Small Craft Advisory 587, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...

A broad surface low is spinning up well south of Kodiak Island
this morning. Its associated front is oriented west to east and is
slowly lifting north from the southern Gulf. Both features are
well-defined on infrared and water vapor imagery and the placement
of the front is further defined by the most recent ASCAT
scatterometer pass, indicating southwesterly winds of 15 kts over
the southern Gulf and easterly winds up to 20 kts over the
northern Gulf, ahead of the front. The combination of the easterly
flow with a shortwave stretching from Kodiak Island to Yakutat is
bringing rain and gusty northeasterly surface winds to the
eastern half of the island this morning as well as showers to the
immediate Southcentral coast. Farther north, an upper-level ridge
over the interior of the state is elongating and sliding
southwest in response to the vertically stacked low to the south
moving slowly to the north and east. Between these two features,
there is cyclonic flow along with weak pulses of energy in the
mid-levels. One is moving near Bristol Bay with shower activity
from this system persisting from Kipnuk to Port Heiden this
morning. The second is near the Al-Can border with most of the
cloud cover and shower activity from this feature staying north of
the Alaska Range.

Farther west, the main weather feature is a deepening surface low
west of Attu. This feature is producing widespread rain and gusty
southeasterly winds along its associated warm front draped over
the western Aleutians. The surface low is getting plenty of
upper-level support as it sits in the left exit region of a 120
kt jet streak at 30,000 ft (300 mb). This system is also aided by
a vigorous short wave at 500 mb diving southeast from the Russian
coast.

&&

.MODEL DISCUSSION...

Models remain in good agreement through the short term, although
there are some minor, yet persistent, differences between models
and model runs. One difference remains the evolution of the
surface low developing southeast of Kodiak Island. The American
Models (NAM/GFS) continue to keep the feature slightly stronger,
but the 00z runs of the EC and GEM have trended to the NAM and
GFS. The differences become greater by Wednesday afternoon as the
low weakens and moves to the eastern Gulf. The NAM and GEM keep
this feature stronger and more compact than the EC and GFS, which
open this feature into a wave. By Thursday, the NAM is the only
one to keep a closed low south of Sitka. In the upper-levels, this
translates to the NAM holding onto a stronger and more elongated
feature along the AK Panhandle coast. Given this, the NAM is also
bringing stronger easterly waves around this feature into the
Copper River Basin and across a larger area of Southcentral.
Overall, the other models are weaker with the waves and have
trended weaker. Thus, the NAM is seen as the outlier.

Out west, models are in very good agreement with the strength and
track of the surface low from west of Attu this morning to the
west coast by Thursday. Guidance, however, appears to be
downplaying the strength of the winds wrapping underneath it as a
secondary upper-level shortwave dives south of the main feature
in conjunction with cold air advection. Deterministic runs of the
global models keep winds just shy of gales. Boundary layer winds
and hi-res models, though, suggest an area of gale-force winds
southwest of the center of circulation developing by Wednesday
morning over the central Aleutians and progressing east to the
AKPEN by Thursday. Given a trend in stronger winds with the most
recent GFS and NAM (00z) runs, and gales in the hi-res models,
gale force winds have been introduced over the aforementioned
areas in the morning forecast package.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

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

A weakening low over the Southern Gulf of Alaska is bringing rainy
conditions to Kodiak Island today. An associated elongated trough
and surface front will lift northward across the Gulf today. This
will bring some showers to the Gulf coast, but the bulk of rain
will remain over the Gulf. As the low continues to the eastern
Gulf this afternoon and evening, rain and winds will diminish
across the western Gulf and Kodiak Island. The rest of
Southcentral will remain under weak cyclonic flow aloft. With
some sunshine expected today this will lead to some afternoon
and evening showers, particularly along the mountains. As the
low in the Gulf continues eastward tonight, weak upper waves
rotating around the north side of the low will cross the eastern
Copper River Basin and rotate southwestward to eastern Prince
William Sound. While none of these waves appears particularly
strong, this will result in numerous showers through this corridor
tonight through Wednesday.

A weak upper ridge will move overhead of Southcentral on
Wednesday. A thermal trough will strengthen beneath it, leading
to isolated thunderstorms across inland areas. The overall warmer
and drier conditions today and tomorrow will be short-lived as a
strong vertically stacked low tracks to the eastern Bering Sea
Wednesday night, pushing a front into the western Gulf. An east-
west oriented ridge will build along the north Gulf coast ahead
of this front, increasing the Turnagain Arm winds and bringing
gusty winds to the Anchorage and Matanuska Valley Areas on
Wednesday night. The upper trough over the eastern Bering will
then dig eastward across the Gulf Thursday, causing the front to
re-orient east-west across the central Gulf. This in turn 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. Thus, the strongest winds with this
system (not counting gap winds) will be over the Gulf waters.

Expect moderate to heavy rain with the arrival of the front in
Kodiak Wednesday night. This rain will then spread to the southern
to eastern Kenai Peninsula on Thursday. Based on the aforementioned
change in orientation of the front, precipitation over eastern
Prince William Sound will much lighter. For the rest of
Southcentral, strong low level southeasterly and easterly flow
will lead to downslope drying, so most areas will remain dry or
will see only a brief period of light rain.

&&

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

An active pattern will evolve over the next few days, bringing
multiple rounds of potential convection, with rainfall becoming
more widespread Wednesday into the first half of Thursday, before
a more showery type pattern takes hold. As for convection, look
for the potential to develop during the afternoon and evening the
next three days, with the best chance being east of a Sleetmute to
King Salmon line (minus the Lake Iliamna area due to a lake-
induced stable layer near the lake). Here, similar to yesterday,
models project LI's (lifted indices) of -2 to -3 with 1000-500
MUCAPES (another instability parameter, most unstable convective
available potential energy) of 400 to 900 J/kg today and
Wednesday. However, relatively meager low and mid level lapse
rates may help mitigate this.

Another area or two that may see a few storms develop would be the
Kilbuck Mountains, and the Alaska Peninsula. However, instability
is weaker here, but still sufficient that it can't be ruled out.
This convective potential is more uncertain for Thursday, as how
much airmass recovery (aided by breaks in cloud cover) can occur.

The other item of interest will be a front reaching the coast
Wednesday evening, pushing inland on Thursday while weakening as
frontolysis (the weakening of a front) ensues as it enters an
occlusion phase (essentially where the cold front catches up to
and merges with the warm front). Widespread rainfall will
accompany this feature, some of which may be of moderate intensity
with decent rain rates, due to a semi-tropical connection.

&&

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

This morning's water vapor imagery shows tropical moisture being
pulled up into the western Aleutians, aided by a low to the
southeast of Japan. This has led to fairly decent cloud top
cooling in the infra-red satellite imagery (indicative of decent
rain rates), with Shemya having bounced around between 3/4 of a
mile and 3 miles visibility. This moisture feed aided by warming
just above the surface, will interact with additional shortwave
energy diving into the base of a developing trough, helping to
allow the storm to maintain its current strength being in the mid
980 mb range as it treks eastward, reaching Bristol Bay by early
Thursday morning.

As it does so, a strong front will sweep eastward while
accompanied by a steady rainfall. The system will also begin to
become vertically stacked in the Wednesday through Thursday.
Strong dry advection behind the front and respectable cold air
advection aloft will tighten the pressure gradient enough to allow
sustained gales to develop during this time, mainly along and
south of the east central Aleutians and Alaska Peninsula (AKPEN).
Here, given a long dynamic fetch, waves building into the 15 to 20
feet range are a distinct possibility.

&&

.MARINE (Days 3 through 5:Thursday through Saturday)...

Bering Sea/Aleutians

A low over the eastern Bering on Thu will move onshore over the
AK mainland Fri. Forecast confidence has increased that there
should be Gale force gusts from the Pribilofs south towards the
eastern Aleutians and AKPEN through Fri morning. These winds will
be amplified in the terrain gaps. By Fri afternoon forecast
confidence is moderate that a low will enter the western Bering,
with the primary warm front reaching the central Aleutians on Sat.
Winds should be lighter with some small craft advisory winds
possible over the central Aleutians.

Gulf of Alaska

A front will move east across the Gulf of Alaska through Fri night
with a triple point low forming to the east of Kodiak Island Fri
morning. Confidence is moderate on Fri in regards to a barrier jet
setting up along the Northern Gulf Coast north of the low. This
would bring small craft easterly winds to the coastline.
Otherwise, forecast confidence is good that small craft westerly
winds should persist to the south of Kodiak through Fri night. By
Sat the low should move onshore, which will help keep winds light
and westerly across the Gulf.

&&

.LONG TERM FORECAST (Days 4 through 7:Friday through Monday)...

A broad longwave trough extending from far eastern Siberia to
Southeast Alaska should continue the cool and wet pattern. The
main, or strongest, upper level low should be over Southwest
Alaska on Friday with a front extending into the Gulf of Alaska.
This system should push east through Sunday, bringing rain and
cooler temperatures to the Alaska mainland. Model uncertainty
increases on after Sunday, with the GFS solution indicating strong
ridging in the wake of the aforementioned system while the
ECMWF/Canadian models have a more zonal jetstream. The GFS
solution would bring a return to summer-like weather of warm
temperatures and sunny skies from the Akpen to Southcentral,
while the other solutions would likely keep things cooler and
continue the fall-like pattern. Regardless, guidance is in good
agreement on Mon as the next system arrives in the Western Bering.
The exact weather impacts of this system are still uncertain but
long term guidance does indicate a long, deep moisture fetch
coming into the region from the Central Pacific early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 176.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...SEB/KM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
MARINE/LONG TERM...CJ