Small Craft Advisory 441, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
330 PM AKDT Wed May 27 2020

.ANALYSIS AND UPPER LEVELS...
The upper level pattern shows a cold trough aloft over Southwest
mainland and the northern Bering Sea and a ridge axis over the
Yukon just east of the Alcan border. This is producing southerly
flow aloft over southern Alaska with some convection in Southwest
Alaska due to the colder temperatures aloft and proximity of the
trough.

Visible satellite shows a spectacular-looking occluded low is in
the North Pacific to the south of the Gulf of Alaska. The leading
front associated with this low is approaching the Gulf and will
ring some Gale force winds to the Gulf Offshore waters tomorrow,
but not be able to make much more headway to the north.

&&

.MODEL DISCUSSION...
Models remain in good synoptic agreement with the main lows into
the weekend. The area of concern resides in the timing and
amplitude of a series of easterly waves that will be traversing
Southern Alaska from the Alcan border to the Kuskokwim Valley late
Thursday through the weekend. In general, the first of these
waves does look to be coming faster than indicated yesterday with
it entering the Copper River Basin by Thursday evening which is
sooner than expected yesterday. As models all seem to be trending
this direction will generally bring in clouds and showers sooner
with this forecast package.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist. There is a
chance for northerly winds to be around 10 kt Thursday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
A vertically stacked low slowly tracking into the Gulf combined
with surface high pressure over the Arctic will lead to offshore
flow and thus mostly sunny conditions across Southcentral through
Friday. Warmer temperatures are likewise expected as an upper
level high builds over the Yukon. Widespread high temperatures in
the 70s are likely Thursday and Friday. These in conjunction with
the aforementioned stronger offshore flow and low relative
humidity have constituted the issuance of a Red Flag Warning for
the Susitna Valley Thursday afternoon and evening.

The upper low in the Gulf will continue to move northward by
Thursday, with associated easterly shortwaves propagating over the
region. Initially, these waves will be rather weak, only leading
to a few showers possible near higher terrain. However, more
potent shortwaves on Friday combined with the instability from
daytime heating will lead to the potential for scattered showers
and thunderstorms developing in the afternoon/evening,
particularly over the Copper River Basin. Brief gusty winds are
also expected along Turnagain Arm around the same time. A wetter,
more active pattern is expected this weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
As the upper level trough lifts north across southwest Alaska
tonight, a ridge of high pressure moves in behind it and builds
across the Kuskokwim Delta through Thursday. Some weak instability
can be expected over southwest Alaska this evening and again
Thursday. As we move into Friday, we will see some low level
warming over Bristol Bay into the middle Kuskokwim Valley coupled
with a cold upper trough advecting across the region from the
east. This will set the stage for increased instability and likely
produce scattered thunderstorms across the region late Friday
afternoon through Friday evening. We will likely have enough
moisture to keep any thunderstorms on the wet side. The low level
north to northeast flow will factor in to increases temperatures
to around 70 across the middle Kuskokwim Valley and interior
Bristol Bay.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
The Bering sea remains under the influence of the upper to
surface low pressure anchored over the western Aleutians to
western Bering Sea through Saturday. The first low just west of
Shemya will weaken as it moves toward the northern Kamchatka
Peninsula Thursday night and swing its associated front east
through the Aleutians to just west of the Pribilof Islands.
Another low will move in from the North Pacific to the central
Aleutians Thursday night. This low will move into the central
bering Sea by Friday evening and weaken. Overall, this will hold
the Aleutians from Dutch harbor west and the Bering Sea from the
Pribilofs west in the clouds and potential for rain the next few
days. The Alaska Peninsula will remain under weak high pressure
after the current trough moves out of the area this evening.

&&

.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 175 176 185.
FIRE WEATHER...Red Flag Warning 145.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SA
MARINE/LONG TERM...EZ/MK