000
FXAK68 PAFC 030036
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
436 PM AKDT Mon Oct 2 2023

.ANALYSIS AND UPPER LEVELS...

An upper-level trough continues to track eastward across Alaska,
with its attendant cold front now bringing colder air into
southcentral Alaska. A weak surface low remains ahead of the main
trough, with its center over the eastern Prince William Sound.
Precipitation continues to spread across portions of southcentral
Alaska, primarily east of the Cook Inlet. Snow levels have fallen
to around 2000ft behind the push of colder air and remain above
3000ft in the Wrangells and eastern Chugach out ahead of the cold
front. Further west over the Bering Sea and Aleutians, benign
weather conditions prevail under the influence of a high pressure
ridge. The pressure gradient between the aforementioned trough and
ridge are promoting gusty northwesterly gap winds along the
Alaska Peninsula and through the Barren Islands.

&&

.MODEL DISCUSSION...

Models remain in good agreement with the overall synoptic pattern
over the next couple days. Model spread has decreased with the upper-
level low and surface front that enters the western Bering and
Aleutians starting Tuesday. This will bring a brief period of gale
force winds to the western Islands. Another low deepens as it enters
the AK Peninsula and eastern Aleutians Wednesday into Thursday.
Model spread remains large with regards to the depth and placement
of this low by the end of the week, ultimately affecting
precipitation placement and intensity.

&&

.AVIATION...
PANC...VFR will prevail for the TAF period as rain continues to
taper off and east of the terminal as drier air moves in behind a
trough moving through this afternoon. Clouds will start to scatter
out this afternoon/evening, and winds are expected to intensify
out of the north before subsiding in the morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3 This
afternoon through Thursday night)...

The arctic trough and associated cold front will continue to move
eastward this afternoon and tonight. The trough axis is just about
over the Anchorage area as of 4PM this afternoon. As the best
lifting for precipitation moves east, conditions will begin to dry
and clear out this evening. Meanwhile, the Copper River Basin
remains in the favorable position for precipitation, east of the
trough axis until late tonight/early Tuesday morning. Temperatures
continue to be cold enough to support snow in the northern Copper
River Basin where a Winter Weather Advisory remains in effect
until 4AM Tuesday morning. Up to an additional 2 inches of
accumulation is possible from Paxon northward to Isabel Pass, as
well as along the Glenn Highway and Tok Cutoff. As cold air
advection continues, so will gusty northerly winds out of Isabel
Pass; this could cause blowing snow for Paxon and areas northward.
Precipitation will end from west to east through Tuesday morning.
The rest of Tuesday and most of Wednesday will feature building
pressure heights and mostly quiet conditions over Southcentral.
However, portions of the northern Susitna Valley could get clipped
Tuesday by a shortwave dropping southward out of Siberia; most of
the energy associated with this shortwave should remain north and
west of the area though.

The weather pattern becomes active again for the second half of
Wednesday as a North Pacific low pushes its gale-force front
northward over Kodiak Island initially. Gusty winds and moderate
rain with pockets of heavier precipitation overspread the island
Wednesday afternoon. The front will continue to lift northward and
reach the northern Gulf Coast Thursday morning. In doing so, an
area of northeasterly storm-force winds will be possible in the
vicinity of the Barren Islands Thursday morning, with easterly and
northeasterly gales elsewhere along the north Gulf coastal
waters. Deep southerly flow and warmer air will accompany this
front. Confidence is high that coastal locations will see mostly
rain with this event. However, there might be enough cold air in
place in interior locations such as the Copper River Basin and
Susitna Valley Thursday morning that precipitation could start as
snow before mixing with and changing over to rain mid-day
Thursday. There are still questions regarding precipitation-type
as well as precipitation totals that will not be answered until
the event draws a little closer. Therefore, forecast confidence
for Thursday is not as high. Please stay tuned.

&&

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

Key Messages:

-High Chance of Gale force winds for the western and central
Aleutians tonight through Tuesday night

-Medium Chance of Storm force winds for the western Bering tonight
through Tuesday night

-Medium Chance of Gale force winds for the AKPen & Bristol Bay area
on Wednesday

Discussion:

For Southwest AK, a weakening ridge and associated surface high
should continue to keep things relatively dry and clear across much
of the Southwest AK through Tuesday night. Under clear skies,
expecting temps to reach down tonight into the low to mid 20s for
interior AK and into the upper 20 to mid 30s along the AKPen. This
high pressure should continue shifting eastward through tonight
towards the Pribilof Islands then moving on-shore before being
absorbed into the mid/upper level flow. Next impactful system
expected by late Tuesday night as a low pressure pushes up from
the N Pacific towards the AKPen. Expecting this system to bring
light to moderate rain/showers and winds reaching Gale strength
along the southern portions of the AKPen and into Bristol Bay by
Wednesday.

For the Bering & Aleutians, a large front moves into the western
Bering Sea tonight ahead of a deepening surface low. As this low
deepens, expecting the associated front and increased pressure
gradient to bring a high potential for Gale Force Winds (>=34kt),
and medium chance of Storm force winds (>=48kts) primarily west
of Adak through Tuesday night. Winds should start tapering off for
the Western Aleutians by late Tuesday night/Wednesday as the low
pulls north towards the Gulf of Anadyr.

&&

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

An upper-level longwave trough, with embedded closed low centers,
will remain in place across southern mainland Alaska for the long
term forecast period. At the surface, model guidance suggests
this corresponds to a broad low pressure centered near the Alaska
Peninsula gradually shifting into the Gulf of Alaska. However, the
surface details are highly uncertain, with different model
solutions depicting differing low strengths and various individual
waves propagating through the flow.

At the beginning of the long term on Friday, a low centered over
the Alaska Peninsula is accompanied by a front spreading
precipitation across much of southern mainland Alaska, while northeasterly
winds are drawn from Interior Alaska out across the Kuskokwim
Delta. These winds are a point of disagreement in the model
guidance, as a result of uncertain strength in the pressure
gradient between the Alaska Peninsula low and high pressure over
the Chukchi Sea. As a result, while strong northeasterly winds are
likely to set up through the Kuskokwim Delta, there is moderate
uncertainty in what their intensity will be. Low end guidance
suggests sustained winds peak out near 25 mph, while the higher
end suggests gale force winds (>39 mph) are possible. The current
forecast favors the mid to higher end of this range.

Northeasterly winds flow out across much of the Bering Sea for
the end of the week, before a ridge moves in from the west and low
pressure shifts into the Gulf by Sunday. This reorients the winds
more northerly, while they become more limited to the eastern
Bering Sea, but affect a greater portion of Southwest Alaska. The
front over southern mainland Alaska weakens, leading to more
showery conditions, while southerly onshore flow continues to
wring out precipitation along the coast from Kodiak to Cape
Suckling through Sunday. The front may linger over Southcentral as
it pivots in response to an Arctic trough dipping southward
across Southwest Alaska. Model inconsistencies abound, so
generally expect continued above seasonal normal rain chances,
with no particular geographical focus among inland locations.

For Monday, there is moderate potential for winds out of gaps
along the Alaska Peninsula and western Gulf coast as low pressure
increasingly focuses in the Gulf and cold air looks to push south
across the Aleutian Range. Models also generally indicate a weak
(below gale force), disorganized low entering the western Bering
Sea at the start of next week.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...MB
SOUTHCENTRAL ALASKA...DN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...NM
LONG TERM...CQ

000
FXAK67 PAJK 022253
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
253 PM AKDT Mon Oct 2 2023

.SHORT TERM...A weakening low in the NE Gulf moved in more
showers to the AK Panhandle Monday. The low will linger overnight
then shift east into Canada through Tuesday. Rain showers and
winds will diminish from west to east as the front moves inland.
High pressure will pass over the area Tuesday night with some
clearing skies and a short dry period. Watching for fog
development due to the residual moisture and radiational cooling.
That said also watching if temps to drop a bit lower than model
guidance, especially for the northern half of the panhandle due to
the potential clear skies. A tightening N-S pressure gradient
overnight increases winds for the northern inner channels. The
next system and warm front will just track into the SW Gulf late
Tuesday night. Better indication of lower winds and cloud cover
for Tuesday night, otherwise any forecast changes were minor.

.LONG TERM.../Wednesday to the weekend/...
Quick notes:
-No real change in the weather pattern - still rainy and cloudy.
-Mid to late week rain is expected to be moderate to heavy.
-Gusty winds around 25 to 35 mph are possible for some inner
channel areas late Wednesday into Friday.
-25 to 35 knot wind speeds likely for the eastern and northeast
Gulf late Wednesday into through Thursday.


The Details:
EURO and GFS 500mb ensemble means are in good agreement
about the long term starting with a passing trough and switching to
a ridge.

At the surface, Wednesday looks to start off mostly dry with partly
sunny skies. But an incoming front will bring rain and wind to
the area beginning Wednesday lasting into the weekend.

Rain would start in the south and move northward with the heaviest
rainfall amounts happening in the northern half. Starting with the
assumption that the timing doesn`t change, it looks like Thursday
has the highest chance for the heaviest rain with over 2 inches
likely along the northeast gulf coast. Friday could see widespread 1
to 2 inch amounts.

Using the latest NBM data, there is about a 20-30% chance for
rainfall amounts to be greater than a 2 year return interval. Some
isolated locations have a 5-10% chance for rainfall amounts to be
greater than a 5 or 10 year return interval.

But remembering that this dataset usually has a wet bias, still
thinking that while yes, this next round of rain will be heavy,
the impacts should be minimal.

&&

.AVIATION.../Through Tuesday afternoon/...Expect more of the same
for the next 24 hours: Marginal VMC due to CIGs and VSBY with
periods of IMC in continued shower activity across all of
Southeast Alaska. Onshore flow perpendicular to ridge tops will
produce some turbulence at and below the terrain, but no LLWS is
expected - as was the case today based on PIREPS received. Typical
Autumn weather for Southeast Alaska, except not too windy right
now.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-013-022-053-641>644-651-652-
661>664-671-672.

&&

$$

SHORT TERM...PRB
LONG TERM....GJS
AVIATION...Ferrin

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