214
FXAK68 PAFC 061340
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
540 AM AKDT Thu Jun 6 2024

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

In the upper levels, a large closed low centered just north of
Dutch Harbor is the dominant feature now, anchoring a long-wave
trough over Alaska. The short-waves which crossed Southcentral
yesterday have exited to Interior Alaska. The vertically stacked
low that had been over the Gulf the past few days, is headed
across Prince William Sound and onshore of Southcentral early
this morning - as it gets absorbed into the vast cyclonic
circulation around the aforementioned Aleutian low. The upper low
over Prince William Sound is opening up into a trough, with a
potent vorticity maximum poised to head inland. Widespread showers
are being observed along the coast, with just some isolated to
scattered showers inland at the moment. Temperatures are in the
40s most spots, with some upper 30s in the Copper River Valley.

The upper level short-wave/vorticity maximum will curve northwestward
as it crosses Southcentral, leading to increasing showers this
morning across the Kenai Peninsula, Anchorage, and Mat-Su. An axis
of instability ahead of the short-wave will extend across much of
the Susitna Valley northeastward across the Talkeetna Mountains
into the northern Copper River Basin/Alaska Range. While it would
be preferable to have a little later arrival to maximize solar
radiation/surface heating, very cold air aloft with the short-wave
(roughly -29 degrees Celsius at 500 mb) combined with upward
vertical motion ahead of the short-wave should be enough to
trigger isolated to widely scattered thunderstorms in the Susitna
Valley. Model guidance uniformly supports the thunderstorm
potential, showing negative LIs and several hundred J/kg of CAPE.
Furthermore, there could be thunderstorms later in the afternoon
behind this short-wave, with additional weaker short-waves
embedded in the upper flow that can take advantage of continued
surface warming. Showers will generally be most widespread this
morning through early afternoon, with a diminishing trend mid
afternoon into the evening hours.

Meanwhile, the surface low will move onshore early this morning
and dissipate. A surface ridge will then build inland and strengthen.
The combination of an unstable airmass with pressure rises will
lead to widespread gusty winds. Coastal bays and mountain gaps
will see a brief surge of winds coincident with the pressure
rises. Have upped winds across the board in the latest forecast.

Southcentral will gradually dry out tonight as the short-waves
exit to the north. Showers will linger into Friday along the coast
thanks to upslope flow. A weak short-wave will quickly track up
from the Pacific to Southcentral Friday afternoon, but the airmass
will be much drier and more stable by then, so there may be some
mid to high clouds but likely no precipitation with this. Thus,
expect a warmer and dry day Friday across Southcentral.

Well to the south over the North Pacific (~37N latitude) at the
base of the long-wave trough, a low has formed and is deepening
and helping to amplify the flow over the Northeast Pacific. This
low will track up toward the Gulf Friday, with rain spreading well
out ahead of it to Kodiak Island. A ridge out ahead of the low
will build northward across SE Alaska and then into Southcentral
Saturday. The newly placed ridge combined with the Aleutian low
will help pull this storm system westward across the Gulf and
toward Kodiak on Saturday, bringing lots of rain and wind (gale
force winds possible across the Kodiak and western Gulf coastal
waters). Some of the warmest 850mb temperatures of the summer will
move into Southcentral with the ridge. Thus, it looks like a warm
and dry day for Saturday, with temperatures well into the 70s
inland. The one area of uncertainty in the forecast for
Southcentral is the extent of cloud cover from the Gulf storm and
how far north precipitation will ultimately make it. At this
point in time, it does look like rain will reach the north Gulf
coast/Prince William Sound regions by Saturday night.

-SEB

&&


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

The arctic low which brought cold temperatures and snow south to
the Pribilofs, the Alaska Peninsula, and the eastern Aleutians is
currently located between the Pribilof Islands and Unimak Island.
The low has stalled though, maintaining the potential for light
showers with snow and/or mixed precipitation. This low is expected
to meander westward along the Aleutian Chain and weaken through
tomorrow morning. However, a North Pacific low moving toward the
Gulf of Alaska could interact with this low and lead to increasing
southeasterly winds and precipitation chances for the Alaska
Peninsula and across the eastern Bering and Bristol Bay tonight
through Friday. How much precipitation and easterly winds
influence the Pribilofs, will depend on the location of the wobbly
low and the position of the incoming Pacific low as they interact
over the next couple days. Any remaining snow showers should
transition to rain showers by Friday and will persist for at least
a few more days as the upper low remains nearby and the cold
airmass slowly moderates. Showers will continue for Southwest
Alaska with southeasterly flow. Highest probability of
precipitation will be along the coast, the interior Bristol Bay,
and upsloping along the Kuskokwim Mountains. Winds will also
increase through Kamishak gap and Bristol Bay later today through
Friday with gusts up to 30 mph.

An easterly shortwave moving across the western Alaska Range
today, coupled with some atmospheric instability, could lead to
isolated thunderstorms near the western Alaska Range. Greatest
chances will be just west of the Alaska Range, around Sparrevohn
and north. Chances drop off between Lime Village and Stony River.

rux

&&


.LONG TERM FORECAST (Days 4 through 7) Sunday through Wednesday...

The Alaska Weather Map is becoming a bit busier through the
forecast period. A dual centered upper level low in the North
Pacific merges to form a single closed low in the Gulf of Alaska
by midweek. An upper level ridge on Sunday moves across the Bering
and flattens out for Wednesday, as a well developed surface low
moves out of the North Pacific. A new low out of Siberia pushes
its way into the Bering, settling into the Central Bering by
Wednesday. A broad ridge remains over the Northern two- thirds of
Mainland Alaska, and continues its support of surface thermal
troughs. Forecast confidence remains good through the period,
starting with a deterministic blend before changing to a UK/EC
ensemble means.

The upper level low in the Gulf hooks up with a North Pacific
surface system to spread considerable rainfall across the
Southcentral areas from the Canadian Border, across Prince
William Sound and the Eastern Kenai to Kodiak Island and parts of
the Alaska Peninsula for Sunday and Monday. Mostly offshore gusty
winds will occur over the Gulf, but move over Kodiak Island and
the Southern Kenai Peninsula through early Tuesday. Unsettled
conditions will linger across Southcentral, bringing showers over
the area. Transient systems will bring locally heavy rains over
the Aleutians and Bering through Wednesday.


-Kutz

&&
.AVIATION...

PANC...VFR conditions will persist throughout the TAF period.
Another low pressure trough is expected to move northwest across
the Kenai Peninsula early this afternoon. Rapid pressure rises
behind this trough will lead to a sudden redevelopment of
southeast Turnagain Arm winds bending into the terminal as early
as 18z. Peak wind gusts during the afternoon could exceed 30 kts
at times, but should weaken considerably by late this evening.
Showers could also move near the terminal as the trough moves
through during the early afternoon, but significant reductions to
ceiling/visibility with this activity is not anticipated.

&&


$$