gradient
force.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 145.
MARINE...Gales 130 131 132 136 138 139 140 150 155 160 165 172 176
177 178 180 352 411 414.

&&

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