Small Craft Advisory 617, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
542 PM AKDT Sat Aug 11 2018

.ANALYSIS AND UPPER LEVELS...
The main weather feature today 9and will be for a few days) is the
strong low in the eastern Aleutians that is the remnants of former
typhoon Shanshan. This storm continues to re-intensify as it has
entered its extra-tropical phase and will likely reach a minimum
pressure in the 970s early Sunday morning. Strong jet support is
helping to intensify this low and is also enhancing the lifting
over the area which will mitigate the downslope on the lee side of
the mountains which should allow periods of both strong winds and
rain to the typically dry lee side of the mountains.

&&

.MODEL DISCUSSION...
Models are in good agreement through the next few days with all of
them locked-on to the strong low and keeping it near Southwest
mainland Alaska for a few days.

&&

.AVIATION...
PANC...VFR conditions will give way to MVFR ceilings this evening
as rain moves into the area and intensifies. Southeast winds
should remain at 10 kt or less through most of the night but then
rapidly increase toward morning.

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

An atmospheric river will help to transport a series of upper
level waves that will impact the region through early in the week.
Through the early evening hours, an upper level wave will push a
warm front across the area. Showers are currently riding along
this frontal boundary and will sweep to the northeast this
evening. The source of this front is the remnant of the now post
tropical system, Shanshan. These overnight rain showers will
persist into the morning. There will be a small window on Sunday
where the rain will end but the winds will increase.

Attention then turns towards an approaching cold front slide east
from Bristol Bay. This front will bring rains to all areas
through Monday. The heaviest will be along the coast and the
western Susitna Valley. Cross barrier flow will help to dry out
the atmosphere along the usual downsloped areas in Anchorage and
Palmer. Winds ahead of this front will begin to really ramp up
and blow through channeled terrain and areas of the Anchorage
hillside. The strongest of winds is likely going to be confined to
a 4-6 hour window late Sunday afternoon and into the early
evening in the Anchorage Bowl. Guidance is still indicating that
winds will be strong, but they will likely not be damaging. With
wet soils and fully-leaved trees, winds could be strong enough in
the Anchorage area to produce some isolated downed branches or
trees.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Rain will continue over the Southwest Mainland this evening as
the front ahead of the deepening north Pacific low tracks through
the area through tonight. As the low shifts northward tonight, the
secondary and more impactful front will sweep across the Alaska
Peninsula overnight tonight and into the Southwest Mainland.
Winds and precipitation will increase tonight into Sunday morning
as the low moves toward the southwest coast Sunday afternoon. The
front will quickly traverse the area during the day Sunday with
the low moving onshore by Sunday evening. A slight risk of
thunderstorms is possible ahead of the front Sunday afternoon in
the middle Kuskokwim basin. Gusty southerly winds ranging from 40
to 55 mph ahead of the low will shift to the northwest Sunday
evening and persist through Monday evening. High surf and areas of
coastal erosion are possible Sunday evening through Monday night,
especially from Cape Newenham to Goodnews Bay and from Port
Heiden to Pilot Point, where water levels are forecasted to peak
at one to three feet above high astronomical tide.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The main focus through the forecast period is the development and
rapid deepening of a strong low moving northeast into the eastern
Aleutians & Bering through Sunday evening. Gale to storm-force
winds will develop tonight both ahead of the low and behind the
system as it rapidly intensifies and moves into the Bering west
of Dutch Harbor. Colder air advecting in from the north behind the
low will produce gusts of 55 to 65 mph from the central Bering
and Pribilofs south through the central and eastern Aleutians.
These winds will continue through Sunday before slowly diminishing
from west to east as the low moves onshore late Sunday. Gusty
northerly winds will continue across the the eastern Bering and
portions of the eastern Aleutians and Alaska Peninsula through
Monday. Farther west, high pressure will settle over the western
Bering and Aleutians. This will keep winds light, but low clouds
and areas of fog should remain in place through Monday as a result
of the stabilized airmass.

&&

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

The long-term forecast is fraught with uncertainty straight from
Wednesday on. In general, a trough or upper level low will be over
southern Alaska to start out the forecast, this should promote
isolated to scattered shower development during the day Wednesday,
especially through the mountains. The GFS opens the upper level
low into a trough quickly, the EC maintains a much deeper upper
level low that remains nearly stationary over Bristol Bay into
Friday, while the GEM moves the low into the northern Panhandle by
Thursday afternoon. Such wildly differing solutions mean
speculating on many specifics anywhere in the long-term would be
pointless, as they're highly likely to change in the coming days.

A couple broad generalities that can be surmised are that there
are no deep, big systems producing widespread rain anywhere in
southern Alaska until possibly Saturday. Thus, it appears likely
that most areas in both Southwest Alaska and Southcentral see a
generally dry period through Friday. The persistent longwave
trough over Southern Alaska should mean daily afternoon showers,
especially in the mountains, through Friday. Those recreating
outdoors should plan on the potential for brief showers. Winds
should be generally light through the period, and temperatures
will remain seasonable as well.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storms 155 172. Gales 130-132 136-139 150 160 165 170 171
173-175 179-181 185 352 412-414.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...SS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...JW