NWS Anchorage, AK wrote its last area forecast discussion before the first event at Aug 9, 4:31 PM AKST, 12 hours 17 minutes earlier.
Area forecast discussion, Aug 9, 4:31 PM AKST, 12 hours 17 minutes before
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
531 PM AKDT Fri Aug 9 2024
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
This afternoon through Monday afternoon)...
Southcentral Alaska saw some much needed sunshine today as the
atmospheric river responsible for this week's rain exited the
region. Get out and enjoy the sun while you can, because another
round of moderate to heavy rain and strong winds is forecast for
this weekend. Here are the hazard we're currently monitoring:
- Flood Warning for the Lake Creek, Skwentna, and Yentna River
Basins, in effect through tomorrow afternoon
- Flood Watch for Eastern Kenai Peninsula/Western Prince William
Sound, in effect from Saturday morning through Sunday evening
- Flood Watch for Eastern Prince William Sound, in effect from
Saturday morning through late Sunday night
- Strong winds in Anchorage and Matanuska Valley from about Saturday
afternoon through Sunday morning could lead to downed trees and
powerlines due to saturated soils.
Diving into the details... for the remainder of today, expect rain
showers along the foothills of area mountains (including the
Alaska Range, Talkeetna Mountains, and Western Chugach range) as
upper level shortwaves move through. With decent daytime heating
under clearer skies, there is a slight chance of thunderstorms
along Western Susitna Valley and the Copper River Valley this
afternoon and evening.
By tomorrow, much stormier weather is on the way as a
strengthening Bering Sea low pushes its front into Southcentral
Alaska. Models have come into better alignment since the previous
forecast package, lending greater confidence to the forecast.
Let's start first with the rain forecast. This storm system will
be moving through our area much faster than our recent atmospheric
river, leading to lower overall rain amounts. However, favorable
dynamics and abundant moisture will still lead to periods of
moderate to heavy rain. Eastern Kenai Peninsula and Prince William
Sound will be the locations to watch, both because southwesterly
flow leads to favorable upsloping, and because the development of
a triple point low will lead to a longer duration of heavier
rainfall at these locations. Confidence is very high that
communities in these locations will see at least an inch of
rainfall, with locations like Whittier, Tatitlek, and Cordova
likely seeing over 2 inches. Of lower confidence are rain amounts
for favored downslope areas in Matanuska Valley, Anchorage, and
Western Kenai Peninsula. Strong winds aloft will very likely
reduce rain amounts, but whether they will be strong enough to shut
off rain altogether is something we'll be monitoring in upcoming
forecast packages.
Along with the rain, expect strong winds with this storm system.
The strongest winds will be at higher elevation in the mountains
directly bordering the northern Gulf and Prince William Sound.
However, much of Eastern Kenai Peninsula, Anchorage, Matanuska
Valley, and Copper River Valley will see their fair share of gusty
winds as well. Of note will be areas that tend to see strong
southerly to southeasterly gap winds: Turnagain Arm and into
Anchorage, out of Eagle River, through Knik River Valley into
Palmer, and Copper River Valley. With the recent rainfall and more
on the way, this could lead to downed trees and powerlines. Be
sure to also use caution if recreating or traveling at higher
elevations as this is where winds will be strongest. And if you've
made it this far, here's a treat: colder air moving in aloft, in
combination with incoming moisture, will likely lead to a fresh
dusting of snow on our mountains.
-KC
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3)...
Moderate to heavy rainfall with gusty winds is expected with a
weak system currently located in the southern Bering and a
developing low merging with it and strengthening near the Pribilof
Islands early Saturday morning. This low will deepens as it
progresses into the eastern Bering. Rain picks up in intensity
across Southwest through the day on Saturday and works into
interior Southwest through Saturday afternoon and evening. By
Saturday evening, the storm will be in the vicinity of Kuskokwim
Bay and will move inland through the day Sunday. Steady rain will
begin to taper off from west to east through Sunday with interior
locations the last to see steady rain become more showery.
In addition to the heavy rain threat, gusty southeasterly winds
will also occur through the gaps, and bays/passes of the southern
AKPEN, especially Friday evening and Saturday morning ahead of the
storm center. Winds will eventually turn westerly Saturday
afternoon as the storm passes to the north and east in the Bering
Sea. As the storm moves inland late Saturday night into Sunday, a
corridor of gale-force westerly winds looks to develop in Bristol
Bay during this time. These winds will slowly diminish back to
small-craft late Sunday morning into the afternoon. Wave heights
also look to build Sunday morning to around 9 to 12 feet in the
open waters of Bristol Bay with heights of around 7 to 10 feet
towards the Bristol Bay coast. Waves begin to subside through the
afternoon and evening. With that, a Special Marine Weather
Statement has been issued to address the wind and wave height
concerns Saturday night and Sunday with this system across the
Bristol Bay marine zones.
As this powerful fall-like system continues moving inland Sunday,
impacting Southcentral Alaska, another shortwave looks to drop
southward from Siberia and into the Kuskokwim Delta and Bristol
Bay Sunday evening through Monday. More light to moderate rain and
small-craft marine winds are anticipated with this system Sunday
evening into the day on Monday. Meanwhile, out across most of the
Bering Sea and the Aleutian Chain, a ridge builds in behind the
storm system starting Saturday morning across the western
Aleutians and western Bering. This ridge migrates to the eastern
Aleutian Chain through Monday as a weak system passes to the south
of the Chain in the North Pacific through Tuesday. Another trough
looks to reach the western Aleutians late Monday and to the
central Aleutians for Tuesday. A ridge will build over Southwest
Alaska for Tuesday.
&&
.LONG TERM FORECAST (Days 4 through 7/Tuesday through Friday)...
A closed upper level Arctic low extends a longwave trough through
Eastern Mainland Alaska Tuesday. This trough continues across the
Northern Canadian Provinces through the forecast period. An
Eastern Bering ridge gains amplitude while moving into the Eastern
Mainland. The main focus is on a second longwave trough that sets
up over in the Russian Far East, then deepens as it moves into
the Central Bering through Friday. A low center forms in the
Western Bering for Thursday, and is nearly stationary through
Friday. A number of shortwaves also rotate through the pattern
throughout the forecast. The ensemble means continue in good
agreement in positions and strengths all week.
Heaviest rainfall exits the Southcentral coastal zones from the
Copper River Basin to the Canadian border, with lesser amounts
across the Interior of Southcentral Alaska to the Alaska Range
through Tuesday. There is a decaying front moving along the
Central Aleutians into the North Pacific through Tuesday. A new
low and front develops in the far West, spreading heavier rains
and increasing winds over the Western Aleutians and Bering for
Tuesday. This low will deepen and its front will spread from the
Chukchi Sea across St Matthew Island and the Pribilofs into
Western Alaska and over the Central and Eastern Aleutians by
Thursday. Winds increase to low-end gale force across the Eastern
Bering and Bristol Bay late Wednesday through Thursday, with
higher gusts out of bay and passes along the Eastern Bering and
Alaska Peninsula.
- Kutz
&&
.AVIATION...
PANC...VFR conditions expected to continue as lower cloudcover
continues to scour out this afternoon. Mid and upper between 10-20
kft will increase late this afternoon into the evening as another
upper wave pushes through. May see a few scattered showers develop
in the vicinity of the terminal, but most showery activity will
remain tied to the higher terrain. A more potent system will allow
for rainfall to spread northwards up the Cook Inlet Saturday
morning...nearing the terminal by 21Z. A strengthening gradient
over the Chugach and Kenai Mountains will lead to increasing
cross-barrier flow and a stout Turnagain Arm wind Saturday night
through Sunday morning.
&&