Small Craft Advisory 744, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
Issued by National Weather Service Juneau AK
311 PM AKDT Wed Sep 30 2020

.ANALYSIS AND UPPER LEVELS...

The assault on the Gulf of Alaska continues as a barrage of power
Pacific storms continues to pummel the region as a longwave
trough is entrenched across the Bering Sea down to 30N across the
NPAC. This represents a busy Alaska fall storm cycle, with more to
come as the subtropical jet stream remains active. Currently, a
weakening former storm force low (now gale force) is moving
northwest towards the Alaska Peninsula. The warm front is moving
inland along the North Gulf Coast, with copious rain falling over
this area, including Kodiak Island where over 3" of rain has
fallen over the past 72 hours. "Slopover" rainfall is moving over
the Chugach Mountains into Anchorage, the Mat Valley, and Cook
Inlet. Out west, the cyclonic flow is favoring more of an offshore
northeast flow, with northerly flow over the Bering Sea. However,
heavy downslope flow off the Aleutian/Alaska Ranges is favoring a
much drier pattern with only spotty light rainfall.

&&

.MODEL DISCUSSION...

The numerical models are now in excellent agreement with the
timing of the low moving into Bristol Bay this afternoon. It is
now a bit slower and deeper than it was expected to be yesterday,
thus the forecast was updated to reflect this, as this will impact
timing of rainfall and local winds across Cook Inlet (slower onset
of southeast winds in Cook Inlet. Otherwise, little change to the
first day of this forecast package.

The next concern is a basal shortwave trough wrapping around the
larger longwave trough, with the associated warm front and surface
low moving into the Gulf of Alaska then landfalling over the
Bristol Bay Coast on Friday. This storm is stronger than the
current storm and will reach storm force along the Gulf Coast (48
knots to 55 knots) and through the Barren Islands. Timing is still
slightly different in the models, but overall agreement is good as
all models now take the low through the Kuskokwim Delta by Friday
evening/overnight. This will bring copious rainfall to the Gulf
Coast and into Kodiak Island, strong coastal winds, and near
hurricane force winds down Turnagain Arm, with some of those gusty
southeast winds blowing through Anchorage Bowl as the low tracks
to the northwest. The favored models were the NAM and GFS for this
forecast package.

&&

.AVIATION...
PANC...VFR conditions will persist with showers mostly ending this
evening. Low level wind shear will abate late this evening as
northerly winds diminish and winds aloft and southeast winds
develop. Gusty southeast winds will last through most of Thursday
afternoon.

&&

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

A gale force low is moving into the Alaska Peninsula this evening
with the associated warm front pushing into the inland areas of
Southcentral. Heavy rainfall along the coast will end this evening
as the front falls apart moving northward, along with light
"slopover" rainfall on the lee slopes of the Chugach Mountains.
Current 48 rainfall totals exceed 3.5" for Portage Valley, 2.5"
for Seward, 1.50" for Cordova, and 2.10" for Kodiak Island. Inland
areas, behind this front, will see very efficient downslope
shadowing and will likely remain dry with only periodic sprinkles,
if anything. As the front moves inland, the low level pressure
gradient will become strong enough up Cook Inlet to bend gusty
southeast winds into South Anchorage, the Hillside, and down
Turnagain Arm. Expect gusts to 65 MPH down Turnagain Arm and
across the upper Hillside, with gusts to 20-40 MPH through South
Anchorage persisting through Thursday afternoon before winds
quickly bend down Cook Inlet.

Attention very quickly turns to the next powerful low poised to
bring more wind and rain to the North Gulf Coast, Kodiak, and
COok Inlet. This storm force low, a remnant of a former tropical
low, will bring storm force winds to the Gulf by Thursday evening
and overnight. This will bring a second round of copious rainfall
to the coastal areas of the Gulf. This storm will be more compact
and fast moving, as well as stronger, so another round of near
hurricane force winds are expected down Turnagain Arm, which will
occur as gale force winds cross Prince William Sound, storm force
winds develop across Southern Cook Inlet and through the Barren
Islands. The low will take a track up Bristol Bay through the
Kusko Delta, thus another round of windy conditions will sweep
across Anchorage, with northeast winds in the Mat Valley switching
to southeast, on Friday afternoon/evening. Gusty south winds will
also sweep through the Copper Basin as the front tracks north. In
addition, copious rainfall will plaster the coast, with 4-7"
rainfall expected for Seward by Saturday evening and 2-3" for
Kodiak Island. See Special Weather Statement for more details
regarding local flooding potential along the Kenai and
landslide/mudslide threats across Kodiak.

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

Yet another fall storm is impacting the region, with this low
currently tracking right near the Alaska Peninsula this evening as
it continues northwest into Bristol Bay. The associated warm front
has already wrapped into the Bering Sea, leaving a pretty large
downslope shadow over most of the Southwest mainland. Northeast
winds 15-30 MPH will gradually shift to southeast winds as the
low moves into Bristol Bay. The upper level low will over tonight
and Thursday, which should be enough for some periodic showers.
ANother warm front will sweep across the region Thursday night and
Friday, bringing more gale force winds and a round of rain. Winds
may reach 40-60 MPH through Iliamna Gap on Thursday
evening/overnight. The low will quickly track northwest from
Bristol Bay to the Kusko Delta Friday night and Saturday, bringing
a shot of strong south-southwest winds to the region, along with
periods of rain as the upper trough moves through.

&&

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

The first weakening low is moving across the Alaska Peninsula into
Bristol Bay this evening, and will continue to slowly move
northwest into the N Bering Sea by tomorrow afternoon. Near gale
force northerly winds over the Eastern Aleutians and Pribilof
Islands gradually taper off through this same period. The next low
moves into Bristol Bay on Thursday night and Friday, bringing
another round of winds and rain to the Eastern Aleutians while
broad cyclonic flow persists elsewhere, with typical Bering Sea
stratus and showers.

&&

.MARINE (Days 3 through 5)...

Bering/Aleutians

The remnants of a storm moving up Bristol Bay on Friday evening
will quickly move to the Bering Strait Saturday, leaving a region
of enhanced fetch northerly winds and building seas 15-20 feet
over the western half of the Bering Sea. A brief reprieve in the
stormy weather will develop over the eastern half of the Bering
Sea and Bristol Bay through Sunday with the next strong low
expected midweek.

Gulf of Alaska

The barrage of powerful storms continues on Saturday night as yet
another fall storm barrels into the Gulf of Alaska, bringing a
gale to near storm force low up to the coast by Sunday
morning/afternoon. There is high confidence in this storm track,
so mariners should be ready for yet another strong fall storm.
This storm exits Sunday night.

&&

.LONG TERM FORECAST (Days 4 through 7)...
The final in a series of fall storms will make landfall along the
Gulf Coast on Saturday night through Sunday evening. This will
bring another bout of heavy coastal rain and strong winds to the
coast, with lighter rain inland. This low track will generally
keep Southwest Alaska and the Bering away from the impacts of this
low. Expect rainy and cloudy weather to persist through Tuesday,
with a brief period of high pressure and drying on Tuesday night
before the next Bering front quickly moves into Southwest Alaska
on Wednesday. Inland blocking high is looking more and more
likely, thus the storm track looks to finally shift south of the
Gulf of ALaska by mid to late week, allowing cooler high pressure
to dry out Southcentral and the Gulf Coast for the second half of
the week, although Kodiak will continue to see wetter weather.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 125 127 140 141 150 155 160 172
         STORM 110 120 130 131 132 136 137 138 139

&&

Ahsenmacher