Small Craft Advisory 704, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
502 AM AKDT Fri Sep 18 2020

.ANALYSIS AND UPPER LEVELS...

A large stacked low is continuing to work its way north into
Norton Sound and the Seward Peninsula this morning with a
weakening dry slot now approaching the AlCan border. Areas of
showers are working their way back into the the southwestern
mainland and Mat-Su Valleys behind the drier air that was potent
enough to temporarily clear out nearly all cloud cover over
Southcentral earlier on Thursday afternoon. The tap of tropical
moisture along and ahead of the low's front has now shifted to the
east over the AK Panhandle, and this will mark the end to the
heavier/steady precip that tracked through much of the area on
Thursday. Local effect winds are still going strong with up inlet
flow becoming solidly established. The gap winds through the Knik
and Turnagain Arms that were earlier gusting up to 60-70 mph in
mainly the far southeast Anchorage hillsides are gradually
beginning to decrease with the low pulling away and the pressure
gradient slowly relaxing.

A couple shortwaves are now moving along the Aleutians out west in
the broad cyclonic flow now in place around the low centered to
the north with limited impacts in the immediate term aside from a
few bands of lower clouds. Streaks of low clouds are also
drifting over much of the Bering around and south of the low
center. Overall, the most dominant impacts with the main system
appear to be well on the downswing this morning.

&&

.MODEL DISCUSSION...

Models are in good agreement that the low near the Seward
Peninsula will continue to weaken and drift northeast towards the
Beaufort Sea, though there are some subtle discrepancies with the
next couple shortwaves that will move from the Aleutians to the
AK Gulf Saturday to Sunday. Some models show a deformation band of
rain (with possible high elevation snow) develop along the
eastern Kenai Peninsula and western Chugach Mountains to the north
of a low that should be moving well into the Gulf by Sunday.
Determining precip placement and even precip type near mountain
peaks will be a potential forecast challenge for near the end of
the weekend. Additionally, another Western Bering low will come
into play from the west by Saturday that could produce gale to
storm force winds as it moves in.

&&

.AVIATION...

PANC...VFR conditions will mainly continue, though there could be
occasional MVFR conditions with passing showers until Friday
afternoon. LLWS around 20 kts may persist through midday Friday.
Gusty south to southeast winds will persist through the end of the
forecast period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Sunday Night)...

Southcentral is on the southeast side of a large upper level low,
which formed following the passage of the strong low that caused
yesterday's winds. The low is directing southwesterly flow across
the area. This is allowing upslope flow to produce rain shower
activity from Anchorage east, as the winds run up against the
Chugach and Talkeetna Mountains. As this large-scale weather
scenario will be very slow to change, expect the best chances of
precipitation in areas near the mountains, especially in the Mat-
Su Valleys and the northern and eastern Prince William Sound. In
other areas, especially Anchorage and the Western Kenai
Peninsula, today will be a mostly dry day, though a stray shower
can't be ruled out. This will be mixed with a bit of sunshine as
well. Winds across the area persist this morning, but will
continue to gradually diminish through the day. This will remain
steady-state into Saturday morning.

Then on Saturday, a robust upper level trough will approach
Southcentral from the west, circulating around the upper low that
will be retreating to the Arctic coast. There is significant model
disagreement about how strong the upper level trough will be, and
how quickly it moves through the area. For the most part, the
models agree that the trough will be weakening as it moves across
the Cook Inlet region Saturday and Saturday night. Since the
trough will be negatively tilted, which supports the development
of precipitation, most of the models develop an area of
precipitation anywhere from Anchorage east to western Prince
William Sound Saturday evening through the overnight. That is
where the agreement ends. The westernmost models developing
precipitation over Anchorage would result in a period of cold rain
for the Bowl. The easternmost keep the band east of Girdwood. The
middle ground solution favored with this forecast package develops
the precipitation from the upper Hillside over Palmer, then up to
Hatcher Pass. Thus, lowered some of the precipitation chances for
the Bowl Saturday and Saturday night, while raising them along the
mountains. During the day, the precipitation will be all rain,
however, as slightly cooler air moves in overnight, snow levels
will drop to allow termination dust for higher elevation areas
from Glen Alps to Hatcher Pass. In Hatcher Pass, there is
potential for several inches of snow accumulation by Sunday
morning.

Shower activity will continue on Sunday across Prince
William Sound on through the Copper River Basin, while rain shower
activity continues in the mountains north and east of Anchorage.

&&

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

The low pressure system that brought the region widespread rain
yesterday is now over Norton Sound. This low will continue to
slowly drift northward over the next few days. As it continues to
meander near Nome while weakening the next few days, upper level
waves of energy will continue to rotate around the upper level
low through Southwest Alaska. This will keep the risk for rain
showers in the forecast each day of the forecast period. 850 mb
temperatures will slowly cool each day resulting in slightly
cooler morning temperatures for Saturday and Sunday morning.
Saturday morning low will drop into the mid 30s to near 40 degrees
in the Middle Kuskokwim Valley and Y-K Delta with mid to upper 40s
for the coastal regions. Sunday morning will be even cooler with
low to mid 30s across the inland areas and low 40s along the
coastal regions.

Breezy conditions will continue through today as the pressure
gradient remains in tact. This pressure gradient will begin to
lessen late today into this evening resulting in diminishing winds
across the area.

&&

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

A low pressure system continues to move quickly southeastward
across the southern Bering and the eastern Aleutian Chain bringing
a quick burst of rain to these areas as well as an area of small
crafts to the marine zones to the south of the Chain. This low
will be moving into the western Gulf by Saturday morning.

The next big weather maker will be a storm system that will be
developing off a 120-140 kt jet racing northward from the western
Pacific towards the western Aleutians. The warm front from this
system will begin to spread rain across the western Aleutian
Chain this afternoon and this evening with increasing winds. As
the baroclinicity strengthens, this storm will begin to rapidly
intensify overnight tonight into Saturday morning. The front end
of this system will mainly bring gale force winds to the region as
the air will be a little more stable resulting in less mixing of
the higher winds aloft to the surface. On the backside of the low,
storm force sustained winds and storm force gusts are expected for
the western to central Aleutian Chain beginning Saturday night and
continuing through the day Sunday. Cold air advection will take
over on the backside creating a more unstable atmospheric profile
resulting in the higher winds aloft being mixed down to the
surface. Widespread rain is expected for the western to central
Chain beginning late this afternoon through Saturday. This system
will continue moving eastward into the Bering Saturday night into
Sunday spreading rain to the Pribilofs and the eastern Aleutians
including Dutch Harbor. With the increasing winds, wave heights
will be on the rise Saturday afternoon into Sunday morning with
wave heights of 20-27 feet on the Pacific side of the Chain and
wave heights of 10-15 feet on the Bering side.

&&

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

A strong low entering the Bering Sea on Saturday seems to be
trending stronger with the newer model runs. Thus, expect gale
force winds will more widespread in nature across the Bering Sea
and Aleutians.Saturday night through Monday. There is also the
possibility of a brief period of storm force winds along the
Aleutian Chain as colder feeds into the back side of the
storm Saturday night into Sunday. By Tuesday the low crosses
over the Aleutians and moves into the Northeast Pacific/Gulf of
Alaska. There is disagreement on the track, but with the latest
models generally farther south, suspect the bulk of small
craft/gale force winds may be confined to the southern Gulf/Kodiak
Island.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday through Thursday)...

A strong low in the vicinity of the Eastern Aleutians/Alaska
Peninsula on Monday will continue southeastward into the
Gulf/Northeast Pacific by Tuesday. Thus, rain and wind over
the central to eastern Bering will shift south and eastward to
Kodiak Island and the Gulf. Cold air advection behind the low
will maintain robust northerlies across the eastern Bering and
Southwest Alaska coastal waters all the way through Wednesday.
Much of Southwest and Southcentral look drier on the northern
periphery of the low Monday through Tuesday, with steady rain
likely confined to the Gulf coast. However, it may not be
completely dry over Southcentral, as most models indicate a
short-wave trough moving through sometime Monday.

The forecast for late next week looks potentially "winter-like".
Most models show a trough dropping out of the Arctic mid week,
though there is disagreement on how far south the trough will
make it. The GFS is most aggressive in driving cold air (down
to -10 degrees C at 850 mb) down across Southwest Alaska, then
potentially interacting with energy being pulled from the
Gulf/Northeast Pacific up to Southcentral. At a minimum, could
be a round of mountain snow. Stay tuned, as forecast confidence
is quite low at this point in time.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 177 178 411 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...KM