Small Craft Advisory 440, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...

An upper level ridge resides over the Alaska Yukon border with a
col of inactivity over much of southcentral Alaska as evidenced by
the weak winds and absence of sensible weather. A stacked low is
approaching the southern Gulf from the north Pacific. A weak
shortwave is tracking through southwest Alaska and the Kuskokwim
Valley with some light rain showers falling in Kalignek and
Aniak. An upper trough is located over the eastern Bering and
another trough is entering the western Aleutians bringing fog and
rain to Shemya this morning.

&&

.MODEL DISCUSSION...

Models have come back around this morning with a general
consensus that a stacked low will park itself in the Gulf of
Alaska and generate a series of easterly waves across the southern
half of the state over the coming days. Model agreement is pretty
strong with this feature. Moving towards Saturday there is some
agreement in a low tracking along the Aleutian chain and
eventually interacting with the Gulf low, confidence is low on how
these two features will impact one another.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist through the day.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Friday night)...
A pattern change is underway which will bring notably warmer and
drier conditions to Southcentral - at least for the next couple
days. The low which had been anchoring a trough in the Bering Sea
for the past week is exiting across the northern Bering Sea while
a short-wave trough which lifted across Southcentral on Tuesday
exits across northern Alaska. A short-wave ridge will build
overhead today and merge with a stronger ridge along the ALCAN
border. Meanwhile, a vertically stacked low over the northeast
Pacific will track slowly northward toward the Gulf of Alaska.
This will set up offshore flow today, which will strengthen
tonight through Thursday as the low makes progress into the Gulf.
Offshore flow combined with warming beneath the upper ridge will
lead to much warmer temperatures across the region through the
remainder of the work week.

The precipitation forecast is a little trickier. As the Gulf low
makes more northward progress, short-waves rounding this low will
gradually make their way into Southcentral. These "easterly
waves" are difficult to forecast very far out in time. The initial
waves moving through on Thursday look relatively weak, so expect
isolated to scattered convection, mainly inland. It looks like one
or more significant waves could move through on Friday. With
continued low level warming, stability indices also indicate
potential for thunderstorms on Friday afternoon/evening. Putting
it all together, expect warm and mostly dry conditions over the
next couple days followed by a likely increase in cloud cover and
convective activity on Friday.

&&

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

An upper level trough lifts northward over the Kuskokwim Delta
Coast today. Showers are expected over the Kuskokwim Mountains
this afternoon and evening, and a rumble of thunder cannot be
ruled out. A pattern change arrives Thursday as a ridge over the
eastern Bering moves eastward. The first of several upper level
easterly waves arrives over Southwest early Thursday, which will
generate offshore (northerly) winds across the area. This will
allow for a warming and drying trend. Clearing skies will allow
for increased daytime heating, which will drive high temperatures
on Thursday into the upper 60s across most interior locations. By
Friday, several waves rotating around a low centered in the Gulf
of Alaska are ejected westward over Southwest. Meanwhile,
strengthening northerly flow in the midlevels will aid the warming
and drying trend, with temperatures expected to climb into the
70s Friday afternoon across much of Southwest. There is increasing
confidence that isolated thunderstorms will develop over the
Kuskokwim Valley Friday afternoon.

&&

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

A low centered west of Shemya strengthens as it lifts northward
today. The associated front tracks eastward across the Aleutians
bringing gusty southerly winds and rain. This front weakens and
dissipates just before reaching the Pribilof Islands on Thursday.
Meanwhile, a second low develops south of Shemya and moves into
the Bering Friday morning. Southerly winds and rain are expected
to continue across the Chain Friday as this second front moves
eastward. By Saturday morning, the front weakens and stalls over
the central Bering.

&&

.MARINE (Days 3-5: Friday through Sunday)...
Gulf of Alaska: No significant winds or seas are expected. A weak
low skirting the Gulf Friday however could briefly produce high
end easterly small craft winds over the southern Gulf through
Friday evening.

Bering Sea/Aleutians: A weather front will bring sub-gale
southerly winds to the eastern Bering Sea/east Aleutians Friday.
Winds will then diminish through Sunday.

&&

.LONG TERM FORECAST (Days 4-7: Saturday through Tuesday)...
The upper level Rex block over the mainland and the Gulf starts
to break down Saturday as the upper ridge retreats north and
splits. This allows the Upper low over the Gulf to track slowly
north through early next weak. The increased easterly flow aloft
will drive easterly waves west across Southcentral. This will
result in cloudy conditions, increased chances of rain/showers,
and cooler temperatures across much of Southcentral into early
next week.

For Southwest Alaska, an upper level trough will setup over the
southern mainland this weekend. This will generally bring
increasing clouds and chances of showers. Any stronger upper
waves could set off isolated thunderstorms in the afternoons,
depending on how much instability and sunshine develop earlier in
the day.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 185 352 412 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...LTB
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...RC