Small Craft Advisory 546, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
515 AM AKDT Wed Jul 15 2020

.ANALYSIS AND UPPER LEVELS...

Starting in the west, an upper level low and associated warm
front are located near the western Aleutians, while a shortwave
trough is moving off the northeastern Bering Sea into the western
part of Alaska. Sandwiched between these two features is a
vertically stacked ridge from the eastern Aleutians extending
northwest into the Bering Sea. Another upper level ridge extends
from Southwest Alaska into the Interior, with an associated
surface high just offshore near Kuskokwim Bay. A low level thermal
trough is located from the Copper River Basin westward toward the
MatSu Valleys. Finally, an upper level low and surface low over
the northeastern Gulf is moving out of the forecast area.

&&

.MODEL DISCUSSION...

Models are in good agreement with an upper level ridge building
into Southwest and Southcentral Alaska Wednesday and then
weakening Thursday and Friday. Meanwhile a shortwave trough moving
east-southeast toward Southcentral Alaska on Wednesday and
Thursday is handled fairly well with some minor differences on
position and strength. An upper level low moving east-northeast
across the Aleutians is handled well through Thursday, while on
Friday some differences emerge on the location as it slows down
and the low elongates. The Canadian model is currently showing
two separate lows while the other model solutions indicate different
variations of an elongated low with a new low forming in the
southwest quadrant of the parent low. A broad ridge forms over
the Gulf from a previous ridge over Southwest and Southcentral
Alaska. This ridge is slightly stronger with the Canadian model
extending it a bit farther inland into Southcentral while other
solutions are slightly weaker, keeping much of its influence over
the Gulf and coastal areas of Southcentral Alaska.

Surface features match up well through Thursday with a warm front
and associated precipitation moving into the AKPEN, Southwest
Alaska, and northern Gulf of Alaska. This front weakens quickly
Thursday night through Friday morning. The thermal trough will
continue Wednesday and Thursday across interior parts of
Southcentral and appears to weaken a bit on Friday as surface
ridging develops near the Southcentral coastal areas north of a
warm front moving into the Gulf on Friday. Only issue is seen on
Friday as two separate surface lows develop south of the Aleutians
with fairly significant differences on location of the low
centers.

&&

.AVIATION...
PANC...Light southwesterly to westerly seabreeze expected through
the evening hours with VFR conditions continuing through the
period.

&&

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

An upper-level wave will move across interior Southcentral this
afternoon. Instability associated with this system will increase
shower and thunderstorm potential, mainly over the higher terrain
of the Alaska, Talkeetna, and Chugach Ranges this afternoon and
evening. This is due to the west-northwesterly flow aloft
providing an upslope flow across the mountains which will aid in
the lift necessary for convective development. Locations across
the Matanuska Valley and lower Susitna Valley may see an isolated
thunderstorm later this afternoon and evening as storms move off
the higher terrain. Places like Talkeetna and Anchorage, however,
may miss out on any thunderstorm activity due to the westerly
flow with the best chance for convective development along the
elevated terrain east of both locations. A few widely scattered
showers may linger into the overnight and early morning hours on
Thursday as this first wave continues to track east.

A second wave will then push across Southcentral on Thursday.
This system will trigger another round of showers and
thunderstorms across the region Thursday afternoon and evening.
Thunderstorm activity may be more widespread with storm
development possible along the Kenai Mountains and over the Copper
River Basin in addition to the aforementioned areas today. The
flow aloft with this system will shift to the west-southwest,
allowing more moisture to advect over the region. The flow aloft,
however, will likely keep the brunt of the thunderstorm activity
away from places like Anchorage, Kenai, and possibly Talkeetna,
once again.

The pattern will then shift late Thursday into Friday as the
thermal trough lifts north in response to a trough moving into the
northern Gulf. This system will induce a coastal ridge which will
increase gap winds across Turnagain Arm, the Knik River Valley,
and Copper River Basin. Rain will overspread the Southcentral
coast by Friday as the trough continues to lift north and the flow
aloft switches from the west-southwest to the southeast.

&&

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

An upper level ridge remains in control across Southwest Alaska
today with warm temperatures expected. Some regions in Bristol
Bay may push the 77-78 degree F mark today. The ridge remains in
place through Thursday, though a squeeze play will begin to take
shape in the upper levels with an upper level trough dropping
southward and an approaching frontal system moving northeastward
along the Aleutian Chain. As a result, today will be the last
quiet day for much of the region. A few weak upper level waves
will move through the flow again today and may allow for cumulus
(Cu) to build up across the Kuskokwim Mountains and the western
Alaska Range. One of these may yet again grow enough to produce a
brief shower over the mountains.

Beginning Thursday, a more potent upper level wave is expected to
move into the region. Therefore, the risk for thunderstorms will
be higher than previous days due to the ridge breaking down.
Instability parameters such as CAPE (Convective Available
Potential Energy) values are between 100-500 J/Kg and LIs (Lifted
indices) are around 0. As a result, we have a trigger which is
the upper level wave moving southward, instability, and we have
moisture streaming into the area to work with. Therefore, all the
pieces of the puzzle are there to produce a thunderstorm. Right
now, it appears the Kuskokwim Mountains and the Alaska Range will
have the best chance of seeing a thunderstorm. The steering flow
is westerly through a decent depth of the atmosphere for the Lower
Kuskokwim Valley including Aniak, Sleetmute, and Lime Village for
storms to move off the mountains from west to east across the
region. This forecast is still being fine tuned, and it is
possible that thunderstorms will be more widespread.

For Friday, the region will remain under the influence of the
aforementioned upper level trough with the threat for
thunderstorms looming once again for Friday afternoon/evening.
Instability looks to be on par with the day before. Though, some
guidance shows the instability even more on Friday with CAPE
values of 200-800 J/Kg and LIs of -1 to -2. The steering flow
looks to turn more southwesterly for Friday resulting in any
storms that form on the mountains first moving from the southwest
to northeast direction. All the thunderstorm activity is expected
to remain to the east of the Yukon-Kuskokwim Delta both days,
though a shower cannot be ruled out for Friday afternoon/evening.

Bristol Bay will largely remain clear of all the activity with
another very warm day on tap for today with temperatures
decreasing a little bit each day as the ridge slowly breaks
down. As the front associated with a low pressure in the Bering
moves up the Aleutian Chain it will largely undergo frontolysis
(the dissipation of the front) as the upper level energy gets
stretched apart.

&&

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

A frontal system currently over the Aleutian Chain this morning
will continue moving eastward along the Chain bringing frontal
precipitation in the form of rain. Dutch Harbor will likely see
rain later this morning into this afternoon. Any small craft
winds with the front will largely remain over the water in the
Bering and the Pacific near Adak. The southeasterly gap winds are
expected out of the normal gaps and passes from Dutch Harbor
eastward today.

An upper level ridge is otherwise expected to dominate the western
Bering and the western Aleutians for Thursday and Friday with the
normal low stratus and patchy fog.

&&

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

Models are in decent agreement today with a low south of the
central Aleutians and high pressure over the eastern and central
Bering for Friday. Small craft winds with the low pressure system
will taper off Friday night. After this, models have come into
better alignment since yesterday with high pressure holding over
the eastern and central Bering through Saturday. Confidence has
increased that a polar low will drop down on Sunday, clipping the
western Bering with small craft winds.

A front associated with a complex low in the North Pacific is on
track to enter the southern Gulf Saturday. The current forecast
challenge is if the low maintains its center or weakens into a
trough. The former has the potential to bring high end small craft
to gale criteria winds across the Gulf on Sunday.

&&

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

Chances for precipitation will increase across the region
beginning Saturday, as moisture associated with a low south of the
eastern Aleutians works its way into the eastern Bering,
Southwest, along the Gulf coast, and into Southcentral through
Sunday. A similar low seems to be on the tail of the first,
bringing similar weather conditions to the aforementioned areas
through Monday. All in all, expect a wet pattern for the forecast
period.

For the western Aleutians and Bering Sea, expect a showery pattern
as multiple low pressure systems traverse the area. Beginning
Sunday, a polar low will drop down and clip the western Bering,
bringing ample rainfall. This will then move across the Aleutian
Chain through Tuesday.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...RC
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...CK