Small Craft Advisory 656, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
458 AM AKDT Wed Sep 11 2019

.ANALYSIS AND UPPER LEVELS...

Two large vertically stacked lows are impacting portions of
Southern Alaska this morning. The one in the far southern Gulf
will be moving towards the Panhandle, with little in the way of
impacts for Southcentral. The other low has been very well
advertised for the past several days, and is over the far western
Bering. This Bering low will be the primary weather maker for all
of Southern Alaska for the next several days. To the east of the
low center, abundant tropical moisture is streaming northeastward
across the central and eastern Aleutians, extending northward
across the eastern Bering. The associated high cloud cover ahead
of the front has already enveloped much of Southwest Alaska. So
far this morning, the strongest winds with the front have been in
Adak, where gusts reached 45 kt (50 mph) yesterday evening. Gusts
to that level will become much more common over the eastern Bering
and the Southwest Alaska coast today.

Across Southcentral, the area is largely in the calm before the
storm. A weakening upper level wave moving eastward across the
Cook Inlet region continues to produce scattered shower
activity across Prince William Sound and extending north over the
Talkeetna Mountains. Clouds remain in place east of this wave over
the Copper River Basin, meanwhile where skies are clearing over
Cook Inlet behind this wave, low stratus and fog are already
forming in some areas.

&&

.MODEL DISCUSSION...

The models remain in good agreement for the short term period,
with decent agreement into Friday, but still enough difference
that significant forecast changes can result. Thus, forecast
confidence rapidly decreases by Friday. This is especially the
case with winds across Southcentral, particularly the timing of
when the Turnagain Arm, Knik River Valley, and other gap winds
come down when the front causing the winds dissipates.
Precipitation amounts also remain a point of uncertainty, as there
is some discrepancy in places like Seward as to whether the town
itself gets 2 inches or up to 4 inches of rain, that difference
having a large say as to potential flooding concerns. The timing
of the downsloping across Anchorage, how strong the Hillside winds
are, and when those winds dissipate and allow for more substantive
rainfall are also forecast details still being refined this
morning.

The center of the low has come into much better agreement
compared to previous days for Southwest Alaska. As such, there is
better confidence across Southwest Alaska and the Bering as to
the timing and strength of the winds. There remains some question
as to rainfall amounts over portions of Southwest Alaska, as that
will depend on how the infusion of the remnants of former typhoon
Faxai become incorporated into the leading front. There is
increasing confidence that areas around Dillingham could pick up
closer to 2 inches of rain for the whole event. This is due to the
slowing movement of the low center, which will prolong the period
of rainfall, and the fact the models do poorly with rainfall
totals when the origins of the moisture producing the rain are
tropical.

&&

.AVIATION...
PANC...VFR conditions will persist. This evening, the Turnagain
Arm wind will increase south of the airport, at times associated
gusts will impact the south side of town and into the airport.
Chances of showers will increase after midnight as a front moves
into the coast. By the predawn hours Thursday morning, the rain
shadowing effect should result in reduced chances of rain, though
the wind threat from the Turnagain Arm wind will persist until
midday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Thursday night)...
The upper level short-wave trough over Cook Inlet this morning
will lift northeastward across Southcentral today. Based on trends
in water vapor satellite imagery, moisture will be pulled
northward from the closed low over the southern Gulf. The
interaction of the trough and moisture will produce showers over
eastern Prince William Sound (Cordova/Valdez) on up into the
Copper River Basin. Showers elsewhere will be isolated to
scattered in nature.

As the short-wave exits to Interior Alaska late this afternoon,
a leading upper wave out ahead of the Bering low will enter the
western Gulf and Cook Inlet. While low level southeast flow will
be strengthening ahead of the approaching surface front, upper
level flow with this wave will be out of the south to southwest.
Expect rain to develop out ahead of the wave, with upslope flow
enhancing rain across the eastern Kenai Peninsula and northern to
western Susitna Valley. The western Kenai, Anchorage and the
Matanuska Valley will also have a shot of rain, though increasing
downslope flow should ensure anything that does fall is light and
brief in nature.

All eyes will then shift to what is an impressive system out over
the Bering Sea. A strong upper level short-wave on the leading
edge of the trough will enter the western Gulf overnight tonight,
along with a strong frontal system. Strong dynamics combined with
anomalously high amounts of moisture will lead to heavy rain from
Kodiak to Prince William Sound to Cook Inlet and the western
Susitna Valley as this front progresses north and east Thursday
and Thursday night. At this point in time do not expect any river
flooding, though ponding of water on roadways and more localized
street flooding are possible. Strong and gusty southeast winds
will develop out ahead of the front, and will be strongest
through terrain gaps of the Kenai and Chugach Mountains. This will
lead to some downsloping, though as a general rule, strong
dynamics and tropical moisture often overcome the downslope flow
and produce some rain in the lee of the mountains.

The big complicating factor with the forecast is the track of a
triple point low which develops in the vicinity of Shelikof Strait
on Thursday. The upper level wave will become negatively tilted,
with the northern portion of the wave rotating back toward the
Bering low while the southern portion continues north and east
across the Gulf. This will cause the forward motion of the low
and frontal system to slow. There is uncertainty in where this
slowdown will occur. This will affect the strength of the
aforementioned southeast winds, including for Turnagain Arm and
the Anchorage Hillside, with weaker winds if the front stalls to
the south and stronger winds if it moves onshore. For now have
taken a middle ground with this forecast in between the weakest
and strongest solutions.

&&

.FIRE WEATHER...
The focus for fire weather will be on the strong front on
Thursday. This will bring gusty east to southeast winds to the
Interior Kenai Peninsula as well as the Copper River Basin. The
Kenai Peninsula is more likely to see periods of rain while the
Copper River basin remains mostly dry.

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

Southwest Alaska is in store for a wet pattern as a strong front
associated with a fall storm moves inland today. Heavy rains and
strong south-southeast winds will accompany this system as it
moves across the region through Thursday. Heaviest rains will
occur this afternoon through evening, which could bring around 2
inches of rainfall to areas of the Kuskokwim Delta and Bristol
Bay, including Cape Newenham to Dillingham. Minor coastal erosion
is possible with this storm along the Kuskokwim Bay, but the main
focus will be on the winds that could see gusts up to 60 mph
along these coastal areas.  By Thursday evening, the front will
have weakened and shifted eastward, leaving a cooler and showery
pattern stretched across the Southwest through the end of the
week.

&&

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

The strengthening gale force low will continue to progress
eastward. As it does, the front will bring heavy rains and
possible storm force gusts as it moves into the eastern Bering
towards the Southwest coast. On the backside of this low, gale
force winds will continue as cold air advection gets wrapped in,
keeping these strong winds through early Friday.

There will be a brief period of high pressure building in behind
this low before a new front enters the western Bering by Friday
afternoon. While this low isn't as strong as the current one,
small craft advisory winds are already in the forecast, which
could increase to gale force as forecast confidence increases with
time.

&&

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

The forecast is on track for the low in the Bering Sea on Thursday
with its frontal system extending into the Gulf of Alaska. We are
confident in the wide swath of gale force winds in the Bering and
Aleutians, as well and the gale force winds associated with the
front in the Gulf. High seas are still expected with the strong
winds. The system will weaken as progresses east across the Alaska
Peninsula Friday night, with just some small craft advisory winds
in the Gulf of Alaska by Saturday.

&&

.LONG TERM FORECAST (Days 4 through 7/Friday through Monday)...

The long-term starts with the strong Bering Sea storm weakening
and exiting the Bering Sea over the Alaska Peninsula. It will
still be strong enough at this point to produce some gale force
winds, but will lose its strength as it crosses land Friday night
and moves into the western Gulf of Alaska. Another weaker low
pressure system is expected to move east through the Bering Sea
Saturday into Sunday, while weak low pressure continues to exist
in the gulf. This means generally cool and cloudy weather with
some rain showers for the weekend. Models do indicate a shortwave
ridge briefly in the Gulf on Saturday, so there is a chance of
some clearing, but hedge your bets on mostly cloudy with showers.

Models have come into decent enough agreement over the past 24 hrs
to be confident in this trend, however smaller features like
placement of areas of precipitation still need to be resolved, so
there is moderate confidence in the forecast into the weekend.
To start next week, models are still loathe to agree with each
other so forecast confidence remains low. Right now it looks like
general low pressure for our forecast area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 181.
Gale Warning: 120 130-132 136-139 141 150-170 172-180 185 351 352
              411-414.
FIRE WEATHER...NONE.

&&

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

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
159 PM AKDT Wed Sep 11 2019

AKZ101-111-121-125-145-130100-
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
159 PM AKDT Wed Sep 11 2019

...STRONG WINDS AND HEAVY RAINS EXPECTED FOR SOUTHERN ALASKA
THROUGH THURSDAY...

A strong Fall storm over the Bering Sea will drive a front into
Southcentral AK starting late tonight and into tomorrow. 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 tomorrow.

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.

For more information on the forecast visit our website at
www.weather.gov/Anchorage.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ANCHORAGE AK
220 PM AKDT WED SEP 11 2019

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0147 PM     COASTAL FLOOD    KWIGILLINGOK            59.88N 163.16W
09/11/2019                   BETHEL             AK   PUBLIC

            EXTREME FLOODING IS HAPPENING BECAUSE OF THE
            STORM.

&&

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
221 PM AKDT Wed Sep 11 2019

AKZ155-161-181-122100-
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
221 PM AKDT Wed Sep 11 2019

...STRONG WINDS, HEAVY RAINS, AND COASTAL FLOODING IN SOUTHWEST
ALASKA THROUGH THURSDAY...

Strong onshore flow will persist into Thursday and will continue
the threat of flooding along the coast. Areas around Kuskokwim Bay
currently receiving coastal flooding can expect this to continue
into tomorrow. Minor coastal erosion will also be possible with
flooding along the coast.

In addition, expect gusty south to southeast winds to continue
over the marine areas along Bristol Bay and the Kuskokwim Delta.
Winds will also be gusty across Interior Bristol Bay and Lake
Iliamna.

Heavy rain is also expected for much of Southwest Alaska.
Rainfall amounts of 1 to 2 inches are possible across much of
Bristol Bay and the Kuskokwim Delta through Thursday.