Small Craft Advisory 671, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 AM AKDT Thu Aug 30 2018

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low near Saint Paul is slowly moving to the
southeastern Bering. The associated leading front has lifted over
the central Gulf and radar imagery indicates showers spreading in
across the southern Mainland along this weakening frontal
boundary. Light rain is also falling over Southwest Alaska and
along the northern AKPEN where the occluded front continues to
stall out. A large area of high pressure persists over the Gulf
and will soon have a stronger influence over Southcentral after
the shortwave moves through today.

&&

.MODEL DISCUSSION..

Models remain in good synoptic agreement with the timing and
placement of the Bering low. Model solutions also agree with the
timing and placement of the ridge over the Gulf. The main
forecast challenges are timing of the precipitation over the
southern Mainland today, and areas of patchy fog across
Southwest Alaska.

&&

.AVIATION...

PANC...Southerly winds will shift more southeasterly and increase
early this morning. Winds will remain breezy but shift more
southerly through midday and southwesterly this afternoon and
evening. Occasional brief MVFR conditions are possible in showers
on today, but VFR conditions are expected to predominate.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Saturday)...

A shortwave trough moving over the Southwest Mainland is already
bringing periods of light rain from Anchorage northward into the
Susitna Valley. This rain is expected to become more widespread
and heavier as the shortwave moves overhead this afternoon,
bringing wetting rains to most areas to the north of Kenai
Peninsula through this evening. As this shortwave moves into the
Gulf on Friday morning, rainfall will begin to diminish throughout
the area, but with weak cyclonic flow remaining aloft, cloud
cover is expected to persist for much of the day Friday before
clearing for this weekend.

The second impact of this shortwave will surround a low that will
rapidly develop under it in the Western Gulf this morning. Signs
of this development are already being observed with gale force
winds that rapidly developed through Shelikof Strait overnight.
These winds are expected to spread northward into the Barren
Islands and areas of the northern Gulf this morning, where they
will persist for much of the evening. This low is also expected to
bring rapidly increasing outflow winds in particular to
Resurrection Bay later this morning. Gap winds along the North
Gulf will then begin to diminish today as the low center races
into the Eastern Gulf. Winds throughout the Gulf will then slowly
diminish on Friday as the pressure gradient slowly weakens ahead
of incoming high pressure. This trend will continue into Saturday,
leaving much of the area with a period of dry and benign weather
to begin the Labor Day weekend.

&&

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

An elongating upper trough over the Bering Sea, with shortwaves
rotating inland, will dominate the pattern for the next two days.
Currently, strong southwesterly onshore flow will continue through
today then weaken slowly overnight. A shortwave trough embedded
within the flow will move inland later this afternoon and tonight,
bringing more organized but still light rain to the Kuskokwim
Delta and Valley.

Thereafter, the upper low and associated surface pressure gradient
will weaken and fall apart with Friday remaining showery and
cloudy, but less windy. Complete drying and weak offshore flow
develops Friday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A strong gale force low over the northwest Bering will slowly
weaken today and drift to the southeast Bering Sea Fri night
before weakening and moving into the Gulf of Alaska Saturday. The
flow over the Bering Sea will gradually shift to northerly. Upper
ridging will rapidly build in so winds will slacken off to less
than 20 knots over the whole Bering Sea by Friday night.

&&

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

An extended period of dry, quiet weather is on tap for all of
Southern Alaska through the entire long-term forecast period. High
pressure centered over the central Bering on Sunday builds
eastward across the area through Wednesday. As the center of the
high moves from the Bering into the Gulf Tuesday and Wednesday,
there may be some stronger offshore gap winds through Seward,
Whittier, and Valdez. The front mentioned in previous discussions
moving in from the north has once again been pushed back to
Monday evening, and is only being depicted by the GFS. Thus, even
with that one feature, a dry forecast appears the most likely.
Another front and associated precipitation moves into the western
Aleutians Sunday evening, but since it's bumping up against a
strong and persistent high, it only drifts eastward. The GFS and
EC only get the leading edge of the precipitation into the eastern
Aleutians by Wednesday evening. Enjoy the beautiful early
September weather, Southern Alaska!

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 119 120 130 131 138 139 179 185 351 411 412 413 414.

&&

SYNOPSIS AND MODEL DISCUSSION...KO/JER
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...JW