Small Craft Advisory 612, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
417 PM AKDT Thu Aug 13 2020

.ANALYSIS AND UPPER LEVELS...

An upper-level low over the northern Gulf, combined with a ridge
of high pressure building across western and northern Alaska, is producing
a general northeasterly flow across Southcentral and Southwest
today. A shortwave embedded within the flow is sliding over
Southcentral today. This feature will aid in the development of
showers and isolated thunderstorms, mainly over the higher terrain
this afternoon and evening. Convective development along with
shower activity can already be seen over parts of the Chugach and
Wrangell Mountains via satellite and radar imagery. Areas of fog
this morning from Talkeetna south to Kenai and Homer have lifted
and diminished in coverage; however, there is still some low
stratus sitting over parts of the Susitna Valley and Cook Inlet
this afternoon. This will bear watching as clearing conditions
and a weak up inlet flow will likely produce another round of fog
overnight tonight. To the south, an area of low pressure well
south of Kodiak Island is moving east and is pushing a warm front
toward the southwestern Gulf. Conditions are also mainly clear
and dry over the Southwest. Valley fog that developed overnight in
places like King Salmon and Aniak has also dissipated.

For the Bering and Aleutians, a vertically stacked low continues
to slowly drift east and is near Dutch Harbor this afternoon.
Plenty of low clouds and showers accompany this feature.
Widespread low stratus also sits over the western and northern
Bering, trapped underneath the ridge. Mid-level clouds are moving
over the far western Aleutians in association with a front
attached to a Kamchatka low.

&&

.MODEL DISCUSSION...

Models remain in decent agreement going into the weekend with some
subtle differences. The first is the movement of the upper-level
low over the eastern Aleutians and the surface low south of Kodiak
Island. Both the American and International models now have these
features pushing east on Friday in association with a
strengthening and amplifying jet. The upper-level then partially
phases with the low in the Gulf and ejects as a shortwave over the
Alaska Peninsula. In addition, there are differences in the
position of the Gulf low and timing in the transit of the
shortwave and just how far east the ridge to the north builds over
the interior of the state, but any solution looks to keep
unsettled weather well out of the forecast area with an overall
dry, warm, and windy solution for Southwest and Southcentral. The
position of these features will have an outcome on just how strong
the winds peak through Saturday. Hi-res guidance is also
indicating the development of strong and gusty outflow winds late
Friday through Saturday.

Out west, the upper-level ridge will strengthen and amplify in the
wake of the aforementioned low and ahead of a deepening upper-
level low and surface Kamchatka low. Again, there are some
differences in the strength of these features and timing of
when its associated front reaches the eastern Bering Sunday.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through the
nighttime hours. A light westerly sea breeze will develop for
this afternoon and evening. A stray shower is also possible in the
late afternoon and evening hours. Winds will remain light but
turn southerly by late evening into the overnight hours. This will
help draw moisture and low clouds up Cook Inlet. Low stratus and/
or patchy fog is expected to move up inlet through the overnight
hours. Right now, it appears more likely for low stratus to
advect over the terminal; however, there is the potential, albeit
low confidence, for an hour of two of fog (14z-16z).

&&

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

A weather pattern that will provide hot and windy conditions is
developing headed into the weekend. High pressure will move into
Southcentral AK, while the low pressure system currently over the
Alaska Peninsula moves into the Gulf of Alaska on Friday. This low
will merge with and reinvigorate a weak low already in place
along the Gulf Coast. This low will hold in place into Sunday.

The setup of high pressure inland and low pressure in the Gulf
will tighten the pressure gradient through Southcentral AK and
produce strong north to northeasterly winds through areas like the
Susitna Valley including Broad Pass, Palmer and the lower
Matanuska Valley, and gap winds along the coast in places like
Whittier and Valdez. Also of note with this setup, strong high
pressure will create very warm temperatures for this time of year,
with the highest temperatures in the Lower Susitna Valley.
Temperatures there will push 80 degrees over the weekend. Daily
maximum temperatures in some areas through the Susitna Valley and
Anchorage Bowl are in danger of being broken this weekend.

Low pressure in the Gulf is expected to shift south on Sunday,
easing the pressure gradient and allowing winds to relax a bit,
though it will still be breezy Sunday. High pressure will still be
in place, keeping it warm and dry through the weekend. All in
all, a resurgence of summer is in store this weekend, with some
wind. Be careful if you are out this weekend due to dry and windy
conditions. Otherwise, enjoy this one last hurrah of summer as we
can expect that fall conditions will soon return.

&&

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

An upper level ridge is expected to dominate the pattern for the
forecast period with practically all of Southwest remaining dry
with gorgeous weather besides for a possible shower in the
western Alaska Range late today to this evening. Therefore, the
main story will be sunny conditions with warming temperatures
(upper 60s to 70s) each day. There may even a spot of near 80F
Saturday afternoon. However, along with the nice weather will come
increasingly windier conditions for Friday and Saturday. As the
upper level ridge changes orientation tomorrow to more north to
south, this will setup a tight pressure gradient across Southwest
Alaska. The tight pressure gradient will be caused by an upper
level low in the Gulf of Alaska and another upper level low
approaching the western Aleutian Chain. As a result, breezy
conditions are expected for Friday with minimum relative
humidities in the 30s to 40s for inland locations. Saturday is the
better chance for windier conditions along with lower minimum
relative humidity values in the 20s in which the criteria for a
Red Flag Warning may be met. Fuels do not meet the criteria at
this time for a Red Flag Warning to be issued but guidance is
trending in the direction that Saturday could need a Red Flag
Warning. Clear skies at night will help aid radiative cooling but
with the windy conditions, this may keep relative humidity values
from reaching there full potential.

&&

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

The upper level low that has been slowly moving eastward through
the southern Bering Sea will finally move out of the area and into
the North Pacific. As it does so though, it will continue to bring
periods of rain through this evening to Dutch Harbor with rain
continuing for areas east including Sand Point and Cold Bay
through Friday morning.

An upper level ridge that has been sitting across the northern
Bering Sea in an east to west orientation will shift to a more
north to south orientation over the next two days. As a result,
day cloud phase distinction satellite is already showing the back
edge of the clouds heading for the Pribilof Islands. With this
pattern in place, mostly sunny skies are expected for the
Pribilofs for Friday into Saturday.

Further west, a gale force frontal system will move through the
western Aleutian Chain beginning this evening. The gale force
winds are expected to begin early Friday afternoon through Friday
night for the western Aleutian Chain before pushing north into the
Bering early Saturday. Accompanying the frontal system will be
rain showers with the steadiest and heaviest rain expected for
the western Aleutian Chain Friday afternoon through Saturday
afternoon. This system has also had plenty of lightning associated
with it over the North Pacific. As a result, have added in the
slight chance for a thunderstorm mainly on the Pacific side of the
Chain for Friday morning through Friday evening. This risk is very
slim but due to the dynamics with this system coming northward,
there is the potential for a lightning strike south of the Chain
on Friday.

&&

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

Aleutians/Bering: A North Pacific low and front move across the
Aleutians and Bering through Tue. While the track and development
of the low are mixed through the end of the forecast, confidence
is good for showers and small craft winds and waves to move with
the front over the Aleutians and Bering. Some local gales may be
possible West of St Matthew Island and diminishing Sun as the
winds wrap around the low through Tue. As the front enters the
Eastern Bering Mon, the winds dissipate. Northerly small craft
winds and waves continue over the Western Bering behind the low
through Tue.

Gulf of AK: A Southern Gulf low dissipates over the North Pacific
by Tue. Confidence is fair for showers and Northerly small craft
winds and waves over the Western Gulf for Sun. Several troughs
move around the dissipating low through Tue. Confidence is good
for local Easterly small craft winds to develop along the North
Gulf Coast through Tue. A new front enters the Western Gulf for
Tue. Confidence is good for showers and Southerly small craft
winds over the Western Gulf for Mon and Tue.

&&

.LONG TERM FORECAST (Days 4 through 7: Mon through Thu)...

There is good model agreement that the amplified ridge will begin
to break down on Monday, with one piece lifting northeast and
phasing with the North American ridge and the second retreating
south over the North Pacific. This transition comes as a result of
the Bering low sliding southeast over the AKPEN, lifting a second
upper-level wave into the Gulf of Alaska. Upstream, yet another
upper-level low dives into the Bering from Siberia. These
features will all work to establish a long-wave trough right
where the ridge was over the weekend. The resulting flow will turn
southeasterly over the Gulf and Southcentral, ushering in
moisture and increasing clouds and chances for showers late Monday
into Tuesday. The pattern is similar over the Southwest where a
southerly flow ahead of a frontal system will also bring increased
chances for clouds and precipitation early next week. By mid-
week, the upper-level low in the Gulf remains as a ridge tries to
build over the interior from the north. This pattern would result
in a building of the thermal trough with warmer and drier
conditions under a northerly flow. Periodic easterly waves
transiting the region would bring a chance for afternoon and
evening showers across Southcentral mid-week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE... Gale 411 177 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...BJB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...MK/TM