Small Craft Advisory 578, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 PM AKDT Fri Jul 27 2018

.ANALYSIS AND UPPER LEVELS...
An upper level low centered over Bristol Bay continues to drive
moisture from the Gulf into the southern Mainland. At the surface,
a weaker low has formed just south of Kodiak Island. Radar
imagery shows scattered showers across the Cook Inlet region and
areas in and around Prince William Sound. Surface observations
also confirm some lingering showers in the southwestern part of
the state.

To the west, the leading front of the Kamchatka low has pushed
across the Bering and is clearly defined on water vapor satellite
imagery over the central Aleutians. Also of note is the upper
level jet over the western Aleutians with max values near 90
knots that is working to steer the Kamchatka low southeastward.

&&

.MODEL DISCUSSION...
Models remain in good agreement through the early weekend. Model
solutions indicate the upper level Bristol Bay low will phase into
the upper level Kamchatka low in the southern Bering beginning
Sat afternoon. For early next week, models disagree slightly on
the exact location of this large Bering low by Mon afternoon. At
this time, the GFS and NAM place the low center just west of Saint
Paul while the EC places the low center northwest of Saint Paul.

&&

.AVIATION...
Showers this afternoon and evening will taper off overnight. With
the lower atmosphere remaining moist there is a possibility of
patchy fog forming anywhere with breaks in the clouds overnight,
However, for PANC a persistent VFR ceiling will likely keep skies
cloudy and fog less likely.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3...Sat
through Mon)...
Several waves of rain will push from south to north through the
area until ridging starts to build back in early next week. First
things first, scattered rain showers have once again established
residency across much of Southcentral. These will be on and off
through most of the evening before finally waning overnight. A
slightly stronger area of low pressure then starts to approach
Kodiak Island during the night. Models have trended a tad stronger
with this low, so we have increased winds to gale-force for the
Eastern Barren Islands Sat. Because of the fairly tight gradient
associated with it, southeast flow over the Kenai and Chugach
Mountains look to set up the typical downslope range shadow across
much of Cook Inlet. So while some scattered showers are possible
in these areas, the steadiest and heaviest rainfall will certainly
be across the Gulf Coast into Prince William Sound.

Rain will linger across Prince William Sound and much of the Gulf
Coast to start Sun. Conditions will be very slow to improve as
onshore flow persists ushering in many weak pieces of energy.
Expect widespread cloud cover to persist. But higher heights
start to slide back over the area from the Eastern Gulf as we
head toward the start of the work week. This should allow for a
drying trend across just about all areas. The further east in the
domain you are (Copper River Basin), the better chance of seeing
sunshine. This sunshine will likely help enhance instability and
could lead to some gusty southeast winds and better chance for
convection along the mountains by Mon afternoon.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
The long-wave trough over the Bering will remain in place through
the weekend. At the surface, a series of surface lows pinwheeling
around the periphery of the trough will keep clouds and periodic
showers over the AKPEN and southwestern coast. The first of these
lows will continue to lift north away from the coast tonight,
limiting showers to the immediate coast. A second low move into
the eastern Bering early Saturday bringing another round of rain
across the Alaska Peninsula and western coast with scattered
showers for the middle Kuskokwim Valley. As the low approaches,
the pressure gradient between it and an amplifying ridge over
the Gulf of Alaska will tighten, resulting in an increase in
southeasterly wind gusts across the region. This repeats for
Sunday as the second low lifts northwest and is absorbed into
a third strengthening low moving into the southern Bering. This
system will swing a front along the southwest coast and produce
another round of rain and gusty southerly winds. Gale-force winds
are possible in line from western Bristol Bay to south of Nunivak
Island Sunday afternoon and evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A low northeast of Atka will continue to track toward the eastern
Bering overnight. This will bring rain and gusty southerly winds
for the eastern Bering through Saturday. A weak ridge over the
central Aleutians will weaken and slide south tonight as a
developing low over the western Aleutians advances east. This low
will continue to strengthen through Saturday as it lifts north
into the southern Bering by early Sunday morning. Widespread
precipitation and small-craft advisory winds will develop around
the low as it slows and becomes nearly stationary through Sunday.
Winds begin to diminish Monday as the low fills and lifts to the
northwest. A building ridge upstream of the low slides over the
western Aleutians for Monday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term outlook is showing a bit of a pattern change as a
more progressive pattern is taking shape over Alaska next week.
Starting Monday night, a persistent upper level low will continue
to rotate over the Bering Sea while an upper ridge will dominate
the eastern half of Alaska and the panhandle, with axis located
directly over southcentral. This means cloudy and wet weather will
continue over the Aleutians, Bering, and western Alaska, while
dry conditions will be seen east of the Alaska Range. By Wednesday
morning, a ridge begins to build over the western Aleutians which
will push the upper low east over mainland Alaska spreading rain
from southwest east across southcentral Wednesday into Thursday.
Models begin to diverge some at this point as to where the low
goes, though there is some consensus that a secondary upper low
will ride over the top of the ridge building over the Bering and
drop southeast down across the western mainland and southcentral
towards the end of the work week. A stout cold front looks to
sweep across the area ahead of the low which would bring colder
air (850 mb temps below 5 C) and widespread precipitation. The
good news is models do show the Bering ridge pushing east over
the mainland by the weekend which could bring a few days of
sunny warm weather.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 131.

&&

SYNOPSIS AND MODEL DISCUSSION...JR
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...KVP