Gale Warning 426, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
559 PM AKDT Tue Aug 4 2020

.ANALYSIS AND UPPER LEVELS...
A vertically stacked low centered just south of the Gulf continues
to slowly fill as it drifts east, while its associated front has
brought Small Craft winds and rain to much of the Gulf Coast
today. Impacts from this system are confined to the Gulf Coast, as
a weak mid level ridge over the Mainland is allowing for generally
sunny skies and light offshore winds for Southcentral and
Southwest. Some weak shortwaves rotating around the outer fringes
of this ridge is allowing for convective development along the
Alaska Range and Talkeetna Mountains. The next system for the
Mainland is beginning to move eastward into the Central Bering,
bringing widespread clouds, gusty winds, and rain to the region.
This system will continue tracking towards the Mainland where it
will begin impacting the West Coast late Wednesday.

&&

.MODEL DISCUSSION...
Synoptic model agreement remains quite good with both the
dissipation of the Gulf low and eastward progression on the
Bering low through Wednesday. Afterwards, the guidance begins to
diverge with the development of a lee side low in the Gulf later
in the week. The last few runs of the NAM and GFS have
consistently indicated stronger solutions than the EC and GEM,
which is not surprising given the mesoscale nature of this
development. The afternoon forecast will continue to trend towards
the stronger NAM/GFS solutions, however we held off on
significant changes and finer scale details until there is better
synoptic scale agreement.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist into Wednesday.
There may be shower activity in the vicinity of the terminal this
afternoon, especially to the east over the Chugach, but at this
point it appears that the majority of the activity will remain
away from the airport.

&&

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

A weakening low over the southern Gulf of Alaska continues to
move east with areas of light rain and showers over the
northeastern Gulf coastal areas up to the Prince William Sound. A
shortwave rotating around the north side of low this will produce
showers and isolated thunderstorms across the Copper River Basin
through this evening. Otherwise surface heating and unstable air
will allow showers and a couple of thunderstorms to form over the
Alaska and Talkeetna Mountains this afternoon and this evening.
Tonight most of the showers will end with the exception of eastern
areas of the Copper River Basin where the low pressure system and
another wave rotating around the north side of the low will keep
showers going through tomorrow.

A transitory upper level ridge will move over Southcentral on
Wednesday allowing for slightly warmer temperatures and a thermal
trough to form beneath it. This will allow isolated thunderstorms
to develop mainly over the Alaskan Range, the Talkeetna and
Wrangell mountains. The warmer and drier conditions will
unfortunately end as a strong vertically stacked low tracks to the
eastern Bering Sea Wednesday night, pushing a front into the
western Gulf. A coastal ridge will develop along the north Gulf
coast ahead of this front, increasing the Turnagain Arm winds and
winds into the Anchorage and Matanuska Valley on Wednesday night.
As the upper trough moves into the Gulf Thursday, this will cause
the front to re-orient east-west across the central Gulf. This
will lower pressure over Cook Inlet and cause the Turnagain Arm
jet to bend southward. Winds will ramp up Thursday along Turnagain
Arm and the Anchorage Hillside, but these winds will remain out
of the Anchorage Bowl. The front will stall over the northern Gulf
as a surface low forms in response to the upper low arriving from
the Bering Sea Thursday night. The strongest winds with this
system (not counting gap winds) will remain over the Gulf waters.

Expect moderate to heavy rain as the front moves into Kodiak
Island Wednesday night. This rain will then spread to the Southern
and Eastern Kenai Peninsula on Thursday morning. The rest of
Southcentral will see downsloping flow develop on Thursday with
very little rain for most valley areas with cloudy conditions and
only a brief shower possible.

&&

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

An extensive area of patchy fog has rolled right past Nunivak
Island, Kusko Bay, Kusko Delta and Bethel since this morning. Have
introduced this area into the grids through late tonight, and
adjusted thunderstorms to remain farther east of Bethel but south
of the Kusko Valley and over the Alaska Range for the remainder
of this evening, and again on Wednesday afternoon and Wednesday
night. The highly anticipated wetting rains from the frontal zone
will keep temperatures inland a little bit cooler on Wednesday and
Wednesday night. Computer models show impulses of shower bands
trailing over the region from the upper level low mentioned in the
Bering Sea/Aleutians discussion. This situation could bring
another series of showers between Thursday and Friday, but for
now, will monitor this situation before making new updates.

&&

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

Strong gales associated with the frontal system remain headlined
for several Aleutian marine zones for Wednesday and Wednesday
night. This is thanks to a steeping upper level low over the
Western Bering Sea which will stay south of Pribilof Islands and
much closer to central Aleutians. This will help strengthen the
pressure and wind gradients over an area including Unalaska and
Adak until the upper low shifts toward the Alaska Peninsula on
Thursday morning. The presence of fog and low clouds, small craft
conditions and steady rain will persist over much of the area the
next few days, with the exception of gale force winds over Central
Aleutians between Wednesday and Wednesday night.

&&

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

Bering Sea/Aleutians

A Southwest Alaska low moves into the Gulf of Alaska by Sat.
Confidence is good for rear flank small craft winds and waves over
the Eastern Bering and Bristol Bay diminishing Sat. A low exiting
Anadyr moves across the Bering through Sun. Although model tracks
are mixed, confidence is good for westerly small craft winds to
move across the Bering and diminish Sun. A North Pacific low
approaches the Western Aleutians and Bering for Sun. Confidence is
good for southerly small craft winds with local gales over the
Western Aleutians through Sun.

Gulf of Alaska

A low and its front to the East of Kodiak Island tracks into the
Northeastern Gulf for Sun. With consistent model tracking,
confidence is good for easterly small craft winds and waves
through Sun with local gales near capes ahead of the front for
coastal zones diminishing Sat, and widespread westerly small
craft winds through Sun with local gales diminishing Sat to move
across the Gulf.

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday through
Tuesday)...

A fairly active and progressive longwave pattern looks to set up
this weekend as a series of systems move eastward from the Bering
Sea/Aleutians into the middle of next week. The first of these
will be a developing low diving from Siberia into the Gulf,
bringing cloud cover to much of the South Mainland and rain
primarily to the Gulf Coast. The next low will quickly follow the
Siberian Low, and will be the first in a series of lows moving
into the Bering from Kamchatka. This pattern will allow for
generally cloudy, cool conditions for much of the area, with
storminess for the Bering Sea and Gulf of Alaska.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES: 171-176.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...RC/MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...MK/DK