899
FXAK68 PAFC 111312
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
512 AM AKDT Wed Sep 11 2024

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

Three shortwave troughs moving east over the area will effect the
weather over the next 48 hours. The light to moderate rain
currently over Southcentral this morning is being generated by a
shortwave currently moving east through the Interior.
South to southwesterly winds are causing rain to favor the western
and southwestern faces of the region`s mountain ranges. However,
this will change starting this afternoon as two troughs over the
Bering begin to shift east and alter the current weather regime.

The stronger of these two features is a deep upper low and
strengthening surface low currently near Unalaska. As this trough
moves east this afternoon it will take on a negative tilt as it
phases with an upper level shortwave currently near St. Lawrence
Island. This will strengthen the surface low as it gets split by
the Alaska Peninsula this afternoon, before eventually
consolidating east of the Barren Islands Thursday morning. There
has been, and continues to be, differences as to where the low
center will be. For most aspects of the forecast this wont matter,
but for the marine areas south of the Kenai Peninsula, it will
play a large role in the length that gales are present.
Additionally, some guidance brings the front into the Sound, which
would result in gales, but given the track of the low and the
shape of the upper level low and no strong northward progression,
this forecast package is leaning away from this. Moving into
Friday, models are in relatively good agreement that the low will
slowly dissipate as it moves towards Yakutat, and some ridging
builds in ahead of another broad trough in the Bering.

For the precipitation forecast from this afternoon through Friday
morning, expect periods of light to moderate rain for most areas.
Due to the phasing of the two troughs, it`s quite possible that
the downsloping and low level drying expected this afternoon and
through Thursday morning may not be enough to keep the rain at
bay. Additionally, it looks like deformation may set up Thursday
into Friday, keeping the light rain around for a bit longer.
However, some riding and a brief period of dry conditions remains
slated from Friday evening to sometime on Saturday. Large amounts
of uncertainty remain with the potentially large system moving
into western Alaska on Saturday, which may send it`s front into
Southcentral sometime late Saturday or Sunday.

-CJ

&&


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

Yet another round of rainfall is expected for Southwest Alaska
today as a pronounced wave moves through the area into tonight.
The system is also showing more of a northward trajectory, which
will likely increase the rainfall to be expected across the
Kuskokwim Delta and Lower Kuskokwim Valley, with the highest
rainfall signal remaining over Bristol Bay peaking in the early
afternoon. Steady rain becomes more showery in nature for Thursday
as the storm moves to the central Gulf and occludes.

Southerly to southeast gap winds this afternoon and evening
through the eastern Aleutians and AKPEN will turn northwesterly
through the day today as the storm continues eastward. Pressure
rises and a little cold air advection behind the storm will
promote northwesterly gales through the gaps and passes of the
eastern Aleutians and AKPEN. There could be gusts to storm- force
at times through the favored gaps and passes of those areas
today and this evening. Embedded in the system driving the
elevated winds, isolated lighting-producing thunderstorms are
possible over the marine areas of Bristol Bay this afternoon.

Following this Bristol Bay system, a weak ridge is building across
the Central Bering which will allow for a brief reprieve from the
active pattern. However, behind this ridge, a front from a
Kamchatka low will deliver a round of rain and gusty southerly
winds to the western Bering and western Aleutians today. This
front will weaken as it travels eastward across the central
Aleutians Thursday and the Pribilof Islands Thursday night.
Isolated showers will continue across the higher terrain of
mainland southwest for Friday while most of the valleys should
remain dry. Another Kamchatka front looks to move into the Bering
Friday with more wind and rain across the western and central
Aleutians. This front looks to have relatively more moisture than
the recent system. Therefore, the rain from the Thursday/Friday
system looks to be a little more widespread across the Aleutians
and Pribilof Islands.

Following this, there is yet another low pressure system aiming to
push into the Central Bering Sea by early Friday afternoon from
the Northern Pacific Ocean. This system initially appears to be
much more organized, with a more well-defined moisture tap, and
structure that will allow for heightened winds. Details on timing
and intensity regarding this system are still coming together, but
current projections show a favorable onshore wind direction, wave
action, and rainfall that may lead to minor coastal flooding
along the Southwest Coastline.

-CL

&&

.LONG TERM FORECAST (Days 4 through 7: Saturday through
Tuesday)...

An upper level low will move out of the Northern Gulf early
Friday morning which will allow a ridge of high pressure to
briefly build into the Gulf and South Central Alaska. This will
provide a period of drier air and reduced chances of precipitation
across the region through early Saturday before the next frontal
system moves across the area over the weekend.


For Friday and Saturday, a consensus of model guidance bring a
strong upper level low and associated fronts into the Eastern
Bering Sea on Saturday. The front will continue into the South
Central and Gulf region over the weekend. Models diverge on the
track going into Sunday and Monday. The GFS brings the center of
the low farther south but is not consistent with the Canadian or
European models. More weight is being given to the more northern
track which could produce strong prolonged southwesterly onshore
winds across the southwestern coast of the state, allowing for the
potential of minor coastal flooding and high seas across the
Kuskokwim Delta and low-lying coastal communities through early
Monday.


A North Pacific low will approach the Western Aleutians late on
Monday. It is uncertain whether this system will enter the Bering
Sea or remain south of the islands. Regardless of the eventual
track, the potential for strong northerly gap winds is not out of
the question for much of the Western and Central Aleutian
Islands.

-DD

&&

.AVIATION...

PANC...A weak Turnagain Arm wind will likely remain over the
terminal through ~18Z with occasional gusts in the 15 to 20 kt
range possible. Light rain and increased moisture this morning
will likely bring ceilings to MVFR through the early afternoon
when the winds in the midlevels switch direction to southeasterly
and produce some drying, returning conditions to VFR.
Intermittent light rain is expected from this afternoon through
Thursday.

&&


$$