Small Craft Advisory 909, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
1154 PM AKST Tue Nov 28 2017

.UPDATE...

Made a quick update to zone 185 in the Aleutians to mention warning
level winds confined to the Nikolski area. Further east, winds were
trended down in the Dutch Harbor area as this region remains a
little displaced to the east of the stronger cold air advection,
where better mixing and downward momentum transfer will take place.
Although some increase in winds here are still expected in the next
few hours, they should remain below headline criteria.

.ANALYSIS AND UPPER LEVELS...
The main center of action continues to revolve around the Bering
Sea as another powerful low (approximately 952 mb) lifts north
into the Southern Bering Sea from the Alaska Peninsula. Near
hurricane force gusts continue to channel through parts of the
central Aleutians with a large area of storm force marine winds
impacting parts of the Aleutian Chain. Near Southcentral, there is
a weak upper trough moving eastward toward the ALCAN, with this
feature supporting an area of rain and snow showers over Prince
William Sound and the local surrounding mountains.

&&

.MODEL DISCUSSION...
The numerical weather models are in excellent agreement with the
general synoptic pattern through Friday before the details of
another front moving toward Southcentral become murky. However, in
the short range, the models are still struggling with a strong low
moving toward Bristol Bay, with the GFS being a weaker outlier
compared to current observations and the Canadian and ECMWF
models. Therefore the GFS was given less consideration as the low
retrogrades west with a second maximum of strong winds expected
to impact Unalaska. For Southcentral, a blend to higher resolution
models was favored to capture local winds and heavier
precipitation along the coastal mountains where heavy snow is
likely as the front moves inland along the North Gulf Coast.

&&

.AVIATION...
PANC...VFR conditions and light north winds will persist through
tonight. Chances for scattered snow showers will increase at the
terminal after 15z Wednesday as a front moves over the north Gulf
coast, with cigs lowering to 5000 ft or lower.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Wednesday
and Thursday)...
Active weather is on its way back to mainland Southcentral Alaska
after a break today. A front is working its way northward across
the Gulf, with the leading cloudcover heading towards Anchorage
and points north. Strong east to southeast flow associated with
the front will keep Anchorage, the western Kenai Peninsula, and
the Matanuska Valley largely dry due to downsloping. But for
coastal locations, it will bring about another round of
occasionally heavy precipitation. Temperatures will warm with the
front so most coastal locations, including Seward and Homer, will
at least mix with rain as the front passes by. In addition to the
precipitation, winds along the north Gulf Coast will briefly get
to gale force with a barrier jet that develops ahead of the front.
As the front pushes inland and rapidly weakens, the winds will
diminish in a hurry early Wednesday morning.

Behind the front on Wednesday, a brief period of southerly wind
flow behind the front will allow temperatures at the coast to get
into the mid 30s on Wednesday, keeping accumulations from any
precipitation during much of the day minimal. Aloft, colder air
races in behind the front, causing atmospheric instability to
rapidly increase. That means the precipitation will become much
more showery and scattered to numerous in coverage. Thus, coastal
communities along the north Gulf Coast and Kenai Peninsula can
expect the precipitation to change back over to all snow Wednesday
afternoon and overnight as freezing levels come back down. That
being said, when it is precipitating, the showers will very
possibly feature occasionally heavy snow as the falling snow
locally lowers snow levels.

This same pattern will continue into Wednesday night, with
additional waves of moisture moving into the north Gulf Coast,
particularly the Kenai Peninsula, which will be closer to the
center of the low over Southwest Alaska. Thus, in the Seward area
in particular, snow showers will continue overnight, possibly
becoming heavy at times with a "shovellable" accumulation
expected by Thursday morning. Fortunately, due to the convective
nature of the snowfall, any periods of heavy snow should only be
brief as the parent snow shower passes by.

The parent low causing all of the rain and snow across
Southcentral through Wednesday night will begin quickly moving
southeast into the North Pacific by Thursday, resulting in
improving conditions area-wide during the day.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Low pressure across the eastern Bering slowly meanders west
tonight then dives south across the Eastern Aleutians Wednesday
morning. Strong gusty easterly flow ahead of this low becomes
northerly on Wednesday as it moves into the North Pacific. Light
precipitation continues to develop across the region through late
Thursday as the Bering storm system sends short waves along its
periphery on its slow southerly track across the North Pacific.
Only minor accumulations are expected with only weak support at
the surface.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Strong winds are expected to increase overnight across the Bering
with the Eastern Aleutians in the path for the gustiest flow
through Wednesday morning. The storm system spinning across the
southern Bering sends a strong short wave south across the central
waters this evening. At the same time, a ridge across the
Northern Pacific advances toward the western Bering. The pressure
gradient forcing between these two systems along with cold air
advection on the backside of the low supports stronger flow which
tapers down briefly as transient short wave ridging crosses the
western Aleutians Wednesday night. A high wind warning was issued
with the afternoon package for the Eastern Aleutians for tonight
as the powerful storm system dives south. The next front follows
quickly into the Western Chain Thursday morning. This storm system
pushes the frontal boundary across to the central Bering by
Thursday afternoon.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast beginning with Thursday night starts with a
low over the western Bering that will push its front over
southwest Alaska on Friday and then Southcentral Alaska Saturday.
After that the forecast gets messy. Models are having a difficult
time handling a North Pacific low that will track toward the
Alaska Peninsula Sunday.

Some models are bringing it east toward various locations around
the Gulf while the majority bring it into the eastern Bering. The
timing of this low has also been inconsistent as well as its
strength. Some models have it dropping into the 950 mb range just
before it reaches the Alaska Peninsula while others are higher.
Uncertainty with this system is high.

By late Monday into Tuesday there are large differences with the
operational model runs leading toward an ensemble mean approach
for much of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 185.
MARINE...Storm Warning 170 172>175.
 Gale Warning 119 120 130 131 139 150 155 160 165 171 176>178 180.

&&

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