Small Craft Advisory 562, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
503 PM AKDT Sat Aug 3 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 very moist air is bringing significant rains in between these
two features, stretching from the central/eastern Aleutians
through the Kuskokwim Delta and Lower Kuskokwim Valley areas.
There is also a fairly strong surface front associated with this
moisture stream, with gale force winds from the central/eastern
Aleutians northward through the Pribilofs and eastern Bering. 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, and
some showers are occurring 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. Some lower
clouds could develop with marine influence overnight into Sun, but
should remain above 3,000'.

&&

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

A large rather warm upper level ridge centered south of the
western Gulf will remain in place through Sunday before starting
to slowly drift east on Monday. An associated surface ridge will
continue to bring southwesterly flow to the Gulf and Southcentral
Alaska through Monday before the flow begins turning more
westerly Monday night and Tuesday. Rather wet frontal systems
pushing through interior Alaska will continue to periodically push
clouds into Mainland of Southcentral Alaska as well as bringing
precipitation near and along the Alaska Range. Further to the
south, scattered to numerous showers are possible each afternoon
and evening across the Susitna Valley, Copper River Basin and
Talkeetna mountains with isolated showers possible further south
over the Chugach Mountains.

&&

.FIRE WEATHER...

While the pattern will remain warm, it will otherwise continue to
be fairly mild fire weather conditions for Southcentral through
Tuesday. An upper level ridging strengthening and sharpening over
Southwest Alaska Tuesday night and Wednesday will allow northerly
offshore flow to increase over Southcentral Alaska for Wednesday
and Thursday. Once the northerly flow develops, skies should clear
out rapidly on Wednesday with RHs dropping and N-NW winds
increasing. Winds will be generally out of the north through the
Susitna Valley with the strongest winds in Broad Pass and breezy
N-NW winds continuing south to the western Kenai Peninsula.
Northerly gap winds will channel from Moose Pass into Seward and
through Resurrection Bay, with westerly gap winds increasing
through Whittier and Passage Canal.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Saturday
night through Tuesday)...

A cold front stemming from a mature low in the Bering Strait and
draping across the Kuskokwim Delta this evening will become
stationary overnight before lifting back to the north as warm
front tomorrow. Copious moisture is streaming in from the
southwest along this boundary in the form of an atmospheric
river, helped in part by a second broad double-barrel low spinning
over the southern Bering Sea. This is producing widespread
rainfall from the Kuskokwim Delta and the Kilbuck Mountains east
along the Kuskokwim Valley. With this feature slow to move in any
direction, moderate to heavy rain at times rain will continue over
these locations through Monday. An additional 1 to 2.5 inches of
rain is expected between tonight and Monday. Precipitation will
finally wind down on Tuesday as a ridge of high pressure south of
the AKPEN lifts a bit to the north, just enough to allow the low
and its associated front to retreat to the north and west.
Weakening onshore flow, however, will keep scattered low clouds
and showers along the coast and mountain locations. South of the
stalled front, onshore flow will keep areas of low stratus and
scattered rain showers over the Bristol Bay region. However, with
more influence from the ridge to the south, there may be some
breaks in the cloud cover, allowing for slightly warmer
temperatures.

Gusty southwesterly winds will persist along the front through
late Sunday. These winds will continue to push water up along the
Kuskokwim Delta Coast through the weekend. Elevated water levels
could lead to minor coastal erosion. Residents and those with
interests along the coast are encouraged to move property away
from the shore. A Special Weather Statement remains in effect to
address this.

&&

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

A broad double-barrel low over the southern Bering and its
associated fronts stretching from the central Aleutians northeast
across the eastern Bering will continue to produce a large swath
of southerly small-craft winds and moderate rainfall over the
eastern half of the Aleutians and Bering through Monday. This
system weakens and retreats to the north and west on Tuesday as a
ridge south of the AKPEN strengthens.

&&

.MARINE (Days 3 through 5)...
(Tuesday through Thursday)

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

High pressure will build across the eastern Aleutians, AKPEN, and
across portions of the Bering...mainly south and east of a
Pribilof Islands to Nunivak Island line. This will promote mainly
dry conditions with sustained winds and seas remaining below small
craft conditions.

As the high builds westward from the Gulf during this time, the
associated storm track will lie across the western Aleutians and
Bering, where periods of rain are likely. However, here too
sustained winds and waves look to remain below headline criteria,
with no real impacts expected.

...Gulf of Alaska...

High pressure across the north Pacific will place much of the Gulf
on its northern periphery, with west to northwesterly flow
dominating. These winds will be enhanced through the gaps. As
such, sustained winds in the 20 to 33 knot range (small craft
criteria) are likely from Kamishak Bay to east of the Barren
Islands, with seas topping out near 11 feet (also small craft
criteria of 8 feet or higher). Periods of sustained winds near 25
knots appear likely south of Marmot Island as well.

&&

.LONG TERM FORECAST (Days 3 through 7)...
(Tuesday through Saturday)

An unusually strong north Pacific/southern Gulf of Alaska area of
high pressure will retrograde as its center repositions to south
of Dutch Harbor by Friday evening. Meanwhile, an active Arctic
jet stream will try to send a surface trough southeastward. But
the main effects of it look to remain across the east central
interior and points north. The net affect of these two systems
will be to promote offshore flow, with 700mb RH progs shows a
distinct drying trend, putting the end to any hopes of shower
development across much (if not all) of Southwest and
Southcentral Alaska.

The real questions are how much will the pattern amplify, as the
ridge develops into an Omega Block pattern across the central
Bering and Aleutians, aided by strong low pressure development
(for this time of year) across the eastern Siberia/Kamchatka
Peninsula region. The European Model (ECMWF) is much more
aggressive with this than the American GFS model. The Canadian
Global shows the high amplitude ridge in this same area, but it
dives Siberian energy way too far south into the AKPEN, which
makes no sense given the ridge strength, orientation, and
progression. As such, it was discounted.

The other curious detail is it appears the ECMWF/GFS cool the
850mb temps too quickly given the lack of any real intrusions of
colder air aloft as the high retrogrades. This could very well be
a parameterization issue with both models, as this high is
unusually strong for this time of year. Plus, the models tend to
flatten ridges too quickly often. The trend this summer has been
for the more amplified pattern to verify in time, with warming
850mb (and as a result, surface temperatures) as it gets closer in
time (the models have tended to warm, temps as you get closer to
a point where your within 2 or 3 days of the event).

&&

.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