Small Craft Advisory 698, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
552 AM AKDT Wed Sep 16 2020

.ANALYSIS AND UPPER LEVELS...
A clear transition to southwest flow has become established behind
a broad upper wave drifting north into the central interior. The
front that has been steadily advancing eastwards from the
Aleutians to more recently the Mat-Su valley and Kenai Peninsula
has all but sheared out now, but lingering moisture in the low
levels is allowing showers to persist for the greater Anchorage
area and along the east shores of the Cook Inlet. The Turnagain
wind had surprisingly mostly held off for the PANC terminal over
the past several hours with only sporadic gusts to 20 kts reported
at any point recently. Temperatures are unsurprisingly holding up
mostly in the 40s area-wide with ample clouds in place.

Two lows rotating about a common center are now located out west
over the Bering with one located over St Lawrence Island and the
other nearing the Central Aleutians. A wide variety of cloud
levels are evident in the nighttime microphysics satellite
product both near the two lows and over much of the remainder of
the forecast area. Low stratus is sprawled out over most of
Southcentral and a plume of cirrostratus is spreading into the
Gulf and AKPEN with the upper jet. Some areas in the Southwest are
beginning to report fog which is likely being aided by the
moderate rainfall in the past 24 hours with the frontal passage.

&&

.MODEL DISCUSSION...
Models have come into better alignment with the track of a low
expected to move from the North Pacific to eventually well inland
over the interior by Friday. Most solutions now depict the surface
low approaching Sand Point from the south before crossing or
redeveloping past the AKPEN on the Bering side as it moves north
between Wednesday and Thursday before moving inland Friday. This
should result in a lower rainfall threat for most of Southcentral
than earlier model scenarios that showed the low possibly tracking
as far east as Kodiak Island. Unfortunately, this low track will
have a more favorable setup for strong gap and lee hillside winds.
The exact timing, behavior an strength of these winds stands out
as the primary forecast challenge. Additionally, a surface trough
or low in the southern Interior may linger after the primary
forcing from the wave aloft passes, and this may result in
difficulty determining the end time for stubborn areas of rain
that could last well into Friday and beyond.

&&

.AVIATION...
PANC...Periods of showers will persist through Wednesday morning,
but visibility and ceilings should remain VFR, though a short
period of MVFR ceilings remains possible. Areas of rain showers
will return Wednesday night with mainly VFR ceilings and
visibilities persisting, but brief MVFR conditions are again
possible around this time. LLWS around 20 kts will begin
Wednesday night and last through Thursday morning. Strong and
gusty Turnagain winds are expected to potentially return at or
shortly after the end of the valid forecast period, or by Thursday
afternoon and evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Friday Night)...

A dissipating front over Southcentral this morning is producing a
few rain showers over the Mat-Su Valleys, but most of the area is
under cloudy conditions. For most of the day, most of the area
will remain dry with mostly cloudy skies and highs in the 50s.
Late this afternoon, an approaching North Pacific low with a
subtropical moisture tap will cause rain to break out all across
the western Gulf. The track of the low has shifted since yesterday
to the west. The low will track near Sand Point late this
afternoon, move onshore in Bristol Bay near Dillingham around
midday Thursday, then will transfer its energy to a new low near
Norton Sound Thursday evening. Due to the track having moved west,
there have been several changes to the forecast.

First is the heavy rain threat. Most of the heavy rain is expected
along the Gulf Coast, and in the western Susitna Valley. Both
areas will have upslope flow in the southeasterly winds, which
will enhance rainfall rates. Along the Gulf Coast, the heavy rain
will be associated with the leading front, which will be drawing
moisture from near Hawaii. The front will move northeast along the
Gulf Coast through Thursday evening. The heaviest rain begins late
tonight along the Gulf Coast, and will persist in Prince William
Sound through late Thursday night. Rainfall amounts of 2 to 3
inches in 12 hours are expected from Seward north and east along
Prince William Sound. See the Special Weather Statement for more
details. The heavier rain will persist for longer in the western
Susitna Valley, which will have the benefit of both the rain
associated with the leading front, and also the continued upslope
flow over the western Susitna Valley into Friday morning.

The other primary threat with this storm will be strong winds,
particularly along Turnagain Arm and higher elevations. A Special
Weather Statement has also been issued for the Anchorage area as
well, with focus on the strong winds. There are a couple factors
that suggest that prolonged warning level winds are not likely
with this storm. First, the low will be weakening through the
event, and on Thursday transferring its energy to a developing
low to the north. This should help keep the wind speeds down a
bit. Secondly, the low isn't expected to be particularly strong,
staying above 980 mb, so that will reduce the pressure gradient
force that would support stronger winds if it were stronger. With
that said, the forecast winds have been increased from previous
forecasts. This is primarily due to the westward track of the low
into Bristol Bay. This will allow the leading front, which will be
the primary forcing for the strong winds, to persist longer
aligned with the coast and helping direct the strong winds through
Turnagain Arm. Thus, frequent gusts of 65 to 70 mph are expected,
but gusts must stronger than that should be isolated, confined to
the windiest areas.

Once the primary low becomes the one near Norton Sound late
Thursday afternoon, the Turnagain Arm wind should be pushed
northward into Anchorage. Fortunately by this point the wind
strength should be greatly diminished from the peak winds Thursday
morning, but once again, gusty conditions are likely across the
Anchorage Bowl and the Matanuska Valley Thursday night, which
will persist into Friday morning. Gusty winds are also likely from
Thursday into Friday through the Copper River Basin. While mostly
cloudy conditions will persist over much of the area into Friday
night, overall rain and shower activity will diminish Friday
night. Finally another North Pacific low will enter the
southwestern Gulf early Saturday morning. Most of the models keep
the low well south of Kodiak and track it into the northeastern
Gulf, but it will have the potential to cause rain and low
ceilings around Kodiak City late Friday night into Saturday
morning.

&&

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

As one system slowly departs, another is quickly following on its
heels. A slow-moving low over the Chukchi Sea continues to
feature a pair of surface troughs, the first of which extends
southeast to near Iliamna. A rapidly approaching North Pacific
low is already spreading mid and high level cloud cover across the
region, and these cloud bases will continue to lower as moisture
advection and lift rapidly increases with the approaching system.
Showers will mark the more substantial leading edge of this low,
before rapidly changing over to a steady rain as it spreads from
southwest to northeast.

The models have basically locked in on a track of the low which
tacks it from King Cove to New Stuyahok, before lifting it in a
more northerly direction. Locations along and east of this low
will likely experience periods of moderate to occasionally heavy
rainfall as the center of the low approaches/passes by.

Then, as the associated surface front pushes eastward, dry
slotting will allow for the rainfall to end for a short period of
time, before wrap around showers cyclonically rotate around the
low as they overspread most of Southwest Alaska once again,
mainly on Thursday. Then on Friday, as the low pulls away, showers
will continue to affect the region, although they'll slowly
decrease in coverage with time from southwest to northeast.

&&

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

The main item of interest will be an approaching North Pacific
low that looks to travel from near King Cove north to
northeastern Bristol Bay (just east of Dillingham) for Wednesday
into Thursday morning. The models have generally locked in on this
track with just minor differences. As such, sustained winds were
increased for the southern coastal waters to gales east of
Shishaldin Volcano, with a few marginal storm force gusts
possible. This system will also have a tropical feed originating
from near Hawaii, so mariners and coastal residents can expect
moderate to at times heavy rainfall along and east of the track of
the storm, which could reduce visibilities to less than 3 miles
for mariners. Thought about adding patchy fog to account for this,
but, since the visibility reduction is more so due to heavy rain
than a pure fog by itself, held off on this. That said, the day
shift may need to reconsider this as the event unfolds and we get
better observations on this. As the storm moves ashore, winds will
drop to below gales.

For the remainder of the Bering Sea and the Aleutians, a broad
trough will occupy the region, with a surface trough extending
southward from the Chukchi Sea towards Adak. Along this axis, wind
shifts are expected (although wind speeds below small craft
conditions will have a minimal impact), before a developing low
dives southeast from the western Bering on Thursday, reaching the
central Aleutians (probably near Nikolski) by Friday morning. As
it does so, sustained winds will increase into the 20 to 25 knot
range, with rainfall becoming more widespread.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...

Lingering small craft winds and waves over Bristol Bay diminish
Fri night. A front south of the Aleutians moves into the Gulf of
Alaska for Sat. Confidence is good for small craft winds over and
south of the Aleutians through Sat night. A North Pacific low and
front move into the western Aleutians and Bering through Sun.
While track and development of the low are mixed, confidence is
good for gale force winds to wrap the low with local storm force
winds over the Aleutians through Sun.

A front stretches across the Gulf through Sun. Confidence is good
for small craft winds and waves with local gales around Kodiak
Island dissipating Sat. A second low on the front south of the
Alaska Peninsula moves across the Gulf through Sun. While track
and development vary, confidence is good for small craft winds and
waves to move across the Gulf through Sun.

&&

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

Saturday will see a cool upper level trough entrenched over
western Alaska from the Arctic coast southward to the Alaska
Peninsula. With a cut- off low at the tip of this trough tracking
into the southern Gulf of Alaska, it will keep a cloudy, cool and
showery regime over southern Mainland Alaska for most of the
weekend.

On Saturday night, a large low will move into the western Bering
Sea and the south of the Alaska Peninsula by Tuesday. While there
is uncertainty to the exact track of this low, the general
pattern shows the strongest winds and heaviest rain will remain
over coastal and marine areas in the Bering Sea and Gulf of Alaska
for the first half of next week.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 150 155 119 125 130 131 132.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...MK/EZ

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1142 AM AKDT Wed Sep 16 2020

AKZ101-171230-
Anchorage-
Including the cities of Anchorage, Eagle River, Indian,
and Eklutna
1142 AM AKDT Wed Sep 16 2020

...STRONG WINDS EXPECTED ALONG TURNAGAIN ARM AND HIGHER ELEVATIONS
LATE TONIGHT THROUGH THURSDAY...
...GUSTY WINDS ACROSS THE REMAINDER OF THE ANCHORAGE AREA AND
MATANUSKA VALLEY ON THURSDAY...

The third strong wind event of the Fall season will bring another
round of windy conditions to Anchorage and the Matanuska Valley
on Thursday.

Turnagain Arm and higher elevations of Anchorage (including the
Upper Hillside)...
East to southeast winds will increase overnight tonight, with
peak wind gusts to 60 to 70 mph late tonight through early
Thursday morning. Gusts of 45 to 60 mph will then persist all
the way through Thursday night before diminishing on Friday.

Remainder of the Anchorage area...
The Eagle River Valley and areas close to the mountains could
see southeast wind gusts to 30 mph Thursday morning. The
southeast winds will become more widespread and stronger across
all of Anchorage Thursday afternoon, with gusts as high as 50 mph
near the mountains (including East Anchorage and Eagle River)
and gusts to 35 mph as you head away from the mountains. Winds
will diminish Thursday evening, but remain gusty through Thursday
night.

Matanuska Valley...East winds will increase late tonight and
peak during the day Thursday, with gusts of 25 to 40 mph. The
strongest winds will occur along the Knik River Valley on into
Palmer. Winds will gradually diminish Thursday night.

Precautions should be taken today to secure loose objects. The
winds will also have the potential to bring down branches from
trees.

AKZ125-145-171230-
Western Prince William Sound-Susitna Valley-
Including the cities of Whittier, Seward, Girdwood, Moose Pass,
Talkeetna, Willow, and Cantwell
1142 AM AKDT Wed Sep 16 2020

...HEAVY RAIN WILL LEAD TO ELEVATED RIVER LEVELS IN THE SEWARD
AREA ON THURSDAY AND THE WESTERN SUSITNA VALLEY ON FRIDAY...

A strong front moving across Southcentral late tonight into
Thursday will produce periods of heavy rain in Seward and
surrounding mountains as well as across the western Susitna
Valley and bordering areas of the Alaska Range.

For Seward...
The heaviest rain will be overnight tonight through midday
Thursday, with a total of 2 to 2.5 inches of rain expected through
Thursday afternoon. Additional rainfall of around half an inch is
likely Thursday night. This will lead to elevated water levels in
the Resurrection River and other area rivers and streams.

Western Susitna Valley...
The heaviest rain will be late tonight through Thursday afternoon,
with around 1 inch of rain expected in the valley and 2 inches in
the headwaters of the Yentna and Skwentna Rivers. This will lead
to elevated river levels beginning Thursday night into Friday, as
these rivers are slower to respond to the rainfall.

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1226 PM AKDT Wed Sep 16 2020

AKZ101-171230-
Anchorage-
Including the cities of Anchorage, Eagle River, Indian,
and Eklutna
1226 PM AKDT Wed Sep 16 2020

...STRONG WINDS EXPECTED ALONG TURNAGAIN ARM AND HIGHER ELEVATIONS
LATE TONIGHT THROUGH THURSDAY...
...GUSTY WINDS ACROSS THE REMAINDER OF THE ANCHORAGE AREA AND
MATANUSKA VALLEY ON THURSDAY...

The third strong wind event of the Fall season will bring another
round of windy conditions to Anchorage and the Matanuska Valley
on Thursday.

Turnagain Arm and higher elevations of Anchorage (including the
Upper Hillside)...
East to southeast winds will increase overnight tonight, with
peak wind gusts to 60 to 70 mph late tonight through early
Thursday morning. Gusts of 45 to 60 mph will then persist all
the way through Thursday night before diminishing on Friday.

Remainder of the Anchorage area...
The Eagle River Valley and areas close to the mountains could
see southeast wind gusts to 30 mph Thursday morning. The
southeast winds will become more widespread and stronger across
all of Anchorage Thursday afternoon, with gusts as high as 45 mph
near the mountains (including East Anchorage and Eagle River) and
gusts to 30 mph as you head away from the mountains. Winds will
diminish Thursday evening, but remain gusty through Thursday
night.

Matanuska Valley...East winds will increase late tonight and
peak during the day Thursday, with gusts of 25 to 40 mph. The
strongest winds will occur along the Knik River Valley on into
Palmer. Winds will gradually diminish Thursday night.

Precautions should be taken today to secure loose objects. The
winds will also have the potential to bring down branches from
trees.

AKZ125-145-171230-
Western Prince William Sound-Susitna Valley-
Including the cities of Whittier, Seward, Girdwood, Moose Pass,
Talkeetna, Willow, and Cantwell
1226 PM AKDT Wed Sep 16 2020

...HEAVY RAIN WILL LEAD TO ELEVATED RIVER LEVELS IN THE SEWARD
AREA ON THURSDAY AND THE WESTERN SUSITNA VALLEY ON FRIDAY...

A strong front moving across Southcentral late tonight into
Thursday will produce periods of heavy rain in Seward and
surrounding mountains as well as across the western Susitna
Valley and bordering areas of the Alaska Range.

For Seward...
The heaviest rain will be overnight tonight through midday
Thursday, with a total of 2 to 2.5 inches of rain expected
through Thursday afternoon. Additional rainfall of around half
an inch is likely Thursday night. This will lead to elevated
water levels in the Resurrection River and other area rivers
and streams.

Western Susitna Valley...
The heaviest rain will be late tonight through Thursday afternoon,
with around 1 inch of rain expected in the valley and 2 inches
in the headwaters of the Yentna and Skwentna Rivers. This will
lead to elevated river levels beginning Thursday night into
Friday, as these rivers are slower to respond to the rainfall.