High Wind in Alaska, August 30, 2020

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
642 AM AKDT Sun Aug 30 2020

.ANALYSIS AND UPPER LEVELS...

A ~170 kt upper level jet extends across the North Pacific. Its
left exit region is strengthening a 500 mb low near Unalaska
leading to a deep surface low and very strong winds over the
central Bering/Aleutians. These are further enhanced by a nearly
100 kt 700 mb low-level jet (LLJ). This will track eastward into
Bristol Bay through today. An upper level trough over the Gulf is
leading to a surface front along the coast bringing moderate rain
and gusty southeasterly winds through terrain gaps. The overall
active pattern will continue over the next several days.

&&

.MODEL DISCUSSION...

Models are in generally good agreement regarding the well-
advertised deep low tracking across the eastern Bering into
Bristol Bay. The GFS and RDPS are currently the only models
indicating hurricane force winds south of Unalaska Island which
have been preferred. Forecast winds are significantly higher than
models in Dutch Harbor itself due to possible funneling of
westerly winds. However depending on the exact track of the low,
these may or may not verify. A few mile shift in either direction
could drastically affect peak gusts. All models maintain the
central pressure in the low-to-mid 960 mb range and bring it
onshore near Togiak. As was the case last night, the GEMs are
still slightly slower than the American models. Likewise, the wind
forecast is also higher than models with hurricane force gusts in
Bristol Bay. Again confidence is moderate, however if the low
maintains its central pressure all the way to landfall, these
would be most probable south of King Salmon. As the low tracks
inland, gusty southeasterly winds along Turnagain Arm are expected
to develop and peak this afternoon. As is usually the case, high-
res models are most aggressive with these. While it is somewhat
borderline, sporadic gusts exceeding 75 mph are possible given
that even the larger scale models show a tight pressure gradient.

&&

.AVIATION...

PANC...Gusty southeasterly winds will develop this afternoon,
peaking this evening (possibly between 35-40 kt gusts) before
diminishing slightly overnight into Monday. As winds are
unidirectional in the low levels, shear should not be an issue.
Showers will end this morning, before redeveloping this afternoon
and will continue through Monday morning. Ceiling will likely
switch between MVFR/IFR and visibilities may briefly be reduced as
heavier showers move over the airport.

&&

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

The first of two lows is barreling into the north Gulf coast just
south of Prince William Sound early this morning. Rainfall will be
mostly relegated to coastal areas with downsloping keeping the
areas on the lee side of the mountains mostly dry at least through
the morning.

The second, and much stronger, low is moving quickly towards
Bristol Bay and will send its front into the lower Cook Inlet
area this afternoon with it moving into the northern Susitna
Valley to Prince William Sound by late evening. This is the main
low and front that we have been talking about for the past number
of days. Strong and gusty winds will develop across the area and
especially through channeled terrain. Once again, there will be a
lot of downsloping on the lee of the mountains this afternoon as
the front moves across the area. There will probably still be some
rain that makes it through, in spite of the downsloping, due to
the amount of lift with this front. The main rain will move into
the area this evening behind the front as the low center shifts
winds to the southwest, and the low center is closer, bringing in
abundant moisture without the downsloping.

There was a High Wind Watch for the Anchorage Hillside and
Turnagain Arm that was canceled with this package. The reason is
that the winds are not expected to reach warning criteria due to
the first low mentioned in the discussion (the one currently near
Prince William Sound). Typically when there is a low as strong as
the one in southwest Alaska, it is pushing into a ridge over
Southcentral. This creates a strong pressure gradient that is the
root cause of the high winds. With the first low moving the ridge
out of the way already, the pressure gradient should not be strong
enough to produce the high wind warning level winds. That said,
there will still be a period of strong southeasterly winds not
only in Turnagain Arm and the Hillside, but winds gusting between
35 and 45 mph are also expected in town late this afternoon into
this evening as well. These southeast winds will likely be the
strongest winds in the Anchorage Bowl since last winter. By
midnight winds should shift to the south-southwest and remain
gusty at times through Monday. Precautions should still be made to
secure loose objects or ones that can be damaged in the wind.

&&

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

The strong low pressure system will be moving into southwest
Alaska today with increasing winds and plenty of rain. High wind
Warnings are in effect for southwest Alaska with the strongest
winds in Bristol Bay, along the coast mainly east of Dillingham
and from King Salmon and areas southward. Expect the strongest
winds to hit the area from midday through the afternoon and
evening hours. Much of Bristol Bay will see storm force winds
Sunday afternoon especially, however hurricane force gusts are
possible across eastern areas. Winds will ease up as we head into
Monday morning as the low pressure moves inland and weakens.

This storm also will bring a good amount of rain as it has
copious moisture from tropical regions with 1 to 1.5" of rain
possible across the Bristol Bay region and near King Salmon and
areas northward. Less rain is expected in the YK delta region with
up to a half inch in most areas. Rain will decrease Monday and
Tuesday as the system moves towards the Interior of Alaska.

Coastal flooding is also expected in Bristol Bay due to strong
southwesterly onshore flow creating the potential for water levels
to rise 2 to 4 feet above high astronomical tide. Areas with a
concave coastline (where the coastline curves into the land) are
the most prone to coastal flooding and erosion as the onshore flow
will help funnel the water in these areas. Areas with a convex
coastline (i.e. Cape Newenham where the land points outward) are
less prone to coastal flooding as the water is less likely to
funnel with the onshore flow in these regions.

&&

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

A strong low pressure system will move across the Aleutians today
and towards Bristol Bay by afternoon. Storm and hurricane force
winds will track just south of Dutch Harbor this morning. These
storm force winds will reach Cold Bay later this morning as the
low continues to move northeastward towards Bristol Bay. Winds
are expected to gust to hurricane force over the western Alaska
Peninsula on Sunday as well. Locally higher winds and gusts are
expected in the gaps due to the localized funneling of winds. A
High Wind Warning remains issued for the western Alaska Peninsula
from 8am to 8pm AKDT today. Winds will gradually ease from west
to east across the Aleutians Sunday evening through Monday
afternoon.

High seas from 20-30ft are expected south of the central Aleutians
(Pacific side) due to the storm and hurricane force southwesterly
winds combined with ample fetch for these seas to build. Seas will
be locally higher while wave periods will be locally shorter at
times as the west-southwesterly winds and west-southwesterly
primary swell direction will oppose the east-northeasterly
currents and locally stack the seas. The highest seas will be
south of Dutch Harbor (Pacific side of the Aleutians) due to this
area having hurricane force winds while Real Time Ocean Forecast
System (RTOFS) data shows the east-northeasterly currents being
strongest in that region. Due to those factors, the area south of
Dutch Harbor will have seas closer to 30ft at times Sunday
morning. Seas will gradually subside and lengthen in period from
west to east across the Bering/Aleutians Sunday evening through
Monday evening.

Ample rainfall is expected from this storm in the Aleutians. The
highest rainfall amounts are expected in the mountains due to
upsloping. Rainfall amounts will total between 0.75" and 1.00" in
those areas through Monday afternoon.

&&

.MARINE (Days 3 through 5/Tuesday through Thursday)...

For the forecast period, a fairly benign pattern is expected
across the Bering Sea and the Gulf of Alaska. The only item to
note is a possible low pressure center moving up towards the
eastern Gulf of Alaska Wednesday into Thursday morning that may
bring an increase in winds. However, latest model guidance today
is suggesting a farther east track and weaker storm. Therefore,
this system may not affect the western Gulf of Alaska but we will
continue to monitor.

&&

.LONG TERM FORECAST (Days 4 through 7/Wednesday through Saturday)...

Model guidance for the long term forecast period continues to
suggest upper level troughing across much of the state of Alaska.
As a result, a cool, wet, and cloudy pattern remains expected for
the foreseeable future. Towards the very end of the forecast
period on Saturday, almost all model guidance is showing an upper
level ridge over the North Pacific. Some guidance shows this
being a fairly strong ridge. However, the placement and intensity
of this ridge will be crucial as an upper level trough is
expected to move southward through the Bering and into the North
Pacific as this ridge builds northward. In between, the potential
exists for an atmospheric river type pattern to develop. Model
guidance shows multiple systems moving northward and then tracking
across the Gulf of Alaska on the northern periphery of this
ridge. Therefore, the long term forecast period will feature near
normal to slightly below normal temperatures and above average
rainfall for the southern mainland of Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Coastal Flood Advisory: 161.
         High Wind Warning: 161 181 185
MARINE...Hurricane Force Wind Warning: 172.
         Storm Warning: 155 160 165 170 171.
         Gale Warning: 119 120 125 130-132 138-141 150 174 180
                       351 352 411-414.
FIRE WEATHER...NONE.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
420 AM AKDT Sun Aug 30 2020

AKZ152-155-161-310100-
Lower Kuskokwim Valley-Kuskokwim Delta-Bristol Bay-
Including the cities of Aniak, Sleetmute, Bethel, Hooper Bay,
Nunivak Island, King Salmon, Dillingham, Naknek, and Pilot Point
420 AM AKDT Sun Aug 30 2020

...WET AND WINDY CONDITIONS EXPECTED AS THE FIRST BIG STORM OF
THE FALL SEASON MOVES INTO SOUTHWEST ALASKA SUNDAY INTO MONDAY...

A strong low moving in from the Bering on Sunday into Monday will
produce strong winds, periods of heavy rain, and potentially high
water levels along the Bristol Bay Coast.

Rainfall will move into Southwest Alaska Sunday afternoon
associated with this system, which will continue through Monday.
Areas across interior Bristol Bay and the Kuskokwim Valley can
expect to see over an inch of rain through this time period.
Strong southwesterly winds will move into Bristol Bay by Sunday
afternoon impacting coastal locations from Togiak to Port Heiden,
with the possibility for high water levels. Elevated water levels
are also expected along the Iliamna River late Sunday into
Monday.

Please stay tuned for the latest forecast throughout the weekend.

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
539 AM AKDT Sun Aug 30 2020

AKZ101-310100-
Anchorage-
Including the cities of Anchorage, Eagle River, Indian,
and Eklutna
539 AM AKDT Sun Aug 30 2020

Southeast winds gusting between 30 and 45 mph are expected to
move into the Anchorage Bowl by late this afternoon and continue
through the evening hours. These winds will likely be the
strongest ones in town since last winter so precautions should be
made to secure loose objects. Garden plants sensitive to strong
winds should also be protected if possible. By midnight, the
winds should shift to the south-southwest but remain gusty around
20 to 30 mph through Monday.

AKZ125-145-310100-
Western Prince William Sound-Susitna Valley-
Including the cities of Whittier, Seward, Girdwood, Moose Pass,
Talkeetna, Willow, and Cantwell
539 AM AKDT Sun Aug 30 2020

Additional rainfall will be moderate to heavy at times along the
Gulf Coast in the Seward area and over the western Susitna
Valley with an additional 1 to 2 inches of rain expected through
Sunday night in these areas.

Rivers and streams (especially the Yentna and Skwentna) will
experience elevated water levels beginning Sunday night and
continuing into Tuesday.

AKZ131-135-310100-
Northeast Prince William Sound-Southeast Prince William Sound-
Including the cities of Valdez, Thompson Pass, and Cordova
539 AM AKDT Sun Aug 30 2020

Periods of moderate to heavy rain will persist over eastern
Prince William Sound through Monday night. A total of 2.5 to 3.5
inches of rain is possible in and around Valdez and Cordova
through Monday night. Be alert for elevated water levels on area
rivers and streams into Wednesday.