Small Craft Advisory 476, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...

The pattern is dominated by a weakening upper-level low approaching
the Alaska Panhandle and an upper-level ridge upstream extending
from the central Aleutians northeast to central portion of the
state. A shortwave wave trough is moving south from the Copper
River Basin into the northern Gulf, embedded in the north-
northeasterly flow between these two large-scale features. Rain
this morning from Gulkana south is also moving over the Gulf;
however, cloud cover continues to linger in the wake of the
precipitation. Elsewhere over Southcentral, conditions are
generally clear and dry with a few cumulus clouds developing
along the higher terrain. Temperatures are already pushing well
into the 70s under mostly sunny skies.

For Southwest Alaska, subsidence ahead of the ridge axis is
keeping conditions across the Middle Kuskokwim Valley and greater
Bristol Bay clear and dry. Elsewhere, a stable maritime airmass
associated with the ridge is advecting over the Kuskokwim Delta
and Alaska Peninsula due to a westerly flow in the lowest levels
of atmosphere. Temperatures for these locations are struggling to
reach 50 degrees.

Farther west, the low stratus deck encompasses much of the Bering
Sea this afternoon. A stalled frontal system lingers across the
western Bering along with southerly winds. A North Pacific low
continues to advance toward the western Aleutians.

&&

.MODEL DISCUSSION...

Models still depict a shortwave dropping south across Southcentral
today with the potential for shower and thunderstorm activity.
Again, the biggest challenge through this evening will be whether
there is enough instability to overcome the weak subsidence from
the building ridge upstream. Hi-res guidance all show convective
develop along the higher terrain of Southcentral, but disagree as
to the extent of development. The next challenge will be the
timing and location of a two troughs moving from western AK into
Southcentral for Wednesday into Thursday. The 12z NAM and GFS are
now indicating slightly stronger troughs with a more southern
extent into Southwest Alaska. If this trend holds, it could denote
a better potential for convective activity across the Alaska
Range for Wednesday and over Southcentral for Thursday.

&&

.AVIATION...
PANC...VFR conditions with ceilings above 5000 ft are expected
through the TAF period. A westerly sea breeze will continue
through this evening. Isolated rain showers are possible in the
vicinity of the airport during the afternoon and evening
timeframe. While thunderstorms are not currently expected to
directly affect the terminal, 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, but 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 slightly lower temperatures Thursday as
a weak front moves across the state. Northerly gap winds will
likely increase later today, especially along the Alaska Range.
Winds will diminish for Wednesday as the ridge moves overhead,
only to pick up again for Thursday with passage of the front. Hot,
dry, windy conditions don't look to line up in conjunction anywhere
in Southcentral, but certainly very warm conditions will be
widespread. There is some uncertainty as to how dry it will get
Wednesday and Thursday with such warm temperatures.

Thunderstorms are expected across parts of Southcentral over the
next several days. All thunderstorms on all three days will be
wet. Tuesday will likely have the most coverage by far, especially
over the Talkeetna and Chugach Mountains, due in part to the
instability of a shortwave trough moving south across the region.
Capping on Wednesday with the ridge moving overhead will help
limit convective development and likely keep any storms isolated.
By Thursday, a front moving through will allow the threat for
isolated thunderstorms to persist. While instability parameters
may be weaker, lift out ahead of the front should provide enough
forcing for more widespread shower and thunderstorm activity than
Wednesday.

For Southwest Alaska, the building ridge should keep any
convective activity isolated and mainly confined to mountain
slopes. A warming trend over interior Southwest will also take
place over the next couple of days in association with the ridge.
The biggest challenge will be the extent of a more stable marine
layer advecting over the Kuskokwim Delta and Alaska Peninsula.
Expect cooler temperatures, low stratus, and areas of mist/fog
for locations under the influence of this airmass. Winds are
forecast to remain generally light over the next couple of days;
however, northwesterly wind gusts approaching 25 mph will be
possible Tuesday afternoon from Sparrevohn to Iliamna. Calmer
winds are forecast Wednesday for the remainder of the week as
high pressure shifts atop the region.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Tonight through
Friday Evening)...

A vertically stacked low in the Gulf is moving east out of the
region, shifting the flow to northerly for Southcentral tonight.
Steady precipitation associated with the aforementioned low has
moved out of Southcentral and is being replaced by convective
activity approaching from inland Alaska. Widely scattered
thunderstorms can be seen in the Copper River Basin and north of
the Alaska Range and are currently moving south.

A growing ridge is approaching from the Bering Sea. This high
pressure will bring a warm, dry weather regime this week for most
of Southcentral. Winds will remain calm over much of Southcentral
with the ridge in place, except for gusty NW winds expected for
the Shelikof Strait and around the Barren Islands. Isolated to
widely scattered thunderstorms are possible Wednesday afternoon
over the Mat-Su Valley and Copper River Basin given the daytime
heating. The growing ridge may also provide ideal conditions for
marine stratus to develop over Cook Inlet and into the coastal
Gulf areas. A shortwave originating from a polar low is expected
to approach Southcentral by Thursday which may, depending on
timing, augment thunderstorm potential.

&&

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

High pressure continues to build over the Southwest this forecast
period resulting in a warming trend. Warm air over cold water is
inherently stable so expect areas of fog and low clouds along the
coast, the AKPEN and in Bristol Bay especially in the mornings.
Yesterday, there were over 5,000 lightning strikes in the state of
Alaska most of which were in the interior. This evening there is
potential for isolated wet thunderstorms along the Western Alaska
Range. Late Wednesday and Thursday a shortwave trough will push
across Nunivak and the upper level ridge axis will re-orient.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

There is a mature low south of Attu Island that will move towards
the Western Aleutians this evening. This low will actually start
to retrograde on Thursday as the dome of high pressure remains
steadfast over the Central/Eastern Bering Sea and the North
Pacific. Expect low stratus and fog being trapped by high pressure
over the Bering Sea with little improvement forecast over the
next few days. A re-enforcing shortwave will move through the base
of the upper level trough late Thursday and a widespread batch of
unsettle weather will move into the Western Aleutians. Expect
small craft to gale force winds from the Western Aleutians into
the Central Aleutians as the front moves eastward through Friday.

&&

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

Bering/Aleutians: Eastern Bering high pressure moves inland
through Saturday. A Western Bering low moves through the Gulf of
Anadyr, while its front extends through the Central Bering and
Eastern Aleutians to a another low in the North Pacific South of
Dutch Harbor for Saturday. Forecast confidence is fair due to the
movement of the front. Widespread small craft winds with areas of
gales with the front move into Eastern Bering, diminishing Sunday.
Seas to 14 feet, subsiding.

Gulf of Alaska: High pressure lingers across the Gulf through
Sunday. Forecast confidence is good. Generally light winds over
the Gulf. Seas mostly 6 feet or less.

&&

.Long Term Discussion (Days 4 through 7: Saturday through Tuesday)...

A ridge will continue to build over the state this week in a Rex
block pattern with a stalled, large low located south of the Gulf
over the Pacific Ocean. An embedded shortwave will develop along
the ridge axis later in the week as it moves east. By Saturday
morning, the trough will be located over eastern Alaska near the
AlCan border. As the trough becomes influenced by the blocked flow
pattern, it may slow and drop south throughout the day Saturday.
The ridge however will remain in place over Alaska so while the
overall weather pattern is expected to lean warmer and drier than
normal through the entire extended forecast, this shortwave may
allow for slightly cooler weather going into this weekend with a
brief period of unsettled weather possible, particularly for the
Copper River Basin and possibly into the Anchorage Southcentral
area. There is some uncertainty with regards to how far south the
shortwave will influence Southcentral, the timing, strength of
this trough, and its extent westward along its way south. The
latest model runs have it edging further towards the Anchorage
area than in previous runs. The strength of this ridge will
quickly push the low south. For many, the unseasonably warm
temperatures will continue. Any easterly waves over Southcentral
may introduce opportunities for thunderstorm activity.

Over the Aleutians and the Bering sea, a large low will move
through from the west towards the end of the week bringing gusty
winds and rain to the eastern Aleutians through Saturday.
Circulation could develop around the eastern Aleutians and AKPen
area Sunday afternoon that could lead to some unsettled weather
for the eastern Aleutians, AKPen, and toward Bristol Bay through
Tuesday. Model guidance beyond Sunday breaks down substantially
and there is low certainty in the progression of this circulation
as it will be influenced by the strength of the ridge over Alaska
and the shortwave over the east.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 176 177 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...CL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS
MARINE/LONG TERM...KZ/VR