Gale Warning 407, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
528 AM AKDT Sun Jun 4 2023

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low is positioned south of the Alaska
Peninsula with a short-wave ridge out ahead of it from the Gulf of
Alaska into southern Alaska and another short-wave ridge upstream
over the central Bering Sea. Short-waves exiting the upper low
along its eastern periphery are moving through the short-wave
ridge up across Southwest Alaska and the Cook Inlet to Susitna
Valley corridor in Southcentral. This is producing cloudy skies
and areas of light rain showers. A short-wave rounding the base of
the upper low is picking up a weak surface low over the Pacific
and steering it northward toward Kodiak Island and the Alaska
Peninsula.

Out west, a deep vertically stacked low is nearly stationary just
east of the Kamchatka Peninsula. A strong upper level jet streak
and short-wave have led to cyclogenesis, with a new deepening low
tracking passing just west of the westernmost Aleutian Islands.
The associated frontal system is crossing the western Aleutians
into the Bering Sea, bringing rain and a band of southeasterly
gale force winds. In addition to strong dynamical forcing, there
is a long fetch of deep tropical moisture from the west Pacific
(and remnants of Typhoon Mawar) feeding into this storm. As a
result, rain could be heavy at times.

&&

.MODEL DISCUSSION...

Despite a fairly complex pattern, with multiple low pressure
systems and numerous short-waves aloft, model guidance is in
fairly good agreement through Tuesday. Minor differences do exist
in low tracks and frontal timing, but this will have minimal
impact on the overall forecast.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist. Expect winds
to shift to the west by this afternoon before becoming southerly
again tonight.

&&

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

An area of weak vorticity advection embedded in the southwesterly
flow between an upper trough over Northwest Alaska and a ridge
over the Gulf is providing enough lift and instability for upslope
showers to continue this morning across the Susitna Valley.
Isolated showers also developed overnight across parts of Cook
Inlet and the western Kenai Peninsula, but are diminishing as the
vorticity lobe continues to move away from region.

Elsewhere, a series of weak upper-level waves will move over
Kodiak Island throughout the day, with rain for the area. The rest
of Southcentral looks to remain mostly dry, with warming
temperatures, as the ridge builds overhead through Tuesday.
The warmest and driest locations look to be across the Copper
River Basin Monday afternoon where temperatures are expected to
top out in the lower 70s and minimum relative humidity values
falling to between 15% and 25%. For Anchorage and the Mat Valley,
temperatures are expected to warm into the middle and upper 60s,
respectively, with RH values falling to between 20% and 30%. Along
with the warm and dry conditions, southerly and southeasterly gap
winds are again expected to redevelop through Turnagain Arm, the
Knik River Valley, and the Copper River Valley Monday afternoon.
These winds will start to diminish late Monday night before
picking up again Tuesday afternoon.

Meanwhile, a negatively tilted trough enters the southern Gulf of
Alaska on Monday and lifts northward to the north Gulf coast by
Monday night. Kodiak Island, especially eastern portions of the
island, will receive enhanced precipitation from this system due
to favorable easterly to southeasterly flow through the
atmosphere off the Gulf. The eastern Kenai Peninsula and Prince
William Sound will also see steady rain as the system pushes
north by Tuesday morning. Anchorage and the Mat-Su Valleys look
to remain mostly dry due to downslope drying flow from the
southeast.

This pattern is expected to persist through Wednesday as another
wave moves into the Western Gulf and maintains the threat for rain
for Kodiak Island and the Southcentral coast.

&&

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

Overall, few major changes to the forecast in the short-term. A
blustery and showery pattern persists across much of the region
with two separate lows (one near Kamchatka Peninsula and one near
Unalaska) anticipated to have the dominant impacts on sensible
conditions in the near term. These two features will eventually
merge to form one large, complex upper low spanning nearly the
entire outlook area by Tuesday.

As guidance hones in on the Kamchatka low, several adjustments
have been made to the forecast. Winds are forecast to weaken more
quickly along the front this afternoon, leading to a shorter
duration of gales in the Bering Sea. However, with the low center
curving towards the Aleutian Chain as it occludes, forecast winds
have been increased to gale force along the Western Aleutians for
early Monday morning. The other major change was to increase
rain chances and amounts along coastal Southwest Alaska for
Monday as a shortwave swings northward around the Unalaska low.

As the two lows merge and weaken through Tuesday, expect
widespread rain showers along the Bering Sea and Aleutians as
well as diminishing winds. However, Southwest Alaska looks likely
to see a similar set-up to Monday as a triple point low forms and
moves into Southwest Alaska later on Tuesday.

&&

.LONG TERM FORECAST (Days 4 through 7/Wednesday through Saturday)...

The odds tilt in favor of the cool and damp pattern continuing
across Southern Alaska as a deep upper level low is expected to
remain in place over the eastern Aleutians. This will result in
the continuation of southerly onshore flow across Southcentral
which means increased cloud cover, cooler temperatures and a
higher chance for precipitation. This pattern is expected to
persist Wednesday through Saturday.

&&

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC
LONG TERM...ED