Small Craft Advisory 764, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 AM AKDT Mon Oct 14 2019

.ANALYSIS AND UPPER LEVELS...

The extratropical low that was once Typhoon Hagibis is finally
over Alaska waters, after multiple days of advertising. The low
crossed the western Aleutians yesterday evening and is rapidly
intensifying over the western Bering. The storm has a classic
comma shape, with the leading front having recently crossed
through Adak and Atka.

A separate smaller, but also quite visually impressive low is over
the North Pacific well south of Kodiak Island and the Alaska
Peninsula. It too has a classic comma shape. Its northern edge is
causing windy conditions with some rainy spells across Kodiak
Island and the Alaska Peninsula. Some high cloud cover associated
with the low is moving east across Bristol Bay and the southern
Alaska Peninsula.

Weatherwise most of the rest of the area is having another night
with clear skies and light winds, allowing temperatures to plummet
in those areas. 10s and 20s are common away from the coastal
areas. Meanwhile near the coast and under the high clouds,
temperatures are generally in the 30s. A broad area of stratus
clouds is trapped in the western and southern part of the Copper
River Basin.

&&

.MODEL DISCUSSION...

The models are in generally good agreement through Tuesday night.
The models are finally very well clustered regarding the future
track and strength of extratropical storm Hagibis, with the storm
expected to turn north and then northwest, with the center reaching
Eastern Russia by Tuesday evening. With the tight clustering in
the track, forecast confidence of winds and precipitation with the
storm is high.

A secondary impulse of energy zips across the Bering south of
Hagibis Tuesday evening. While the low will be much smaller, it
will also be capable of producing locally strong storm force
winds, along with 40 foot waves south of the low being aided by
dynamic fetch. There is a fair bit of disagreement between on
where the final track of this low is going to be. This will be
exceptionally important for the Pribilofs especially, because if
the center tracks north of the islands, then the full brunt of the
strong winds and high waves hits the islands. For now, the model
consensus takes the center right through the islands, which will
keep the worst conditions to the south. The compact nature of the
low and the high sensitivity of the track will require interests
in the Pribilofs to keep a close eye on future forecast updates
for this low for the Tuesday night time frame.

Across the Gulf, the models are in agreement that the low moving
through the Southern Gulf will create multiple meso-lows along its
front. The models are having a very hard time with these meso-
lows, and each could produce locally gusty winds and briefly heavy
rain, so the timing and intensity of precipitation around the Gulf
and Kodiak Island is also low confidence.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

High pressure is still over Southcentral Alaska this morning, but
it is being slowly being eroded by the large low moving toward
the Gulf of Alaska from the south. This is bringing gales to the
Gulf waters and Shelikof Strait. The front associated with this
low will bring some rain to the North Gulf Coast Tuesday
afternoon as a low develops along it in the eastern Gulf and then
moves along the front to the northwest. This is the area of the
forecast with greatest uncertainty. While models are all
recognizing this feature will develop, the strength and speed of
it vary a bit. This is not a surprise as compact lows of this type
generally spin up fast and are difficult for the models to handle
until they have really begun to develop. The other item to note
is that just because this will be a rather small and compact low,
that does not necessarily mean it will be weak. It has the
potential to spread gales as far north as Hinchinbrook entrance to
Prince William Sound should it develop properly. That looks
unlikely at this point so currently forecast does not show that
solution. However, the development and movement of this low
overnight should be watched.

&&

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

At the onset of the forecast period expect high pressure from
Bethel to the Brooks Range. 24 hrs of dry weather are anticipated
across much of the region. Then the Bering low and the associated
frontal boundary will stall along the coast, bringing rain to
Bristol Bay and the coastline on Tuesday. However, as time elapses
the upper level high will push north of the Brooks Range which
will allow energy from the Bering to work its way into Southwest
Alaska. A second impulse from the Bering low will be ejected into
Southwest AK midweek.

&&

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

The main event for the Bering and Aleutians will be the extra-
tropical remnants of Hagibis. The surface low this morning was
northwest of Adak at about 960mb. The precipitation shield spans
from the northern Bering into the north Pacific. There is high
confidence in storm force to hurricane force winds and rough seas.
The models indicate that the low will continue to deepen today
and track northeastward. Tuesday, the low will move north and then
retrograde...all the while beginning to fill. There are multiple
lobes of vorticity associated with the main upper low and
secondary impulses will impact the Aleutian chain and southern
Bering.

&&

.MARINE (Days 3 through 5)...

Bering Sea/Aleutians: As the extratropical remnants of Typhoon
Hagibis exit to the northwest Bering late Tuesday, seas will
slowly subside and winds will diminish over the central and
eastern Bering. Models have come into better agreement that a
potent secondary low that will develop over the western Aleutians
midday Tuesday. This feature will quickly track eastward across
the Bering north of the Aleutians late Tuesday into Wednesday,
likely reaching the vicinity of the Pribilof Islands Wednesday
morning. Expect an area of storm force winds just south and west
of the low's center, with a broader swath of gales in its wake.
Considering the speed and compact nature of this low, seas will be
wind wave dominant, with combined seas in the 25 to 35 foot
range. While some models have indicated brief hurricane force
gusts in that core of strongest winds, model discrepancies persist
with track and strength of the low. Thus, we didn't have the
confidence to include such strong gusts in the forecast, but it
certainly bears watching. As the cold air mass behind the
associated front moves across the warmer SSTs of the Bering,
instability will increase thus isolated thunderstorms are expected
over the southern Bering Sea Wednesday. Conditions will become
more benign Thursday, though areas of gales still won't be out of
the question. Exactly where the highest winds develop will depend
whether the remnants of Hagibis set up residence as a garden
variety low in the central Bering, or if that system weakens in
favor of the secondary low in the eastern Bering. In any case,
broad low pressure is likely in the Bering through the end of the
week.

Gulf of Alaska: Longwave troughing will continue over the
Gulf/north Pacific. An upper low will cross the AKPEN then
redevelop a surface low in the Gulf late Wednesday into
Thursday, and dive southeastward across the Gulf/North Pacific.
Exactly where that low tracks is uncertain, and depends on the
synoptic setup upstream. Given the broad range of solutions for
the preceding time frame in the Bering, will have to wait for a
few more cycles to zero in on the Gulf low placement. In any case,
expect increased winds and precipitation along this low... but
the primary impacts may be in the North Pacific.

&&

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

The excitement in the long term continues to be focused in the
Bering Sea and Aleutians. As post-tropical cyclone Hagibis exits
to the northern Bering Tuesday, focus turns to what happens in
its wake. Models have come into better agreement that a potent
low will develop in the baroclinic zone under a 160 to 170 kt jet
midday Tuesday. It will quickly traverse the southern Bering (just
north of the Aleutians) from west to east and likely bring a
brief return of winds in the 40 to 50 kt range over the Aleutians
and Pribilof Islands late Tuesday into Wednesday, with another
round of precipitation. Isolated thunderstorms are expected behind
the associated front Wednesday as modified Arctic air flows over
relatively warm water.

Meanwhile, weather continues to look relatively quiet over the
Southwest Mainland. Surface high pressure over the interior will
persist, thus offshore winds and mostly dry conditions should
prevail. As the secondary low approaches the coast, precipitation
will be possible in Bristol Bay...mainly along the coast. While
models agree that general low pressure will settle over the Bering
by the weekend, there's indication of roughly two possible
scenarios. The remnants of Hagibis could slightly restrengthen in
northwest Bering, given the healthy source of cold continental
air from eastern Siberia. Or, that secondary low could become the
dominant feature in the Bering and stall near the west coast. The
latter projection would result in a trend toward wetter weather by
the weekend over the Southwest mainland. On the other hand, should
the former situation come to fruition, the mainland will probably
continue offshore flow and cold, dry weather while cold air
advection around the backside of a Bering low triggers widespread
showers, or at least enhanced cloud cover.

Over Southcentral, surface high pressure over northern Alaska will
persist, maintaining generally dry weather and offshore winds.
Models place a series of lows in the Gulf or North Pacific later
in the week. If these lows develop far enough north, an associated
front could bring precipitation to the North Gulf Coast, or even
inland...but uncertainty remains high with these features due to
the chaotic range of nature of the upstream setup.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Hurricane Force Wind Warning: 411, 412, 413.
Storm Warning: 173, 175-179, 181, 185.
Gale Warning: 120, 131, 132, 138, 150, 155, 165, 170, 171, 172,
			  174, 180, 351, 352, 414.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
MARINE/LONG TERM...MM