Gale Warning 477, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
651 PM AKDT Mon Aug 28 2023

.ANALYSIS AND UPPER LEVELS...

A fairly wide swath of moderate to heavy rain is spreading northward
this afternoon along the eastern flank of the Alaska Range, the Cook
Inlet, and the Kenai Peninsula with moderate rainfall ongoing across
Kodiak Island. Scattered rain showers were also increasing in
coverage across the Prince William Sound and Southcentral Coast, and
inland from the Susitna and Matanuska Valleys eastward to the Copper
River Valley. The shortwave trough driving this current round of
widespread precipitation will continue to lift north towards Kodiak
and the Alaska Peninsula this evening as it remains tapped into a
plume of moisture being drawn up and over broad ridging established
over western Canada.

A closed low over the southern Bering Sea is slowly translating east
this afternoon as the next shortwave trough starts to to take shape
along the southeast quadrant of the parent low as the front pushes
towards the southern end of the Alaska Peninsula this evening. A
corridor of 25 to 30 knot winds are still being realized for
portions of the Central Aleutians and near the center of the
occluded low. Southwest Alaska remains quiet with partly to mostly
cloudy skies and light winds.

&&

.MODEL DISCUSSION...

Models are in good agreement on the synoptic-scale evolution of
the forecast through the short term with a low tracking into
Southwest Alaska tonight and its associated moisture plume. There
is still a bit of uncertainty in terms of the exact rainfall
amounts and the timing of heavier precipitation rates, but models
are in good agreement that flow will shift from a southwest to
southeast orientation between now and Tuesday, focusing higher
rainfall amounts where upslope flow sets up along the Chugach
Range, Kenai Mountains and Alaska Range. The main challenge will
be weak shortwave troughs lifting along the western periphery of a
broad ridge extending from Western Canada. These weak impulses
will produce difficult-to-time intermittent periods of heavier
rain along the north Gulf coast, overspreading past the coastal
mountains to inland areas which are expected to be generally
drier.

Out west, models increasingly agree on the track and intensity of
the remnants of Tropical Storm Damrey as they approach the
Aleutian Islands on Wednesday. While ensembles have shifted to
further favor the stronger, northerly track into the Bering Sea for
ex-Damrey, the likely storm-force low looks to reach peak
intensity over the North Pacific, occluding and beginning a
weakening trend just before it crosses into the eastern Bering
Sea Thursday morning. Additionally, since yesterday, models have
shifted eastward with the track, suggesting reduced risk for
coastal flooding with a much shorter fetch across Bristol Bay,
limiting storm surge potential.

.AVIATION...
PANC...Rain showers and MVFR ceilings with periods of IFR ceilings
will prevail this evening and are expected into early morning
Tuesday. Light northerly winds will switch to southeasterly and
increase late this evening with an expected improvement in
ceilings to MVFR or low VFR Tuesday morning. Ceilings will
decrease again Tuesday afternoon when the next system arrives.

&&

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

Dreary weather continues for much of southcentral Alaska as
rainfall associated with a warm front extending out from a low
near Kodiak Island spreads northward. Anomalously high moisture
content has been observed with this system w/ precipitable water
values above an inch, which is solidly above 90th percentile for
this time of year. The heaviest rainfall will occur across the
higher terrain of the Kenai Peninsula and Prince William Sound,
where 2-4 inches (locally higher) are expected. The Susitna Valley
will also receive heavy rainfall today with the due south flow
aloft moving over the Valley and upsloping along the Alaska Range.
Rainfall will be lighter and generally more intermittent across
the downsloping areas such as the Anchorage Bowl, western Kenai
Peninsula, and the Copper River Basin.

In addition to rainfall, southeasterly gap winds across Turnagain
Arm, Knik River, and Copper River winds are expected to intensify
once again later today and into Tuesday. However, they are not
expected to be nearly as strong as they were Friday and Saturday
of last week.

The low will track north of Kodiak Island into southwest Alaska.
The attendant cold front will bring yet another round of rainfall
to much of southcentral Tuesday night into Wednesday. A brief
break in rainfall is likely in the latter half of Wednesday before
yet another low pressure system approaching southwest Alaska bring
more widespread rain to the area on Thursday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3: This afternoon through Thursday afternoon)...

There will be no shortage of weather in Southwest Alaska and the
Bering Sea/Aleutian Islands in the coming few days.

Through the remainder of today, an upper level shortwave trough
and its attendant surface low will bring in a deep plume of
moisture along the Western Alaska Range. The heaviest rain is
expected along and to the east of a line extending from Lime
Village to about Iliamna Lake. Steady moderate to heavy rain
through early Tuesday afternoon could lead to rain totals as high
as 1-2 inches. As we head through Tuesday, expect steady rain to
become more showery and spread westward across Southwest Alaska.

Another low is currently present near the Central Aleutians, where
it is bringing stronger winds and rain to the Eastern Aleutians.
The low will gradually weaken as it tracks along to Alaska
Peninsula, reaching Bristol Bay by Wednesday morning.

The biggest story are the remnants of what is currently Tropical
Storm Damrey in the West Pacific. As the system moves east across
the North Pacific Ocean, favorable upper level dynamics in the
form of a 160+ kt jet streak and a robust shortwave trough will
allow it to strengthen rapidly as it approaches the Central
Aleutians. Forecast confidence has increased since yesterday with
regards to the magnitude and probable location of strong winds,
high waves, and moderate to heavy rain. Overall, the prognosis
looks better today than it did yesterday. With the low expected to
move at a decent pace and to weaken right before it hits the
Aleutian Islands, hazards look less severe than they did
yesterday. The strongest winds are still expected over the
Aleutian Islands and Alaska Peninsula, though much of Southwest
Alaska will also see elevated winds along the low's front.
Dynamic fetch along the southeast quadrant of the low will help
build waves of about 20-30 feet. This will impact the Pacific
coast of the Alaska Peninsula, which will serve as a barrier to
help keep the strongest waves out of the Bering Sea. The last
hazard is rain amounts - though this system will bring a decent
fetch of moisture, the front should move through pretty quickly.
This will help limit rain totals for all but the Central Aleutians
and Pacific coast of the Alaska Peninsula, where persistent
northerly and southerly flow, respectively, will lead to a longer
duration of rain and greater rain totals. While confidence is
improving in the forecast, there remains some degree of
uncertainty. Please stay tuned as we continue to update the
forecast.

&&

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

Confidence is above average for the long term forecast, as despite
a pattern shift toward more zonal, less amplified flow, models are
in fairly good agreement through the weekend. Beginning Friday,
the extratropical remnants of Tropical Storm Damrey move inland
over Southwest Alaska, and quickly make their way into the
Interior as an upper trough over the Bering Strait works to
eject it north and east. Winds associated with the low diminish
quickly, but a large-scale pressure gradient between a North
Pacific ridge and broad low pressure over Northern Alaska will
continue to drive gusty southwesterly winds across Southwest
Alaska, as well as up Cook Inlet. Various southerly gap winds will
also be active. Westerly gap winds out of Kamishak Bay will
briefly reach gale force Friday night through the Barren Islands
as remnant troughing from ex-Damrey moves into the Gulf of Alaska.
Aside from this, the main impacts from ex-Damrey (strong winds
and coastal flooding potential) are expected to occur Thursday.
These are discussed in previous sections of the short term
forecast.

With the development of a zonal, westerly flow regime, expect
several progressive, quick hitting systems bringing rain to low
elevations, with more prolonged upslope precipitation likely to be
the main hydrologic concern. Southerly flow ends on Friday,
ending the bulk of precipitation for the Prince William Sound
area. The westerly flow that takes over will shift the focus to
the Western Alaska Range and the western slopes of the Talkeetna
Mountains, bringing more rainfall to the basins in the Susitna
Valley which are already dealing with high water levels this week.

A broad low tracks across the northern Bering Sea Friday night,
bringing a large swath of gale force winds as far south as the
Aleutian Chain through Saturday, when the low moves into mainland
Alaska. Precipitation with this low sweeps quickly east from
Southwest Alaska on Saturday into Southcentral for Sunday. This
coincides with the development of a triple point low in the
northern Gulf Sunday morning, drawing another round of westerly
gap winds, potentially as strong as storm force, through the
Barren Islands, aided by cold advection with 850 mb temperatures
dipping below 0 Celsius streaming across the Alaska/Aleutian
Ranges. Winds diminish through Monday as the low exits into the
eastern Gulf. Confidence in the long term forecast decreases by
Monday, though model guidance suggests that the next low is
likely to be approaching the region in the vicinity of the
western Bering Sea/Far East Russia.

&&

SYNOPSIS AND MODEL DISCUSSION...PP/CQ
SOUTHCENTRAL ALASKA...MJB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
LONG TERM...CQ