Small Craft Advisory 475, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
546 AM AKDT Tue Jun 21 2022

.ANALYSIS AND UPPER LEVELS...

An upper-level low with a weakening surface component spins in
the eastern Gulf of Alaska. An upper-level shortwave crossing from
the Copper River Basin into the Gulf of Alaska has taken
advantage of the moist airmass wrapping around the low, bringing a
band of stratiform rain across the Copper Basin overnight with
cloudy skies and a few showers as far west as the Susitna Valley.
Over the past twelve hours, wetting rains of greater than one
tenth of an inch have been observed across portions of the Copper
Basin, particularly in the east and higher elevations.

To the west, dry, northerly flow has contributed to much clearer
skies across Southwest Alaska and western portions of
Southcentral. A shortwave trough diving southward across the
Alaska/Aleutian Range is supporting a few remnant rain showers
over the mountains' western slopes, while a ridge centered over
the eastern Aleutians builds northeastward into Southwest Alaska,
bringing drier air and a more stable, capped environment. As a
consequence, westerly flow around the surface anticyclone has
advected a marine layer of low stratus and fog into the Kuskokwim
Delta and the northwestern side of the Alaska Peninsula. Further
west, a front with small craft winds extends across the western
and central Bering.

&&

.MODEL DISCUSSION...

Guidance remains in good agreement with regard to the large-scale
synoptic features through the week. Models depict the ridge over
the Bering building eastward into Southwest Alaska and then
extending across much of the state. The biggest challenge in the
short term will be the timing of weak waves dropping south across
the region in advance of the building ridge. Convective
parameters become favorable today for Southcentral Alaska,
with the potential for widely scattered afternoon and evening
thunderstorms. The question, however, will be how much subsidence
results from the advancing ridge. Warming in the mid-levels due
to any subsidence could produce a cap and limit convective
activity. The extent of convective activity will also be dependent
on the timing of the aforementioned waves, though there is
increased agreement that a weak shortwave will cross the Alaska
Range in the morning and reach the Gulf late Tuesday night.

&&

.AVIATION...
PANC...VFR conditions with ceilings above 5000 ft will persist
through the TAF period. A westerly sea breeze is expected to
develop by midday, with isolated rain showers possible in the
vicinity of the airport during the afternoon and evening
timeframe. While thunderstorms are not currently expected to
affect the terminal at this time, the atmospheric conditions and
expected storm motion today lend to a slight potential for a few
lightning strikes in the vicinity of the airport during the
evening hours. However, this would be contingent on a strong cell
surviving crossing over Knik Arm. Light and variable winds return
for the overnight hours.

&&

.FIRE WEATHER...

The drying and warming trend will continue through Wednesday
across Southcentral, with a slight reversal of that trend on
Thursday as a front crosses the area. Today will be the windiest
day of the week, followed by a break on Wednesday, then breezy
northerly winds return on Thursday with the front. For now, think
the hot, dry, and windy won't line up anywhere in Southcentral,
but certainly hot conditions will be widespread. There is some
uncertainty as to how dry it will get Wednesday and Thursday with
such warm temperatures. Today's significantly warmer highs with
downsloping winds over some of the area should be the tell as to
how dry it will get on the subsequent days.

Thunderstorms are expected over portions of Southcentral the next
3 days. All thunderstorms on all 3 days will be wet. Today will
have the most coverage by far, especially over the Talkeetnas and
the Chugach. This is due to significant moisture remaining from
the past couple days, lots of forcing, and a lot more instability
due to the warmer temperatures. While the temperatures warm more
on Wednesday with a slight drop-off on Thursday, many areas will
be over 75 degrees the next 3 days. Capping on Wednesday with the
ridge moving overhead will keep any storms isolated. That said,
any storms that form today and on Wednesday could be strong due to
quite a bit of instability developing under the building cap. By
Thursday, a front moving through will reduce the instability a
fair bit, but the front will add to the forcing, so storms should
be more numerous on Thursday as compared with Wednesday, but
should not be as strong as today or Wednesday.

In Southwest Alaska, the threat for showers and isolated
thunderstorms across the region are winding down today as high
pressure continues to strengthen. A low chance for an isolated
wet thunderstorm will briefly exist over the Western Alaska Range
later this evening with a potential southward moving shortwave
trough over the Alaska Range. Should any development occur,
however, low level winds will steer it southeastward and out of
the area. Instead, a warming trend over Southwest will be the
bigger story over the next couple of days. Daytime heating will
see temperatures climb into the low to mid 70's across interior
Bristol Bay through the end of the week, with Friday afternoon
Highs climbing into the upper 70's. Winds are forecast to remain
generally light over the next few days but Tuesday afternoon
northerly winds approaching 20 mph will be possible from
Sparrevohn to Iliamna. Calmer winds are forecast Wednesday into
Friday as high pressure shifts atop the region.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Thursday)...

Southcentral's weather pattern features a ridge encroaching from
the west, and a large upper level low centered over the eastern
Gulf retreating to the east. Nonetheless with the area stuck in
between these two features, expect another round of showers and
thunderstorms to develop this afternoon and evening. The storms
will mainly develop from the Copper River Basin in the east, the
Parks Highway to the west, and a line connecting Turnagain Arm and
the north coast of Prince William Sound to the south. Widely
scattered thunderstorms are expected over the Talkeetnas and the
Chugach mountains mainly, and any storms that develop will move
south off those mountains. Temperatures will be warming
significantly today, especially over the Cook Inlet region, while
cloud cover holds tougher further east. The warmth and sunshine
will be thanks to the aforementioned encroaching ridge. Northerly
gusty gap winds out of the Alaska Range and Talkeetnas are also
expected today.

On Wednesday, the ridge moves directly overhead, which should make
this the warmest day of the week overall. The ridge will reduce
the winds area-wide. Hot conditions are a near certainty, while
RH values will likely approach critical dry weather conditions.
Expect widespread lower 80s over inland areas for the Kenai and
the Mat-Su, with coastal areas and the Copper River Basin warming
into the mid to upper 70s. Despite the capping inversion that
will significantly reduce overall thunderstorm activity on
Wednesday, most guidance suggests isolated thunderstorms are still
possible over many of the same areas getting the most
thunderstorms today, namely the Talkeetnas and Chugach mountains.

On Thursday, a weakening disturbance will slide over the northern
portions of the area. Expect increased cloud cover for most areas
to start the day Thursday, with clearing following the front by
Thursday afternoon in western areas. The Copper River Basin may
take longer for the clouds to clear, namely because the
aforementioned front will likely take advantage of the additional
solar heating out ahead of it to set off another round of showers
and thunderstorms. Due to the preexisting cloud cover, the
instability over the Copper River Basin Thursday afternoon will be
lesser, but the cap will be eliminated, so expect more
thunderstorm coverage Thursday than Wednesday, but the storms will
be weaker in general.

&&

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

High pressure continues to strengthen over Southwest Alaska this
morning, which should help to eventually bring widespread clearing
to the region later this afternoon. In the short term, however,
many locations will start the day under low stratus and fog,
especially over the Kuskokwim Delta and over portions of Bristol
Bay. Fog and low clouds have become prevalent from Quinhagak to
Tuntutuliak to Tuluksak across the Kuskokwim Delta. Fog also
stretches southwestward into Cape Newenham. Both areas will
eventually see improvement in visibility later this morning as
daytime heating promotes vertical mixing and the eventual
dissipation of the fog layer. Across the Lower Kuskokwim Valley
and eastern portions of Bristol Bay residual cloud cover from
yesterday's shower activity continues to advect southward into the
Alaska Peninsula. This too should begin to clear through midday.
Precipitation chances through Wednesday will remain slim to none
with morning fog and areas of stratus possible each morning. An
exception may exist over the Western Alaska Range where an
isolated shower or two, and perhaps a wet thunderstorm, may
develop later this evening. Should any development occur it will
quickly drift southeastward and out of the area.

By Thursday a shortwave trough is forecast to push across the
northern extent of high pressure and dig southeastward across
Southwest. A dry air mass will be in place and should preclude any
significant shower chances. Initial rain may overspread Nunivak
Island and Toksook Bay before drying up further inland. Finally,
areas of fog and stratus will pester the Alaska Peninsula through
the middle of the week beneath the ridge of high pressure as
trapped moisture over the Bering, due to an inversion, drift
southward.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Today through Thu night)...

A front has stalled over the Western Aleutians this morning with
scattered light showers west of Adak. Shemya is reporting fog and
reduced visibility down to one half mile. Low stratus and fog are
being maintained by high pressure over the Bering Sea with little
improvement forecast over the next couple of days. The front over
the Western Aleutians will gradually fall apart through Tuesday
afternoon. The next North Pacific low will lift across the Western
Aleutians on Wednesday and bring southeasterly small craft and
gale force winds from the Western Aleutians into the Central
Aleutians as the front moves eastward through Thursday. A pattern
change occurs over the Bering Sea and the Aleutian Chain, as high
pressure over the southern Bering is forecast to slowly break
down while getting pushed eastward into Southwest Alaska by the
end of the week.

&&
.MARINE (Days 3 through 5: Thursday through Saturday)...

Bering/Aleutians: Eastern Bering high pressure moves inland through
Saturday. A North Pacific low moves over the Western Aleutians
while its front pushes into the Central Aleutians through Saturday.
Forecast confidence is fair with the movement of the low. Gusty
Southerly winds increase with areas of gales with the front moving
into the Central Aleutians, diminishing Saturday. Seas build to
18 feet on the Pacific side of the Central Aleutians, and to 12
feet over the Central Bering for Friday, subsiding.

Gulf of Alaska: A trough over the Western Gulf dissipates by
Saturday. High pressure spreads across the Gulf for Saturday.
Forecast confidence is good. Generally light winds over the Gulf,
except for gusty Northwest winds over the Barren Island through
Saturday. Seas mostly 6 feet or less, but rougher through the
Barren Islands through Saturday.

&&

.Long Term Discussion (Days 4 through 7: Friday through Monday)...

Looking ahead into the extended forecast, the latest operational
model runs are in good agreement with the evolution and
progression of the synoptic scale features, but diverge in the
specific details towards the end of the period. After a shortwave
trough and associated cold front passes through and exits the
eastern interior, a strong upper ridge builds from the eastern
Bering Sea into Mainland Alaska. Deterministic models seem to
agree on this pattern by the weekend into next week, with
temperatures expected to be above normal for interior southern
Alaska. Out west, a couple of North Pacific lows lift toward the
western Bering Sea from late this week into early next week. Even
though there are some timing differences, much of the western and
central Aleutians can expect increasing rain chances and possible
gusty winds along and ahead of the attendant fronts. The
aforementioned ridge will prevent the eastward progression of
these systems, leaving the rest of the Aleutians and western
mainland Alaska relatively warm and dry.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 177 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CQ
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BL
MARINE/LONG TERM...KZ/MF