643
FXAK68 PAFC 041318
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
518 AM AKDT Tue Jun 4 2024

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

A high amplitude trough encompasses all of Alaska and extends well
down into the North Pacific (around 40N latitude). Two deep upper
level closed lows are the dominant weather features, one vertically
stacked low spinning off the British Columbia coast and another
(upper center) dropping southward along the west coast of the
mainland. Conditions are quiet at the moment over Southcentral and
Kodiak Island, with just a few weak vorticity-maxima transiting
overhead producing some mid to high level clouds but little or no
precipitation. Temperatures vary from the mid 30s to upper 40s,
coldest over inland areas where skies are clear.

The low off the B.C. coast will lift northward across the eastern
Gulf through tonight. A series of short-waves will rotate across
Southeast Alaska, the Yukon, and into Southcentral beginning this
afternoon. A short-wave ridge will amplify ahead of the first
short-wave and overhead of Southcentral today. Thus, expect a
warm day with some sunshine for most of the region. Clouds and
showers will develop over the Copper River Basin this afternoon,
becoming more widespread and heavier through the afternoon and
into the evening.

While model runs in prior days had been indicating one fairly
strong short-wave, they all now seem to indicate a steady stream
of short-waves. Solutions of course vary on exact timing/track
of short-waves, though the end result is similar. Showers will
transition to steady rain overnight tonight and spread westward
across all of Southcentral through Wednesday. There will still
be a convective aspect to the precipitation as some atmospheric
instability persists. Thus, expect areas of moderate to heavy rain
embedded with the larger rain shield. The challenge is trying to
hone in on timing and rainfall amounts for any given location.
Based on the general east to west track of the short-waves over
inland areas of Southcentral tonight through Wednesday, it looks
favorable for some of the heaviest rain to fall from the western
Copper River Basin to the Mat-Su, with roughly 0.50-0.75" through
Wednesday afternoon. Future forecasts will continue to fine-tune
rainfall amounts.

Model guidance continues to struggle with the track and strength
of the Gulf low Wednesday through Wednesday night. Most of the
operational runs along with ensemble means shows the same general
idea of a weakening surface low tracking westward across the
northern Gulf while the upper low opens up into a trough and heads
inland across Southcentral. The operational GFS is a consistent
outlier with a deeper surface low lingering in the northwestern
Gulf into Thursday and a much slower process of opening into a
trough aloft. In any case, it becomes even more challenging to
track specific features Wednesday night through Thursday. The
pattern generally favors continued cool and wet conditions,
especially along the coast where southerly upslope flow will
strengthen. By Thursday night, upper level forcing looks very
weak, which should lead to a drying trend. Showers could linger
along the coast with continued upslope flow.

As far as the wind forecast, there will be some robust sea breezes
along the coast today as inland areas warm up and a thermal trough
strengthens over the Interior. As short-waves cross southern
Alaska Wednesday the thermal trough will retreat northward across
the Mainland with a surface ridge building over Southcentral
ahead of the Gulf low. This will lead to gusty winds spreading
onshore and particularly through the usual gaps - and likely
spreading further inland due to the synoptic support. While it
will be a much cooler (and wetter) day, forecast soundings
indicate some instability right off the surface during the
afternoon and evening hours which will help promote the gusty
winds. There could be a repeat on Thursday, depending on where
the remnants of the Gulf low track.

-SEB

&&


SHORT TERM FORECAST SOUTHWEST ALASKA, THE BERING SEA AND THE
ALEUTIANS (Days 1 through 3)...

Radar this morning shows a persistent convergence band of
precipitation just south and east of the Kuskokwim mountains.
Scattered showers are also lingering across portions of the
Kuskokwim Valley and Bristol Bay, near Iliamna. There is a chance
that the band of precipitation will linger into this afternoon,
though showers are expected to taper off until later in the day
when the foothills of the western Alaska Range bring more cumulus
convection showers. Showers look to linger across the Alaska
Peninsula today and partly cloudy to mostly sunny skies are
expected in the western Bering Sea, western Aleutians to the
central Aleutians.

Temperatures will be cooler today, especially for the Kuskokwim
Delta and the Bering Sea. An artic upper low is expected to track
south, ushering in an anomalously cold airmass with it. Midlevel
temperatures at its coldest will be around minus 12 to 13 degrees
Celsius. A surface shortwave trough originating out of eastern
Russia will accompany on the western periphery of this upper low.
As it skirts west of the Pribilofs today, the trough will develop
into a weak closed low at the surface as it continues into the
southern Bering Sea. Energy distribution within the developing low
will determine the location of the gustiest winds and the highest
chances for precipitation. Models have slight variations in the
outcome, but the core of the weather will be over the Bering Sea
through tonight. The Pribilof Islands would be cold enough for
snow, should a shower develop later this afternoon and evening.

By Wednesday morning, the low will be over the eastern Aleutians
and the Alaska Peninsula, bringing the cool air with it under
northerly flow. Snow showers are expected behind the main trough
for the Pribilofs and could potentially extend as far west as the
central Aleutians Wednesday night into Thursday morning.
Precipitation for the Aleutians and the Alaska Peninsula will
likely start off as rain, but as the cool air advects southward,
rain will switch to snow sometime Wednesday night and Thursday
morning. Timing of the switch is a little uncertain as some models
vary and it will mostly depend on the development of the surface
low. A weak, easterly shortwave could swinging towards the western
Alaska Range from Southcentral Thursday. Some chances of
precipitation could spill over those mountains into the Kuskokwim
Valley. Overall though, showers will gradually decrease throughout
Thursday for most places across the Bering, Aleutians, and
Southwest Alaska as the low wobbles as it weakens.

High temperatures for the Pribilofs and Aleutians will struggle
to make it out of the upper 30s to lower 40s. Temperatures for
Southwest Alaska however, will trend warmer. Highs today across
Southwest Alaska will be in the 40s to lower 50s (mid to upper 30s
near the Kuskokwim Delta coast), and for this weekend, nudge into
the 50s and 60s for this weekend.

rux

&&


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

A generally unsettled pattern remains for much of southern Alaska
for the beginning of the period as a large upper low sits over
the Bering. Through the period, this low will pull another upper-
level low up from the North Pacific, into the Gulf of Alaska. At
the surface Friday morning, a rainy pattern across mainland
Southern Alaska is expected, with models advertising varying
levels of intensity. However, by Saturday morning the showers are
expected to ease across the area, while a surface low pressure
system builds and moves into the Gulf of Alaska. Generally, this
system will maintain the unsettled pattern across the Southcentral
coastline this weekend.

More specifically, deterministic model solutions by early this
weekend disagree regarding the location of the surface low, and to
what degree it influences the Southcentral coastline. The GFS/EC
solutions bring the precipitation into Kodiak Island as early as
Saturday afternoon, while the Canadian solution keeps it in the
eastern Gulf of Alaska near Sitka at that time. Model solutions
continue to diverge beyond Saturday afternoon, leading to low
confidence regarding the timing and intensity of escalated rain
impacts.

-CL

&&
.AVIATION...

PANC...VFR conditions and light southwesterly winds are expected
to persist through the TAF period.

&&


$$