Small Craft Advisory 730, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
542 AM AKDT Fri Sep 16 2022

.ANALYSIS AND UPPER LEVELS...

All eyes are on the powerful, near 940 mb Bering Sea low (the
extratropical remnants of Typhoon Merbok) which continues to move
northward across the west-central Bering. Its front currently
sweeps across the Alaska Peninsula and into southwest Alaska.
Bethel and King Salmon radars are both picking up on areas of rain
associated with the front. Very early this morning, Adak recorded
a 81mph wind gust as the core of strongest winds moved through.
There is a deep, tropical Pacific connection with this system as
noted in the jet stream configuration on water vapor imagery. With
this tropical connection also comes moisture content with the
system. the CIMSS MIMIC satellite-based precipitable water product
shows a large corridor of 1" Total Precipitable Water (TPW) which
extends along the Alaska Peninsula and eastern Aleutian Chain.
Within this corridor also resides an areas of over 1.5" TPW near
and east of Nikolski extending northeastward to Nunivak Island.

Meanwhile, the jet stream currently extends northeastward from
the sub-tropical Pacific to over the central Bering and curves
southeastward over southern Alaska and into the Gulf of Alaska. A
weak surface low resides in the northern Gulf of Alaska while a
shortwave noted in water vapor imagery continues to dig
southeastward over the southern Gulf and heading to the North
Pacific. Middleton Island radar shows areas of rain showers in the
north-central Gulf in association with the surface low.

&&

.MODEL DISCUSSION...

Model guidance still exhibits strong agreement with regards to
strength and positioning of synoptic scale features in the short
term. Of particular note is the strong model agreement with the
handling of the major Bering Sea low. Forecast confidence
continues to be high with timing and impacts. The timing and exact
degree of storm surge for the Kuskokwim Delta coast continues to
be a spot of uncertainty. However, given the track of the storm
and its strength, this could be an impressive storm surge event
for that area.

With regards to Southcentral Alaska, guidance points to a
transient upper ridge moving over the area through the day today
before moist, southwesterly flow and associated precipitation
chances take back over for the day on Saturday. While ridging will
be moving over head, there appears to be upper level shortwaves
that will ride around the ridge. The forecast challenge here is
exactly who gets some rain and who stays dry. There are
indications that a more zonal (west to east) flow could occur for
Sunday and provide some clearing and dry weather, but confidence
is only average for that given the persistent pattern the area has
been in.

&&

.AVIATION...

VFR conditions look to remain predominant. However, fog could be
an issue this morning given low level moisture present from rain
overnight and relatively calm winds. However, lower ceilings
should scour out by mid to late morning as surface flow turns
southeasterly in response to the major storm in the Bering Sea. As
the east to west oriented pressure gradient tightens this
afternoon into the evening, gusty Turnagain winds will be the
result. There is potential for light precipitation this afternoon
as a series of weak upper shortwaves travel atop a transient
shortwave ridge moving eastward. Since flow will remain
southeasterly, ceilings should stay VFR, although there could be
a brief period of MVFR ceilings.

&&

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

The upper level low that brought rain to the area yesterday has
moved into the northeastern Gulf with some lingering rain this
morning near Cordova. Meanwhile, a ridge begins to build across
Southcentral today which will allow for dry conditions early
today for most areas except Kodiak Island. However, a quick moving
shortwave will move from Southwest Alaska and northward from
Kodiak Island bringing chances of rain for Friday afternoon and
evening. As this wave moves north and encounters the ridge it will
weaken. Widespread rain is not expected, and a few locations may
remain dry throughout the day on Friday. A frontal system will
then quickly move towards the area on the front side of the ridge
with a quick shot of rain Friday night through Saturday morning.
As this front moves through the area the pressure gradient will
tighten up and up inlet flow will develop Saturday as the
Hurricane Force Low moves north. This will allow southerly winds
to develop across parts of the area with gusty winds along
Turnagain Arm, west Anchorage and the Copper River Valley. This
front will quickly move east spreading showers across the Copper
River Basin by Saturday afternoon. Meanwhile, the rest of
Southcentral will see dry conditions develop Saturday afternoon
with sunshine expected in most areas. The nice weather will round
out the end of the weekend with a dry and sunny day on tap for
Sunday with cooler morning temperatures.

&&

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

The forecast remains on track and the low should be reaching peak
intensity near 940 mb this morning over the Western Bering. There
are still a few minor differences in low position as it nears St.
Matthew Island this afternoon. Minor differences in the east-west
position of the low could tweak the gusts by 10 mph here or
there, but overall high winds with gusts between 75 and 85 mph are
still expected along the coast to the west of the mouth of the
Kuskokwim River. To the east and south of the river and along the
coast, a few gusts to 75 mp are possible should the low veer more
to the east. Winds will continue to increase throughout the Delta
region today, with peak intensity expected this evening and into
the overnight hours. The Coastal Flood Warning also remains in
effect through 4pm Saturday with surge/tide expected to be 5 to 8
ft above the normal highest tide line. It remains a possibility
that surge will make it through the mouth of the river and into
the Bethel area. Widespread shower activity is expected along with
the high winds, which will decrease visibility and add risk to
travel.

For the Bristol Bay and interior areas, expect elevated winds
from Cape Constantine north to Aniak. High surf remains a
possibility for the northern Bristol Bay Coast from Cape
Constantine west. Otherwise, these regions should be calmer, with
moderate south to southwesterly winds and rain showers. The system
will move far enough north overnight Saturday that it's influence
on the region will begin to wane. For Sunday and Monday, expect
mostly calm conditions, with some light rain showers possible as a
few weak shortwaves move through the area.

&&

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

The strong extra-tropical low crossed over the Western Aleutians
and is now comfortably in the Western Bering where it should be
reaching peak intensity this morning. Adak recorded gusts between
75 and 80 mph through the night and buoy 71 to the south of the
Rat Islands recorded waves/swell up to 41 ft. Winds will continue
to diminish over the Aleutians today. Focus now shifts to the
Central Bering, where the High Wind Warning remains in effect for
the Pribilof Islands through 1am Saturday. Gusts between 80 and 90
mph are expected today and through the evening hours. Some
solutions indicate that gusts up to 100 mph are possible should
all the pieces fall together. Out over the water, seas are
expected to reach as high as 54 ft. Buoy 35 recorded an increase
of 28 ft in wave height over a three hour period as swell and wind
action moved over the area. Over the Alaska Peninsula and Eastern
Aleutians, warm temperatures and gusty southwesterly to southerly
winds are expected today.

On Saturday the influence of this storm on the land masses of the
Bering Sea and Aleutian Chain will be minimal, but over the
water, gale force winds will persist through the evening north of
the Pribilofs. Calm weather will return throughout the region
Sunday and Monday with light westerly winds and a few passing
fronts.

&&

.MARINE...

Merbok remains the main concern through the end of this week. As
this system enters the Bering Sea it is expected to re-intensify
and go through the extra-tropical transition as is ingesting cold
air from eastern Russia. There is very minor uncertainty regarding
the exact track and strength of this storm as the center of the
low expected to pass over Shemya this afternoon. It will then
track from the Western Bering Sea to the vicinity of Norton Sound
on Saturday. Expect widespread storm force winds across the
Bering with areas of hurricane force winds. The hurricane force
winds are most likely from the Western Aleutians to the Central
Bering Sea between this afternoon and Friday evening. Seas are
also expected to build rapidly with widespread 30 to 40 ft seas
across much of the Bering with 40 to 50 ft seas associated with
the strongest winds. This is a major Bering Sea Storm and mariners
are encouraged to enact necessary precautions.

&&

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

Unsettled conditions are expected to continue for Southern Alaska
in the extended period, with multiple waves approaching from the
north Pacific and western Bering Sea. Several bouts of intense
winds and rainfall are expected for both the Aleutian chain and
Southcentral.

By Sunday evening, Typhoon Merbok remnants in the Eastern Bering
will have moved north out of the area, however will still be
inducing onshore flow into the southwest Alaska coastline. The
concern of strong winds and rainfall associated with this low
will have diminished significantly by Monday, however showery
activity across Southwest Alaska is still possible. Following
Merbok in Southwest Alaska, a new low originating from the north
Pacific looks to enter the Eastern Bering Monday. Timing and
positioning of this system has yet to reach a consensus in the
guidance. Thursday's 12Z Canadian and Euro models advance this
north Pacific low across the Eastern Aleutians into the Bristol
Bay area Monday morning. Additionally, a low forming off of the
Kamchatka Peninsula and, with moderate model uncertainty, will
deepen and approach the western Aleutians by Tuesday morning. The
Kamchatka low will move across the Aleutian Chain providing
lingering precipitation in the Central Bering as well as strong
winds throughout the region through the middle of next week.

From Sunday afternoon, a respite in precipitation for
Southcentral is expected to persist through Monday afternoon. With
some discrepancies, guidance then suggests the introduction and
intensification of a North Pacific low south of the Alaska
Peninsula. This system will provide heavy precipitation across the
Southcentral coastline and eventually move inland. Precipitation
associated with this system is expected to persist into Wednesday.
Southeasterly flow will develop a Barrier jet wind along the
northern Gulf coast, including Prince William Sound Tuesday
morning. Winds up the Kamishak Gap into the Greater Bristol Bay
area will also be a concern Thursday afternoon.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning: 195, 155.
         Coastal Flood Warning: 155.

MARINE...Hurricane Force Wind Warning: 411-413, 185.
         Storm Warning: 414, 177-181, 175.
         Gale Warning: 176, 173, 172, 170, 165, 160.

FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DAN
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...CL