Gale Warning 546, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 PM AKDT Mon Oct 21 2019

.ANALYSIS AND UPPER LEVELS...
Short-waves continue to transit around the anchor low sitting
over the Bering Sea, though this circulation has broken down
a bit as flow over the Gulf and Southcentral becomes more
progressive. Weak short-waves crossing Southwest Alaska and
Southcentral are producing a few showers. Otherwise, it's
relatively quiet across the southern mainland with a mix of
sun and clouds and localized areas of fog. The Bering Sea and
Aleutians are also fairly benign with scattered clouds and
showers. The focus is over the North Pacific where a digging
trough from the Bering Sea is beginning to interact with a
storm crossing the North Pacific. This will ultimately lead to
amplification of the upper level pattern and rapid cyclogenesis,
with a storm force low tracking northward toward mainland Alaska.

&&

.MODEL DISCUSSION...
Models have largely locked on to a common solution with the deep
low tracking to the Alaska Peninsula Tuesday night/Wednesday, so
forecast confidence has greatly increased. Models are still
showing some fluctuations from run-to-run, with minor changes in
track and strength. However, even small changes will affect the
details of the wind forecast, especially for gap winds such as
Turnagain Arm. We will continue to hone in on these details based
on trends of latest model guidance.

&&

.AVIATION...
PANC...Weak upper level waves will continue to transit the area,
which should maintain a VFR cloud desk through most of the period.
However, there could be a break in clouds between upper waves
this evening. If this does occur, conditions would be favorable
for some patchy fog and stratus to form. This is a low
probability, so have left out of the TAF for now.

Easterly flow will increase across the Chugach mountains and
Turnagain Arm Tuesday afternoon as a deep low over the North
Pacific approaches the Alaska Peninsula. The pressure gradients
down Cook Inlet will also increase, which means the strongest of
the winds will bend southward as they come out of Turnagain Arm.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The main challenge in the short range forecast through mid week is
a rapidly developing storm system that moves across the southern
Gulf Tuesday night and then to the northern gulf early Wednesday.
Storm and gale force winds are expected with this storm as the
powerful low lifts from the North Pacific to Bristol Bay. Strong
easterly winds increase through Whittier, Portage Valley,
Turnagain Arm and the Upper Hillside Tuesday night...peaking
Wednesday afternoon. The Matanuska Valley and Western Kenai will
see gusty winds from the northeast through the same time frame.

For tonight...relatively benign conditions set up across
Southcentral as a weak boundary washes out along the coast. Patchy
fog may develop after midnight with light winds and a moist
surface layer in the wake of the latest frontal passage. Another
weak front will bring light rain across Kodiak Island early
Tuesday morning spreading to Cook Inlet, the eastern Kenai and to
the northeast Gulf during the afternoon. A few isolated showers
are possible inland, but no accumulations are expected.

The storm that is the primary focus, pushes a front to the
southern Gulf late Tuesday afternoon which quickly moves to the
northern coast Tuesday night. The associated low deepens to around
960 mb as it tracks to Kodiak Island Wednesday morning and then
crosses over to Bristol Bay Wednesday evening. During this time
expect gusty winds to peak with easterly flow along the
boundary...although northerly flow sets up across the Matanuska
Valley to Cook Inlet as the pressure gradient brings a down-Inlet
flow with the location of the surface low to the southwest.
Moderate to heavy rain is expected across the Gulf and surrounding
coasts. Locations west of the mountains will be downsloped due to
strong low level easterly flow...so a dry forecast is still
expected along the western Kenai, Anchorage Bowl and Mat-Su
Valleys. The front slowly moves from the coast to the Copper River
Valley Wednesday afternoon. The locations from Glennallen east
will see rain from this system as a warmer maritime air mass pulls
north...areas to the west may see rain and snow as the cold pool
on this side of the basin is slower to erode.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1-3; Tonight through
Thursday)...

The frontal system that brought light snow to the Kuskokwim Delta
through the morning hours has lifted north away from the forecast
area. A secondary trough associated with low pressure west of
Cape Newenham is pushing onshore this evening, reinforcing the
moist onshore flow and yielding scattered showers from King Salmon
to Bethel. The scattered shower activity will diminish in
coverage as it lifts north with the trough Tuesday morning.

Southeasterly winds and widespread precipitation will be on the
increase for the AKPEN and Bristol Bay Tuesday afternoon as a
deepening North Pacific low tracks north toward Southwest Alaska.
This system then tracks over interior Bristol Bay by Wednesday
with precipitation and gusty northeasterly winds on the increase
for the lower Kuskokwim Valley and Kuskokwim Delta. Although some
residual colder air may remain in place across interior locations
during the onset of precipitation, any snow that does fall
initially is expected to change quickly to rain. The exception to
this may be locations in the Kuskokwim Delta where the colder air
may linger just enough for a light accumulation of snow before
the warmer air works in. For areas along the lee of the Alaska and
Aleutians Ranges such as King Salmon and Iliamna, the cross
barrier flow over the higher terrain will produce downsloping,
helping to limit the amount of rain that falls.

This system will begin to weaken as it stalls near Bristol Bay
through early Thursday morning, then drift slowly east during the
day Thursday. As it does, the crux of the precipitation will
diminish, with only mixed rain/snow showers persisting along the
coast and upslope areas of the higher terrain. Winds will also
shift more northerly behind the low and remain quite gusty,
especially across the AKPEN.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1-3; Tonight
through Thursday)...

An area of low pressure east of the Pribilof Islands will continue
to weaken through the overnight hours as it lifts to the
northwest. As it does, gusty easterly winds and areas of showers
will diminish in coverage and intensity. Farther west, areas of
small-craft advisory northwesterly winds will continue over the
western Bering and Aleutians through midweek.

The main focus, though, is on a quickly deepening low that will
track from the North Pacific towards the AKPEN on Tuesday. This
system will bring widespread gale-to-storm force winds and rain
to the eastern Aleutians, AKPEN, Bristol Bay, and eastern Bering
as it lifts north to Bristol Bay late Tuesday and lingers over
the area through Thursday. As this system stalls inland of Bristol
Bay, southeasterly gales ahead of the low will switch to
northerly gales behind the system. South of the low, an area of
storm-force winds with seas over 30 ft will impact the coastal
waters south of the AKPEN late Tuesday through late Wednesday. Any
lingering precipitation behind the low will also transition to
showers by Thursday as the low moves east toward Southcentral
Alaska. Storm- force winds and high seas are expected for waters
south of the AKPEN starting late Tuesday into Wednesday.

&&

.MARINE (Days 3 through 5)...
For the Gulf side, the main concern on Thursday is with the
progression of a low across the Gulf. Models vary on the track of
the low resulting in differences of northwest winds near the
Barren Islands ranging from gale force to light winds. Currently a
more northerly low track seems more favorable resulting in small
craft winds through the Barrens, but confidence is low.

The focus then shifts to a system approaching the western
Aleutians Thursday. There is high confidence with this system
being storm force into Friday. Confidence diminishes over the
weekend as the low moves into the Bering and the models diverge
with its track.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term, beginning on Thursday, has one low exiting the
Gulf and a new low approaching the western Aleutians from the
North Pacific. The North Pacific low is bringing a long fetch of
warm air and moisture with it. As the low tracks east on Friday,
ridging will build in over the southern mainland. On Saturday,
the ridge will block the progression of the low and the low will
start to track north into the Bering. Model confidence is low with
how the low will move into the Bering, however there is high
confidence of an atmospheric river setting up between the low and
the ridge. Models are also starting to converge on a more western
setup of the atmospheric river which places it closer to the
Alaska Peninsula as opposed to Southcentral Alaska where previous
model runs had it.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm 138 150 155.
		 Gale 119 120 125 130-137 139 160 172 180.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...DK