Flooding in Alaska, September 1 to 2, 2023

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 AM AKDT Fri Sep 1 2023

.ANALYSIS AND UPPER LEVELS...

On satellite imagery there is a deep fetch of moisture from Prince
William Sound to the west Pacific basin. The remnants of Damrey
has brought rain to Southwest and Southcentral Alaska. In the wake
of the low, there is weak ridging over the Central/Eastern Bering
and another low moving towards the Western Bering. The jet is
barreling over Prince William Sound with an estimated strength of
160 knots at the core.

&&

.MODEL DISCUSSION...

The models are highly progressive. The run to run continuity has
been decent for the global models. Models remain in good
agreement at the synoptic level through the end of the week. There
are some subtle differences with the track of the next low moving
into the Bering which will impact the timing of the precipitation
shield.

&&

.AVIATION...
PANC...Turnagain Arm winds with gusts approaching 30 kts will
be possible. Cigs will generally vary between 4-6 kft. The
potential for LLWS is actively being monitored.

&&

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

Winds have come down overnight in many locations across
Southcentral, but with predominantly southwest flow, winds up Cook
Inlet is keeping gusty winds through Kenai and Anchorage. Where
this is not a stratus layer with precipitation, mountain waves
appear on satellite and hint of a westerly component in the winds
aloft. This westerly wind over the mountain tops of the western
Alaska Range is downsloping enough to allow for skies to clear
this morning in the Kenai area. Some of this clearing could move
into Anchorage area and lead to some periods of sunshine today,
despite the continuation of southerly gusty winds at the surface.

The majority of the leftover rainfall associated with this system
will remain over the eastern Prince William Sound area and higher
terrain near the Alaska Range and Wrangell Mountains. Glennallen
could see some precipitation later this evening. The driest day
over the short term forecast will be Saturday. A period of gusty
winds will be possible out of the Whittier area and west-east
terrain gaps as a trough passes from the north.

Late Saturday night into Sunday morning another system approaches
from the west. A band of precipitation could set up on the front
range of the Chugach and the Talkeetnas, south across the Kenai
Peninsula. Precipitation will gradually fill in during the day
Sunday and southerly wind could increase through the Copper River
as the low passes east, but there is still uncertainty in the
details.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 2: Today through Sunday morning)...

Light scattered showers are ongoing across Southwest Alaska this
morning. A weak trough is pushing through the Greater Bristol Bay
region, which has resulted in a few areas of light rain from King
Salmon into Iliamna. For the rest of the day, rain chances will
not only linger but increase across Southwest Alaska as a second
upper low emerges from the Seward Peninsula and clips the Susitna
Valley further east through tonight. The next weather maker is a
trough moving into the western Bering. The trough and an
associated axis of southerly gales will slide across the Western
Aleutians later this afternoon and eventually reach the Central
Aleutians by Saturday morning. What's notable about the axis of
gales is its length. By Saturday morning gales will possible from
Saint Matthew Island all the way to Adak and further south on the
Pacific side of the Aleutians. Gales will cross the Pribilof
Islands by midday Saturday before spreading offshore of Cape
Newenham and also overspreading the Eastern Aleutians Saturday
afternoon and evening. Due to the favorable orientation of west-
southwest through Dutch Harbor, some funneling and enhancement of
the winds are likely. Peak winds of 35 knots gusting to 45 knots
are forecast with higher gusts possible before midnight. As a
whole, small craft winds will be fairly widespread across the
Bering through Saturday night. The good news is that thermal
gradient maintaining the stronger winds will weaken late Saturday
and overall winds should decrease to below small craft speeds
region-wide by Sunday morning.

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

Confidence remains high that the active, yet progressive pattern
looks to continue through the long-term period. For Monday, there
is reasonable agreement between deterministic and ensemble
guidance that Gulf low will depart to the southeast for Monday
which will bring most rain to an end over Southcentral, with
perhaps scattered showers left over in the mountainous areas. A
Bering front that previously moved eastward over Southwest Alaska
will also decay, ending most rain there. Like Southcentral, the
best chance for precipitation Monday will be against the western
Alaska Range and the Aleutian Range on the Alaska Peninsula
(AKPEN).

Southcentral remains mostly dry for Tuesday. However, another
Bering front sweeps eastward over the Aleutian Chain to coastal
Southwest and the AKPEN, bringing another round of rain with it
for Tuesday and Wednesday. The system moves eastward throughout
the day Tuesday and will bring rain to Southcentral for
Wednesday. Confidence decreases somewhat for Wednesday evening as
guidance depicts the formation triple- point low forming
somewhere in the northwestern Gulf of Alaska in the vicinity of
Prince William Sound. Rainfall looks to be enhanced for coastal
areas of Southcentral Wednesday night and Thursday, particularly
for the northern Gulf coast and Prince William Sound. Inland areas
of Southcentral could also see periods of enhanced rainfall due
to favorable southerly to southwesterly flow in the middle to
upper levels of the atmosphere. Meanwhile, guidance also hints at
a new strong upper shortwave moving eastward from the North
Pacific into the Bering for Thursday. However, there are stark
differences with strength and exact placement of the surface low.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...VR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BL
LONG TERM...DAN