Gale Warning 504, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
427 AM AKDT Tue Sep 6 2022

.ANALYSIS AND UPPER LEVELS...

A broad longwave trough with multiple embedded shortwaves exists
from the Kamchatka Peninsula to Southeast Alaska. The primary
upper level features of note are an occluded low moving south into
the Bering Sea from the Gulf of Anadyr, a narrow shortwave ridge
over central and northern Alaska, a shortwave trough and surface
low over the southeastern Gulf, and a developing low south of the
Aleutian Chain. No significant areas of clear skies can be
observed on satellite this morning other than over the northern
Gulf coast, offshore areas of the Gulf of Alaska, and portions of
the Copper River Basin. With ample moisture from recent rains,
most stations are observing humidities above 85 percent, though
high levels of cloud cover have prevented fog from developing in
most areas.

A few light showers can be observed on the Kenai Radar this
morning, primarily over the western faces of the Chugach and Kenai
Mountains, as well as the western and northern Susitna Valley. A
very weak, and nearly stationary shortwave over the Kenai
Peninsula is capitalizing on some weak instability to produce
these showers. Steering flow is generally out of the south between
5 and 10 kts, resulting in slow moving showers from Anchorage
north to the Mat-Su Valleys.

&&

.MODEL DISCUSSION...

Overall model consensus over the next two days is good. Minor
differences in timing and placement of shower activity over
Southcentral remains a challenge. More significant and impactful
model differences arise late Wednesday into Thursday as a
currently developing low south of the Aleutian Chain merges with
the existing trough over the Bering and amplifies the overall
synoptic pattern. The low track and its deviations within the
models will impact the core of heaviest precipitation with the
upcoming atmospheric river event on Thursday. East-west
deviations and overall placement of the shortwave will impact the
wind field in the mid-levels, which could enhance or diminish
rainfall totals in the Seward area. A farther east solution would
allow for a shorter period of heavy rain for the Kenai Peninsula,
whereas a western solution may prolong the event. The surface low
track will also dictate the potential for strong winds through the
Turnagain Arm, and the timing of the low passage north of ~60N
will dictate the timing and strength for potentially elevated
winds over the Anchorage Hillside. Currently the track would keep
winds over the arm when the core of the strongest winds are
expected, but this may change depending on the low track.

&&

.AVIATION...

PANC...Light southerly winds will persist through most of the day
with a light Turnagain Arm wind developing later in the afternoon.
Showers are expected before noon, which may temporarily lower
ceilings to MVFR conditions, but overall VFR conditions are
expected over the next 24 hours. MVFR ceilings will likely become
dominant Wednesday morning as stratus moves up the inlet.

&&

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

A couple of short-waves sitting over Southcentral will exit this
morning, with a short-wave ridge moving in behind. However, this
ridge will largely be a non-player, as a long-wave trough over the
Bering Sea and Southwest Alaska arrives this afternoon. Broad
cyclonic flow with weak embedded short-waves will combine with
existing atmospheric instability to cause an overall uptick in
shower activity, especially from the western Kenai Peninsula to
Anchorage and the Mat-Su. Showers will largely diminish tonight,
but linger in the Susitna Valley in upslope flow.

The precipitation forecast for Wednesday is a bit more challenging.
The Bering trough will amplify, with a steady supply of weak
short-waves and abundant cloud cover headed for Southcentral.
The unorganized/unfocused nature of the vertical motion combined
with a marginally more stable airmass makes it hard to identify
the best location for showers. Generally expect increasing showers
later in the day as stronger forcing arrives, especially along
the Gulf coast.

More significantly, the digging trough will lead to strengthening
of a surface low crossing the North Pacific, with a deep low
headed northward across the Alaska Peninsula and into Bristol Bay
Wednesday night/Thursday. A strong meridional jet streak and
atmospheric river will form on the leading edge of the trough and
become pointed directly toward Southcentral by Thursday. While
there is quite a bit of uncertainty in the track and depth of the
surface low out west, there is high confidence that this storm
system will produce heavy precipitation across much of the region
Wednesday night through Thursday night. The Gulf coast/Prince
William Sound region will see the heaviest rain, with multiple
inches of rain likely. This could exacerbate flooding being
observed along portions of Kenai Lake/Kenai River from the Snow
Glacier-Dammed Lake release and introduce the risk of minor
flooding to other areas bordering Prince William Sound. Will
continue to evaluate this threat as details of the storm evolution
become clearer.

In addition to the heavy rain, also expect fairly strong winds
across the Gulf waters and through mountain gaps in Southcentral.
The low track and depth will determine whether some of the
stronger gap winds move into the population centers. This pattern
is particularly favorable for strong winds along Turnagain Arm,
so there is potential for "fall-like" winds along the Anchorage
Hillside. The stronger the low, the slower the northward
progression into Southwest Alaska would be. In this scenario, the
strongest winds would remain in Turnagain Arm. If the lower is a
bit weaker and takes a faster northward track, winds would be more
likely to impact Anchorage. At this point in time, the timing of
strongest winds looks to be late Wednesday night through Thursday
morning.

&&

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

The longwave pattern features a trough over the Bering along with
ridging over the Central Pacific. On satellite imagery (CIMSS
MIMIC PW) there is a deep fetch of moisture from Adak to the
Mariana Islands. In this plume of moisture, satellite imagery
detected relative humidity values nearly 300 percent of normal
just south of the chain. The models show the upper level trough
deepening with a 140 knot jet core moving over the eastern
Aleutians Wednesday morning and a round of cyclogenesis. By
Thursday morning there will be a 995mb low over Bristol Bay. This
low will be coincident with a widespread swath of rain. Locations
such as Dillingham and Levelock will get over 1 inch of rain with
this system while portions of the Alaska Peninsula may get upwards
of two inches of new rainfall.

&&

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

The longwave pattern features a trough over the Bering along with
ridging over the Central Pacific. On satellite imagery (CIMSS
MIMIC PW) there is a deep fetch of moisture from Adak to the
Mariana Islands. In this plume of moisture, satellite imagery
detected relative humidity values nearly 300 percent of normal
just southwest of Adak. The models show the upper level trough
deepening with a 140 knot jet core moving over the eastern
Aleutians Wednesday morning and a round of cyclogenesis. By
Thursday morning there will be a 995mb low over Bristol Bay. This
low will be coincident with a widespread swath of rain. Also of
interest there is a second distinct plume of moisture associated
with Typhoon Hinnamnor which is west of Japan this morning. This
system will move inland and then the remnants of Hinnamnor will
move into the Western Bering on Thursday.

&&

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

Gulf of Alaska: A low tracking into the southeastern Bering will
move southwesterly flow along a front into the Gulf Thursday. Gale
force winds are expected through the western Gulf, across the
Barren Islands, and into Kamishak Gap. Southerly gales could point
towards Prince William Sound and the Gulf coast by Thursday
afternoon. The southeasterly flow through this area could spill
into Turnagain Arm. Winds may diminish some, but overall forecast
confidence is moderately low through this time frame. Seas in the
Gulf build Thursday through Friday up to 15 feet, then gradually
subside to around 10 feet or less Saturday.

Aleutians/Bering: A low will track into the southeast Bering
Thursday. While a widespread area of small craft winds is expected
around the low, there is a possibly for gale force winds. By
Friday afternoon, the low is expecting to track inland and the
potential for gales moves towards the Southwest Alaska coast.
Winds could also approach gales along the Aleutians Thursday
afternoon with another trough moving into the western Bering. Seas
build Thursday night over the western Aleutians and southwestern
Bering from 10 to 13 feet and spread east along the entire
Aleutian chain by Friday night. Forecast confidence decreases
during this time frame, but as a low develops in the western
Bering by Saturday morning, it could prolong the potential for
gales. Seas with this late week low would continue to build from
10 to 15 feet.

&&

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

Model agreement is fair at the synoptic scale on Friday, though
it deteriorates rapidly as we head through the weekend and into
Monday. Several systems are expected to impact our forecast area
through the extended period. The first low system will be over the
Y-K Delta by early Friday. Given the large fetch of tropical
moisture along its front, the primary threat with this low will be
long duration heavy rain along Prince William Sound. For now,
there is consensus among the deterministic models and model
ensembles that the heaviest rain will fall late Thursday night
into early Friday morning, with rain tapering off around Friday
afternoon. Otherwise, rain is expected to be widespread over much
of the Southern Mainland through Friday as several upper level
shortwaves move across the region. Winds will also be elevated
with this system; colder air wrapping around the backside of the
low will lead to gusty westerly to southwesterly winds along the
Southwest Alaska coast and Western Gulf. The southwesterly
orientation of winds also favors breezier conditions up Cook Inlet
and over Anchorage.

The next surface low will be a weaker one. The center itself will
linger near Russia, though a surface trough looks to extend over
the Pribilof Islands. Lower pressure over the Bering Sea, in
conjunction with higher pressure in the North Pacific Ocean,
should keep westerly winds elevated around the Aleutian Chain
through Saturday morning.

The final system, and one that could be rather impactful,
approaches the Western Aleutians and Western Bering Friday night
before tracking eastward through Monday. Confidence is low at this
time due to model disagreement regarding the strength and track
of this low. However, models generally agree that the low will
reach Southwest Alaska by Sunday, that wind speeds will be small
crafts or greater, and that there will be widespread rain across
the Southern Mainland on Sunday and Monday. The GFS is a bit of an
outlier among the global models as it hints at a deeper low, with
winds at gale force and greater rainfall intensity as the front
tracks along the Aleutians.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CJ
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...KC