Small Craft Advisory 718, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKDT Sun Sep 29 2019

.ANALYSIS AND UPPER LEVELS...
Ridging remains over eastern Alaska, extending up from the North
Pacific. A high amplitude trough extends through the central
Bering while it tracks to the east. This setup has helped to pull
up tropical moisture over southern Alaska bringing rain across the
area with the heaviest over southwest Alaska. Additionally, a
strong push of warm air was also brought into the southern
mainland in this setup.

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.MODEL DISCUSSION...
Models are in good agreement with large scale features over the
next couple days and overall forecast confidence is high.

&&

.AVIATION...
PANC...Strong southeasterly Turnagain Arm winds combined with
light surface winds will bring windshear to the airport through
the morning. As the Turnagain Arm wind diminishes in the
afternoon, wind shear will diminish as well. A front will push
through this afternoon which will drop the ceiling and visibility
until the evening when the front clears. There is high confidence
that the visibility will rise after the front leaves, but there is
some uncertainty as to how high the ceiling will be overnight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Sun and
Mon)...
The forecast remains on track with windy and wet conditions
persisting today before a slight drying trend moves in for Mon.
The initial warm front moved through late Sat evening. This
brought some light rain to much of the area and opened the door
for the strong southeast winds to start to blow through the gaps.
A brief break in the rain will continue through the early morning
hours as those southeast winds create some downsloping conditions
in the lee of the Kenai and Chugach mountains. However, this break
is slated to end in a dramatic way by early this afternoon. A
broad trough over the West Coast will push a shortwave up Cook
Inlet. This will switch flow to S-SW at nearly all levels. This is
a very wet pattern for the Anchorage Bowl and much of the Susitna
and Matanuska Valleys with nothing to impede the long fetch of
subtropical moisture into the area. So as winds die down, expect
rain to start to fall in earnest. QPF amounts were only adjusted
slightly for this forecast package. Thus we are still expecting
around 0.50" for much of the Anchorage Bowl with closed to 1.5"
for coastal locations through early Mon morning.

This system will create a weak surface low over the Northern Gulf
heading into Mon. This will help to keep some light rain over the
Cordova area and leave inland locations vulnerable to showers.
However, even with those showers, Mon will be a much drier day
than Sun. This low will keep some gusty northwest winds through
the Barren Islands with some associated weak cold air advection.
The most interesting aspect of the low is that as it slowly
retrogrades (moves west) late Mon into Tue, it will actually
deepen significantly by Tue. This is due to a strong upper level
vorticity maximum coming over it from the west and providing
healthy upper level support. As the low deepens somewhere just
east of Kodiak Island, expect more rain to move into all of
Southcentral. The heaviest rain will once again be in Prince
William Sound. On the western periphery of the low, expect gusty
northwest winds to develop for Kodiak Island and Shelikof Strait
as some cooler air is pulled through the Aleutian/Alaska Ranges.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
The frontal system currently stretched across the Lower Kuskokwim
Valley and Greater Bristol Bay area and bringing abundant
rainfall to those areas will continue to move slowly to the east
through morning and cross the Alaska Range this afternoon. Cooler
upper level troughing will become established along the coast
tonight with southwesterly flow bringing scattered showers
onshore. As the upper level trough swings inland on Monday, the
general flow will turn more northwesterly. The northwesterly flow
will then increase Monday afternoon through Tuesday morning as
upper level ridging strengthens over the eastern Bering and a low
spins up in the western Gulf. The northwest winds will begin
gradually diminishing Tuesday afternoon through Tuesday night as
the upper ridge flattens and the low moves farther away to the
east.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
An upper level ridge will build over the western Aleutians today
with the broad upper level trough currently stretching across the
Bering and Aleutians shifting to the east. The upper level ridge
will progress east to reach the Central Bering Monday and eastern
Bering Monday night. Northwesterly winds will increase along the
east side of the ridge Monday with winds strengthening across the
Alaska Peninsula Monday night through Tuesday morning. The flow
across the Bering will become more westerly Tuesday and Wednesday
as the upper ridge flattens.

&&

.MARINE (Days 3 through 5)...

A deep low will develop in the northern Gulf of Alaska by early
Tuesday. While there's still some uncertainty with location of
the low's center, confidence is growing that there will be
widespread gales, with potential for storm force winds in some
areas. In particular, expect storm force gusts, if not sustained
winds out of Kamishak Bay and over the Barren Islands. As the low
exits to the east Wednesday, a barrier jet could develop along the
North Gulf Coast. This will depend on how far north the low
tracks. By Thursday, expect mostly benign conditions over the
Gulf.

Over the Bering/Aleutians, no impactful storms expected until late
Thursday at the earliest. A low will develop in eastern Siberia
early Thursday, possibly bringing gales to the central Bering late
Thursday night as it progresses eastward and strengthens.

&&

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

A deep low will be positioned in the northern Gulf of AK Tuesday.
Strong winds on Kodiak Island and along the North Gulf Coast are
likely. As the associated warm front and trailing cold front
traverse Southcentral from south to north, expect precipitation to
make it inland. That said, heaviest precipitation will be along
the North Gulf Coast and Kodiak Island. While the low exits to the
east, a highly amplified ridge over the Bering will cut off a
high, and a shortwave will develop over eastern Siberia. This
feature isn't well handled in the latest models, but in general
expect increased west winds and precipitation along the Kuskokwim
Delta Coast Wednesday into Thursday as the cold front pushes
inland. As the trough progresses to the east Thursday into Friday,
Southcentral will get another chance of precipitation. A stronger
shortwave will develop over eastern Siberia Thursday into Friday.
Again, not handled all that well by models given the upstream
extratropical transition of (current) tropical storm Mitag. But,
as of now gusty winds and another round of precipitation look
likely in Southwest Friday as the low tracks inland, and
precipitation will spread to Southcentral Saturday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales...119, 120, 125, 130, 131.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
MARINE/LONG TERM...MM