733
FXAK68 PAFC 240043
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 PM AKDT Sun Jun 23 2024

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: This
afternoon through Wednesday afternoon)...

An upper level ridge extends across Southcentral Alaska,
surrounded by upper lows in Bristol Bay, the North Pacific, and
the southern tip of the Southeast Panhandle. Clearer skies under
the ridge and weak upper level shortwaves moving around these
features are producing an environment favorable for diurnally-
driven thunderstorms. With little influence from other synoptic
features in the short-term, isolated to scattered wet
thunderstorms will remain the primary hazard over Southcentral
Alaska through the middle of the week. Daytime highs will also
remain relatively warm, ranging from the upper 60s to the upper
70s, and even as high as the low 80s. Smoke or haze may be visible
on the horizon, but the thinking is that southerly, up-Inlet flow
will keep it mostly at higher elevations and/or confined to near
the Alaska Range.

Diving into the details... radar currently shows thunderstorms
developing across much of Southern Alaska, driven largely by
thermal instability. The 00Z observed soundings have yet to come
in at the writing of this forecast discussion, but even the 12Z
Anchorage and Kodiak soundings show a weak, low-level inversion
with lapse rates of about 8 degrees Celsius per kilometer aloft.
With excellent daytime heating under this morning and early
afternoon`s clear skies, the inversion has mixed out, allowing
developing convection to tap into environmental instability and
show some impressive growth. Radar shows multiple thunderstorms
developing in parts of the Mat-Su Valleys, Kenai Peninsula,
Chugach Front Range, Copper River Basin, and even Kodiak Island.
These thunderstorms have been producing a good amount of
lightning, with high reflectivities (50-60 dBz) hinting at heavy
rain and even hail. Thunderstorm motion is generally east-to-west
across the Mainland, except north-to-south in the Copper River
Basin and south-to-north for Kodiak Island. However, with steering
flow remaining relatively weak, most of these storms will be
relatively short-lived as they build up, then collapse on
themselves. Still, interactions between various storms may
contribute to longer-lived clusters.

Warmer temperatures and thunderstorm potential continue through
much of the week. Expect temperatures to modulate a little for
Monday and Tuesday before rising again for Wednesday.
Thunderstorms will develop over a similar area tomorrow, with
slightly expanded coverage in the Copper River Basin. Tuesday may
see thunderstorms limited more to the Mat-Su and Copper River
Valleys as high pressure builds into the northern Gulf coast,
suppressing convection in these areas. By Wednesday, forecast
confidence diminishes regarding thunderstorm potential. Have kept
forecast coverage for Wednesday about the same as for Tuesday, but
there will be a chance for greater thunderstorm
intensity in the Copper River Basin for Wednesday afternoon than
is currently in the forecast.

Aside from the convective forecast, the one major update to the
previous forecast package has been to greatly diminish winds, wave
heights, and precipitation for Kodiak Island and the Western Gulf
on Wednesday. Where models had previously shown a gale-force
front moving in from Southwest Alaska, they are now trending
towards a more robust ridge of surface high pressure in the Gulf,
which will weaken the front and keep it at bay.

-KC

&&


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

An area of low pressure located over Bristol Bay has been
providing favorable lapse rates and lift for shower and
thunderstorm activity today, more specifically across the Greater
Bristol Bay region. The morning sounding out of King Salmon
showed a capping inversion at around 2000 feet in elevation.
However, just enough forcing downstream of the low was able to
break through the inversion earlier this morning. This resulted in
a single, stationary storm (likely rooted on a surface boundary)
on the western edge of Lake Iliamna, which persisted through the
morning and afternoon hours along with numerous lightning strikes.
Furthermore, the single storm`s outflow spread southwestward
contributing to additional showers and thunderstorms developing
during peak diurnal heating across Greater Bristol Bay. Currently,
numerous wet thunderstorms are being observed from north of
Koliganek, down to just east of Dillingham, and then eastward to
just north of King Salmon. For the rest of today into the evening
hours, showers and thunderstorms will remain possible across the
Greater Bristol Bay area. Recent satellite imagery shows bubbling
cu across the Lower Kuskokwim Valley, which could develop into
additional wet thunderstorms later this evening.

Looking ahead, shower and thunderstorm chances will redevelop on
Monday in the afternoon and evening. The biggest change will be
the movement of the upper low to the south and east. Thus,
development of precipitation will be driven more so by daytime
heating and the passage of any subtle shortwaves through Southwest
Alaska. Tuesday, models are showing less instability for
convective activity as a cold front approaches the coast of
Southwest Alaska and high pressure becomes shunted further north
and east into interior Alaska.

Elsewhere, models are trending stronger with a low and front
moving to the Western Aleutians/southern Bering on Monday and
Tuesday. Have increased winds with a broader area of gales with
the leading front, but even more so with northerly winds on the
backside of the low. The fact that models have the low
strengthening even after crossing the Aleutians is increasing
confidence in widespread small craft and gales across the entire
Bering late Tuesday. Some areas between the Pribilof Islands and
Central Aleutians may even see storm force gusts. Gusty southerly
winds can also be expected along the Kuskokwim Delta coast and
across Bristol Bay on Tuesday. Offshore, mariners can expect
significant wave heights approaching 20 feet on the Bering and
Pacific side of the Central Aleutians, and 8 to 15 foot seas
across the eastern Bering.




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

The long term begins with a ridging from western Canada north to
the high Arctic and a deep upper level low and trough over the
Bering. The ridging is likely to keep any notable systems from
reaching the Mainland, with most consolidated lows remaining over
the Bering or passing through the southern Gulf. For the Southern Mainland,
the broad southerly flow between the trough in the west and
ridging to the east will keep temperatures cooler than what has
been observed recently, but will notably increase winds through
the coastal terrain gaps. Shortwaves riding north on the southern
periphery of the ridge may keep convection active, with afternoon
and evening showers possible over the terrain. Overall, conditions
will remain similar to the past week, just slightly cooler and
windier.

&&

.AVIATION...

PANC...VFR conditions will persist. Winds should remain southerly
for the next day, though they could have a few higher gusts to 15
or 20 kt at times due to convergence of wind out of Turnagain Arm
with wind moving up the Inlet. There will be some showers and
possibly a thunderstorm over the Chugach Mountains this evening,
but they are expected to remain mostly over the mountains but one
could reach the airport area. There is also a slight chance for
some low level stratus to develop early tomorrow morning as
southwesterly winds up Cook Inlet and bring in a marine layer.
This looks unlikely at this time, but will be something to watch
the next few days.

&&


$$