Small Craft Advisory 1032, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
513 AM AKST Tue Dec 25 2018

.ANALYSIS AND UPPER LEVELS...

A vertically stacked, elongated low just south of the eastern
Aleutians is moving northeastward toward the Gulf and weakening
this morning. This low is causing northeasterly flow and cold air
to advect over the Bering. At the surface, gusty conditions
persist south of the AKPEN which is observed by an ASCAT
satellite pass. A front, associated with this low, is pushing
through the central Gulf and advancing towards the northern
coastline. While weather through most of Alaska is fairly quiet
and clear, some showers continue along the frontal boundary in the
northern Gulf. Through the eastern Gulf and extending into
Southcentral, a ridge is building which is causing skies to
scatter out through the Anchorage area and Mat- Su. Light and
variable winds over Southcentral and the AKPEN, combined with a
deep inversion layer and low temperatures, caused dense fog to
form in the Turnagain Arm, Cook Inlet, the Northwestern Kenai
Peninsula and the AKPEN tonight.

&&

.MODEL DISCUSSION...

Models are in fairly good synoptic agreement through early
Thursday. Thursday morning, a North Pacific low advances towards
the southwestern Gulf and this is when models significantly
differ. There is a wide spread between the models as they approach
Kodiak Island with the GFS being the more central solution. By
Thursday afternoon, the EC has become the fastest and most
northern solution, while the NAM is directly south of the EC, and
the GFS to the west.

&&

.AVIATION...
PANC...With northerly flow increasing, it looks likely the that
PANC will remain fog free this morning with VFR conditions
prevailing.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A front is moving into the area from the south and is bringing
rain to Kodiak as it pushes through. It will keep tracking north
through the morning before stalling over the Kenai Peninsula this
afternoon. The front looks to be bringing enough warm air for a
mix of snow and rain to areas along the coast with the heaviest
amounts of precipitation over the eastern Kenai Peninsula. The
front will diminish tonight, but linger through much of Wednesday.
Wednesday night a new system will approach the Gulf but models are
struggling with the track of this system. How far to the east or
west it moves as it makes its way to the north Gulf coast will
play a big factor in determining which areas of Southcentral
Alaska get the most precipitation. The further east it tracks, the
less likely it is that areas around Cook Inlet will see much
snow.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Monday
night through Wednesday night)...

Between two separate upper level areas of low pressure, Southwest
Alaska is starting out this Christmas morning in relatively quiet
weather. However, the low to the south of the Alaska Peninsula is
moving into the area, which in turn will increase southeasterly
winds moving over the mountains and into Bristol Bay today. Gusty
northeasterly winds and snow showers may make for areas of blowing
snow along the Bering coast today. However, most of the
precipitation will stay south and east of the Aleutian Range to
start out the day. As the low continues to move north and a
shortwave trough tracks westward around it, more moisture will
begin the upslope snow along the Kuskokwim Mountains, which could
extend as far south as Dillingham this afternoon and evening. As
the upper low continues moving north this evening, it is likely to
provide lift for a semi-stationary snowband to develop out of the
Kamishak Gap. For the most part, this band will be oriented
southeast to northwest, so areas along the foothills of the Alaska
Range, including Iliamna, Lake Clark, Sparrevohn, and potentially
even as far as Sleetmute will get some accumulating snow tonight
through into the day on Wednesday. There remains significant
uncertainty as to how stationary the band will be. If it stays
over those aforementioned towns, then it's likely some of them
will reach advisory criteria snow, despite it likely being a wet
snow with temperatures just below freezing. If it waves around and
is more intermittent, then lower amounts of snow are expected. The
middle ground that looks most reasonable at this point is a
general 3 to 5 inches of snow in those areas.

Eventually by Wednesday evening, the entire band will move
northeastward and dissipate over the mountains. The other upper
level low, a stationary one sitting over St. Lawrence Island is
expected to begin slowly digging into Southwest Alaska on
Thursday. Ahead of it, more widespread snow showers are likely
over nearly all of Southwest Alaska Wednesday night into Thursday.
Nothing quite as organized as the snow band tonight into
Wednesday is expected.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/Monday
night through Wednesday night)...

Continuous cloud cover persists across all of the Bering Sea this
morning. This is mostly due to a feed of moisture and cold air out
of the Arctic from an upper level low over the Chukotski
Peninsula. Cyclonic flow of the winds from near northerly in the
Bering Strait to near easterly over the Aleutians, is also helping
add to the lift keeping the cloud cover in place. A vertically
stacked low and its associated front is spreading rain and snow
along the Alaska Peninsula. This front will weaken as it tries to
cross the Aleutian Mountains. The parent low looks to remain south
and east of the Alaska Peninsula, so the northerly winds over
Bristol Bay and the Bering will continue. A longwave trough
emanating from the low over St. Lawrence Island will dig southward
over the central Bering Wednesday and Wednesday night as cold air
digs further south into the central Aleutians.

Some of this cold air will begin to wrap around a new low south of
Dutch Harbor on Thursday morning, which will begin to track
eastward on the Pacific side of the Alaska Peninsula through
Thursday. This may cause more snow across this area into Thursday
night, but there remains enough model disagreement on the track
that where the worst weather associated with this low is still
uncertain. Upper level ridging along with another low and its
associated strong front will approach the western Aleutians
Thursday night.

&&

.LONG TERM FORECAST (Days 3 through 7...Thu Dec 27 through
Mon Dec 31)... The pattern will become quite a bit more active
during this time period. This is due to a deep cold trough
extending south through the Bering Strait ferrying some warm,
moist air up through the Gulf of AK. (This set up is actually
quite a bit like the one we had in the middle of December where
Anchorage had many snow chances.) The first of these systems
approaches the Northern Gulf of AK on Thu. Model agreement with
this system is actually pretty good at this time. They all paint
at least a gale- force low working from Kodiak Thu morning towards
the Prince William Sound by early Fri. This system will certainly
bring precipitation to the Gulf Coast, along with some gusty
easterly winds. Precipitation looks to start out as snow at least,
but could slowly become mixed with rain during the event. For
inland locations (to include the Cook Inlet), this will be the
best shot for widespread snow since the middle of last week. There
are still a lot of details to iron out and this storm track has
proven extremely challenging for models this past month, however,
the system does bear watching.

Out west, snow chances will also return for SW AK while a warm
occluded front impinges upon the Aleutians. The cold trough
digging through the Bering Strait looks to bring a couple shots of
light snow across the SW Mainland. As the trough continues to dive
into the weekend, expect another round of gusty winds and snow
showers across parts of the Alaska Peninsula and through bays and
passes on the Pacific side. Out over the Western Aleutians, the
warm occlusion will bring a mix of rain/snow Fri-Sat. This front
will set the stage for next week's weather.

As the front elongates over the Central Aleutians, the models
start to differ. The GFS wants to develop a potent triple-point
low along the boundary. This would serve to cut off the warm air
advection and keep the storm track primarily through (west to
east) the Gulf of AK. However, the European and Canadian models
(and their various ensembles) present a solution with much better
warm air advection moving north. This in turn builds a sharp ridge
over Eastern/SE AK. With the ridge in place, the storm track
starts to become much more SW to NE (picture Alaska Peninsula
towards central AK Range) allowing much warmer air into the
southern mainland. Individual pieces of energy and associated
surface lows are too hard to lock onto at this time. But the main
idea to start 2019 would be a much warmer and somewhat wetter
pattern with the possibility of some rain mixing with snow over
Southern AK.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale: 130 131 132 138 139 173 176 351.
         Heavy Freezing Spray: 140 160 180 181.

&&

SYNOPSIS AND MODEL DISCUSSION...RA
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JPW
LONG TERM...MSO