Small Craft Advisory 1022, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
416 PM AKST Sat Dec 22 2018

.ANALYSIS AND UPPER LEVELS...
A weakening vertically stacked low is nearly stationary south of
Kodiak Island. There is no longer a discernible front associated
with this low, though tight pressure gradients continue to produce
Small Craft to Gale force winds across the Gulf coastal waters.
Broad and deep instability combined with a series of weak upper
level waves rotating around the low center are producing showers
across the Gulf, Kodiak Island, Prince William Sound and over to
Bristol Bay. Weak deformation between this system and a short-
wave trough sitting off the west coast of Alaska has set up in the
vicinity of Bethel and the Kuskokwim Delta, leading to a fairly
narrow band of light snow. Quiet conditions dominate interior
portions of both Southcentral and Southwest Alaska.

Out west, a deeper vertically stacked low just south of the
western-most Aleutian Islands is slowing tracking off to the south.
An occluded front extends northward across the western Bering,
then eastward across the central Bering. Rain and snow showers and
gusty winds persist in the vicinity of the low center as well as
along the occluded front.

A seasonably strong undulating upper level jet remains south of
Alaska, across the North Pacific. Expect several storm systems to
develop along the jet, one of which will be far enough north to
affect the Alaska Peninsula and vicinity within 2 or 3 days.

&&

.MODEL DISCUSSION...
Models have a fairly good handle on the large scale pattern,
though are struggling with some of the weaker and smaller scale
features transiting around the Gulf low. In particular, the
precipitation forecast for Bristol bay and the Kuskokwim Delta is
a challenge. Fortunately, any snow that does fall is expected to
be on the light side with little in the way of accumulation.

More significant differences develop with the storm system
tracking south of the Eastern Aleutians and Alaska Peninsula
Monday into Tuesday. This looks like a case of multiple triple
point development, with each low forming and rotating back
westward over top of the original parent low. Run-to-run
consistency is poor and forecast confidence is low in such
a complex scenario.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

A mix of rain and snow showers along a weakening occlusion will
diminish from north to south across Southcentral overnight as the
parent low south of Kodiak Island continues drift south before
sliding east across the Gulf tomorrow. Locations along the
immediate coast that saw mostly rain with this system should see a
brief changeover back to snow, with little to no accumulation,
before the precipitation comes to an end. By late tomorrow, only a
few isolated showers will remain over the marine areas. With the
low moving east through the Gulf, a northerly flow over the
southcentral mainland will be on the increase, allowing colder air
to move back over the region Sunday night through Christmas Eve.
The development of a weak trough along the coast near Yakutat
could spread a few scattered snow showers across the eastern half
of Prince William Sound for Monday as well. Otherwise, conditions
should remain cold and dry across the southcentral mainland. The
next low moves south of the AKPEN late Monday sweeping a front
first over Kodiak Island then across the Gulf. Strong
southeasterly flow will advect plenty of moisture ahead and along
the front, with areas of snow changing to rain for locations like
Kodiak City Christmas Eve night into Christmas morning. Farther
north, snow is expected to develop along the eastern Kenai
Peninsula and western Prince William Sound by early Christmas
morning. With a strong southeasterly push of warmer air, a
changeover to rain by Christmas evening along the immediate coast
cannot be ruled out. The cross-barrier flow will also limit the
the chance for snow along the lee of the Kenai and Chugach
Mountains, including the Anchorage Bowl.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Monday Night)...

Areas of light snow showers will push west across the southwest
mainland tonight through tomorrow as easterly mid-level flow
pushes moisture across the Aleutian and Alaskan Ranges from a
surface low south of Kodiak. The low will also lead to mostly
cloudy conditions and thus somewhat warmer air through the
weekend, with temperatures near the Bristol Bay coast and AKPEN
warming to the mid 20's to even lower 30's in spots. Colder air
will persist over the Lower Kuskokwim River Valley and Y-K Delta
with temps generally in the negative to positive single digits. By
Monday, skies should clear some as high pressure briefly builds
in. This will also bring a return to cold northerly flow and
temperatures dropping around 10 degrees lower than on Sunday,
though not as cold as they were this past week. By Tuesday, cloud
cover will increase ahead of the next system tracking east, just
south of the Aleutians. This system will be a bit stronger and
will pump in warmer temperatures to the area. Increasing
easterly/cross barrier flow should lead to downsloping and keep
precipitation relegated mostly to the higher terrain.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Today through Monday Night)...

A ridge of high pressure will drift east over the Eastern Bering
and Aleutians through Monday leading to fairly quiet weather with
just some isolated snow showers on the Bering side of the islands
tonight. Out west, a 973 mb surface low centered south of Attu
will continue swinging a front north across the Western Aleutians
and Bering before dying out. Winds will also continue to diminish
tonight and tomorrow as the low continues to move to the south.

By Monday, two Pacific lows look to merge a couple hundred miles
south of Adak and track east-northeast towards the eastern
Aleutians and AKPEN. Model runs have been struggling with this
system and have failed to lock onto the placement and strength of
multiple surface lows embedded within the upper level trough.
Trends have shown the general track to stay just south of the
islands before clipping the south side of the AKPEN on Tuesday.
Low placement is important, especially with precip type, as a more
southern track will keep warmer air south, allowing mostly snow
to develop. On the other hand, a more northerly track will bring
mostly rain, especially to the Pacific side of the Aleutians and
AKPEN. Right now, it looks like colder air will remain on the
Bering side of the islands bringing periods of snow and blowing
snow from Cold Bay east on Tuesday, while the Pacific side should
see a quick transition from snow to rain as southerly flow advects
in bringing warmer air with it.

&&

.LONG TERM FORECAST (Days 4 through 7)...

At the onset of the forecast period there are a series of surface
lows from the Gulf Of Alaska to the North Pacific to the Sea Of
Okhotsk. At 12z Wednesday, the first surface low is south of
Middleton Island, the second low is south of Adak and the third
low is near Kamchatka. Additionally, at the surface there is a
dome of high pressure over Central and Eastern Siberia. The GFS,
ECMWF and the Canadian all showcase these synoptic features.
However, even at the onset of the forecast period, the precise
location of the surface low with each storm is not in sync. As
time evolves and these storms progress eastward, the model
divergence increases. There is high confidence that the southern
tier of Alaska will be in a wet pattern. But due to the vagaries
of the global models, there is a hefty amount of uncertainty
establishing the start/stop times for the precipitation shields
associated with each cyclone. The two biggest items of concern are
determining whether these events will be straight snow or a
wintry mix. And how much additional snow will fall across portions
of Prince William Sound and Passage Canal which has been dumped
on the past couple of weeks; at sea level as well as in the higher
elevations.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Heavy Freezing Spray 180 181.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
LONG TERM...PJS