Gale Warning 603, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 AM AKDT Tue Sep 10 2024

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

Widespread rain is expected for Southcentral through Thursday as a
series of features move through the area. For today and tonight,
two upper level features, one currently over the Norton Sound, and
another over the central Aleutians, will phase over Southcentral.
The flow aloft will be primarily out of the southwest, allowing
for the only downsloping effect to be the Copper River Valley, but
even then the upper level support and marginal instability should
allow for at least some showers by Wednesday morning. In many
ways, we can expect today to be similar to the past couple of
days, with light southerly gap winds and mild, seasonal
temperatures.

On Wednesday, the strongest of the upper level and surface
features currently slated in the broad area that is our forecast
area will enter the western Gulf. There is a moderate level of
uncertainty with the overall track and shape of the upper level
features, but what we do know is that the system will likely track
over southern Kodiak Island Wednesday afternoon, and continue to
strengthen as it moves south of Seward into Thursday morning.
Widespread gales are of moderate to likely chance along the
coastal waters from the Sound to Kodiak Island with this system.
Additionally, widespread rain is expected for coastal areas and
the Kenai Peninsula, but there is some uncertainty for communities
north of Anchorage and into the Copper Valley. This precipitation
forecast uncertainty stems from the difference in the upper level
forcing (vorticity and omegas), with some models continuing to
revolve shortwaves north, and far into Southcentral, while others
keep most of the energy farther to the east. Additionally,
northerly surface winds will likely develop from the Mat-Su south
into the Cook Inlet, which will help induce some low level
drying. All this to say periods of light rain are possible for
the Cook Inlet communities on Thursday, while the Sound enjoys
more steady rain. Moving into Friday, a shortwave ridge looks to
grace our presence for a rain free day, with even the possibility
of sunshine should the fog and stratus be kept at bay.

-Jones

&&

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

Light showers persist across eastern Bristol Bay and the Lower
Kuskokwim Valley interior this morning. Similarly, shortwaves
rotating up and over a weak ridge in the Central and Eastern
Aleutians/AKPen are persisting showers and allowing some areas of
fog to develop over the area this morning. This feature will
continue it's track eastward, moving over the Alaska Peninsula,
merging with the trough still settled over the southern Alaska
Range.

Behind the weak ridge currently over the southwestern Bering and
Western Aleutians, a system is assembling over the area, bringing
light rain. While this system does have a potent upper-level
trough supporting it's development, the development seen at the
surface is not expected to be very impressive, with even the
latest model runs backing off on the projected strength of this
system as it moves eastward. As suggested, this surface low will
likely begin to shift eastward today, reaching the Central
Aleutians and Alaska Peninsula by early this evening. An initial
round of rain showers is expected with this system, followed by a
moderate push (or pull) of gusty northerly winds winds down into
the Eastern Aleutians and Alaska Peninsula by tonight and into
Wednesday morning. Gusts from this are expected to reach up to 45
mph through the gaps and passes of the Alaska Peninsula and nearby
Aleutians. This forcing surface low will likely spread across
mainland Southwest, focused in the Bristol Bay area, through the
day Wednesday.

Following this is another series of stop-and-go weather with a
weak ridge expected to build in on the heels of the aforementioned
surface low, with yet another frontal boundary to move in beyond
that, bringing another (seemingly more transient) round of
rainfall to the western Aleutians by late Wednesday reaching the
Central Aleutians by early Thursday morning.

-CL

&&

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

A strong upper low will move from the western Gulf to near Prince
William Sound on Wednesday and Thursday, setting the stage for
ongoing showery conditions across Southcentral to start the long
term forecast period on Friday. Model guidance is in good
agreement with the general placement of the Gulf low and broader
trough. Above normal precipitation chances will exist along with
above normal temperatures for Southcentral through much of the
period. A short-lived ridge of high pressure is forecast to
briefly overspread Southcentral on Friday, which may see a short
reprieve in precipitation. Otherwise, above normal precipitation
chances are expected to persist for Southcentral through Monday.

For Southwest Alaska, diminishing precipitation chances will
start the long term period through early Friday, as the region
sits on the backside of a departing upper low to the east. Friday
afternoon may present a break in precipitation across Southwest
before the arrival of the next Bering front. The main weather
feature for the Bering, Aleutians, and Southwest Alaska through
the long term period will be a substantial Kamchatka low. While
the low itself will remain well to the north, it is forecast to
bring a strong north-to-south oriented front across the Bering and
Aleutians, and into Southwest Alaska by Sunday. The front should
be accompanied by widespread showers and gusty conditions. The
steady progression of the the front will allow for improving
conditions by Monday evening as weak high pressure returns to the
Bering.

&&

.AVIATION...

PANC...MVFR conditions are possible late this morning or early
this afternoon as rain develops over northern Cook Inlet and
moves over the terminal. These will become more likely as rain
intensifies overnight. Southwesterly winds will continue to be on
the gusty side through the forecast period.

&&