995
FXAK68 PAFC 101313
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
513 AM AKDT Fri May 10 2024

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

An east-west oriented upper level short-wave is lifting northward
across Southcentral this morning. There are a few lingering showers
along the Gulf coast and Alaska Range, partly due to upslope flow.
Otherwise, much of Southcentral has seen at least partial clearing
overnight. With a cold airmass in place, surface temperatures
have dropped to freezing or below for nearly all of Southcentral.
Some very localized gap winds are responsible for the locations
still sitting above freezing.

Looking upstream, a large vertically stacked low pressure system
centered over the Eastern Aleutians and southern Alaska Peninsula
is tracking northeastward toward southern Alaska, with a short-
wave ridge building out ahead of it. Rain and wind will quickly
spread to Kodiak Island today and then to the Gulf coast and
Prince William Sound and Eastern Kenai Peninsula regions late
this afternoon through this evening. The storm system will be
slow moving, with the low drifting into the western Gulf Saturday
afternoon. Thus, look for a prolonged period of rain for these
areas. The CIMSS satellite-based Total Precipitable Water product
shows a tropical tap of moisture feeding into this system, with
values just under an inch along the leading edge of the upper
low/trough. A series of short-waves rounding the low will provide
plenty of lift and produce some hefty precipitation totals for
Kodiak Island and the coastal zone of Southcentral. The heaviest
precipitation will be along western Prince William Sound/eastern
Kenai Peninsula, enhanced by strong southeasterly upslope flow.
Have generally upped precipitation amounts in the latest forecast.
While the airmass in place right now is quite cold - and
supportive of snow - strengthening winds ahead of the storm will
lead to quickly rising snow levels today, resulting in primarily
rain for communities and the road system. The one place that could
see some light wet snow accumulation is Turnagain Pass to Summit
Lake along the Seward Highway. Snow levels will continue to rise
Saturday, resulting in a change to all rain for this area.

Meanwhile, for inland areas of Southcentral, a glancing blow from
upper level short-waves combined with airmass instability (due to
cold air aloft) will lead to some showers each afternoon/evening.
However, these will be brief in any given location and much of the
region will remain completely dry. Temperatures will slowly warm
each day. In addition, surface ridging ahead of the low combined
with the low level instability will lead to gusty gap winds,
strongest today.

The low will weaken over the northern Gulf on Sunday, with the
upper trough then lifting northward across Southcentral. Forcing
mechanisms for precipitation will become weaker and less focused.
As a result, there could be a shower just about anywhere on
Sunday, though the track of the main part of the trough will
favor eastern Prince William Sound up to the Copper River Basin.
Broad low pressure across the region on Sunday will result in
light winds for most areas.

-SEB

&&

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

A stacked, weakening low is now moving into the southern tip of
the AKPen this morning just to the north of Cold Bay. This low is
embedded in a much larger longwave trough extending across nearly
all of Mainland Alaska and out across the eastern Bering Sea.
Gale force winds continue to wrap around the north side of the low
between the AKPen and the Pribilofs, as well as along its
occluded front approaching Kodiak Island from the west. Areas of
rain and wet snow will continue moving up from the Aleutians and
AKPen into Bristol Bay this morning as southeast winds pick up as
well across much of Southwest today. This low will slowly drift
into the western Gulf as it fills in and weakens over the
weekend, resulting in flow turning more northeasterly and offshore
across Southwest through Sunday. This should finally allow
temperatures to climb out of the late season deep freeze back to
something a little more seasonable. For the most part, highs will
reach back into the low to mid 50s (and mid 40s along the coast)
both Saturday and Sunday. There may even be enough low level
instability developing for a few rounds of afternoon showers where
breaks form in cloud cover and allow for more direct surface
heating.

Out across the Pribilofs and Aleutians, little change to the
overall cool and blustery pattern is expected over the next few
days. Gusty northerly winds to the northwest of the primary low
center should drop off across the eastern Aleutians and Pribilofs
as the low weakens and pulls away into the western Gulf through
Saturday. At the same time, a weaker North Pacific low will skirt
south of the western Aleutians, pushing across easterly winds up
to small craft range and periods of rain/snow as it progresses
steadily eastwards through Sunday. Consensus for the track of this
low has shifted a bit farther south in latest guidance, so have
lowered precipitation amounts and winds along/north of the Chain
with this package to reflect this trend.

-AS

&&

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

A high amplitude, positively tilted trough extends from the
Arctic across Western Alaska into the North Pacific South of the
Aleutians. This trough weakens and become more shallow through the
forecast period. This allows a number of energetic shortwaves to
gain energy from the North Pacific and pass through the nearly
zonal flow for Thursday. New features from Siberia brings some
undefined changes after Wednesday. Model agreement is reasonable
during the first half, but changes to more uncertain conditions
through midweek, especially in the Western half of the forecast
area.

The extended forecast opens somewhat quietly as Gulf low move the
active weather into Southeast Alaska. Locally heavy rain over
Prince William Sound and the Copper River Basin dissipates
Tuesday. A broad surface ridge over the Bering begins to flatten
through Tuesday. Showery precipitation is expected to spread over
the Aleutians, Bering and Alaska Peninsula ahead of the ridge. In
the far West, the first Siberian low brings gusty winds and
precipitation starting as snow, but quickly changing to rain
across the Aleutians and Bering. This low and front pushes some
mixed precipitation into Western Alaska, trailing over the
Aleutians for Tuesday and Wednesday before diminishing over the
AKPEN Thursday. Locally heavy rainfall moves inland over the West
as the low moves into the Lower Yukon Valley through Thursday.

-Kutz

&&

.AVIATION...

PANC...VFR conditions will persist through the TAF period.
Southeast winds out of Turnagain Arm will pick up this morning out
ahead of a frontal system in the southwestern Gulf of Alaska.
Winds peak this afternoon coincident with the greatest amount of
instability present. Southeast winds may gust as high as 30 mph
this afternoon before down-inlet flow strengthens by the evening
hours and persists into Saturday. This will help to keep the
southeast winds over Turnagain Arm and out of the terminal with
northerly winds prevailing this evening. The majority of
precipitation will remain along the coast and the eastern facing
slopes of the coastal mountain ranges due to the downslope drying
effects of the southeasterly flow.

&&


$$