Small Craft Advisory 1031, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
535 AM AKST Mon Dec 26 2016

.ANALYSIS AND UPPER LEVELS...
Nearly the entire state is under the influence of a broad longwave
trough that is based over the Bering Sea. Within this trough, the
remnant warm occlusion from several days ago continues to fall
apart over SW AK. Several weak lows have been able to form along
this boundary. One of these is pushing some light snow through
Bethel and the Kuskokwim Delta. The other is wreaking havoc with
the aviation forecasts in Bristol Bay. Places near the low center
or just east of it continue to see a light rain and snow mix.
Places west of it have onshore flow ushering in widespread fog.
But there is some weak cold advection along the immediate coast
that is helping to break up the fog in Dillingham and King Salmon.
This cold air might be just enough to keep the surface visibility
from dropping, but low clouds should linger through the day. The
last low we will mention has formed just north of Kodiak Island
overnight. This low will fall in behind the main upper-level wave
that is moving through South Central this morning. It will
continue to keep plenty of moisture moving into the Gulf Coast and
provide some additional lift. Therefore, the snow over portions of
the Kenai Peninsula will continue through a good portion of the
day. It is estimated that Turnagain Pass accumulated roughly 2
feet of snow from the Christmas storm. Snow for today should be
much lighter in nature and should start to wind down late today as
the low tracks towards Prince William Sound.

The last feature of interest is a vorticity max breaking off from
a strong low well up into the Arctic. This piece of energy will
play a critical role in our forecast as we go through the week.
It will be a key feature for the potential for some snow across
South Central.

&&

.MODEL DISCUSSION...
Across the west, models remain in good agreement and forecast
confidence remains moderate to high. There are some strength and
placement differences associated with the surface low near the
Kamchatka Peninsula on Tue. But these differences will have little
bearing on the sensible weather associated with the system.

In the east, the higher-resolution NAM was favored due to some
minor, yet important differences among the models. Forecast
confidence is high that a low will track along the north Gulf
Coast through the early part of the week. However, forecast
confidence is low in precipitation type (rain vs snow) and how
much will fall. Models all indicate some form of a deformation
zone setting up along the Western Kenai Peninsula this morning and
then drifting east through the day. Yet even with impressive radar
returns as of 5am, we have yet to see any precipitation reach the
ground. We will keep an eye on the radar and observations to
adjust as needed.

&&

.AVIATION...
Conditions are expected to remain VFR through the day, even if
some light snow does fall. As downsloping continues to wane
tonight, it will leave the area vulnerable to some light
precipitation wrapping around the low near Prince William Sound.
Confidence is low in the timing and intensity of tonight's snow,
but if it does get going, it won't take much to drop conditions
to MVFR.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

The steady and heavy precipitation over the eastern Kenai
Peninsula and western Prince William Sound has begun to diminish
to showers as the front falls apart and tries to push inland. Meanwhile,
the next feature taking shape in the area is a low pressure center
deepening between Kodiak Island and the Kenai Peninsula this
morning. Winds across the Kodiak area have begun to gust out of
the west, and overall flow across the region will slowly become
more offshore in nature as this low moves slowly toward Middleton
Island through tonight. The big question mark continues to be how
much precip will fall over the western Kenai Peninsula and the
Anchorage area today and tonight as the cross- barrier winds
weaken and general shower activity is allowed to spread over the
area. Things don't look too impressive at this point, but a
dusting of snow can't be ruled out in any areas. No significant
warm or cold temperature advection during the day today, so
precipitation type will continue to be a challenge along the gulf
coast locations, but the trend by this evening with any ongoing
shower activity should definitely be toward an all-snow scenario
as cold advection begins to spread in from the west.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
A broad trough currently stretches from a surface low in the
central Bering, inland across the Kuskokwim Delta and through
Southwest Alaska to another low east of the Barren Islands. With
much of Southwest Alaska in a region of relatively flat pressure
gradient and warm and moist air, fog and low clouds as well a mix
of rain and snow continue to be a forecasting challenge for
Southwest Alaska. Cooler air aloft will spread in from the
southwest during the day today associated with an upper level
trough tracking east along the Aleutians and Alaska Peninsula.
With this cooling, showers should turn to predominantly snow
this afternoon through tonight.

The pressure gradient will further flatten across Southwest Alaska
late Monday night through midday Tuesday as a weakening Bering low
tracks into southern Bristol Bay across the Alaska Peninsula.
Behind this surface low, a cold arctic trough digging south
across the Bering Strait and over the Seward Peninsula will push a
significantly colder air mass with increasing northwesterly winds
over the Kuskokwim Delta Tuesday afternoon and across the rest of
Southwest Alaska Tuesday night. This cold snap, however will be
short lived with a strong warm front reaching the Southwest Alaska
coast Wednesday afternoon and lifting inland Wednesday night.
This frontal system is warm enough that most areas along the
Kuskokwim Delta and greater Bristol Bay area will switch over to
rain quickly, however do have some concerns for precipitation
starting off as freezing rain in the sheltered inland valleys
along the Kuskokwim Valley.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
The broad upper level low over the Bering will shift east today
through Tuesday as a flat ridge builds in from the west across
the Aleutians. As the arctic low swings into western Alaska
Tuesday night, the ridge will continue to amplify and shift east
over the eastern Bering. A strong low will rapidly spin up over
the western Bering Tuesday night and Wednesday along a very strong
thermal gradient offshore of the Kamchatka Peninsula. This low
will send a strong warm front across the Bering Tuesday night
through Wednesday bringing widespread gales to storms and a rapid
warm up.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A big pattern change with possible warmer air and rain is on the
way for much of Southern Alaska. Beginning on Tuesday night, the
upper level low pressure currently responsible for active weather
over the northern Gulf region will still be in the area, but on
it's way out to the east. Isolated to scattered showers will
linger around the Gulf, while inland areas will not see much
outside of cloud cover. Winds will remain relatively light with a
weak outflow pattern along the coast taking hold. Out west, the
instrument of pattern change will be entering the Bering. A large
deepening low pressure system will be sending a warm front from
west to east, pushing onshore the mainland Wednesday night,
starting as snow, quickly changing to rain. These events set us up
for the extended forecast.

A big pattern change is progged by the models with the
aforementioned system. It will pump up a large ridge over the
Bering Sea first, then over Southcentral Alaska. Guidance is in
agreement with this longwave feature. The uncertainty comes with
the extent of warm air involved and it's push onto the mainland.
The two camps of solutions at this point are as follows: one
bringing the warm front through southcentral AK from southwest to
northeast which would bring much warmer air and rain, and the
other, shunting the warm air just to the south in the Gulf which
would severely limit the amount of moisture available, but
precipitation would stay snow rather than rain. The high-
confidence portion of the forecast would be pattern amplification,
followed by a strong cold-front passage. Guidance all hang on to
an amplified pattern for the remainder of the forecast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

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