Small Craft Advisory 762, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 PM AKDT Sun Oct 13 2019

.ANALYSIS AND UPPER LEVELS...

The 2 major systems across the area are a developing low south of
Kodiak Island and the remnants of former Typhoon Hagibis. The
southern mainland of AK is under the influence of transient high
pressure making for mostly dry and sunny conditions. The first
system will slide along the southern half of the Gulf of AK and is
already starting to increase winds and seas, especially in
Shelikof Strait. A strong upper level shortwave and a 70 kt 300 mb
(30,000') jet will cause this system to deepen quickly tonight.
However, the upper level jet will also keep most of the impacts
associated with it south of the mainland.

The remnants of Hagibis are currently undergoing extratropical
transition and becoming baroclinic. They are just starting to
move into the strong westerlies. A 125 kt upper level jet is just
pushing over the Western Aleutians. As the remains of this system
start to interact with this impressive upper level support and
cold air from Siberia, it will deepen rapidly over the Central
Bering.

&&

.MODEL DISCUSSION...

Models are struggling with several details of the forecast in the
next 60 hours. However, the general synoptic idea remains
consistent enough that forecast confidence remains moderate to
high. One area to note is over the Gulf of Alaska by Tue morning.
As energy swirls through the trough over the Gulf, the GFS wants
to develop a strong surface low over the Northern Gulf of AK. The
NAM is starting to latch onto this idea, while the other primary
forecast models are not as aggressive and hold the energy further
south. For this forecast package, we have elected to make minimal
changes and lean away from the American solutions at this time.

Hagibis is a recurving ex-typhoon undergoing extratropical
transition. The models notoriously have a hard time resolving
exactly what this looks like. In general, the models all continue
to paint a very consistent story. The exception to this is the NAM
which has become a clear outlier with a deeper and further north
track. This forecast package will be based mostly on the GFS as it
continues to be the most consistent.

The final piece for the models is a "new" secondary storm
developing in the wake of Hagibis late Tue. The EC has led the
way with this energy and now the GFS is trending towards a
stronger solution as well. We have increased winds and waves as a
result and we will monitor this area closely for further updates.

&&

.AVIATION...

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

&&

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

Surface high pressure continues to dominate over the state. A
shortwave trough tracking over the Copper Basin into the Yukon is
enhancing that high pressure and mostly clear skies across the
region, and this will also play a role in enhancing fog and
stratus tonight and Monday morning across the Copper Basin. Beyond
this, a gale force front moving into the southern Gulf of Alaska
will lead to increasing rain across Kodiak Island, and will
continue to support just enough down Cook Inlet flow to keep fog
threats at bay across that region. The front over the Gulf of
Alaska will continue to lift north Tuesday, bringing increasing
chances for rain (mixed snow over the Copper Basin) for locations
from Prince William Sound eastward. Rain and snow will make it
into the Chugach mountains around Anchorage by Tue evening.

&&

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

High pressure will continue to dominate southwest Alaska keeping
the area clear of precipitation until Monday night when a front
moves in over the coast from the Bering. The front will stall
along the coast, keeping rain isolated to the coastline. Under the
effects of the high pressure, winds will stay out of the east
through Monday. Monday night winds near the coast will increase
ahead of the approaching front and begin to switch to the west
Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

The main impact for the Bering and Aleutians will be from the
remnants of Typhoon Hagibis as it re-intensifies tonight as it
moves into the Bering. A shortwave from the west will phase with
the remnants of Hagibis when it passes the western Aleutians and
help it tap into the cold air to the north. This will intensify
the low and drop the surface pressure from around 980 mb this
afternoon to near 950 mb Monday afternoon in the central Bering.

This setup will bring strong winds and high seas to coastal areas
around the Aleutians and in the Bering. The western and central
Bering will see the highest winds and seas with hurricane force
winds around 65 kt with seas as high as 50 ft Monday and Monday
night. Tuesday the low will track to the north towards Saint
Matthew Island and winds will start to diminish.

&&

.MARINE (Days 3 through 5)...

Bering Sea/Aleutians: As the extratropical remnants of Typhoon
Hagibis exit to the northeast 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 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 20 to 30 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: 173-179, 185, 414.
Gale Warning: 119, 120, 131, 132, 138, 150, 155, 165, 170, 171,
     	      172, 180, 181, 351, 352.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
MARINE/LONG TERM...MM