Small Craft Advisory 661, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
414 AM AKDT FRI AUG 19 2016

.ANALYSIS AND UPPER LEVELS...

Little has changed with the upper air pattern since yesterday with
the main feature of interest being a positive tilt upper level
trough centered over the Alaska Peninsula and a shortwave ridge
building into the western Bering Sea. Water vapor and IR satellite
imagery is showing a shortwave trough tracking around the
periphery of the positive tilt upper trough into the Northern Gulf
of Alaska. Inland, there are showers and light rain across
southwest and Southcentral in association with weak and subtle
disturbances aloft, aided by regions of elevated instability which
remain from the previous low. The low moving into the Gulf will
skirt along the coast before it moves east by late this afternoon.

&&

.MODEL DISCUSSION...

The models are in excellent agreement with the large scale
synoptic pattern through Monday as the region begins to transition
into another wet pattern. They are in less agreement with just how
fast the trough amplifies as it moves into the Eastern Bering and
eventually the Gulf of Alaska which impacts the speed of
precipitation onset and local winds. For now, the slower and more
amplified solutions from the ECMWF and GFS were favored.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The short-wave trough lifting northward across the western Gulf
early this morning will quickly slide eastward across the northern
Gulf this afternoon, then exit this evening as the upper level
flow becomes progressive with a zonal sub-tropical jet driving eastward
into the Bering Sea. The track of this short-wave is farther
south and faster than previous forecast so have backed off PoPs
over the Prince William Sound region. The atmosphere across the
remainder of Southcentral remains supportive for isolated to
scattered convection.

Of particular note, localized low level convergence and upper
diffluence will continue to produce showers from the Kenai
Mountains northward to Anchorage, the Matanuska Valley and eastern
Susitna Valley through the morning hours. Showers will then diminish
this afternoon into the evening as the trough exits to the east
and a short-wave ridge builds in from the west. Expect some breaks
of sun as the ridge first moves in.

While the upper levels will dry out with arrival of the ridge, low
levels will remain quite moist. Also, flow in the Cook Inlet to
Susitna Valley corridor will quickly back to the west to southwest
overnight tonight producing upslope flow across the Susitna Valley
and along the western slopes of the Talkeetna, Chugach, and Kenai
Mountains. All of this is pointing toward low clouds with areas
of very light rain or sprinkles and likely some patchy fog.
Increasing isentropic lift and upslope flow ahead of an approaching
trough and surface front will lead to development of steady rain
through this corridor Saturday Afternoon through Saturday Night,
with eastward progression toward the rest of Southcentral by early
Sunday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...
The low that has been near the Alaska Peninsula will slowly drift
off to the east as weak ridging builds in from the west. This
ridging will allow for fog formation around Bristol Bay tonight
that will be aided by weak onshore southwest flow. The ridging
will also help briefly clear out some of the rain but that will be
short lived as the next system will move in Saturday and bring
steady rain through the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
Ridging over the western Bering will swing south and east
throughout the day as the next system makes its way into the
Bering from the west. This system has a strong push of warm air
that has helped enhance the inversion over the western Aleutians
bringing dense fog to the area. As the warm air eventually moves
south conditions will improve, but patchy fog will remain through
Saturday. This system will also bring more rain to the Aleutians
Saturday that will last through the weekend.

&&

.LONG TERM FORECAST (Days 3 through 7)...
An active weather pattern headlines the extended forecast over the
weekend and through Wednesday, as subtropical and polar lows send
a series of systems into the Bering and Gulf. The broad trough
setting up over the Bering and southwest Saturday through Monday
will suppress any ridging building into the areas. The only
exception being brief ridging over the Gulf Saturday into Sunday
afternoon, which will quickly be replaced by the upper level
trough tracking eastward on Sunday night. Once this trough begins
to stall over the Southern Mainland Monday, ridging aloft will
push in behind it over the western Aleutians/Bering by monday
night. Overall, this set up will continue the wet pattern to
southern Alaska and the eastern Bering through next Wednesday with
areas of fog for the western Bering and Aleutians. The next system
will enter the Western Bering from the west mid next week,
bringing more wet and windy weather to the Bering and Aleutians
through late next week.

Model agreement is generally good with the synoptic pattern
through Sunday morning, with increasing spread between the
solutions early next week through mid next week. Even though
agreement is relatively in a good range for this weekend, there is
still higher than average uncertainty as models typically struggle
when multiple low centers are embedded within a broad trough.
Models continue to struggle with the timing and strength of these
systems, with the more middle ground 12Z GFS solution being the
preferred solution.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...JRA
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DWK
LONG TERM...PEPE