Small Craft Advisory 242, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
414 AM AKDT Thu Mar 22 2018

.ANALYSIS AND UPPER LEVELS...

A classic omega block in the upper level pattern has formed over
Alaska and the adjacent waters. An upper level low is centered off
the coast of Vancouver Island and the other one is located over
the northern Kamchatka Peninsula. A strong negatively tilted
trough is over the western Bering, helping fuel the development of
a low that will track back towards eastern Russia by this evening.
In between these two features, an expansive ridge whose axis is
just inland of the west coast of Alaska completes the omega block
as the jet stream navigates around all of these features to
create the look of a Greek capital letter omega. Weatherwise, a
block of this type, as the name implies, means the weather will
not be changing much from current status quo. Continued
seasonable, sunny, dry, windy weather is expected with strong
offshore flow continues to the east of the ridge axis, while
stormy unsettled weather remains the forecast to the west side.

Locally, the jet stream turning to a northeast to southwest
orientation will support the continuation of the strong winds
through Thompson Pass, Valdez and the nearby waters, the Copper
River Delta, and Resurrection Bay near Seward. Additionally, a
modest amount of cold air in the Copper River Basin in conjunction
with the jet is helping "push" that cold air down the Matanuska
Valley, resulting in gusty winds through Palmer-Wasilla.
Fortunately, being late March, that cold air is not nearly as cold
as it could be if it were deeper into winter, and as such damaging
winds will be isolated at best in that area. A similar pattern in
December or January with colder air would be a far bigger threat.

The High Wind Warning continues through Thompson Pass, which saw
an 89 mph gust recorded by the DOT station located there earlier
yesterday evening.

&&

.MODEL DISCUSSION...

The models all remain in very close agreement through Saturday
morning, so confidence remains high. Forecast challenges remain
regarding winds through the gaps. The GFS also moves the low over
the far western Bering a bit faster than the other models by
Saturday morning, but as far as differences to the broad weather
pattern, they are few. The high resolution models were preferred
for this forecast package for their better handling of the winds.

&&

.AVIATION...
PANC...VFR conditions will persist through Sunday. Winds will
diminish through this afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level ridge sitting to the west of Southcentral will
incrementally shift eastward toward Southcentral through tomorrow,
leading to a very gradual weakening of ongoing offshore winds
across the region. A series of short-wave troughs moving in from
the east will push up against the ridge this morning, enhancing
subsidence in eastern Prince William Sound and maintaining periods
of warning level winds (75 mph) in the Valdez/Thompson Pass area.
Winds will also remain quite robust in the Matanuska Valley,
with more favorable low level flow out of the northeast now well
established with the low level ridge centered over the eastern
Interior. Winds should begin to diminish for all areas by around
midday today, then continue the aforementioned slow downward trend
tonight through Saturday as the upper jet exits to the south and
the ridge nudges in from the west.

A rapidly weakening front crossing Southwest Alaska Thursday night
does not look like it will survive the crossing into the western
Gulf. Maintained chance Pops for southern Kodiak island, but even
that looks suspect. The upper level short-wave trough looks more
impressive, so do think will see some mid level clouds moving into
Southcentral Friday through Friday night. This will also lead to
development of a weak surface low over the northern Gulf Friday
night, which will combine with weak cold air advection to bring
another round of winds to Gulf coastal areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A frontal system approaching from the west will reach the
Kuskokwim Delta this afternoon and then fall apart as it continues
inland tonight through Friday morning. An upper level trough
bringing a colder airmass with it will swing inland Friday
afternoon. Flat ridging will build Friday night and Saturday
before the next frontal system swings inland around Sunday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A rapidly deepening low is lifting north through the western
Bering while the associated strong frontal system pushes west into
the central Aleutians. With the surface low still strengthening
and strong cold air advection pushing in behind it, conditions
look favorable for storm force winds associated with the bent-
back occlusion to follow the low north-northeast into the Bering
today. The cold frontal portion of the storm will track east today
but weaken rapidly as it reaches the southwest Alaska coast late
this afternoon and evening. A broad upper level low and several
small short waves will keep cool showery weather in place over the
Bering tonight and Friday. The next incoming frontal system will
track from the North Pacific across the Aleutians and into the
Bering Friday evening through Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The extended portion of the forecast beginning Friday night will
see outflow winds diminishing across Southcentral as the Bering
ridge shifts eastward to the mainland and breaks down by Saturday.
This will bring about a return to a more progressive pattern
going into next week as several systems track across the region
from west to east. The next good chance for precipitation across
Southcentral looks to occur on Sunday as a low moves into the
eastern Bering and a front extends across the Alaska Peninsula
into to the Gulf. Forecast confidence decreases after Monday as
guidance begins to diverge more substantially. There is a general
consensus on a more active and somewhat wetter pattern for the
southern mainland going through next week, however the individual
details remain unresolved at this time as models continue to
struggle after Day 5.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 131.
MARINE...Storm 179 181 185 412.
 Gales 126-128 165 170 172 173 175 177 178 180 411 413 414.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JER
LONG TERM...CB