996
FXAK68 PAFC 081306
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
506 AM AKDT Tue Oct 8 2024

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

The low that has been in the western Gulf the last couple days has
shifted to the eastern Gulf, with showers associated with the low
now out over the Gulf waters. The story for Southcentral has been
weak features with weak and confused steering flow. This is
exemplified in the 12Z Anchorage sounding, with less than 10 kts
of wind through the first 35,000 feet of the atmosphere. At the
moment mesoscale ridging is dominating, leading to mostly dry
conditions across Southcentral this morning, with just a few
isolated showers noted in radar imagery and surface observations.

Looking upstream, a neutrally tilted short-wave trough is marching
eastward across western Alaska. The southern portion of the trough
is gradually being overtaken by anticyclonic flow on the leading
edge of a short-wave trough and an associated polar jet streak.
As the weakening trough approaches Southcentral, the trough will
begin to lift northward as it reaches aforementioned meso high
over Southcentral. The best forcing will stay north and west of
the Alaska Range, with a weak trough passage for the Cook Inlet
region up to the Susitna Valley. The best chance of seeing some
light precipitation with this will be the northern Susitna
Valley/Broad Pass area late this afternoon through tonight, with
some light wet snow accumulation possible in Broad Pass. Cold air
advection will lead to an uptick in winds for Kodiak Island and
the western Gulf coastal waters, primarily through bays and
passes.

The forecast becomes a lot more interesting as we continue through
mid week. A series of short-waves will then cross the northern
Bering  and dig down the front side of an amplifying upstream
ridge in the Bering Sea. This will lead to consolidation of the
short-waves into a much larger and stronger negatively tilted
trough over the northwestern Gulf on Wednesday. A weak surface low
will form ahead of the trough along the north Gulf coast, with
the low and associated moisture and low level instability being
pulled northwest across the Kenai Peninsula toward Anchorage and
the Mat- Su as the trough lifts northward across Southcentral
Wednesday night through Thursday. This could lead to locally
heavier bands of precipitation. Meanwhile, the airmass will have
cooled a little bit with weak cold advection behind the previous
trough, leading to lowering snow levels. If you then introduce
steady (and potentially locally moderate to heavy) precipitation
into this airmass, the end result would be snow levels dropping
to near sea level. Model guidance all shows the same general idea,
with variations in the movement of individual vorticity maxima
about the trough axis, leading to differences in precipitation
amounts and distribution. This does lower forecast confidence,
with potential for a single stronger vort-max to affect the larger
trough track/position and shift location of heaviest
precipitation. Have trended toward a wetter forecast from the
northern Kenai Peninsula to Anchorage and the southern Mat-Su.
Have also cooled temperatures and depicted either a mix of rain
and snow or a change to snow for lower elevations late Wednesday
night through early Thursday morning. Precipitation intensity will
ultimately be the key to whether snow accumulation is observed at
sea level.

As the upper trough exits northward on Thursday, a major storm
system will be taking shape in the Bering Sea. The set-up looks
quite dynamic, with a rapidly amplifying pattern supported by a
170 kt upper level jet, and development of one or more very deep
surface lows. Stay tuned to forecast updates as this type of
pattern has good potential to produce widespread high winds and
heavy rain - especially for Kodiak Island and the Gulf coast -
beginning as early as Thursday night for Kodiak Island and the
western Gulf Thursday night.

-SEB

&&


.SHORT TERM FORECAST SOUTHWEST ALASKA, AND THE BERING
SEA/ALEUTIANS (Today through Friday morning)...

The forecast for the short term continues to be somewhat shambolic
in nature as we are in transitory period from one major low
pressure system to the next. Currently, low amplitude ridging over
the Bering Sea continues to flatten. However, cold northerly flow
also continues this morning into the Southwest mainland and
Alaska Peninsula (AKPEN). Due to this cold air advection, a
continuation of gusty northwesterly gap winds are expected across
the AKPEN through this morning before the pressure gradient
begins to relax later today and gap winds slowly diminish. As
this cold air moves across a relatively warmer sea surface, steep
lapse rates will result in an unstable atmosphere, giving us
precipitation more convective in nature and scattered showers
across the AKPEN through today; especially along the Bering Sea
side of the AKPEN. Expect less coverage of showers today compared
to yesterday.

This afternoon and evening, a weak front from Kamchatka looks to
come ashore of the Kuskokwim Delta and perhaps northwestern
portions of Bristol Bay by Wednesday morning. With cold air
initially in place, portions of the Kuskokwim Delta and
northwestern Bristol Bay could see a brief period of snow or rain
snow/mix early Wednesday morning before precipitation fizzles out
Wednesday afternoon.

After this weak front fizzles out Wednesday afternoon over the
Southwestern mainland, our attention quickly turns to a powerful
low that is set to strengthen in the central Bering by Thursday
morning. Moderate rain is expected to accompany gale force winds
for the western and central Aleutians on Thursday. By Friday
morning, gusts from 50-75 mph are possible for Bristol Bay and the
Alaska Peninsula. Confidence is increasing with regards to track.
Global guidance and ensemble systems project the storm tracking
from central Bering Sea southeastward to around the Eastern
Aleutians/Southern AKPEN by Thursday evening. The system then
transfers it energy across the AKPEN and into the North Pacific
Thursday evening where the low meanders and remains about nearly
stationary through Friday morning as it continues to strengthen.
Stay tuned as we continue to iron out the finer details of this
developing system.

&&

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

A very strong, large and dynamic low pressure system will be in
the midst of some changes near the start of the outlook period
early on Friday. This large area of low pressure centered over
the eastern Bering Sea will be embedded within a deep longwave
trough stretching from the Beaufort Sea down to the Aleutians and
parts of the North Pacific. Just about the entire outlook area
will be feeling impacts from this system in some way by this
point, with an impressively widespread area of strong winds, rain
and mountain snow encompassing the low and its frontal structure
well away from the center. A new surface low will likely be
developing along the low`s occluded front stretching from the old,
occluded center in the Bering across the AKPen and out over the
western Gulf early on Friday as well. This new low is expected to
quickly consolidate and deepen rapidly south of Kodiak Island and
the AKPen throughout the day on Friday.

Some notable disagreement between global deterministic solutions
is now appearing for the exact timing and location for the
development of the aforementioned secondary low. The latest GFS
and ECMWF project the low developing somewhere between Chignik
and the southern tip of Kodiak Island, while the Canadian NH model
shows the low forming farther out along the front and maturing
over the southern Gulf. However, there is still broader agreement
that this will remain a large and powerful low pressure system
through much of the weekend as the secondary center takes over as
the primary low and then slowly begins to weaken over the central
Gulf from Saturday to Sunday.

One of the more notable concerns with this low will be the very
broad wind field extending quite a ways out from the center,
particularly as the center south of the AKPen is deepening to peak
intensity (likely below 960 mb) on Friday. Gale to storm force
sustained winds could affect a large portion of the Bering Sea,
AKPen and Aleutians on the western side of the low as cold air
originating from the Arctic spills southward and begins to wrap up
into the system as it slowly drifts eastwards from the end of the
week into this weekend. Meanwhile, a corridor of very strong winds
will also move into the Gulf on Friday, particularly near and
along the low`s front as it lifts north throughout the day.
Easterly gusts to Hurricane force could very well materialize
along this corridor, possibly affecting parts of Kodiak Island on
Friday morning as the front moves past, then shifting up towards
the far southern end of the Kenai Peninsula and much of the Gulf
Coast later in the day into Friday night as the front continues to
lift north. The front should rapidly weaken as it approaches the
coast, so this could limit the potential for higher end
winds/gusts to extend farther north into southern parts of
Southcentral past the immediate coastline.

By Monday, the low will continue to weaken considerably as it
slowly fills in and stalls out over the central Gulf. Meanwhile,
another low with tropical origins will fall short of reaching the
Bering Sea as it pulls north and then curves east before it is
able to reach the western Aleutian Chain. This system will bring
minimal impacts, with easterly winds and rain spreading over the
central/eastern Aleutians north of the low from Sunday into
Monday.

-AS

&&

.AVIATION...

PANC...VFR conditions and light winds will persist.


&&


$$