Small Craft Advisory 966, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 PM AKST Thu Dec 6 2018

.ANALYSIS AND UPPER LEVELS...
A vertically stacked, negatively tilted low is sitting at the
base of a broad trough that encompasses the Bering and Aleutians.
This low has been the weather driver for most of the state and
continues to do so. The jet stream is in more of a meridional
pattern with a jet streak flowing just below the western Aleutians
reaching speeds of 170 kts at 300 mb. The jet is driving this
upper level low southeastward into the North Pacific. A second jet
streak, albeit weaker, is over the central Gulf. This jet is
driving southerly flow through the Gulf and is bringing ample
moisture to Southcentral and southern mainland Alaska. Coupled
with the jet is a strong band of increased vorticity at 500 mb. At
the surface, a low pressure system rotates just south of Kodiak
Island. This low is enhancing the southerly flow and intensifying
the pressure gradient throughout the Gulf, which is setting the
stage for quick northerly progression of the low and strong winds.
The influx of moisture, both at the upper and low levels, is
covering much of mainland Alaska in stratus. The AK- PEN and north
Gulf Coast are seeing most of the precipitation which can be seen
on radar.

&&

.MODEL DISCUSSION...
Models continue to struggle with the next batch of systems. In
general, models are consistent through Friday morning but quickly
diverge as the NAM moves the low in the western Gulf northward
faster than the EC, GFS and Canadian. As this initial low travels,
a second gale force low develops on the leeward side Friday
morning in the southern Gulf. This low is not well handled by the
models. As this low moves northward, the GFS and Canadian are
similar tracking the low through the eastern Gulf then into
Southcentral on Saturday morning. The GFS is the most robust of
the solutions bringing widespread stronger winds into the northern
Gulf. In opposition, the NAM and EC are in decent agreement and
push the low into the western Gulf at a much faster pace on Fri
night and remain weaker. As the next low enters the southern Gulf
Saturday afternoon, there is a lot of disagreement between models.
They are all over the place with the EC being the fastest and the
NAM becoming the slowest solution.

&&

.AVIATION...

PANC...After a period of MVFR and even some IFR conditions in
mixed precipitation later this afternoon and early evening, VFR
conditions should prevail through tomorrow. Some ceilings in the
3500 to 5000 ft range may occur with precipitation overnight.
Precipitation type will be a challenge and will consist of it
switching between snow and rain and freezing rain before settling
on rain Friday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
Another day, another low moving into Southcentral from the warm
North Pacific. It seems we are dealing with weather for October,
not December, but here we are. Another Pacific front is spreading
mixed precipitation across parts of interior, including sleet and
freezing rain across the Glenn Highway corridor. For this reason,
decided to issue a freezing rain advisory for the Anchorage Bowl
northward to the Matanuska Valley. Snow will shift north into the
Susitna Valley late this evening and overnight with accumulations
of 1-3 inches, higher amounts closer to the Alaska Range.

Later tonight into tomorrow, another strong gale force to storm
force front will move up along the North Gulf Coast, switching the
flow aloft more easterly which will likely dry most inland areas
out with heaviest precip along the coast and the coastal ranges.
The storm force front moves inland Friday night with another near
storm force low moving along the coast Saturday evening and Sunday
morning. Here the forecast gets real tricky as some models show a
deformation band forming in cold air over parts of the interior
with snow accumulations. For now, took a very conservative
approach with light accumulations possible as it is more likely
the warm air moves in and the deformation band likely shifts west
over Cook Inlet before the trough shifts east again. We shall
see...stay tuned.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Tonight through Friday a mix of precipitation will continue to
develop across the Southwest mainland between low pressure systems
over the Gulf and Bering Strait. The northern low picks up
momentum on Friday pushing a cold front to the west coast in the
morning which advects eastward through the day. This front
interacts with an occluded system which pushes westward across
Bristol Bay during the afternoon. A deformation zone sets up
between the two systems with moderate to heavy snow expected to
occur across the eastern areas of Bristol Bay and the Kuskokwim
Valley through Saturday morning. A winter weather advisory for
snow was issued with the afternoon forecast package to highlight
this event Friday afternoon through Saturday morning with snow
totals 4 to 6 inches expected with localized amounts up to 10
inches.

Snow tapers off Saturday with showers remaining in the forecast as
low pressure spins along the Yukon Delta. The next front to
monitor is a fast paced system from the Bering expected to push to
the coast Sunday morning bringing increased snow chances for the
start of next week.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Storm force westerly winds along the Pacific side of the Central
Aleutians taper down late tonight. The next focus for active
weather surrounds a low currently developing just south of the
Bering Strait. This system itself will be relatively slow moving,
however it is expected to increase winds to gusty gale force with
higher gusts. Freezing spray will be an issue across the northern
waters through the weekend. Winds begin to impact the northern
Bering tonight with the surface front diving south to the Alaska
Peninsula Friday morning. Gusty winds across the eastern half of
the Bering taper down Saturday afternoon as the next strong front
pushes quickly across from the North Pacific. This front pushes to
the Western Aleutians late Friday night with the frontal boundary
moving across to the central Bering Saturday night. Snow may
develop along the boundary initially, but quickly changes over to
rain with this warm core system. The one exception to this will be
over the Pribilof Islands where the elongating boundary may allow
colder temperatures to slowly erode, so added a mention of
blowing snow for this area Saturday night.

&&

.LONG TERM FORECAST (Days 4 through 7)...
A very active pattern will continue through the week, with
forecast confidence growing as model continuity continues to
improve. Monday will feature a rapidly deepening low which
undergoes bombogenesis (defined as a rapidly deepening cyclone
with a pressure decrease of 24 mb in 24 hours), as its pressure
lowers from 970 mb southeast of Sand Point late Monday morning, to
947 mb south of Seward by early Tuesday morning. This storm will
also draw up tropical moisture, with current water vapor values
forecast to be 300 to 400 percent above normal. Thus, some of the
precipitation will probably be of a moderate to heavy intensity at
times. Another important aspect associated with this system is
wind speeds, with current indications suggesting some hurricane
force winds possible for the coastal waters, as a strong barrier
jet develops.

Next in the active pattern will be another area of low pressure
that will track along or just south of the Aleutians in the Monday
through Tuesday time frame, This system will also undergo
bombogenesis, as mid-level energy drives into the backside of a
central Bering trough, with current indications showing this low
dropping from the around 983 mb to 949 mb (34 mb drop in 24 hours)
as it moves to just east of Kodiak Island by Wednesday morning.
This storm will also tap the subtropics with a similar moisture
feed in terms of percentage above normal. Similar to its
predecessor, warm air will be pumped north with very strong winds,
especially for the marine waters. This storm will weaken as it
fills across the northern Gulf, with yet another rapidly deepening
and moisture-laden storm moving into the west central Aleutians
on Thursday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storm Warning 119 177.
 Gale Warning 120 125 130>132 138 139 150 155 160 178 185.

&&

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