Small Craft Advisory 789, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
455 PM AKDT Sat Oct 13 2018

.ANALYSIS AND UPPER LEVELS...

The upper level semi-stationary low over the eastern Bering, that
once brought significant weather to southern Alaska, is shifting
northward and is beginning to weaken. South easterly flow aloft is
creating drier conditions and thus, patchy skies lie over the
central and eastern Aleutians as well as the western mainland.
The next Bering low is on the way as it has moved just south of
the western Aleutians from Kamchatka. This low sits in a favorable
jet position in the left exit region where winds are approaching
110 kt. This low is deepening as it moves eastward. A second
section of the jet stream at 300 mb is ridging over eastern Alaska
with a jet max sitting over Prince William Sound. The placement
of the jet combined with residual energy from the low in the
eastern Bering is increasing instability over Southcentral and the
AKPEN.

At the surface, convective style rainfall continues to fall along
the western coast as the surface low slowly migrates northward.
Yesterday's afternoon sounding in King Salmon showed high
instability which aided in several thunderstorms popping up over
Bristol Bay Friday evening. Another front, associated with the
approaching Bering low, has moved over the western Aleutians and
is brining cloud cover to the area. In the east, heavy rain in
Southcentral is dissipating as the warm front moves inland.
Satellite imagery shows widespread cloud cover over most of
Southcentral and the eastern Gulf while the west remains patchy
ahead of the front associated with the next surface low.

&&

.MODEL DISCUSSION...
Models continue to be in excellent agreement with synoptic
features through roughly Tuesday. Differences at that time revolve
around the progression of an anchored low in the Bering and the
timing/intensity of waves rotating around it, as well as the
weakening of the northwest Pacific ridge. By Thursday, models are
struggling to have any consistent forecast messaging.

&&

.AVIATION...
PANC...the airfield will flirt with occasional IFR conditions
early on this afternoon as the rain moves out of the area.
Otherwise persistent MVFR cigs are expected into the evening. If
clearing skies are realized before the evening, fog chances
increase. At this point, expect IFR VSBY and CIG in fog and low
clouds Sunday morning.

&&

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

A brief period of calm weather is in store for Southcentral Alaska
for the next 24 hours or so as upper level ridging builds into the
area. Clearing skies behind today's rains are gradually making
their way eastward across the area. Winds are also rapidly
diminishing as well. Thus, with very recent rainfall, clearing
skies, and diminishing winds in place, many of the needed
ingredients for fog are now present. Areas of fog are in the
forecast for nearly all of Southcentral as a result. Most of the
area should see a good amount of sunshine for the day Sunday as
well. Enjoy the sunshine Sunday because it doesn't return to the
forecast after that for quite a long time.

A large low tracking eastward across the Bering will bring a front
across the Gulf and into Southcentral Sunday night into Monday.
As usual, Kodiak will be first to see rain starting up Sunday
afternoon, and then the rain will spread northward along all of
the Gulf Coast by Monday morning. The usual downsloping winds
through the day Monday may help to keep Anchorage and portions of
the western Kenai Peninsula drier for much of the day, but the
rainfall will be around the area, affecting the western Kenai and
north through the Susitna Valley. Turnagain Arm and the Hillside
should not see winds as strong as they have been the past couple
events, with 30-40 mph winds currently expected. The winds will
diminish by Monday evening allowing nearly all areas to get in on
the rainfall action during the overnight hours, before slowly
diminishing in coverage Tuesday. Ongoing onshore flow from the
southwest will continue through the day Tuesday, making off-and-on
showers a likelihood for all areas throughout the day.

Behind the front, for a period of about 24 hours starting midday
Monday, there will be a moisture feed straight out of the tropics
that is expected to impact Prince William Sound and points east.
This feature we meteorologists call an "atmospheric river".
Atmospheric rivers are known to bring periods of heavy rain to
areas in their path, and high rainfall totals can be possible if
it remains in one place for too long. Rainfall amounts of 3-4
inches for Valdez and 4-5 inches in Cordova are possible through
midday Tuesday, with the heaviest rain expected late Monday
afternoon through Monday night. Residents of those areas should be
prepared for rising river levels to bankfull along with the
potential for localized flooding. Stay tuned to future forecasts
for more information.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
General showery conditions will continue tonight as energy
rotates around an area of low pressure in the Bering. Weak
ridging may lead to some areas of inland fog this evening and
overnight, but another approaching front will see the return to
southerly winds and rain Sunday evening and into Monday. Highs
through Monday continue to be unseasonably high, ranging 7 to 10
degrees above normal. Overnight lows should normally be in the
upper 20s bu mid October, but the cloud cover and warm wind will
keep overnight lows in the upper 30s to lower 40s.

Water levels will remain high along the SW Coast into Sunday. Due
to the duration of S-SW flow along the coast over the past few
days, water has been pushed up against coastal areas, mainly the
Kuskokwim Bay. Water levels could rise 2-3 feet above the normal
highest tide line on tonight's high tide. Water levels should then
slowly lower late Sun into the early part of the week as the flow
turns offshore. Please take caution if you are in or around
coastal areas.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A weakening low in the northern Bering will be replaced by a
slowly deepening low that will move from near Atka to just east
of the Pribilof Islands by Monday. The back (western) side of
this low will see fairly strong northerly gale force winds across
the sea, and continued windy and rain conditions along the
Aleutian chain. To the east of the low, conditions will be
similar, though winds will be a little weaker and out of the
south.

&&

.LONG TERM FORECAST (Days 4 through 7)...

At the onset of the forecast period, the longwave pattern features
ridging over Copper River Basin, a trough over SW Alaska and the
AKPEN, ridging over the western Bering and a new low near Sapporo
Japan. Anticipate a very progressive pattern. By 00z Friday, the
model guidance starts to diverge with the low near Kodiak/AKPEN
and the cyclone near Kamchatka. Given the lack of run to run
continuity and the lack of model consensus, the track and the
timing of both lows is still uncertain. Generally expecting wet
and unsettled weather with both systems. Stay tuned for more
details.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 130 131 155 160 165 170-178 180 413.

&&

SYNOPSIS AND MODEL DISCUSSION...SS/RJA
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...PS