Small Craft Advisory 670, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
442 PM AKDT Wed Aug 29 2018

.ANALYSIS AND UPPER LEVELS...
A gale force low remains over the central Bering with its front
moving over the AKPEN and Southwest Alaska. Showers have been
spotted on radar over Southwest Alaska as this front is
stretching out and weakening. A southwesterly 150 kt jet sits over
the Aleutians and southern Bering. Ridging persists over the Gulf
into Southcentral, leaving Anchorage and surrounding areas with a
gorgeous summer day with dry conditions and mostly blue skies
this afternoon.

&&

.MODEL DISCUSSION...
Models continue to be in good agreement with the timing and
placement of the Bering low as it moves eastward across the Bering
through Friday. The main forecast challenge remains the precipitation
amounts as the shortwave moves into Southcentral on Thursday.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist into tonight.
Gusty southerly winds will begin late tonight and continue through
Thursday afternoon. Occasional MVFR conditions are possible in
showers on Thursday, but VFR conditions are expected to
predominate.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...

Clear and dry conditions will persist through the evening hours
as high pressure crests over the region. Tonight, clouds and
the chance for showers increase in advance of an upper-level
shortwave moving into the region from the west. Gap winds along
Turnagain Arm and along the Copper River will also increase
tonight as the pressure gradient between the ridge and a
developing surface low off Kodiak Island increases. These winds
are not expected to be all that strong, but an increasing
southwesterly flow up Cook Inlet this evening will push some of
the gusty winds across Anchorage overnight tonight through
Thursday morning. These winds will quickly turn back offshore (to
the west along Turnagain Arm) by Thursday afternoon as the low
moves into the northern Gulf and a ridge builds across Southwest
Alaska.

Rain will also accompany the shortwave as it moves through
on Thursday. The majority of the precipitation is expected to
fall from the Mat-Su Valleys south to Eagle River and east from
Prince William Sound through the Copper River Basin. However,
some light rain could fall across the Anchorage Bowl early
Thursday as a southwesterly flow ahead of the trough draws
moisture up the Cook Inlet. Although, a general clearing trend
begins Thursday night, a few showers will linger across the
higher terrain Thursday night into Friday as the trough axis
loses momentum and lingers along the northern Gulf. High pressure
then begins to build back over Southcentral for Saturday, with a
return to drier conditions and an offshore flow.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Saturday)

A somewhat wet pattern will be in place for parts of the region
through Friday. A weakening low will continue to track toward
Saint Matthew Island through the night, with it's attendant
front, currently stretching from the central Kuskokwim Valley to
Iliamna, with widespread showers and gusty winds developing
following frontal passage, and this looks to continue for tonight.
On Friday, as the initial low weakens, a vort lobe swinging
southeast towards the Pribilofs from the northwest, helping to
deepen the trough and induce new cyclogenesis in this area, then
tracking into Bristol Bay by late evening. A moist cyclonic flow
will remain in place given this scenario, with showers continuing
across much of the region.

Heading into your weekend, high pressure will build east across
the area, allowing for partly to mostly sunny skies to develop
with drier conditions ensuing.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Saturday)

The latest ASCAT pass continues to indicates Gales of 35 to 40
knots for much of the Bering near the area of low pressure, and
this should continue through the evening with seas up to 22 feet.
As the low continues to weaken overnight near Saint Matthew
Island, these winds will drop to small craft criteria. A new low
will form tomorrow morning near the Pribilof Islands, scooting off
to Bristol Bay by late evening, but even so the wind fields will
continue to weaken. The other main affect from these systems will
be to keep a general showery pattern in place as cyclonic flow
continues for the Bering, becoming more isolated towards the AKPEN
and Aleutians.

All good things must come to an end (if you like rain and wind) by
Saturday, as an area of high pressure develops across western
portions of the chain/Bering, allowing dry advection to take hold
with skies clearing some.

&&

.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 120 179 185 411-414.

&&

SYNOPSIS AND MODEL DISCUSSION...ALH/BL
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JW