Small Craft Advisory 770, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
453 PM AKDT Tue Oct 15 2019

.ANALYSIS AND UPPER LEVELS...

There is an upper level low over the far western Bering Sea, and
another low center developing just north of the western Aleutians.
There is an upper low over the southern Gulf. High pressure is
over central and northern Alaska. There is a front along the
southwest coast of Alaska, stretching back to a surface low over
the northwest Bering. This surface low is the remnants of
"Hagibis", which is still fairly deep at 960 mb but is slowly
filling and moving west. There is a rather strong surface low
(around 966 mb) vertically stacked under the low north of the
western Aleutians, and this low is still deepening and moving
rapidly eastward. Winds on the south side of this low are
approaching hurricane force. Some moisture associated with the
Gulf low is along the north Gulf coast, moving very slowly
westward.

&&

.MODEL DISCUSSION...

The numerical models are in fairly good agreement on the synoptic
pattern through Thursday afternoon. The main differences involve
the handling of a low over the eastern Bering Wednesday morning as
it splits into several centers and rapidly moves eastward across
the Alaska Peninsula into the Gulf Thursday and Thursday night.
This doesn't impact the wind fields that much, and the forecast
confidence is near normal.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through
tonight. Conditions could drop to MVFR Wednesday morning with
periods of flurries or very light snow.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tuesday
night through Friday)...

High pressure continues to exist over much of mainland AK while
disorganized low pressure spins in the Gulf of Alaska. An area of
cloud cover has worked its way over the Copper River Basin with
enough instability to provide some light snow for Gulkana.
Otherwise, the only thing of note is the weak low pressure system
working its way over the eastern Kenai Peninsula and Prince
William Sound. Expect this will produce rain along the coast and
snow over higher elevations. There is still a chance some of this
moisture will spill over the Chugach and into Anchorage tomorrow
morning. Right now, it looks like flurries or a period of very
light snow with only a dusting at most expected, as it will remain
cold enough to snow at valley level. After the exit of this
system, its pretty quiet for Southcentral AK through the rest of
the week.

Disorganized and weakening low pressure will produce some gale
force winds in the western Gulf of Alaska tonight but will taper
off tomorrow. The next front enters the western Gulf of Alaska
tomorrow night from the AKPEN with gale force winds. This front
stays south of Kodiak Island though so much of the weather will
stay well offshore in the southern Gulf of Alaska.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Friday evening)

A front will approach the coast of the mainland before stalling
later tonight. Widespread precipitation will develop along and
ahead of this feature, as an over-running setup develops. As of
right now, the best chance to see any rain or snow looks to be
southwest of a line from Iliamna to Mekoryuk. Further north and
east, look for abundant cloud cover to dominate your weather,
which should help mitigate any concerns for fog development. As
the system moves away from the region on Thursday and Friday,
cloud cover will continue to be prevalent.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Friday evening)

This afternoon's forecast remains on track. The remnants of
Hagabis continue to shift away from the region towards Siberia,
with our next system of interest continuing to strengthen.
Currently, the storm is northeast of Shemya, where instability
continues to build as cold air advects into the region on the
backside of the storm. This is very evident in this afternoon's
satellite imagery, with the weather observer at Erickson Station
in Shemya reporting CB (cumulus nimbus) clouds in their most
recent observation. this is the type of cloud most commonly
associated with any thunderstorm development. This cold air will
continue to spread eastward, with the best chance of seeing any
thunderstorms being located along the southern flank of the
system.

Of bigger concern will be the winds and waves. Today's model runs
continue to be in excellent agreement, as they portray a compact
storm trekking across the southern Bering Sea. Widespread
sustained storm force winds are expected, with some hurricane
force winds along the immediate southwestern and southern flank
of the low. Similar to yesterday's storm, a very long fetch length
will be conducive to significant wave heights, which look to top
out near 40 feet.

Conditions look to improve for Thursday and Friday though, as the
storm exits the region to the east/southeast while weakening, with
winds dropping below headline criteria and wave heights
decreasing as well.

&&

.MARINE (Days 3 through 5)...

Westerly gales along the coastal waters near the eastern
Aleutians and Alaska Peninsula will diminish gradually Thursday
through Thursday night with winds across the Bering and Aleutians
expected for the most part to remain below gales through Friday
and Saturday. There are, however, a couple of smaller troughs
moving through the broader troughing on Friday and Saturday that
do bear watching with the potential to produce brief periods of
localized stronger winds in the central and eastern Aleutians as
they spin up.

For the Gulf, westerly winds near gale force are possible at times
in the southern Gulf Thursday and Friday as small troughs rotate
through the broader westerly flow but otherwise winds are expected
to remain below gales through Saturday.

&&

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

By Friday, upper level troughing extending from a broad vertically
stacked low over the Bering will stretch east across much of
mainland Alaska, extending to a second vertically stacked low in
the central Gulf. A cut off upper level low over the Bristol Bay
area embedded in the broader troughing will lift to the northwest
over the Kuskowim Delta Friday night and Saturday while the low in
the Gulf moves slowly north.

A developing North Pacific frontal system located well to the
south of the Alaska Peninsula Friday night will track to the
northeast across the southern Gulf Saturday to reach the Southeast
Alaska Panhandle Sunday. In response, the upper low in the
central Bering will rotate southeastward over the eastern
Aleutians on Sunday before turning into a open trough and
continuing to the east just south of the Alaska Peninsula. Also
caught up in the same rotation, the upper low over the Kuskowim
Delta will swing to the northwest, moving offshore Sunday and then
turning south to swing by St Matthew Island and the Pribilofs
Sunday night and Monday before becoming quasi-stationary over the
eastern Bering offshore of the Kuskokwim Delta through midweek.

The next potentially rather strong low will track in from the
west, crossing the North Pacific well to the south of the western
Aleutians Sunday night and then beginning to turn to the
northeast Monday and Monday night while it is south of the
central and eastern Aleutians. While all the models are in good
agreement with the potential for this to be a powerful storm and
in fair agreement regarding the speed of its eastward progress,
there are very large differences regarding its north to south
position. The 12Z GFS has the furthest south track, keeping the
low well clear of the Aleutians as it rapidly deepens. Both the
ECMWF and the Canadian models curve the low to the northeast more
rapidly, bringing it just south of the western Alaska Peninsula
by Tuesday morning while the GFS solution's low remains roughly
350 miles further south. The ECMWF and Canadian models both then
bring low into Bristol Bay while the GFS tracks the low into the
Gulf for Wednesday and Thursday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning: 195.
MARINE...Hurricane Force Wind Warning: 413, 414.
Storm Warning: 155, 165, 170-173, 175, 176, 179, 412.
Gale Warning: 131, 132, 150, 160, 174, 177, 178, 180, 352, 411.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...JR