Gale Warning 529, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 AM AKDT Sat Oct 12 2019

.ANALYSIS AND UPPER LEVELS...

Two weak, vertically stacked lows are impacting Southern Alaska
this morning. One is in the northern Gulf, and the other is in the
southern Bering north of Adak. The northern Gulf low is a shell of
its former self, but is a compact, wrapped up system with
convective clouds swirling south of the center, with the
stratiform cloud shield dissipating over Southcentral, and moving
east across the Copper River Basin and Prince William Sound. Thus,
skies are undergoing a clearing trend. As such, most of
Southcentral is in the 30s, with the only 40s in Anchorage and
along the Gulf Coast. Stronger, gustier winds are continuing in
Seward and Whittier as cold air drains through the valleys.

Except for some trapped moisture in the Alaska Range, all of
Southwest Alaska is under clear skies with temperatures in the
20s except in a few coastal locations. The second low over the
southern Bering has a much less defined signature in the otherwise
broad stratus field that persists across much of the Bering. The
northern edge of the cloud shield that is associated with a
deepening North Pacific low is approaching the southern Alaska
Peninsula. This low will be the primary weather maker for that
area and over to Kodiak the next few days.

&&

.MODEL DISCUSSION...

The models are in good agreement through Sunday night, allowing
forecast confidence to be high. There are some minor track
differences by Sunday night in what will be a rapidly deepening
low, the former Typhoon Hagibis by that point. Those differences
do not amount to much change in the forecast. It is beyond Sunday
night regarding where Hagibis goes that the models are still
struggling to resolve. All but the Canadian now take the center of
Hagibis looping north, and never getting east of 170 west
longitude. With the historically poor accuracy of the models to
handle extratropical lows that are former typhoons, it is typical
for there to be large changes in the track from run to run. As
such, more updates to the forecast are likely. With that said,
confidence remains very high that regardless of where the center
goes, very strong winds and dangerous seas are expected around the
low. More on Hagibis below.

Quiet weather is expected elsewhere with the main forecast concern
being persistent offshore gap winds along the north Gulf Coast.

&&

.AVIATION...
PANC...VFR conditions and northerly winds will persist. Gusts are
still possible, especially this morning, but the trend in the
speed will be downward through the day.

&&

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

The low near Middleton Island is starting to track eastward as
seen on radar. High pressure building into the area is bringing in
colder and drier air into the region.
Winds will generally diminish with the low moving away from the
area, however the pressure gradient will remain tight along the
coastal mountains so areas such as Thompson Pass will continue to
see strong winds.

With clearing skies over the next few days and winds diminishing,
expect the coldest temperatures so far this season across much of
the region the next few nights.

&&

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

Mostly sunny skies will persist over Southwest Alaska through
this weekend under high pressure. The clear skies will cause
overnight lows to fall to near 20 over inland areas. The
extratropical low associated with Typhoon Hagibis will approach by
Monday afternoon. Rain and gusty southerly winds are likely along
the immediate coastline in the evening, however inland areas will
see little to no impacts.

&&

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

A weakening low in the southern Bering will bring periods of rain
to the central and eastern Aleutians today. This feature will
merge with a stronger low approaching from the south on Sunday
which will cause rain to continue over the same area, tracking
eastward through Monday. Farther west, an extratropical low that
is currently Typhoon Hagibis will move into the western Bering
Sunday evening. It will intensify Monday as it moves into the
central Bering. Although the strongest winds will remain offshore
near the low center, very gusty northwesterly winds up to 75 mph
are possible over the western Aleutians, with gusts of 60 mph
reaching as far east as Unalaska by Monday evening. Periods of
heavy rain are likely as well.

&&

.MARINE (Days 3 through 5)...

Bering Sea/Aleutians:There is high confidence that the
extratropical remnants of Typhoon Hagibis will approach the
western Aleutians by Monday and rapidly deepen to around 955mb.
Sustained hurricane force winds and heavy rains are likely in the
western Aleutians late Sunday night into early Monday, then in the
central Aleutians/southern Bering by Monday evening. Combined
seas up to 50 feet are expected. While models have varied widely
run to run, the current expectation is that the low will take a
northward turn late Monday into Tuesday, and its center will be
roughly along 60N by Wednesday. Based on the expected track,
hurricane force winds are not expected in the Pribilof Islands,
but storm force winds are likely. Winds will generally diminish
and seas will subside as the low exits to the north, however
models continue to depict a series of potent lows skirting from
the western to central Aleutians in the Tuesday to Wednesday
timeframe. But, there is high uncertainty with location. So, while
hurricane force winds should be limited to the post-tropical
cyclone, gales, or even storm force winds are likely in parts of
the Aleutians/Bering even after that feature exits.

Gulf of Alaska: Easterly gales are expected in the Gulf Monday as
a North Pacific low deepens and pushes an elongated front north.
Conditions will improve Tuesday into Wednesday, though a secondary
low may develop and bring another round of small craft winds to
the central Gulf Wednesday.

&&

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

The main focus in the long term is Typhoon Hagibis, which will
undergo extratropical transition in the rough Sunday timeframe,
then bomb out over the western Aleutians/Bering early Monday.
Besides the marine hazards mentioned in the marine discussion,
expect strong winds over the western Aleutians with gusts in the
70 to 80 knot range. In addition, heavy rain is likely over the
Aleutians. While models have come into better agreement with the
track of the former typhoon, there is a lot of uncertainty with
what comes after. Models generally depict a secondary low (or
perhaps series of shortwaves or closed lows) that form under the
polar front of a 160 to 170 knot jet streak oriented roughly
along 50N from the Kamchatka Peninsula to the Aleutians. In any
case, unsettled weather will continue over the Aleutians and
Bering at least through midweek, likely through the end of the
week as broad cyclonic flow continues and several weak shortwaves
traverse the Bering from eastern Siberia.

Given the expected northward track of the former typhoon, weather
in the Southwest mainland will be relatively quiet in the long
term. Strong southeast winds along the west coast and heavy
precipitation at times is expected as the weakening front
associated with the typhoon reaches the mainland late Monday into
Tuesday. Otherwise, expect several weakening fronts to approach
the mainland as the week goes on, bringing bouts of precipitation.

Over Southcentral, generally dry weather is expected through
midweek. High pressure will settle over northern Alaska, and cold
conditions inland with offshore winds will dominate through
midweek. A series of fronts will move from the Gulf northward.
This will potentially bring precipitation to the North Gulf
Coast, but generally expect precipitation to stay over the marine
area. During the later part of the week, the ridge will retrograde
to the north and a wetter pattern is possible.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 132, 138, 150, 155, 178, 352.

&&

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