Small Craft Advisory 653, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
526 AM AKDT Tue Sep 10 2019

.ANALYSIS AND UPPER LEVELS...

An upper level low over the Yukon Delta region south of Norton
Sound is helping increase instability leading to some shower
activity over the Kuskokwim Delta. The showers are largely
scattered in nature. Shower activity is also ongoing over the
Susitna Valley, with more isolated activity further south into the
Kenai Peninsula and Anchorage. This is because a weak upper level
disturbance is moving northeastward across the Alaska Range, and
the shower activity is at the leading edge of that disturbance.
The upper level flow remains out of the south and southwest ahead
of the low and associated trough over Southcentral. Thus, despite
there being somewhat less shower activity this morning than there
was yesterday morning, cloud cover and atmospheric moisture
content remain largely the same. Thus, there are very few areas of
Southern Alaska where skies are mostly clear. This pattern
continues to favor seasonably warm and wet conditions.
Temperatures across nearly all of Southern Alaska are in the mid
40s to mid 50s range once again.

Meanwhile out west, a developing low over the far western Bering,
off the coast of the Kamchatka Peninsula is the extratropical
remnants of former West Pacific typhoon Lingling. On its
southwestern periphery, the remnants of former West Pacific
typhoon Faxai are both on the move eastward towards the eastern
Bering and the west coast of Alaska, and will be the focus for by
far the most significant weather in the state for much of the
upcoming week. More on that below.

&&

.MODEL DISCUSSION...

The models remain in excellent agreement through the short-term
regarding the track and positioning of the Bering low, and the
movement of its associated occluded front into Southwest Alaska
Wednesday afternoon through Wednesday night. On Wednesday night,
the remnants of former West Pacific typhoon Faxai reach Southwest
Alaska and move into Bristol Bay along the leading front. The
interaction of this moisture with the low could very well lead to
future uncertainty, despite the good agreement right now. The
models traditionally have had a very difficult time resolving
extratropical transition and the absorption of those remnants
into bigger lows. Precipitation amounts with the rainfall are
likely to come up with time along the coast and into Southwest
Alaska due to the large amounts of moisture that former tropical
cyclones infuse into their surrounding environments.

Across Southcentral, the chances of showers will remain the
largest point of uncertainty, since most of the disturbances
causing the shower activity will be relatively weak. By Thursday,
the aforementioned leading front, containing Faxai's remnants move
into Southcentral. How the rainfall interacts with the terrain
regarding precipitation amounts will be a highly impactful piece
of the forecast in the coming days, which remains uncertain in a
number of ways. These include the amount of downsloping/rain
shadowing to the west of the Chugach and Kenai Mountains, and how
much rain falls on the coastal side. Rainfall amounts in Seward
could possibly lead to flooding concerns as multiple inches of
rain over a short time are possible. Fortunately, the firehose of
moisture associated with the front and Faxai remnants is well-
agreed upon in the models to continue moving eastward across the
Gulf through Thursday night, somewhat limiting rainfall amounts
in any one location.

Behind the front, the models have mostly changed to suggest a
lee-side low develops in the northern Gulf Thursday night. The
timing of the development of the low, its intensity, and where it
moves will have large impacts as to how strong the gap winds
become, particularly the Turnagain Arm wind, and whether or not
they impact Anchorage. Thus, forecast uncertainty is increasing by
Thursday, and confidence, while generally high through Wednesday,
decreases quickly by Thursday and beyond.

&&

.AVIATION...
PANC...MVFR ceilings over Anchorage are expected to persist into
the morning. Isolated showers will become more scattered in
coverage this afternoon, but there is significant uncertainty as
to where the showers will develop and if they will impact the
airport, so specific mention of a period of time with showers were
left out for now.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Tue and
Wed)...Weather will remain relatively quiet for the next couple
of days before a strong front arrives late Wed into Thu. For
today, a broad trough over the west coast will continue to keep
the area in moist, unstable flow. This will keep the potential for
showers high for just about all areas west of the Richardson
Highway. There should be just enough breaks in the clouds to help
generate some weak instability. Thus, expect the showers to be
more widespread this afternoon during peak heating and especially
focused over the mountains. Showers will dissipate overnight into
Wed for inland areas. Wed should be a mostly dry and rather mild
Fall-like day for many locations. However, a small area of low
pressure will develop over the Northern Gulf and bring more light
rain to Cordova and the Eastern Prince William Sound.

By Wed night and Thu, a strong front will sweep quickly from west
to east towards Southcentral AK. This system will bring some of
the heaviest rains and strongest winds we have seen for several
months. Rain should start along the southern coast of the Kenai
Peninsula late Wed night and then make it into Prince William
Sound by mid Thu morning. Rainfall will be heavy for an extended
period of time along the coastal areas, to include Seward and
Whittier. Downsloping conditions will make for lower QPF amounts
inland.

One of the areas of uncertainty with this system is the
development of a triple point low near Kodiak Island. Models have
trended slightly stronger with this feature. If this were to be
the case, the southeast winds along the Anchorage Hillside would
be lower while the northeast winds in the Matanuska Valley would
be stronger. An SPS has been issued to address some of these
concerns.

&&

.FIRE WEATHER...The Copper River Basin will continue to be the
primary fire weather concern. Temperatures will remain in the 60s
while minimum RH values slowly climb higher each day into the
40-50% range. Gusty southeast winds will develop in the river
valley again Wed afternoon. For all other areas, showers will
persist today before conditions dry out on Wed. A strong front
will bring gusty southeast gap winds late Wed into Thu. While rain
will be likely over inland areas, amounts will generally be less
than a tenth of an inch through Thu night.

&&

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

Showers will continue across Southwest Alaska this morning through
the afternoon as an upper level low weakens along the Kuskokwim
Delta coast. There will be a brief dry period later this evening
through the early morning hours before the pattern shifts. A
strong front associated with the remnants of typhoons Lingling
(low in the Bering) and Faxai (merging from the south) will
approach the Southwest coast by Wednesday morning. As this front
continues to push inland, it will bring strong winds and
widespread rainfall across the area. Southwest will stay in an
active showery pattern through Friday once the weakening low
starts to move inland.

The largest concern with this active pattern will be the strong
southerly winds and amount of precipitation that will impact the
coastal regions. This could lead to high surf near Quinhagak and
the Kuskokwim Bay.

&&

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

A low near Kamchatka will continue to extend its front over the
western Bering today as a second front approaches the western
Aleutians. These two features will merge in the central Bering
Wednesday morning, as gale force winds advances towards the
Southwest coast. A second area of gales will persist across the
western and central Bering on the backside of this low, which
could possibly bring storm force wind gusts to the area. Widespread
rain and high seas are expected with this active pattern.

&&

.MARINE (Days 3 through 5/Wednesday through Friday)...
We are still expecting a large low pressure system to track
across the Bering Sea Wednesday through Friday. This system will
bring gale force winds with it and wave heights of 15-20 ft. Low
pressure in the Bering during this time period will be the result
typhoon remnants working their way through the Bering Sea. In the
Gulf of Alaska, small craft advisory winds are expected to be
present as low pressure pushes east from the Bering and crosses
the Alaska Peninsula. There will also be some small areas of gale
force winds in areas of channeling. Overall though, winds and
waves will be weaker in the gulf than the Bering.

&&

.LONG TERM FORECAST (Days 4 through 7/Thursday through Sunday)...
It's still looking wet and windy to start the long term. The
remnants of of two typhoons are expected to move into the Bering
Sea. Both systems, being tropical in origin, will bring plenty of
moisture and wind with them. This means heavy rain for southwest
Alaska on Thursday and then southcentral Alaska on Friday as these
systems progress east across our forecast area. After both
systems move through, showery precipitation looks like it will
persist with general low pressure present. The models agree
fairly well on the entrance of the typhoon remnants, but
substantially disagree outside of that area. So, we are confident
we will be impacted by a large storm with gale force winds and
heavy rain. However, there is low confidence in day to day weather
past the end of this week.

There does not appear to be much of a threat of storm surge for
our coastal area with the incoming weather. Current guidance
indicates there will be some high water, but nothing out that
wouldn't be typical with a Bering Sea storm this time of year.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 155 160 165 170 172 173 175-185 411-414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...BB

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
231 PM AKDT Tue Sep 10 2019

AKZ155-161-181-112245-
Kuskokwim Delta-Bristol Bay-Alaska Peninsula-
Including the cities of Bethel, Hooper Bay, Nunivak Island,
King Salmon, Dillingham, Naknek, Pilot Point, Cold Bay,
and Sand Point
231 PM AKDT Tue Sep 10 2019

...STRONG WINDS, HEAVY RAINS, AND COASTAL EROSION POSSIBLE IN
SOUTHWEST ALASKA WEDNESDAY INTO THURSDAY...

A strong Fall storm will move through the Bering the next couple
of days, impacting Southwest Alaska starting Wednesday morning
into Thursday. Expect gusty south to southeast winds to develop
over the marine areas along Bristol Bay and the Kuskokwim Delta.
The strongest winds will impact the Kuskokwim Delta Wednesday
morning with gusts to 60 mph. Winds will also become gusty across
Interior Bristol Bay and Lake Iliamna.

The strong onshore winds starting Wednesday and persisting into
Thursday will make minor coastal erosion possible along the
Kuskokwim Delta coast, especially at high tide.

The storm will also bring heavy rain to much of Southwest Alaska.
Rainfall amounts of 1 to 2 inches are possible across much of Bristol
Bay and the Kuskokwim Delta through Thursday.

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
300 PM AKDT Tue Sep 10 2019

AKZ101-111-121-125-145-112245-
Anchorage-Matanuska Valley-Western Kenai Peninsula-
Western Prince William Sound-Susitna Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, Chickaloon, Kenai, Soldotna, Homer,
Cooper Landing, Whittier, Seward, Girdwood, Moose Pass,
Talkeetna, Willow, and Cantwell
300 PM AKDT Tue Sep 10 2019

...STRONG WINDS AND HEAVY RAINS EXPECTED FOR SOUTHERN ALASKA
LATER THIS WEEK...

A strong Fall storm over the Bering Sea will drive a front into
Southcentral AK starting late Wednesday night and into Thursday.
This front will cause gusty southeast winds to develop over the
marine areas and into Turnagain Arm. Winds will also become gusty
across the Anchorage Hillside and the Matanuska Valley. Winds will
reach their peak on Thursday.

The system will also bring heavy rain to many coastal areas. Seward
and Whittier could see several inches of rain by the end of the
week. Inland areas will also see rain, but amounts should be
significantly less.

Many of the details of this system are still uncertain, so
individuals with interests in these areas are advised to closely
monitor the forecast at www.weather.gov/Anchorage.