Small Craft Advisory 564, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 PM AKDT Sun Aug 4 2019

.ANALYSIS AND UPPER LEVELS...
There is an upper level trough over the central to western Bering
and Aleutians. There is a strong upper level ridge that extends
from the Gulf of Alaska to south of the Alaska Peninsula. A stream
of moist air is bringing rain in between these two features,
stretching from the eastern Aleutians through the Kuskokwim Delta
and Lower Kuskokwim Valley areas, then through the central and
northern Susitna valley to the northwest Copper River basin.
The interior Bristol Bay area is seeing decent onshore flow, and
thus, have overcast skies and some showers. Over Southcentral, the
westerly flow aloft is allowing some clouds to cross the area,
with showers limited to areas north of Anchorage.

&&

.MODEL DISCUSSION...
The numerical models are in rather good agreement for the short
term portion of the forecast. Therefore the forecast confidence
remains above average.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

Strong ridging at both the surface and upper levels persists
across the Gulf of Alaska with the upper level ridge centered over
the Southwest Gulf. Its influence extends across the entire area.
For the Gulf, the net effect will continue to be large areas of
marine stratus with southwesterly onshore flow increasing each
afternoon and evening up Shelikof Strait and Cook Inlet. Over
Kamishak Bay and the Barren Islands, the northern two thirds of
Kodiak Island and the southern end of the Kenai Peninsula, west to
southwest winds will persist with the strongest gusts in the 35
to 40 knot range continuing near the Barren Islands. These west to
southwest winds with more of an offshore and downsloping
component will also tend to keep these downwind areas more stratus
free. Over mainland Southcentral Alaska, the primary influence of
the ridge will be to keep temperatures warm over much of the area.
This will be in spite of clouds and some precipitation from the
frontal system spilling over the Alaska Range and into the
Susitna Valley and northern Copper River Basin.

Minimal changes are expected in the synoptic pattern tonight
through Monday. While the interior warm front lifts slightly
further north as it progresses slowly east, there will still be
enough moisture and instability around for showers develop along
and near the mountains from the Anchorage area north Monday
afternoon and evening with increasingly higher shower chances
further north along the Talkeetna Mountains, Susitna Valley and
Copper River Basin with the possibility of a few isolated wet
thunderstorms as well.

On Monday night and Tuesday, the upper level ridge in the Gulf
begins slowly shifting to the east and then south with a sharper
ridge axis building in over western Alaska Tuesday night. This
progression of the ridge, as well as an upper level trough
digging into northwest Canada will put Southcentral Alaska into
more of a northerly flow pattern with offshore flow increasing
Tuesday night into Wednesday. While the upper levels of the
atmosphere will cool as a result of this pattern shift, both the
downsloping effects of the offshore flow and more abundant
sunshine due to clearer skies will keep surface temperatures warm.
Also, the offshore flow looks more than sufficient to overpower
the diurnal sea breezes and cut the area off from the moderating
marine influence.

&&

.FIRE WEATHER...

For the near term, minimal changes area expected in the pattern
with temperatures remaining warm but RHs staying moderated by the
persistent onshore southwesterly flow. Light precipitation
continues to spread into the Susitna Valley and northern Copper
River Basin from the front pushing into Interior Alaska with
diurnally enhanced shower activity continuing through Tuesday.
Beginning Tuesday evening, increasing offshore flow will end the
shower activity and bring clearer skies and drier conditions to
the area for Wednesday and Thursday.

North to northwest winds will increase Tuesday night into
Wednesday which will favor stronger gusts through the gaps. North
to northeasterly winds increase through Broad Pass in the
northern Susitna Valley with winds more out of the northwest in
the Copper River Basin. Offshore gap winds through the mountains
near the Gulf will be likely be the strongest and the driest as
they will be further enhanced by a lee side trough stretching from
Prince William Sound to the east into the northern Gulf.

&&

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

A stalled frontal boundary over the YK-Delta will begin a
northward trek overnight. However, a strong southwesterly flow
ahead of a decaying frontal boundary over the eastern Bering will
allow the conveyor belt of moisture streaming over the Kuskokwim
Delta and Valley to persist through Monday. By early Tuesday, the
ridge that has been sitting south of the AKPEN providing
relatively dry conditions as far north as the Bristol Bay region
will lift to the north and west. As it does, the atmospheric river
stretching from the central Aleutians to Kuskokwim Bay will shut
off the as the southwesterly winds shift more westerly. Any
lingering precipitation will diminish quickly on Tuesday with only
a few scattered showers remaining over the higher terrain by
Wednesday. The onshore flow, however, may keep plenty of low
clouds around, especially along the coast and over the YK-Delta
through mid-week.

&&

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

The double-barrel low over stretching across the central
Aleutians and Bering will continue to weaken as it slowly drifts
west in response to a strengthening ridge across the eastern
Aleutians and AKPEN. Moisture streaming northward along a decaying
cold front over the eastern Bering will persist from the Dutch
Harbor to Kuskokwim Bay through Monday before this boundary breaks
down the ridge builds from the east. The gradient between these
two features will continue to produce southerly small-craft winds
through Monday evening. Southerly flow around the backside of the
ridge will draw an area of weak low pressure northward across the
western and central Aleutians Tuesday into early Wednesday. The
center of circulation tracks north into the western Bering on
Wednesday with scattered showers lingering along its front draped
over the central Bering.

&&

.MARINE (Days 3 through 5)...
(Wednesday through Friday)

...Bering Sea and the Aleutian Coastal Waters...

The battle of systems will be the main weather theme, as a ridge
of high pressure becomes anchored south of Cold Bay and stretches
through the central Bering. One weakening front will bump into
this ridge on Wednesday and weaken, with a much stronger front
entering the picture across the western Bering and Aleutians on
Friday.

Relatively quiet conditions look to continue through the period,
with sustained winds remaining under 20 knots in most areas, and
waves less than 10 feet expected. The exception to this being east
of the Shumagin Islands south of the AKPEN, where offshore
northwesterly flow on the eastern edge of the ridge will promote
sustain winds of 20 to 25 knots. Otherwise, mainly periods of
showers with the fronts mentioned above look to be the main item
of interest.

...Gulf of Alaska...

With high pressure repositioning to the west by the middle of the
week, a strengthening offshore flow pattern will develop. This
will promote winds increasing into the 25 to 30 knot range from
Kamishak Bay, through the Barren Islands, and into the central/
southern Gulf for Wednesday into Thursday. It will be possible to
see a few low- end gales develop in later forecasts.

A separate area of winds near 25 knots looks to develop south of
Sitkinak Island during the same time period as well. Otherwise,
look for a dry northerly flow to keep a mainly quiet weather
pattern in place.

&&

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

The weather forecast is starting to look more interesting, as
model agreement and continuity is showing signs of improvement as
we head into the weekend. In the mean time, an area of high
pressure will retrograde (move west) from the north
Pacific/southern Gulf to south of Cold Bay for the middle of the
week, while extending to eastern Siberia. As this occurs,
offshore flow will increase, helping to decrease cloud cover with
temperatures warming into the mid 70s to lower 80s for southcentral.
For southwest Alaska, this warm up will be muted somewhat, due to
the high centered overhead creating less of a pressure gradient
(weaker wind field), helping limit heating via down-sloping
effects and allowing for some marine stratus.

A rather significant shift has been seen with today's model runs,
as both the European model (ECMWF) and Canadian Global have
shifted their solutions towards the GFS for the start of the
weekend. They now all show a western Pacific disturbance riding
the periphery of the low, reaching eastern Siberia Thursday
evening, then diving southeast on Friday into Saturday. They (the
models) all differ on strength and positioning, with the Global
being centered near McGrath by Saturday, the GFS near Goodnews
Bay, and the Euro near King Salmon. In any event, should this
trend continue, cooling conditions and a return to a showery
pattern could re-emerge for next weekend.

&&

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

&&

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