Small Craft Advisory 637, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
515 AM AKDT FRI AUG 12 2016

.ANALYSIS AND UPPER LEVELS...

The dominant upper level feature on water vapor imagery this
morning continues to be the upper low pressure system anchored
over western Alaska. The most active weather lies on the south and
east sides of this upper low. A sharp, amplified upper trough
extends from the upper low center across the western Alaska Range
through the upper Cook Inlet region south through the western Gulf
of Alaska (and even extending well south into the North Pacific).
A southerly jet streak ahead of this trough continues to push
copious amounts of rain across Southcentral Alaska ahead of the
trough axis. The water vapor satellite depicts strong subsidence
across Cook Inlet and the western Gulf this morning, and this has
allowed for LCLs to lower to near the surface with fog formation
during the overnight hours.

In the Bering, between the upper low to the north and a westerly
stinger jet moving moving across the south-central Bering is the
remnants of a West Pacific typhoon (Omais). The front extends
north and east ahead of the low bringing rain and some winds to
the region, with gales primarily along the southern side of the
low across the southern Bering and Aleutian chain. Another upper
low is located between Shemya and the Kamchatka Peninsula and is
becoming absorbed as a shortwave as it tracks east to southeast
toward the Western Aleutians into the Bering system.

&&

.MODEL DISCUSSION...

Synoptic guidance is in excellent agreement in moving the Bering
low to the Gulf of Alaska this weekend and then slowly
diminishing through the beginning of next week, with a highly
amplified ridge building along the east coast of the Kamchatka
Peninsula. Some model disagreement begins to emerge with
disturbances dropping south through the Bering between the Gulf
low and aforementioned upstream amplified ridge.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

An unsettled pattern remains in place across Southcentral as an
upper level low remains in the Eastern Bering. This morning an
upper level wave is progressing through the Susitna Valley, and
has been supporting locally heavy rain showers in southern and
central Susitna Valley. A weak surface low in the eastern Gulf of
Alaska is helping to enhance southerly winds across Prince William
Sound and has been supporting steady rain overnight. This surface
low and associated upper level wave will push northward today and
enhance rainfall across the northern Gulf Coast by this evening.
The surface pressure gradient will increase as well due to coastal
ridging ahead of the low, and some local gusts are possible
through local gaps this evening such as Turnagain Arm. While this
cross barrier will partly limit precipitation, moisture is
expected to be deep enough to support light rainfall in typically
downsloped areas such as the Anchorage Bowl.

Wet weather will persist into Saturday, as the main upper level
low tracks from the Eastern Bering towards Kodiak Island. This
will continue to support broad moist onshore flow across the Gulf
Coast. Rainfall will be less intense, however, with weaker upper
level waves pushing across the region. &&

.SHORT TERM FORECAST SOUTHWEST ALASKA (through day 3)...

Plenty of rain and showers will affect Southwest Ak as the low
moves into the Gulf by the weekend. Upper level disturbances will
interact with moisture rich unstable air west of the Alaska and
Aleutian ranges. In addition to showers, a slight chance of
afternoon and evening thunderstorms exists Sat and Sun, focused
over the Kuskokwim river valley.

&&

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

As the low moves across the Akpen into the Gulf by the weekend,
it will spread rain and showers across the eastern Aleutians and
Bering. High pressure will dry out the western Aleutians, except
for a few showers as small disturbances move over the area. Drying
conditions then reach the eastern Bering later on Sun. Gale force
wind and significant seas west of the low will gradually diminish
and subside through the weekend.

&&

.LONG TERM FORECAST (Days 3 through 7)...
By Monday the closed upper level low previously in the Bering
Sea will be positioned just east of Kodiak Island. This synoptic
feature tracks into the northern Gulf before filling, and
dissipating by Tuesday. Therefore, look for the bulk of the
precipitation along the Kodiak Island northward into the northern
coastal communities. While, the Anchorage Bowl will have drier
conditions associated with downsloping with predominantly
southeasterly/easterly winds along the Kenai/Chugach Mountains.
Meanwhile, the Bering will have northwesterly winds with high
pressure located west of the Western Aleutians will build into
the region through the midweek. This ridge will be accompanied by
marine layer stratus, and fog over the Western Aleutians/Bering.
These northerly winds will bring in cooler temperatures into the
northern Bering, and filter into the central Bering through the
next several days. By Wednesday a 991 mb low drops down off the
Kamchatka Peninsula resulting in southwesterly flow with a swath
of warmer temperatures, and moisture streaming into the Western
Bering/Aleutians. While, the Southwest Alaska region will have
offshore winds except for northeasterly/easterly along the Alaska
Peninsula. On the Pacific side a gale force low will be parked
near the Shumagin Islands, with abundant moisture streaming into
the eastern Bering. The models in the extended period were in
better agreement with the different synoptic features. Therefore,
a blend of WPC, GEFS Mean, and the ECMWF Ensemble Mean models were
used through this weekend extending into the midweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE 170 171 172 173 174 175 176 177.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CC
SOUTHCENTRAL ALASKA...ELN
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DS
LONG TERM...PD