Small Craft Advisory 620, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
547 AM AKDT Mon Aug 8 2022

.ANALYSIS AND UPPER LEVELS...

A large upper-level low is centered over Western Alaska this
morning, producing unsettled weather across the vast majority of
the southern half of Alaska. A broad surface low sits under the
upper-level low, centered in Kotzebue Sound, but extending its
influence far southward along a series of fronts. Its occluded
front wraps across Interior Alaska into Southcentral, producing
upslope showers along the southern side of the Alaska Range. A
warm front spans out ahead of the low across Southcentral, roughly
from northern Cook Inlet to along the north Gulf coast, fed by a
modest stream of moist, southwesterly flow across Cook Inlet and
the western Gulf. As a result, moderate to heavy upslope rainfall
has been persisting on the south and western slopes of mountain
ranges across the Mat-Su Valleys, Anchorage Bowl, northern Kenai
Peninsula, and Prince William Sound. In addition, low pressure in
the Interior has induced a strong southerly flow up Cook Inlet,
as well as through the Copper River Basin, with gusts up to 35
mph consistently observed overnight in Kenai and Gulkana,
respectively.

Behind this, a cold front stretches westward across Southwest
Alaska and across the Bering Sea, bringing rain and south to
southwesterly winds up to 30 mph to the region. A broad,
disorganized low south of the western Aleutians is producing
marginal wind and precipitation impacts as its front grazes the
marine zones south of the Chain. High pressure over the Bering
Sea, sandwiched between the front to the north and low to the
south, is keeping the Aleutians and Alaska Peninsula under a layer
of stratus.

&&

.MODEL DISCUSSION...

Models are generally in good synoptic agreement that the large
upper-level low over Western Alaska will exit to the Canadian
Arctic by Wednesday as an amplified shortwave trough dives south
from Chukotka into the southern Bering through Tuesday night.
Models diverge in the evolution of this shortwave as it cuts off
into a closed upper low. In particular, the speed of the eastward
progression of the upper low is uncertain as it moves into the
western Gulf with an associated surface low for Thursday. This
decreases confidence in the timing of gap winds through the
eastern Aleutians/AKPEN on Wednesday, as well as the duration and
placement of heaviest rain over Southcentral through midweek.

&&

.AVIATION...
PANC...VFR conditions are expected through the TAF period. As
rain continues through this evening, ceilings will periodically
drop below 5000 ft. Southerly winds will continue, with winds
generally 15-20kts and gusts to 25-30kts. The timing for the end
of this rain event is somewhat uncertain, though rain is expected
to decrease in intensity late this evening through Tuesday morning
and cease later on Tuesday. Gusty southerly winds also gradually
diminish along with rain for the same time period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Wednesday night)...

Persistent precipitation driven by a deep and cold upper-level
low over the Seward Peninsula will continue into midweek. A brief
respite in the precipitation on Wednesday will be followed by a
wave approaching from the Gulf. This persistent rainfall,
augmented by rainwater drainage from surrounding mountains has
encouraged a Flood Watch to be issued for the Mat-Su valley.
Strong southerly winds will be consistent across Southcentral
through Tuesday as the low moves through the Interior and
weakens. Gusty conditions in places such as Turnagain Arm, Knik
Valley, and Copper River Basin, are expected through today.
Southerlies up to gale force are also expected over the Cook Inlet
waters.

&&

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

An upper-level low located near Norton Sound this morning extends
its associated surface front across Southwest Alaska. Gusty winds
and rain with the front will taper off throughout the day today as
the front exits the domain. As a result, expect clearing skies,
light winds, and drier conditions to begin on Tuesday for most of
Southwest. The exception lies with the Alaska Peninsula, as a
North Pacific low will track northeast along the region and
maintain cloud cover and rainfall through Wednesday night.

&&

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

The main feature of note is an upper-level trough rotating across
the Bering Sea while its associated surface front extends from
west to east across the domain. The Pribilof Islands will
continue to see rain through this evening while the front
gradually moves south to the eastern Aleutians. Areas of fog and
low stratus will persist across most of the Bering and Aleutians
today. By Tuesday, an upper-level ridge of high pressure moves
east over the domain. As a result, expect clearing skies, light
winds, and drier conditions to unfold through Wednesday night. The
exception lies with the eastern Aleutians, as a North Pacific low
will track northeast along the region and maintain cloud cover
and rainfall through Wednesday morning.

&&

.MARINE (Days 3 through 5: Wednesday through Friday)...

Gulf of Alaska: An area of low pressure is expected to track from
south of the Alaska Peninsula into the western Gulf between
Wednesday and Thursday. Forecast confidence for this system is
improving slightly, but model disagreement for the progression of
this low persists. The best potential for any winds reaching gale
force near or north of the low will be where terrain funneling is
favored, or south of the Kenai Peninsula to the Barren Islands and
Shelikof Strait, depending on low track and intensity for
Wednesday and Thursday. Otherwise, seas less than 15 ft and small
craft winds or less are anticipated, extending to the entire Gulf
region as the low weakens on Friday.

Aleutians/Bering: Confidence remains high for most of the Bering
and Aleutians to remain under the influence of large scale high
pressure, with seas less than 15 ft and small craft or lower winds
generally prevailing. The exception will be near portions of the
Alaska Peninsula and far eastern Aleutians, where a North Pacific
low will track northeast for Wednesday and reach the western Gulf
by Thursday. The chance for northerly winds to reach gale force on
the back side of the low near the Alaska Peninsula is increasing
somewhat, but model disagreement for the low is still leading to
decreased forecast confidence for the higher winds. Expect any
gale force winds to diminish Thursday as the low weakens and
shifts east, but higher gusts could persist through gaps and
passes along and south of the Alaska Peninsula and eastern
Aleutian Chain through Thursday.

&&

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

Overall, the larger scale pattern based on deterministic models
and respective ensemble suites for the extended period is becoming
a little better resolved, but discrepancies with a couple key
details still leave trends in sensible conditions somewhat in
question. Forecast confidence is once again highest out across the
Bering/Aleutians, where a very inactive, calm pattern should
persist through at least the first part of the weekend due to a
longwave high pressure ridge sitting in place. Relatively dry and
increasingly warmer conditions will persist near and downstream of
the upper ridge across the Aleutians and Southwest through at
least Saturday, with some hint at a ridge breakdown and more
active conditions looming by Sunday.

For the eastern portions of the forecast area, much is hinging on
the eventual fate of a trough and attendant surface low swinging
into the Gulf from the AKPen region on Thursday. Recent guidance
is coming into better agreement for the trough to close off into
an upper low over the Gulf by late Wednesday, where it will likely
aimlessly drift in place for several days thereafter. Weak upper
ridging will develop north of the center over portions of the
southern mainland, potentially leading to seasonably warm and dry
conditions for the end of the week across at least northern parts
of Southcentral. The consensus is that the upper low stalled out
over the Gulf will begin to shift south and away from the coast
between Friday and Saturday, allowing rain near the Gulf coast and
Prince William Sound on Thursday/Friday to mostly abate by
Saturday. Recent ECMWF ensemble guidance depicts this shift south
happening earlier, with the operational GFS and ensemble mean
still keeping the low and associated clouds/rain in place near the
coast for longer into the weekend. In short, the difference
between a warmer and drier weekend and a cooler/wet one for
Southcentral will heavily depend on when and if the Gulf low
pressure retreats south quickly enough.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Watch 111 145.
MARINE...Gale Warning 139 140.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CQ
SOUTHCENTRAL ALASKA...AP/CL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AF
MARINE/LONG TERM...AS