Small Craft Advisory 1027, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
422 PM AKST Sun Dec 23 2018

.ANALYSIS AND UPPER LEVELS...
The low which has been sitting south of Kodiak Island the past
couple days is now exiting off to the east (across the Gulf of
Alaska). Most of the remaining shower activity is over the
western Gulf and Kodiak Island. A few light showers can be found
over Southwest Alaska, but these are diminishing with time. Thus,
weather across most of Southwest and Southcentral Alaska is quiet.

Meanwhile, out west a weak low which had been in the vicinity
of the far western Aleutian Islands is dropping southward into
the North Pacific with just a few showers lingering across the
Aleutians and Bering Sea. To the south and east of this low, a
second deeper low (around 980 mb) along with its associated
frontal system and triple point low are lifting northward just
south of the central Aleutians. A jet streak and low level cold
air feeding into the back side of the trough is leading to
amplification of the flow across the north central Pacific. This
is a scenario which leads to development of multiple triple point
lows as the trough and frontal system progress eastward. For the
time being these systems are not really affecting the Aleutians,
but this will change over the next couple days as the flow
continues to amplify helping to bring systems further northward
toward the Aleutians and Alaska Peninsula.

Lastly, high pressure over the eastern Bering Sea down to the
southern Alaska Peninsula is producing benign conditions across
this region.

&&

.MODEL DISCUSSION...
With quiet conditions across most of the forecast area, there are
few model problems in the very short-term. Most of the focus is on
the amplifying trough and associated surface lows over the north
central Pacific. There is still a large spread in solutions for
location and track of triple point lows over the next couple days.
Despite these differences, there is better agreement on the
northern periphery of these systems, with the placement of the
frontal system as it approaches the Alaska Peninsula and Kodiak
Island Monday through Monday night and moves into the Gulf of
Alaska on Tuesday. Changes to the forecast will focus on the track
and strength of this frontal system. Significant changes to
development and tracks of surface lows could still change exactly
how the frontal system interacts with these areas, so overall
forecast confidence is average to below average.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist for the early
portion of the TAF. An isolated area of fog has formed along the
Knik River across the Glenn Highway and into the northern tip of
Knik Arm. Conditions area favorable for this to expand overnight.
With a weak north wind in the low levels expect the fog will drift
south along Knik Arm. For now have included VCFG in the terminal
forecast, though there is a chance fog could actually affect the
airport. If so, would expect it after 12Z.

&&

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

A trough in the northern Gulf will continue to drift slowly to
the northeast tonight, spreading a few scattered shower along the
coast east of Prince William Sound. Patchy fog may develop
tonight from along Cook Inlet and over the Copper River Basin as
an upper-level ridge remains in place, trapping some lingering low
level moisture. Otherwise, conditions will remain cold and dry
over the southcentral mainland. A second upper-level shortwave and
associated surface trough will lift north from the western Gulf
tomorrow bringing another round of scattered rain/snow showers
from north of Kodiak and along the eastern Kenai Peninsula north
across Prince William Sound. This trough is moving north in
advance of the next significant weather maker, a low south of the
AKPEN producing gales and sweeping an occluded front over the
western Gulf and across Kodiak Island beginning tomorrow night. By
Christmas Day, the front will begin to slow but still continue to
lift toward the northern Gulf with persistent gales and
widespread precipitation from Kodiak Island north along the
eastern Kenai Peninsula. Models are in better agreement with the
overall evolution of this system; however there is still some
discrepancy on how far north the front will advance and how much
warm air will advect in from the south. Current thinking is that
there will be enough warm air transition snow for the onset of the
event to at a mix of rain and snow along the immediate coast by
the late afternoon/evening, limiting snowfall amounts in place
like Seward and Whitter. The cross-barrier flow will limit
precipitation along the lee of the Chugach and Kenai Mountains.

&&

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

Satellite imagery depicts a subtle deformation zone aloft over
Southwest Alaska with continued lingering snow showers over parts
of the Kuskokwim Delta and Bristol Bay. Otherwise, weak northerly
flow is keeping rather extensive sea stratus over the coast with
patchy areas of dense fog in Bristol Bay...which will continue
into tomorrow morning. Quiet weather continues through Monday
night before a front moves toward the Alaska Peninsula on
Christmas Day. There is still quite a bit of uncertainty with how
far north precipitation moves north, but there will be increasing
threats for light snow through Christmas Day, especially across
Bristol Bay and into the higher local mountainous terrain north of
Dillingham.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
High pressure centered over the north Bering Sea will remain in
place as the storm track remains south of the Aleutian Chain. The
next gale force front will bring gale force winds to the Central
and Eastern Aleutians tonight through Monday with another gale
force front moving into the Western Aleutians Tuesday. Both of
these fronts will be rather moisture starved, so the main impact
will be gale force winds along the chain with a general N-NE flow
over the remainder of the Bering Sea through Christmas.

&&

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

At the onset of the forecast period there are a series of lows at
the surface from SE Alaska to Kamchatka. At 00z Thursday, the
first surface low is over the eastern Gulf Of Alaska, the second
storm is southwest of Unalaska and the third low is over the Sea
of Okhotsk near Kamchatka. Additionally, at the surface there is a
dome of high pressure over Central and Eastern Siberia that is
beginning to nose its way into the western Bering. The GFS and
ECMWF both show this menagerie of wintry synoptic features.
However, even at the onset of the forecast period the precise
location of the surface low with each storm is slightly offset. As
time evolves and these storms progress eastward, and the model
divergence increases. High confidence that the southern tier of
Alaska will be in a wet pattern. But due to the variation in the
global models, there is a fair amount of uncertainty establishing
the start/stop times for the precipitation shields associated with
each cyclone in the long term forecast period. The biggest
forecast challenge will be determining whether these events will
be straight snow or a wintry mix or even rain.
&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 132 150 155 172-176.
		 Heavy Freezing Spray 180.

&&

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