Small Craft Advisory 958, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 AM AKST Tue Dec 4 2018

.ANALYSIS AND UPPER LEVELS...

A large vertically stacked low centered east of Adak this morning
is the feature controlling all of the weather across Southern
Alaska. Its strongest occluded front is moving northeastward into
coastal Southwest Alaska, with some warmer air further east across
the Gulf. Rain associated with the front has begun in Kodiak, and
the King Salmon and Bethel radars are filling in with
precipitation across Southwest Alaska. Temperatures are well above
freezing everywhere across Bristol Bay, ensuring that much of what
falls there at lower elevations will be rain. Further north some
cold air remains at lower elevations, while temperatures for the
Kuskokwim Delta are rising through the 20s into the 30s. For
Southcentral, nearly all areas are below freezing except those at
the immediate coast, such as Seward and Homer. The high cloud
cover associated with the front are helping keep fog and low
stratus more on the patchy side. This morning's 12Z Anchorage
sounding shows a shallow cold layer only about 1,000 to 1,500 feet
deep that remains near saturation, which helps explain the
localized fog concerns. Meanwhile above it temperatures are closer
to freezing, but also very dry. Thus, it is likely to take all
morning and into the afternoon before the atmosphere moistens up
enough for precipitation to a sufficiently deep depth. Upper level
shortwave ridging is moving northeast ahead of the front, and its
departure should result in a diminishing of the strong inversion
currently over the area.

&&

.MODEL DISCUSSION...

The models remain in excellent agreement initially as they all
track the large Aleutian low and most of its frontal features are
strong and large-scale enough to be easily followed by all of the
models. Disagreement significantly increases by Wednesday night as
a secondary low moves north out of the Pacific around the east
side of the weakening Aleutian low which by then will be centered
south of the Eastern Aleutians. Since the old low will be
transferring its energy to the deepening new low, the details of
how that will come about are very different in all the models,
leading to large discrepancies in the new low center position by
Thursday morning. Otherwise, confidence is lower regarding
precipitation type across Southwest Alaska, despite the good
agreement on timing, as temperatures continue to warm steadily
across the area.

&&

.AVIATION...
PANC...Patchy fog in the vicinity banked up against the Chugach
mountains should diminish this morning. Thicker higher level
clouds will work to disrupt the fog regeneration process and
winds will increase around mountain-top level. As a front moves
into Southcentral off the Gulf, southeast winds aloft will
increase, resulting in wind shear concerns later this morning as
northerly winds persist near the surface. There may be a brief
period of a snow and rain mix this afternoon as a weak, fast-
moving wave associated with the front moves northward across
the region. This may be followed by some of the Turnagain Arm
wind impacting the airport for a relatively brief time Tuesday
evening. The Turnagain Arm wind will shift back to the south as
northerly winds redevelop over Cook Inlet by midnight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A front approaching the area from the southwest will bring another
round of warm air to Southcentral Alaska. It looks to spread
across the southern mainland through this evening while first
bringing a period of snow to many locations before transitioning
over to rain. There are several challenges with the precipitation
for this front, with one of the greatest being the timing of the
transition of snow to rain. Models have been struggling with this
for many of the systems coming in from the south and are
demonstrating a lack of consistency with this one as well.

Another challenge will be precipitation type for the western
Kenai Peninsula north into the Mat Valley. As the front passes
there is a chance for light freezing rain but this is dependent
on how long cold air can hang on at the surface. This is another
area that the models are struggling with. After the front pushes
through tonight, broad southerly flow will linger over the area
keeping most of the precipitation along the coast through
Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Tue and Wed)...
A large warm occluded front is starting to blast into nearly all
of SW AK. This is courtesy of the vast low parked out over the
Central Aleutians. This system is ushering in plenty of warm air
and precipitation with it. First, winds have been on the increase
through the night. Places across Bristol Bay have seen gusts in
the 40-50 mph range while locations further inland are just
beginning to see some gusty conditions develop. The SE winds are
helping to usher in some of this warm air and should be fairly
effective at mixing it through the column down to the surface.
Before that happens though, inland protected valleys of the YK-
Delta and the Lower Kuskokwim Valley will still have a brief
chance to see some freezing rain later this morning. The chances
of this look a bit lower than they did yesterday as the transition
to rain should be rather quick. However, it can't be ruled out
that some of the colder/deeper spots hold onto sub-freezing
temperatures through the day. The strong winds could also lead to
some pockets of blowing snow along the northern YK-Delta
coastline. Again, this will be contingent on how long any cold air
can hold on. There will certainly be periods of heavy snow which
will reduce visibility.

Tonight, all models have come into much better agreement on
developing shortwave moving from south to north across Bristol
Bay. This system will actually recirculate some cold air back into
the pattern (unusual from the south). So while much of the area
from King Salmon to Koliganek will see light-moderate rain, some
snow could mix back in farther north, especially as the night
wears on.

This pattern looks to repeat on Wed with another south to north
moving system moving quickly through the area. 850 mb (5000')
temperatures show signs of colder air moving back in from the
west, especially for the YK-Delta. This would result in rain
further inland near Igiugig, with some snow mixing in further to
the west.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Tue and
Wed)...
The story remains the very broad low over the Central Aleutians.
Strong cyclonic (counter-clockwise) flow continues around this
storm, with the strongest winds back towards the cold air
advection near the extreme Western Aleutians. This cold air should
also allow rain to change over to more snow later today. East of
the center, southerly gale-force winds behind the front continue
to usher in moderate rain from the North Pacific.

Over the next few days, the low will slowly drift east as it
weakens. Expect showery conditions to encompass all of the area as
cold air aloft settles in. A new warm occlusion will try to
impinge upon the extreme Western Aleutians by late Wed bringing a
change in wind direction and another round of rain.

&&

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

A high degree of uncertainty will make for an interesting forecast
for the end of the week and into early next week. The models are
in very poor agreement (to put it mildly) with regard to a Bering
Strait trough that tries to dig down into the eastern Bering and
Southcentral, but gets deflected to our north due to a series of
vort maxes on the backside of a full latitude trough that
stretches from the Strait south through the central Aleutians and
into the Pacific. As these pieces of energy round the base of
said trough, cyclogenesis commences as they initially head north
before curing northwest into the Prince William Sound Region.
Added to the mix will be a deep tropical connection with the
Integrated Water Vapor Transport (IVT) from the GFS model showing
values some 400 to 600 percent above normal.

However, the GFS/ECMWF/Canadian Global height fields and surfaces
pressures are all over the place, making the spaghetti plot truly
look like spaghetti. About the only thing they do agree on is the
colder air associated with the disturbance near the Bering
Strait will shift eastward across the northern half of the state.
So in short it looks like an active and wet period, but the
details are murky due to model disagreement and inconsistency
being far below normal.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 152 155.
MARINE...
Gale Warning: 130-132 137 139 150-160 170-172 174-178 180-185 411-413.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
946 AM AKST Tue Dec 4 2018

AKZ101-111-050700-
Anchorage-Matanuska Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, and Chickaloon
946 AM AKST Tue Dec 4 2018

...WINTRY MIX EXPECTED FOR THE AFTERNOON AND EVENING COMMUTE
ACROSS THE ANCHORAGE BOWL AND MATANUSKA VALLEY...

A weather front heading north will bring a mix of snow and areas
of light freezing rain beginning this afternoon and continuing
through the evening commute. The good news is the freezing rain is
expected to be fairly light and localized. That said, conditions
will become a little slicker for the commute.

Please allow extra time and exercise caution when out and about.