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FXAK68 PAFC 010536
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
936 PM AKDT Tue Apr 30 2024

.PREV DISCUSSION... /issued 348 PM AKDT Tue Apr 30 2024/

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

The upper-level ridge in place over the past several days is
breaking down over Southcentral as its retreats to the east.
Developing southwesterly flow is advecting moisture into the
region, as seen by the increasing PWAT values in the most recent
soundings, as well as the expansion of low stratus across the Gulf
and Alaska and patchy fog over northern Cook Inlet. A vorticity
maxima embedded in the southwesterly flow on the western periphery
of the ridge is helping to keep the lowest 10,000 ft or so of the
atmosphere unstable. This feature, along with daytime heating
across the Anchorage Bowl into the Mat-Su Valleys, is helping
provide the lift necessary to produce convection across these
areas. As of 3:30 pm, radar returns are showing showers beginning
to develop near Hatcher Pass and along the foothills of the
Chugach and Kenai Mountains.

These showers will lift north tonight as the flow aloft turns more
southerly; an isolated shower is not out of the question from
Kenai to Anchorage and Talkeetna. A second, slightly stronger
shortwave will move north across Cook Inlet early Wednesday
morning. This feature will keep the potential for isolated to
scattered showers in the forecast across the same locations
through the first half of Wednesday. This wave will push the ridge
out of the area, allowing a more potent wave and surface front to
move from Kodiak on Wednesday to the Kenai Peninsula for Thursday
before stalling. Showers will transition to more widespread rain
across coastal locations. Showers across interior locations
Wednesday afternoon/evening will taper off as advecting airmass
becomes more stable. The southeasterly flow ahead of the front
will also create more favorable downsloping conditions, limiting
precipitation potential in the lee of mountains for Thursday.

Southeasterly gap winds will increase later today as the pressure
gradient along the coast tightens and the warmer airmass over the
interior provides lift. These gap winds will be stronger for
Wednesday as the approaching front induces a coastal ridge,
further tightening the pressure gradient. Gusts of 25 to 35 mph
are not out of the question across Turnagain Arm, West Anchorage
and the Hillside, the Kink River Valley, and the Copper River
Valley.

-TM

SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Thursday night)...

A weakening ridge over southern mainland Alaska starts to get
forced out of Southwest as a western Bering low moves eastward
tonight. Areas of patchy fog are still possible through tomorrow
morning along the Bristol Bay and Kuskokwim Delta coastline, as
well as near the Pribilof Islands. A series of shortwave troughs
will continue to pass over Southwest through this evening,
allowing a chance for showers to occur at lower elevations with
some limited instability present.

Southeasterly winds will increase for much of the Bering through
tomorrow morning as the low continues to move eastward. This
direction of flow will help support upslope precipitation along
the eastern Aleutians and Alaska Peninsula through Wednesday
night. The front will start to weaken as it moves closer to
Southwest, providing an opportunity for showers along Bristol Bay
and the Kuskokwim Delta Wednesday night into Thursday evening.

Beginning Wednesday night, a new low begins to form in the
western Bering. Its front will begin extending eastward Thursday
morning and reach the western Alaska coastline by Friday
afternoon. Widespread precipitation mainly in the form of rain is
expected across the Aleutians and Pribilofs as the front passes
through. The western Aleutians will be impacted Thursday and the
eastern Aleutians/Pribilof Islands will be impacted Thursday night
into Friday. Recent model guidance is slowing down the front`s
eastward progression starting Thursday afternoon through Friday,
which could impact conditions across Southwest as the system
starts to weaken Friday afternoon.

BS

LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)...

A weakening upper level ridge slips towards the Arctic Coast and
recedes into Canada through the forecast period. A very broad
upper level trough stretches from the Russian Far East across the
Bering and Alaska Peninsula and Gulf of Alaska. This trough is
well supported by an elongated jet along the Aleutians into the
Gulf throughout the weekend, while the body of the trough persists
over the Northern Bering. Ensemble means maintain good confidence
through Tuesday. A well clustered upper low entering Bristol Bay
Saturday moves into the Gulf of Alaska for Monday before shifting
to the upper Southeast Panhandle by the end of the period. This
low will also initiate moderate cold air along the Alaskan West
Coast and bring somewhat below normal surface temperatures across
Western Alaska.

The main low brings gusty Westerly winds along the Aleutians and
AKPEN, with a second wind area moving along the West Coast from
the North through Sunday. A large area of rainfall spreads across
the Aleutians and Bering across Southwest and Southcentral Alaska,
with locally heavier amounts expected across Southcentral zones
through Tuesday. Another round of widespread precipitations moves
over the Western Aleutians and Bering on Sunday and moves East
through Tuesday.

- Kutz

AVIATION...

PANC...Turnagain Arm winds have not developed due to weaker than
expected pressure gradient. As such, light and generally
northerly winds to continue tonight. Southeasterly Turnagain Arm
winds still on track to pick up Wednesday afternoon. May see periods
of cloud ceilings lower than 6000` tonight with diurnally driven
convection continuing to diminish this evening.

&&

$$