Flooding in Alaska, May 13 to 18, 2023

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
542 AM AKDT Sat May 13 2023

.ANALYSIS AND UPPER LEVELS...

A large upper-level low continues to reside in the North Pacific
this morning. Numerous shortwaves continue lifting northward
through the front-stretch that is the eastern Gulf of Alaska and
pivot westward into turn 2 towards the north Gulf Coast. The
shortwaves then exit turn 3 into the backstretch heading southward
in the vicinity of Kodiak Island and back to the North Pacific at
the finish line. The resultant weather from this "shortwave race"
is continued very light shower activity for Prince William Sound
and portions of the north Gulf Coast, as seen on Middleton Island
radar.

Out west, an upper-level ridge extends from the eastern Aleutian
Chain through the eastern Bering Sea and into mainland Southwest
Alaska. The eastern Aleutian Chain observes mostly high clouds
while the Alaska Peninsula, especially on the Bering Sea side,
sees low stratus clouds and even fog, which is the case in Cold
Bay as seen on the Nighttime Microphysics satellite product.
Mainland Southwest see mostly clear conditions. Meanwhile, an
upper-level low between the western Aleutians and Kamchatka
ejects a strong shortwave eastward through the western and central
Bering this morning. Portions of the central Aleutian Chain
observe rain this morning as the front moves through and to the
east.

&&

.MODEL DISCUSSION...

Guidance continues to be in mostly good agreement through the
first half of the weekend as the Bering Sea front progresses
eastward along the Aleutian Chain. However, solutions diverge on
the exact orientation of the front as it reaches mainland
Southwest Alaska for Sunday and the beginning of the new work
week. The 00z GFS and 00z NAM show a more south-to-north frontal
orientation and keep most precipitation confined to the coast and
slightly inland of the coast. Meanwhile, the 00z Canadian and 00z
ECMWF show a more southwest-to-northeast orientation and bring
heavier precipitation inland quicker than the GFS or NAM. Thus,
the precipitation amount continues to be a main challenge to the
forecast and confidence remains low at this time. Disagreements
are also abundant with this system as it approaches Southcentral
Alaska towards the end of the short-term period on Tuesday.
Therefore, confidence also remains somewhat low in the
precipitation amount forecast for Southcentral. However, there are
some indications that coastal locations could receive enhanced
precipitation due to favorable southerly to southwesterly flow in
the middle and upper levels. Portions of interior Southcentral
could also get in on precipitation as well for Monday evening into
Tuesday; but details still need to be ironed out with time.

&&

.AVIATION...

PANC...VFR conditions will persist. The coastal ridge responsible
for the strong Turnagain Arm jet southeasterly winds over the
terminal yesterday will remain once again today. The strength of
these winds will fluctuate this morning. However, as daytime
heating gets underway and instability builds during the afternoon,
expect these southeast winds to once again become strong and gusty
with gusts up to 30 kt. and perhaps a reading or two around 35
kt. There are indications that the southeast winds could relax and
become variable for a time Sunday morning at the end of the TAF
period. However, with daytime heating, southeast winds are once
again expected to bend over the terminal for Sunday mid-day and
afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3:
Today through Monday night)...

A pair of easterly waves will move east to west across the
northern Gulf and immediate Southcentral coast through the
weekend. The first of these will move to the western Gulf this
morning, with a southeasterly flow behind the trough keeping
widespread shower activity across Prince William Sound and the
eastern Kenai Peninsula. The shower activity begins to fall apart
this afternoon due to the development of a transient ridge out
ahead of a second wave that moves to the northern Gulf for early
Sunday morning. This second system will bring another round of
showers to Prince William Sound and eastern Kenai Peninsula Sunday
morning through Monday. While most of the shower activity will be
confined to the coast, there is still the outside chance for an
isolated shower along the foothills of the Alaska Range and
Talkeetna Mountains through the weekend.

The bigger story is the development southeasterly gap winds due to
the tightening pressure gradient between a surface coastal ridge
in the wake of the shortwave and lower pressure across the
interior. Gusty winds developing out of the Knik Valley,
Turnagain Arm and Copper River last night may diminish slightly
after the trough passes to the west this morning, but otherwise
expect the peak of the stronger wind and gusts to follow a
typical diurnal pattern, with a peak winds during the afternoon
and evening hours, along with a decrease in winds through the
overnight hours.

The flow then turns more southerly for Sunday afternoon through
Monday as a frontal system approaches from the west. This
southerly flow will advect more moisture toward the region, with
steadier rain developing along the coast for Monday. Rain is then
expected to move over Cook Inlet and across the Mat-Su Valleys by
Tuesday as both the surface front and the upper-level trough move
through Southcentral.

&&

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

A high amplitude upper-level ridge is centered over Southwest
Alaska while an upper trough and embedded low are located near the
western Aleutians. An attendant surface low over the Bering
extends its front across the Pribilof Islands and central/eastern
Aleutians this morning. While the low center tracks towards the
Bering Strait through Sunday, the front will continue lifting east
to Southwest. The front looks to briefly weaken tonight before an
incoming upper trough and surface shortwave revive it near the
Southwest coast around Sunday morning. The front is expected to
bring unseasonably wet conditions to the region as it stalls
through the weekend and into early next week, promoting a
prolonged period steady rain and gusty southerly winds. Winds
across the region will gradually increase through the day today,
peaking this evening through early Sunday morning but remaining
elevated through much of Monday. The highest precipitation amounts
are expected along/near the western facing slopes of the
Kuskokwim Mountains and Aleutian Range.

After the front exits into Southwest Alaska by Sunday night,
quieter conditions are anticipated across the Bering and
Aleutians. Although on Monday, a weak low pressure feature is
expected to track southeastward across the Bering and near the
Pribilof Islands.

&&

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

Models continue to struggle in the long-term with poor run-to-run
consistency regarding an upper-level trough centered over the
eastern Bering Sea and western Alaska, with a series of strong
short-waves moving through the flow. Models disagree on handling
of specific short-waves, location of surface low(s) and resultant
distribution of precipitation and wind over southern Alaska. This
general pattern looks unseasonably wet and windy, especially
early in the week. Colder air aloft with the trough combined with
clouds and precipitation will lead to near to slightly below
normal temperatures for southern Alaska. Models also disagree on
how the pattern evolves later in the week, with solutions varying
from keeping the trough over mainland Alaska, progressing it
eastward, and even retrograding it back westward into the Bering
Sea. Therefore, forecast confidence is extremely low as we head
through the work week.

&&

SYNOPSIS AND MODEL DISCUSSION...DAN
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AF
LONG TERM...NSC