Heavy Freezing Spray Warning 157, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 AM AKDT Sat May 1 2021

.ANALYSIS AND UPPER LEVELS...

An upper-level trough over the Copper River Basin continues to
lift northeast this morning. Low stratus remains entrenched over
the basin, aided by a persistent southerly flow ahead of the
trough. However, this cloud deck is slowly diminishing in coverage
as the trough starts to exit the region. Upstream of the trough,
an upper-level ridge is building across much of the Southern
Mainland. Subsidence underneath the ridge is keeping conditions
clear and dry with temperatures this morning dropping below
freezing for many locations from interior Bristol Bay and the
Kuskokwim Valley to areas around Cook Inlet and the Susitna
Valley.

An expansive area of low stratus covers much of the western two
thirds of the Bering this morning. This region is situated
underneath a col in the mid and upper levels, a region of reality
weak flow bounded by the aforementioned upper-level ridge; a
second ridge centered over the far western Bering; and two areas
of low pressure, one well north of the Pribilofs and a second
southeast of Atka. In addition to these features, a weak surface
trough is lifting north toward the eastern Aleutians this morning.

&&

.MODEL DISCUSSION...

Models remain in good agreement through the first 36 to 48 hours
before breaking down. The main issue centers on the exact track
of a surface low as it enters the southwestern Gulf by late
Sunday. The Canadian is the most bullish in moving the low center
toward the central Gulf before retrograding it west toward Kodiak
Island Monday morning. The EC is similar to the Canadian, but
farther east. The GFS keeps the center well south and west and the
NAM depicts a broader low centered between the GFS and Canadian
solutions. Despite these differences, all models show that
retrograde motion toward Kodiak Island as the low weakens and a
secondary low spins up just to the southeast of the island. Even
with the noted differences, all solutions are keying in on a
period of wet and windy weather for Kodiak Island through early
next week.

Differences in placement of surface lows also show up over the
Bering and Aleutians. Again, despite the differences, the overall
weather picture is quite similar: low pressure developing near
the Pribilofs with plenty of cold air advection yielding rain/snow
showers and strong northerly wind gusts for the Bering and
Aleutians Sunday into Monday.

&&

.AVIATION...

PANC...VFR conditions will persist. Southerly winds are expected
to develop over the airport complex by early to mid afternoon and
persist through the evening hours. These winds are then expected
to diminish and become variable by late evening or early morning
Sunday.

&&

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

An upper level ridge will build overhead of Southcentral today,
then exit to the northeast Sunday through Monday. This will lead
to dry conditions over most of Southcentral this weekend. The
airmass in place remains quite chilly, with 850 temperatures
roughly -2 to -6 degrees Celsius. Thus, daytime temperatures will
max out near seasonal norms despite full sunshine while nighttime
lows will be at or below freezing for all except the Gulf coast.

As the ridge tracks northeastward, a vertically stacked low will
lift northward from the North Pacific and into the Gulf of Alaska.
This will lead to a long duration rain on Kodiak Island, beginning
tonight and continuing through at least Monday. Rainfall amounts
will be enhanced along the eastern side of the island thanks to
persistent east to northeast winds. Expect some showers to
develop across the eastern Kenai Peninsula Sunday as the leading
edge of the trough arrives. However, the low circulation over the
Gulf will hold steady rain to the south. This will change on
Monday as the upper low opens into a trough and lifts northward
into Southcentral. While this trough will rapidly weaken, a new
trough from the Bering Sea will quickly move into the Gulf Monday
through Monday night, serving to reinforce rain across Kodiak and
the Gulf. Rain will spread onshore across the eastern Kenai
Peninsula Monday afternoon and evening and become heavier Monday
night. A few showers could spread inland, but it wouldn't be much
as most of the dynamics remain focused over the Gulf with the new
trough.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)

An upper level ridge over Southwest will begin to depart eastward
today, while a broad upper level low over the northern Bering
moves southward. Increasing cloud cover over Southwest is expected
today as a North Pacific low lifts northward into the eastern
Bering. Rain and southerly flow will begin to spread in across
the southern Alaska Peninsula this morning. By tonight,
precipitation will reach the Kuskokwim Delta coast, though the
heaviest rain is likely to remain offshore. As the low center over
the eastern Bering begins to move southward, colder air on the
backside of the low will allow for snow showers across the higher
elevations of the Alaska Peninsula on Sunday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)

A broad upper level trough over the northern Bering will move
southward today, introducing colder northerly flow over the
western Bering. Snow showers are expected across the western
Bering waters as well as the western Aleutians. Meanwhile a North
Pacific low lifts northward tonight, bringing warm southerly flow
and rain to the central and eastern Aleutians. Precipitation is
expected to change over to snow showers by late Sunday, as colder
air on the backside of the low fills in across the central
Aleutians. Northerly gale force winds are expected over the
central Bering Sunday night into Monday as the low tracks to the
southeast.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...

Gales will reach the southern Bering from Unalaska to Adak by
early Monday morning. Waves in association with the winds will
build to between 15 and 20 feet. The gales will weaken and waves
subside by Monday afternoon as the surface low crosses the Chain
and moves into the north Pacific. Monday into Tuesday, expect a
front to move across the western Aleutians and into the central
Aleutians by Tuesday night before moving into the eastern
Aleutians by Wednesday. The low center is to follow closely
behind, so anticipate higher winds and waves with the front and
low. For the Gulf, easterly winds 20 to 30 kt will dominate the
northern Gulf Monday as a front moves north across the region.
Conditions will calm and remain so Tuesday through Wednesday.

.Long Term Forecast (Days 4 through 7: Tuesday through Friday)...

A stalled front across the north Gulf coast will bring rain and
snow to higher elevations to this area through Wednesday before
showers diminish. Expect a mix of rain and snow as cold air is
advected across the Bering Sea and the Aleutian Chain on Tuesday.
After this, a ridge is anticipated to move across the Bering,
increasing temperatures and reducing precipitation. Unsettled
weather is anticipated to return Thursday for the western
Aleutians and the northern Gulf as fronts move into both regions.
Friday looks to be drier for both areas as their respective
systems weaken.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 185 412.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...CK