Small Craft Advisory 629, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
403 PM AKDT TUE AUG 9 2016

.ANALYSIS AND UPPER LEVELS...
The upper level low that has been dominating the pattern is slowly
drifting through the Kuskokwim Valley. It continues to advect
plenty of moist southwest flow across the southern half of Alaska.
With this, there was plenty of fog and low clouds around this
morning, especially near Prince William Sound. Most of the fog has
since burned off or mixed out, but the low clouds will continue to
linger. The Polar Front Jet is well north of the mainland and will
not play a role in this forecast period. But the Subtropical Jet
is moving from the Bering into the Panhandle in patchwork fashion.
The jet is strongest over the Western Bering at 120 kts supporting
a robust upper level wave. It then moves from Southwest to
northeast across the Northern Gulf of AK at 85 kts helping to
ferry the energy from Mondays' rain towards the ALCAN border.

&&

.MODEL DISCUSSION...
Models are good agreement through the short term and then diverge
by early Thu in handling the remnants of Tropical Storm Omais. The
higher resolution models were favored for the first 48 hours or so
in the timing of any gaps winds and showers. After this, the
forecast package leans more towards the GFS as it has shown better
run-run consistency. The 12z ECMWF was notably slower in bringing
Omais' energy across the Bering compared to the 09/00z run. This
trend was towards the slower NAM solution and could potentially
result in a slightly longer dry period for Southcentral. We will
be monitoring future models runs to see if this trend continues.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Wed and
Thu)...
Broad southwest flow will remain entrenched over the area. This
will keep abundant cloud cover across the region with the
continuing possibility of showers for the next several days. The
best forcing will be up in the Susitna Valley, closer to the low
center itself. So expect the best chance for more steady rain and
heavier showers to be up there. But with some weak instability
throughout the area, all areas will see a chance for showers,
especially along the mountains in the afternoon and evening. By
late Wed into Thu, the remnants of the previously mentioned
western Bering wave make their way into the Northern Gulf. There
are signs that this wave could interact with the Kuskokwim upper
level low. This should serve to increase the steady rain along
coastal areas and bump up the chances of some of this steady rain
tracking over the Cook Inlet by mid-day Thu.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Wed and Thu)...
The vertically stacked low centered over the Lower Kuskokwim
Valley will continue to control the weather for the next several
days. It will keep very moist southwest flowing moving off the
Bering Sea all the way up against the Aleutian and Alaska Ranges.
There will be a enough instability to support isolated thunderstorms
along the Kuskokwim Mountains this evening. The instability will
spread further east tomorrow, so the Kuskokwim Mountains and
Alaska Range could see some isolated thunderstorms late in the
day.

Most places could see a break in the wet weather on Fri as the
upper level low breaks apart and moves out of the area. There will
likely still be plenty of cloud cover as flow is mainly onshore
and 500mb heights will not change much. There will still be
enough residual instability for one more round of isolated
thunderstorms along the Alaska Range Fri afternoon and evening.
The next system looks to start tracking inland by late Fri
bringing more widespread showers.

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...
The forecast for the Bering Sea and Aleutians will be
characterized by two lows tracking through the area through the
rest of the week. The first is over the western Bering that is
pushing east and will dissipate as it moves over the Central
Aleutians Wednesday.

The second low will be the remnants of Tropical Storm Omais and
it will move into the western Bering Thursday. As it tracks tracks
east through the end of the week its impacts will be primarily
confined to gale force winds and an increase in rain over the
Bering. The exception will be the Western Aleutians that will see
gale force winds Wednesday night into Thursday while the winds for
the Central and Eastern Aleutians look to stay below gale force
late Thursday and Friday as the low passes.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Heading into Saturday a Bering storm system, within a broad
longwave trough over Alaska and the Bering, slides to the
southwestern Gulf and continues slowly eastward through the
weekend. To the west, a North Pacific Ridge amplifies northward
toward the western Bering. This set up allows multiple shortwaves
to dive down from the northern Bering and then wrap around the
base of the the trough in the Gulf. This synoptic regime keeps
the highest chances for rain and stronger persistent winds across
the eastern half of the Bering and along the Alaska Peninsula. The
gulf coastal areas will stay in a wetter pattern with inland areas
staying in a showery pattern. Models are in very good agreement
with this long range forecast so a blend of the GFS and ECMWF,
including ensemble means, was used to update the forecast Saturday
through Tuesday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MO
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO/DK
LONG TERM...KH