Small Craft Advisory 571, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
257 PM AKDT Thu Jul 26 2018

.ANALYSIS AND UPPER LEVELS...
The large upper level low that has been the main story for the
week has drifted north and is now positioned over the AKPEN. A
weak surface low near Kodiak Island continues to deliver a large
swath of moisture to the southern Mainland. As a result, rain
continues to fall across the AKPEN and along the coastal areas of
Prince William Sound. This ample stream of Gulf moisture also
brought some early morning fog across the Cook Inlet region, and
rain showers as far north as the Susitna Valley. Radar imagery
indicates additional rain moving through eastern Kenai Peninsula
and PWS areas this afternoon.

Looking out west, the strong upper level ridge that is in place
just south of Shemya is quickly losing its influence over the
Bering as a large upper level Kamchatka low moves in. The leading
front associated with this low is producing widespread cloud
cover, as satellite imagery indicates plenty of low-level marine
stratus over the area.

&&

.MODEL DISCUSSION...
Model solutions remain in great agreement with the upper level
low over the AKPEN through the next 24 hours. There is generally
good agreement regarding timing and location of the advancing
upper level Kamchatka low into the western Bering within the next
36 hours. By Saturday, models indicate the AKPEN low and the
Kamchatka low will merge over the southern Bering. At this time,
minor disagreements on the timing are evident, as the EC keeps the
lows separated through Sat evening, while the GFS and NAM show a
single low center earlier on Sat afternoon.

&&

.AVIATION...
PANC...Light winds and VFR conditions with intermittent showers
are expected through Friday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
With surface low pressure hovering south of Kodiak Island and an
upper-level low moving into the Bering Sea, Southcentral remains
in the wet-weather pipeline. However, the steady and widespread
nature of the precipitation seems to be thinning somewhat with
some patchy areas of drier air showing up in the water vapor loop.
Additional waves of rain will develop if persistence is the rule,
with the next key rainmaking feature rotating off the base of the
trough.

The short story will be: while some breaks in the rain are
occurring, we expect the region to continue with showers or light rain
in the forecast through the weekend. More persistent, heavier rain
amounts will remain focused on the Prince William Sound region.

Winds will remain generally light with the exception of a few
locations in channeled terrain within the Copper River Basin.

&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
An active and unsettled weather pattern will remain through the
short term for the Eastern Bering, AKPEN, and Southwest Mainland.
Tonight, low pressure south of the Alaska Peninsula will lift
north to a position west of Bristol Bay. Clouds and showers will
linger across the southwest mainland overnight as the broad,
cyclonic flow around the low continues to usher in moisture from
the Bering Sea. This feature will track along the coast then
retrograde back into the central Bering by Friday night. As it
does, a second low will dive southeast through the southern
Bering, undercutting the first system. A weakening occluded front
associated with this low will approach the western AKPEN with
another round of clouds and showers Friday night through Saturday.
As the front lifts north, it will steer another low well southwest
of Kodiak Island north across the AKPEN and southwest mainland
Friday night through Saturday.

As these systems lift to the north and west Saturday, their
energy will be absorbed by yet another low sliding east across the
Aleutians. The center of this low will slow as it reaches the
Pribilofs; however, its associated front will move across the
AKPEN and into the southwest mainland for Saturday night and
Sunday. As this low intensifies and inches closer to a ridge
downstream, gusty southeasterly winds will increase along the west
coast, over the Kilbuck Mountains, and in gap locations along the
Alaska and Aleutian Ranges for Saturday and Sunday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Weak ridging over the central Bering is bookended by a weakening
low south of the AKPEN and a second, mature low in the far
northwestern Bering with a front stretching south across the
western and central Aleutians. This ridge breaks down by Friday
morning as the front occludes and moves into the central Bering
while the low downstream moves northwest parallel to the coast of
western Alaska. A new, triple point low will form on this front
Friday night as it moves into the southern Bering. This system
will weaken rapidly on Saturday as it drifts north toward the
Pribilofs. Upstream, a third, Kamchatka low will strengthen as it
moves over the western Aleutians late Friday. This low will then
quickly move across the central Bering Saturday and stall over the
southern Bering Saturday night. Unsettled weather will remain
across the Bering and Aleutians through the remainder of the
weekend as the long-wave trough pattern holds over the region.

&&

.LONG TERM FORECAST (Days 3 through 7)...
There is little change to the long term outlook today which
features a fairly simple and stagnant pattern over the region.
Beginning Sunday night, a large upper low will be situated over
most of the Bering while an upper ridge will be centered over
Southeast Alaska. This will promote broad southwest flow over most
of the southern mainland leading to a continuation of the
unsettled weather for next week. There are some model differences
in the ridge axis placement over southcentral which will
determine how far east precipitation will spread. The Euro and
GFS models slightly amplify the ridge axis west of Anchorage
Monday night through Tuesday, which would keep most of
southcentral dry. By Wednesday there is decent agreement that the
ridge slides east allowing a short wave trough to swing up from
the Gulf of Alaska into southcentral bringing another round of
rain to the area. The low center then moves east over the
southwest mainland as a ridge builds in over the western Bering
by Thursday bringing cooler temperatures and overall cloudy and
wet conditions to the mainland.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...KO
SOUTHCENTRAL ALASKA...DS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...KVP