Small Craft Advisory 17, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 PM AKST Fri Jan 5 2018

.ANALYSIS AND UPPER LEVELS...

A very large trough has carved itself out over the North-central
Pacific and Bering Sea. The anchor low is now vertically stacked
and centered over the western Bering Sea, with the deep surface
low measured around 946 mb. There is a second low which spun up
along the North Pacific jet stream in the vicinity of the Eastern
Aleutians. This low is around 965 mb and is enhancing a storm
force warm front as it moves into Bristol Bay. Over mainland
Alaska, weather is much quieter as a blocking high strengthens
over the central part of the state. There is a remnant and
weakening upper trough over the eastern Copper Basin which is
continuing a weak off shore pressure gradient from Prince William
Sound eastward. With the weak inland pressure gradient, low
stratus and fog remain an issue across Southcentral. Over
southwest Alaska, strong cross-barrier flow has induced strong
southeast winds as the warm front tracks through Bristol Bay.

&&

.MODEL DISCUSSION...

The numerical weather models are in excellent agreement through
Tuesday as the state remains under the influence of a blocking
high while the storm track remains over the Bering Sea. There is
still some slight disagreement with the exact position of a triple
point closed low that will form in the Gulf of Alaska, but the
overall impact of these differences is very minor. The GFS and
ECMWF are in close agreement, so those models were favored over
Southcentral. Over the Bering, another strong storm force front
will move in Sunday and Monday, and the models have been locked in
on that low for a couple days, so no model preference was favored.

&&

.AVIATION...
PANC...Low stratus and some patchy fog will remain a threat
through the evening before higher clouds ahead of a front help to
erode the low level inversion clouds tonight. Along with a
strengthening pressure gradient, VFR conditions are expected to
return tonight and continue through Saturday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An area of high pressure currently dominating the weather across
most of Southcentral is moving to the north as it gets pinched off
between two lows to the west and east. Fog and low stratus will
continue through the evening over NW Kenai, the Anchorage Bowl,
and parts of the Matanuska Valley before being mixed out as the
next system moves in overnight. A weather front will move into the
Northern Gulf tonight bringing heavy precipitation and gale force
winds to coastal areas tomorrow. A closed low looks to form
behind the front and will drift over the Southern Kenai before
weakening by Sunday. Due to the location of the system, flow will
be predominately NE'ly over inland areas which will help keep
things dry for populated areas along the Western Kenai north
through the Mat-Su Valley. Along the coast, precipitation will
start off as snow before warmer air is pulled north changing
precipitation over to rain later in the afternoon on Saturday.
Total snow accumulations look to be around 1-2" for Seward while
Whittier and Portage could see up to 6". Precipitation will
slowly taper off along the coast through the day on Sunday as the
low weakens and high pressure begins to build in again.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
As the surface low centered just to the south of the Alaska
Peninsula tracks east-northeast and gradually weakens through the
evening and the broader parent low pulls back to the west in the
Bering, the storm force front stretched along the Southwest Alaska
will begin to weaken as well as it starts to shear apart. Downsloping
east to southeasterly winds will stay strong through much of the
night, particularly in the lee of the gaps of the Alaska and
Aleutian Ranges. By early Saturday morning, the remnants of the
surface low will lift north from Bristol Bay and inland across
Southwest Alaska as an open trough. Following this trough, winds
will shift more southerly ending the downsloping and spreading
rain showers to the coast and snow showers inland. Precipitation
will turn over to all snow Saturday evening and then taper off to
scattered snow showers Sunday and isolated snow showers Sunday
night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The large vertically stacked low centered over the western Bering
will begin to gradually fill tonight as it tracks to the west in
response to a fast moving, cold upper level trough tracking from
west to east south of the Aleutians around 45 N. On Saturday and
Saturday night the upper trough will slow and start curving to the
northeast bringing cold advection showers to the eastern
Aleutians and Western Alaska Peninsula. A second upper level
trough staggered to the north and slightly behind this feature
will swing across the southern Bering Saturday and over the
Pribilof Islands early Sunday morning also bringing showers. The
next rather strong frontal system will track in from the north
Pacific and reach the Aleutians around Sunday afternoon. The large
frontal system will continue north over Bering and east to the
Alaska Peninsula Sunday night and Monday while the large parent
low tracks into the central Aleutians.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning Sunday night, a deep storm force low will be making its
way northward across the Aleutian chain and into the southern
Bering Sea with an occluded front spreading north and east across
the Bering Sea. Strong height rises out ahead of this system will
re-enforce ridging over northern Alaska, though it will re-center
farther west toward Siberia. The GFS is a big outlier in
maintaining two separate upper level high centers which results
in a fundamentally different solution as we head through the week.
For the moment, will stick with the idea that ridging will
persist over mainland Alaska, keeping storminess confined to the
Bering Sea and Gulf. Thus, most of Southcentral and Southwest
Alaska will be seasonably cool and dry, with coastal areas seeing
occasional clouds and a chance of precipitation. The Bering Sea,
Aleutians, Alaska Peninsula, Kodiak Island, and the Gulf will see
continued active weather, which means warm windy and wet for much
of this region.

One feature to keep an eye on is an arctic trough dropping down
the east side of the upper ridge on Monday, southward across the
Yukon Territory. Some models, led by the ECMWF dig this southwestward
into Southcentral Alaska, around the bottom side of the upper ridge.
This would result in colder temperatures and perhaps an offshore
wind event along the Gulf coast. For now, trended slightly in that
direction, but with so much uncertainty in position of morphing
upper level ridge, will have to wait until models get a better
lock before considering further changes to the forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storms 160 180. Gales 119 120 130-132 138 139 150 173-178
181-185 351 352 411-413.

&&

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...KJP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...SEB