Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 212353
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
353 PM AKDT Thu Mar 21 2024

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

The high amplitude upper level ridge which has been parked over
Southcentral and the Gulf of Alaska is shifting slowly eastward
today. High cirrus clouds are moving up and over the ridge while
areas of low clouds this morning from the western Kenai to
Anchorage and Mat-Su have largely dissipated. Thus, it is another
warm and partly to mostly sunny day across the region.

To the west of the ridge, a storm system is lifting northward
across the eastern Bering Sea with a trough beginning to lift
northeastward toward southern Alaska as the ridge shifts eastward.
This short-wave remains on track to cross Southcentral on Friday.
However, all model guidance depicts this feature a little
differently now, with a lower amplitude short-wave and less of
a negative tilt. There is also a separate weak trailing short-wave
which moves through late Friday afternoon through Friday evening.
While upper level forcing looks less impressive (if a bit prolonged
due to trailing short-wave), low level forcing in the form of
upslope flow looks a little stronger. All in all the weather
message for Friday is similar, with light precipitation spreading
northward across the western tier of Southcentral, from the Kenai
Peninsula/Prince William Sound to Anchorage and the Mat-Su. With
low level flow out of the south, the best chance for steady
precipitation is the Kenai Peninsula Gulf coast and the northern
Susitna Valley. Precipitation-type remains a challenge, but expect
rain or a rain/snow mix most places. The best chance of seeing
some very light snow accumulation is the northern to western
Susitna Valley. A Turnagain Arm jet will form on Friday as a
dissipating occluded front moves into the western Gulf. As the
upper level short-wave lifts inland Friday afternoon (and surface
low tracks up the west coast of Alaska), pressure gradients become
favorable to bend some of the southeasterly Turnagain winds into
south to west Anchorage.

The storm system for this weekend is largely on track. In the
latest forecast have adjusted rainfall amounts (primarily for
Kodiak Island) and timing of occluded front lifting northward
toward the north Gulf coast. Rain and wind will overspread Kodiak
Island and the western Gulf Friday night and continue all the way
through Saturday night as the surface front moves through and a
series of short-waves track overhead. The heaviest rain will occur
Saturday afternoon and evening. Storm total amounts will be
highest along the east side of Kodiak Island thanks to persistent
easterly flow, with around 1.5" for Kodiak City and as much as 3"
as you head down to the coast south of Chiniak Bay. Models differ
in depiction of how far north the occluded front will lift as well
as timing of precipitation spreading into coastal Southcentral.
The GFS was an outlier in a much slower northward progression, so
have leaned heavily toward the faster solutions (NAM/ECMWF/Canadian).
There is good agreement that the upper trough will dig eastward
across the Gulf, which means the occluded front will weaken over
the northern Gulf (and won`t push onshore of Southcentral. As a
result, the vast majority of impacts will be confined to coastal
Southcentral, with the highest precipitation amounts once again
along the eastern Kenai Peninsula region. Strong downslope flow
will keep most of the western Kenai Peninsula, Anchorage, and
Mat-Su dry. The Copper River Basin will also remain dry far from
the key features producing precipitation. Temperatures will remain
warm, with a little narrower diurnal ranges due to cloud cover.
Precipitation will be in the form of rain at and near sea level
along the coast, with mountain snow. Inland areas may still see a
daily freeze-thaw, though night-time temperatures won`t be nearly
as cold as the past couple mornings.

&&

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

Key Messages:

* Additional snow accumulations of up to 1 inch for Southwest
  Alaska through tonight

* 3 to 5 inches of snow and blowing snow possible for the
  Pribilofs this weekend

Discussion:

Light to moderate rain across the eastern Aleutians will slowly
wind down and lift north through early tonight. Areas of light
snow associated with a low pressure and boundary off the coast of
Southwest Alaska continues across the western Kuskokwim Delta and
Nunivak Island. As the main shortwave pushes through tonight, a
brief period of light snow will push through much of Southwest
Alaska, with additional accumulations of up to 1 inch.

A large surface low and associated upper level low will then push
into the Aleutians late Friday into Saturday. The main boundary
with this low will settle itself just south of the Aleutian Chain,
bringing periods of rain and snow. Strong vorticity advection
near the eastern end of the boundary will form a new parent low
just south of the eastern Aleutians, pushing north near the
Pribilofs by Saturday afternoon. This low will draw additional
moisture northward, with at least 3 to 5 inches of snow possible
for the Pribilofs Saturday into Sunday. Wind gusts of at least 40
mph may also cause areas of blowing snow over the weekend. The
main front will push into Southwest Alaska as well late Saturday
bringing periods of snow through at least this weekend, where
southeasterly flow will hinder precipitation in typical
downsloping areas.

ME

&&

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

The upper level ridge over the Mainland continues to weaken
slowly through the forecast period. Forecast models are well
clustered to start, but become undone through midweek. Over
Western Alaska and the Eastern Bering, an elongated low stretching
from the Arctic to the North Pacific oscillates East-West with
the transient shortwaves flowing through the system. A weak
Russian Far East high wobbles over the Western Bering through
Thursday. The main track of the surface systems coincides with the
tighter gradient across the Southern portions of the state.

Southerly airflow over the Interior continues, but is being
eroded in the West somewhat by Arctic air with the Bering trough.
A well developed surface low in the Southeastern Bering anchors a
front that extends across the Gulf of Alaska, and trails over most
of the Aleutians through Tuesday. A second well developed North
Pacific low and front enter the Alaskan waters for Tuesday and
moves along and just South of the Aleutians through Thursday,
before slipping into the Eastern North Pacific.

Gusty Northerly winds spread over the Central and Eastern Bering
for Monday and Tuesday, crossing the Eastern Aleutians by
Wednesday. Snow will predominate over Western Alaska and the
Pribilofs, becoming mixed with or changing to rain across from the
Aleutians,and Alaska Peninsula through Tuesday. The second low
and front bring another round of rain or mixed rain and snow late
Tuesday through Thursday across the Aleutians. The front extending
across the Gulf will spread mostly rain from the AKPEN and Kodiak
Island across to the Canadian Border. Rain, possibly mixed with
snow invades the Southcentral Interior through Wednesday.

Kutz

&&

.AVIATION...

PANC...Southwesterly flow continues up Cook Inlet this evening
which is keeping ceilings right around the VFR and MVFR break
point. Visibility should remain VFR with light winds. A weak
frontal system will lift northward Friday with the possibility of
more persistent MVFR ceilings at the end of the TAF period and
into Friday evening. However, low level southeasterly flow may be
enough to keep ceilings VFR in spite of the weak front.

&&


$$


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