Small Craft Advisory 963, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
405 PM AKST Wed Dec 5 2018

.ANALYSIS AND UPPER LEVELS...
A broad occluding low, that once remained stationary over the
central Aleutians, is slowly shifting south. This vertically
stacked low sits at the base of a long wave trough spread over the
central Bering and Aleutians. The jet stream is taking on a
similar troughing pattern over the central Aleutians, with a jet
streak over the AK-PEN. This southerly flowing portion of the jet
is coupled with a surface front on the leading edge of the upper
level trough. The jet stream and front are advecting moist air
into the southwestern mainland Alaska and western Gulf, which can
easily be seen in IR/water vapor satellite imagery. The flood of
moisture is bringing precipitation to the AK-PEN, Bristol Bay, and
much of Southern Alaska as seen by radar. In addition to the
front over mainland Alaska, a second front is moving into the
western Bering. A weak ridge is building over Southcentral brining
a brief reprieve in precipitation to the area. However, southerly
flow from the Pacific is creating a second stream of moisture
through the central Gulf and creating precipitation along the
northern Gulf coast.

&&

.MODEL DISCUSSION...
Models area still struggling with low centers tracking northwest
across the Gulf. On Thursday night, differences in the low
positions still exist, however the general position and
orientation of the synoptic pressure fields are similar. Things
get a bit more complicated Friday/Friday night with the next big
low to track NW across the Gulf. The GFS/EC while differing in the
low position at least have similar pressure fields. So did a
blend of the GFS/EC in the Gulf for Friday night/Saturday morning.

&&

.AVIATION...
PANC...LLWS will be a concern into the early evening hours.
Ceilings will lower this evening, but most likely remain VFR.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Tonight through Friday)...
Cloudy, warm and generally benign weather will continue into
Thursday as a band of deformation associated with an upper low
over the west coast slowly dissipates as it moves northward. The
most significant feature in the early term to speak of is a weak
wave moving through the North Gulf Coast which will bring
continued areas of rain and snow generally from Prince William
Sound eastward through this evening. As this wave makes landfall,
precipitation will spread northward into the Copper River Basin
and produce a few inches of new snowfall, mainly along the
Richardson Highway from Glennallen southward.

The focus of the forecast then shifts to a developing low well
south of Kodiak Island that will continue intensifying as it
tracks into the Western Gulf on Thursday. This low will bring gale
to storm force winds as well as widespread rain to most of the
Gulf Coast into Friday morning (becoming wet snow at higher
elevations). This system then becomes much more uncertain as a
secondary low quickly develops in the Eastern Gulf late Friday, as
is typically the case with fast moving and rapidly developing
North Pacific features. There is increasing confidence however
that this low will bring a second round of storm force winds to
the North Gulf as the low tracks into the vicinity of Prince
William Sound, however with much lower confidence on when this
would occur on Friday or possibly as late as Saturday morning.
What is clear is that continued active weather will dominate the
Gulf Coast for the remainder of the work week, while areas in the
lee of the Chugach should remain generally warm and cloudy from
persistent downsloping.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Precipitation is expected to continue through the end of the week
as two low pressure systems are about the region...one moving
from the western Gulf and the other dropping south along the
Bering Strait. Fog conditions will be a challenge beginning
tonight as low level moisture is abundant and temperatures cool to
dew point values. Visibilities may periodically improve as the
bands of precipitation shift position.

The most challenging element during the short range forecast
through Friday is precipitation types. The low pressure system
moving to the western Gulf is steered by the subtropical jet
stream. This influence modifies the temperatures warm enough to
rain with a jet max lined up from south to north along eastern
Bristol Bay to the western Alaska Range. On Friday, the low over
the Bering Strait advects a colder air mass inland which modifies
temperatures back below freezing, changing precipitation back over
to snow by Friday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Low pressure along the eastern Aleutians moves east tonight as a
front from the North Pacific advances to the western Chain. Gale
force southeasterly winds and rain are expected along the boundary
as it pushes across tonight. A quick change of wind direction
occurs Thursday morning as a fast paced cold front follows,
bringing westerly gales on the backside of the low. At the same
time, low pressure over the Bering Strait slowly pushes a cold
front into the northern waters. This front advects a colder air
mass to the southern waters by Thursday evening. The latter front
catches up with the front along the western half of the Chain and
then pushes south of the Alaska Peninsula on Friday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
An active pattern will continue for the region through the
upcoming week, with southwest/southcentral Alaska becoming the
"battle zone". It'll definitely be interesting to see if the
models, which continue to come into better agreement, push some
Arctic air into southcentral, or keep it bottled up west of the
Alaska Range. Conversely, how much tropical-laden moisture gets
thrown into a developing deformation band near the Alaska Range
and into southwestern parts of the state.

Now for the details. Model continuity remains good regarding a
strong mid-level low dropping south from the Chukchi Sea. There
is a new wrinkle though in that they now stall the system near
Norton Sound on Friday and hold it there through the weekend. As
the surface reflection develops, much colder air will plunge into
the eastern two-thirds of the Bering and southwest Alaska, through
the eastern half of the Aleutians, and into the north Pacific.
The further north positioning (compared to near Nunivak Island
yesterday) then in turn allows a pair of strong storms to move
north to northwest into the north central Gulf. With the further
west track, warmer air will push across most of Southcentral.
This mean rain will likely be the predominant precipitation type,
with snow across southwest Alaska and the eastern half of the
Aleutians, and a mix for Kodiak Island.

These Gulf lows will weaken while moving northeast, as a new
system enters the picture to start the week. This storm will
undergo significant deepening, as it drops from 980 mb (per the
GFS) south of Adak late Sunday morning to 958 mb south of Kodiak
Island late Monday morning. This will put the AKPEN and the
southern part of southwest Alaska in a potential for some fairly
decent snowfall accumulations if the current forecast holds. It'll
then take up residence in the southern Prince William Sound region
for Tuesday and wednesday, as the storm slowly fills and weakens.
This is perhaps the "best" potential to see some snowfall for
Southcentral, but even then, seems like the dynamics and moisture
will be on the downward trend before enough cold air makes in
into the region. Still, with it being several days out things can
change. Stay tuned!

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 119 120 125 130 131 139 177 178.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/RC
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...PD