Small Craft Advisory 650, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
523 AM AKDT Wed Aug 18 2021

.ANALYSIS AND UPPER LEVELS...

A split flow pattern continues to dominate with a longwave trough
and embedded cold upper low in the polar jet stream extending
from the Arctic Ocean to most of interior state of Alaska. Along
the polar jet, a strong upper level wave and associated surface
low from the Kamchatka Peninsula is crossing the northern Bering
Sea and is expected to trek onshore near the Kuskokwim Delta
tonight. Another shortwave embedded in the westerly flow is
crossing the Alaska Range and bringing scattered showers from the
western Kenai Peninsula into Anchorage Bowl, and over the Copper
River Basin.

Further south, the subtropical jet stream extends from the
western Pacific to the south of the Aleutian Chain and into
southeastern Gulf of Alaska, with a 100+ kt jet streak in the left
exit region. A weak perturbation with an associated surface low
south of Cordova is providing light steady rain along eastern
Prince William. Based on the CIMSS MIMIC imagery, a deep fetch of
tropical moisture of 1-2" of precipitable water extends
northeastward to southeast Alaska.

Meanwhile, a ridge of high pressure extends from the northeast
Pacific to western Canada, with a shortwave trough downstream of
the ridge moving into the Pacific Northwest. A transient ridge
ahead of the aforementioned Kamchatka low is building across the
eastern Bering Sea and into the southwest mainland, leading to
benign conditions. The Nighttime Microphysics (RGB) this morning
shows localized areas of low clouds and fog, with partial clearing
elsewhere.

&&

.MODEL DISCUSSION...

Models are in consolidated agreement with the synoptic scale
features in the short term. As the transient ridge continues to
build over the southern mainland today, the bulk of moisture and
precipitation will be concentrated over Southeast Alaska. Out
west, the latest model runs are consistent with the overall track
and intensity with a North Pacific low moving south of the
Aleutian Chain by Friday morning, but deviate on the placement as
we head into the weekend. Otherwise, forecaster confidence remains
high with this upcoming system that will bring cool, wet, and
windy conditions across the western and central Aleutians for the
week ahead.

&&

.AVIATION...

PANC...A weak upper level wave combined with a moist environment
will bring scattered showers with MVFR conditions. Rain will
taper off later this morning with VFR conditions and light winds
for the rest of today.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2:
Today through Friday)...

A weak upper-level wave moving across the Alaska Range this
morning will keep clouds and rain over the Copper River Basin and
higher elevations of Southcentral through late afternoon or early
evening before this system exits the region. This feature is also
producing a few scattered showers across the Anchorage Bowl early
this morning. These showers are expected to diminish by mid-
morning as the trough moves east, with a general clearing trend
from Cook Inlet to the Mat-Su Valleys as a transient upper-level
ridge builds in from the west. Rain will also linger across parts
of Prince William Sound and Cordova through late morning / early
afternoon as a weak surface low moves from Middleton Island to
south of Yakutat.

With the ridge building in later today, the next forecast
challenge will be the development and extent of fog and low
stratus over the northern Gulf and Cook Inlet. The 12z VIIRS
nighttime microphysics satellite image shows an area of fog and
low stratus over the Barren Islands and just west of Kenai. Right
now, it looks like there will be enough subsidence and stable air
with this ridge to allow fog to expand from east of Kodiak Island
to Prince William Sound by this evening. A weak surface trough
will also allow for a light southerly wind flow across the
northern Gulf this evening. This should advect any fog and low
stratus into Prince William Sound toward Cordova by early morning
Thursday. Likewise, a southwesterly flow up Cook Inlet could
advect fog and low stratus northward toward Kenai and Anchorage
for Thursday morning. Any fog and low stratus should begin to
erode by late Thursday morning or early afternoon as the next
weather system advances from the west. This system will bring an
increase in gap winds for Turnagain Arm and the Knik and Copper
River Valleys by Thursday afternoon as a deepening southwesterly
flow lifts moisture northward. Expect rain to develop from the
Anchorage Bowl north by early morning Friday, advancing east
through the day as the gap winds begin to wind down by early
Friday afternoon.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Today and
Thursday)...

Mainly fair weather is expected across much of Southwest today as
a weak transient upper level ridge moves through the region. As a
result, expect a fair amount of sunshine and temperatures in the
mid to upper 50s for the Kuskokwim Delta nearing 60 degrees in
places such as Bethel. For Bristol Bay and the Middle Kuskokwim
Valley, even warmer conditions can be expected with temperatures
in the mid to upper 60s. There may be some spotty areas of patchy
fog especially around Bristol Bay but expect any patchy fog to
quickly mix out as the sun rises later on this morning.

Next, a potent storm system currently located to the west of Saint
Matthew Island will continue moving eastward across the northern
Bering Sea this morning. The front associated with this low is
expected to move onshore in the Hooper Bay area beginning late this
afternoon with periods of rain and increasing winds. The front
will continue moving inland tonight reaching Bethel around 12z/4am
Thursday morning. Winds will also be on the increase in the Bethel
area this evening and overnight with gusts as high as 35 mph from
around 12z Thursday through 12z Friday. As a result, these winds
are expected to impact the pedestal replacement ongoing at the
Bethel radar. Inland areas are expected to largely remain dry
still for Wednesday night with the front and its associated
moisture making it over the Kilbuck/Kuskokwim Mountains for
Thursday. This will bring widely scattered showers to much of the
area with the steadiest rain expected across the Middle Kuskokwim
Valley. Breezy conditions are also expected across much of
interior Southwest for Thursday as the front moves through.

One last item of note is that on the back side of this system
Thursday morning to Thursday evening/night, there is the
potential that there may be some high surf as gale force winds are
expected for an approximately 6-12 hour period Thursday around
Hooper Bay. Current storm surge guidance is showing water getting
1 to 1.5 feet above HAT (highest astronomical tide) during the
above time. Therefore, stay tuned for future updates as a product
may be warranted for this.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2:
Today and Thursday)...

Semisopochnoi Volcano located 72 nm east of Kiska continues to
have occasional brief eruptions. As a result, a Marine Weather
Statement has been sent to alert for trace ashfall in the vicinity
of the volcano especially to the north and east. See marine
weather statement for further details as more eruptions may occur
at any time.

The main story across the Bering will be the current storm system
located to the west of Saint Matthew Island this morning. This low
will continue moving to the east throughout the day reaching the
Kuskokwim Delta Coastline by late this afternoon with rain and
increasing winds. Small craft winds will be associated around this
low pressure as it traverses the northern Bering Sea today. As the
system moves northeastward towards Nome, Alaska, cold air wrapping
around on the back side of the system will produce gale force
winds through a narrow swath on the backside of the system. These
gale force winds are mainly expected to the northeast of Saint
Matthew Island beginning Thursday morning with the threat
spreading eastward to the Kuskokwim Delta Coastline including
Hooper Bay through Thursday evening. Wave heights of 10 to 13 feet
are also expected in these marine areas north and east of Saint
Matthew Island as this system moves through. Periods of rain will
also accompany this system especially with the leading front.

Elsewhere across the Bering, expect mostly westerly flow as
a generally weak upper level high persists through Thursday.
Underneath this ridge, the usual low stratus and patchy fog can be
expected.

&&

.MARINE (Days 3 through 5/Friday through Sunday)...

A surge of cold air advection, indicative of the lateness of the
summer season, will cross the western Gulf on Friday along the
south side of a storm system crossing interior Alaska. Confidence
is now high for winds to reach gale force from Kamishak Bay, the
Barren Islands and out into the Gulf. This looks to be short-
lived, with winds expected to quickly diminish Friday night.

Meanwhile, a new storm will track from the north-central pacific
northward to the Aleutians on Friday, then slowly make it's way to
the eastern Bering Sea on Sunday. It looks like this storm will
reach it's strongest point as it nears the Aleutians. Expect high
end small craft advisory winds, with some potential for gale
force winds near the low center. The low will be in a weakening
state as it continues into the Bering Sea, so winds should remain
safely below gale force.

&&

.LONG TERM FORECAST (Days 4 through 7/Saturday through Tuesday)...

Beginning Saturday, a high amplitude pattern will be in place with
a vertically stacked low and trough centered over the southern
Bering Sea and a downstream ridge crossing southern Alaska. With
progressive upper level flow, expect the Bering storm system to
track toward the southern mainland, with widespread rain spreading
across Southwest Alaska Saturday afternoon through Saturday night,
then across Southcentral Alaska on Sunday. This system will
originate in the tropical Pacific and dynamics look moderate to
strong. Thus, would expect moderate to heavy rainfall and some
large rainfall amounts, as we heave seen with many storm systems
this month.

The leading ridge will connect up with a separate ridge over
Canada Sunday night. This new higher amplitude ridge will serve
to block the flow. As a result, the upper trough will stall over
southern Alaska early next week. A series of short-wave troughs
moving through the long-wave trough will maintain a wet pattern
across the eastern Bering Sea and southern Alaska. Forecast
confidence starts out high with the storm system over the weekend,
then drops a bit Monday-Tuesday, largely due to the inability to
track specific short-waves this far out. However, as mentioned,
this pattern supports wet conditions and cool temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 181 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MF
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...SB