Small Craft Advisory 712, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 AM AKDT Fri Sep 27 2019

.ANALYSIS AND UPPER LEVELS...
High pressure continues to dominate southern Alaska, keeping clear
skies over Southcentral Alaska. Out west, low pressure sits over
the western Bering with its front tracking through the central
Bering on its way towards the west coast. Between the ridge and
the low is southerly flow bringing up a strong push of warm air
and a long fetch of moisture from the Pacific.

&&

.MODEL DISCUSSION...
The models are in good agreement with the main synoptic features
through the mid term with the exception of some minor timing
differences of a surface low tracking through southwest Alaska
late Saturday. The GFS and EC have been the most consistent and
were preferred.

&&

.AVIATION...
PANC...Light winds will persist. There is the potential this
morning for fog to form along Knik Arm and move in over the
airport. However, confidence is low considering how dry it has
been around Southcentral Alaska through the night. Otherwise, VFR
conditions will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Fri and
Sat)...
The tale of 2 days is upon us. High pressure will slowly traverse
Southcentral today before a frontal system arrives by the middle
of the weekend. For today, expect mostly sunny skies with just
some increasing high clouds late in the day. There is still a good
chance that some fog will form around open water ways (especially
the Knik Arm) around sunrise this morning. Any fog that does form
should dissipate by early afternoon. Winds will generally remain
light across the area.

By late Sat morning, there will be a significant change from the
tranquil weather. A broad trough over the Bering will shift to the
east. This trough will tap into some moderate subtropical moisture
from south of 40N. The trough will ferry this moisture over the
prominent North American Ridge that resides over the Gulf of AK
and will push it into southern AK as a robust warm front. The
first noticeable component of this feature will be increasing
southeast to northwest winds through the usual gaps (Turnagain and
Knik Arms, the Copper River Valley, Passage Canal, etc). These
winds could initially provide some downsloping over the Chugach,
Kenai, and Coastal Mountains and hold some of the rainfall amounts
in check, but the Cook Inlet will still see rain. Along the coast,
rain will start on Sat morning and continue through the weekend.
There could be periods of heavy rain for the Seward area with this
system as well. The other area that will receive heavier QPF will
be the northern Susitna Valley as southerly flow leads to upslope
in that region.

The weather models are still struggling with the evolution of this
large system as we head into Sun. The higher resolution models
show a stronger surface low developing over the southern Cook
Inlet. This could keep the gustier winds at bay for most of the
lower elevations of Anchorage. However, a slight change in track
could lead to some of the Turnagain and Hillside winds mixing into
town for a brief period. Deep level southwest flow on Sun will
ensure everyone sees rain.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Sunday)...

A front is currently positioned between the Pribilofs and the
mainland coastline, bringing showers and gusty southerly winds to
coastal communities along the Kuskokwim Delta. Strong high
pressure to the east will inhibit its impacts to inland areas
today, thus although cloud cover will increase, no precipitation
is expected through this evening. The aforementioned front will
merge with a low approaching from the south by Saturday.
Associated moist southerly flow will lead to steady moderate to
heavy rain over non-downsloped areas and gusty winds across the
Bristol Bay region in the evening through Sunday morning. Rain
will taper to scattered showers Sunday afternoon in the wake of
the low, with winds likewise diminishing.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Today
through Sunday)...

A broad area of low pressure will bring mostly cloudy skies and
scattered showers to much of the western and central
Aleutians/Bering through Sunday. Its associated front extends
south of the Pribilofs and is leading to rain and gusty southerly
winds to areas farther east. This will continue to slowly track
eastward through today before merging with another low approaching
from the south on Saturday, bringing periods of heavy rain
through the day east of Unalaska. Rain will gradually end on
Sunday as a dry slot builds in the wake of the low.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

Near gale force wind in advance of a frontal boundary is possible
south of Kodiak Sunday. These strong winds will move east across
the Gulf of Alaska to the North Gulf Coast region through Sunday.
Monday a strong low in the northern Gulf of Alaska will have a
strong wrap around northwest flow across the Alaska Peninsula that
will likely produce gale force wind south of the Alaska Peninsula
and possibly in Bristol Bay. The storm in the Gulf is expected to
intensify through Tuesday and storm force north to northwest
winds are possible south of the eastern Alaska Peninsula across
the Kodiak Island waters with gale force winds very likely over
the Gulf of Alaska waters.

A front is expected to move into the western Bering Sea Monday
that may produce gale force wind in advance of it. As the front
progresses east it will weaken as will the wind in advance of it.

&&

.LONG TERM FORECAST (Days 3 through 7: Sunday through Thursday)...

The models remain in good synoptic agreement through Tuesday.
Larger synoptic scale differences arise Wednesday with respect to
a weak low in the Bering Sea and another low approaching the
central Aleutians. The models agree with a moderately strong low
in the Gulf of Alaska Wednesday, although the position of the low
center does vary between the models. Moving into Thursday the
model solutions diverge significantly with the strength and
location of a low impacting the eastern bering Sea and Alaska
Peninsula, thus confidence in the forecast for day 7 is low for
this region.

A front from a low near Kodiak Island with a long southerly
fetch into Kodiak Island and the north Gulf Coast will bring
ample rainfall to Kodiak Island and Prince William Sound Sunday,
then move off to the east. A large deep upper level trough over
the Bering Sea will bring broad southwest flow to the mainland,
essentially keeping the mainland in cloudy or wet conditions.
This trough will bring cooler air into the mainland, though not
cool enough to support snow. A low forming along the frontal
boundary near the Barren Islands intensifies in the Gulf of Alaska
Tuesday then moves off to the east late Wednesday as a broad flat
ridge of high pressure moves into the Gulf.

Ridging builds into the eastern Bering Sea late Monday with the
low in the Gulf of Alaska, resulting in drier northerly flow
across the southwest mainland along with colder air moving in.
This will also result in strong north to northwest wind across the
Alaska Peninsula into the western Gulf of Alaska. The Bering Sea
will be relatively benign for this time of year with a front
moving into the western Bering Sea Monday with possible gale
force wind. This front weakens as it elongates to the east and the
wind associated with it will drop off. The parent low to this
front moves to the northern Bering Sea Tuesday, leaving a broad
zonal westerly flow across the Bering Sea through Wednesday.
Thursday model solutions diverge significantly, with a low
possible in the northwest Bering Sea with a trough across the
Bering Sea to the Alaska Peninsula, or a strong low over the
eastern Aleutians with a weak trough to the northwest Bering Sea.
Suffice it to say, confidence in the forecast for Thursday is
very low at this time.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales...181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...SA