Small Craft Advisory 472, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
527 AM AKDT Mon Jun 20 2022

.ANALYSIS AND UPPER LEVELS...

A broad, occluded low centered southeast of Kodiak Island continues
to gradually weaken. As the low tracks further eastward into the
Gulf of Alaska, its moisture advection, which has been the theme
the past few days across southern mainland Alaska, has come to an
end. The cool, moist airmass wrapping around the low has ceased
pushing onshore, with flow in the low and mid-levels of the
atmosphere taking on a direction more parallel to the coast. In
addition, shortwave ridging over the central and eastern
Bering/Aleutians has begun to nudge in over Southwest Alaska, with
a weak ridge bridging across interior Alaska to meet a well-
established ridge centered over the Canadian Northwest
Territories. This changing regime has allowed cloud cover over
inland portions of the domain to begin to show some breaks on
infrared satellite imagery and heralds a shift to the warmer,
drier weather expected over the next several days. As a result,
shower coverage has decreased inland, while more widespread rain
remains over the western Gulf.

Out west, widespread stratus remains the status quo. Late last
evening, a ship south of the central Aleutians reported areal fog
with visibility as low as one eighth of a mile, which appears to
correspond to a thicker patch of stratus which persists on
infrared satellite imagery along and south of the central
Aleutians in association with the strong ridging in place. Lastly,
another weak low in the western Bering Sea is tracking northward
accompanied by a broad swath of southerly small craft winds.

&&

.MODEL DISCUSSION...

Models are in good agreement regarding the progression of the
synoptic-scale pattern across the domain, with all models
depicting a ridge in the central Bering building eastward into
Southwest Alaska and then extending across much of the state. The
biggest source of uncertainty in the forecast is the conflict
between model QPF and instability. Model QPF fields are depicting
storms with limited coverage in comparison to areas forecast to
have sufficient instability for thunderstorm development. This
results in decreased confidence in the potential for and areal
coverage of thunderstorms, especially on Tuesday in Southcentral.

&&

.AVIATION...
PANC...VFR conditions with ceilings above 5000 ft will persist
through the TAF period. Southeasterly winds diminish this morning
with a west to southwesterly sea breeze developing this
afternoon. Otherwise, winds are expected to remain light at the
terminal through Tuesday morning.

&&

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

A vertically-stacked low currently southeast of Kodiak Island will
move east into the central Gulf by this evening. On this track,
the flow will become northeasterly with time. Meanwhile an
easterly wave circulating around the low will cause widespread
wetting rains across the Copper River Basin. In contrast, much of
the Cook Inlet region will continue drying out today under a mix
of sun and clouds. Temperatures will be significantly warmer
today through the Cook Inlet region, and fairly cooler than
yesterday in the Copper River Basin.

The upper low over the eastern Gulf to start the day Tuesday will
move near or over the southern Panhandle by Tuesday evening. As
this low departs, a strong ridge over Southwest Alaska will
continue building into Southcentral from the west. This will make
for MUCH warmer conditions area-wide on Tuesday as the ridge is
associated with a much warmer air mass. Tuesday is also the
Summer Solstice so we will have maximized the intensity and
duration of solar heating as well. With lots of sunshine
especially over the Cook Inlet region, the ground and atmosphere
will get to take full advantage of the nearly 20 hours of
daylight. There is some uncertainty as to thunderstorm coverage
Tuesday afternoon and evening. More on that below, in the fire
weather section.

On Wednesday, the ridging will be fully over the area with the
axis of the ridge going right up Cook Inlet to the Copper River
Basin. Expect plenty of sunshine once again, perhaps even moreso
than Tuesday. In addition, the warming air mass in general will
allow highs to spike near 80 in many inland locations while
coastal locations hover near 70. Any thunderstorm activity will be
significantly limited by the capping associated with the ridge,
but with ample instability, any storms that do form could become
strong.

Looking ahead, the models are falling in line with yesterday's GFS
solution going into the weekend. First a quick break from the heat
on Thursday as a strong shortwave skirting by north of the area
potentially results in more widespread shower and thunderstorm
activity from Anchorage north. Then a large upper level high moves
in from the Aleutians and sits over Southcentral right through the
weekend, and perhaps longer. Thus, 70s and lower 80s are quite
probable with lots of sunshine.

&&

.FIRE WEATHER SOUTHCENTRAL ALASKA (Today through Wednesday)...

Today, expect minimal activity as isolated wet thunderstorms are
possible on the mountains on either side of the Susitna Valley.
Widespread broken cloud cover should hinder too much instability
from developing.

On Tuesday, uncertainty increases as to how much shower and
thunderstorm activity can develop as a strong ridge begins to
build into the region. On the one hand, the ridge will allow for
a lot more sunshine, warmer temperatures, and thus, instability to
develop by the afternoon. On the other, the ridge will cap all of
this instability, which increases the chances of just some fair
weather cumulus development that is not allowed to vertically
develop into thunderstorms. Since the main ridge axis stays west
of the area on Tuesday, think isolated to widely scattered wet
thunderstorm development is likely, especially over the Talkeetnas
and the Chugach east of Anchorage. Significantly more uncertainty
exists for thunderstorms in the Kenai Mountains. Regardless, with
lots of instability, any storms that are able to overcome the cap
have the potential to become strong, with frequent lightning,
gusty winds, heavy rain, and small hail possible.

By Wednesday, the ridge axis moves right overhead, which will
really enhance the capping. Most models suggest something will
form Wednesday afternoon, but they will be quite isolated unless
any associated gust fronts improve the lift to allow new storms to
form. For sure, less thunderstorm activity is expected Wednesday
as compared with Tuesday.

Hot, dry, and windy conditions are also possible on Tuesday,
especially over the Susitna Valley as gusty northwesterly gap
winds funnel through the Alaska Range. The gustiest conditions
will be through Broad Pass and along the Denali Highway corridor.
Also expect gusty conditions in the western Susitna Valley from
Skwentna west. These downsloping winds will warm the temperatures
and also dry out the atmosphere. Gusty winds are also expected in
the Copper River Basin as well, but with more cloud cover
lingering there, think hot and dry conditions are unlikely.

Looking ahead, hot and dry conditions are likely again Friday
through this weekend. As of now, the upper high moving right
overhead will limit the wind threat during this time.

&&

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

Weak onshore flow along the Kuskokwim Delta is bringing in a
blanket of low stratus and fog from Toksook Bay southward into
Cape Newenham this morning. A ridge of high pressure over the
southern Bering will slide eastward and into Southwest over the
next couple of days with warmer and clearer conditions forecast.
In the short term, however, daytime heating on Monday will allow
instability to build over the Middle Kuskokwim Valley with the
same airmass responsible for showers and wet thunderstorms
experienced on Sunday. The greatest threat for wet thunderstorms
will exist from the Western Alaska Range, southwestward, through
the Middle Kuskokwim Valley and into Bristol Bay late this
afternoon and evening. Beyond Monday, precipitation chances
diminish considerably.

&&

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

Satellite reveals that the marine layer is extensive over the
eastern Bering Sea this morning and stretches southward into the
Aleutians and Alaska Peninsula. A ridge of high pressure over the
southern Bering Sea is responsible for the widespread cloudiness,
which will persist beneath the ridge as it gradually expands
eastward and into Southwest Alaska over the next couple of days.
This will promote continued low stratus and areas of fog over the
eastern Bering Sea.

Further west, southerly small craft winds are ongoing across the
Western Aleutians and extend just west of Saint Matthew Island as
a shortwave trough and front continue northward across the Western
Bering Sea. The axis of small craft winds will persist into early
Tuesday before dissipating from south to north while also
shifting eastward over Saint Matthew Island briefly. Main concerns
for the Aleutians will entail the next upper low and front, which
is forecast to overspread the Western Aleutians late Tuesday into
Wednesday. Scattered showers will accompany the low's arrival
along with southerly gales over the Western Aleutians. Seas up to
15 feet may devlop on the Pacific side of the Western and Central
Aleutians coincident with the higher winds.

&&

.MARINE (Days 3 through 5: Wednesday through Friday)...

Bering/Aleutians: Southerly winds around 30 kts are expected
by early Wednesday morning, as a North Pacific high resides over
the Central and Eastern Aleutians and a 998 mb low approaches
Shemya. Seas up to 14 ft in the Bering Sea, and up to 18 ft on
the Pacific side of the Aleutians, from Wednesday through
Friday.

Gulf of Alaska: The Barren Islands will have gusty winds and rough
seas this forecast period. Expect weak cyclonic flow over the Gulf
Of Alaska on Wednesday through Friday. While wave heights will be
rough by the Barren Islands and SE of the Barren Islands, most of
the Gulf can expect wave heights less than 6 feet from Wednesday
through Friday. Prince William Sound the wave heights will average
around 2 feet or less.

&&

.Long Term Discussion (Days 4 through 7: Thursday through
Sunday)...

Thursday there is high confidence of a ridge from the Central and Eastern
Bering into Mainland Alaska. The GFS and the ECMWF have this ridge
over the Eastern Aleutians and Western Alaska through Saturday.
The GFS actually keeps the positive height anomalies over western
Alaska and much of the Aleutian chain through June 27th (Monday).
There is a North Pacific low that approaches the Western Aleutians
Thursday and Friday, but it loses its gusto pretty quickly. The
ridge to the east will impede its eastward progression. By Sunday
morning the surface low is still west of Adak.

Overall confidence is decent, the major synoptic features are in
good agreement among the global models. The greatest uncertainty
is pin pointing the areas of fog and low stratus over the Bering
and the timing/tracking of shortwave energy over the Southern
Mainland.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

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