Small Craft Advisory 442, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
434 AM AKDT Thu May 28 2020

.ANALYSIS AND UPPER LEVELS...

An upper level ridge from the Yukon border to the Alaska Peninsula
is pushing back against a semi-stationary stacked upper low in the
Northeast Pacific. Another upper ridge is building into the
eastern Aleutians from the North Pacific. An upper level low off
the Kamchatka Peninsula is pushing a front into the Central Bering
and Aleutians. The abundance of high pressure and weak pressure
gradients is keeping sensible weather in short supply this
morning. Some gusty southerly winds and rain in Adak and Atka
are the main topic of weather conversation.

&&

.MODEL DISCUSSION...

Models continue to agree on the Northeast Pacific low approaching
the Gulf of Alaska as high pressure recedes some. Thus, allowing a
series of easterly waves to propagate through the mean flow
between these two features. This will be the primary pattern for
the Thursday through Saturday time frame. Sunday things begin
shifting as an approaching low from the Aleutians begins to
interact with the Gulf low. There is some vague agreement between
models that this will continue the pattern of easterly shortwaves
impacting southern Alaska.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist at terminal
through the period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Saturday night)...
Offshore (northerly) flow will strengthen today across Southcentral
in response to a vertically stacked low moving into the Gulf of
Alaska. This will continue the warming and drying trend that began
on Wednesday. As inland areas warm up and become unstable expect
gusty winds and drier air to mix down to the surface, especially
over the northern Susitna Valley and northern Copper River Basin.
Offshore flow will also delay sea breezes along the coast which
will allow coastal communities to join in on the warm temperatures.
While the low levels will be quite dry, a leading upper wave
rounding the top of the Gulf low will combine with steep lapse
rates to initiate some isolated to scattered convection near the
mountains in the Copper River Basin. Storm motion will take these
showers (and possibly thunderstorms) westward across portions of
the Mat-Su.

The thermal trough will strengthen over interior Southcentral
on Friday while weak upper level waves continue to traverse
the region on the northern periphery of the Gulf low. Expect
a marginal increase in cloud cover and convection. The primary
corridor for showers and thunderstorms will remain the same,
from the Copper River Basin to the Mat-Su. However, will also
likely see a few showers for coastal areas as well.

Forecast confidence drops significantly for Friday night and
into the weekend due to a high level of uncertainty in the
amplitude and track of successive upper level short-waves.
The pattern favors cloudier and wetter conditions with a
likelihood of stronger "easterly waves" producing rounds of
steady rain. Stay tuned to forecast updates for the weekend
as we get closer and are able to better resolve where these
upper waves will track.

&&

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

An upper level ridge has built over southwest Alaska this morning.
A warming and drying trend associated with this feature will
continue for the forecast period. Expect weak instability
associated with a strengthening thermal trough following the
Alaska Range. This will increase the potential for thunderstorm
development this afternoon and into this evening for west of the
Alaska Range and into the Middle Kuskokwim Valley. The thermal
trough will continue to strengthen into Friday allowing for
increased instability across the region. This will lead to greater
chances for thunderstorms from west of the Alaska Range to the
lower Yukon Delta for Friday afternoon.

&&

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

A front associated with a low pressure system northwest of the
Bering Sea is bringing rain and increased winds to the western
Aleutians this morning. This feature will continue to propagate
eastward before it starts to dissipate Friday. Expect the front
to bring rain to the central Aleutians Friday and up into the
Pribilofs on Saturday before it completely dissipates. Another
low will move across the Aleutians to the south beginning
Saturday. This will bring increased winds, waves and chances for
precipitation to the Chain as it propagates to the east through
the end of the forecast period.

&&

.MARINE (Days 3 through 5)...
By Saturday, the low south of the Gulf will have weakened
significantly and then get absorbed by a new low late Monday.
There is a chance this new low could bring a barrier jet capable
of gale force winds to the north Gulf coast late Monday into
Monday night. This is uncertain as the low and front would have to
develop just right, but models are indicating this as a
possibility.

A front along the low on Sunday has the potential to bring an
area of gales along the eastern Aleutians on Sunday, but they
should be short-lived if they do occur.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The upper level low will have located itself along the Alaska
Arctic coast by Sunday. It will then remain there through Monday
and then move northward into the middle of next week. As this
happens, the upper level trough which will extend across Southwest
Alaska through the weekend will move south of the state setting
up easterly flow aloft for the weekend through the middle of next
week.

This pattern sets us up for easterly waves to move across
Southcentral and Southwest mainland Alaska. This will produce
more cloudiness with chances for showers over the region. Some
stronger upper level waves coupled with local areas of instability
and sunshine may spur isolated thunderstorms during the
afternoons, but details are too far out at this time to specify.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 174 175 176 413.
FIRE WEATHER...Red Flag Warning 145.

&&

SYNOPSIS AND MODEL DISCUSSION...LTB
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CK
MARINE/LONG TERM...EZ/MK